Optical Nonlinearities in Bulk GaAs Determined by EL2 Defect

Abstract

Time-resolved studies of light diffraction on free carrier phase gratings and light absorption in subnanosecond time domain were carried out in two distinct areas of semi-insulating GaAs with high and low growth-defect density. Numerical analysis was performed in order to reveal the role of ΕL2 defect in carrier generation and transport. The possibility of transient grating technique to study various defect-governed carrier relaxation processes were demonstrated experimentally.PACS numbers: 72.20.JvOptical illumination of GaΑs:EL2 at 1.06 m leads to the change of its complex refractive index via free carrier (FC) nonlinearity, described by the DudeLorentz model. Modulation of refractive index Δn is governed by non-equilibrium carrier concentration ΔN, Δn -n ehΔN, where neh 2 x 10 -21 cm-3 is refractive index modulation by one electron-hole pair, and absorption index modulation is due to free carriers and transformation of defect charge state. In a case of spatially modulated light interference pattern, the electrical and optical properties of medium are spatially modulated as well.The solution of continuity equation yields the dynamics and simple relationship between the modulating FC concentration ΔN(t) = Nmax (t) -Nmin (t) and optically measured values where α, R are absorption and reflection coefficients, I0 is the incident beam intensity, f(t) describes the temporal shape of the laser pulse. The grating decay time τe is governed by recombination and diffusion 1/τe = 1/τr + 1/τd. The rate of diffusion decay can be monitored by changing the grating period Α = λ/ sin θ = 2π(τdD) 1 / 2 , where θ is the angle between interfering beams, and D is the diffusion coefficient. Hence, measurements of grating decay time at different Α enable one to separate the erasure processes.(939)