H. Cao

Observation of dressed‐exciton oscillating emission over a wide wavelength range in a semiconductor microcavity

We have observed the dressed‐exciton oscillating emission in the time domain and the associated spectral splitting in the frequency domain from a GaAs single quantum well microcavity over a very broad range of cavity resonant wavelengths. The spectral splitting and temporal oscillation period have been measured to be nearly constant over two orders of magnitude variation of pump intensity, which confirms the linear bosonic feature of Wannier excitons in the weak excitation regime.

COLLAPSE AND REVIVAL OF EXCITON-POLARITON OSCILLATION IN A SEMICONDUCTOR MICROCAVITY

We measured temporal evolution of the coherent emission from a semiconductor microcavity by a very sensitive ac balanced homodyne detection system. We observed collapse and revival of the exciton–polariton oscillation due to a three-mode beating.

Wide band AC balanced homodyne detection of weak coherent pulses

Abstract A wide band AC balanced homodyne detection technique was applied to measure the amplitude of coherent pulses. The ultimate sensitivity given by a peak signal to noise ratio (S/N)=1 of our system is estimated to be about 0.25 fW (3.3×10 −24 J/pulse or a photon number of 1.25×10 −5 photons/pulse). As a demonstration for probing very weak ultrafast coherent optical processes, an exciton-polariton oscillation in a GaAs single quantum well microcavity was observed up to nine oscillation periods.

Theory of inhomogeneous microcavity polariton splitting

A quantum theory of microcavity exciton polariton for an inhomogeneous distribution of exciton is presented for the linear excitation regime. Calculations show that the apparent vaccum Rabi splitting is reduced when the inhomogeneous linewidth exceeds the intrinsic splitting without inhomogeneous broadening.

Effect of dephasing on exciton superradiance and exciton cavity polaritons

Quantum-well exciton superradiance and exciton cavity polarization formation both require a substantial spatial coherence area in order to become the dominant exciton field interaction. In both interaction processes the quantum-well (in-plane) momentum is conserved. Momentum scattering, which is due, e.g., to interaction with a phonon reservoir, quickly localizes an initially delocalized exciton and randomizes the excitation momentum. We take a close look at how momentum scattering influences measurements of excitonic superradiance and of exciton cavity polariton splitting. An important conclusion is that, in general, measurements of the emitted light do not correspond to the evolution of the system as a whole. Therefore, e.g., the measured decay rates do not correspond to the true decay rate of the system.

TUNNELING SPECTROSCOPY FOR QUANTUM WELL EXCITONS

We have demonstrated a technique of directly measuring the exciton binding energy and the valence band split in the quantum well through a tunneling process. We have also measured the emission efficiency of quantum well heavy-hole excitons and light-hole excitons into the normal direction.

Microcavity laser physics

Microcavity lasers have been predicted to offer low threshold current, high quantum efficiency and high modulation bandwidth. In this report we review the physics underlying microcavity device behavior. Specifically we cover dipole-field coupling for both localized (point) dipoles and extended dipoles. In general, optical pumping of the devices is required to create extended dipoles. We also outline the difference between the weak (irreversible) coupling regime and the strong (reversible) regime. For photonic application the intermediate, superradiant regime is perhaps more interesting than the strong coupling regime. Finally, we describe our recent experimental efforts to make high quantum efficiency devices by creating extended excitonic dipoles in electrically pumped devices.