Contains a report on one research project.U.S. Air Force - Office of Scientific Research (Contract F49620-80-C-008

Aggarwal, Roshan L.

Gamble, Edward

Jagannath, Chiravurri

Kash, Kathleen

McManus, J. Barry

Warnock, James

Wolff, Peter A.

Wong, Stephen

Youngdale, Eric P.

Yuen, Y. C. Sunny

Contains a report on one research project.U.S. Air Force - Office of Scientific Research (Contract F49620-80-C-0008

DSpace@MIT

Infrared Nonlinear Optics10. Infrared Nonlinear OpticsAcademic and Research StaffProf. P.A. Wolff, Dr. R.L. Aggarwal, Dr. F. Brown, Dr. C. Jagannath, Dr. L.R.Ram-Mohan, Dr. Y.C.S. YuenGraduate StudentsE. Gamble, K. Kash, J.B. McManus, J. Warnock, S. Wong, E. Youngdale10.1 Infrared Nonlinear Processes in SemiconductorsU.S. Air Force -Office of Scientific Research (Contract F49620-80-C-0008)Roshan L. Aggarwal, Peter A. Wolff, Chiravurri Jagannath, Y.C. Sunny Yuen, Edward Gamble,Kathleen Kash, J. Barry McManus, James Warnock, Stephen Wong, Eric P. YoungdaleIn doped, narrow gap semiconductors there is a sizeable nonlinear (Raman) interaction betweentwo laser beams and plasma waves. This interaction can be used to coherently excite plasmons ofwell-defined wave vector. An experiment to test this idea was performed 1 in thin n-InSb plates;plasmon excitation was detected via FIR radiation (= 100[L) emitted at the plasma frequency by thethin-film modes. The FIR power observed was lower than anticipated, but in other respects themeasurements confirmed theory.Though acoustic plasma waves were predicted many years ago, they have not been observed incrystals until recently.2 Light scattering spectra of optically excited electron-hole plasmas in GaAsexhibit a novel, low frequency resonance. The position and carrier-density variation of this line agreewith those predicted by RPA theory of the acoustic plasmon. In previous light scattering studies ofoptically excited electron-hole gases, the acoustic plasmon resonance was not observed, presumablybecause of plasma inhomogeneity. Uniform plasmas were assured in this work by creatingelectron-hole pairs in thin (= 4000 A) GaAs epilayers, bounded by transparent (Ga,AI)As layers. Theinvestigation was performed in collaboration with Dr. A. Pinczuk and Dr. J. Shah of Bell Laboratories.Intense, degenerate four wave mixing has been observed3 at room temperature in p-type(Hg,Cd)Te with a CO2 laser pump. The experiments yield a nonlinear susceptibility, X(3) = 5 x 10.6esu, with p = 2.5 x 1017 holes/cc. This value exceeds, by several orders of magnitude, any previouslyobserved nonresonant, nonlinear susceptibility. The large X(3) is attributed to intervalence bandtransitions which modulate the plasma contribution to the dielectric function. The nonlinearsusceptibility is proportional to the light-to-heavy hole relaxation time, which is estimated to be (2- 4)x 10-12 sec.RLE P.R. No. 125Infrared Nonlinear OpticsTheoretical calculations4 suggest that, in semiconductor crystals with spatially varying effectivemass, there is a large enhancement of the free-carrier nonlinear susceptibility. Structures of this typecan be grown via MBE techniques. The variable-mass effect is most pronounced when the massvaries rapidly in space, though large amplitude fluctuations are not required. Nonlinear coefficients inthe 10-4 - 10-5 esu range are anticipated at 10.6 I.Nonlinear optical studies of donors in Ge and Si are continuing. Four-wave mixing spectroscopyhas been used to study5 the stress dependence of the ground state multiplet of phosphorus donors inSi. This work was made possible through the development of a quantitative stress cryostat. Theexperiments determine the stress deformation potential,.- -'a, characterizing the is ground statemultiplet.The stress cell has also been coupled to a superconducting magnet to measure the combinedstress (F) and magnetic field (B) dependence of the donor levels. With F constant one observesfour-wave resonances, as a function of B, for various values of the difference frequency, AW,between two CO02 laser beams. Preliminary experiments on As donors in Ge with stress F 11 [110] yielda value 0.03 cm-1/T 2 for the diamagnetic coefficient of the is (A1) -+ 1s(B1 ) transition in the highstress limit. This value is in excellent agreement with a theory which takes into account both valleyre-population and the change in size of the ground state, ls(A1), wave function.References1. J.B. McManus, Appl. Phys. Lett. 41,692 (1982).2. A. Pinczuk, Jagdeep Shah, and P.A. Wolff, Phys. Rev. Lett. 47, 1487 (1981).3. S.Y. Yuen, Appl. Phys. Lett., accepted for publication.4. S.Y. Yuen, Appl. Phys. Lett., 42, 331 (1983).5. C. Jagannath and D.M. Larsen, Bull. APS 28, No. 3, 534 (1983).Other PublicationsJagannath, C., to be published.Larsen, D., (private communication).Kash, K., P.A. Wolff, and W.A. Bonner, "Nonlinear Optical Studies of Picosecond Relaxation Times inn-GaAs and n-GaSb," Appl. Phys. Lett. 42, 173 (1983).Ram-Mohan, L.R. and P.A. Wolff, "Joint-Density of States in Interband Transitions in Semiconductorsin a Magnetic Field," Phys. Rev. B 26, 6711 (1982).Kash, K., "Nonlinear Optical Studies of Electron Dynamics in Semiconductors," Ph.D. Thesis, PhysicsDepartment, M.I.T., September 1982.RLE P.R. No. 125

Infrared Nonlinear Optics

Infrared Nonlinear Optics9. Infrared Nonlinear OpticsAcademic and Research StaffProf. P.A. Wolff, Dr. R.L. Aggarwal, Dr. F. Brown,Dr. C. Jagannath, Dr. L.R. Ram-Mohan, Dr. Y.C.S. YuenGraduate StudentsE. Gamble, K. Kash, J.B. McManus,J. Warnock, E. Youngdale9.1 Infrared Nonlinear Processes in SemiconductorsU.S. Air Force - Office of Scientific Research (Contract F49620-80-C-008)Roshan L. Aggarwal, Peter A. Wolff, Chiravurri Jagannath, Y.C. Sunny Yuen, Edward Gamble,Kathleen Kash, J. Barry McManus, James Warnock, Eric P. YoungdaleThe difference-frequency variation of the free-carrier-induced, third-order nonlinear susceptibility[X3] has been studied in n-lnSb, n-GaAs, n-GaSb, and n-(Hg,Cd)Te. Two CO2 laser beams (offrequencies w1 and w2) were focused onto the samples. The four-wave signal, at frequencyW3 = 2w1 -2' was measured as a function of Aw _ (W1 - 2 ) in the range 1.5 cm' < Aw (<100 cm1'. Adramatic enhancement of P(w 3), as Aw-- 0, was seen in all cases. This effect had not previously beenobserved,nor was it predicted by theories of free-carrier-induced, four-wave mixing. In n-lnSb, and inn-GaAs and n-GaSb at low temperatures, we attribute the effect1 to temperature fluctuations of theelectron gas in the crystal. The model explains the Aw variation of X3 and yields values for theelectron-lattice thermalization times. At higher temperatures, valley-transfer processes contribute tothe nonlinear susceptibility2 in n-GaAs and n-GaSb, modifying the Aw variation of X3 . We havedeveloped coupled equations for the temperature and carrier-density fluctuations, which give anexcellent fit to the data, and yield values for both the electron thermalization and valley-transfer times.Finally, in (Hg,Cd)Te, the four-wave signal is attributed3 to the electron-hole plasma generated by theintense CO02 laser beams. This work suggests that four-wave mixing may prove to be a versatiletechnique for studying short electron-relaxation times (in the 10-12 sec range) in semiconductors.Nonlinear optical studies of donors4 in n-Ge are continuing. Four-wave mixing has been used tostudy the magnetic field dependence of the valley-orbit splitting for B 11[111] in Ge:P and Ge:As. Inthis geometry, the valley-orbit split manifold contains a singlet (with large diamagnetism) and adoublet (with small diamagnetism). We observe transitions, for electrons of both spin directions, fromthe ground state to the singlet. The data determine the donor diamagnetism (hence the ground-stateradius) and the g-value of the excited state, and give information concerning the magnetic fieldcompression of the wave functions.RLE P.R. No. 124Infrared Nonlinear OpticsFar ilfrared generation via coherent excitation of thin-film plasmons has been observed in n-inSbsThin plates of the semiconductor were irradiated with two CO02 laser beams whose differencefrequency, Aw = (w1-W2 ), was near the plasma frequency (Wp) of the electron gas in the crystal.Plasma waves, at frequency Aw, were coherently excited by the nonlinear interaction responsible forplasmon-Ramnan scattering. The wave vector of the waves was controlled by varying the anglebetween the laser beams. Plasma waves in thin plates radiate efficiently at an angle determined bytheir wave vector. The observed far infrared signals behave, in all respects, as predicted by thetheory of coherent plasma-wave excitation. Powers in the 100-microwatt range are measured. Thispower can be enhanced via band-gap resonance of the pump lasers.References1. S.Y. Yuen and P.A. Wolff, Appl. Phys. Lett. 40, 457 (1982).2. K. Kash, P.A. Wolff, S.Y. Yuen, C. Jagannath, and W.A. Bonner, Bull. APS 27, 175 (1982); K. Kashand P.A. Wolff (to be published).3. S.Y. Yuen (to be published).4. C. Jagannath, E. Youngdale, D.M. Larsen, R.L. Aggarwal, and P.A. Wolff, Opt. Commun. (in press).5. J.B. McManus, P.A. Wolff, and R.L. Aggarwal, Bull. APS 27, 369 (1982); also J.B. McManus (to bepublished).RLE P.R. No. 124

http://dspace.mit.edu/bitstream/1721.1/56824/1/RLE_PR_124_09.pdf

Infrared Nonlinear Optics

Abstract

Similar works

Full text

Available Versions

DSpace@MIT

DSpace@MIT