© (2006) ACS

Quantum optics has long been known for its exciting effects in the interactions of light with matter. Cavity quantum electrodynamics (QED) uses optical resonators to enhance these interactions. To take advantage of such enhanced properties in a practical device, a solid-state approach is sought, and semiconductors provide a real possibility. However, owing to the fragility of exposed etched structures, inefficient heat sinking and incompatibility with electrical injection, existing cavity systems are impractical for real applications. Spatial and spectral alignment of a solid-state light source and the cavity has also proved problematic.

A new approach that isn’t afflicted by these limitations has been adopted by A. Muller and colleagues from the University of Texas1 . Their system consists of an optical microcavity with bottom and top mirrors for vertical light confinement and mesas (plateau structures) etched before the growth of the top mirror to provide lateral confinement. The light sources, quantum dots, are in the mesa and so are self-aligned to the cavity.

With the aid of time-resolved photoluminescence spectroscopy, the team demonstrates that increasing the optical confinement in the plane of growth by reducing the diameter of the mesa containing the quantum dot, allows control over the wavelength and the mode spacing of the emitted light. In addition, better coupling and fine tuning can be achieved by overgrowing a second mirror layer onto the sample. The researchers also demonstrate a cavity-QED effect — Purcell spontaneous-emission enhancement — and observe that it has a strong spectral and spatial dependence. This highlights the importance of the spatial overlap between a single quantum dot and a single cavity mode. This proposed system offers tremendous technological advantages over existing microcavity designs and is particularly attractive for quantum-dot lasing.