Free-Carrier Induced Optical Nonlinearities in Narrow Bandgap Semiconductors
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FREE-CARRIER INDUCED OPTICAL NONLINEARITIES IN NARROW BANDGAP SEMICONDUCTORS D. Walrod(S), S. Y. Auyang and P. A. Wolff(b) Francis Bitter National Magnet Laboratory Massachusetts Institute of Technology Cambridge, MA 02139
ABSTRACT Free-carrier induced third-order optical nonlinearities can be both large and fast in narrow-gap semiconductors. We have studied a variety of mechanisms in bulk semiconductors and heterostructures using CO 2 lasers and found third-order susceptibilities as large as 2xI0-s esu with picosecond relaxation times. These mechanisms saturate at much higher intensities than do slower mechanisms and hence induce huge modulations of the dielectric function. In addition, most of these processes are nonresonant so they do not require the exact matching of material parameters and are relatively insensitive to temperature.
1 INTRODUCTION The first observation of a third-order nonlinearity caused by a free-carrier process in semiconductors was made by Patel et al.1 These modest nonlinearities in n-InSb, n-GaAs and n-InAs were explained by Wolff and Pearson as resulting from the nonparabolicity of the conduction band. 2 The nonlinearity was very fast, relaxation times in the picosecond range, but fairly weak. Hence, for many years, it was believed that fast processes such as these were intrinsically weak. Our studies have shown that this is not the case. There are several free-carrier processes which are both fast and moderately large. We have observed X(3) of 2x10-3 esu with picosecond relaxation times in Hg 0.84Cd 0 .16Te epilayers with a near-zero bandgap. Similar nonlinearities have been found in other II-VI epilayers. The major free-carrier processes that we have investigated rely on a modulation of the free-carrier dielectric function. In narrow bandgap semiconductors, the important mechanisms are 1. Nonparabolicity of the energy bands. 2. Excitation of interband transitions. 3. Excitation of intervalence band transitions. 4. Energy dependence of carrier scattering. These mechanisms have been observed in both bulk and epitaxially grown semiconductors. Enhanced nonparabolicity due to superlattice structure has also been observed in various narrow 3 gap superlattices. Free-carrier nonlinearities have a number of features that are very desirable for device applications. Most processes have very fast relaxation times, so they are well suited to devices where speed is critical. They also do not tend to saturate at high power. Since most of the processes are non-resonant, there are not stringent requirements placed on material parameters and they are fairly insensitive to temperature changes. Mat. Res. Soc. Symp. Proc. Vol. 161. ©1990 Materials Research Society
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Table 1. Figures of Merit for Third-Order Optical Nonlinearities in Semiconductors
X(3) T(K) (esu)
Nonlinear Mechanism
Medium Hg 0 .gCd0 16Te HgTe n-InSb p-Hg0 .78Cd0 .22Te p-GaAs n-Si:P Hgo. 77 Cdo. sSe:Fe InSba Hgo.78Cdo.22Te p-Hgo.¢7 Cdo.22Te Ge GaAs/GaAlAsb
4
interband t interband nonparabolicityl° 6 intervalence 7 intervalence
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