Optical microcavity
Encyclopedia
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 laser
s 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 wavelength
s, 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 atom
s 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 computer
s.
An overview is given in Nature.
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s this forms an optical cavity
Optical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
or optical resonator, allowing a standing wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
to form inside the spacer layer.
The thickness of the spacer layer determines the so-called "cavity-mode", which is the one wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
of the microcavity, a long range of wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s, that is reflected and a single one being transmitted. (usually in the centre)
The fundamental difference between a conventional optical cavity
Optical cavity
An optical cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light. They are also used in optical parametric...
and microcavities is the effects that arise from the small dimensions of the system. Quantum
Quantum
In physics, a quantum is the minimum amount of any physical entity involved in an interaction. Behind this, one finds the fundamental notion that a physical property may be "quantized," referred to as "the hypothesis of quantization". This means that the magnitude can take on only certain discrete...
effects of the light's electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
can be observed. For example, the spontaneous emission
Spontaneous emission
Spontaneous emission is the process by which a light source such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to a state with a lower energy, e.g., the ground state and emits a photon...
rate and behaviour of atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s is altered by such a microcavity. One can imagine this as the situation that no photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
is emitted, if the environment is a box that is too small to hold it. This leads to an altered emission spectrum
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....
, which is significantly narrowed.
There are different means of fabricating microcavities, either by evaporating alternating layers of dielectric media to form the mirrors (DBR
Distributed Bragg reflector
A distributed Bragg reflector is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with varying refractive index, or by periodic variation of some characteristic of a dielectric waveguide, resulting in periodic variation...
) and the medium inside the spacer layer or by modification of semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
material or by metal mirrors.
Microcavities have many applications preferably in optoelectronics, where vertical cavity surface emitting lasers VCSEL
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 which emit from surfaces formed by cleaving the individual chip out of a...
are probably the best known. Recently, a single photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
emitting device was demonstrated by placing a quantum dot
Quantum dot
A quantum dot is a portion of matter whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. They were discovered at the beginning of the 1980s by Alexei...
in a microcavity. These light sources are interesting for Quantum cryptography
Quantum cryptography
Quantum key distribution uses quantum mechanics to guarantee secure communication. It enables two parties to produce a shared random secret key known only to them, which can then be used to encrypt and decrypt messages...
and Quantum computer
Quantum computer
A quantum computer is a device for computation that makes direct use of quantum mechanical phenomena, such as superposition and entanglement, to perform operations on data. Quantum computers are different from traditional computers based on transistors...
s.
An overview is given in Nature.