An optical microcavity or microresonator is a structure formed by reflecting faces on the two sides of a spacer layer or optical medium, or by wrapping a

waveguide A waveguide is a structure that guides waves, such as electromagnetic waves or sound, with minimal loss of energy by restricting the transmission of energy to one direction. Without the physical constraint of a waveguide, wave intensities de ...

in a circular fashion to form a

ring Ring may refer to: * Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry * To make a sound with a bell, and the sound made by a bell :(hence) to initiate a telephone connection Arts, entertainment and media Film and ...

. The former type is a

standing wave In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect ...

cavity, and the latter is a

traveling wave In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (re ...

cavity. The name ''micro''cavity 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

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

s, this forms an optical cavity or ''optical resonator'', allowing a

to form inside the spacer layer or a traveling wave that goes around in the ring.

Applications and effects

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 In physics, a quantum (plural quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantizati ...

effects of the light's

electromagnetic field An electromagnetic field (also EM field or EMF) is a classical (i.e. non-quantum) field produced by (stationary or moving) electric charges. It is the field described by classical electrodynamics (a classical field theory) and is the classical c ...

can be observed. For example, the spontaneous emission rate and behaviour of

atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, gas, and ...

s is altered by such a microcavity, a phenomenon that is referred to as inhibited spontaneous emission. One can imagine this as the situation that no

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...

is emitted, if the environment is a box that is too small to hold it. This leads to an altered

emission spectrum The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an electron making a atomic electron transition, transition from a high energy state to a lower energy st ...

, which is significantly narrowed. Moreover, nonlinear effects are enhanced by orders of magnitude due to the strong light confinement, leading to the generation of microresonator frequency combs, low-power parametric processes such as down-conversion,

second-harmonic generation Second-harmonic generation (SHG, also called frequency doubling) is a nonlinear optical process in which two photons with the same frequency interact with a nonlinear material, are "combined", and generate a new photon with twice the energy of ...

four-wave mixing Four-wave mixing (FWM) is an intermodulation phenomenon in nonlinear optics, whereby interactions between two or three wavelengths produce two or one new wavelengths. It is similar to the third-order intercept point in electrical systems. Four-wave ...

and

optical parametric oscillation An optical parametric oscillator (OPO) is a parametric oscillator that oscillates at optical frequencies. It converts an input laser wave (called "pump") with frequency \omega_p into two output waves of lower frequency (\omega_s, \omega_i) by means ...

. Several of these nonlinear processes themselves lead to the generation of quantum states of light. Another field that harnesses the strong confinement of light is

cavity optomechanics Cavity optomechanics is a branch of physics which focuses on the interaction between light and mechanical objects on low-energy scales. It is a cross field of optics, quantum optics, solid-state physics and materials science. The motivation for re ...

, where the back-and-forth interaction of the light beam with the mechanical motion of the resonator becomes strongly coupled. Even in this field, quantum effects can start playing a role. Microcavities have many applications, frequently at present in optoelectronics, where vertical cavity surface emitting lasers

VCSEL The vertical-cavity surface-emitting laser, or VCSEL , is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also ''in-plane'' lasers) which ...

are probably the best known. Recently, a single

emitting device was demonstrated by placing a

quantum dot Quantum dots (QDs) are semiconductor particles a few nanometres in size, having light, optical and electronics, electronic properties that differ from those of larger particles as a result of quantum mechanics. They are a central topic in nanote ...

in a microcavity. These light sources are interesting for

quantum cryptography Quantum cryptography is the science of exploiting quantum mechanical properties to perform cryptographic tasks. The best known example of quantum cryptography is quantum key distribution which offers an information-theoretically secure solution ...

and

quantum computer Quantum computing is a type of computation whose operations can harness the phenomena of quantum mechanics, such as superposition, interference, and entanglement. Devices that perform quantum computations are known as quantum computers. Though ...

s. An overview is given in the review article published in the journal

Nature Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...

Types

Standing-wave

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 one

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...

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 transmission

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of colors i ...

of the microcavity, a long range of

s, 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 of

semiconductor A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...

material or by metal mirrors.

Traveling-wave

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 of whispering-gallery resonators, or as integrated ring resonators. Typical materials from which they are made could be semiconductors like

Silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

Silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...

silicon nitride Silicon nitride is a chemical compound of the elements silicon and nitrogen. is the most thermodynamically stable and commercially important of the silicon nitrides, and the term "silicon nitride" commonly refers to this specific composition. It ...

, crystalline fluorides ( CaF₂, MgF₂, SrF₂) or

lithium niobate Lithium niobate () is a non-naturally-occurring salt consisting of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other linea ...

. The material is chosen such that it is low-loss and transparent in the wavelength of application desired. Typically, such structures are fabricated by either

diamond turning Diamond turning is turning using a cutting tool with a diamond tip. It is a process of mechanical machining of precision elements using lathes or derivative machine tools (e.g., turn-mills, rotary transfers) equipped with natural or synthetic d ...

or micromachining a cylindrical rod of a material (especially for fluorides and lithium niobate), or by

photolithography In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer, to protect ...

and

electron-beam lithography Electron-beam lithography (often abbreviated as e-beam lithography, EBL) is the practice of scanning a focused beam of electrons to draw custom shapes on a surface covered with an electron-sensitive film called a resist (exposing). The electron b ...

to produce a patterned resonator on chip (for silicon-based materials). When an integer 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 the Finesse Coefficient of the resonator. This also leads to an ultrahigh

quality factor In physics and engineering, the quality factor or ''Q'' factor is a dimensionless parameter that describes how underdamped an oscillator or resonator is. It is defined as the ratio of the initial energy stored in the resonator to the energy los ...

, meaning that light travels around the circumference many million times before decaying into the surroundings.

References

{{DEFAULTSORT:Optical Microcavity Optical components Laser science

Applications and effects

Types

Standing-wave

Traveling-wave

See also

References