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CHAPTER – I
INTRODUCTION TO CRYSTAL GROWTH BY
UNIDIRECTIONAL SR TECHNIQUE AND NONLINEAR OPTICS
1.1 INTRODUCTION
Crystals have interested man because of their beauty and rarity. The
enchanting colours, the smooth surfaces with scintillating reflections of light,
the definite and varied shapes with sharp edges, the deep transparency of some
perfect crystals, all together aroused the aesthetic sense of early man who used
them as ornaments. The fantasy of their external beauty was understood more
thoroughly through the natural laws of physics, mathematics and chemistry.
The contents of the crystals and their insides were explored, analyzed and
understood by modern methods of diffraction as well as with the help of
spectroscopic and electron microscopic techniques. The external shapes, planes
and colours were correlated with the internal atomic content and their
arrangements in unequivocal terms. Thus grew a science, the study of “crystal
growth and characterization”.
Crystal growth is a multidisciplinary field covering physics, chemistry,
electrical engineering, metallurgy, crystallography, mineralogy etc. In the past
few decades, there has been a growing interest in crystal growth process,
particularly in view of the increasing demand for materials for technological
applications [1-3]. The strong influence of single crystals in the present day
technology is evident from the recent advancements in the fields of
semiconductors, polarizers, transducers, infrared detectors, ultrasonic
amplifiers, ferrites, magnetic garnets, solid state lasers, nonlinear optic,
piezoelectric, acousto-optic, photosensitive materials and crystalline thin films
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for microelectronics and computer industries. Hence, in order to achieve high
performance from the device, good quality single crystals are needed.
This chapter deals with the various methods of growing single crystals
and in particular, the solution growth method, and the development of the
nonlinear optical crystals along with the theory of nonlinear optics.
1.2 METHODS OF CRYSTAL GROWTH
The phenomenon of crystal growth is widely observed in nature and it is
found to occur in different ways, depending upon the material involved. Over
the period of time, a better understanding of the process has led to the
development of several techniques for the growth of single crystals. The
methods of growing crystals are very wide and mainly dictated by the
characteristics of the material and its size. The methods of growing single
crystals may be classified according to their phase transformations.
Growth from Solid → Solid−solid phase transformation
Growth from liquid → Liquid−solid phase transformation
Growth from vapour → Vapour−solid phase transformation
The above methods have been discussed in detail by several authors [2],
[4, 5]. The different techniques of each category are found in reviews and
books by Faktor and Garrett [6] on vapour growth, Brice [7] on melt, Henisch
[8] on gel growth, Buckley [9] on solution growth and Elwell and Scheel [10]
on high temperature solution growth.
For a successful crystal growth experiment, it is necessary to know the
following information.
(i) Proper examination of the physical and chemical properties of the
material under consideration. This step is essential to decide the most
suitable technique.
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(ii) To identify the process parameters that are likely to influence the growth.
(iii) To identify the constraints that might be faced in the optimization of the
processing parameters.
An efficient process is the one, which produces crystals adequate for
their use at minimum cost. The growth method is essential because it suggests
the possible impurity and other defect concentrations. Choosing the best
method to grow a given material depends on the material’s characteristics.
1.2.1 Low temperature solution growth
The NLO materials chosen for the present study have been grown from
low temperature solution growth technique. Solution growth is particularly
suited to those materials, which suffer from decomposition at high
temperatures and undergo phase transformations below the melting point. The
method of crystal growth from low temperature aqueous solutions is extremely
popular in the production of many technologically important crystals. The
growth of crystals by low temperature solution growth involves weeks, months
and sometimes years.
Among the various methods of growing single crystals, solution growth
at low temperatures occupies a prominent place owing to its versatility and
simplicity. Materials having moderate to high solubility in temperature range,
ambient to 100 oC at atmospheric pressure can be grown by low temperature
solution growth method. This method is the most widely used method for the
growth of single crystals, when the starting materials are unstable at high
temperature [11]. This method is widely used to grow bulk crystals, which have
high solubility and have variation in solubility with temperature. Growth of
crystals from solution at room temperature has many advantages over other
growth methods though the rate of crystallization is slow. Since growth is
carried out at room temperature, the structural imperfections in the solution
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grown crystals are relatively low [12]. After undergoing so many modification
and refinements, the process of solution growth now yields good quality
crystals for a variety of applications.
Low temperature solution growth can be subdivided into the following
methods:
(i) Slow cooling method
(ii) Slow evaporation method and
(iii) Temperature gradient method
1.2.1a Slow cooling method
Slow cooling is the easiest method to grow bulk single crystals from
solution. However, the major disadvantage of slow cooling method is the need
to use a range of temperatures. The possible range of temperature is usually
narrow and hence much of the solute remains in the solution at the end of the
growth run. The use of wide range of temperatures may not be desirable
because the properties of the grown crystals may vary with temperature.
Temperature stability may be increased by keeping the solution in large water
bath or by using a vacuum jacket. This technique needs only a vessel for the
solution in which the crystals grow. The height, radius and volume of the
vessel are so chosen as to achieve the required thermal stability. Achieving the
desired rate of cooling is a major technological hurdle. This method also has
the difficulty of requiring a programmable temperature controller. In spite of
these disadvantages, the method is widely used with great success [7]. The
temperature at which crystallization begins is usually within the range 40-70 oC
and the lower limit of cooling is the room temperature.
Hiroaki Yuan et al [13] have grown organic nonlinear optical
4-dimethylamino-4-methyl-4-stibazolium tosylate (DAST) crystals by slow
cooling method. KDP crystals were grown by Rajesh et al [14] from aqueous
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solutions with organic additives by slow cooling method for obtaining better
nonlinear properties. Nixon et al [15] have reported the growth of 4-nitro-4-
methoxy benzylidene aniline (NMOBA) by employing restricted evaporation
and slow cooling methods. In the case of KDP, the addition of 5 M % of KCl
resulted in the rapid growth of the crystals and also suppressed the
incorporation of metal ion impurities into the crystal lattice Li et al [16].
A new chelating agent diethylenetriaminepentaacetic acid (DTPA) was
employed by Haja Hameed et al [17] to investigate its effect on the metastable
zone width, crystal growth and characterization of 4-dimethylamino-N-methyl-
4-stilbazolium tosylate (DAST) single crystals. Bulk crystals of two new
organic nonlinear optical (NLO) materials of hydroxyethylammonium-L-
tartarate monohydrate and hydroxyethyl ammonium-D-tartrate monohydrate
have been successfully grown by slow-cooling method. Its structural,
spectroscopic, nonlinear and thermal properties were also discussed [18].
Bhaskaran et al [19] have synthesized a new nonlinear optical material,
namely, tetrakis thiourea nickel chloride (TTNC) and single crystals were
grown from mixed solvent of water and isopropanol by both slow evaporation
and slow cooling methods. The growth habits and transparency of KDP crystals
doped with different concentrations of sulphate were studied by Jianqin Zhang
et al [20], the crystals were grown by both the traditional temperature lowering
method and the rapid growth method. Single crystals of triglycine sulfate
(TGS) with L-glutamine and L-methionine were grown in aqueous solution by
a slow cooling method by Bharthasarathi et al [21]. Ravikumar et al [22] have
grown a new nonlinear optical active inorganic crystal of cadmium iodate
(CDI) by slow cooling method. The effects of the addition of L- lysine
monohydrochloride dehydrate (L-MHCl dehydrate) on the growth and various
properties of ammonium dihydrogen orthophosphate (ADP) single crystal
grown by slow cooling method have been studied [23]. Single crystals of
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Cesium hydrogen L-maleate monohydrate, have been grown by the
conventional slow cooling technique from aqueous solution, the grown crystals
display both platelet and prismatic morphologies depending on the imposed
supersaturation [24].
1.2.1b Slow evaporation method
This technique is similar to the slow cooling method in terms of
apparatus requirements. In this method, the saturated solution is kept at a
particular temperature and provision is made for evaporation. The Manson Jar
Crystallizer used for the solution growth technique is shown in Figure 1.1. If
the solvent is non-toxic like water, it is permissible to allow the evaporation
into the atmosphere. Typical growth conditions involve a temperature
stabilization of about 0.05 oC and rate of evaporation of a few mm3/h. The
evaporation technique has an advantage viz. the crystals grow at a fixed
temperature. But inadequacies of the temperature control system still have an
effect on the growth rate. This method can effectively be used for materials
having moderate temperature coefficient of solubility. Evaporation of solvent
from the surface of the solution produces highly local supersaturation and
formation of unwanted nuclei. Small crystals are also formed on the walls of
the vessel near the surface of the liquid from the material left after evaporation.
These tiny crystals fall into the solution and hinder the growth of the crystal.
Another disadvantage lies in controlling the rate of evaporation. A variable rate
of evaporation may affect the quality of the crystal. In spite of these
disadvantages, this is a simple and convenient method of growing single
crystals of large size.
Single crystals of pyridinium perchlorate (PyClO4) were grown by slow
evaporation of the water-ethanol (1:1) solution at constant room temperature
[25]. The habits of NMBA (4-nitro-4-methyl benzylidene aniline) crystals
grown with 10 different organic solvents were studied by Srinivasan et al [26]
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by restricted evaporation method. The growth of crystals of pure and doped
(Sr2+ and Mn2+) ammonium tetrachlorozincate (AZC) in different
concentrations by slow evaporation technique was reported by Gaffer et al [27].
The morphology, optical absorption and dc conductivity studies indicated the
influence of dopants in the crystal lattice.
Dhanuskodi et al [28] have grown 1-Ethyl-2, 6-dimethyl-4-hydroxy
pyridinium chloride dihydrate and bromide dihydrate salts by the slow
evaporation of aqueous solution at 30 °C. Single crystals of 2-amino-5-
chlorobenzophenone (2A-5CB) were grown by employing slow evaporation
technique using acetone as solvent [29,30].
Figure 1.1 Manson Jar Crystallizer
Liu et al [31] have grown L-arginine trifluoroacetate (LATF) from
aqueous solution and studied the influence of pH value on the crystal growth
using the micro crystallization method. Single crystals of cadmium thiourea
sulfate (CTS) and magnesium cadmium thiourea sulfate (MCTS) have been
successfully grown from aqueous solution by slow evaporation technique using
predetermined solubility data. The basic growth parameters of the crystal
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nuclei of the grown crystals of CTS and MCTS were evaluated based on the
classical theory of homogeneous nucleation [32]. Cadmium mercury
tetrathiocyanate single crystals were grown from acetone-water (4:1) mixed
solvent by slow evaporation solution technique [33]. An organometallic
material of mercury chloride thiocyanate (MCCTC) was synthesized in water-
methanol mixed solvent [34]. The growth of a new inorganic mixed borate of
barium strontium borate (BSB) has been reported by solution growth technique
using slow solvent evaporation method [35]. Allylthiourea cadmium bromide
(ATCB), a promising organometallic second order nonlinear optical material
was grown by isothermal solvent evaporation as well as by conventional
temperature lowering methods. The growth mechanism and surface features of
the as grown single crystals were analyzed by chemical etching analysis [36].
Single crystals of L-phenylalanine L-phenylalaninium perchlorate (LPAPCl), a
semiorganic nonlinear (NLO) material have been successfully grown upto a
size of 14 X 5 X 3 mm3. Nonlinear optical study reveals that the SHG
efficiency of LPAPCl is nearly 1.4 times that of KDP. The laser damage
density is found to be 7.4GW/cm2 [37]. Metal complexes of thiourea with
group II transition metals (Zn, Cd) as central atom and period III elements (S,
Cl) were synthesized by chemical reaction method and single crystals were
grown from aqueous solution by slow evaporation method [38]. The
investigation by Parikh et al [39] indicates that when KDP is doped with amino
acid L-alanine the SHG efficiency increases; where as the thermal stability of
the sample decrease. Bis thiourea zinc bromide (BTZB) a semiorganic NLO
material, has been synthesized and single crystals of size in the range of
5x4x3mm3 have been grown from aqueous solution by slow evaporation
method and the z-scan studies it is found that the material has large nonlinear
response [40]. Recently semiorganic compound, L-proline strontium chloride
monohydrate (L-PSCM) was grown from its aqueous solution at room
temperature [41].
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1.2.1c Temperature gradient method
This method involves the transport of the materials from hot region
containing the source material to be grown to a cooler region where the
solution is supersaturated and the crystal grows.
The main advantages of the method are that:
i) Crystal grows at fixed temperature
ii) This method is insensitive to changes in temperature, provided
both the source and the growing crystal undergo the same change
iii) Economy of solvent and solute
On the other hand, changes in the small temperature difference between
the source and the crystal zones have a large effect on the growth rate.
1.3 CRITERIA FOR OPTIMIZING SOLUTION GROWTH PARAMETERS
The growth of good quality single crystals by slow evaporation and slow
cooling techniques require the optimized conditions and the same may be
achieved with the help of the following norms: (i) Material purification
(ii) Solvent selection (iii) Solubility (iv) Solution preparation (v) Seed
preparation (vi) Agitation (vii) Crystal habit and (viii) Cooling rate.
1.3.1 Material purification
An essential prerequisite for success in crystal growth is the availability
of material of the highest purity attainable. Solute and solvents of high purity
are required, since impurity may be incorporated into the crystal lattice
resulting in the formation of flaws and defects. Sometimes impurities may slow
down the crystallization process by being adsorbed on the growing face of the
crystal which changes the crystal habit. A careful repetitive use of standard
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purification methods of recrystallization followed by filtration of the solution
would increase the level of purity.
1.3.2 Solvent selection
A solution is homogeneous mixture of a solute in a solvent. Solute is the
component, which is present in a smaller quantity and the one which gets
dissolved in the solvent. For a given solute, there may be different solvents.
The ideal solvent should yield a prismatic habit in the crystal and should have
the following characteristics [42].
(i) high solubility for the given solute
(ii) high positive temperature coefficient of solubility
(iii) low viscosity
(iv) low volatility
(v) less corrosion and non-toxicity
(vi) density less than that of the bulk solute
(vii) cost advantage
Solvent commonly used include water, both light (H2O) and heavy
(D2O), ethanol, methanol, acetone, carbon tetrachloride, hexane, xylene and
many others. Solvents having all the above characteristics together, however,
do not exist. Almost 90% of the crystals produced from low temperature
solutions are grown by using water as a solvent. Probably no other solvent is as
generally useful for growing crystals as is water. Some properties that account
for this are its high solvent action, which is related to its high dielectric
constant, its stability, its low viscosity, its low toxicity and its availability.
1.3.3 Solubility
Solubility is an important parameter, which dictates the growth
procedure. If the solubility is too high, it is difficult to grow bulk single crystals
and too low solubility restricts the size and growth rate of the crystals. Neither
a flat nor a steep solubility curve will enable the growth of bulk crystals from
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solution. If the solubility gradient is very small, slow evaporation of the solvent
is the other option for crystal growth to maintain the supersaturation in the
solution. The solubility characteristics of the solute in a given solvent have a
considerable influence on the choice of a method of crystallization.
Low temperature solution growth is mainly a diffusion-controlled
process; the medium must be less viscous to enable faster transfer of the
growth units from the bulk solution by diffusion. Hence a solvent with less
viscosity is preferable. Supersaturation is an important parameter for the
solution growth process. The solubility data at various temperatures are
essential to determine the level of supersaturation. Hence, the solubility of the
solute in the chosen solvent must be determined before starting the growth
process [43].
The solubility of the solute can be determined by dissolving the solute in
the solvent maintained at a constant temperature with continuous stirring. On
reaching saturation, equilibrium concentration of the solute can be determined
gravimetrically. A sample of the clear supernatant liquid is withdrawn by
means of a warmed pipette and a weighed quantity of the sample is analyzed.
By repeating the above procedure for different temperatures, the solubility
curve can be plotted. Solubility of most substances increases with temperature
(the temperature coefficient of solubility is positive).
1.3.4 Solution preparation and crystal growth
For solution preparation, it is essential to have the solubility data of the
material at different temperatures. Sintered glass filters of different pore size
are used for solution filtration. The clear solution, saturated at the desired
temperature is taken in a growth vessel. For growth by slow cooling, the vessel
is sealed to prevent the solvent evaporation. Solvent evaporation at constant
temperature can be achieved by providing a controlled vapour leak. A small
crystal suspended in the solution is used to test the saturation. By varying the
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temperature, a situation where neither the occurrence of growth nor dissolution
is established. The test seed is replaced with a good quality seed. All unwanted
nuclei and the surface damage on the seed are removed by dissolving it at a
temperature above the saturation point. The temperature is then lowered to the
equilibrium temperature and the growth commences. Solvent evaporation can
also be helpful in initiating the growth. The quality of the grown crystal
depends on the (a) nature of seed (b) cooling rate employed and (c) agitation of
the solution.
1.3.5 Seed preparation
Seed crystals are prepared by self-nucleation under slow evaporation
from a saturated solution (Figure 1.2). Seeds of good visual quality, free from
any inclusion and imperfections are chosen for growth. Since strain free reface
of the seed crystal results in low dislocation content, a few layers of the seed
crystal are dissolved before initiating the growth. Defects present in an
imperfect seed propagate into the bulk of the crystal, which decreases its
quality. Hence, seed crystals are prepared with care. The quality of the bulk
crystal is usually slightly better than that of the seed.
Figure 1.2 Apparatus for the preparation of seed crystals
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1.3.6 Agitation
To have a regular and even growth, the level of supersaturation has to be
maintained equally around the surface of the growing crystal. An uneven
growth leads to localized stresses at the surface generating imperfection in the
bulk crystals. Moreover, the concentration gradient that exist in the growth
vessels at different faces of the crystal cause fluctuations in supersaturation,
seriously affecting the growth rate of individual faces. The gradient at the
bottom of the growth vessel exceeds the metastable zone width, resulting in
spurious nucleation. The degree of formation of concentration gradients around
the crystal depends on the efficiency of agitation of the solution. This is
achieved by agitating the saturated solution in either direction at an optimized
speed using a stirrer motor.
1.3.7 Crystal habit
The growth of a crystal at approximately equivalent rates along all the
directions is a prerequisite for its accurate characterization. This will result in a
large bulk crystal from which samples of any desired orientation can be cut.
Further, such large crystals should also be devoid of dislocation and other
defects. These imperfections become isolated into defective regions surrounded
by large volumes of high perfection, when the crystal grows with a bulk habit.
In the crystals which grow as needles or plates, the growth dislocations
propagate along the principal growth directions and the crystals remain
imperfect [42]. Needle like crystals have very limited applications and plate
like crystals need to be favourably oriented.
Changes of habit in such crystals which naturally grow as needles or
plates can be achieved by any one of the following ways:
(i) Changing the temperature of growth
(ii) Changing the pH of the solution
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(iii) Adding a habit modifying agent and
(iv) Changing the solvent
Achievement in this area is of great industrial importance where such
morphological changes are induced during crystallization to yield crystals with
better perfection and packing characteristics.
1.3.8 Cooling rate
Supersaturation, the driving force which governs the growth of a crystal,
is achieved by lowering the temperature of a solution. Temperature and
supersaturation have to be precisely controlled for desirable results. The growth
rate is maintained linear in order to grow large crystals. This requires an
increase in the supersaturation level and linear cooling will not provide this.
Hence, after the initial growth, the rate of temperature lowering is increased.
Operation within the metastable limit occurs without any spurious nucleation in
the solution. A large cooling rate changes the solubility beyond the metastable
limit. Further, fluctuations in supersaturation may encourage solution
inclusions of flaw in growing crystals. Hence, a balance between the
temperature lowering rate and the growth rate has to be maintained.
1.3.9 Factors that influence the perfection of the crystal
Using the techniques described in the above sections, large size, well-
faceted, optically clear crystals can be produced. There are four basic factors
that determine the perfection of the grown crystal.
The perfection of the final crystal is based on:
(i) The purity of the starting materials
(ii) The quality of the seed crystal
(iii) Cooling rate employed and
(iv) The efficiency of agitation
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Hence, high quality single crystals can be grown from quality seeds in
an efficiently stirred solution.
1.3.10 Need for growing bulk size single crystal
The molecular structure and bonding interactions of the crystals chosen
for the present study could be perceived even with the help of tiny crystals
from X-ray measurements. But, for the effective use of these crystals in the
field of opto-electronics as an electro-optic modulator, frequency doubler and
for optical parametric amplification, bulk crystal with good optical quality are
needed. The dimensions of the crystals for such device fabrications range from
centimeters to inches. Though there are different methods based on different
principles for growing good crystal, the following considerations seem to be
common to all of them.
(i) Temperature fluctuation should be avoided
(ii) Growth should proceed at a constant rate
(iii) Rate of growth should be as low as possible
(v) Large temperature gradient should be avoided
(vi) Starting material should be of high purity
1.4 UNIDIRECTIONAL SANKARANARAYANAN-RAMASAMY (SR) TECHNIQUE
1.4.1 Limitations of conventional slow evaporation method
In the conventional slow solvent evaporation growth, all crystals
bounded by planar habit faces contain separate regions common to each facet
having their own sharply defined growth direction known as growth sectors.
The boundaries between these growth sectors are more strained than the
extended growth sectors due to mismatch of the lattices on either side of the
boundary as a result of preferential incorporation of impurities into the lateral
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section [44]. Further, in solution growth method, many of the commonly
observed characteristic growth-induced defect structure comprising growth
sectors and boundaries, growth banding, solvent inclusions, dislocations, twins
and stacking faults can be attributed to impurities [45]. Beside the relatively
low growth efficiency, the common problem the above method is that it is very
difficult to control the growth of crystal along a given direction. In other words,
the grown crystals must be cut and polished to obtain the specific crystal faces
before they are applied as a linear or nonlinear optical device. Many methods
have been developed for cutting cylindrical crystals. Unfortunately, the cutting
and polishing in a desired direction as phase matching angle are difficult for
organic NLO crystals because of their poor chemical stability and brittleness.
Cutting and machine working of single crystals result in the appearance of
structural defects and, what is most important, in these processes the expensive
material ends up as scraps. To minimize the above problems, crystals must be
grown with specific orientation in a growth vessel at room temperature. Hence,
there is a need to investigate a possible single crystal growth technique to grow
high quality crystal with a reasonable yield. Further, growth of crystal with
specific orientation has tremendous value in terms of its significance towards
device application.
1.4.2 Unidirectional growth of crystals: Common issues
High quality phase matched second harmonic generation (SHG) single
crystal is the current interest in the field of nonlinear optical materials [46].
Various attempts to grow bulk single crystals for optoelectronic applications
have been reported in the literature using solution method, vapour method,
the Bridgman–Stockbarger method [47], Czochralski method and recently
micro-tube Czochralski method [48]. In particular, novel growth methods from
melt such as seed-oriented under cooled melt growth method [49], the indirect
laser heated pedestal growth method [50], and horizontal Bridgman–
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Stockbarger method [51] with specially designed glass cell as crucible are the
specialized growth methods to grow oriented crystal towards a phase-matched
direction. All the above mentioned crystal growth methods involve growth at
elevated temperature and there by lead to thermally induced growth defects,
and in addition these methods employ complicated equipment and multi-step
processes. These problems limit the growth of large size crystals and the
growth of crystals in unidirection is a challenging task. In this connection, a
novel unidirectional crystal growth method was reported by Sankaranaryanan
and Ramasamy [52]. It offers a solution growth method at room temperature
involving less sophisticated equipment to grow unidirectional single crystal
with cylindrical morphology, 100% solute crystal conversion efficiency and
ease in scaling up of crystal diameter. In contrast to the conventional slow
solvent evaporation technique, the crystal is restricted to grow with a specific
direction and inside a growth ampoule. This yielded a crystal with cylindrical
morphology in contrast to the crystal with planar habit face. The solution
growth method is the basic method which is economically more viable and
simple and there are no problems with thermal decomposition.
1.4.3 Fundamentals of Sankaranarayanan and Ramasamy (SR) tehcnique
Crystallization from the solution is an important process and is a two-
step process i.e the nucleation and crystal growth. The driving force for
crystallization is the degree of supersaturation which has been commonly
expressed as the difference in concentration between the supersaturated and
saturated solutions. Compared to other crystal growth techniques, it has been
widely used to grow several types of crystals at ambient temperature.
Probably no other solvent is as generally used for growing crystals as water. If
the solvent is volatile, precautions must be taken to prevent volatilization which
promotes spurious nucleation due to temperature and concentration changes.
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In the case of organic materials, significant efforts have been made to
grow large sized crystals due to their potential application in optoelectronics
and nonlinear optical applications. However, their low melting point, weaker
mechanical properties, low thermal conductivity and the ease of supercooling
of organic materials lead to growth related problems while growing crystals
from supersaturated solutions or from melt. Extremely low growth rate and
thermal gradients are usually required for growing single crystals from melt.
From this point of view, a novel crystal growth method has been
proposed to grow organic single crystal with specific orientation in an ampoule
at room temperature [52].
1.4.4 Experimental set-up
The schematic diagram of the experimental set up is shown in
Figure.1.3. It consists of a growth ampoule made out of glass with seed
mounting pad. An outer glass shield tube protects and holds the inner growth
ampoule. A ring heater positioned at the top of the growth ampoule is
connected to the temperature controller and it provides the necessary
temperature for solvent evaporation. The temperature around the growth
ampoule is to be set based on the solvent used and is controlled with the aid of
the temperature controller. Depending on the growth rate of the crystal, the ring
heater is moved downwards using a translation mechanism. In a typical
procedure, the required solution of optimized saturation is prepared in a solvent
and then it is transferred to the growth vessel and the entire experimental set-up
is placed in a dust free hood [52].
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Figure 1.3 Schematic diagram of the SR experimental set-up
1.4.5 Growth Procedure
Before conducting the actual crystal growth experiments, solubility
experiment is carried out in different solvents at room temperature and the
suitable solvent is identified. According to the solubility data, saturated
solution is prepared and transferred to conventional growth vessel (beaker) for
preparing seed crystal by the slow solvent evaporation technique. Based on the
quality of the grown crystals, a suitable seed crystal having a reasonable size is
selected for single crystal growth with specific orientation. Based on the
observations related to induction period, growth rate and the solvent properties,
a particular solvent is selected for conducting the single crystal growth
experiment with specific orientation. Prior to growth, care has been taken to
avoid to any contamination from the growth vessel which can lead to spurious
nucleation. Also, special care is to be taken for the preparation of the solution.
The seed crystal is chemically polished and a specific orientation is
selected to impose the orientation in the growing crystal. The glass ampoule for
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conducting the single crystal growth is carefully mounted along with the seed
of a particular plane to face towards the solution poured into the vessel. The
entire set-up is porously sealed and placed in a dust-free hood.
Once the system attains equilibrium, the growth is initiated with a
suitable temperature provided by the ring heater at the top of the supersaturated
solution. The effective zone width of the solution and the maximum
temperature of the ring heater determine the effective evaporation rate of the
supersaturated solution for a given diameter of the ampoule. Due to the
transparent nature of the solution and the experimental set-up, real time close-
up observation will help to find out the solid-liquid interface. Under optimized
condition highly transparent crystal growth is seen. The shape of the solid-
liquid interface is monitored during the growth process and it was observed that
unlike in melt growth technique any change in the growth parameters of this
technique such as effective zone width temperature and the lowering rate of the
ring heater did not have any visible effect in the shape of the solid-liquid
interface. However, in few cases (as reported for benzophenone crystal); rapid
growth rate is also observed as evidenced from the less transparent region of
the ingot than the other regions of the ingot grown under optimized conditions
[52]. This fact is proved again by the existence of another less transparent
region of the ingot where fast growth was deliberately introduced. Depending
on the values of these growth parameters, the solvent evaporation rate can be
controlled more effectively since controlling of solvent evaporation results in
the controlling of degree of supersaturation in the solution.
Impurities affect the growth of different faces of the crystal to different
extents. The presence of impurities may accelerate, retard or even stop the
further outgrowth of individual crystal faces, sometimes producing a habit
change. As it it now possible to achieve growth on any desired face, the
influence of specific impurities on different faces can be found out. As
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impurity concentration varies in different faces it is possible to achieve
minimum impurity concentration in the grown crystals by choosing the
appropriate growth faces.
Microbial growth has been causing serious concern in solution growth
experiments largely due to aging of the solution. However, as fresh solution
can be constantly fed during the crystal growth, the problems associated with
microbial growth can be avoided in this method. Thus the present method
encourages the growth of amino acid crystals, which usually suffer by
microbial growth. Another added advantage is; when a suspension thread is
used in crystal growth, the region close to the thread often affects its quality.
But this situation is avoided by SR method. Sankaranarayanan and Ramasamy
method offers an elegant way to evaluate the growth rate of different faces of a
crystal by choosing the appropriate seed face for conducting growth.
1.4.6 Modified SR methods
The Sankaranarayanan and Ramasamy method, which was originally
designed to grow large size organic crystals, is now turning out to be the best
method not only for the organic crystals but also for the inorganic and
semiorganic crystals. The method has been modified by few researchers
according to the choice of the materials and other requirements.
Balamurugan et al [44] have successfully grown potassium acid
phthalate (KAP) crystal of length 140 mm and diameter 20 mm, from aqueous
solution using modified SR method setup. The modifications were mainly
focused for growth from aqueous solution. The SR method setup has been
modified in some aspects in order to grow KAP crystals. In the original form of
the SR method setup [52], depending on the growth rate of the crystal, the ring
heater has to be moved downwards using a translation mechanism. But, it is
difficult to translate the heater at the rate of crystal growth. Also the top of the
growth container (ampoule) was of the same diameter as the middle of the
22
ampoule. In the modified assembly, the top of the ampoule has a bigger
diameter compared to the middle, so that from the surface of the solution more
and more evaporation will take place and also the ring heater is not translated
but fixed on the top of the ampoule. The crystallizer is kept in a water bath to
avoid the temperature fluctuation of the daily variation. The modified SR
method setup is shown in Figure 1.4. The ring heater is connected to
temperature controller and it provides the necessary temperature. The mercury
thermometer shows the temperature near the seed. The top cover is preventing
the evaporation of water from bath and it allows the evaporation of the solvent.
An advantage of this method is that it can be used to grow single crystals of
substances that have a positive, zero, or negative coefficient of solubility.
Additionally, it is suitable for crystal growth of solids that have a narrow
temperature range for thermal stability since this process is carried out
isothermally.
Figure 1.4 Modified SR method experimental setup
23
Another modified SR method procedure has been reported by Bairava
Ganesh et al [53]. An efficient semiorganic nonlinear optical crystal
L-Glutamic acid hydrochloride has been grown by using the novel uniaxial
crystal growth method of Sankaranarayanan and Ramasamy with a slight
modification in the experimental setup. This method allows the crystals to grow
in one specified axis with well developed facet. In this method, the ring heater
is replaced with a furnace-like arrangement. It can be compared with a single
zone Bridgman furnace. The heater assembly consists of a 6 cm diameter, 30
cm long cylindrical glass tube with the heating element wound over it. The
assembly is designed in order to obtain a temperature profile with maximum
temperature at the top. The temperature reduces as we move from top to bottom
of the furnace creating a gradient along the axis. The gradient is adjusted
according to the requirement by varying the spacing between each winding.
The growth ampoule with a seed fitted at the bottom is filled with the saturated
solution of L-Glutamic acid hydrochloride. The ampoule is placed along the
axis of the growth assembly. The temperature gradient creates a concentration
gradient along the growth ampoule, with a maximum supersaturation at the
bottom of the ampoule and a minimum at the top of the tube, thereby avoiding
any possibility of a spurious nucleation along the length of the ampoule. The
temperature of the furnace is controlled with an indigenously developed
controller with an accuracy of ± 0.1 ˚C. The excess the solute generated by
evaporation of the solution is driven down the ampoule by the temperature
gradient of the furnace setup. Thus the growth commences upon the seed fixed
at the bottom of the ampoule with desired orientation. After one week,
cylindrical shaped crystal with good optical quality has been obtained. With the
modified apparatus, L-Glutamic acid hydrochloride crystals of length 60 mm
and 10 mm diameter was successfully harvested [53].
24
Justin Raj et al [54] have employed an assembly of alternating 40 W
filament lamps to replace the ring heater and also the bottom seed fitted thistle
funnel replaced the ampoule for growing KDP crystal. The schematic
arrangement is shown in Figure. 1.5. The thistle funnel was placed along the
axis of the growth assembly maintaining steady temperature around it. The
temperature gradient (0.5 K per cm) has been adjusted according to the
requirement by varying the spacing between the lamps placed alternately. The
seed-fitted funnel was filled with saturated solution of the KDP salt. The
temperature near funnel was maintained at 313 K using a temperature
controller setup for the evaporation of saturated solution at the top of the
funnel. The seed fused portion of the funnel is immersed in water coloumn to
maintain the required temperature gradient. The outer surface of the growth
assembly was maintained at room temperature. The temperature gradient
makes the concentration gradient maximum at the bottom and minimum at the
top of the funnel for avoiding the spurious nucleation along the axial dimension
of the funnel. The growth rate of the crystal was found to be around 5 mm per
day. KDP crystal of 5 mm diameter and 60 mm length has been grown
successfully within a period of 30 days. The grown crystal exhibited the
cylindrical morphology same as that of the growth vessel.
25
Figure 1.5 Crystal growth assembly
Dinakaran and Jerome Das [55] reported on optically transparent bulk
single crystal of lithium para-nitrophenolate trihydrate (NPLi) along (1 1 0)
plane using the uniaxial crystal growth method of Sankaranarayanan–
Ramasamy with a slight modification in the growth assembly. The schematic
representation of the apparatus is shown in Figure 1.6. It consists of heating
coil, thermometer, inner container, temperature controller, growth vessel and
water bath. A ring heater fixed at the top of cylindrical glass tube of diameter 6
cm and height 40 cm was used as inner container and a short cylindrical
constant temperature water bath acted as the outer container. The assembly was
designed in such a way to obtain a maximum temperature profile at the top.
The ring heater connected to microprocessor controlled thermocouple provides
a constant temperature of 318 K at the top of ampoule. A seed was fixed at the
bottom of the ampoule and filled with the saturated solution of NPLi which
was mounted along the axis of the inner cylinder. The ampoule was designed
with an inner L-bend, which controls spontaneous nucleation on the top wall of
the ampoule and prevents poly crystallization. The water level inside the water
bath was increased with respect to the growth in the ampoule. The temperature
gradient creates a concentration gradient along the growth ampoule, having a
26
maximum super saturation at the bottom of the ampoule and a minimum at the
top of, thereby avoiding any possibility of a spurious nucleation along the
length of its ampoule. The excess solute generated by evaporation of the
solution is driven down the ampoule by the temperature gradient of the setup.
With this modified apparatus, NPLi crystal of 80 mm length and 12 mm
diameter has been grown successfully within a period of 12 days.
Recently, Rajesh et al [56] used another modified SR method setup to
segregate the impurities present in the solution. Many identical slots were made
in the ampoule with equal distance above the seed mounting pad. A crystal
when it is growing segregates the impurities and all the segregated impurities
are staying near to the growing crystal and there is no way to go away from the
region in the normal SR method experimental assembly. When the
concentration of the segregated impurities increases, the quality of the crystal
will become bad. It is well known that the matter diffuses from regions of high
concentration to regions of low concentration.
Figure 1.6 Modified SR Crystal growth setup
The slots made in the ampoule allow diffusion of impurities from the
high concentration to the low concentration medium, i.e the impurities present
near the crystal diffuse to the outer ampoule and several slots were made to
continue this process throughout the crystal growth process. The slots are in
27
rectangular shape and 6 mm in length and 2 mm in breadth. The diagram of the
ampoule with slots is shown in Figure. 1.7. It is expected that the impurities
segregated by the growing crystal diffuse away from the crystal vicinity to the
outer ampoule thus avoiding the defects in the crystal and other deleterious
effects of the impurities. The ampoule is placed into another big ampoule.
Saturated solution of ADP (900 ml) was used for growth. The solution was
prepared at 33 оC and it was overheated to 35 оC for few hours and again
reduced to 33 оC. Filtered solution was carefully transferred into the growth
vessel. Both the inner and outer ampoules are now filled with the solution. It is
arranged such that the inner ampoule is just taller than the outer ampoule
because, the inner ampoule is covered with porously sealed cover and allows
controlled evaporation and the outer ampoule is covered fully and no
evaporation is allowed. The ring heaters placed on the top and bottom of the
outer ampoule provide the necessary temperatures. The growth rate was
approximately 1.5 mm/day for the given ampoule of diameter 45 mm. After 40
days of the growth a good quality crystal of size 40 mm in diameter and 45 mm
in length was harvested.
28
Figure 1.7 Slotted ampoule diagram of modified SR method
1.4.7 A review on SR method grown crystals
The SR method, introduced in the year 2005 was originally designed to
grow large size organic crystals. As the growth process in organic crystals by
other conventional methods present various growth related problems, an
attempt was initially made to grow organic nonlinear optical crystal of
benzophenone and Sankaranarayanan and Ramasamy were successful in
growing unidirectional crystal of 60 mm diameter. Table 1.1 presents the list of
various crystals grown by SR method. A brief review of the work done by the
crystal growers on SR method is also highlighted.
29
Table.1.1 List of crystals grown by SR method
Harvested size Crystal Diameter
(mm) Length (mm)
Reference
Benzophenone 20 50 Sankaranarayanan and Ramasamy (2005) [52]
ADP 20 60 Sethuraman et al (2006) [57]
BMZ 15 40 Vijayan et al (2007) [58]
L-Glutamic hydrochloric acid
10 50 Ganesh et al (2007) [53]
KAP 20 140 Balamurugan et al (2007) [44]
NPLi 12 80 Dinakaran et al (2008) [55]
HA 10 23 Vijayan et al (2008) [60]
TGS 18 150 Senthil Pandian et al (2008) [59]
KDP 10 110 Balamurugan et al (2009) [61]
L-LMHCl 17 80 Senthil et al (2009) [63]
Ammonium chloride doped ADP
10 100 Rajesh and Ramasamy (2009) [62]
SA 35 150 Senthil Pandian et al (2010) [64]
LAM 18 52 Mohd. Shakir et al (2010) [65]
BTCZC 12 95 Uthrakumar et al (2011) [66]
BTCA 15 45 Ganesh et al (2011) [67]
30
Uniaxial benzophenone crystals along (1 1 0), (0 1 0) and (1 0 0)
orientation were grown by uniaxially solution-crystallization method of
Sankaranarayanan–Ramasamy (SR). A transparent uniaxial benzophenone
crystal having dimension of 500 mm length and 55 mm diameter was grown at
room temperature [68]. In contrast to the conventional solution growth method,
the growth rate along each direction was measured at ease during the respective
growth experiment by monitoring the elevation of the solid–liquid interface.
The scaling up was found to be less complicated compared to hitherto known
crystal growth methods.
A transparent uniaxial benzophenone crystal was grown at room
temperature by Arivanandhan et al [69]. In contrast to the conventional solution
growth method, the growth rate along each direction was measured at ease
during the respective growth experimental by monitoring the elevation of the
solid-liquid interface and found to be 2,4 and 6 mm /day along the (1 1 0),
(0 1 0) and (1 0 0) directions, respectively, for a chosen supersaturation.
(1 0 0) oriented benzophenone single crystal with large size (500 mm length,
55 mm diameter) have been grown by uniaxially solution-crystallization
method. A constant growth rate 100mm/day along (1 0 0) direction was
achieved at room temperature with 100% solute-crystal conversion efficiency.
X-ray topographic studies illustrate the absence of growth sectors and solvent
inclusion which are the prime inherent problem in the conventional solution
grown crystals.
Arivanandhan et al [70] have grown unidirectional benzophenone single
crystals grown by vertical Bridgman (VB), microtube-Czochralski (mT-CZ),
uniaxially solution crystallization method of Sankaranarayanan–Ramasamy
(SR) and made a systematic investigation on the growth parameters and
compared the three methods. The crystals grown by the three methods were
characterized using X-ray diffraction (XRD), high resolution XRD (HRXRD),
31
laser damage threshold (LDT) studies and the results were compared. The
structural perfection and LDT of the benzophenone grown by VB, mT-CZ and
SR methods were compared. HRXRD studies reveal that the SR grown sample
has relatively high crystalline perfection than the samples grown by other
methods. The VB grown crystal was found to have low crystalline perfection
due to the difference in thermal expansion of the growing crystal and the
ampoule which may lead to the occurrence of plastic deformation in the grown
crystal during the post-growth annealing process. The SR grown sample has
high LDT than the crystals grown by other methods, probably due to low
dislocation density in the SR grown ingots.
Good quality single crystal of hippuric acid (HA) has been grown by
Vijayan et al [60] using SR method. The crystalline perfection has been
evaluated from the HRXRD analysis and it is found to be reasonably good. The
grown crystal is found to have 1.54 times relative SHG efficiency than that of
KDP.
Unidirectional (0 0 1) bulk ferroelectric Tri glycine sulphate (TGS)
single crystal of diameter 18 mm and length 150 mm was successfully grown
by SR method. The value of Meyer's index for conventional and SR grown
TGS crystal was estimated as 2.8 and 1.8 respectively. The TGS crystals grown
by SR method are found to have higher hardness than the conventional method
grown crystals [59].
Balamurugan et al [61] successfully employed SR method in growing
inorganic NLO crystal of KDP directed along (0 0 1) with dimensions of
10 mm diameter and 110 mm length. The HRXRD analysis indicates that the
crystalline perfection is excellent without having any very low angle internal
structural grain boundaries. The better laser damage threshold value indicates
that the direction controlled KDP crystal has high damage resistance and hence
the grown crystals are useful in high powder frequency conversion application.
32
The SR method grown KDP has higher transmittance and higher hardness
value compared to conventional method grown crystals. The improved
transparency and size make this crystal suitable for SHG and optoelectronic
device fabrication.
Rajesh and Ramasamy [62] investigated the growth of (0 0 1) directed
ammonium dihydrogen phosphate (ADP) single crystal with the addition of
1 mol% of ammonium chloride in the mother solution by Sankaranarayanan-
Ramasamy method. The presence of ammonium chloride in the growth
medium suppressed the metal ion impurities and improved the quality of the
crystal. The grown crystal is found mechanically harder and thermally more
stable than the pure ADP. Higher decomposition temperature as well as
development of cracks at higher loads seems to be in conformity with each
other. Ammonium chloride doped crystals show higher intensity of green
signals than pure ADP crystals because of its better optical quality.
Senthil et al [63] have developed large diameter semi-organic
L-lysinemonohydrochloridedihydrate (L-LMHCl) single crystal using SR
method. The width of metastable zone was determined which was helpful for
the growth of larger diameter L-LMHCl dihydrate single crystal. The gravity
driven concentration gradient effect in the SR method was analyzed at different
heights. The faces which have higher attachment energy grow faster. The
results from various characterization studies demonstrate the suitability of this
method to obtain nonlinear element right during crystal growth, thus,
decreasing material consumption when making products for nonlinear optical
applications. Similarly, another unidirectional bulk semi-organic NLO single
crystal of L-LMHCl (18mm diameter and 70mm length) with the growth rate
of 5 mm/day was reported by Ramesh Babu et al [71] employing SR method.
The microbial growth was avoided and the growth conditions were optimized.
33
The SR method grown L-LMHCl dihyrate has transmittance of 96% and
moderately good hardness in the (1 0 0) plane.
Recently, Senthil Pandian et al [64] have grown a uniaxial sulphamic
acid single crystal having dimensions of 35 mm diameter and 150 mm length
by SR method. Etching behaviour of the (1 0 0) plane of conventional and the
Sankaranarayanan–Ramasamy method grown sulphamic acid crystals was
investigated with different etchants. Vickers microhardness test on the (1 0 0)
plane confirmed the mechanical stability of the conventional and the
Sankaranarayanan–Ramasamy method grown sulphamic acid crystals. High
resolution X-ray diffraction results show that the crystalline perfection of
sulphamic acid single crystals grown by the Sankaranarayanan–Ramasamy
method is extremely good compared to the conventional slow evaporation
method grown sulphamic acid crystal.
Bulk single crystal of semi-organic bis (thiourea) cadmium zinc chloride
was grown aqueous solution by a modified SR method and its high resolution
XRD measurements substantiate the excellent quality of the crystal free from
major defects like structural grain boundaries and inclusions [72].
L-asparagine monohydrate (LAM), an organic compound from the
amino acid family has been grown by slow evaporation solution technique as
well as SR method. The crystal perfection was assessed by high resolution
XRD and etching studies and it was found that the quality of the SR crystal is
better than those grown by slow evaporation solution technique [65].
Ganesh et al [67] developed (1 1 1) oriented bis thiourea cadmium
acetate (BTCA) crystal of diameter 15mm and length 45mm by a unidirectional
SR method and compared the high resolution X-ray diffraction (HRXRD),
chemical etching, Vickers microhardness, UV-Vis, dielectric studies and
differential scanning calorimetry of the sample with those grown by
conventional slow evaporation method.
34
1.5 NONLINEAR OPTICS
Almost all real systems are nonlinear. Nonlinear optical (NLO)
materials play a major role in nonlinear optics and in particular, they have a
great impact on information technology and industrial applications. In the last
decade, however, this effort has also brought its fruits in applied aspects of
nonlinear optics. This can be essentially traced to the improvement of the
performances of the NLO materials. The understanding of the nonlinear
polarization mechanisms and their relation to the structural characteristics of
the materials has been considerably improved. The new development of
techniques for the fabrication and growth of artificial materials has
dramatically contributed to this evolution. The aim is to develop materials
presenting large nonlinearities and satisfying at the same time all the
technological requirements for applications such as wide transparency range,
fast response and high damage threshold.
1.5.1 Nonlinear optical phenomenon
Nonlinear optics (NLO) deals mainly with various new optical effects
and novel phenomena arising from the interactions of intense coherent optical
radiation with matter. One of the most intensively studied nonlinear optical
phenomena and specifically the NLO property studied in the present thesis, is
second harmonic generation. Nonlinear optics (NLO) is the study of the
interaction of intense electromagnetic field with materials to produce modified
fields that are different from the input field in phase, frequency or
amplitude [73].
Second harmonic generation (SHG) is a nonlinear optical process that
results in the conversion of an input optical wave into an output wave of twice
the input frequency. The process occurs within a nonlinear medium, usually a
crystal. Such frequency doubling processes are commonly used to produce
green light (532 nm) from, for example, a Nd:YAG (yttrium-aluminium-
35
garnet) laser operating at 1064 nm. The light propagated through a crystalline
solid, which lacks a center of symmetry, generates light at second and higher
harmonics of the applied frequency. This important nonlinear property of non-
centrosymmetric crystals is called second harmonic generation (SHG) and this
phenomenon and the materials in which it occurs are the subject of intense
study.
Nonlinear optics is completely, a new effect in which light of one
wavelength is transformed to light of another wavelength. The creation of light
of new wavelength can be best understood, as we think about the electrons in
nonlinear crystal. Electrons in a nonlinear crystal are bound in potential well,
which acts like a spring, holding the electrons to lattice the points in the crystal
(Figure 1.8). If an external force pulls an electron away from its equilibrium
position the spring pulls it back with a force proportional to the displacement.
Figure 1.8 Electrons in a nonlinear crystal are bound in a potential well, holding the electrons to lattice points in a crystal
36
The spring’s restoring force increases linearly with the electron
displacement from its equilibrium position. The electric field in a light wave
passing through the crystal exerts a force on the electrons and pulls them away
from their equilibrium position. In an ordinary optical material ie., linear
optical material the electrons oscillate about their equilibrium position at the
frequency of this electronic field.
The nonlinear material is different from the linear material in several
aspects. We can think of a nonlinear material as the one whose electrons are
bound by very short springs. If the light passing through the material is intense
enough, its electric field can pull the electrons so far that they reach the end of
their springs. The restoring force is no longer proportional to the displacement
and then it becomes nonlinear. The electrons are jerked back roughly rather
than pulled back smoothly and they oscillate at frequencies other than the
driving frequency of the light wave. These electrons radiate at the new
frequencies, generating the new wavelength of light. The exact values of the
new wavelengths are determined by conservation of energy. The energy of the
new photons generated by the nonlinear interaction must be equal to integral
multiple of the energy of the photon used. Figure 1.9 shows the photons
involved in the second harmonic generation process.
Figure 1.9 Two photons are welded together to produce a single photon with the energy of both original photons
IR jjjNNNNN SR
IR
Two photons in one photon out
NON
LINEAR MEDIUM
37
When the electromagnetic field of a laser beam is illuminated on an
atom or a molecule, it induces electric polarization, which gives rise to many of
the unusual and interesting properties that are optically nonlinear. In a
dielectric material, the influence of an electric field causes distortion in the
spatial distribution between the electrons and the nucleus. These distortions
cause electric dipoles, which in-turn manifest as polarization [74]. At very low
fields, the induced polarization is directly proportional to the electric field.
However, at intense electric fields, polarization becomes independent of the
field and the susceptibility becomes field dependent. The induced polarization
is capable of multiplying the fundamental frequency to second, third order and
even higher harmonics. The re-radiation from the oscillating dipoles differs in
amplitude with respect to the incident sinusoidal electric field. As a
consequence, the distorted reradiated waves contain different frequencies from
that of the incident wave.
1.5.2 Theoretical explanation of nonlinear optics
The explanation of nonlinear effects lies in the way in which a beam of
light propagates through a solid. The nuclei and associated electrons of the
atoms in the solid form electric dipoles. The electromagnetic radiation interacts
with these dipoles causing them to oscillate, which by the classical laws of
electromagnetism, results in the dipoles themselves acting as sources of
electromagnetic radiation.
If the amplitude of vibration is small, the dipoles emit radiation of the
same frequency as the incident radiation. As the intensity of the incident
radiation increases, the relationship between irradiance and amplitude of
vibration becomes nonlinear resulting in the generation of harmonics in the
frequency of radiation emitted by the oscillating dipoles. Thus frequency
doubling or second harmonic generation (SHG) and indeed higher order
frequency effects occur as the incident intensity is increased.
38
In a nonlinear medium the induced polarization is a nonlinear function
of the applied field. A medium exhibiting SHG is a crystal composed of
molecules with asymmetric charge distributions arranged in the crystal in such
a way that a polar orientation is maintained throughout the crystal.
At very low fields, the induced polarization is directly proportional to
the electric field.
P�
= εoχ E�
(1.1)
Where χ is the linear susceptibility of the material, E�
is the electric field
vector, εo is the permittivity of free space.
At high fields, polarization becomes independent of the field and the
susceptibility becomes field dependent. Therefore, this nonlinear response is
expressed by writing the induced polarization as a power series in the field.
P�
= εo χ (1) E�
+ χ (2) E�
. E�
+ χ (3)E�
. E�
. E�
+... (1.2)
In nonlinear terms, product of two or more oscillating fields gives
oscillation at combination of frequencies and therefore the above equation can
be expressed in terms of frequency as:
P�
(-ωo) = εo χ (1) (-ωo; ω1). E�
(ωo) + χ (2) (-ωo; ω1, ω2). E�
ω1. E�
ω2 +
χ (3) (-ωo; ω1, ω2, ω3). E�
ω1. E�
ω2. E�
ω3 +…. (1.3)
Where, χ(2), χ (3) …. are the nonlinear susceptibilities of the medium. χ(1)
is the linear term responsible for material’s linear optical properties like
refractive index, dispersion, birefringence and absorption. χ(2) is the quadratic
term which describes second harmonic generation in noncentrosymmetric
materials. χ (3) is the cubic term responsible for third harmonic generation,
stimulated Raman scattering, phase conjugation and optical instability. Hence
the induced polarization is capable of multiplying the fundamental frequency to
second, third and even higher harmonics. The coefficients of χ (1), χ (2) and χ (3)
39
give rise to certain optical effects. These are listed in Table 1.2. If the molecule
or crystal is centrosymmetric then χ(2) = 0. If a field +E�
is applied to the
molecule (or medium), equation 1.3 predicts that the polarization induced by
the first nonlinear term is predicted to be +2E�
, yet if the medium is
centrosymmetric the polarization should be –2E�
.
This contradiction can only be resolved if χ(2) = 0 in centrosymmetric
media. If the same argument is used for the next higher order term, +E�
produces polarization +3E�
and –E�
produces – 3E�
, so that χ(3) is the first non-
zero nonlinear term in centrosymmetric media. In second harmonic generation,
the two input wavelengths are the same 2ω1 = ω2 or (λ1= 2 λ2).
During this process, a polarized wave at the second harmonic frequency
2ω1 is produced. The refractive index, n1 is defined by the phase velocity and
wavelength of the medium. The energy of the polarized wave is transferred to
the electromagnetic wave at a frequency ω2.
Table 1.2 Optical effects of nonlinear materials
Order Susceptibility Optical effects Applications
1 χ (1) Refraction Optical fibers
2 χ (2) SHG (ω+ω = 2ω)
Frequency mixing (ω1±ω2=ω3)
Pockels effect (ω+o =ω)
Frequency doubling
Optical parametric oscillators
Electrooptical modulators
3 χ (3) 4 wave mixing Phase gratings
Kerr effect Optical amplitude
Raman Coherent spectroscopy Real time holography
Ultra high speed optical gates
Amplifiers, choppers etc.
40
The phase velocity and wavelength of this electromagnetic wave are
determined by n2, the refractive index of the doubled frequency. To obtain high
conversion efficiency, the phase vectors of input beams and generated beams
are to be matched.
( ) 02
12
=−
=∆nn
kλ
π (1.4)
Where, k∆ represents the phase–mismatch. The phase–matching can be
obtained by angle tilting, temperature tuning or other methods. Hence, to select
a nonlinear optical crystal, for a frequency conversion process, the necessary
criterion is to obtain high conversion efficiency.
The conversion efficiency η is given by
2
2
.
sin
∆∆
=Lk
kLdLP eff�
η (1.5)
Where, deff is the effective nonlinear coefficient, L is the crystal length,
P�
is the input power density and k∆ is the phase – mismatching.
In general, higher power density, longer crystal, large nonlinear
coefficients and smaller phase mismatching will result in higher conversion
efficiency. Also, the input power density has to be lower than the damage
threshold of the crystal. Table 1.3 lists the laser and crystal parameters for
selecting a NLO crystal for a particular application.
41
Table 1.3 Parameters for selecting a NLO crystal
Laser parameters Crystal parameters
NLO process Type of phase matching
Power, Repetition rate Damage threshold
Divergence Acceptance angle
Band width Spectral acceptance
Beam size Crystal size, Walk off angle
Pulse width Group velocity mismatching
Environment Moisture, temperature acceptance
1.6 CRITERIA FOR SELECTING USEFUL NONLINEAR OPTICA L MATERIALS
The “ideal” nonlinear crystal does not exist. The applicability of a
particular crystal depends on the nonlinear process used, the desired device
characteristics and the pump laser. Special material properties that are important
in one application may not be significant in another. For instance, efficient
doubling of very high power lasers having poor beam quality requires a material
with large angular bandwidth. A crystal, which has a smaller nonlinearity but
allows noncritical phase matching, will perform better than one which is more
nonlinear, but is critically phase matched. On the other hand, for the doubling of
femtosecond optical pulses, the preferred material will be one with a large
nonlinearity so that a very thin crystal can be used to avoid dispersive broadening
of the second harmonic output pulses [75].
For a material that has favourable features such as large nonlinearity, high
damage threshold, favourable crystal growth habits etc., an application can
invariably be found that uses the crystal efficiently. From a material point of
42
view, only general criteria can be established to gauge the usefulness of a
nonlinear crystal. For specialized applications where device performance
requirements are well established, quantitative criteria for the selection of
suitable nonlinear crystals can be obtained which are often invaluable in aiding
system design.
Nonlinear frequency converters are most commonly used with an
efficient nontunable laser source. Obviously, the nonlinear crystal should have
good transparency at the pump laser wavelength. Specific applications of
nonlinear crystals currently of interest can be divided into the following
efficient harmonic generation and up-conversion, optical parametric oscillator,
frequency conversion of ultrashort pulses, frequency conversion of high
average power sources, frequency conversion of low average power sources,
and laser fusion.
The resulting urge of interest in the development of other materials with
superior optical quality and improved nonlinear properties soon led to the
discovery of a number of early materials, including ammonium dihydrogen
phosphate (NH4H2PO4), potassium dihydrogen phosphate (KH2PO4), lithium
niobate (LiNbO3), barium sodium niobate (NaNbO3), lithium iodate (LiIO3),
lithium formate monohydrate, potassium niobate (KNbO3) and barium titanate
(BaTiO3), potassium pentaborate (KB5O8.4H2O) and ammonium pentaborate
(NH4B5.4H2O), urea, potassium titanyl phosphate, beta barium borate and
lithium borate [76]. Crystal growth and further characterization of these
materials has been identified as high-priority research areas by a high level
expert committee in the workshop on nonlinear optical materials [77]. These
crystals played an important role in the establishment of nonlinear optics as a
major area of laser science. Subsequently, intensive efforts were expanded and
are continued till today in search for new and better nonlinear materials.
43
Kurtz and Perry powder SHG method was introduced at the end of
1960’s. In this method, a powdered sample is irradiated with laser and scattered
light is collected and analyzed for its harmonic content with the use of suitable
filters. For the first time, rapid and qualitative screening for second order NLO
effect was possible. The stage was set for a rapid introduction of new materials,
both inorganic and organic.
1.7 TRENDS IN ORGANIC NLO MATERIAL DEVELOPMENT
For the past two decades, the search for new NLO materials has
concentrated primarily on organic compounds owing to their large nonlinearity.
The NLO properties of large organic molecules and polymers have been the
subject of extensive theoretical and experimental investigations during the past
two decades and they have been investigated widely due to their high nonlinear
optical properties, rapid response in electro-optic effect and large second or
third-order hyperpolarizabilities compared to inorganic NLO materials.
The low temperature solution growth technique is widely used for the
growth of organic compounds to get quality single crystals. Vijayan et al [78]
have grown p-hydroxy acetophenone (C8H8O2), one of the potential organic
NLO materials. Nagaraja et al [79] showed that benzoyl glycine (BG), an
organic nonlinear crystal grown by slow evaporation from DMF solution has
the advantages of both the organic and inorganic NLO materials. Owing to
high nonlinear efficiency, high melting point, good chemical stability, less
sublimation problems and improved hardness and cleavage properties (unlike
other organic materials), benzoyl glycine is found to be a promising material
for NLO applications. Lakshmana Perumal et al [80] further extended the effort
in synthesizing 4-methoxy benzaldehyde-N-methyl-4-stilbazolium tosylate
(MBST), which is a derivative of stilbazolium tosylate. The Kurtz powder SHG
measurements on MBST showed that the peak intensity is 17 times more than
that of urea.
44
Urea has been used in an optical parametric oscillator to generate
tunable radiation throughout the visible region but intrinsic absorption and
phase matchability considerations make it unsuitable for wavelengths longer
than 1000 nm (Rosker and Tang) [81]. The efforts made to resolve the
problems associated with urea have not been successful. The newly grown
binary UNBA crystal by Rai et al [82] is thermally and mechanically harder
than the crystal of the parent components. It is quite transparent almost in the
entire the UV region and hence it can be used for producing green/blue laser
light. Lin et al [83] have synthesized two component urea-mNBA systems and
urea-L-malic acid systems with different urea compositions. Jun Shen et al [84]
have grown single crystals of L-tartaric acid-nicotinamide and D-tartaric acid-
nicotinamide by the temperature lowering method from aqueous solution.
Single crystal of 3-methyl 4-nitropyridine 1-oxide (POM) was grown by
Boomadevi [85]. Manivannan and Dhanushkodi [86] have grown 3-[(1E)-N-
ethylethanimidoyl]-4 hydroxy-6-methyl-2H-pyran 2-one, by slow evaporation
technique and found that the SHG efficiency is close to that of urea.
A new ligand N-(3-fluorophenyl) naphthaldimine has been synthesized
by Unver et al [87]. The electric dipole moment (µ) and the first
hyperpolarizability (β) values of the N-(3-fluorophenyl) naphthaldimine have
been computed and the results reveal that the synthesized molecule might have
microscopic nonlinear optical (NLO) behaviour with non-zero values.
L-arginine acetate (LAA) is an organic nonlinear optical material and has a
wide optical transmission window between 220 and 1500 nm. Its laser damage
threshold and SHG efficiency are comparable with that of KDP. The N-(3-
nitrophenyl) phthalimide (N3NP) is a phase-matchable NLO crystal and can be
used as an efficient frequency doubler and optical parametric oscillator due to
its high SHG conversion efficiency, which was grown by slow evaporation
technique using DMF solvent [88]. Shaokang Gao et al [89] have synthesized
45
the N-(4-nitrophenyl)-N-methyl-2-aminoacetonitrile (NPAN) material and
single crystals of dimensions 36 x 8 x 8 mm3 were harvested. Second-harmonic
generation (SHG) in the NPAN crystal was observed using Nd:YAG laser with
a fundamental wavelength of 1064 nm. An organic NLO material, 4-OCH3-4′-
nitrochalcone (MNC), has been synthesized and grown by Patil et al [90] which
has NLO efficiency 5 times more than that of KDP.
A new organic crystal of semicarbazone of 2–amino–5–chloro–
benzophenone (S2A5CB) has been grown and characterised by proton nuclear
magnetic resonance by Sethuraman et al [57] and its second harmonic
generation property was confirmed by Kurtz powder method. Vibrational
spectral analysis of the non-linear optical material, L-prolinium tartrate (LPT)
was carried out using NIR-FT-Raman and FT-IR spectroscopy by
Padmaja et al [91].
Jagannathan et al [92] have synthesized the organic material
4-Ethoxybenzaldehyde-N-methyl 4-Stilbazolium Tosylate (EBST), a derivative
in Stilbazolium Tosylate family. Its NLO efficiency is 11 times greater than
that of urea. Studies on the nucleation kinetics of Sulphanilic acid (SAA) single
crystals were reported by Mythili et al [93]. The laser damage threshold values
of the SAA crystals are found to be 7.6 and 6.6 GW/cm2 for single and multiple
shots, respectively. Single crystals of pure and Cu2+ and Mg2+ doped L-arginine
acetate (LAA) were grown by Gulam Mohamed et al [94] using slow
evaporation method. It is observed that both Cu2+ and Mg2+ dopants have
increased the percentage of transmission in LAA. Ravindra et al [95] reported
that the solution grown NLO crystals of p-chloro dibenzylideneacetone
(CDBA) are thermally stable up to 250 ºC. Ammonium malate (AM), an
organic nonlinear optical material has been synthesized from aqueous solution.
The structural perfection of the grown crystals has been analyzed by high
resolution XRD rocking curve measurement [96]. Single crystals of L-lysine
46
acetate, an organic nonlinear optical material were grown by Sun et al [97]
using the controlled evaporation of its aqueous solution. Good quality
benzophenone (BP) crystals were grown by solution technique using CHCl3 as
solvent by adopting slow evaporation method at room temperature was
reported by Madhurambal et al [98]. The growth of a organic nonlinear optical
(NLO) crystal of 2-aminopyridinium maleate (2APM) in larger size has been
reported by [52] by slow evaporation method. A new semi-organic nonlinear
optical crystal, L-Phenylalanine L-Phenylalaninium perchlorate (LPPAPC) has
been grown through synthesis between L-Phenylalanine and perchloric acid.
Bulk crystals of dimension 5.5 x 0.4 x 0.3 cm3 were obtained by submerged
seed solution method [99]. A new nonlinear optical single crystal L- alaninium
fumarate (LAF) belonging to the amino acid group was grown by slow
evaporation solution growth technique [100]. The organic nonlinear optical
crystal of amino-carboxyl acid family, L-lysinium trifluoroacetate (LLTF) was
successfully grown from its aqueous solution by the temperature-lowering
technique. Its growth morphology was investigated by X-ray single diffraction
data and the growth habits were studied using micro-crystallization method
[101]. Urea ninhydrin monohydrate (UNM) was synthesized and grown from
aqueous solution employing the slow evaporation method [102]. Recently,
Jerald Vijay et al [103] have investigated the rapid growth of DAST by
adopting the slope nucleation method and by rapidly evaporating the solvent.
Thin plates of organic NLO crystal of DAST are grown within a period of 72 h
by carefully optimizing the growth conditions. Picric acid and its complexes
with amino acids viz., L-prolinium picrate, L-valinium picrate and
L-asparaganium picrate show very high SHG efficiency [104]. A novel
nonlinear optical crystal of tris (glysine) calcium (II) dichloride (TGCC) of
dimensions 34 x 23 x 5 mm3 was grown by slow evaporation technique and the
second harmonic conversion property of TGCC was identified by the Kurtz and
Perry technique [105]. Recently Gupta et al [106] studied the synthesis and
47
growth of a new nonlinear optical material, L-proline strontium chloride
monohydrate (L-PSCM) of amino acid family and found that SHG efficiency
of the crystal is nearly one tenth of the KDP. L-Arginine meleatate dihydrate
(LAMD, C6H14N4O2.C4H4O4.2H2O), a complex of strong amino acid has been
successfully grown by SR method by Urit Charoen-In et al [72]. SHG
measurements indicate that the SHG efficiency of LAMD crystal is roughly 4
times that of KDP. A recent study by Amalanathan et al [107] throws a new
interest in the possible application of amino acid crystals in terahertz (THz)
technology. The molecular structure, vibrational spectra and NLO properties of
L-valine hydrobromide are reported. The charge transfer interaction, calculated
first order hyperpolarizability and the HOMO-LUMO energy gap explain the
NLO activity of the molecule. The calculated first order hyperpolarizability is
found to be 6.617e-30e.s.u., which is 25 times that of urea. Vibrational and NBO
analysis confirms the N-H…..Br hydrogen bonding. The increase of β value is
additive property to exhibit nonlinear optical activity. Hence the L-valine
hydrobromide crystal can be a better entrant for the THz application [107].
1.8 IMPORTANCE OF L-TARTARIC, L-ALANINE AND L-PHENYLALANINE BASED NLO CRYSTALS
On the search towards new NLO materials with better mechanical
properties, many researchers have focused on the small organic molecules
having a large dipole moment and a chiral structure. These molecules are
usually linked through the hydrogen bond. The tartaric acid molecules are
bonded into a layer by O…..O-type hydrogen bonds to generate a two
dimensional frame work. Tartaric acid forms a broad family of hydrogen-
bonded crystals. The tartrates structure can be used to discuss a design strategy
for the engineering of crystals with predesigned architecture. The salts of
tartaric acid were intensively studied by means of structural, spectroscopic,
optical, dielectric methods. Some tartrates exhibit structural phase transisions
and in many cases vibrational spectroscopy was effectively used to study them
48
[108]. The influence of the strong and very strong hydrogen bonds on the
nonlinear optical properties of the crystal was already considered.
The multidirectional hydrogen-bonded tartrate anions provide a
conformational rigid environment for the incorporation of cations to form
acentric crystalline salts of SHG materials. Few examples for this category are;
L-lysine-L-tartaric acid, L-tartaric acid-nicotinamide and L-alanine tartrate
complex [108,109].
Because of the ability of enhancing the macroscopic nonlinearity in a
synergistic mode and initiating multidirectional hydrogen bonds, tartaric acid
was chosen to synthesize nonlinear materials. Hence, in the present thesis the
growth of selected salts of tartaric acid are attempted with success and the main
emphasis is to grow L-tartaric acid (LTA), L-tartaric acid-nicotinamide (LTN),
L-alaninium tartrate (LAT), L-phenylalanine hydrochloride (LPHCl) and
L-phenylalaninium maleate (LPM) single crystals by unidirectional SR
method.
L-tartaric acid-nicotinamide (LTN) is yet another interesting category of
new organic NLO material, which crystallizes in the monoclinic crystal system
with space group P21. The lattice parameter values are a =7.650 Ǻ, b =15.499Ǻ
and c = 10.506 Ǻ. The transmission range of LTN is found to be better than
KDP [109].
L-alanine is an efficient organic NLO compound under the amino acid
category. Single crystals of L-alanine was grown and characterized by Razzetti
[110]. Vijayan et al [111] have reported the bulk growth and characterization
studies of L-alanine single crystal. Rajan Babu et al [112, 113] reported the
growth of single crystals of L-alanine derivatives such as
L-alanine tetrafluoroborate (L-A1FB) and studied their fundamental growth
properties. The linear optical properties showed that L-alanine family crystals
have lower cut-off wavelength in the UV-region. Among all L-alanine
49
derivatives, L-A1Ac possesses high transmittance of 80 %. The powder SHG
test confirms that the nonlinear optical property of the grown crystals of
L-alanine derivatives is comparable with other semiorganic crystals.
Dhanuskodi and Vasantha [114] have reported the structural, thermal and
optical characterization of L-alaninium oxalate (LAO). LAO has its
transparency window from 230 nm onwards, suggesting the suitability of LAO
for SHG of the 1064 nm radiation and for other applications in the blue violet
region. Solution grown single crystals of LAO have produced SHG efficiency
of 1.2 times that of KDP [115]. Photoacoustic studies and thermal properties of
the NLO compound, L-alaninium maleate were reported by Martin Britto Dhas
et al [116].
The crystal structures of amino acids and their complexes have provided
a wealth of interesting information to the patterns of their aggregation and the
effect of other molecules and ions on their interactions and molecular
properties. Among them, L-phenylalanine is an essential protein amino acid,
which is used by the body to build neurotransmitters. The phenylalanine
molecules are related by a non-crystallographic pseudo two fold symmetry.
There have been several spectroscopic studies on the behaviour of many amino
acids and peptides including phenylalanine and on complexes involving amino
acids, organic molecules and metal ions [117].
1.9 SCOPE OF THE THESIS
The ever increasing demand for highly efficient nonlinear optical (NLO)
crystals for visible and ultraviolet regions is extremely important for laser and
material processing. In this context, the design and growth of single crystals
suitable for such requirements, assumes centre stage.
Organic NLO crystals formed with L-tartaric acid, L-alanine and
L-pheynyl have been identified as potential candidates for replacing KDP in
nonlinear optical applications. Hence attempts are made to grow L-tartaric acid
50
(LTA), L-tartaric acid-nicotinamide (LTN), L-alanine tartrate (LAT),
L-phenylalanine hydrochloride (LPHCl) and L-phenylalaninium maleate
(LPM) single crystals. Keeping in view, the importance attached to the
materials chosen for the present thesis, the growth of these crystals has been
carried out with the focus on unidirectional method along with the conventional
low temperature method. The grown crystals are characterized to identify the
physicochemical properties for possible exploitation of the developed materials
for future applications.
The present investigation is aimed at
(i) Synthesizing the chosen materials for the growth of single crystals
(ii) Determining the solubilities of the materials
(iii) Growing bulk size single crystals by unidirectional and conventional low temperature techniques
(iv) Identifying the crystal structure by single crystal X-ray diffraction analysis
(v) Estimating the crystalline quality by high resolution X-ray diffractrometry (HRXRD) study
(vi) Characterizing the grown crystals by Kurtz powder NLO test
(vii) Laser damage threshold measurement
(viii) FT-IR and Optical transmission/absorption studies
(ix) NMR spectral studies
(x) Studying of thermal behaviour of the grown crystals
(xi) Determining microhardness values
(xii) Measuring the dielectric constant and dielectric loss of the grown crystals
(xiii) Carrying out ac/dc conductivity study of the grown crystals and
(xiv) Carrying out photoconductivity study of the grown crystals.
51
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