Silicon photonics is the study and application of
photonic
Photonics is a branch of optics that involves the application of generation, detection, and manipulation of light in form of photons through emission, transmission, modulation, signal processing, switching, amplification, and sensing. Though ...
systems which use
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 ...
as an
optical medium
An optical medium is material through which light and other electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it.
Properties
The ...
.
The silicon is usually patterned with
sub-micrometre precision, into
microphotonic components.
These operate in the
infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...
, most commonly at the 1.55 micrometre
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, tr ...
used by most
fiber optic telecommunication
Fiber-optic communication is a method of transmitting information from one place to another by sending pulses of infrared light through an optical fiber. The light is a form of carrier wave that is modulated to carry information. Fiber is pre ...
systems.
The silicon typically lies on top of a layer of silica in what (by analogy with
a similar construction in
microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture (or microfabrication) of very small electronic designs and components. Usually, but not always, this means micrometre-s ...
) is known as silicon on insulator (SOI).
Silicon photonic devices can be made using existing
semiconductor fabrication
Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuit (IC) chips such as modern computer processors, microcontrollers, and memory chips such as NAND flash and DRAM that are ...
techniques, and because silicon is already used as the substrate for most
integrated circuits, it is possible to create hybrid devices in which the
optical
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...
and
electronic components are integrated onto a single microchip.
Consequently, silicon photonics is being actively researched by many electronics manufacturers including
IBM and
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
, as well as by academic research groups, as a means for keeping on track with
Moore's Law, by using
optical interconnects to provide faster
data transfer both between and within
microchips.
The propagation of
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
through silicon devices is governed by a range of
nonlinear optical
Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typica ...
phenomena including the
Kerr effect
The Kerr effect, also called the quadratic electro-optic (QEO) effect, is a change in the refractive index of a material in response to an applied electric field. The Kerr effect is distinct from the Pockels effect in that the induced index cha ...
, the
Raman effect,
two-photon absorption and interactions between
photons and
free charge carriers.
The presence of nonlinearity is of fundamental importance, as it enables light to interact with light,
thus permitting applications such as wavelength conversion and all-optical signal routing, in addition to the passive transmission of light.
Silicon
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 ...
s are also of great academic interest, due to their unique guiding properties, they can be used for communications, interconnects, biosensors, and they offer the possibility to support exotic nonlinear optical phenomena such as
soliton propagation.
Applications
Optical communications
In a typical optical link, data is first transferred from the electrical to the optical domain using an electro-optic modulator or a directly modulated laser. An electro-optic modulator can vary the intensity and/or the phase of the optical carrier. In silicon photonics, a common technique to achieve modulation is to vary the density of free charge carriers. Variations of electron and hole densities change the real and the imaginary part of the refractive index of silicon as described by the empirical equations of Soref and Bennett. Modulators can consist of both forward-biased
PIN diodes, which generally generate large phase-shifts but suffer of lower speeds,
as well as of reverse-biased
PN junctions.
[
] A prototype optical interconnect with microring modulators integrated with germanium detectors has been demonstrated.
[
]
Non-resonant modulators, such as
Mach-Zehnder interferometers, have typical dimensions in the millimeter range and are usually used in telecom or datacom applications. Resonant devices, such as ring-resonators, can have dimensions of few tens of micrometers only, occupying therefore much smaller areas. In 2013, researchers demonstrated a resonant depletion modulator that can be fabricated using standard Silicon-on-Insulator Complementary Metal-Oxide-Semiconductor (SOI CMOS) manufacturing processes. A similar device has been demonstrated as well in bulk CMOS rather than in SOI.
On the receiver side, the optical signal is typically converted back to the electrical domain using a semiconductor
photodetector. The semiconductor used for carrier generation has usually a band-gap smaller than the photon energy, and the most common choice is pure germanium. Most detectors utilize a
PN junction for carrier extraction, however, detectors based on
metal–semiconductor junctions (with
germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbo ...
as the semiconductor) have been integrated into silicon waveguides as well.
More recently, silicon-germanium
avalanche photodiode
An avalanche photodiode (APD) is a highly sensitive semiconductor photodiode detector that exploits the photoelectric effect to convert light into electricity. From a functional standpoint, they can be regarded as the semiconductor analog of ...
s capable of operating at 40 Gbit/s have been fabricated.
Complete transceivers have been commercialized in the form of active optical cables.
Optical communications are conveniently classified by the reach, or length, of their links. The majority of silicon photonic communications have so far been limited to telecom
and datacom applications, where the reach is of several kilometers or several meters respectively.
Silicon photonics, however, is expected to play a significant role in computercom as well, where optical links have a reach in the centimeter to meter range. In fact, progress in computer technology (and the continuation of
Moore's Law) is becoming increasingly dependent on faster
data transfer between and within
microchips.
Optical interconnects may provide a way forward, and silicon photonics may prove particularly useful, once integrated on the standard silicon chips.
In 2006,
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the devel ...
Senior Vice President - and future CEO -
Pat Gelsinger stated that, "Today, optics is a niche technology. Tomorrow, it's the mainstream of every chip that we build."
The first microprocessor with optical input/output (I/O) was demonstrated in December 2015 using an approach known as "zero-change" CMOS photonics.
This first demonstration was based on a 45 nm SOI node, and the bi-directional chip-to-chip link was operated at a rate of 2×2.5 Gbit/s. The total energy consumption of the link was calculated to be of 16 pJ/b and was dominated by the contribution of the off-chip laser.
Some researchers believe an on-chip
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 firs ...
source is required. Others think that it should remain off-chip because of thermal problems (the quantum efficiency decreases with temperature, and computer chips are generally hot) and because of CMOS-compatibility issues. One such device is the
hybrid silicon laser, in which the silicon is bonded to a different
semiconductor
A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. ...
(such as
indium phosphide) as the
lasing medium.
Other devices include all-silicon
Raman laser or an all-silicon Brillouin lasers
wherein silicon serves as the lasing medium.
In 2012, IBM announced that it had achieved optical components at the 90 nanometer scale that can be manufactured using standard techniques and incorporated into conventional chips.
In September 2013, Intel announced technology to transmit data at speeds of 100 gigabits per second along a cable approximately five millimeters in diameter for connecting servers inside data centers. Conventional PCI-E data cables carry data at up to eight gigabits per second, while networking cables reach 40 Gbit/s. The latest version of the
USB standard tops out at ten Gbit/s. The technology does not directly replace existing cables in that it requires a separate circuit board to interconvert electrical and optical signals. Its advanced speed offers the potential of reducing the number of cables that connect blades on a rack and even of separating processor, storage and memory into separate blades to allow more efficient cooling and dynamic configuration.
Graphene
Graphene () is an allotrope of carbon consisting of a Single-layer materials, single layer of atoms arranged in a hexagonal lattice nanostructure. photodetectors have the potential to surpass germanium devices in several important aspects, although they remain about one order of magnitude behind current generation capacity, despite rapid improvement. Graphene devices can work at very high frequencies, and could in principle reach higher bandwidths. Graphene can absorb a broader range of wavelengths than germanium. That property could be exploited to transmit more data streams simultaneously in the same beam of light. Unlike germanium detectors, graphene photodetectors do not require applied voltage, which could reduce energy needs. Finally, graphene detectors in principle permit a simpler and less expensive on-chip integration. However, graphene does not strongly absorb light. Pairing a silicon waveguide with a graphene sheet better routes light and maximizes interaction. The first such device was demonstrated in 2011. Manufacturing such devices using conventional manufacturing techniques has not been demonstrated.
Optical routers and signal processors
Another application of silicon photonics is in signal routers for
fiber optic telecommunication, optical communication. Construction can be greatly simplified by fabricating the optical and electronic parts on the same chip, rather than having them spread across multiple components.
A wider aim is all-optical signal processing, whereby tasks which are conventionally performed by manipulating signals in electronic form are done directly in optical form.
An important example is all-
optical switching An optical transistor, also known as an optical switch or a light valve, is a device that switches or amplifies optical signals. Light occurring on an optical transistor's input changes the intensity of light emitted from the transistor's output ...
, whereby the routing of optical signals is directly controlled by other optical signals.
Another example is all-optical wavelength conversion.
In 2013, a
startup company
A startup or start-up is a company or project undertaken by an entrepreneur to seek, develop, and validate a scalable business model. While entrepreneurship refers to all new businesses, including self-employment and businesses that never intend ...
named "Compass-EOS", based in
California
California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the ...
and in
Israel
Israel (; he, יִשְׂרָאֵל, ; ar, إِسْرَائِيل, ), officially the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, label=none, translit=Medīnat Yīsrāʾēl; ), is a country in Western Asia. It is situated ...
, was the first to present a commercial silicon-to-photonics router.
Long range telecommunications using silicon photonics
Silicon microphotonics can potentially increase the
Internet
The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consists ...
's bandwidth capacity by providing micro-scale, ultra low power devices. Furthermore, the power consumption of
datacenters may be significantly reduced if this is successfully achieved. Researchers at
Sandia,
Kotura,
NTT,
Fujitsu
is a Japanese multinational information and communications technology equipment and services corporation, established in 1935 and headquartered in Tokyo. Fujitsu is the world's sixth-largest IT services provider by annual revenue, and the la ...
and various academic institutes have been attempting to prove this functionality. A 2010 paper reported on a prototype 80 km, 12.5 Gbit/s transmission using microring silicon devices.
Light-field displays
As of 2015, US startup company
Magic Leap is working on a
light-field chip using silicon photonics for the purpose of an
augmented reality
Augmented reality (AR) is an interactive experience that combines the real world and computer-generated content. The content can span multiple sensory Modality (human–computer interaction), modalities, including visual, Hearing, auditory, hap ...
display.
Physical properties
Optical guiding and dispersion tailoring
Silicon is
transparent to
infrared light with wavelengths above about 1.1 micrometres.
Silicon also has a very high
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, o ...
, of about 3.5.
The tight optical confinement provided by this high index allows for microscopic
optical waveguide
An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber waveguides, transparent dielectric waveguides made of plastic and glass, liquid light ...
s, which may have cross-sectional dimensions of only a few hundred
nanometer
330px, Different lengths as in respect to the molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re, ...
s.
Single mode propagation can be achieved,
thus (like
single-mode optical fiber
In fiber-optic communication, a single-mode optical fiber (SMF), also known as fundamental- or mono-mode, is an optical fiber designed to carry only a single mode of light - the transverse mode. Modes are the possible solutions of the Helmholt ...
) eliminating the problem of
modal dispersion.
The strong
dielectric boundary effects that result from this tight confinement substantially alter the
optical dispersion relation. By selecting the waveguide geometry, it is possible to tailor the dispersion to have desired properties, which is of crucial importance to applications requiring ultrashort pulses.
In particular, the ''group velocity dispersion'' (that is, the extent to which
group velocity varies with wavelength) can be closely controlled. In bulk silicon at 1.55 micrometres, the group velocity dispersion (GVD) is ''normal'' in that pulses with longer wavelengths travel with higher group velocity than those with shorter wavelength. By selecting a suitable waveguide geometry, however, it is possible to reverse this, and achieve ''anomalous'' GVD, in which pulses with shorter wavelengths travel faster.
Anomalous dispersion is significant, as it is a prerequisite for
soliton
In mathematics and physics, a soliton or solitary wave is a self-reinforcing wave packet that maintains its shape while it propagates at a constant velocity. Solitons are caused by a cancellation of nonlinear and dispersive effects in the mediu ...
propagation, and
modulational instability
In the fields of nonlinear optics and fluid dynamics, modulational instability or sideband instability is a phenomenon whereby deviations from a periodic waveform are reinforced by nonlinearity, leading to the generation of spectral-sidebands and ...
.
In order for the silicon photonic components to remain optically independent from the bulk silicon of the
wafer
A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...
on which they are fabricated, it is necessary to have a layer of intervening material. This is usually
silica
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 o ...
, which has a much lower refractive index (of about 1.44 in the wavelength region of interest
), and thus light at the silicon-silica interface will (like light at the silicon-air interface) undergo
total internal reflection
Total internal reflection (TIR) is the optical phenomenon in which waves arriving at the interface (boundary) from one medium to another (e.g., from water to air) are not refracted into the second ("external") medium, but completely reflecte ...
, and remain in the silicon. This construct is known as silicon on insulator.
It is named after the technology of
silicon on insulator
In semiconductor manufacturing, silicon on insulator (SOI) technology is fabrication of silicon semiconductor devices in a layered silicon–insulator–silicon substrate, to reduce parasitic capacitance within the device, thereby improving perfo ...
in electronics, whereby components are built upon a layer of
insulator in order to reduce
parasitic capacitance and so improve performance.
Kerr nonlinearity
Silicon has a focusing
Kerr nonlinearity, in that the
refractive index
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, o ...
increases with optical intensity.
This effect is not especially strong in bulk silicon, but it can be greatly enhanced by using a silicon waveguide to concentrate light into a very small cross-sectional area.
This allows
nonlinear optical
Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typica ...
effects to be seen at low powers. The nonlinearity can be enhanced further by using a
slot waveguide A slot-waveguide is an optical waveguide that guides strongly confined light in a sub wavelength-scale low refractive index region by total internal reflection.
A slot-waveguide consists of two strips or slabs of high-refractive-index (nH) mate ...
, in which the high refractive index of the silicon is used to confine light into a central region filled with a strongly nonlinear
polymer
A polymer (; Greek ''poly-'', "many" + '' -mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...
.
Kerr nonlinearity underlies a wide variety of optical phenomena.
One example is
four wave mixing, which has been applied in silicon to realise
optical parametric amplification,
parametric wavelength conversion,
and frequency comb generation.,
Kerr nonlinearity can also cause
modulational instability
In the fields of nonlinear optics and fluid dynamics, modulational instability or sideband instability is a phenomenon whereby deviations from a periodic waveform are reinforced by nonlinearity, leading to the generation of spectral-sidebands and ...
, in which it reinforces deviations from an optical waveform, leading to the generation of
spectral-sidebands and the eventual breakup of the waveform into a train of pulses.
Another example (as described below) is soliton propagation.
Two-photon absorption
Silicon exhibits
two-photon absorption (TPA), in which a pair of
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 particle, massless ...
s can act to excite an
electron-hole pair
In the solid-state physics of semiconductors, carrier generation and carrier recombination are processes by which mobile charge carriers (electrons and electron holes) are created and eliminated. Carrier generation and recombination processes are ...
.
This process is related to the Kerr effect, and by analogy with
complex refractive index, can be thought of as the
imaginary-part of a
complex Kerr nonlinearity.
At the 1.55 micrometre telecommunication wavelength, this imaginary part is approximately 10% of the real part.
The influence of TPA is highly disruptive, as it both wastes light, and generates unwanted
heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
.
It can be mitigated, however, either by switching to longer wavelengths (at which the TPA to Kerr ratio drops),
or by using slot waveguides (in which the internal nonlinear material has a lower TPA to Kerr ratio).
Alternatively, the energy lost through TPA can be partially recovered (as is described below) by extracting it from the generated charge carriers.
Free charge carrier interactions
The
free charge carriers within silicon can both absorb photons and change its refractive index.
This is particularly significant at high intensities and for long durations, due to the carrier concentration being built up by TPA. The influence of free charge carriers is often (but not always) unwanted, and various means have been proposed to remove them. One such scheme is to
implant
Implant can refer to:
Medicine
*Implant (medicine), or specifically:
** Brain implant
** Breast implant
**Buttock implant
**Cochlear implant
**Contraceptive implant
**Dental implant
** Fetal tissue implant
**Implantable cardioverter-defibrillator ...
the silicon with
helium
Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
in order to enhance
carrier recombination.
A suitable choice of geometry can also be used to reduce the carrier lifetime.
Rib waveguide
An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber waveguides, transparent dielectric waveguides made of plastic and glass, liquid ligh ...
s (in which the waveguides consist of thicker regions in a wider layer of silicon) enhance both the carrier recombination at the silica-silicon interface and the
diffusion
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
of carriers from the waveguide core.
A more advanced scheme for carrier removal is to integrate the waveguide into the
intrinsic region of a
PIN diode, which is
reverse bias
Reverse or reversing may refer to:
Arts and media
* ''Reverse'' (Eldritch album), 2001
* ''Reverse'' (2009 film), a Polish comedy-drama film
* ''Reverse'' (2019 film), an Iranian crime-drama film
* ''Reverse'' (Morandi album), 2005
* ''Reverse'' ...
ed so that the carriers are attracted away from the waveguide core.
A more sophisticated scheme still, is to use the diode as part of a circuit in which
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge t ...
and
current are out of phase, thus allowing power to be extracted from the waveguide.
The source of this power is the light lost to two photon absorption, and so by recovering some of it, the net loss (and the rate at which heat is generated) can be reduced.
As is mentioned above, free charge carrier effects can also be used constructively, in order to modulate the light.
Second-order nonlinearity
Second-order nonlinearities cannot exist in bulk silicon because of the
centrosymmetry of its crystalline structure. By applying strain however, the inversion symmetry of silicon can be broken. This can be obtained for example by depositing a
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 ...
layer on a thin silicon film.
Second-order nonlinear phenomena can be exploited for
optical modulation
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultra ...
,
spontaneous parametric down-conversion
Spontaneous parametric down-conversion (also known as SPDC, parametric fluorescence or parametric scattering) is a nonlinear instant optical process that converts one photon of higher energy (namely, a pump photon), into a pair of photons (namely, ...
,
parametric amplification,
ultra-fast optical signal processing and
mid-infrared
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
generation. Efficient nonlinear conversion however requires
phase matching between the optical waves involved. Second-order nonlinear waveguides based on strained silicon can achieve
phase matching by
dispersion-engineering.
So far, however, experimental demonstrations are based only on designs which are not
phase matched.
It has been shown that
phase matching can be obtained as well in silicon double
slot waveguide A slot-waveguide is an optical waveguide that guides strongly confined light in a sub wavelength-scale low refractive index region by total internal reflection.
A slot-waveguide consists of two strips or slabs of high-refractive-index (nH) mate ...
s coated with a highly nonlinear organic cladding
and in periodically strained silicon waveguides.
The Raman effect
Silicon exhibits the
Raman effect, in which a photon is exchanged for a photon with a slightly different energy, corresponding to an excitation or a relaxation of the material. Silicon's Raman transition is dominated by a single, very narrow frequency peak, which is problematic for broadband phenomena such as
Raman amplification, but is beneficial for narrowband devices such as
Raman lasers.
Early studies of Raman amplification and Raman lasers started at UCLA which led to demonstration of net gain Silicon Raman amplifiers and silicon pulsed Raman laser with fiber resonator (Optics express 2004). Consequently, all-silicon Raman lasers have been fabricated in 2005.
The Brillouin effect
In the Raman effect, photons are red- or blue-shifted by
optical phonons with a frequency of about 15 THz. However, silicon waveguides also support
acoustic phonon excitations. The interaction of these acoustic phonons with light is called
Brillouin scattering Brillouin scattering (also known as Brillouin light scattering or BLS), named after Léon Brillouin, refers to the interaction of light with the material waves in a medium (e.g. electrostriction and magnetostriction). It is mediated by the refr ...
. The frequencies and mode shapes of these acoustic phonons are dependent on the geometry and size of the silicon waveguides, making it possible to produce strong Brillouin scattering at frequencies ranging from a few MHz to tens of GHz. Stimulated Brillouin scattering has been used to make narrowband optical amplifiers as well as all-silicon Brillouin lasers.
The interaction between photons and acoustic phonons is also studied in the field of
cavity optomechanics, although 3D optical cavities are not necessary to observe the interaction. For instance, besides in silicon waveguides the optomechanical coupling has also been demonstrated in fibers and in chalcogenide waveguides.
Solitons
The evolution of light through silicon waveguides can be approximated with a cubic
Nonlinear Schrödinger equation,
which is notable for admitting
sech-like
soliton
In mathematics and physics, a soliton or solitary wave is a self-reinforcing wave packet that maintains its shape while it propagates at a constant velocity. Solitons are caused by a cancellation of nonlinear and dispersive effects in the mediu ...
solutions.
These
optical solitons (which are also known in
optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...
) result from a balance between
self phase modulation Self-phase modulation (SPM) is a nonlinear optical effect of light–matter interaction.
An ultrashort pulse of light, when travelling in a medium, will induce a varying refractive index of the medium due to the optical Kerr effect. This variation i ...
(which causes the leading edge of the pulse to be
redshifted and the trailing edge blueshifted) and anomalous group velocity dispersion.
Such solitons have been observed in silicon waveguides, by groups at the universities of
Columbia
Columbia may refer to:
* Columbia (personification), the historical female national personification of the United States, and a poetic name for America
Places North America Natural features
* Columbia Plateau, a geologic and geographic region in ...
,
Rochester
Rochester may refer to:
Places Australia
* Rochester, Victoria
Canada
* Rochester, Alberta
United Kingdom
*Rochester, Kent
** City of Rochester-upon-Medway (1982–1998), district council area
** History of Rochester, Kent
** HM Prison ...
,
and
Bath
Bath may refer to:
* Bathing, immersion in a fluid
** Bathtub, a large open container for water, in which a person may wash their body
** Public bathing, a public place where people bathe
* Thermae, ancient Roman public bathing facilities
Plac ...
.
References
{{DEFAULTSORT:Silicon photonics
Nonlinear optics
Photonics
Silicon