χ3 enhancement by carrier multiplication in semiconductor junctions

Abstract

Two important mechanisms for the nonlinear refractive index in bulk semiconductors are: (1) free carrier plasma interactions and (2) bandfilling and the associated Burstein-Moss shift. Both effects require the creation of excess free carriers by photons having energies slightly less than the bandgap of the semiconductor. This produces a fundamental limit to the strength of the nonlinear optical interaction, since high absorption (required for a high density of free carriers) significantly limits the interaction length over which switching or other functions may be accomplished. One way to exceed this fundamental limit in bulk semiconductors is by carrier multiplication. It is well known that suitable avalanche photodiode structures can enhance the free carrier population by a factor equal to the mean avalanche multiplication. This results in a direct enhancement of the nonlinear refractive index in both direct and indirect semiconductor junctions. The performance of such junctions in quantum-limited optical switching is examined. We show that enhancements in the nonlinear refractive index ranging from 102 to 109 are possible, depending on the choice of materials and operating conditions.