Physics of semiconductor microcavities

Abstract

Semiconductor microcavities have emerged to present abundant opportunities for both device applications and basic quantum optics studies. Here we investigate several aspects of the cw and ultrafast optical response of semiconductor quantum well microcavities. The interaction of a high-finesse semiconductor microcavity mode with a quantum well (QW) exciton leads to normal mode coupling (NMC), where a periodic energy exchange develops between exciton and photon states, appearing as a double peak in the cavity transmission spectrum and a beating in the time resolved signal. The nonlinear saturation of the excitonic NMC leads to a reduction of the modulation depth of the NMC oscillations and corresponding transmission peaks with little change in oscillation period or NMC splitting. This behavior arises from excitonic broadening due to carrier-carrier and polarization scattering without reduction of the oscillator strength. The nonlinear NMC microcavity luminescence exhibits three excitation regimes, from reversible normal mode coupling, through an intermediate double-peaked emission regime, to lasing. The nonlinear PL spectrum is governed by density-dependent changes in both the bare QW emission and in the microcavity transmission. The temporal evolution of the microcavity emission is analogous to the density-dependent behavior, and can be attributed to a time-dependent carrier density which results from a combination of carrier cooling and photon emission. A strong magnetic field applied perpendicular to the plane of a QW confines electrons and holes to Landau orbits in the QW plane, transforming the QW into a quantum dot (QD) whose radius shrinks with increasing magnetic field strength. This strong magnetic confinement enhances the normal mode coupling strength in the microcavity via an increase in exciton oscillator strength. The time-resolved stimulated emission of a QW microcavity which has been transformed to a QD laser by magnetic confinement reveals a fast relaxation which is uninhibited by the magnetic field, indicating the absence of a phonon bottleneck. As a novel manifestation of cavity-modified emission, we demonstrate synchronization of the stimulated emission of a microcavity laser to the electron spin precession in a magnetic field, achieved by modulating the optical gain for the circularly polarized emission via the Larmor precession. The oscillating laser emission is locked to the completely internal electron spin precession clock, and the GHz oscillation frequencies depend only on the magnetic field strength and the QW material parameters.