Silicon photonics
Encyclopedia
Silicon photonics is the study and application of photonic
systems which use silicon
as an optical medium. The silicon is usually patterned with sub-micrometre precision, into microphotonic
components. These operate in the infrared
, most commonly at the 1.55 micrometre wavelength
used by most fiber optic telecommunication systems. The silicon typically lies on top of a layer of silica in what (by analogy with a similar construction
in microelectronics
) is known as silicon on insulator (SOI).
Silicon photonic devices can be made using existing semiconductor fabrication
techniques, and because silicon is already used as the substrate for most integrated circuit
s, it is possible to create hybrid devices in which the optical
and electronic
components are integrated onto a single microchip. Consequently, silicon photonics is being actively researched by many electronics manufacturers including IBM
and Intel, as well as by academic research groups such as that of Prof. Michal Lipson
, who see it is a means for keeping on track with Moore's Law
, by using optical interconnect
s to provide faster data transfer both between and within microchip
s.
The propagation of light
through silicon devices is governed by a range of nonlinear optical
phenomena including the Kerr effect
, 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 waveguides are also of great academic interest, due to their ability to support exotic nonlinear optical phenomena such as soliton propagation
.
) is becoming increasingly dependent on ultra-fast data transfer between and within microchips
. High speed optical interconnect
s are seen as a promising way forward, and silicon photonics is seen as particularly useful, due to the ability to integrate electronic and optical components on the same silicon chip. Intel senior vice president Pat Gelsinger
has stated that, "Today, optics is a niche technology. Tomorrow, it's the mainstream of every chip that we build." IBM also has an extensive research program in the field.
Optical interconnects require the integration of a range of technologies. Firstly, an on-chip laser
source is required. One such device is the hybrid silicon laser
, in which the silicon is bonded to a different semiconductor
(such as indium phosphide) which acts as the lasing medium. Another possibility is the all-silicon Raman laser
, in which the silicon itself acts as the lasing medium.
There must also be a means to modulate
the light, thus causing it to carry data in the form of optical pulses. One such technique is to control the density of free charge carriers, which (as described below) alter the optical properties of the waveguide. Modulators have been constructed where the light passes through the intrinsic region
of a PIN diode
, into which carriers can be injected or removed by altering the polarity
of an applied voltage
. By using an optical ring resonator with a built in PIN diode, data transmission rates of 18 Gbit/s have been achieved. By constructing devices where the electrical signal co-moves with the light, data rates of 30 Gbit/s have been achieved. By constructing devices where the multiple wavelengths are used to scale the modulation capacity 50 Gbit/s modulation capacity has been achieved. A prototype optical interconnect with microring modulators integrated with germanium detectors has also been demonstrated
After passage through a silicon waveguide
to a different chip (or region of the same chip) the light must be detected
, converting the data back into electronic form. Detectors based on metal-semiconductor junction
s (with germanium
as the semiconductor) have been integrated into silicon waveguides. More recently, silicon-germanium avalanche photodiode
s capable of operating at 40 Gbit/s have been fabricated.
'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
and various academic institutes have been attempting to prove this functionality. A prototype 80 km, 12.5 Gbit/s transmission has recently been reported using microring silicon devices.
, of about 3.5. The tight optical confinement provided by this high index allows for microscopic optical waveguides, which may have cross-sectional dimensions of only a few hundred nanometers. This is substantially less than the wavelength of the light itself, and is analogous to a subwavelength-diameter optical fibre
. Single mode propagation can be achieved, thus (like single-mode optical fiber
) 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 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
propagation, and modulational instability
.
In order for the silicon photonic components to remain optically independent from the bulk silicon of the wafer
on which they are fabricated, it is necessary to have a layer of intervening material. This is usually silica, 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
, and remain in the silicon. This construct is known as silicon on insulator. It is named after the technology of silicon on insulator
in electronics, whereby components are built upon a layer of insulator in order to reduce parasitic capacitance
and so improve performance.
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
effects to be seen at low powers. The nonlinearity can be enhanced further by using a slot waveguide, in which the high refractive index of the silicon is used to confine light into a central region filled with a strongly nonlinear polymer
.
Kerr nonlinearity underlies a wide variety of optical phenomena. One example is four wave mixing, which has been applied in silicon to realise both optical parametric amplification and parametric wavelength conversion. Kerr nonlinearity can also cause modulational instability
, 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.
(TPA), in which a pair of photon
s can act to excite an electron-hole pair. 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
. 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.
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
the silicon with helium
in order to enhance carrier recombination. A suitable choice of geometry can also be used to reduce the carrier lifetime. Rib waveguides (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
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 biased 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
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.
, but is beneficial for narrowband devices such as Raman laser
s. Consequently, all-silicon Raman lasers have been fabricated.
, which is notable for admitting sech-like soliton
solutions. These optical solitons (which are also known in optical fiber
) result from a balance between self phase modulation (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
, Rochester, and Bath
.
Photonics
The science of photonics includes the generation, emission, transmission, modulation, signal processing, switching, amplification, detection and sensing of light. The term photonics thereby emphasizes that photons are neither particles nor waves — they are different in that they have both particle...
systems which use silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
as an optical medium. The silicon is usually patterned with sub-micrometre precision, into microphotonic
Microphotonics
Microphotonics is a branch of technology that deals with directing light on a microscopic scale. It is used in optical networking.Microphotonics employs at least two different materials with a large differential index of refraction to squeeze the light down to a small size. Generally speaking...
components. These operate in the infrared
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, most commonly at the 1.55 micrometre 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...
used by most fiber optic telecommunication systems. The silicon typically lies on top of a layer of silica in what (by analogy with a similar construction
Silicon on insulator
Silicon on insulator technology refers to the use of a layered silicon-insulator-silicon substrate in place of conventional silicon substrates in semiconductor manufacturing, especially microelectronics, to reduce parasitic device capacitance and thereby improving performance...
in microelectronics
Microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...
) is known as silicon on insulator (SOI).
Silicon photonic devices can be made using existing semiconductor fabrication
Semiconductor fabrication
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer...
techniques, and because silicon is already used as the substrate for most integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s, it is possible to create hybrid devices in which the optical
Optics
Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...
and electronic
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
components are integrated onto a single microchip. Consequently, silicon photonics is being actively researched by many electronics manufacturers including IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
and Intel, as well as by academic research groups such as that of Prof. Michal Lipson
Michal Lipson
Michal Lipson is an American physicist known for her work on silicon photonics. She is an associate professor at Cornell University in the school of electrical and computer engineering and a member of the Kavli Institute for Nanoscience at Cornell...
, who see it is a means for keeping on track with Moore's Law
Moore's Law
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....
, by using optical interconnect
Optical interconnect
Optical interconnect is a way of communication by optical cables. Compared to traditional cables, optical wires are capable of a much higher bandwidth, from 10 Gb/s up to 100 Gb/s....
s to provide faster data transfer both between and within microchip
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s.
The propagation of light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
through silicon devices is governed by a range of nonlinear optical
Nonlinear optics
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
phenomena including the Kerr effect
Kerr effect
The Kerr effect, also called the quadratic electro-optic 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 change is directly proportional to the square of the electric...
, 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 waveguides are also of great academic interest, due to their ability to support exotic nonlinear optical phenomena such as soliton propagation
Soliton (optics)
In optics, the term soliton is used to refer to any optical field that does not change during propagation because of a delicate balance between nonlinear and linear effects in the medium. There are two main kinds of solitons:...
.
Optical interconnects
Future progress in computer technology (and the continuation of Moore's LawMoore's Law
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....
) is becoming increasingly dependent on ultra-fast data transfer between and within microchips
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
. High speed optical interconnect
Optical interconnect
Optical interconnect is a way of communication by optical cables. Compared to traditional cables, optical wires are capable of a much higher bandwidth, from 10 Gb/s up to 100 Gb/s....
s are seen as a promising way forward, and silicon photonics is seen as particularly useful, due to the ability to integrate electronic and optical components on the same silicon chip. Intel senior vice president Pat Gelsinger
Pat Gelsinger
Patrick P. Gelsinger is the President and Chief Operating Officer, EMC Information Infrastructure Products at EMC Corporation. Before joining EMC, he was the first Chief Technology Officer of Intel Corporation and Senior Vice-president and General Manager of the Digital Enterprise Group at...
has stated that, "Today, optics is a niche technology. Tomorrow, it's the mainstream of every chip that we build." IBM also has an extensive research program in the field.
Optical interconnects require the integration of a range of technologies. Firstly, an on-chip laser
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...
source is required. One such device is the hybrid silicon laser
Hybrid silicon laser
A hybrid silicon laser is a semiconductor laser fabricated from both silicon and group III-V semiconductor materials. The hybrid silicon laser was developed to address the lack of a silicon laser to enable fabrication of low-cost, mass-producible silicon optical devices...
, in which the silicon is bonded to a different 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...
(such as indium phosphide) which acts as the lasing medium. Another possibility is the all-silicon Raman laser
Raman laser
The Raman laser is a byproduct of Raman scattering, discovered in 1928 by Nobel laureate Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids and independently by Grigory Landsberg and Leonid Mandelshtam in crystals. When light hits a substance, it causes the atoms in the...
, in which the silicon itself acts as the lasing medium.
There must also be a means to modulate
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
the light, thus causing it to carry data in the form of optical pulses. One such technique is to control the density of free charge carriers, which (as described below) alter the optical properties of the waveguide. Modulators have been constructed where the light passes through the intrinsic region
Intrinsic semiconductor
An intrinsic semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities...
of a PIN diode
PIN diode
A PIN diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts....
, into which carriers can be injected or removed by altering the polarity
Electrical polarity
Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...
of an applied voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
. By using an optical ring resonator with a built in PIN diode, data transmission rates of 18 Gbit/s have been achieved. By constructing devices where the electrical signal co-moves with the light, data rates of 30 Gbit/s have been achieved. By constructing devices where the multiple wavelengths are used to scale the modulation capacity 50 Gbit/s modulation capacity has been achieved. A prototype optical interconnect with microring modulators integrated with germanium detectors has also been demonstrated
After passage through a silicon waveguide
Waveguide
A waveguide is a structure which guides waves, such as electromagnetic waves or sound waves. There are different types of waveguides for each type of wave...
to a different chip (or region of the same chip) the light must be detected
Photodetector
Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:*Active pixel sensors are image sensors consisting of an integrated circuit that contains an array of pixel sensors, each pixel containing a both a light sensor and an active amplifier...
, converting the data back into electronic form. Detectors based on metal-semiconductor junction
Metal-semiconductor junction
In solid-state physics, a metal–semiconductor junction is a type of junction in which a metal comes in close contact with a semiconductor material...
s (with germanium
Germanium
Germanium is a chemical element with the symbol Ge and atomic number 32. It is a lustrous, hard, grayish-white metalloid in the carbon group, chemically similar to its group neighbors tin and silicon. The isolated element is a semiconductor, with an appearance most similar to elemental silicon....
as the semiconductor) have been integrated into silicon waveguides. More recently, silicon-germanium avalanche photodiode
Avalanche photodiode
An avalanche photodiode is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche multiplication. From a functional standpoint,...
s capable of operating at 40 Gbit/s have been fabricated.
Optical routers and signal processors
Another application of silicon photonics is in signal routers for 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, whereby the routing of optical signals is directly controlled by other optical signals. Another example is all-optical wavelength conversion.Long range telecommunications using silicon photonics
Silicon microphotonics can potentially increase the InternetInternet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
'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
Sandia National Laboratories
The Sandia National Laboratories, managed and operated by the Sandia Corporation , are two major United States Department of Energy research and development national laboratories....
, Kotura, NTT
Nippon Telegraph and Telephone
, commonly known as NTT, is a Japanese telecommunications company headquartered in Tokyo, Japan. Ranked the 31st in Fortune Global 500, NTT is the largest telecommunications company in Asia, and the second-largest in the world in terms of revenue....
, Fujitsu
Fujitsu
is a Japanese multinational information technology equipment and services company headquartered in Tokyo, Japan. It is the world's third-largest IT services provider measured by revenues....
and various academic institutes have been attempting to prove this functionality. A prototype 80 km, 12.5 Gbit/s transmission has recently been reported using microring silicon devices.
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 indexRefractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
, of about 3.5. The tight optical confinement provided by this high index allows for microscopic optical waveguides, which may have cross-sectional dimensions of only a few hundred nanometers. This is substantially less than the wavelength of the light itself, and is analogous to a subwavelength-diameter optical fibre
Subwavelength-diameter optical fibre
Subwavelength-diameter optical fibre is an optical fibre whose diameter is less than the wavelength of the light being propagated through the fibre. An SDOF usually consists of long thick parts at both ends, transition regions Subwavelength-diameter optical fibre (SDF or SDOF) is an optical fibre...
. Single mode propagation can be achieved, thus (like single-mode optical fiber
Single-mode optical fiber
In fiber-optic communication, a single-mode optical fiber is an optical fiber designed to carry only a single ray of light . Modes are the possible solutions of the Helmholtz equation for waves, which is obtained by combining Maxwell's equations and the boundary conditions...
) eliminating the problem of modal dispersion
Modal dispersion
Modal dispersion is a distortion mechanism occurring in multimode fibers and other waveguides, in which the signal is spread in time because the propagation velocity of the optical signal is not the same for all modes...
.
The strong dielectric boundary effects
Interface conditions for electromagnetic fields
Maxwell's equations describe the behavior of electromagnetic fields; electric field, electric displacement field, magnetic field and magnetic field strength. The differential forms of these equations require that there's always an open neighbourhood around the point they're applied to, otherwise...
that result from this tight confinement substantially alter the optical dispersion relation
Dispersion (optics)
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency, or alternatively when the group velocity depends on the frequency.Media having such a property are termed dispersive media...
. By selecting the waveguide geometry, it 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
Group velocity
The group velocity of a wave is the velocity with which the overall shape of the wave's amplitudes — known as the modulation or envelope of the wave — propagates through space....
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
Soliton
In mathematics and physics, a soliton is a self-reinforcing solitary wave that maintains its shape while it travels at constant speed. Solitons are caused by a cancellation of nonlinear and dispersive effects in the medium...
propagation, and modulational instability
Modulational instability
In the field of nonlinear optics, modulational instability is a phenomenon whereby deviations from an optical waveform are reinforced by nonlinearity, leading to the generation of spectral-sidebands and the eventual breakup of the waveform into a train of pulses.-Initial instability and...
.
In order for the silicon photonic components to remain optically independent from the bulk silicon of the wafer
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
on which they are fabricated, it is necessary to have a layer of intervening material. This is usually silica, 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
Total internal reflection is an optical phenomenon that happens when a ray of light strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is...
, and remain in the silicon. This construct is known as silicon on insulator. It is named after the technology of silicon on insulator
Silicon on insulator
Silicon on insulator technology refers to the use of a layered silicon-insulator-silicon substrate in place of conventional silicon substrates in semiconductor manufacturing, especially microelectronics, to reduce parasitic device capacitance and thereby improving performance...
in electronics, whereby components are built upon a layer of insulator in order to reduce parasitic capacitance
Parasitic capacitance
In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance , is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other...
and so improve performance.
Kerr nonlinearity
Silicon has a focusing Kerr nonlinearity, in that the refractive indexRefractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
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
Nonlinear optics is the branch of optics that describes the behavior of light in nonlinear media, that is, media in which the dielectric polarization P responds nonlinearly to the electric field E of the light...
effects to be seen at low powers. The nonlinearity can be enhanced further by using a slot waveguide, in which the high refractive index of the silicon is used to confine light into a central region filled with a strongly nonlinear polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
.
Kerr nonlinearity underlies a wide variety of optical phenomena. One example is four wave mixing, which has been applied in silicon to realise both optical parametric amplification and parametric wavelength conversion. Kerr nonlinearity can also cause modulational instability
Modulational instability
In the field of nonlinear optics, modulational instability is a phenomenon whereby deviations from an optical waveform are reinforced by nonlinearity, leading to the generation of spectral-sidebands and the eventual breakup of the waveform into a train of pulses.-Initial instability and...
, in which it reinforces deviations from an optical waveform, leading to the generation of spectral
Frequency spectrum
The frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...
-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 absorptionTwo-photon absorption
Two-photon absorption is the simultaneous absorption of two photons of identical or different frequencies in order to excite a molecule from one state to a higher energy electronic state. The energy difference between the involved lower and upper states of the molecule is equal to the sum of the...
(TPA), in which a pair of 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...
s can act to excite an electron-hole pair. This process is related to the Kerr effect, and by analogy with complex refractive index
Mathematical descriptions of opacity
When an electromagnetic wave travels through a medium in which it gets absorbed , it undergoes exponential decay as described by the Beer–Lambert law. However, there are many possible ways to characterize the wave and how quickly it is absorbed...
, can be thought of as the imaginary
Imaginary number
An imaginary number is any number whose square is a real number less than zero. When any real number is squared, the result is never negative, but the square of an imaginary number is always negative...
-part of a complex
Complex number
A complex number is a number consisting of a real part and an imaginary part. Complex numbers extend the idea of the one-dimensional number line to the two-dimensional complex plane by using the number line for the real part and adding a vertical axis to plot the imaginary part...
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
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
. 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 carriersCharge carriers in semiconductors
There are two recognized types of charge carriers in semiconductors. One of them is electrons, which carry negative electric charge. In addition, it is convenient to treat the traveling vacancies in the valence-band electron population as the second type of charge carriers, which carry a positive...
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
Ion implantation
Ion implantation is a materials engineering process by which ions of a material are accelerated in an electrical field and impacted into another solid. This process is used to change the physical, chemical, or electrical properties of the solid...
the silicon with helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads 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 waveguides (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
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
of carriers from the waveguide core.
A more advanced scheme for carrier removal is to integrate the waveguide into the intrinsic region
Intrinsic semiconductor
An intrinsic semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any significant dopant species present. The number of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities...
of a PIN diode
PIN diode
A PIN diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts....
, which is reverse biased 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
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
and current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
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.
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 amplificationRaman amplification
Raman amplification is based on the Stimulated Raman Scattering phenomenon, when a lower frequency 'signal' photon induces the inelastic scattering of a higher-frequency 'pump' photon in an optical medium in the nonlinear regime. As a result of this, another 'signal' photon is produced, with the...
, but is beneficial for narrowband devices such as Raman laser
Raman laser
The Raman laser is a byproduct of Raman scattering, discovered in 1928 by Nobel laureate Chandrasekhara Venkata Raman and Kariamanickam Srinivasa Krishnan in liquids and independently by Grigory Landsberg and Leonid Mandelshtam in crystals. When light hits a substance, it causes the atoms in the...
s. Consequently, all-silicon Raman lasers have been fabricated.
Solitons
The evolution of light through silicon waveguides can be approximated with a cubic Nonlinear Schrödinger equationNonlinear Schrödinger equation
In theoretical physics, the nonlinear Schrödinger equation is a nonlinear version of Schrödinger's equation. It is a classical field equation with applications to optics and water waves. Unlike the Schrödinger equation, it never describes the time evolution of a quantum state...
, which is notable for admitting sech-like soliton
Soliton
In mathematics and physics, a soliton is a self-reinforcing solitary wave that maintains its shape while it travels at constant speed. Solitons are caused by a cancellation of nonlinear and dispersive effects in the medium...
solutions. These optical solitons (which are also known in optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
) result from a balance between self phase modulation (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 University
Columbia University in the City of New York is a private, Ivy League university in Manhattan, New York City. Columbia is the oldest institution of higher learning in the state of New York, the fifth oldest in the United States, and one of the country's nine Colonial Colleges founded before the...
, Rochester, and Bath
University of Bath
The University of Bath is a campus university located in Bath, United Kingdom. It received its Royal Charter in 1966....
.