All-optical polarization switching techniques based on the coherent many-body interactions and the virtual excitation of spin-polarized carriers in semiconductor quantum wells.

摘要

All-optical polarization switches have the potential for combining high-speed with high-contrast and, therefore, of playing an important role as the demand for higher bandwidth components continues to increase. In this work, two new techniques for achieving ultrafast polarization modulation are presented that are based on the coherent nonlinear response of unstrained semiconductor multiple quantum wells (MQWs).; The first switch that is presented utilizes resonant excitonic nonlinearities, where the underlying mechanisms are the coherent optical many-body interactions. Proof-of-principle measurements are presented that show that the device turns on and off in ∼1.5 ps at ∼80 K and exhibits a contrast ratio of ∼8:1 using only ten wells. The experimental results are compared with a phenomenological model, which indicates that the many-body effects are solely responsible for the polarization modulation and that the switching times are determined by the dephasing time. However, although the switching times of the device are not restricted by the lifetime of the excited carriers, the carriers do accumulate in the MQW until they recombine or are swept out by an electric field. This carrier accumulation may ultimately limit the repetition rate of the device. By contrast, the second switch presented in this work does not suffer from the same limitation.; The second switch design is based on the virtual excitation of spin-polarized carriers. For this device, the polarization modulation is accomplished by using a circularly polarized control pulse that is tuned near, but below, resonance, to produce a highly spin-polarized population of carriers that only exists while the control is present in the MQW. Using this technique, the switch is shown to achieve contrast ratios of >300:1 and a switching time of ∼415 fs in a 40 well sample cooled to ∼100 K. The switching mechanisms are investigated by performing a combination of differential transmission and ellipsometric measurements to study the spin-dependent absorptive and refractive changes induced by the control. In addition, a microscopic theory is used to examine the nonlinearities contributing to the operation of the switch and to explore possible avenues for device optimization.