Laser-induced birefringence in polar liquids*

J. F. Giulian† and J. Van Laak Electrical Engineering Department, University of Maryland, College Park, Maryland 20742

(Received 10 September 1975; revised manuscript received 19 January 1976) The high-frequency Kerr constants are measured for the first time for a variety of polar liquids relative to

nitrobenzene, a strong Kerr-active liquid. Among the liquids investigated were aromatic compounds, ketones, and fluoroalcohols. The measurements consisted in inducing optical birefringence in the liquid samples by the intense electric fields from a Q-switched Nd:glass laser and simultaneously observing a pulsed signal through crossed polarizers using a low-power He-Ne laser as a light probe at 6328 Å. Comparison of the measured and calculated optical Kerr constants for those liquids where the depolarization ratios and isothermal compressibilities were known showed satisfactory agreement. A number of liquids exhibited high-frequency Kerr constants on the order of nitrobenzene.

I. INTRODUCTION

In recent years Kerr-act ive materials have received much attention in electro-optics. This has been fos­tered by the need to obtain simple, ultrafast light gates for studying transient optical phenomena.1 '2 The prin­cipal figure of meri t in evaluating such materials is the high-frequency Kerr constant. Mayer and Gires3 and Paillette4 - 6 have determined these constants for a num­ber of both polar and nonpolar liquids. In this paper a re presented the high-frequency Kerr constant mea­surements for nitrobenzene and 16 other polar liquids which have not been previously investigated. The gen­eral method used in these studies is taken from Pai l ­let te .6 Optical birefringence was introduced in each of the liquid samples by the intense electric field from a Q-switched Nd: glass laser at 1.06 μm. This optical birefringence was simultaneously probed by a low-pow­er cw He-Ne laser at 6328 Å. It was possible to com­pare the measured values of the high-frequency Kerr constant with the constants computed from theory in only those cases where the depolarization ratios and . isothermal compressibilities of the liquids tested were known. The polar liquids investigated included aromat­ic compounds, ketones, and fluoroalcohols.

II. THEORETICAL

The high-frequency Kerr constant B0 for a gas or liquid is generally defined by the relation6

III. EXPERIMENTAL ARRANGEMENT

Figure 1 shows a schematic of the experimental a r ­rangement. It was necessary to telescope the Q-switched beam to obtain high enough intensities to make measurements . Par t of this 1.06 μm beam was split off and monitored by an ITT (S-l) photodiode and dis­played at one input of a dual beam 551 Tektronix oscil­loscope. The He-Ne probe beam was sent through the liquid sample in a direction opposite to that of the Q-switched beam. Both beams were made collinear and overlapped each other in the liquid sample. A thin-film pellicle at nearly normal incidence with respect to the He-Ne beam reflected about 4% of this light into a suitably apertured and filtered Amprex (S-11) photo-multiplier, whose output was displayed at the second in­put to the dual beam scope. A 99% 1.06 μm dielectric reflector oriented at a slight angle with respect to the Q-switched beam was placed behind the liquid cell in order to protect the He-Ne laser and at the same time transmit the He-Ne light through it in the opposite di­rection. The He-Ne beam was polarized 45° with r e ­spect to the Q-switched beam, and an analyzer in front of the photomultiplier was oriented 90° with respect to the He-Ne polarization in order to produce maximum extinction of the He-Ne light prior to firing the pulsed laser . It was essential in this arrangement that the pellicle beam splitter be oriented near normal incidence

where ne is the induced (extraordinary) refractive index parallel to the intense electric field, n0 is the induced (ordinary) refractive index orthogonal to the intense electric field, E2 is the time-averaged square of the in­tense electric field in the medium, and λi is the wave­length of the probe beam.

Detailed calculations from classical considerations show that B0 for polar liquids can be expressed in t e rms of measurable quantities in cgs units by the relation6

where βT is the isothermal compressibility of the me­dium, nt i s the refractive index at the wavelength of the probe beam, nL is the refractive index at the wavelength of the intense beam, and Δ is the polarization ratio, generally obtained from Rayleigh scattering informa­tion.

FIG. 1. Schematic of the experimental setup used in making measurements of the optical. Kerr constants of various polar liquids.

J. Opt. Soc. Am., Vol. 66, No. 4, April 1976 Copyright © 1976 by the Optical Society of America 372

TABLE I. Tabulation of measured and computed optical Kerr constants for the investigated polar liquids and some pertinent phys­ical constants associated with them.

in order to make as nearly linear as possible the polar­ization of the reflected He-Ne light impinging on the analyzer in the absence of the pulsed beam.

The Nd: glass laser was Q- switched by means of a Pockels cell. The telescoped beam had a uniform cross section (determined by its burn pattern on a piece of Polaroid 410 film) with an area A of 0. 3 cm2 (substan­tially larger than that of the He-Ne beam). The energy S of the beam was measured by a TRG 100 thermopile. The pulse width τ_(full width at half-maximum) was about 20 nsec. E2 was calculated using the formula

where P= E/τ is the power of the telecoped beam. In the present work the intensity was kept below 15 MW/cm2

for all liquids except the alcohols. More will be said about this in the following section.

IV. PROCEDURE

An absolute determination of B0 was made at 6328 Å for a 15 cm long sample of nitrobenzene at a Q-switched laser intensity of 10 MW/cm2. The other liquids were measured at the same wavelength and for the same path length relative to nitrobenzene by normalizing the Q-switched laser intensity to 10 MW/cm2. The absolute value of B0 for nitrobenzene was determined by use of the relations6

and

where AV is the measured signal voltage at 6328 A when the polar izers were crossed and the Q-switched beam was present, V is the measured signal voltage at 6328 Å when the polar izers were parallel and the Q-switched beam was absent, φ is the phase retardation, l is the length of the liquid sample (15 cm), and E2 is the time-averaged square of the intense electric field in the medium.

The fluctuations in E2_produced scatter in ΔV. Gen­erally, the variation in E2 was within about 8%. This resulted in an uncertainty in B0 of at least 8%, and in the less active liquids (HFIP and HFAS) the uncertainty was larger due to the presence of photomultiplier noise. The estimated e r r o r s in B0 a re listed in Table L

For all of the liquids investigated, the measured val­ues of ΔV/V were adjusted for absorption6 at 1.06 μm. The absorption coefficient β for nitrobenzene at 1.06 μm was about 0.028 cm"1, and this increased the mea­sured ΔV/V by a factor of 1.3 for the 15 cm path used. The res t of the liquids had absorption coefficients at 1.06 μm on the order of nitrobenzene, i. e . , β≈0.03 cm"1. All the liquids except nitrobenzene (to be d is ­cussed later in the text) showed negligible absorption at 6328 Å, and appeared to remain thermally stable under intense laser radiation. The measured value of φ for nitrobenzene for a power density of 10 MW/cm2 was 0.25 rad. Substitution of the pertinent values into Eq. (4) gave an optical Kerr constant for nitrobenzene of

373 J. Opt. Soc. Am., Vol. 66, No. 4, April 1976 JOSA Letters 373

2.36×10 - 7 esu cm"1.

Figure 2 shows the measured phase retardation φ in degrees as a function of the mean-square pulsed elec­tric field in a ir from the Nd : glass laser for a 15 cm cell path of nitrobenzene. Note the straight-line depen­dence as expected from Eq. (4). This experimental curve was particularly significant since any additional nonlinear effects, such as self-focusing or stimulated Raman scattering, would have significantly altered the slope as a function of electric field strength up to the values shown in Fig. 2. (See Ref. 6 for a detailed d is ­cussion of the influence of such nonlinearities on the phase retardation.) Thus the power density was pur­posely limited to 10-15 MW/cm2 for all liquids investi­gated except the much weaker Kerr-act ive alcohols, which have shown no evidence of self-focusing up to 30 MW/cm2 in a 15 cm cell.

V. RESULTS

Table I summarizes the present measurements of the 17 polar liquids, which include aromatic compounds, ketones, and fluoroalcohols. It is clearly seen from this table that the aromatic liquids, for which nitro­benzene is typical, possess by far the larger high-fre­quency Kerr constants. Two of the liquids investigated, namely 1-bromonaphthalene and m -nitrotoluene, have values comparable in magnitude to nitrobenzene. Of the ketones investigated, only acetophenone, which is aromatic, possesses a noticeably large B0, whereas the pure ketones appear to show no apparent effect within the sensitivity of the present experimental setup. It is well known that the electrons associated with aromatic molecules a re delocalized, that is, they a re not tightly bound to single nuclear centers . This can explain why a larger optical effect is seen in these molecules than in molecules lacking an aromatic character .

Two compounds from the class of nitrotoluenes, o-

and m - nitrotoluene, exist as liquids at room tempera­ture, while a third compound, p-nitrotoluene, exists as a solid at room temperature. An attempt was made to dissolve a sample of p-nitrotoluene in ethyl alcohol, but it was found that only a few percent of the solid went in­to solution. Both the o- and m -compounds showed strong Kerr activity, but no measurable effect was ob­served for the parasubstituted compound. Comparison of the s tructures of the three nitrotoluenes predicts a comparable effect for the dissolved solid. The lack of response in this last case is therefore evidently due to the known low concentration of sample molecules p r e s ­ent.

The last group of liquids shown in Table I is com­posed of three alcohols: hexafluoroisopropylalcohol (HFIP), isopropyl alcohol, and hexafluoroacetone sesquihydrate (HFAS). It has been shown by one of the authors7 that HFIP produces an unusually strong inten­sity-dependent modulation of an intense laser pulse when placed inside or outside the laser cavity. The present measurements showing birefringence in this same fluoroalcohol give additional evidence of marked nonlinearity in this liquid. It is interesting to compare HFIP with its nonfluorinated analogue, isopropyl alco­hol. Both have the same molecular structure except that the fluorine atom replaces the hydrogen atom in the (CX3) group, where X = F , H. The foregoing obser­vation of HFIP 7 and present Kerr studies of HFIP and isopropyl alcohol indicate that the enhanced laser - in­duced nonlinearities exhibited by the HFIP over the isopropyl alcohol can be directly attributed to the fluo­rine substitution.

VI. DISCUSSION

In Table I a re found calculated values of B0 for acetophenone, bromobenzene, chlorobenzene, and nitrobenzene. These calculations were based on Eq. (2) and were limited to those compounds for which the depolarization ratios and isothermal compressibilities were known. The difference between the computed and measured values varies by 10%-20% for acetophenone, brombenzene, and chlorobenzene, and by less than 10% for nitrobenzene. Table I also shows Paillette 's mea­sured value for nitrobenzene6 extrapolated to 6328 Å. The presently measured value and this extrapolated val­ue agree to within 6%.

FIG. 2. Phase retardation as a function of the mean square 1.06 μm laser field in air for a 15 cm cell length of nitroben­zene.

For these measurements a commercial grade of nitrobenzene was used. Detailed spectral t r ansmis ­sion studies of this material showed significant broad absorption over the red-green region, though very little absorption was measured over this same region in a pure sample of nitrobenzene obtained from the NBS.8 To see if these extraneous bands had any effect on the high-frequency Kerr constant, a comparison was made of the measured ratios ΔV/V for both samples normalized to the same laser density and liquid path length. The observed difference in the ratios between the two samples was less than 3%, indicating that the impurities present in the commercial grade did not s e ­riously alter the measured value of B0.

374 J. Opt. Soc. Am., Vol. 66, No. 4, April 1976 JOSA Letters 374

VII. SUMMARY Measurements of the optical Kerr constants for this

selection of polar liquids have shown a number of Ker r -active materials as efficient as nitrobenzene, and thus may provide useful alternatives to nitrobenzene in spe­cial light modulation or switching applications. Also, the fluoroalcohol HFIP has been shown to exhibit a larger than expected optical birefringence, which may be related to the replacement of hydrogen by fluorine in the alcohol.

ACKNOWLEDGMENTS

The authors wish to thank Professor Chi H. Lee for the many helpful suggestions in the course of these in­vestigations. We also wish to acknowledge Dr. R. E.

Hebner and W. A. Bagley for graciously providing us with a purified sample of nitrobenzene.

*Work supported by NSF through Grant No. ENG75-00262. †On Sabbatical leave from the NavaL Research Laboratory,

Washington, D. C. 1M. A. Duguay and J. W. Hansen, Appl. Phys. Lett. 15, 192

(1969). 2W. Yu and R. R. Alfano, Opto-Electronics 6, 243 (1974). 3G. Mayer and F. Gires, C. R. Acad. Bulg. Sci. 258, 2039

(1964). 4M. Paillette, C. R. Acad. Bulg. Sci. 262 (1966). 5M. Paillette, C. R. Acad. Bulg. Sci. 267, 29 (1968). 7 J. F. Giuliani, Proceedings of the Electro-Optical Systems

Design Conference, New York, 1973, p. 259. 8For this sample, water vapor and other impurities have been

removed by passing reagent-grade liquid through a chromato­graphic absorption column of activated alumina.

375 J. Opt. Soc. Am., Vol. 66, No. 4, April 1976 Copyright © 1976 by the Optical Society of America