Sequential Operations of Quantum Dot/Photonic Crystal All-Optical Switch With High Repetitive Frequency Pumping

A photonic crystal (PC)-based symmetric MachZehnder type all-optical switch (PC-SMZ), previously operated by single pump pulse alone, has been operated newly by a multiple-pump pulse train corresponding to a repetition frequency of 40 GHz at pulse energy as low as 10 fJ. The device involves quantum dots (QDs) in two parallel PC arms as optical nonlinear media and functions as a time-differential phase modulator caused by the pump pulse inducing carriers in the QD. Prior to the switch operation, sequential time response of the phase shift for a probe pulse was investigated in detail by changing the power and repetition rate of the pump pulse in the straight PC waveguide configuration. Besides, PC and QD parameters were explored for possibility of 100% on-off switching ratio. As a result, five QD layers, 40-ps QD relaxation-time, 500-m PC-length and use of as low as 0.05 c PC group-velocity (c; light velocity in vacuum) were found to implement the 100% switching ratio. Since each of these parameters has ever been achieved experimentally, the result will pave a promising way to an ultra-small and ultra-fast integrated all-optical switch.