Investigations of a coherently driven semiconductor optical cavity QED system

TL;DR

This paper investigates a chip-based semiconductor optical cavity QED system with a quantum dot, using fiber taper coupling for precise measurements, and explores the effects of a control laser on system saturation and dephasing.

Contribution

It provides a detailed experimental and theoretical analysis of a fiber-coupled quantum dot cavity system, including measurement techniques and the impact of off-resonant control laser.

Findings

Precise optical coupling and loss measurements enable accurate intracavity photon number determination.

Resonant spectroscopy reveals the coherent coupling rate and dephasing mechanisms.

Off-resonant control laser induces saturation through free-carrier generation, not ac-Stark shift.

Abstract

Chip-based cavity quantum electrodynamics (QED) devices consisting of a self-assembled InAs quantum dot (QD) coupled to a high quality factor GaAs microdisk cavity are coherently probed through their optical channel using a fiber taper waveguide. We highlight one particularly important aspect of this all-fiber measurement setup, which is the accuracy to which the optical coupling level and optical losses are known relative to typical free-space excitation techniques. This allows for precise knowledge of the intracavity photon number and measurement of absolute transmitted and reflected signals. Resonant optical spectroscopy of the system under both weak and strong driving conditions are presented, which when compared with a quantum master equation model of the system allows for determination of the coherent coupling rate between QD exciton and optical cavity mode, the different levels of elastic and inelastic dephasing of the exciton state, and the position and orientation of the QD within the cavity. Pump-probe measurements are also performed in which a far off-resonant red-detuned control laser beam is introduced into the cavity. Rather than producing a measurable ac-Stark shift in the exciton line of the QD, we find that this control beam induces a saturation of the resonant system response. The broad photoluminescence spectrum resulting from the presence of the control beam in the cavity points to sub-bandgap absorption in the semiconductor, and the resulting free-carrier generation, as the likely source of system saturation.