Silicon photonics
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-micron precision, into microphotonic components.[4] These operate in the infrared, most commonly at the 1.55 micron wavelength used by most fiber optic telecommunication systems.[1] 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.[1] Consequently, silicon photonics is being actively researched by many electronics manufacturers including IBM and Intel, who see it is a means for keeping on track with Moore's Law, by using optical interconnects to provide faster data transfer both between and within microchips.[6][7]
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.[8] The presence of nonlinearity is of fundamental importance, as it enables light to interact with light,[9] 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.[10][11][12]
Applications
Optical interconnects
Future progress in computer technology (and the continuation of Moore's Law) is becoming increasingly dependent on ultra-fast data transfer between and within microchips.[13] High speed optical interconnects 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.[1][14] 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."[7] IBM also have an extensive research program in the field.[6]
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.[15] Another possibility is the all-silicon Raman laser, in which the silicon itself acts as the lasing medium.[16]
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.[17] By using an optical ring resonator with a built in PIN diode, data transmission rates of 12.5 Gbit/s have been achieved.[18] By constructing devices where the electrical signal co-moves with the light, data rates of 30 Gbit/s have been achieved.[19][20]
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 junctions (with germanium as the semiconductor) have been integrated into silicon waveguides.[21] More recently, silicon-germanium avalanche photodiodes capable of operating at 40 Gbit/s have been fabricated.[22][23]
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.[24] 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][25] An important example is all-optical switching, whereby the routing of optical signals is directly controlled by other optical signals.[26] Another example is all-optical wavelength conversion.[27]
Physical properties
Optical guiding and dispersion tailoring
Silicon is transparent to infrared light with wavelengths above about 1.1 microns.[28] Silicon also has a very high refractive index, of about 3.5.[28] 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.[8] 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,[8] 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.[8] 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 microns, 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.[29][30] Anomalous dispersion is significant, as it is a prerequisite for soliton propagation, and modulational instability.[31]
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 [32]), 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.[33]
Kerr nonlinearity
Silicon has a focusing Kerr nonlinearity, in that the refractive index increases with optical intensity.[8] 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.[10] 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.[34]
Kerr nonlinearity underlies a wide variety of optical phenomena.[31] One example is four wave mixing, which has been applied in silicon to realise both optical parametric amplification [35] and parametric wavelength conversion.[27] 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.[36] Another example (as described below) is soliton propagation.
Two photon absorption
Silicon exhibits two photon absorption (TPA), in which a pair of photons can act to excite an electron-hole pair.[8] 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.[8] At the 1.55 micron telecommunication wavelength, this imaginary part is approximately 10% of the real part.[37]
The influence of TPA is highly disruptive, as it both wastes light, and generates unwanted heat.[38] It can be mitigated, however, either by switching to longer wavelengths (at which the TPA to Kerr ratio drops),[39] or by using slot waveguides (in which the internal nonlinear material has a lower TPA to Kerr ratio).[34] Alternatively, the energy lost through TPA can be partially recovered (as is described below) by extracting it from the generated charge carriers.[40]
Free charge carrier interactions
The free charge carriers within silicon can both absorb photons and change its refractive index.[41] 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.[42] 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.[43]
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.[44] 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.[40] 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.[17][18][19]
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.[8] Consequently, all-silicon Raman lasers have been fabricated.[16]
Solitons
The evolution of light through silicon waveguides can be approximated with a cubic Nonlinear Schrödinger equation,[8] which is notable for admitting sech-like soliton solutions.[45] 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.[31] Such solitons have been observed in silicon waveguides, by groups at the universities of Columbia,[10] Rochester,[11] and Bath.[12]
External links
- IBM's page on silicon integrated nanophotonics
- Intel's page on silicon photonics
- Uk based project website on silicon photonics
- European project website on silicon photonics
References
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