Investigation of linear and nonlinear optical properties of GaSxSe1–x crystals

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1982 Sov. J. Quantum Electron. 12 947

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Investigation of linear and nonlinear optical properties of~ x crystals

K. R. Aliakhverdiev, R. I. Guliev, E. Yu. Salaev, and V. V. Smirnov

P. N. Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow(Submitted September 9, 1981)Kvantovaya Elektron. (Moscow) 9, 1483-1485 (July 1982)

The refractive index and the birefringence of GaS,Se,_., crystals were determined in the range 0.63-20 fi.Generation of the second harmonic of CO2 laser radiation was used to determine nonlinear opticalcoefficients. Mixing of radiations from a YAG laser and a tunable i^"-center laser generated the differencefrequency in a Gao8SO2 crystal and radiation of this frequency was tunable in the wavelength range 7-12.5/i.

PACS numbers: 78.20.Dj, 78.20.Fm

Nonlinear optical properties of GaSe crystals wereinvestigated in Refs. 1-4 and it was found that thesecrystals could be used in optical parametric mixing.We investigated linear and nonlinear optical propertiesof GaS^Se!^ crystals. A special property of these solidsolutions was a structural phase transition in the com-position range 0.2 «x «0 .3 , from a centrosymmetric(/3) to a noncentrosymmetric (E) modification, which wasaccompanied also by changes in the transmission range,refractive indices, hardness, etc.

MEASUREMENTS OF THE REFRACTIVE INDEXAND BIREFRINGENCE

The refractive indices (n0 and nt) and the birefringencewere determined in the transparency range (0. 63-20 /u.)by two methods. In the range 0.63-3.39 ii the valuesof n0 and ne were determined from the Brewster anglesand the angles of a conoscopic pattern5 at three fixedNe-He laser wavelengths (0.63, 1.15, and 3.39 ti).In the range 3. 39-20 n the refractive indices n0 and ne

were found by an interference method6 using an infraredspectrometer.

Measurements were carried out on crystals with thesulfur concentrations x = 0, 0.2, 0.4, 0.8, and 1. Theresults of the measurements carried out on GaS andGaSe crystals were in good agreement with the pub-lished data.1-7'8 In the 0. 6-20 ii range the indicesw0 and wewere extrapolated with an accuracy of ±0.005using the dispersion relationships:

* + B}-2 -f C -f Dl? + EX.*. MX,-* + N>.~ Ri.\

where A is the wavelength in microns; A, B, C, D, E,M, N, O, P, and R are extrapolation coefficients listedin Table I.

The values of the refractive indices were used to cal-

TABLE I.

ABCDEMNOPR

G " S . , 1 S | V.

—1.841-10-8.994-10-7.718

—1.725-10-—6,200-10"

5.333-10-3.248-10-5.958

—2.932-10-—4.180-10-«

OaS . . .Se . . .

—2.317-10-1

1.0557.0199

— 1.527-10-—3.200-10-- 2.196-10-

9.650-10-5.437

—1.283-10-'—2.38-10-'

G a S . . 8s ' o , ,

5.734-10-26.112-10-1

6.668—1.566-10->—2.250-10-«—3.175-10-'

3.822-10-1

5.298—1.095 -10-3

2.16510-"

OaS

—3.485-10-6.305-10-6.556

—1.304-10-—2.03-10-

—3.54410-3.355-10-'4.954

—8,844-10-— 1.115-10-'

culate the angular frequency conversion curves for theprocess of optical parametric mixing of pump radiationsof wavelengths 1.06 and 5.3 M (Fig. 1), as well as thephase matching angles 6 for second harmonic generation(Fig. 2) as a result of interactions of two types: oo-eand eo-e. The experimental results are represented bypoints.

DETERMINATION OF NONLINEAR OPTICALCOEFFICIENTS

The nonlinear susceptibility tensor of GaS^e^, cry-stals in the E phase (point group 6"m2) has only one non-zero coefficient d22. Interactions of two types are pos-sible in these crystals: oo—e, eo—e; the effective non-linear coefficients for these interactions are dooe

= —d22 cose sin 3cp and deoe = — d22 cos20 cos 3<p, where 6is the angle between the wave vector of the pump waveand the optic axis of the crystal; cp is the angle be-tween the projection of the wave vector of the pumpwave onto the XY plane and the crystallographic axisX.

The nonlinear susceptibility of GaS0>2Se0-8 andGaS0-4Se0>6 crystals was determined by generating thesecond harmonic of CO2 laser radiation under phase-matching conditions. Independent measurements of thenonlinear coefficients were inaccurate (even in the ab-sence of absorption) because they were based on thepoorly known distributions of the power densities of thepump and second harmonic waves over the beam crosssection. The nonlinear coefficient d22 was deducedfrom the known value of this coefficient for GaSe cry-stals .

15 20 2Sfb

FIG. 1. Conversion (tuning) curves of parametric radiationgenerated in a GaSo.2

seo.8 crystal as a result of two Interac-tions: a) e(YAG)o(Fp-'e(IB); b) c(YAG)o(Fj)-(IR).

947 Sov. J. Quantum Electron. 12(7), July 1982 0049-1748/82/070947-02$04.10 © 1982 American Institute of Physics 947

FIG. 2. Dependences of the phase-matching angle for thesecond harmonic on the pump wavelength reaching GaS0>2Se0 (

crystals.

The apparatus used in the determination of the non-linear coefficients is shown in Fig. 3(a). A Q-switchedCO2 laser operated at a repetition frequency of 130 Hzand the pulsed output power was ~400 W. Wavelengthselection was possible, and this enabled us to use thewavelengths 9.6 and 10.6 M. The second harmonic wasmeasured with a Ge: Au detector.

The nonlinear coefficients obtained in this way wered22(GaSo.2Seo.8)/d22(GaSe) = 0.525± 0.05 andd22(GaS0#4Se0.6)/d22(GaSe) = 0.31 ±0.05, where d22(GaSe)= (1.3±0.26)xl07 cm- dyn"1/2 (Ref. 1).

GENERATION OF THE DIFFERENCE FREQUENCYCONTINUOUSLY TUNABLE IN THE RANGE 7-12.5 /xIN A GaSo^Seo^ CRYSTAL

Mixing of radiations from a YAG laser and a tunable^-center laser generated the difference frequency in aGaS0-2Se0.8 crystal and the output radiation was tunablein the range 7-12.5 fi. Our aim was to check the con-version (tuning) curves and to determine whether suchcrystals were suitable for practical applications.

We used the apparatus shown schematically in Fig.3(b). Radiation from the YAG laser (8) passed throughan amplifier (9) and was split by a sapphire plate (6) intotwo beams. The polarization of one of these beams wasrotated by 90° and it was then used to pump the -F^-centerlaser. The second beam was delayed by ~8 nsec and wascombined with the radiation of the J^-c enter laser by adichroic mirror (10). Next, the YAG and F2-center las-er beams, which now had orthogonal polarizations, werefocused in a GaS0>2Se0>8 crystal by lenses (12). The dif-ference-frequency radiation was recorded with a detec-tor (7). The emission wavelength of the -F2-center laserwas measured with a DFS-12 spectrograph after pre-liminary frequency doubling in a LiIO3 crystal. The Q-switched YAG laser emitted the TEM00 mode at a repet-ition frequency of 10-20 Hz. After amplification thepower per pulse was 1.5—2 MW and the pulse durationwas 15 nsec. The .FJ-center laser was excited lon-gitudinally by the YAG laser radiation which passedthrough a dichroic mirror (11) transmitting the pump

FIG. 3. Apparatus used to generate the difference frequency(a) and to determine the nonlinear coefficient (b): 1) CO2

laser; 2) rotatable exit mirror; 3) selector; 4) NaCl lenses;5) investigated crystal; 6) sapphire plate; 7) Ge : Au photo-detector; 8) YAG laser; 9) amplifier; 10), 11) dichroic mirrors;12) lenses; 13) F^-center laser; 14) diffraction grating;15) Investigated crystal.

radiation but reflecting that from the i^-center laser.Frequency selection was provided by a 600 line/mmdiffraction grating (14) and the output wavelength wastuned by rotation of a mirror (16) about its vertical axis.The output power of the i^-center laser at the wave-length ~1.15 ju was ~75 kW in the form of pulses of 10nsec duration when the pump radiation power was 1.25MW. The width of the emission line did not exceed 0.2cm"1. The tuning range of the F^-center laser was 1.11 —1.27 /x.

Figure 2 shows the dependences of the phase-matchingangle for a GaS0>2Se0i8 crystal in the case of interactionsof the eo—o and eo—e types. The infrared radiation atthe difference frequency was recorded with a Ge; Au de-tector. In the tuning range of the difference frequencythe radiation power was 30-100 W.

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Translated by A. Tybulewlcz

948 Sov. J. Quantum Electron. 12(7), July 1982 Allakhverdiev et al. 948