Mat. Res. Bull. Vol. 9, pp. 507-510, 1974. Pergamon Press, Inc. Printed in the United States.

THE REFRACTIVE INDICES OF SOME GARNET CRYSTALS AT 1.15 ~m

John Warner* Naval Research Laboratory Washington, D. C. 20375

(Received February II, 1974; Communicated by J. H. Schulman)

A B S T R A C T Refractive index measurements for Y3Fes012 , (LaY)3Ga.33Fe4.67012 and Ga3Gd5012 were obtained at 1.152 ~m by observing optical

waveguiding in epitaxial thin films.

We would like to report our measurements of the refractive indices of

some garnet single crystals. These result from studies of optical-waveguiding

in thin-films of iron garnets grown by liquid-phase epitaxy on to gadolinium

gallium garnet substrates. Our experimental arrangements is shown in Fig.l.

~inearly polarised light from a 1.152 ~m He-Ne laser passes through two small

holes approximately 1 meter apart on to the input coupling prism (Ref.l). The

priem/waveguide assembly may be rotated about a suitable axis to locate the

angles of incidence at which waveguide modes are exited. After propagating a

certain distance the guided light is coupled out of the waveguide by the out-

put prism and either observed with an ~mage converter tube or, for absorption

coefficient measurements, detected with a silicon photodiode. The refractive

indices and prism angle ~ of the rutile coupling prisms were measured on a

spectrometer. The film thickness and refractive indices o~ the film and

*On sabbatical leave from Royal Radar Establishment, Malvern, WORCS, U.K.

507

508 G A R N E T C R Y S T A L S V o l . 9, No. 4

4

RUTILE PRISMS

BEAM

IRON GARNET FILM

IMAGE CONVERTER

\ ' O0TPUT _ ~ ~ t~___ ~ BEAM

GALLIUM GARNET SUBSTRATE

FIG.I Schematic diagram of optical waveguiding apparatus.

3

2 Z~ /~ A

I

I I i I 0.2 0.4 0.6 0.8 1.0

z(CmS) FIG. 2

TE o o u t p u t power ( a r b . u n i t s ) a s a function of coupling distance z.

1.2

substrate were obtained from fitting

a theoretical mode dispersion curve

(Ref.l) to measured coupling angles

for several waveguide modes.

Absorption coefficients were esti-

mated by coupling a TE o mode into

the waveguide and plotting the

coupled-out intensity as a function

of the distance between the prism-

couplers as shown in Fig. 2.

Two film compositions were

studied, each grown by liquid phase

epitaxy on to a (iii) oriented single

crystal of GdsGa5012. One group of

films was of pure YIG (Y3Fe5012)

whereas the second group was of YIG

V o l . 9, No . 4 G A R N E T C R Y S T A L S 509

TABLE 1 Summary of Experimental Data

(i) Prism data (I=1.152 ~m) Prism angle ~ = 45o29 ' Ordinary refractive index (for TM modes) = 2.4703 ~ .0003 Extraordinary index (for TE modes) = 2.7254 ± .0003

(ii) Waveguide data (I=1.152 ~m)

Item

TE o coupling angle

TM o coupling angle

TE 1 coupling angle

TM I coupling angle

TE 2 coupling angle

TM 2 coupling angle

Y3Fes012 film

(YLa)3Ga.33Fe4.67012

18o2 '

37 ° 22'

2 ° 12'

16 ° 15'

!

19 ° 8

40 ° 51'

13 ° 39'

32 ° 9'

5 ° 17'

20 ° 32'

A least squares fit of above data to mode dispersion equation gives the following:

refractive index of Gd3Gas012 = 1.945 ± .001

refractive index of Y3Fe5012 = 2.2117 ~ .0004

refractive index of (YLa)3Ga.33Fe4.67012 = 2.1783 ~ .0004

From the slopes of the graphs in Fig. 2:

absorption coefficient of Y3Fe5012 = 4.0 cm -I

absorption coefficient of (YLa)3Ga.33Fe4.67012 = 1.3 cm -I

doped with La and Ga, i(YLa)3Ga.33Fe4.67012~ . At least three films (of

different thicknesses) were studied, each giving consistent refractive index

values.

Representative data are su~rlzed in Table i where the error figures on

the refractive indices indicate the RMS deviation of the data points from the

best mode dispersion curve. Coupling angles were typically repeatable to

better than 5 minutes of arc.

Acknowledgements

This work was supported in part by the Advanced Research Projects Agency

under ARPA order no. 2327 and is published by permission of her Brittanic

510 GARNET CRYSTALS Vol. 9, No. 4

Magesty's Stationery Office. The garnet films used in the work were prepared

by Dr. R. Henry of Rockwell International Inc., Anaheim, "California.

References

i. P. K. Tie.n, Applied Optics i0, 2395 (1971).