Contains description of one research project.National Science Foundation Grant EET 87-1841

Auyang, Sunny Y. C.

Walrod, David B.

Wolff, Peter A.

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Chapter 2. Novel Processes and MaterialsChapter 2. Novel Processes and Materials forInfrared Nonlinear OpticsAcademic and Research StaffProfessor Peter. A. Wolff, Dr. Sunny Y. AuyangGraduate StudentDavid B. Walrod2.1 Project DescriptionSponsorNational Science FoundationGrant EET 87-18417Optical systems such as signal processors,limiters, spatial light modulators, and opticalcomputers require large and fast optical non-linearities. Although free carrier-inducednonlinearities in semiconductors have thepicosecond speeds required for use in manyapplications, they were considered too weakto be practical for optical systems applica-tions. In our work, we have demonstratedthat these nonlinearities can be substantiallyenhanced in certain materials and in struc-tures such as superlattices. We have recentlyobserved the first optical nonlinearity causedby subband effects in InSb/AllnSb andHgTe/CdTe superlattices.Superlattices have the potential for gener-ating large optical nonlinearities becausetheir band structures can be tailored to opti-mize particular nonlinear effects. Severalnovel nonlinear optical processes that utilizeeither the large subband nonparabolicity inthe growth direction' or intersubband transi-tions in various configurations 2 specific tosuperlattices have been proposed. To ourknowledge, these processes have not beenobserved.With support from the National ScienceFoundation, we have observed optical non-linearities caused by subband nonparabolicityin InSb/InAISb and HgTe/HgCdTe superlat-tices by using four-wave mixing techniqueswith CO2 lasers. We have observed anenhancement in the third-order susceptibility,X~(3) when the laser light was polarized alongthe growth direction instead of in-plane.This experiment demonstrated that the super-lattice subband structure induced largeroptical nonlinearity than that of its bulkcounterpart.3The coupling between light and electrons isgiven by p.E, where E is the electric field ofthe light and p the momentum of the nthenergy subband S,(p). Therefore, to observenonlinear optical effects caused by subbandstructures peculiar to superlattices, the elec-tric field of the light must be polarized in thesuperlattice growth direction. We designatedthe direction perpendicular to the plane ofthe superlattice as z and the in-plane direc-tion as x. Light polarized along x probesonly subband structures parallel to the super-lattice plane, en(px), which is not differentfrom their bulk counterparts. The interestingsuperlattice effects are contained in Es(pz);large enhancement of pz-nonparabolicity hasbeen predicted. Furthermore, intersubbandtransitions are induced by Ez but not Ex.1 W.L. Bloss and L. Friedman, Appl. Phys. Lett. 41:1023 (1982); S.Y. Yuen, Appl. Phys. Lett. 43:813 (1983); G.Cooperman, L. Friedman, and W.L. Bloss, Appl. Phys. Lett. 44:977 (1984); Y. Chang, J. Appl. Phys. 58:499(1985).2 S.Y. Yuen, App. Phys. Lett. 43:813 (1983); D.J. Newson and A. Kurobe, AppL Phys. Lett. 51:1670 (1987).3 D. Walrod, S.Y. Auyang, and P.A. Wolff, App. Phys. Lett. 56:218 (1989).103Chapter 2. Novel Processes and MaterialsFigure 1.The coupling to en(pz) is difficult to achieve inconventional nonlinear optical experiments inwhich light impinges upon the surface of thesample (figure 1). Because of the largeindices of refraction of semiconductors Ezinside the sample, there was never more thana small fraction of the total field. Even atBrewster angle incidence, less than tenpercent of the light intensity was polarizedalong z since the nonlinear signal is propor-tional to the cube of the input intensity.Therefore, the third-order nonlinear suscepti-bility X(3) due to en(Pz) must be at least fiftytimes larger than the bulk susceptibility to bedistinguished in this geometry. We predictedthese large susceptibilities for intersubbandtransitions when the subband gap wasexactly matched to the photon energy.4Intersubband transition was a resonanteffect, however, and the predicted ;(3) mightnot be achievable if the actual subband sepa-ration failed to match the photon energy.Optical nonlinearities generated by the non-Superloti ceSubstrate \4 S.Y. Yuen, Appl. Phys. Lett. 43:813 (1983); D.J. Newson and A. Kurobe, Appl. Phys. Lett. 51:1670 (1987).5 W.L. Bloss and L. Friedman, Appl. Phys. Lett. 41:1023 (1982); S.Y. Yuen, Appl. Phys. Lett. 43:813 (1983); G.Cooperman, L. Friedman, and W.L. Bloss, Appl Phys. Lett. 44:977 (1984); Y. Chang, J. Appl Phys. 58:499(1985).104 RLE Progress Report Number 132(a) (b)k(c) (d)parabolicity of the lowest subband eo(pz),were nonresonant, not placing as severe ademand on the specification of the sample.However, the enhancement over the bulkwas smaller.5We observed optical nonlinearities caused bysubband structures in the growth direction inan AllnSb/InSb superlattice by using an end-firing technique. The AIInSb/InSb superlat-tice we studied consisted of 50 periods, eachwith 45 A of InSb and 60 A of Alo.o08 ln 92Sb.This superlattice was separated from the 350um Cr-doped GaAs substrate by a 200 AAISb buffer layer. The samples wereundoped. We used a 50 ym InSb epilayergrown under the same conditions on thesame kind of substrate for a control sample.For the end-firing experiments, we cleavedoff a strip about 1 mm in width. The cleavededge provided good transmission. Weobserved about 50 percent transmissionthrough the 1-mm sample; almost all losswas due to reflection. To ensure that theGaAs substrate did not contribute to thenonlinear optical signal, we cut off smallsections of the samples and polished off thesuperlattice. Then we compared thesubstrate with a sample in which the super-lattice was left intact. In all cases, the non-linear signal vanished when we removed thesuperlattice.The end-firing configuration enabled us toprobe the z-direction dispersion relation bychanging the polarization direction. Thefour-wave signal from the InSb epilayer doesnot change as the light polarization is rotated90 degrees. This result suggests that thematerial is isotropic. The four-wave signalfrom the superlattice was comparable to thatof the InSb epilayer when the light waspolarized along x. However, as thepolarization was rotated to the i direction,the four-wave signal increases bytwenty-two times, as depicted in figure 2.Chapter 2. Novel Processes and MaterialsFigure 2.105AZ0x02.0o0co uo03oOL=By comparing the optical nonlinearities gen-erated by different light polarizations, weascertained that the z-direction subbandstructure gave an enhancement of X(3) overthe bulk value. There are several possibleexplanations for the enhancement, but wewere unable to distinguish themquantitatively. Our research is continuing inthis direction.PublicationsAuyang, S.Y., and P.A. Wolff. "Free-Carrier-Induced Third Order Optical Nonlinearitiesin Semiconductors." J. Opt. Soc. Am. A6:2696 (1989).Walrod, D., S.Y. Yuen, P.A. Wolff, and W.Tsang. "Optical Nonlinearities due toSubband Structures in Alo08 ln0.92Sb/InSbSuperlattices." Appl. Phys. Lett. 56(3):218-220 (1989).106 RLE Progress Report Number 132

Novel Processes and Materials for Infrared Nonlinear Optics

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Novel Processes and Materials for Infrared Nonlinear Optics

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