An optical microcavity is a structure formed by reflecting faces on the two sides of a spacer layer or optical medium. The name microcavity stems from the fact that it is often only a few micrometers thick, the spacer layer sometimes even in the nanometer range. As with common lasers this forms an optical cavity or optical resonator, allowing a standing wave to form inside the spacer layer.
The thickness of the spacer layer determines the so-called "cavity-mode", which is the one wavelength that can be transmitted and will be formed as standing wave inside the resonator. Depending on the type and quality of the mirrors, a so-called stop-band will form in the transmission spectrum of the microcavity, a long range of wavelengths, that is reflected and a single one being transmitted. (usually in the centre)
The fundamental difference between a conventional optical cavity and microcavities is the effects that arise from the small dimensions of the system. Quantum effects of the light's electromagnetic field can be observed. For example, the spontaneous emission rate and behaviour of atoms is altered by such a microcavity. One can imagine this as the situation that no photon is emitted, if the environment is a box that is too small to hold it. This leads to an altered emission spectrum, which is significantly narrowed.
There are different means of fabricating microcavities, either by evaporating alternating layers of dielectric media to form the mirrors (DBR) and the medium inside the spacer layer or by modification of semiconductor material or by metal mirrors.
Microcavities have many applications preferably in optoelectronics, where vertical cavity surface emitting lasers VCSEL are probably the best known. Recently, a single photon emitting device was demonstrated by placing a quantum dot in a microcavity. These light sources are interesting for Quantum cryptography and Quantum computers.
An overview is given in Nature[1].