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Optical Materials 27 (2004) 567–572

Characterization of bromocresol green and resin as holographic film

J.C. Ibarra a,*, M. Ortiz-Gutierrez b, P. Alonso-Magana a

a Centro de Ensenanza Tecnica Industrial, Nueva Escocia No. 1885, C.P. 44620 Guadalajara, Mexicob Universidad Michoacana de San Nicolas de Hidalgo, Fco. J. Mujica s/n, C.P. 58060 Morelia Mich, Mexico

Received 7 January 2004; accepted 25 June 2004

Available online 17 September 2004

Abstract

We present a film composed by bromocresol green (BCG) dye and resin mixed with isopropyl alcohol as photosensitive emulsion.

In this film, we have recorded phase gratings with the emission line k = 457nm of an argon laser. The characterization of this film

indicates us a diffraction efficiency of 2.27%; and its simple fabrication and low cost make this film an attractive option for holo-

graphic elements.

� 2004 Elsevier B.V. All rights reserved.

PACS: 42.40.J; 42.70.L; 42.40.EKeywords: Holographic optical elements; Holography recording materials; Diffraction gratings holographical

1. Introduction

The bromocresol green dye� (BCG) is commonly

used as yellow pH(3.8) or blue–green pH(5.4) indicator,

and as a pKa sensor [1]. The fluorescence emission of the

BCG and bovine serum albumin interaction has been

well studied before [2]. The condensed formula ofBCG is C21H14 Br4 O5 S, it presents a composition of

36.14% of C, H(2.02%), 45.79% of Br, 11.46% of O

and 4.59% of S; its molecular weight is 698.02 and has

a fusion point at 218 �C. This substance presents moder-

ate solubility in ether, water, ethyl acetate, and good sol-

ubility in benzene, besides it is sensitive in alkaline

solutions. When the BCG dye is used as a pH sensor,

it is mixed with sodium hydroxide and diluted in250ml of water.

The resin is obtained from Oocarpa Mexican pines [3]

but in many countries it is known as colophony which is

extracted from palustris or pinaceae pines too. The resin

has application in the fabrication of varnish, polish, ce-

0925-3467/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.optmat.2004.06.003

* Corresponding author. Tel.: +52 33 36 41 32 50.

E-mail address: jcibarra@ceti.mx (J.C. Ibarra).

ment, dyes, plastics, paper, etc. [4,5]. We use the same

resin that was employed in previous papers where the

principal characteristics were described [6–8].

In this job we made a photosensitive emulsion com-

posed by the BCG dye and resin diluted in isopropyl

alcohol. The resin was used strictly as the base of the

photosensitive emulsion, to fix the dye and to obtainan appropriate emulsion for recording of diffraction

gratings.

In order to know the behavior of the material when

this is exposed to a laser light, we have developed a char-

acterization which basically consists in the analysis of

the diffraction efficiency (g) of a phase diffraction grating

with relief modulation recorded in the film with the help

of three different wavelengths from an argon laser(k = 457, 476 and 488nm). We present some experimen-

tal results with different concentrations of BGC and

resin, exposure energy, interference angle and intensity

ratio between the beams.

This photosensitive material has several advantages

such as its low cost and its easiness to be produced.

The characteristics of our film permit the recording of

holograms without the need of a dark room; when thefilm has been exposed, its developing process is easy

568 J.C. Ibarra et al. / Optical Materials 27 (2004) 567–572

and only requires 18s to complete. The developer is

composed of water and chlorine in concentration of 50

parts to 1 respectively.

After the developing process we have in the emulsion

a relief modulation grating as Murry�s hypothesis [9] de-scribes. During this process a phase grating is formedbecause the area of the film where constructive interfer-

ence occur becomes hardened, and in the region where

destructive interference happens the film remains soluble

and is easily removed by the developer [10].

In a similar way Austin and Stone developed a peri-

odic structure model as a function of the emulsion thick-

ness and the developing time which are characteristics

that modify the profile of the recorded grating [11]. Theyexpress the phase depth on a film, Dd, as

Dd ¼ T ½r1 � Dre�cE�; ð1Þwhere T is the developing time, r1 the removing speed of

material, Dr the speed difference between the r1 and thenon-affected material, c is a constant and E the exposure

energy.

This phase depth is related to the phase change, /(x, y), and the transmittance function, T(x, y), of the

phase holograms as

T ðx; yÞ ¼ ei/ðx;yÞ ¼ eiknDd ; ð2Þwhere n is the refraction index of the film.

Table 1

Concentration of BGC and resin

BCG (g) 0.01 0.02 0.03 0.05

Resin (g) 0.5 0.5 0.5 0.5

Alcohol (ml) 5 5 5 5

Concentration 50:1 25:1 17:1 10:1

2. Preparation of the film

For the preparation of the photosensitive film, we

mixed the BCG, resin and isopropyl alcohol. The quan-

tities of the mixture are 0.5g of rosin, 0.005g of BCG

and 5ml of isopropyl alcohol to allow us mix up the

250 300 350 400 450.0

0.5

1.0

1.5

2.0

2.5

3.0Resin

Abs

orba

nce

Wavele

Fig. 1. This graphic shows the absorbance for the different concentra

components and obtain a liquid and homogeneous solu-

tion. With these elements, there are not chemical reac-

tions between them.

The solution obtained in the last process is deposited

on a 2 · 2-in. glass plate by the gravity technique; the

plate has been previously cleaned. We let repose the filmduring 12h and then it is introduced in an oven at 100 �Cduring 3h. After it, the film becomes hardened and the

isopropyl alcohol has evaporated [12].

3. Results

Following the procedure above mentioned, we madefour different concentrations of BCG and resin, these

are listed in Table 1.

In Fig. 1, we show the graphic of the absorbance ob-

tained for the concentrations mentioned in Table 1. As

can be observed, in the visible region of the electromag-

netic spectrum, each concentration have a maximum

absorbance about k = 425nm, but our laser has not this

emission line, so we employed the wavelengthk = 457nm that is closer to the maximum absorbance

line of the film to recording the phase grating.

As can be observed, in the line k = 457 the absorb-

ance is increased as the BGC concentration is increased

0 500 550 600 650

= 457.8 nm of Work

and BCG Concentrations 10:1 17:1 25:1 50:1

ngth (nm)

λ

tions of the BCG, resin and alcohol as a wavelength function.

Table 2

Beer�s law parameters

Concentration k (nm) A c (mol/l) l (cm) a (l/molcm)

10:1 457 0.9076 0.3449 6.5 · 10�4 4048

17:1 457 0.6808 0.3391 7.5 · 10�4 2742

25:1 457 0.503 0.3361 9 · 10�4 1661

50:1 457 0.2807 0.3334 17.5 · 10�4 481

Argon Laser

Emulsion

M1

M2

BS

Interferenceθ

Fig. 2. Interferometric setup for the diffraction grating recording; BS is a beam splitter, h the interference angle, and M1 and M2 are mirrors.

J.C. Ibarra et al. / Optical Materials 27 (2004) 567–572 569

too. The absorbance A is given for a specific wavelength

by Beer�s law [13–15]

A ¼ cla; ð3Þwhere c is the molar concentration (mol/l), l the thick-

ness (cm) and a is the absorption coefficient (l/mol cm).

The a coefficient is easily calculated by Eq. (3) and thevalues for the different concentrations of BCG and resin

are shown in Table 2. The thickness l of our films were

obtained with a surfanalizer profile.

2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Resin and BCGconcentrations different

10:1 17:1 25:1 50:1

10° Interference Angle

(%)

Exposure En

η

Fig. 3. This graphic shows the diffraction efficiency vs. exposure energy for th

the highest efficiency.

Employing the experimental setup show in Fig. 2, we

recorded six gratings for each concentration, in each one

we have varied the exposure time with a fixed wave-

length k = 457nm at an angle h = 10�.After the recording process we put the film into a

developer for a time of 18s according to Eq. (1), and

give a dry process. It should be noted that the develop-

ing process is easy and very cheap.To reconstruct the gratings we used on the film a

beam with the same wavelength employed to record

6 7 8 9

ergy (J/cm2)

e different concentrations of BCG and resin. The 25:1 concentration has

1 2 3 4 5 6 7 8 90.0

0.5

1.0

1.5

2.0

2.5

3.0Resin and BCG Concentration 25:1

Interference angle 5o 10o

20o 30o

o 40o 50

Exposure Energy (J/cm2)

(%)

Fig. 4. Diffraction efficiency for gratings with different spatial frequency obtained changing the angle h for the 25:1 concentration.

570 J.C. Ibarra et al. / Optical Materials 27 (2004) 567–572

the grating and observed the diffracted orders. We meas-

ured the diffraction efficiency, g, for each grating and for

all the emulsion. In Fig. 3 we show the diffraction effi-

ciency obtained.

It is obvious, from Fig. 3 that the best diffraction effi-ciencies are obtained for the 25:1 concentration and it

has a value between 0.25% and 1.2%. The 50:1 concen-

tration is the lowest because the film has a saturation

of resin and its photosensitivity response is slow. The

diffraction efficiencies for the 10:1 and 17:1 concentra-

tions have intermediate values.

Now, just employing as a reference the 25:1 concentra-

tion we recordedmore phase grating with different spatialfrequencies changing the angle h between the object and

reference beams (see Fig. 1). Taking the same exposure

2 3 4 5

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Intensity reasonbetwen beams

1:1 2:1 3:1

(%)

Exposure En

Fig. 5. Diffraction efficiencies varying the inten

energy values as before, we record gratings at the angles

h = 5�, 10�, 20�, 30�, 40� and 50�. In Fig. 4 we show the

diffraction efficiencies (g), obtained in this process.

The results in Fig. 4 show the best diffraction effi-

ciency for an angle h = 5� corresponding at 190 lines/mm and a maximum exposure energy for the 25:1 con-

centration. The lowest diffraction efficiency corresponds

to h = 50� corresponding at 1845 lines/mm.

The results above obtained indicate that our film has

better response at low spatial frequencies and low re-

sponse at high spatial frequencies.

Having this in mind, we have changed the intensity

of the interference beam and we recorded phase grat-ings. The diffraction efficiencies obtained are shown in

Fig. 5.

6 7 8 9

ergy (J/cm2)

sity ratio between the interference beam.

2 3 4 5 6 7 8 9 100.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Differents wavelengthλ = 457 nmλ = 476 nmλ = 488 nm

5° Interference angle

η (%

)

Exposure Energy (J/cm2)

Fig. 6. Diffraction efficiency for the wavelengths k = 457, 476 and 488nm the maximum efficiency is obtained at k = 457nm as we expected.

J.C. Ibarra et al. / Optical Materials 27 (2004) 567–572 571

In Fig. 5 it is showed that in the emulsion there are

phase modulation changes, due to the intensity differ-ence between the interference beams, with the optimum

intensity ratio between the beams 3:1, where g varies

since 0.05–0.75%, the other two curves show g below

Fig. 7. Photography of the phase grating recorded in the film

composed of BCG and resin amplified 400 times.

Fig. 8. Showing the first diffracted orders with a g = 1.27%.

0.3% in all exposure range, and the intensity ratio been

lightly more efficient between the beams 2:1.In Fig. 6 we show the diffraction efficiency obtained

for the 25:1 concentration using the wavelengths

k = 457, 476 and 488nm for the reconstructed phase

grating, when the angle h = 5� between the object and

reference beams.

From Fig. 6, it can be observed that the wavelength

k = 457nm is the best line which our film has good

response, showing g = 0.8 at maximum exposure energy.As we expected from Fig. 1, the maximum absorbance is

located for this wavelength, and is not good for the lines

k = 476 and 488nm of our argon laser.

According to the results previously mentioned, we

conclude that the maximum diffraction efficiency ob-

tained was for the concentration 25:1 when we record

a phase grating with k = 457, and at h = 5�, correspond-ing a grating with 190 lines/mm.

Finally, we present the photography of the phase

grating that has been recorded in the emulsion amplified

400 times with a microscope, in Fig. 7, we show uniform

lines of the grating and in Fig. 8 its first diffracted

orders.

4. Sensibility hypothesis

When the light strikes on the photosensitive emulsion

a chemical reaction on it originates. In order to under-

stand this reaction it is necessary to know before hand

the molecular structures of the BCG and the resin di-

lutes in the isopropyl alcohol. The alcohol function is

to help the BCG and resin molecules to join and orient,

using for this hydrogen polar bridge links [16,17] notedby the union of negative and positive poles (Fig. 9).

-

R' +

Br

OH

Br

H C3

Br

HO

Br

O

S

O O

CH 3

CH3

COOH

H

H

3CH

3CH

CH

-

+Rδ δ

δδ

Fig. 9. Showing the union molecules of resin and BCG, that present the polarity phenomenon.

572 J.C. Ibarra et al. / Optical Materials 27 (2004) 567–572

In the molecular structure of our emulsion, there exist

the bromo (Br), and that helps the electrons transfer be-

cause of their oxidation state (�1, 1, 3, 5, and 7). When

a beam of energy E = hm is incident on the emulsion it

ejects a free electron. So we expect that the bromo con-

tained in the BCG dye is the responsible to accept thefree electrons transference. In Eq. (3) it is showed the

radical that have the Br atoms in the reaction process.

R� Brþ hm ! ðR� BrÞþ þ e�: ð4ÞThe free electron is caught by an ion of the molecular

structure of emulsion positive part, Eq. (4).

e� þ ðR� BrÞþ ! R� Br: ð5Þ

5. Conclusions

We have a photosensitive film made of BGC dye andresin mixed with isopropyl alcohol. In this material we

recorded a diffraction grating with relief modulation.

The developing process of this film is very fast and

cheap. The maximum diffraction efficiency was h =

1.27% with a k = 457 nm and 3:1 intensity reason be-

tween the interference beams. The low cost of the film

and developing facility make this a good choice as holo-

graphic elements.

Acknowledgment

J.C. Ibarra and P. Alonso-Magana thanks to Centro

de Ensenanza Tecnica Industrial and M. Ortiz-Gutierrez

thanks to Universidad Michoacana de San Nicolas de

Hidalgo for financial support (projects DCA/03403

and 9.15 respectively).

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