Kerry J. Vahala
California Institute of Technology, Mail Stop 128-95, Pasadena, California 91125, USA (e-mail: email@example.com)
Optical microcavities confine light to small volumes by resonant recirculation. Devices based on optical microcavities are already indispensable for a wide range of applications and studies. For example, microcavities made of active III–V semiconductor materials control laser emission spectra to enable long-distance transmission of data over optical fibres; they also ensure narrow spot-size laser read/write beams in CD and DVD players. In quantum optical devices, microcavities can coax atoms or quantum dots to emit spontaneous photons in a desired direction or can provide an environment where dissipative mechanisms such as spontaneous emission are overcome so that quantum entanglement of radiation and matter is possible. Applications of these remarkable devices are as diverse as their geometrical and resonant properties.
ike its acoustic analogue the tuning fork, the
optical microcavity (or microresonator) has a
size-dependent resonant frequency spectrum.
Microscale volume ensures that resonant
frequencies are more sparsely distributed
throughout this spectrum than they are in a corresponding
‘macroscale’ resonator. An ideal cavity would confine light indefinitely (that is, without loss) and would have resonant frequencies at precise values. Deviation from this ideal
condition is described by the cavity Q factor (which is
proportional to the confinement time in units of the
optical period). Q factor and microcavity volume (V)
figure prominently in applications of these devices, and a
summary of values typical for the devices discussed in this
review is given in Table 1. In addition, representative
examples of the three methods of confinement employed
in microcavities are provided in Figs 1–3 (refs 1–7).
In this review, I consider four applications of optical
microcavities: strong-coupling cavity quantum electrodynamics (QED), enhancement and suppression of spontaneous emission, novel sources, and dynamic filters in optical communication. These areas are just four of several possible, and many topics, such as soliton effects8,9, chaos10 and effects in quantum-well microcavities11, will not be reviewed
because of space limitations. Also, I will not review microcavity types in commercial semiconductor lasers because extensive texts and treaties on this subject already exist12,13. Even for the four applications discussed, there are by necessity omissions. Cavity QED, for example, is a vast topic, and selections have been made on the basis of their importance
and for the interesting design limits they illustrate. I will provide a brief introduction to each area, then describe a few representative applications, their microcavity requirements, and the state-of-the-art for these devices, before outlining the challenges for the future.
Figure 1 Micropost (or micropillar) cavities1,2 have played a major role in recent applications of the Purcell effect to triggered, single-photon sources. They offer small cavity volume and relatively high Q, have an emission pattern that is well suited for coupling and manipulation of emitted photons56 (for example, with optical fibres) and can incorporate quasi-atomic, quantum dots as emitters. In the rendering, Bragg mirrors3 at the output (upper stack near arrow) and below provide one dimension of cavity confinement, whereas airdielectric guiding provides lateral (in-the-plane) confinement. A single quantum dot is shown spontaneously emitting a photon that is subsequently directed via the Purcell effect through the cavity top. The inset shows a scanning electron micrograph of such a micropost cavity used in recent triggered single-photon source experiments. Inset micrograph courtesy of Y. Yamamoto76 (Stanford University, CA).
Strong-coupling cavity QED