NONLINEAR OPTICAL PROPERTIES OF A HETERO-nipi STRUCTURE WITH COUPLED QUANTUM WELLS

Abstract

A light-induced broadening of excitonic absorption features was investigated for a hetero n-i-p-i structure containing coupled quantum wells. Light-induced differential absorption as large as 5000 cm-1 was observed with an irradiance of only 280 mW/cm2. The largest absorption change was positive and occurred at 814 nm, just below the absorption edge for the non-illuminated sample. The nonlinear optical response for this structure is unlike the response for conventional n-i-p-i structures which exhibit decreasing absorption below the absorption edge. A corresponding refractive index change was calculated from absorption measurements using a modified Kramers–Kronig relation. The maximum change in the refractive index was 0.01 at 818 nm, a wavelength where the absorption was small for both the illuminated and non-illuminated samples. The origin of the excitonic broadening is explained with calculations for the transition energy and transition strength of the four lowest energy optical transitions for the coupled quantum wells. The relatively low irradiance required to excite the sample is attributed to long electron-hole recombination time, measured to be between 100 and 700 μs, depending on the irradiance on the sample.