A room temperature all-optical polarization switch based on the excitation of spin-polarized "virtual" carriers in quantum wells

Abstract

Room temperature operation of a polarization switch based on virtual excitation of spin-polarized carriers in quantum wells is demonstrated, which exhibits a contrast ratio of -27:1 and a switching time of -575 fs.High-speed all-optical polarization switches based on the excitation of spin-polarized carriers in semiconductor multiple quantum well (MQW) structures are a part of today's growing interest in spin-based technologies. In such devices [1-4], a strong circularly polarized control pulse excites carriers of a preferred spin, which are subsequently used to change the polarization state of a weaker signal. The induced polarization state changes are converted to transmission changes using additional polarization sensitive elements and persist as long as the spin-polarized population exists. In many of these "spin switches," the lifetimes of the resonantly excited carriers are long compared to the time over which the carriers maintain their spin [1-3], and therefore, it is the spin relaxation times that determine the turn-off times of the devices, while the complete recovery of the material requires the carriers to recombine or to be swept out. Accumulation of the carriers from pulse to pulse can ultimately limit the repetition rate of these devices. By contrast, we recently demonstrated a coherent polarization switch based on near-resonant excitation of spin-polarized virtual carriers (SPVC) that did not suffer from this same limitation [4]. We demonstrated that by taking advantage of the spin-dependent nonlinearities associated with a virtual population (that is, carriers that only exist while the control pulse is in the sample), femtosecond switching times and high contrast polarization modulation could be achieved without relying on fast spin relaxation times and without significant carrier accumulation. However, our original switch measurements were performed with the MQW cooled to -100 K. The added device complexity associated with cooling the switching material is not desirable for practical applications. Here we investigate the room temperature performance of the SPVC switch and study the temporal dynamics of the spin-polarized carriers excited in the MQW during its operation. Polarizer1. 0.8 0.4 MQW 1.44 1.46 1.48 Control Energy (eV) Analyzer V-L _Detector &Lock-inFig. 1. Experimental configuration of the SPVC switch. The inset shows the tuning of the control (dotted line) and signal (solid line) spectra relative to the Is heavy hole exciton (dashed line) absorption resonance.