Silicon photonics is the study and application of photonic systems which use silicon as an optical medium. [1] [2] [3] [4] [5] The silicon is usually patterned with sub-micrometre precision, into microphotonic components. [4] These operate in the infrared, most commonly at the 1.55 micrometre wavelength used by most fiber optic telecommunication systems. [6] 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). [4] [5]
Silicon photonic devices can be made using existing semiconductor fabrication 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 and electronic components are integrated onto a single microchip. [6] Consequently, silicon photonics is being actively researched by many electronics manufacturers including IBM and Intel, 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. [7] [8] [9]
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. [10] The presence of nonlinearity is of fundamental importance, as it enables light to interact with light, [11] 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 unique guiding properties, they can be used for communications, interconnects, biosensors, [12] [13] and they offer the possibility to support exotic nonlinear optical phenomena such as soliton propagation. [14] [15] [16]
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. [17] Modulators can consist of both forward-biased PIN diodes, which generally generate large phase-shifts but suffer of lower speeds, [18] as well as of reverse-biased p–n junctions. [19] A prototype optical interconnect with microring modulators integrated with germanium detectors has been demonstrated. [20] [21] 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. [22] A similar device has been demonstrated as well in bulk CMOS rather than in SOI. [23] [24]
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. [25] [26] Most detectors use a p–n junction for carrier extraction, however, detectors based on metal–semiconductor junctions (with germanium as the semiconductor) have been integrated into silicon waveguides as well. [27] More recently, silicon-germanium avalanche photodiodes capable of operating at 40 Gbit/s have been fabricated. [28] [29] Complete transceivers have been commercialized in the form of active optical cables. [30]
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 [31] and datacom applications, [32] [33] 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. [34] Optical interconnects may provide a way forward, and silicon photonics may prove particularly useful, once integrated on the standard silicon chips. [6] [35] [36] In 2006, Intel 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." [8] In 2010 Intel demonstrated a 50 Gbit/s connection made with silicon photonics. [37]
The first microprocessor with optical input/output (I/O) was demonstrated in December 2015 using an approach known as "zero-change" CMOS photonics. [38] This is known as fiber-to-the-processor. [39] 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 source is required. [40] 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 (such as indium phosphide) as the lasing medium. [41] Other devices include all-silicon Raman laser [42] or an all-silicon Brillouin lasers [43] 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. [7] [44] 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. [45]
Graphene 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. [46]
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. [47] 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. [3] [48] An important example is all-optical switching, whereby the routing of optical signals is directly controlled by other optical signals. [49] Another example is all-optical wavelength conversion. [50]
In 2013, a startup company named "Compass-EOS", based in California and in Israel, was the first to present a commercial silicon-to-photonics router. [51]
Silicon microphotonics can potentially increase the Internet'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, [52] Kotura, NTT, Fujitsu 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. [53]
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 display. [54]
Silicon photonics has been used in artificial intelligence inference processors that are more energy efficient than those using conventional transistors. This can be done using Mach-Zehnder interferometers (MZIs) which can be combined with nanoelectromechanical systems to modulate the light passing though it, by physically bending the MZI which changes the phase of the light. [55] [56] [57]
Silicon is transparent to infrared light with wavelengths above about 1.1 micrometres. [58] Silicon also has a very high refractive index, of about 3.5. [58] 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. [10] Single mode propagation can be achieved, [10] 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 is possible to tailor the dispersion to have desired properties, which is of crucial importance to applications requiring ultrashort pulses. [10] 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. [59] [60] [61] Anomalous dispersion is significant, as it is a prerequisite for soliton propagation, and modulational instability. [62]
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 [63] ), 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. [4] [5] 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. [64] Silicon photonics have also been built with silicon nitride as the material in the optical waveguides. [65] [66]
Silicon has a focusing Kerr nonlinearity, in that the refractive index increases with optical intensity. [10] 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. [14] 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. [67]
Kerr nonlinearity underlies a wide variety of optical phenomena. [62] One example is four wave mixing, which has been applied in silicon to realise optical parametric amplification, [68] parametric wavelength conversion, [50] and frequency comb generation., [69] [70]
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. [71] Another example (as described below) is soliton propagation.
Silicon exhibits two-photon absorption (TPA), in which a pair of photons can act to excite an electron-hole pair. [10] 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. [10] At the 1.55 micrometre telecommunication wavelength, this imaginary part is approximately 10% of the real part. [72]
The influence of TPA is highly disruptive, as it both wastes light, and generates unwanted heat. [73] It can be mitigated, however, either by switching to longer wavelengths (at which the TPA to Kerr ratio drops), [74] or by using slot waveguides (in which the internal nonlinear material has a lower TPA to Kerr ratio). [67] Alternatively, the energy lost through TPA can be partially recovered (as is described below) by extracting it from the generated charge carriers. [75]
The free charge carriers within silicon can both absorb photons and change its refractive index. [76] 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. [77] 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. [78]
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. [79] 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. [75] 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. [18] [19] [80]
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 layer on a thin silicon film. [81] Second-order nonlinear phenomena can be exploited for optical modulation, spontaneous parametric down-conversion, parametric amplification, ultra-fast optical signal processing and mid-infrared 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. [82] So far, however, experimental demonstrations are based only on designs which are not phase matched. [83] It has been shown that phase matching can be obtained as well in silicon double slot waveguides coated with a highly nonlinear organic cladding [84] and in periodically strained silicon waveguides. [85]
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. [10] 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. [42]
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. 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. [86] [87] Stimulated Brillouin scattering has been used to make narrowband optical amplifiers [88] [89] [90] as well as all-silicon Brillouin lasers. [43] 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. [91] For instance, besides in silicon waveguides the optomechanical coupling has also been demonstrated in fibers [92] and in chalcogenide waveguides. [93]
The evolution of light through silicon waveguides can be approximated with a cubic Nonlinear Schrödinger equation, [10] which is notable for admitting sech-like soliton solutions. [94] 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. [62] Such solitons have been observed in silicon waveguides, by groups at the universities of Columbia, [14] Rochester, [15] and Bath. [16]
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 typically observed only at very high light intensities (when the electric field of the light is >108 V/m and thus comparable to the atomic electric field of ~1011 V/m) such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the superposition principle no longer holds.
An optical amplifier is a device that amplifies an optical signal directly, without the need to first convert it to an electrical signal. An optical amplifier may be thought of as a laser without an optical cavity, or one in which feedback from the cavity is suppressed. Optical amplifiers are important in optical communication and laser physics. They are used as optical repeaters in the long distance fiber-optic cables which carry much of the world's telecommunication links.
A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic lattices of semiconductors affect their conductivity of electrons. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, as artificially produced, promise to be useful in a range of applications.
An optical ring resonator is a set of waveguides in which at least one is a closed loop coupled to some sort of light input and output. The concepts behind optical ring resonators are the same as those behind whispering galleries except that they use light and obey the properties behind constructive interference and total internal reflection. When light of the resonant wavelength is passed through the loop from the input waveguide, the light builds up in intensity over multiple round-trips owing to constructive interference and is output to the output bus waveguide which serves as a detector waveguide. Because only a select few wavelengths will be at resonance within the loop, the optical ring resonator functions as a filter. Additionally, as implied earlier, two or more ring waveguides can be coupled to each other to form an add/drop optical filter.
A Raman laser is a specific type of laser in which the fundamental light-amplification mechanism is stimulated Raman scattering. In contrast, most "conventional" lasers rely on stimulated electronic transitions to amplify light.
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 guides, and liquid waveguides.
Nanophotonics or nano-optics is the study of the behavior of light on the nanometer scale, and of the interaction of nanometer-scale objects with light. It is a branch of optics, optical engineering, electrical engineering, and nanotechnology. It often involves dielectric structures such as nanoantennas, or metallic components, which can transport and focus light via surface plasmon polaritons.
A fiber laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium. They are related to doped fiber amplifiers, which provide light amplification without lasing.
A frequency comb or spectral comb is a spectrum made of discrete and regularly spaced spectral lines. In optics, a frequency comb can be generated by certain laser sources.
A photonic integrated circuit (PIC) or integrated optical circuit is a microchip containing two or more photonic components that form a functioning circuit. This technology detects, generates, transports, and processes light. Photonic integrated circuits use photons as opposed to electrons that are used by electronic integrated circuits. The major difference between the two is that a photonic integrated circuit provides functions for information signals imposed on optical wavelengths typically in the visible spectrum or near-infrared (850–1650 nm).
In optics, a supercontinuum is formed when a collection of nonlinear processes act together upon a pump beam in order to cause severe spectral broadening of the original pump beam, for example using a microstructured optical fiber. The result is a smooth spectral continuum. There is no consensus on how much broadening constitutes a supercontinuum; however researchers have published work claiming as little as 60 nm of broadening as a supercontinuum. There is also no agreement on the spectral flatness required to define the bandwidth of the source, with authors using anything from 5 dB to 40 dB or more. In addition the term supercontinuum itself did not gain widespread acceptance until this century, with many authors using alternative phrases to describe their continua during the 1970s, 1980s and 1990s.
A slot-waveguide is an optical waveguide that guides strongly confined light in a subwavelength-scale low refractive index region by total internal reflection.
A subwavelength-diameter optical fibre is an optical fibre whose diameter is less than the wavelength of the light being propagated through it. An SDF usually consists of long thick parts at both ends, transition regions (tapers) where the fibre diameter gradually decreases down to the subwavelength value, and a subwavelength-diameter waist, which is the main acting part. Due to such a strong geometrical confinement, the guided electromagnetic field in an SDF is restricted to a single mode called fundamental. In usual optical fibres, light both excites and feels shear and longitudinal bulk elastic waves, giving rise to forward-guided acoustic wave Brillouin scattering and backward-stimulated Brillouin scattering. In a subwavelength-diameter optical fibre, the situation changes dramatically.
Michal Lipson is an American physicist known for her work on silicon photonics. A member of the National Academy of Sciences since 2019, Lipson was named a 2010 MacArthur Fellow for contributions to silicon photonics especially towards enabling GHz silicon active devices. Until 2014, she was the Given Foundation Professor of Engineering at Cornell University in the school of electrical and computer engineering and a member of the Kavli Institute for Nanoscience at Cornell. She is now the Eugene Higgins Professor of Electrical Engineering at Columbia University. In 2009 she co-founded the company PicoLuz, which develops and commercializes silicon nanophotonics technologies. In 2019, she co-founded Voyant Photonics, which develops next generation lidar technology based on silicon photonics. In 2020 Lipson was elected the 2021 vice president of Optica, and serves as the Optica president in 2023.
The Mamyshev 2R regenerator is an all-optical regenerator used in optical communications. In 1998, Pavel V. Mamyshev of Bell Labs proposed and patented the use of the self-phase modulation (SPM) for single channel optical pulse reshaping and re-amplification. More recent applications target the field of ultrashort high peak-power pulse generation.
Optical rogue waves are rare pulses of light analogous to rogue or freak ocean waves. The term optical rogue waves was coined to describe rare pulses of broadband light arising during the process of supercontinuum generation—a noise-sensitive nonlinear process in which extremely broadband radiation is generated from a narrowband input waveform—in nonlinear optical fiber. In this context, optical rogue waves are characterized by an anomalous surplus in energy at particular wavelengths or an unexpected peak power. These anomalous events have been shown to follow heavy-tailed statistics, also known as L-shaped statistics, fat-tailed statistics, or extreme-value statistics. These probability distributions are characterized by long tails: large outliers occur rarely, yet much more frequently than expected from Gaussian statistics and intuition. Such distributions also describe the probabilities of freak ocean waves and various phenomena in both the man-made and natural worlds. Despite their infrequency, rare events wield significant influence in many systems. Aside from the statistical similarities, light waves traveling in optical fibers are known to obey the similar mathematics as water waves traveling in the open ocean, supporting the analogy between oceanic rogue waves and their optical counterparts. More generally, research has exposed a number of different analogies between extreme events in optics and hydrodynamic systems. A key practical difference is that most optical experiments can be done with a table-top apparatus, offer a high degree of experimental control, and allow data to be acquired extremely rapidly. Consequently, optical rogue waves are attractive for experimental and theoretical research and have become a highly studied phenomenon. The particulars of the analogy between extreme waves in optics and hydrodynamics may vary depending on the context, but the existence of rare events and extreme statistics in wave-related phenomena are common ground.
A hybrid plasmonic waveguide is an optical waveguide that achieves strong light confinement by coupling the light guided by a dielectric waveguide and a plasmonic waveguide. It is formed by separating a medium of high refractive index from a metal surface by a small gap.
Plasmonics or nanoplasmonics refers to the generation, detection, and manipulation of signals at optical frequencies along metal-dielectric interfaces in the nanometer scale. Inspired by photonics, plasmonics follows the trend of miniaturizing optical devices, and finds applications in sensing, microscopy, optical communications, and bio-photonics.
Integrated quantum photonics, uses photonic integrated circuits to control photonic quantum states for applications in quantum technologies. As such, integrated quantum photonics provides a promising approach to the miniaturisation and scaling up of optical quantum circuits. The major application of integrated quantum photonics is Quantum technology:, for example quantum computing, quantum communication, quantum simulation, quantum walks and quantum metrology.
Kerr frequency combs are optical frequency combs which are generated from a continuous wave pump laser by the Kerr nonlinearity. This coherent conversion of the pump laser to a frequency comb takes place inside an optical resonator which is typically of micrometer to millimeter in size and is therefore termed a microresonator. The coherent generation of the frequency comb from a continuous wave laser with the optical nonlinearity as a gain sets Kerr frequency combs apart from today's most common optical frequency combs. These frequency combs are generated by mode-locked lasers where the dominating gain stems from a conventional laser gain medium, which is pumped incoherently. Because Kerr frequency combs only rely on the nonlinear properties of the medium inside the microresonator and do not require a broadband laser gain medium, broad Kerr frequency combs can in principle be generated around any pump frequency.
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