Contains reports on one research project.U.S. Air Force - Office of Scientific Research (Grant AFOSR-76-2894

Kash, Kathleen

Khan, Muhammad A.

Muehlner, Dirk J.

People, Roosevelt

Wolff, Peter A.

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X. INFRARED NONLINEAR OPTICSAcademic Research StaffProf. Dirk J. MuehlnerProf. Peter A. WolffGraduate StudentsKathleen KashMuhammad A. KhanRoosevelt People1. INFRARED NONLINEAR PROCESSES IN SEMICONDUCTORSU.S. Air Force - Office of Scientific Research (Grant AFOSR-76-2894)Dirk J. Muehlner, Peter A. Wolff, Kathleen Kash, Muhammad A. Khan,Roosevelt PeopleStudies of resonant, spin-induced, four-photon mixing in (Cd, Hg)Te are continuing.Some time ago, we reported the observation of several distinct spin resonances in our(Cd, Hg)Te sample - a result which indicated that the crystal is inhomogeneous. Thisexperiment is now being used by the Honeywell group as an analytic technique to deter-mine g-value and alloy composition as a function of position in (Cd,Hg)Te. In our work,we have concentrated on homogeneous portions of the (Cd, Hg)Te crystals, which showexceedingly sharp spin resonances. At higher laser powers (in the 500 W/cm 2 range)the four-photon resonances broaden and saturate in a manner similar to that observedin n-InSb. The values of the spin-relaxation times, T 1 and T 2 , that we infer are com-parable to those measured in InSb. Finally, at the highest laser powers the spin linesplits in a quite unexpected way. We suspect that the sideband is a Rabi frequency, in-duced by coherent oscillation of carriers between the conduction and valence bands.Calculations to test this hypothesis are in progress.We have made extensive studies of carrier excitation - from donor levels to theconduction band - as n-Ge is irradiated by intense (10. 6-4) laser beams. In cold n-Gecrystals, the optical absorption decreases markedly (by more than a factor of 10) in the2power range 100 kW-10 MW/cm . A kinetic model, which balances the photoionizationrate from is levels against the rate of carrier recombination with donors, gives anexcellent fit to the data. This experiment determines photoionization cross sections,the donor recombination cross section as a function of carrier temperature, and thethermalization rate of hot carriers in n-Ge.Studies of resonant, impurity-induced, four-photon mixing in n-Ge and n-Si arecontinuing. During the past year, we have observed2, 3 resonances in the third-ordernonlinear coefficient caused by impurity levels in Si - the resonance being at the valley-orbit splitting energy of the donor levels. To date, the impurity resonances in thesePR No. 121(X. INFRARED NONLINEAR OPTICS)four-photon mixing experiments have been studied by step-tuning the CO 2 lasers. Moreprecise data can be obtained by sweeping the impurity levels with magnetic field. Theseexperiments, which are now under way, will determine:i. magnetic structure of donor levels in n-Ge and n-Si;ii. precise linewidths of valley-orbit transitions;iii. excitation transfer rates between donor levels.Referencesi. R. People, R. L. Aggarwal, and P. A. Wolff, "Intensity-Dependent Nonlinear Absorp-tion in Ge:P at 10.6 ri" (Abstract), American Physical Society, March 1979.2. M. A. Khan, D. J. Mueh]ner, and P.A. Wolff, "Resonant Four-Wave Mixing in n-Type Silicon," Opt. Commun. (in press).3. M.A. Khan, "Four-Photon Mixing in Semiconductors," Ph.D. Thesis, Departmentof Physics, M. I. T. , May 1979.PR No. 121

Infrared Nonlinear Optics

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Infrared Nonlinear Optics

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