Optical nonlinearities of GaAs hetero n-i-p-i structures

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Hetero n-i-p-i crystals are periodic arrays of n-and p-doped layers interspersed with undoped (i-) layers of a different smaller band gap semiconductor.1 We measure the frequency dependence of the nonlinear absorption changes at room temperature. We deduce the nonlinear refractive-index changes associated with the nonlinear absorption changes by a Kramers-Kronig transformation. The hetero n-i-p-i samples are grown by molecular beam epitaxy on GaAs substrates and consist of intrinsic layers of GaAs and doped layers of AIGaAs (x = 0.3). In the experiment we directly measure the nonlinear transmission changes through the sample by a pump and probe technique using an optical multichannel analyzer. The pump beam from the dye laser is tuned inside the band gap of the GaAs for efficient generation of free carriers. The broadband probe and the pump are synchronously modulated by acoustooptic modulators of varying pulse lengths (typically 1 µs) with varying repetition rates. The probe passes through the sample under different pumping intensities and is collected by the optical multichannel analyzer.