Optical feedback and the coupling problem in semiconductor microdisk lasers

The smaller the size of a light-emitting microcavity, the more important it becomes to understand the effects of the cavity boundary on the optical mode profile. Conventional methods of laser physics, such as the paraxial approximation, become inapplicable in many of the more exotic cavity designs to be discussed here. Cavities in the shape of microdisks, pillars and rings can yield low lasing thresholds in a wide variety of gain media: quantum wells, wires and even dots, as well as quantum cascade superlattices and GaN. An overview of the experimental and theoretical status is provided, with special emphasis on the light extraction problem. Light emission from microcavities is a problem of great fundamental and applied interest. A wide range of possible active media can be used to form microcavities, and consequently the properties of the microscopic dipoles which generate the light can vary significantly. One of our goals is to identify common, material-independent features of microcavity emitters, which are strongly determined by the geometric dimensions and shape of the cavity. Properties of interest are the spectrum of optical modes, their internal intensity distribution and the external field profile of the emitter. These characteristics in turn determine technological figures of merit, such as external quantum efficiency in the case of light emitting diodes (LEDs), or pump threshold and maximum output power in the case of lasers. A problem in the design of LEDs is their poor external quantum efficiency, owing to the fact that light generated within the diode is not easily extracted. This is because total internal reflection at the interface between