Frequency dependent dynamics of semiconductor microcavities under ultrafast carrier switching

We present ultrafast reflectivity measurements on the dynamics of optically excited free carriers in semiconductor microcavities. We observe that the relaxation dynamics of the switched cavity is strongly frequency dependent, which points towards multiple carrier populations. The interest in ultrafast all-optical switching of nano-photonic structures has rapidly increased due to the inherent speed of the process. This not only promises new developments in information technology [1-3] but also a better understanding in fundamental electrodynamics as well as in ultrafast cavity QED [4]. The fastest mechanisms for switching are, as recently shown, the electronic Kerr effect [5] and, as more commonly used, the excitation of free carriers [2,3,5-9]. Despite all studies on switching with free carriers, there is still a debate on the dynamics and the recombination processes in these nano-photonic structures. Here, we explore the dynamics of a semiconductor microcavity, which is switched by optically excited free carriers. Our experiments are performed on the well-known GaAs-AlAs planar semiconductor microcavity system over a wide frequency range. To track the dynamics of the relaxation of the switched cavity, we keep the pump frequency constant at ω_pu=5000 cm^-1 (λ_pu=2000 nm) to ensure two-photon absorption in the GaAs layers, while the probe frequency was scanned. Using this scheme, we are able to resolve the time dependent spectral response of the cavity (Fig. 1(b)). We observe that the relaxation process of the cavity, driven by the recombination of free carriers, shows a strong spectral dependence. The latter cannot be explained assuming a single population model for the free carriers in the GaAs of the Bragg-stack and the λ-layer.