1

CHAPTER 1

NON-LINEAR OPTICAL BORATE SINGLE CRYSTALS

– A BRIEF INTRODUCTION AND OVERVIEW

1.1 INTRODUCTION

Good single crystals are essential for a variety of scientific and

commercial applications. The single crystals are much required for the

scientific evaluation of the crystallography, topography and tensor properties

of all crystalline material in organic, inorganic, semi-organic and metallic

categories. Significant advancement in the crystal growth technologies has

allowed the development of many excellent crystals to meet the ever growing

applications in microelectronic and computer industries. Semiconductor based

devices, transducers, infrared detectors, ultrasonic amplifiers, solid state

lasers, frequency convertors, optical devices based on nonlinear optical,

piezoelectric, acousto-optic materials essentially need perfect single crystals.

Hence, growth of good quality single crystals has become inevitable for

further research and technology.

Modern solid state electronics is based on a crystal growth

revolution which has now made possible the commercial scale growth of laser

materials like ruby and sapphire, integrated circuit technology and the

production of magnetic materials and piezoelectronics like quartz and TGS.

Highly perfect crystals of materials like sodium choloride, lithium

fluoride and some organic compounds are needed for optical and

X-ray spectrometry and use in equipment like electron-probe microanalysis

2

which are in turn used to help study the growth of yet more crystal materials

and its properties.

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 (Laudise 1970, Brice 1986, Nalwa 1996). The

methods of growing crystals are very wide and mainly regularised by the

characteristics of the material and its size (Buckley 1951, Mullin 1972).

Method of crystal growth ranges from a simple inexpensive technique to a

complex sophisticated expensive process and crystallization time ranges from

minutes, hours, days and to months.

Many NLO crystals based on organic, inorganic and semi-organic

categories are employed for the non-linear optical processes. Among them

borates are new class of inorganic materials which are used for efficient

harmonic generation, frequency conversion, laser production, and in other

important applications related to non-linear optics and its associated

technologies. For the process of reliable and efficient laser frequency

conversion, following are some of the important properties of NLO crystals

(Sasaki et al 2000).

1. Large NLO coeffiecient,

2. Moderate Birefringence and small walk-off effect

3. Large angular, spectral and temperature bandwidths

4. Wide transparency range for operating wavelength

5. High laser induced damage threshold

6. Ease of growth and Low material cost

7. Good chemical and mechanical stability

3

Because of the large number and diversity of present and projected

applications of non-linear optical crystals, no single material can be

considered to be optimised for all purposes. As a result, the quest for new

compounds continues. In the search for and development of new borates for

application as non-linear optical crystals, one is faced with challenges of

incorporating many distinct physical properties into a single material, while

also producing the material in a specific and useful form.

1.2 NLO BORATE SINGLE CRYSTALS

The development of nonlinear optics (NLO) gained technological

importance and usage from their wide application ever since the first

observation of optical second harmonic generation and is reported (Klienman

1962). Inorganic nonlinear optical materials have been developed very rapidly

during the past decade on the borate series since the chemical bonding and the

mechanical properties of these crystals are superior over other organic and

semi-organic crystals. These inorganic borate compounds are reliable

materials for effective non-linear processes and very good candidates for

device applications. Excellent mechanical strength and temperature

withstanding capabilities lead them to be utilised in high power laser sources

and for frequency conversion applications.

Before the invention of borate series of crystals, most NLO crystals

were based on the P–O, I–O and Nb–O bonds like those in KDP (KH2PO4),

LiIO3 and LiNbO3 respectively. KDP is used for frequency conversion in

high-power lasers. It is particularly suitable for the purpose of a laser fusion

system involving NLO devices with large dimensions of several tens of cm.

Crystals like LiNbO3 and KNbO3 are frequently used for frequency

conversions for semiconductor lasers. The KTP (KTiOPO4) crystals

developed later provide properties that merely combined those of KDP and

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LiNbO3. This makes KTP popular for both high power and semiconductor

laser operations.

Comprehensive references on these materials are compiled in the

handbook on NLO crystals (Dmitriev et al 1991). Because of the relatively

high resistance against laser-induced damage and higher transparency in the

UV region, NLO crystals with B–O bonds are often employed for high-

power UV light generation.

1.3 THE STRUCTURE OF BORATE MATERIALS

Crystalline metal borates, like silicates, have a rich structural

chemistry that extends from the simple orthoanions to their condensation of

complex rings, chains, networks, and frameworks unlike the fixed

coordination number of four Si atoms in silicates; however, the coordination

number of the B atom in borates can be either three or four. This unique

feature allows some structural criteria for the search of new NLO crystals

among the borate class of materials.

1. Structures with isolated triangular groups

2. Structures with pyroborate triangular groups

3. Structures with tetrahedral groups

4. Structures with admixture of triangular BO3 and tetrahedral

BO4 groups

There are four types of structures that consist of isolated BO3

triangular groups. A common packing of planar triangular groups result in the

formation of layers illustrated by the calcite structures like AlBO3, GaBO3,

FeBO3, and ScBO3 are the first type. The STACK family of compounds with

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the formula A6MM (BO3)6 where A = Sr or Ba, and M and M’ with any

cation of formal charge +2, +3 that occupies in the octahedral site like in

BaGdSc (BO3)6 and Sr3Sc(BO3)6 are some example formulations for the

second type. The third type which BO3 groups interlink with other anion

groups like BeO4, BeO3, BiO6, LnO6 (where Ln = La3+

, Gd3+

, Nd3+

etc.) to

form a complexed structures as in SBBO, KBBF, BiBO, GdCOB, YCOB,

GdYCOB respectively. In the fourth type, a large family of materials having

the formula LnM3(BO3)4 (Ln = lanthanide, M = Al, Ga, Fe or Sc). These

materials commonly crystallize in the non-centrosymmetric trigonal form of

the mineral Huntite, CaMg3(CO3)4. In this structure, carbon is substituted by

boron, calcium is replaced by a rare earth element or yttrium, and magnesium

is substituted by scandium, gallium or aluminium. The best borate crystal

under this structure which is used as a self-frequency doubling laser source is

Nd:YAl3(BO3)4 (NYAB) crystal.

Borate structures with pyroborate triangular groups are also of four

types. The example for the first type is the minerals and compounds

containing binuclear trigonal planar units B2O5 occur in Mg2B2O5, Ba5B2O5.

In this group, the terminal B-O distances tend to be some what longer than the

bridging B-O distances, and a wide variation (111.8º - 180º) in the bridging

B-O-B angle is observed. The second type of structure occurs in the family of

compounds like AMOB2O5 (where A = K+, Rb

+, Cs

+ and M = Nb5

+, Ta5

+). The

third type is the condensation of three BO3 groups which is known to yield the

cyclic metaborate ring B3O6. ABO2 (A = Na+, K

+, Rb

+, Cs

+), BaB2O4 and

Ba2M(B3O6)2 (M = Ca2+

, Cd2+, Mg2+

, Co2+

and Ni2+

) are some of the

examples. And the last type is the full condensation to the linear metaborate

chain (BO2)nn-

in the compounds like LiBO2 – Type I, CaB2O4 – Type I and

La(MoO4)BO2 are some of the examples.

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The structures with tetrahedral groups can be classified as the

compounds containing BO4 groups which are not common among borate

crystals. The compounds MgAlBO4, MgGaBO4, SrAlBO4, LiBSiO4, and

NbBO4 are some of the examples. A cyclic dinuclear tetrahedral structure

characterises the peroxo-anion B2(O)2(OH)4 which occurs in NaBO3.4H2O.

Fully three-dimensional frameworks through vertex shared BO4 units occur in

the mineral CaAlB3O7, Zn4O (BO2)3, SrB4O7, and PbB4O7. These compounds

contain B3O5 groups. The B3O9 group consists of three BO4 groups which

occur in crystals like BPO4, BAsO4, BaBSi3O8 and Zn4B6O13.

Many borate compounds exists under the structure with admixture

of triangular BO3 and tetrahedral BO4 groups. In this type of structure, two

BO3 units (a pyroborate group) fuse to a BO4 unit and forms a mixed

structure. Some of the examples for these types of borate crystals are as

follows. The structure of LiB3O5 (LBO), CsB3O5 (CBO), CsLiB6O10, Na4Li

(B3O5)3, SrLi(B3O5)3 are characterised as the condensation of B3O7 rings. Two

B3O7 groups combine with one BO4 group to form the condensation of B5O8

rings as in potassium pentaborate (KB5O8. H2O). Crystals like LaMgB5O10,

La2CaB10O19 (LCB) and Ca2Ba6O11.2H2O are formed by the mixture of one

BO3 unit fuse to two BO4 units.

The optical properties of borate crystals can be related to their

molecular structure. Some of the most used borate NLO crystals for the NLO

and laser applications are given in Table 1.1 as examples. These crystals are

constructed from the three basic structure units: (B3O6)3-

, (B3O7)5-

and (BO3)3-

anionic groups. The (B3O6)3-

, (B3O7)5-

and (BO3)3-

anionic groups are shown

in Figure 1.1. A model called the “anionic group theory” was designed by

Chen and his co-workers and is used to understand the relation between

composition, structure of borate materials, and the related NLO properties

(Chen 1993).

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Table 1.1 Selected properties of some NLO borate crystals

S. No

CrystalsSpace

group

Transparency

range (nm)

Nonlinear

coefficient

(pm/V)

Birefringence

n

Basic

structure

unit

1. -BaB2O4

(BBO)R 3c 190 - 3300 d11 = 1.844

0.12 at

1064 nmB3O6

2.LiB3O5

(LBO)Pnn 21

160 - 2600d31 = 0.94,

d32 = 1.13,

d33 = 0.256

0.04 at

1064 nmB3O7

3. CsB3O5

(CBO)P212121 167 – 3400 d14 = 0.863

0.053 at

1064 nmB3O7

4. CsLiB6O10

(CLBO)142 d 180 - 2750 d36 = 0.95

0.050 at

1064 nmB3O7

5. KBe2BO3F2

(KBBF)R 32 155 – 3660 d11 = 0.8

~0.072 at

589 nmBO3

6. Sr2Be2BO7

(SBBO)P 63 155 – 3780 d15 = 2

~0.062 at

589 nmBO3

7. YCa4O(BO3)3

(YCOB)Cm 220 - 3600 d=1.1

0.041 at

1064 nmBO3

(a) (b)

(c)

Figure 1.1 Basic structure units of (a) BO3 (b) B3O6 (c) B3O7 borates

(Dark circles represent boron and bright circles represent

oxygen atoms)

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Due to the planar hexagonal structure of the (B3O6)3-

anionic group,

borate crystals constructed from this basic unit have greater (2)

compared to

crystals composed of (B3O7)5-

and (BO3)3-

anionic groups. Thus in terms of

NLO coefficients, the (B3O6)3-

group is the most suitable as the basic structure

unit of NLO borate crystals followed by (B3O7)5-

group and then (BO3)3-

group. However, the UV absorption edge of the borate crystals constructed

from the (B3O6)3-

group occurred at a longer wavelength (eg. BBO) when

compared to those constructed from (B3O7)5-

group (eg. LBO, CBO, CLBO).

Calculations showed that -conjugated orbitals of planar (B3O6)3-

tend to shift

the UV absorption edge to toward the red. As one of the boron atoms in

(B3O6)3-

changed from trigonal to tetrahedral coordination, thereby forming

nonplanar (B3O7)5-

groups, the -conjugated orbital system is weakened, as in

the case of LBO and CBO and the UV absorption edge shifts to 160-170 nm.

Both CBO and LBO are constructed from a continuous network of non-planar

(B3O7)5-

groups with interstitial cesium and lithium cations, respectively.

Based on the absorption edge, therefore, (B3O7)5-

group is ideal as the basic

structural unit of deep UV NLO materials. Likewise, if the dangling bonds of

the three terminated oxygen atoms of (BO3)3-

groups are interlinked with

cations, an absorption edge appearing at wavelengths as short as 155 nm is

also feasible as in the cases with KBBF and SBBO.

1.4 RESEARCH BASED ON BORATE SINGLE CRYSTALS

Borate crystals are not only employed for frequency conversion but

also used as self-frequency doubling (SFD) active laser sources. Borate

family of crystals were developed in the recent years. Potassium penta borate

(KB5O8.4H2O) (Dewey et al 1975) was the first borate crystal described for

UV light generation in the borate series. But, the search for new NLO borate

crystals got momentum after the invention and development of -BaB2O4

(BBO) and LiB3O5 (LBO) which are very useful for second harmonic

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generation from the visible to the UV ranges down to 200nm. A large

birefringence of BBO ( n = 0.1) leads to high angular sensitivity of phase

matching. On the other hand, LBO has an absorption edge near 165 nm with

birefringence of n = 0.04 to 0.05. But, LBO has a limitation in SHG through

angular phase matching longer than 255nm. Further CLBO was developed

(Mori et al 1995) to overcome the shortcomings. CLBO crystals are effective

for fourth and fifth harmonic generation of Nd:YAG lasers. Yet CLBO

requires an elevated temperature for long term operation. This led materials

scientists into research for new borate NLO crystals that would possess

properties that can overcome the limitations of BBO, LBO and CLBO.

Though BBO and LBO crystals are well studied materials among

borate compounds, due to the reason that they are grown from flux melts

which yields crystals with lesser dimensions, research on borate crystals

exhibiting congruently melting behavior has gained interest. Czochralski

technique is used to grow borate based NLO crystals which are having

congruent melting behaviour. CsLiB5O10 (CLBO) crystals, which are the

structural derivatives of LBO crystals, are grown in larger dimensions from

their melts. CLBO single crystals for UV light generation with good optical

quality were grown and reported (Sasaki et al 2003). Another derivative of

LBO is CsB3O5 (CBO) compound also melts congruently at the temperature

of 842 °C. Crystals with dimensions of 20 mm in diameter and 30 mm in

length were grown with the pulling rate of 8 mm/day.

Incongruent melting behaviour of the borate compounds are also

also in progress. Due to the fact that a few incongruently melting borate

materials are transparent down to vacuum UV region and the single crystal

growth of these compounds were also carried out across the globe. The family

MBe2BO3F2, with M=Na, K is an example of a structure type with isolated

(BO3) triangles, crystallizing in the non-centrosymmetric space group R32

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(Linfeng Mei 1993). The potassium compound KBe2BO3F2 (KBBF) was first

synthesized in the year 1968. These crystals were grown from flux melts with

KBF4 and BeO as the starting materials and the plate-like crystals with the

dimensions of 10 x 10 x 0.3 mm3 were reported.

In general, a good NLO crystal that can generate deep UV light

effectively is required to have a birefringence between that of BBO ( n =

0.12 at =1064 nm) and LBO ( n = 0.04 at =1064 nm). This allows the

expansion of the phase matching range for SHG at a reasonably low walk-off

effect and significantly large phase-matching bandwidths. The earlier

mentioned CLBO crystal, has n=0.052 at =1064 nm and hence it is widely

used for deep-UV light conversion. Thus several NLO crystals like CLBO

that possess a birefringence between that of BBO and CLBO are desired.

SBBO ( n=0.062 at =589 nm) and KBBF crystals ( n=0.072 at =589 nm)

are attractive candidates in this aspect. However, because of the weak binding

between the layered structural units, KBBF is difficult to grow and is

mechanically fragile (Chen et al 1995). The SBBO has strong covalent bonds

between beryllium atoms and oxygen atoms in adjacent layers. This makes

SBBO mechanically stronger and relatively easy to grow compared to KBBF.

However, beryllium is toxic, which makes crystal growth inconvenient. Thus,

it is important that the Be atoms in SBBO be replaced by a nontoxic element.

Sasaki and his coworkers had attempted to replace the (BeO4)6-

with (AlO4)5-

and had replaced Sr2+

with M+ (M

+=Li

+, Na

+, K

+, Rb

+ and Cs

+) according to

the concept of ionic compensation (Al3+

+ K+

Be2+

+ Sr2+

). In this way,

potassium aluminum borate crystal with the chemical formula K2Al2B2O7

(KAB) was discovered.

The KAB crystal has the spatial arrangement similar to that of

SBBO. The KAB crystals are grown from flux technique. Different fluxes

such as B2O3, K2CO3, K2CO3-B2O3, alkali halides such as KF and NaF were

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used for the growth of KAB crystals and are reported (Chengqian Zhang et al

2001). The KAB crystal is transparent from 180-3600 nm. The linear thermal

expansion coefficient of the KAB crystal along the x, y and z directions are

very lesser. The specific heat values of KAB crystal at 47.6 °C and 294.6 °C

are 1.0084 J/g °C and 1.39 J/g °C (Chengqian Zhang et al 2003).

Improvement of the NLO properties by enhancing the number of NLO active

(BO3) groups per unit volume of the crystal structure was attempted by

creating strontium beryllium borate Sr2Be2B2O7 (SBBO). These crystals

exhibit the hexagonal crystal system and were grown by the TSSG technique.

These types of crystals were grown by slow cooling with the cooling rate of

1-2 °C/day. Crystals with the dimensions of 7 x 7 x 3 mm3

with good optical

quality are obtained and reported.

The first member of the rare earth calcium oxy borate (RECOB)

crystals with the chemical composition of RECa4O(BO3)3 (RE=Sm) was

synthesized by Khamaganova and his co-workers in the year 1991

(Khamaganova et al 1991). Later intense work on the growth and

characterization of this family of crystals with various rare earth elements

such as La, Nd, Gd, Er, Y were carried out and reported. The RECOB family

of crystals appears to be attractive candidates for NLO applications as they

possess the non-centrosymmetric structure which is an essential parameter for

any NLO material. Widely studied RECOB crystals include YCOB, GdCOB

and LCOB crystals. The RECOB crystals with the rare earth ions with

electronic configurations 4fn, n 0,7,14 give rise to electronic transitions in

the visible region that would interfere with the expected NLO properties.

These crystals have proved to be excellent NLO materials for the harmonic

generations of Nd:YAG lasers. YCOB single crystals were conventionally

grown using the melt techniques like Bridgman technique or the Czochralski

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technique (Yiting Fei et al 2006, Jun Luo et al 2001). YCOB crystals grown

from flux method are also reported (Arun Kumar et al 2008).

The RECOB crystals melt congruently and were conventionally

grown by the Czochralski and Bridgman techniques of crystal growth. The

melting temperatures of the RECOB crystals increases with a decrease in the

ionic radii of the rare earth ion present in it (Hiroshi Nakao et al 2006).

Accordingly, the melting points of LCOB, GdCOB and YCOB single crystals

are 1410, 1480 and 1510 °C respectively (Daniel Vivien et al 2002). The

RECOB single crystals exhibit the monoclinic crystal structure with the Cm

space group. They are biaxial crystals. These crystals offer the advantage in

providing suitable sites for doping them with ‘laser-active’ ions, since the

widely used ‘laser-active’ ions such as neodymium, erbium, ytterbium have

similar ionic radii and occur in the trivalent state as that of the rare earth ion

present in the RECOB crystals (Mougel et al 1997, Lupei et al 2002).

YAB is a non-centrosymmetric crystal and as early as in 1974 it

was reported as a very effective second-harmonic generating material.

Furthermore, owing to its good chemical stability and the possibility of

substituting Y3+

ions with other lanthanide ions, namely Nd3+

, Yb3+

, and Er3+

,

it is a good material for laser applications. The nonlinear optical properties of

this material along with lasing properties led to the fabrication of numerous

systems generating red, green and blue lights due to self-frequency doubling

effect (Jaque et al 1999). They also possess relatively large two-photon

absorption. These compounds are promising candidates as second and third

order optical materials (Majchrowski et al 2005). At the same time they are

good matrices for different rare-earth ions (Dominiak-Dzik et al 2004, You

et al 2004). Reports are also available on the Nd3+

, Tb3+

, Yb3+

, and Er3+

doped

YAB crystals (Jaque et al 1998, Jing Li et al 2004a, Jing Li et al 2004b).

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Further, research in the family of borate crystals with huntite

structure, for crystals like NdAl3(BO3)4 (NAB), ErAl3(BO3)4 (ErAB),

YbAl3(BO3)4 (YbAB) were carried out (Leonyuk et al 2007). The melting

points of these materials are below 1300 °C. These crystals are grown by flux

techniques and are mainly employed in SFD purposes.

1.5 APPLICATIONS OF BORATE SINGLE CRYSTALS

Borate crystals, such as -BaB2O4 (BBO) and LiB3O5 (LBO), play

an indispensable role in frequency conversion when one needs a powerful or

tunable laser source in the deep ultraviolet region. Beta-barium-borate

(BBO) crystal has been commonly used in the nonlinear frequency

conversion. It has been demonstrated to be one of the best candidates in

nanosecond, picosecond and femtosecond optical parametric devices. Optical

parametric devices have been widely developed and represent one of the

most successful ways to obtain a widely tunable laser source. The

parametric properties of a type-I phase-matched BBO and optical

parametric generation/ampli cation (OPG/OPA) of BBO are reported

(Dongxiang Zhang et al 2000). Frequency doubling and high power UV light

generation properties of BBO crystals enabling for high repetition rate UV

laser processing and micromachining application (Huot et al 2002).

In Nd:YAG and Nd:YLF laser systems, BBO is an efficient NLO

crystal for the second, third and fourth harmonic generation, and the best

NLO crystal for the fifth harmonic generation at 213nm. Conversion

efficiencies of BBO is more than 70% for SHG, 60% for THG and 50% for

4HG have been obtained. Frequency doubling and tripling of ultra short pulse

lasers are the applications in which BBO shows superior properties over KDP

and ADP crystals. A laser pulse as short as 10fs can be efficiently frequency

doubled with as thin as 0.02mm BBO crystal.

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The transmission property of LBO crystal is very unique that

Li2B4O7 is transparent in the deep ultraviolet region down to 170 nm, which

represents the shortest absorption edge among nonlinear borate crystals for

ultraviolet applications. This striking feature of LBO is ahead of other borate

crystals for large transparency at the practical wavelength, for instance, 200

nm, for the fourth and fth-harmonic generation of the Nd:YAG laser. Also

the laser damage threshold of the Nd:YAG laser was about 40 GW/cm2. Thus

LBO crystal having much higher pulse energy and high average power of the

Nd:YAG laser shows the advantage of Li2B4O7 in the borate series of crystals

for ultraviolet solid-state laser applications (Komatsu et al 1997).

Cesium lithium borate is one of the UV nonlinear crystal

(CsLiB6O10-CLBO), which has shown high obtainability of large, high quality

crystal compared to LBO and BBO. This crystal is used as frequency up-

conversion materials (Mori 1995). CLBO has large angular, spectral and

temperature bandwidths, and damage threshold as compared to BBO crystal.

Further, the smaller walkoff angle and larger angular bandwidth of CLBO

crystal allows the generation of frequency converted beams with low

divergence and good spatial-mode quality, which is an advantage in the

frequency quadrupling process. With these properties CLBO crystals find

applications both as a frequency doubler and a quadrupler, and the best choice

for frequency quadrupling of ultrashort pulses (Sharma et al 1996). Also it is

shown that the CLBO crystals are suitable candidate for the generation of

high power mid-infrared radiations (Chatarjee 2001) and widely tunable deep

ultraviolet generation of Nd:YAG laser (Bhar et al 2000).

KBBF belongs to a deep ultra violet (DUV) NLO single crystal, the

only NLO crystal which has the UV cutoff wavelength as short as 155 nm.

Using this crystal, the fifth harmonic generation of Nd:YAG has been

generated. There are several nonlinear optical crystals that can produce DUV

15

coherent light, such as LBO, BBO, KB5, CBO and CLBO etc., but they can

only be used in DUV region by sum-frequency mixing. These sum-frequency

mixing techniques need two laser beams, which make the system complex,

inconvenient and also the efficiency is low. Till now, KBe2BO3F2 (KBBF)

crystal is the only non-linear optical (NLO) crystal to be capable of producing

deep-UV coherent light through second harmonic generation (SHG). KBBF

has been used to generate shortest second-harmonic (SHG) wavelengths down

to 170.0 nm. Sixth harmonic generation of the Nd: YVO4 laser (from 354.7

nm to 177.3 nm) with 3.5 mW average power output is reported. Very

recently the high output power of 12.95 mW of the 6th

harmonic generation of

the Nd: YVO4 laser at wave length 177.3nm using KBBF crystal is also

reported (Wang et al 2008). Deep ultraviolet (DUV) coherent light attracts

many interests for its promising applications such as semiconductor

photolithography, micro-machining, photochemical synthesis, laser

spectroscopy and photoemission spectroscopy and KBBF is one of the

potential candidate in these fields.

KAB crystal has several advantageous properties and can be used

for the generation of efficient tunable UV and VUV laser radiation by

employing different NLO frequency conversion techniques. The generation of

tunable (390 – 412.5 nm) near UV laser radiation by employing Ti:sapphire

regeneratively amplified laser radiation is achieved using KAB single crystals

(Kumbhakar et al 2003). Also using KAB crystal, efficient ultraviolet beam

generation (266 nm) of Nd:YAG laser output is reported (Lu Jun-Hua et al

2002). Yttrium aluminium borate YAl3(BO3)4 (YAB) is a non-linear crystal

which has proved to be an excellent host for Nd3+

and Yb3+

ions in order to

develop self-frequency doubling and self-frequency-sum solid state lasers

(Jaque et al 1999). Using Nd3+

ions in the YAB crystal, simultaneous

generation of laser radiation in the three fundamental colours; red, green

and blue (RGB) in the same solid state laser is reported (Jaque 2001).

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Applications related to NLO and laser properties of the RECOB

family of crystals are widespread. Among the RECOB family of crystals,

GdCOB, GdYCOB, YCOB, LCOB are some of the potential NLO crystals

which are used for frequency conversion and second harmonic generation

applications. GdCOB is an excellent candidate for applications in low cost

microchip lasers (Aka et al 2003) and a promising NLO crystal to obtain UV

laser at 355nm (Shujun Zhang et al 2000). Also GdCOB single crystal is a

potential material for non-critical phase matching applications in different

laser systems (Zhengping Wang 2001). The GdCOB and Ln: GdCOB single

crystals appear very ef cient materials for non-linear optical and laser

applications. Nd:GdCOB crystals operating as self frequency doubling lasers

are very attractive solid state green coherent sources (Aka et al 2000). Also it

is reported that Nd:GdCOB crystal is a promising SFD material for green and

blue laser (Shujun Zhang et al 1999).

1.6 CRYSTAL GROWTH

A supersaturated mother phase is the primary requirement for the

crystal growth of any material. In this supersaturated system, the initial stage

of crystallization is the formation of nuclei of the crystalline phase. These

nuclei attain macroscopic dimensions during the crystal growth process. In

general, the crystallization process occurs by the following three steps,

i. Achievement of supersaturation or supercooling,

ii. Formation of crystal nuclei and

iii. Single crystal growth.

The methods of growing single crystals may be classified according

to their phase transformation as given below:

17

A. Growth from solid : solid–solid phase transitions process

B. Growth from liquid : liquid–solid phase transitions process

C. Growth from vapor : vapor–solid phase transitions process

The conversion of the polycrystalline material into a single crystal

by causing the grain boundaries to swept through and pushed out of the

crystal in the solid–solid growth of crystals (Mullin 1972). The crystal growth

from liquid falls into four main categories namely, 1. Solid growth: 2. Vapor

growth: 3. Melt growth and 4. Low temperature solution growth

The choice of selecting a specific crystal growth technique depends

on the physical and chemical properties of the material to be crystallized and

the suitability of the technique to grow the required crystal. The parameters

such as the growth kinetics, size, shape, purity, quality and the cost involved

in producing the crystals also play a vital role (Pamplin 1980). The growth of

borate based crystals from the melt technique and by the low temperature

solution growth method was adopted in the present work and hence an

overview of these techniques along with a brief discussion on the crystal

growth systems employed is summarized.

1.7 MELT GROWTH TECHNIQUE

The best method for the growth of large size single crystals of high

perfection with relatively rapid growth rate is undoubtedly is the melt growth

method (Pamplin 1980). Nearly 85% of the crystals that are used in various

applications are being grown from the melt techniques of crystal growth.

Currently this is the most important method available for the growth of near

perfect crystals with large size. The melt growth technique is further

subdivided into four main group of techniques. They are,

1. Normal freezing, directional freezing or Bridgman-Stockbarger

method

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2. Crystal pulling method

3. Zone melting method and Flame fusion or pedestal growth

method

The preferable properties of materials that can be grown by melt

techniques are, congruent melting nature of the material, relatively low vapor

pressures, the desired phase of the material to be crystallized should be

possible and there should not be any reaction between the crucible and the

chemicals at higher operating temperatures. In this present work, both

lanthanum calcium borate and neodymium doped lanthanum calcium borate

were grown using crystal pulling method by employing Czochralski technique.

1.8 MELT GROWTH BY CRYSTAL PULLING METHOD

The basic process of growing a crystal using pulling method is

simple and is shown schematically as shown in Figure 1.2. The material to be

crystallised is placed in a suitable container or crucible. The crucible is then

heated by resistance or induction heating until the taken charge is completely

melted. The homogenisation of the melt will be achieved by keeping the melt

few degrees above the melting point for certain time period and bring down to

the growth temperature.

The temperature of the molten charge is adjusted so that the center

of the liquid is at its freezing point. A seed crystal is introduced into the

liquid so as to create the interface between the bottom of the seed and top of

the melt surface. The crystal growth or “pulling” process is carried out by

slowly withdrawing the seed from the interface created. By careful

observation and proper temperature control of the melt, crystallization on the

seed crystal can be started during the pulling process.

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Figure 1.2 Schematic diagram of Czohralski pulling set up

The adjustments of the melt temperature and pulling rate during the

growth process provide control of the growing crystal diameter. Once the

desired crystal length and dimension is achieved, the crystal is quickly raised

from the melt surface to facilitate the termination of interface between the

growing crystal and melt surface or by slowly increasing the liquid

temperature to reduce the diameter to remove the crystal from its melt. When

the crystal is detached from the melt surface, the temperature is lowered to

room temperature and the crystal is withdrawn from the growth apparatus.

Crystal pulling method is widely used by the industries today for

the production of bulk semiconductor and oxide single crystals. Materials that

are routinely grown using this technique include silicon, sapphire (Al2O3),

gallium phosphate (GaP), gallium arsenide (GaAs), indium phosphide (InP),

Gd3Ga5O12, Nd:Y3Al5O12, germanium, LiNbO3, and BBO are to name a few.

Many of these materials finds applications in the electronic, optical and laser

fields.

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1.9 LOW TEMPERATURE SOLUTION GROWTH

Crystal growth from solution is very important process used in

many applications from laboratory to industry. The material, which

decomposes before its melting point can be grown as single crystals by

aqueous solution growth method (Pamplin 1979). In general, low temperature

solution growth involves seeded growth from a saturated solution. The

driving force i.e. the supersaturation is achieved either by temperature

lowering or by solvent evaporation. This method is widely used to grow bulk

crystals, which have high solubility and have variation in solubility with

temperature (James and Kell 1975, Chernov 1984). Growth of crystals from

solution 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 solution grown crystals are relatively low (Brice

1972). Low temperature solution growth can be subdivided into the following

categories:

A. Slow cooling method,

B. Slow evaporation method and

C. Temperature gradient method.

In this present investigation, growth of potassium tetra borate is

carried out using slow evaporation method. In this method, the temperature is

fixed and provision is made for evaporation. With non-toxic solvents like

water, it is permissible to allow evaporation into the atmosphere.

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1.10 SCOPE OF THE PRESENT INVESTIGATION

In view of the potential interest in the borate family of nonlinear

optical crystals, the present investigation was aimed:

1. To investigate the nonlinear optical properties of the LCB

crystal such as second harmonic generation, phase matching

and optical parametric oscillation theoretically.

2. To synthesise the polycrystalline materials of LCB, NdLCB

and potassium tetra borate.

3. To grow the single crystals of LCB, Nd doped LCB and

potassium tetra borate single crystals

4. To characterize the grown crystals to analyse the structural,

thermal, mechanical, dielectric and optical properties.

In the present investigation, the nonlinear optical properties of

monoclinic LCB crystal are analysed theoretically. LCB is a biaxial crystal,

so phase matching property in the XZ and YZ planes are discussed. SHG,

walk-off angle and OPO characteristics for these planes had been drawn and

compared with the well established crystals like BBO and BiBO. The phase

matching angle for Type - I frequency doubling of fundamental wavelength

= 1.064 µm is calculated to be 37.38 and 37.54 respectively. The OPO

curve for the LCB crystal is drawn for the wavelength ranging from 0.4 µm to

3.2 µm.

The starting materials of pure LCB and neodymium 1% and 5%

doped LCB were prepared stochiometrically. The growth of pure LCB single

crystals and Nd doped LCB single crystals were carried out using crystal

pulling method using Czochralski technique. The grown LCB and Nd:LCB

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crystals were subjected to various characterisation techniques and its

properties were studied.

Potassium penta borate single crystals were grown by low

temperature method using solvent evaporation method. The characteristics of

the grown crystals were analysed through various characterisation studies.