OPTICAL NONLINEARITIES IN N-I-P-I AND HETERO-N-I-P-I STRUCTURES

Optical nonlinearities in n-i-p-i and hetero-n-i-p-i structures with selective ohmic contacts, are reported. In this kind of structures strong changes of the absorption and refractive index are achieved either by field effects and/or with phase space filling. The latter one is always present in normal n-i-p-i structures but relatively weak because of the 3D character of the DOS. In QW structures this effect is much more pronounced because of the reduced real and momentum space and the enhanced excitonic oscillator strength. First the dynamics of n-i-p-i structures is discussed and then first experimental results of electrically tuned absorption changes due to band filling are shown. Field effects are also known to give large transmission changes. In particular, results for the transmission modulation in a n-i-p-i structure with a contrast ratio 2 are shown. It is also demonstrated that fast switching can be achieved in n-i-p-i structures. In a device with a 32 mum distance between the n- and p-contacts. a 3 dB frequency of 16 MHz is obtained in excellent agreement with the theoretical prediction. By scaling down the structure to 3 mum frequencies in the GHz range are expected to be feasible.