Anoptical microcavity ormicroresonator is a structure formed by reflecting faces on the two sides of a spacer layer oroptical medium, or by wrapping awaveguide in a circular fashion to form aring. The former type is astanding wave cavity, and the latter is atraveling wave cavity. The namemicrocavity stems from the fact that it is often only a few micrometers thick, the spacer layer sometimes even in the nanometer range. As with commonlasers, this forms anoptical cavity oroptical resonator, allowing astanding wave to form inside the spacer layer or a traveling wave that goes around in the ring.
The fundamental difference between a conventional optical cavity and microcavities is the effects that arise from the small dimensions of the system, but their operational principle can often be understood in the same way as for larger optical resonators.Quantum effects of the light'selectromagnetic field can be observed.[1] For example, thespontaneous emission rate and behaviour ofatoms is altered by such a microcavity, a phenomenon that is referred to as inhibited spontaneous emission.[2] One can imagine this as the situation that nophoton is emitted, if the environment is a box that is too small to hold it. This leads to an alteredemission spectrum, which is significantly narrowed.
Moreover, nonlinear effects are enhanced by orders of magnitude due to the strong light confinement, leading to the generation ofmicroresonator frequency combs, low-powerparametric processes such asdown-conversion,second-harmonic generation,four-wave mixing andoptical parametric oscillation.[3] Several of these nonlinear processes themselves lead to the generation of quantum states of light. Another field that harnesses the strong confinement of light iscavity optomechanics, where the back-and-forth interaction of the light beam with the mechanical motion of the resonator becomes strongly coupled.[4][5] Even in this field, quantum effects can start playing a role.[6]
Microcavities have many applications, frequently at present inoptoelectronics, where vertical cavity surface emitting lasersVCSEL are probably the best known. Recently, a singlephoton emitting device was demonstrated by placing aquantum dot in a microcavity. These light sources are interesting forquantum cryptography andquantum computers.
An overview is given in the review article published in the journalNature.[7]
For a microcavity supporting a single-mode or a few standing-wave modes, the thickness of the spacer layer determines the so-called "cavity-mode", which is the onewavelength that can be transmitted and will be formed as a standing wave inside the resonator. Depending on the type and quality of the mirrors, a so-called stop-band will form in the transmissionspectrum of the microcavity, a long range ofwavelengths, that is reflected and a single one being transmitted (usually in the centre). There are different means of fabricating standing-wave microcavities, either by evaporating alternating layers of dielectric media to form the mirrors (DBR) and the medium inside the spacer layer or by modification ofsemiconductor material or by metal mirrors.
Often just called "microresonators", traveling wave microcavities have a wave going around in a loop-like fashion in a preferred direction, depending on the input light direction. They can be in the form ofwhispering-gallery resonators, or as integrated ring resonators. Typical materials from which they are made could be semiconductors likeSilicon,Silicon dioxide,silicon nitride, crystalline fluorides (CaF2,MgF2,SrF2) orlithium niobate. The material is chosen such that it is low-loss and transparent in the wavelength of application desired. Typically, such structures are fabricated by eitherdiamond turning ormicromachining a cylindrical rod of a material (especially for fluorides and lithium niobate), or byphotolithography andelectron-beam lithography to produce a patterned resonator on chip (for silicon-based materials).[8][9]
When aninteger number of wavelengths in the material fits in the circumference of the resonator, a resonant wave is excited by constructive interference. At resonance, the light field can be enhanced by several hundred to several million times, quantified by theFinesse Coefficient of the resonator.[10] This also leads to an ultrahighquality factor, meaning that light travels around the circumference many million times before decaying into the surroundings.[11][12]
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