Giant nonlinear phase shift at exciton resonance in ZnSe

We study the optical nonlinearity just at the exciton resonance in ZnSe at 8 K. A sample with thickness of 2.4 μm is grown by molecular-beam epitaxy on a GaAs substrate. Using reflection-type polarization spectroscopy with weak picosecond optical pulses, we observe nonlinear phase shift, being free from incoherent processes caused by the accumulation of excitons. A huge nonlinear phase shift, 0.15 deg cm2/kW, whose effective χ(3) is over 10−2 esu, with a response time shorter than 20 ps is obtained.