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Investigation of optical nonlinearities in n-GaAs based on multivalley distributed hot electronics

Only recently a rigid quantum-mechanical modeling of free- electron induced optical nonlinearities in highly doped n- GaAs has been elaborated. The total theory takes into account non-parabolicity, hot phonons, effective mass modulation due to (Gamma) -L intervalley transfers, scattering due to equivalent intervalley transfers inside the ellipsoidal L-valleys, nonlinear screening, etc. It was shown that this hot free electron nonlinearity is strongest near the plasma resonance and significantly depends on the deformation potential field (Lambda) LL describing transitions of L-valley electrons between equivalent L- minima. For the experiments a very sensitive multi-layer leaky waveguide structure for TM polarized waves was designed and grown by MBE. Measurements were performed with 100 ns duration CO2 laser pulses. For a doping concentration no of 7.6 X 1018cm-3 a nonlinear refractive index value n2 equals (1.0 + 0.12)X10-6 cm2W-1 at (lambda) equals 10.6 micrometers was obtained, which was based on an experimentally derived (Lambda) LL equals (1.0 +/- 0.2) X 109 eVcm-1. With intensities of only several MWcm-2 more than 50% of the electrons cold be transferred to the L-valleys, leading to impressive absorption increases of more than 50%. With respect to bulk samples the nonlinearity could be more than 20 times increased. In combination with an estimated relaxation time of 6 - 7 ps, this nonlinearity exceeds most other results at room temperature for (lambda) equals 10.6micrometers .