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Produced by the NASA Center for Aerospace Information (CASI)
Semi-annual Progress Preport

August 1982 to February 1983

NASA Grant 5-220

GROWTH AND EVALUATION OF AgGaS2 and AgGaSe2

FOR INFRARED NONLINEAR APPLICATIONS

Principal Investigators

Robert L. Byer

Applied Physics Dept.,

and

Robert S. Feigelson

Center for Materials Research

(NASA-CR- 175185) GEOWIH AND EVALUATION OF N84-18066

AgGaS2 AND AgGaSe2 FOE INFRARED NONLINEAR

APPLICATIONS Semiannual Progress Repoit,

Aug. 1982 - Feb. 1983 (Stanford Univ. ), 13 p Unslas

HC A02/MF AUI CSCL 20L G3/76 15300

o%_ '4
GROWTH AND EVALUATION OF AgGaS2 and AgGaSe2

FOR INFRARED NONLINEAR APPLICATIONS

I. CRYSTAL GROWTH

A. Objectives

the objective of this program is to improve the crystal

growth technology for the two chalcopyrite compounds AgGaS 2 and

AgGaSe 2 . These two materials demonstrated their promise as 

important nonlinear crystals for infrared applications ten years 

ago. However, at the time, a number of werious growth related

problems such as cracking, twinning and the occurrence of optical

scattering centers made it difficult to obtain high quality

specimens in sizes exceeding 1 cm.

Using seeding and precision-tapered fused quartz gr

ampoules, we are developing Bridgman/Stockbarger growth technology 

to grow crack and twin-free boules in increasingly larger dimensions

with an ultimate goal of 4 cm crystals harvested obliquely from 

c-axis boules. We are studying the post-growth heat treatment

procedures to understand the solid state chemical reactions and 

to avoid crystal damage which frequently occurrs during these 

annealing procedures.

Initially, experiments will be carried out on AgGaS2

(11P =996°C). AgGaS 2 has the advantage of being transparent in

the visible part of the spectrum. Subsequently, experiments will

begin on AgGaSe 2 (?^.' =856C) whichis slightly more difficult to 

evaluate because it is transparent only in the deep red and near

IR.

- 1 -

v
B. Progress to Date

During the second reporting period, growth experiments

continued n AgGaS 2 crystals. Additional crystal growth experiments

were carried out on 2 cm diameter crystals to further ellucidate

the problem of breakdown to polycrystalline and cracked boules.

Growth experiments on high quality v3 twinned and otherwise damaged

seeds supported our earlier conclusion that seed quality was the

problem. However, at this time the matter has not been absolutely

and unambiguously resolved.

Nonetheless, we have moved ahead to the construction of a

larger scale furnace system capable of growing 28 mm diameter boules

which weigh approximately 200 g (three times the weight of our

earlier boules). Initial growth experiments have been very

successful and boule quality appears to be at least as good as

earlier boules. Boules have so far been either toL._a.1y twin-free,

or at most contain a few in the conical flare-out region where it

is of no consequence. Optical scattering centers are present

as in the smaller scale boules. Some compositional or impurity

banding is usually observed and this may influence the yield of

crystals for subsequent heat treatment to a limited degree. Regrowth experiments, typically used to reduce impurities and

eliminate excess components, appear to hpve slightly beneficial

effects, but they are not totally effective at removing banding

and double the two week growth interval.

We are currently using a single batch,, carefully controlled

synthesis procedure to produce the starting material. Starting,

- 2 -

s
compositions of stoichiometric r.omposition and of 0.1% excess

Ga2 S 3 are being utilized, with so far no apparent differences

in the grown crystals.

Refinements in the heat treatment procedures are being

carried out to optimize the thermal program and to determine

the minimum amount of A9 2 S required to totally eliminate residual

scattering. Occasionally, localized residual inclusion3 remain

after heat treatment procedures have been completed. These defects

appear as internally faceted voids when seen under the optical

microscope. They tend to occur in regions which contained bands

or other macroscopic inclusions after growth and no amount of

additional heat treatment eliminates them. Crystal dimensions

from the 28 -gun diameter boules are considerably larger than

earlier specimens and oriented 1 cm2 crystals exceeding 2 cm

in active length are now in preparation.

Initial growth experiments are also underway on AgGaSe2

in the 28 mm diameter boule size. Starting material is slightly

easier to prepare than .or the case of AgGaS 2 because no excess

vapor pressure probleuas occur during the synthesis procedures.

Only one boule of AgGaSe 2 has been grown so far but seeding was

totally successful and when evaluated using an IR image converter,

it was seen to be free of physical defects.. It displayed 4-fold

symmetry typical of a single crystal c-axi2 boule. Additional. growth

and evaluation experiments on AgGaSe2 are planned.

- 3 -

.

,...^. ++. +. — '+r•r^"" .:^,,.^-,......e...:....^.^.^se!vf1^?s^lt..^"'^..'r..3tsn
C. Crystals Suppliea to N,A.S.A. During This Period

AgGaS2 3

1 cm , Type I , 58°

1 cm , Type 1 , 58°

1 cm , Type 1 , 90*

Additional crystals have been supplied to Processor Bye ,:'s group

during this period.

II. EVALUATION FOR INFRARED NONLINEAR APPLICATIONS -

SECOND HARMONIC GENERATION OF CO 2 LASER RADIATION

Second harmonic measurements were performed using a 1 cm

AgGaS 2 crystal grown at Stanford Center for Materials Research

and a CO 2 TEA laser. The purpose of the measurement was to

obtain experimental verification of the calculated conversion

efficiency and calculated phasematching angle, and obtain an

evaluation of crystal quality. First the CO 2 TEA laser output

was characterized. The angle tuned phasematching curve was

obtained using 10.591 um fundamental radiation. A Laue X-ray

diffraction measurement was made to determine crystal orientation

and phasematching angle. Crystal absorption was measured both

with a scanning spectro-photometer and with the CO 2 laser.

A Tachisto model 215 G CO 2 TEA laser was used for these

measurements. Laser wavelength was tuned with a 150 Z/mm flat

grating used in place of the rear reflector. The output mirror

was 35% reflecting with 10M radius of curvature. An iris

r.kaphrsam was used as an intra-cavity aperture to give transverse

- 4 -

., y .yam.. y.. ^w.w.w - _ .rnhM'f's..n.^M!4

o wa 'yy^^l^^^u_ 

i v^^ ^^ R.IIfAI
mode control. Two Brewster angle windows on the discharge tube

provided a polarized output beam. The grating wavelength

ealibratiin was obtained by monitoring laver output with an

Optical Engineering CO 2 laser spec*rum analyzer. The laser

would oscillate on a single CO 2 vibrational/rotational transition.

Observation of the laser pulse with a fast time response photon

drag detector and oscilloscope showed beating of longitudinal

modes. About half the pulse energy appeared to be in a 400 ns

long nitrogen tail. The structure due to mode beating; changed

from shot to shot. Typically about one quarter of the pulse

energy would be present in seven pulses of 3 nsec duration

separated by the 6 nsec round trip cavity transit time. (Fig.l).

It was difficult to obtain Gaussian transverse intensity distributi

only by adjusting the intracavity aperture. Diffraction rings

usually developed somewhere in th% existing; laser beam. Externally

aperturing the laser beam at the first diffraction minimum, however,

did result in a Gaussian-like distribution. It is estimated that

peak intensity of the resulting laser output is very roughly

6 x 10 6 W/cm2.

Second harmonic conversion efficiency and ohasematching

tuning curves were obtained by simultaneously monitoring incident

fundamental energy and generated second harmonic energy.(Fig.2)

A portion of the incident fundamental energy was reflected by a

beam splitter into a pyroelectric energy detector. The remainder

of the fundamental pulse was incident on the nonlinear crystal.

The transmitted fundamental and harmonic beam were then incident

on 3 mm thick sapphire windows which had no measurable transmissic

- 5 -

x
at 10.6 pin and transmission of 57% at the second harmonic

wavelength 5.3 um. The portion of second harmonic transmitted

through the sapphire filter was measured with a second pyroelectric

energy d4x-actor. The crystal was rotated in a micrometer driven

gimbal mount.

The tuning curve that was obtained (Fig. 3) accurately

follows the sin 2 (Akk /2)/(4k2/2) 2 dependence of the ideal case.

The fact the :five of the secondary maxima are displayed so clearly

is an indication of good crystal homogeneity. The fundamental

wavelength for this measurement was 10.591 um. Peak incident

intensity was approximately 5 x 10 6 W/cm2 . This value is uncertain

because of the mode beating in the CO 2

 laser output. The average

measured internal conversion efficiency at the phasematching

peak is 0.17%. This is consistent with the calculated values

to the extent mode beating structure and proportion of pulse

energy in the long decay low intensity tail are known. Lau.e

X-ray diffraction patterns were made to check the orientation

of this crystal. When X-ray diffraction and angle tuning results

were combined the phasematching angle was found to be 74k° t ^°.

This value is closest to the phasematching angle calculated for

the Sellmeier equation obtained by Herbst by fitting to Boyd`s

dispersion data.

Low crystal absorption combined with good optical quality

is an indication of high quality crystal growth. Absorption of

this AgGaS 2 crystal was measured with an infrared scanning spectrophotometer. Crystal transmission was observed to be constant

between 2.5 and S micrometers indicating losses due only to

- 6 -
Fresnel reflection and aperturing (Fig. 4). The spectrod

photometer recording gives absorption coefficient at 10.591 um

of a = 0.48 cm-l . A measurement of transmission at 10.591 um 

using the CO 2

 laser and energy ratiometer gives transmission of

45.7%. The calculated Fresnel reflection loss at each surface

is 17.1%. This gives an absorption coefficient of 0.43 cm-1

The absorption at 10.59 um in AgGaS 2 is an intrinsic property 

of the material due to a three phonon absorption. The low

absorption of this crystal in the 0.5 to 8.0 micrometer Fpectral 

range indicate good material quality.

_ 

- 7 -

A
FIGURE CAPTIONS 

1. Osci:ilogram of laser pulse. Time increases right to

left. Intensity is displayed on the vertical scale.

.f

2. Schematic drawing of setup used to obtain tuning; curve.

{ 3. phasematching curve for 1 cm long ApGaS 2 crystal used

.'t

for second harmonic generation of 10.591 dim CO laser

radiation. Note change of scale.

4. Infrared transmission of AgGaS 2 crystal used for second

harmonic generation and sapphire window used to block

10.6 urn and transmit 5.3 um,

g_

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