Contains report on one research project.U.S. Air Force - Office of Scientific Research (Contract F49620-84-C-0010

Aggarwal, Roshan L.

Jagannath, Chiravurri

Larsen, David M.

Ram-Mohan, L. Ramdas

Wolff, Peter A.

Wong, Stephen

Youngdale, Eric R.

Yuen, Sunny Y. C.

DSpace@MIT

Infrared Nonlinear Optics6. Infrared Nonlinear OpticsAcademic and Research StaffProf. P.A. Wolff, Dr. R.L. Aggarwal, Dr. C. Jagannath, Dr. D.M. Larsen, Dr. L.R.Ram-Mohan, Dr. S.Y.C. YuenGraduate StudentsE. Isaacs, J. Warnock, S. Wong, E.R. Youngdale6.1 Infrared Nonlinear Processes in SemiconductorsU.S. Air Force - Office of Scientific Research (Contract F49620-84-C-0010)Roshan L. Aggarwal, Peter A. Wolff, Chiravurri Jagannath, David M. Larsen, L.R. Ram-Mohan,Sunny Y.C. Yuen, Stephen Wong, Eric R. YoungdaleFour-wave spectroscopy has been used to study the Raman allowed ls(A1) -+ ls(T2)transitions of As donors in Ge at 1.8 K, using magnetic field B up to 12T. 1 With B II1 [ 11] the ls(T2)states split into six levels (including spin), two with high and four with low diamagnetism. Ananticrossing, predicted by People and Wolff, 2 has been observed between donor levels ofopposite spin. Valley repopulation, due to breaking of the tetrahedral symmetry by the magneticfield, is also observed. A theory has been developed, which, with no adjustable parameters,provides good agreement with experiment.Hole-induced optical nonlinearities have been studied in p-type GaAs and p-type Ge, usingfour-wave spectroscopy. 3 The measured values of the third-order nonlinear susceptibility X (3)agree with those predicted by the intervalence band population modulation mechanism.Dispersion of X (3) yields intervalence band relaxation time in the picosecond regime, inagreement with those calculated for optical phonon emission.With uniaxial stress X parallel to the magnetic field B Ii [111], we have studied stress-inducedelectric dipole spin resonance contribution to the second-order nonlinear susceptibility X (2) viafar infrared generation at the difference frequency in the 100 tpm regime.4 A quantitative analysisof the data yields IC2 1 = 0.35 ± 0.1 eV for the interband strain matrix element. This value of C2 isappreciably smaller than those reported previously. 5 We have also observed a stress-induceddecrease in the electron g-factor which is in agreement with the calculated values.The lineshape for the spin-flip transition in n-type InSb has been investigated by four-wavespectroscopy with CO2 lasers. In contrast to lineshapas obtained previously with pump laserfrequency close to the bandgap, the lineshape in our experiment shows a pronouncedRLE P.R. No. 127Infrared Nonlinear Opticsasymmetry. At low laser powers, an extra peak is also observed. The extra peak and asymmetriclineshape cannot be understood in terms of magnetic joint-density-of-states alone.References1. E.R. Youngdale, D.M. Larsen, and R.L. Aggarwal, Phys. Rev. B., submitted for publication.2. R. People and P.A. Wolff, Phys. Rev. B 24, 4634 (1981).3. S.Y. Yuen, P.A. Wolff, L.R. Ram-Mohan, and R.L. Logan, Appl. Phys. Lett., submitted forpublication.4. C. Jagannath and R.L. Aggarwal, Phys. Rev. B, submitted for publication.5. M. Kriechbaum, R. Meisels, F. Kuchar, and E. Fantner, Proceedings of the 16th InternationalConference on the Physics of Semiconductors, Montpellier, France, 1982 (North-HollandPublishing Co.), p. 444.RLE P.R. No. 127

Infrared Nonlinear Optics

Contains reports on six research projects.U.S. Air Force - Office of Scientific Research (Contract F49620-84-C-0010)U.S. Air Force - Office of Scientific Research (Grant AFOSR-85-0269

Infrared Nonlinear Optics7. Infrared Nonlinear OpticsAcademic and Research StaffProf. P.A. Wolff, Dr. R.L. Aggarwal, Dr. D.M. Larsen, Prof. L.R. Ram-Mohan, Dr.S.Y.C. YuenGraduate StudentsE. Isaacs, C. McIntyre, J. Stark, S. Wong, E.R. Youngdale7.1 Four Wave Spectroscopy of DonorsU.S. Air Force - Office of Scientific Research (Contract F49620-84-C-0010)Roshan L. Aggarwal, David M. Larsen, Eric R. YoungdaleFour wave spectroscopy with CO 2 lasers has been used to study Raman allowed transitionsbetween the valley-orbit split is states of As donors in Ge, using magnetic field to 12 T. With B Ij[111] the ls (T 2) state splits into six levels (including spin), two with high diamagnetism and fourwith low diamagnetism. An anti-crossing predicted by People and Wolff 1 was observed betweendonor levels of opposite spin. A theory has been developed that, with no adjustable parameters,provides good agreement with experiment. This work determines electron g-values, valleyrepopulations, and magnetic wave function compressions. 2 Overall, it constitutes a searching testof donor theory.7.2 Stressed-Induced Far Infrared GenerationU.S. Air Force - Office of Scientific Research (Contract F49620-84-C-00 10)Roshan L. Aggarwal, Chiravurri JagannathFIR generation via spin-induced difference frequency mixing was first demonstrated in n-lnSbsome years ago, but the FIR powers were modest ( - 10 [LW). More recently, Kriechbaum et al.3showed that uniaxial stress dramatically enhances spin resonance absorption in n-InSb.Spin-induced FIR emission should also be enhanced. We have performed experiments to testthis idea; an enhancement factor of 30, in FIR power, was observed with a 2 kbar uniaxial stress.In addition, there was a marked change ( 13% at 2 kbar) in the spin resonance frequency.4Unfortunately, the enhancement is accompanied by severe line broadening. This fact, anddifficulties in phase matching, will probably preclude the use of strain enhancement for tunableFIR generation.RLE P.R. No. 128Infrared Nonlinear Optics7.3 Intervalence Band Relaxation TimesU.S. Air Force - Office of Scientific Research (Contract F49620-84-C-0010)L.R. Ram-Mohan, Peter A. Wolff, Sunny Y.C. YuenFree carriers in semiconductors give rise to large, third order optical nonlinearities that can bestudied via four-wave mixing. In our previous work, the dispersion of X(3 ) was used to determineelectron energy relaxation times and valley transfer times in n-lnSb, n-GaAs, and n-GaSb. Morerecently we have investigated hole-induced optical nonlinearities in p-GaAs and p-Ge.5 Large( : 10 - 7 esu) X(3)'s are observed; they are attributed to intervalence band population modulation.The dispersion (variation with A w = w - W2) of X(3) determines the intervalence band relaxationtimes of the holes, which are in the 3 x 10-12-10 - 13 sec. range. The measured relaxation times arein agreement with those calculated from first principles for optic phonon emission. This workconfirms our mechanism for hole-induced optical nonlinearities, and suggests that the optimalmaterial for devices employing intervalence band transitions should be nonpolar and have a smallband gap.7.4 Nonlinear Optics Near the Metal-Insulator TransitionU.S. Air Force - Office of Scientific Research (Contract F49620-84-C-0010)Peter A. Wolff, Sunny Y.C. YuenFree carriers in semiconductors cause optical nonlinearity whenever the properties of theelectron states vary with energy. The metal-insulator transition is an extreme example of such asituation since the Fermi level then separates localized from delocalized states. This idea isconfirmed by experiment; our four wave mixing measurements have shown that n-Si:P has verylarge nonlinear coefficients (per carrier) for doping levels near the transition (n F 3.7 x 10'8/cc),but small ones in lightly doped crystals.6 The faster than linear variation of X(3 ) with n implies thatelectron-impurity interactions are responsible for the nonlinearity. To our knowledge, this is thefirst example of a sizable optical nonlinearity caused by scattering.We have developed a theory of this effect which shows that it provides a direct measure of theelectron pile-up at impurity sites.6 Experimentally, this quantity varies five times more rapidly (withenergy) than RPA predicts. We attribute the discrepancy to higher order terms in the perturbationtheory, which cause carrier diffusion and localization.The theory also implies that the nonlinearity is sensitive to dopant type. Further studies, with Sicrystals doped and alloyed in alternate ways will be performed to optimize the nonlinearity.RLE P.R. No. 128Infrared Nonlinear Optics7.5 Nonlinear Optics in SuperlatticesU.S. Air Force - Office of Scientific Research (Grant AFOSR-85-0269)Sunny Y.C. YuenYuen has suggested several mechanisms for enhancing free-carrier-induced nonlinear opticalprocesses in superlattices.7 Initially this work was entirely theoretical. Recently, however,structures for testing these ideas have been made available to us. In particular, we have recentlyreceived the following superlattices:1. n-GaAs/(GaA1)As with quantum level spacing adjusted to match the CO 2 laserenergy2. p-lnGaAs/GaAs strained layer superlattice3. InSb/(InAI)SbPreliminary four-wave mixing experimenits have been performed on all of these samples. TheInSb/(InAI)Sb superlattices give large signals (X(3 ) a 10- 7 esu) - despite their thinness - thatwe attribute to intervalence band population modulation. These experiments will continue withbetter characterized InSb/(InAI)Sb superlattices and InSb/In(AsSb) superlattices.The p-(InGa)As/GaAs SLS has only a modest nonlinear susceptibility, with no evidence of thelarge, nonparabolicity effect anticipated in the strain-split valence band. And finally, no signalswere observed in the n-GaAs/(GaAI)As superlattices, despite the resonance of the quantum levelsplitting with the pump laser energy.When suitably doped samples are available, we also plan studies of spin-flip nonlinearities inboth InSb/(InAI)Sb and HgTe/CdTe. Calculations imply that the spin-flip signals should beobservable, and will provide an excellent diagnostic of these otherwise hard-to-characterizestructures.7.6 Faraday Rotation FiltersU.S. Air Force - Office of Scientific Research (Grant AFOSR-85-0269)Peter A. Wolff, Sunny Y.C. YuenSome years ago Romestain et al.8 showed that the free carrier spin flip Raman interactionproduces a large Faraday rotation of light whose energy is just below the band gap of thesemiconductor. The effect was demonstrated in n-CdS, but is expected to be considerably largerin materials such as n-lnSb and n-HgCdTe. Since the effect is resonant at the band gap, it canRLE P.R. No. 128Infrared Nonlinear Opticspotentially be used as the basis for an optical filter to pass near-bandgap radiation but rejectother light.To test this concept we have performed CO 2 laser, Faraday rotation experiments onn-Hgo0 77CDo. 23 Te crystals, whose bandgap matches the laser photon energy. Rotations of 90'were observed in samples 0.5 mm thick containing 2.5 x 1014 electrons/cc. When placedbetween crossed polarizers, such a crystal is a bandpass filter for near-gap radiation. The filtercan be tuned magnetically, and might be switched with electric fields (which heat the carrierspins, thereby reducing the Faraday rotation).These studies will next be extended to HgMnCdTe crystals where the carrier-Mn 2 + exchangeinteraction provides better coupling between electron kinetic and spin temperatures.References1. R. People and P.A. Wolff, Phys. Rev. B24, 4634 (1981).2. E.R. Youngdale, D.M. Larsen, and R. L. Aggarwal, Phys. Rev. B32, 3938 (1985); C. Jagannath,R.L. Aggarwal, and D.M. Larsen, Solid State Commun. 53, 1089 (1985); E.R. Youngdale,D.M. Larsen, and R.L. Aggarwal, Phys. Rev. (in press).3. M. Kriechbaum, R. Meisels, F. Kuchar, and E. Fantner, Proceedings of 16th InternationalConference on the Physics of Semiconductors, Montpellier, France, 1982 (North HollandPublishing Co.), p. 444.4. C. Jagannath and R.L. Aggarwal, Phys. Rev. B32, 2243 (1985).5. S.Y.C. Yuen, P.A. Wolff, L.R. Ram-Mohan, and R.A. Logan, Solid State Commun. 56, 489(1985).6. P.A. Wolff, S.Y.C. Yuen, and G.A. Thomas, Phys. Rev. Lett., submitted for publication.7. S.Y.C. Yuen, Appl. Phys. Lett. 42, 331 (1983); S.Y.C. Yuen, Appl. Phys. Lett. 43, 479 (1983).8. R. Romestain, S. Geschwind, and G.E. Devlin, Phys. Rev. Lett. 35, 803 (1975).ThesesYoungdale, E.R., "Four-Wave Magneto-Piezo Spectroscopy in Hydrogenic Donor Levels inGermanium," Ph.D. Thesis, Physics Department, M.I.T., December 1985.RLE P.R. No. 128

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Infrared Nonlinear Optics

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