Induced focusing of optical pulses in a self-defocusing semiconductor medium

The novel phenomenon of induced focusing predicted to occur in a self-defocusing non-linear medium is reported experimentally. A 10 ps pump pulse optically induces a refractive index spatial profile in a GaAs/AlGaAs planar waveguide which results in induced focusing of a subsequent weak copropagating probe pulse for appropriate relative spatial positions. With pump and probe pulses separated by one beam waist, the probe output peak intensity is increased by 40%. These observations are consistent with two-transverse-dimension beam-propagation computations with the carrier-density-dependent optical non-linearity modelled by the plasma theory.