Verticalcavity

A vertical cavity is an optical resonator in which light is confined in the vertical direction, perpendicular to a planar structure such as a semiconductor wafer, by mirrors placed above and below the active region. The term is commonly used in the context of vertical-cavity surface-emitting devices, in particular vertical-cavity surface-emitting lasers (VCSELs), where the optical axis points normal to the surface.

In semiconductor implementations, vertical cavities are formed by two high-reflectivity mirrors, typically distributed Bragg reflectors (DBRs), surrounding an active gain region that may include quantum wells or quantum dots. The mirrors provide strong reflectivity to confine photons in the vertical axis, while the lateral field distribution is governed by the device geometry.

Operation is determined by the resonance condition set by the cavity length and refractive indices. Emission occurs perpendicular to the wafer, enabling efficient fiber coupling and easy integration with on-chip components. VCSELs are known for low threshold currents, circular mode profiles, and potential for high-speed modulation.

Other uses of the term vertical cavity appear in microwave engineering and other photonic structures where short, vertically confined resonators are desirable. In contrast, optical cavities designed with in-plane confinement emit light parallel to the surface. The concept of a vertical cavity highlights the distinction between vertical and planar resonator geometries.