224 OPTICS LETTERS / Vol. 14, No.4 / February 15, 1989

Opto-optical switching in the infrared using CdTe

William H. Steier, Jayant Kumar, and Mehrdad Ziari

Center for Photonic Technology, Department of Electrical Engineering, University of Southern California, Los Angeles,California 90089-0483

Received November 4, 1987; accepted November 30, 1988

Opto-optical switching of a 1.06-,um signal beam by another 1.06-pm control beam has been observed in CdTe:In.The switching is caused by the photocharge created by the control beam and the resultant electric-field shielding.This effect can be switched in microseconds and takes advantage of the high figure of merit (n

3r/e) of CdTe.

Opto-optical switching, in which an optical controlbeam is used to switch or deflect another optical signalbeam, has a large potential for use in optical intercon-nects, optical communications, and optical signal pro-cessing. The approaches published include Fabry-Perot devices,'- 3 four-wave 4' 5 and two-wave 6 mixing,and optical waveguides.7 -9 We report here a new ap-proach to infrared switching using CdTe. It has thepotential for achieving arrays of infrared switches thatare controlled by one or more infrared control beamsand can be switched in microseconds.

The switch, which is shown in Fig. 1, uses photocon-ductivity and the electro-optic effect. In CdTe withan electric field along the (111) direction the electro-optic birefringence can be observed by a signal beampropagating perpendicularly to the applied field. Acontrol beam perpendicular to the signal beam is at awavelength at which the material exhibits modestphotoconductivity. The photocharges created by thecontrol beam drift into the dark region of the crystal,where they are trapped and create a space-charge elec-tric field that is opposite to the applied field. Thus inthe absence of the control beam, the signal-beam po-larization is altered by the applied field and the beamis deflected by the prism into output 1. When thecontrol beam is present the field seen by the signalbeam is decreased and all or part of the signal beam isdeflected into output 2. The intensity or the wave-length of the signal beam is such that the signal beamitself creates little or no photocharge.

This effect is an example of a charge-transport-as-sisted optical nonlinearity,10 as are photorefractionand the self-electro-optic-effect device. However,this effect requires a material with a much lower elec-tron trap density than efficient photorefractive grat-ing switches require. CdTe is of interest for photore-fraction because its figure of merit,11 a measure of theindex change per photon absorbed, is the highest ofthe ferroelectrics or the semiconductors. However, itsuse has been limited by the low electron trap density inthe available materials.'2 The CdTe:In used in thisresearch showed little photorefraction, since it has anestimated trap density of 1012 cm-3, yet works well inthe switch described here. This type of nonlinearity

takes advantage of the high figure of merit in CdTewithout the requirement of a high trap density.

The CdTe:In showed a broad absorption band ofapproximately 0.40 cm-' extending from 1.3,gm to theband edge at 0.9 gim. This absorption and the photo-conductivity are believed to be due to filled deep elec-tron traps created by In on Cd sites. These trapscompensate for the material and are responsible for itshigh resistivity.'3

In these experiments a 4 mm X 5 mm X 6 mmsample was used. The dc photoconductivity, a, wasmeasured at low intensities as a = 5 X 10-10 + 10-6I(Q-cm)-1, where I is the intensity in watts per squarecentimeter at 1.06 gim. The slope of a versus I can berelated to the material parameters such as electronmobility ,Ue and an effective conduction-band electronlifetime To through Ai-/A[ = geroea3/hv, where Aa/AIis the photoconductivity slope, e is the electron charge,a is the absorption coefficient, hv is the energy of lightquanta, and f is the quantum efficiency. Assumingthat all the measured infrared loss is associated withphotoconductivity (i.e., ,B = 1), one can get a mobility-lifetime product of gero = 3 X 10-6 cm2/V. The valueof ge for high-resistivity CdTe:In is reportedl4" 5 to beapproximately 800 cm2/V sec, which would then pre-dict -ro = 3.6 X 10-9 sec.

The electric field was along the (111) direction (6-mm dimension), and the 1.06-,m signal beam propa-gated along the (110) direction (5-mm dimension).The 1.0-mm-diameter signal beam was polarized at

L~rOW> VOUTPUT 2

SIGNALBEAM

POLARIZE OUTPUTr 1/ / POLARIZATION

CONTROL DIVIDINGBEAM PRISM

Fig. 1. Opto-optical switch in CdTe.

0146-9592/89/040224-03$2.00/0 © 1989 Optical Society of America

February 15,1989 / Vol. 14, No. 4 / OPTICS LETTERS 225

50

401

301

10

C'

I I I IIlli I II lII 1F 11F I 1 1 I 11111111 I 1 1I1111 1 I1111

% Transmission With NoControl Beam

ISIG = 3O W/Cm2

_ I \ _IF' 111111 I 111 111111 1 1111111 I 111111 I I M1111 I I I 11111

C o ntrol -B2 a n3 ns 4 W 0 5Control-Beam Intensity Sa/CM2

Fig. 2. Steady-state signal-beam transmission into output1 as a function of the control-beam intensity. The signal-beam intensity is fixed at 30 ,W/cm 2 , and V = 3000 V.

0

1-gsec switching time. This is the time for the chargepattern to form and is consistent with the gratingformation times observed for photorefraction in semi-conductors.'7 The switch turn-off time is limited bythe time required for the trapped charge to be ther-mally excited from the traps. As shown in Fig. 3, thisis typically a few milliseconds in the dark and could besignificantly reduced by flooding the CdTe with anerase beam.

The switch can be used to select pulses from a trainof optical pulses as shown in Fig. 5. The signal beamhas been modulated by an acousto-optic modulatorinto a train of pulses of 365-gsec duration. A 1.5-mseccontrol pulse deflects two of the signal pulses totallyinto output 2. The switching time was 5 gsec for thecontrol-beam intensity used. After the end of thecontrol pulse the switch recovers in approximately 5msec as the trapped charge is reionized by thermaleffects. We have also demonstrated the selection ofpulses from a train of 1-gsec pulses.

In the steady state the control beam decreases theelectric field seen by the signal beam; however, a tran-sient, during which the electric field increases, hasbeen observed at the onset of the control beam. Theamplitude of the transient signal pulse, which is larg-est when the control beam is near the positive elec-trode, indicates that during the transient the electricfield seen by the signal beam is as much as twice thatgiven by dividing the voltage by the electrode spacing.

0

Fig. 3. Signal beam into output 1 (lower trace) when thechopped control beam fills the sample with an intensity of 30mW/cm2 . The upper trace is the photocurrent, which isnegative going; the control beam is on when the photocur-rent goes negative. The signal-beam intensity is 60 AW/cm2. The horizontal scale is 10 msec/division. The zerolevels are indicated for each trace.

450 to the (111) direction. In some cases silver-paintelectrodes were used, and in other cases deposited goldelectrodes were used. The control beam was also at1.06 gm and was unpolarized. The control beam waslarge enough to flood the sample or could be aperturedto a 1-mm-wide slit as suggested by Fig. 1.

Figure 2 shows the steady-state transmission intooutput 1 of the switch as a function of the intensity ofthe control beam when the control beam flooded thesample and there was 3000 V across the crystal. Thesignal-beam intensity was held at 30 gW/cm2, which asour earlier research showed creates negligible photo-conductivity.16 To demonstrate the switching behav-ior the control beam was chopped. Figure 3 shows thetransmitted beam into output 1 (lower trace) and thephotocurrent (negative-going upper trace) when thecontrol beam floods the sample with an intensity of 30mW/cm 2 .

To measure the switching time the control beam waspulsed by a fast-rise (150-nsec) acousto-optic modula-tor. Figure 4 shows the switch turn-on time as a func-tion of the control-beam intensity. The switchingtime is proportional to I-; 10 W/cm2 is required for a

0

E

a,

U,

3

210 _

101.

100I . ... ..... . ..... . ...... . ........ -1 02 103 1C

Control-Beam Intensity mW/cm2

04 1 05

Fig. 4. Switching time versus control-beam intensity.

Fig. 5. Selection of signal-beam pulses by a control-beampulse. The upper trace is the control pulse; the lower traceis the signal beam in output 1. The horizontal scale is 2.0msec/division.

1�

226 OPTICS LETTERS / Vol. 14, No. 4 / February 15, 1989

Fig. 6. Moving-domain switching showing the signal-beampulse in output 1, which occurs at the onset of the controlpulse. The signal-beam intensity is 6 mW/cm2; the control-beam intensity is 30 mW/cm 2 . The horizontal scale is 10msec/division.

Figure 6 shows this effect when the signal-beam inten-sity (6 mW/cm2) is sufficiently high to shield itselffrom the applied field and therefore in the steady stateis self-switched to output 2. The pulse in output 1,shown in Fig. 6, occurs at the beginning of the controlpulse. The observed pulse width was approximately100 gsec when the control-beam intensity was 1 W/cm2, and its width decreases as the control-beam in-tensity increases.

We believe that this pulse is due to a domain of highfield strength that moves between the electrodes.Current oscillations and moving-field domains havebeen observed in other high-resistivity semiconduc-tors with deep-level traps and have been attributed tothe electron-capture coefficient of the traps increasingwith the electric field.'8 We also observe a pulse in thephotocurrent corresponding approximately in time tothe signal pulse. The observed decrease in pulsewidth with increasing intensity can be explained by anincrease in the domain velocity with increasing inten-sity, which was reported in Ref. 19. The signal-beamdiameter was 1 mm, and therefore a 100-,gsec pulseimplies a domain velocity of 104 mm/sec. For compar-ison, if the data on domain velocity for GaAs of Raj-benbach et al.19 are linearly extrapolated to 1 W/cm2 , adomain velocity of 2.5 X 103 mm/sec is predicted.

This switching should also be observable in othersemiconductors that are photorefractive such as GaAsand InP and in the dielectric photorefractives such asBi12SiO20, Bi12GeO20, and BaTiO3. In contrast to thegrating switches, it is not necessary that the control orsignal beams be coherent, and therefore the switchcould be used with light-emitting diodes.

If the signal wavelength is longer than the controlwavelength, so that the signal beam creates little or nophotocharge, the intensity of the signal beam could beconsiderably larger than in these experiments. Forexample, in CdTe the signal wavelength could be 1.5,um and the control wavelength could be 1.06 gin orshorter. In this case it may be possible for theseswitches to exhibit gain, i.e., the control-beam intensi-ty is less than the signal-beam intensity.

The authors acknowledge the support of the U.S.Air Force Office of Scientific Research under awardAFOSR-87-0338 and the University Research Initia-tive Program under award F49620-87-C-007.

References

1. J. L. Jewell, M. C. Rushford, and H. M. Gibbs, Appl.Phys. Lett. 44, 172 (1984).

2. J. L. Jewell, Y. H. Lee, M. Warren, H. M. Gibbs, N.Peyghambarion, A. C. Gossard, and W. Wiegmann,Appl. Phys. Lett. 46, 918 (1985).

3. D. Sarid, R. S. Jameson, and R. K. Hickernell, Opt. Lett.9, 159 (1984).

4. G. T. Sincerbox and G. Roosen, Appl. Opt. 22, 690(1983).

5. P. D. Henshaw, Appl. Opt. 21, 2323 (1982).6. D. Rak, I. Ledoux, and J. P. Huignard, Opt. Commun.

49, 302 (1984).7. H. Yajima, E. Sudo, J. Yumoto, and K. Kashiwa, Appl.

Phys. Lett. 45, 214 (1984).8. K. Tada, Y. Kuhara, M. Tatsumi, and T. Yamaguchi,

Appl. Opt. 21, 2953 (1982).9. K. Tanaka and A. Odajima, Appl. Phys. Lett. 38, 481

(1981).10. A. M. Glass, Mater. Res. Bull. 16, 36 (1988).11. A. M. Glass, Science 235,1003 (1987).12. R. B. Bylsma, P. M. Bridenbaugh, D. H. Olson, and A.

M. Glass, Appl. Phys. Lett. 51, 889 (1987).13. K. Zanio, Semiconductors and Semimetals. Vol. 13.

Cadmium Telluride (Academic, New York, 1978).14. T. Ido, A. Heurtel, R. Triboulet, and Y. Marfing, J. Phys.

Chem. Solids 48, 781 (1987).15. S. Yamada, J. Phys. Soc. Jpn. 17,645 (1962).16. W. H. Steier, J. Kumar, and M. Ziari, Appl. Phys. Lett.

53, 840 (1988).17. M. B. Klein, Opt. Lett. 9, 350 (1984).18. B. K. Ridley and P. H. Wisbey, Br. J. Appl. Phys. 18,761

(1967).19. H. Rajbenbach, J. M. Verdiell, and J. P. Huignard, Appl.

Phys. Lett. 53, 541 (1988).