Silicon photonics
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.
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.
[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.
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.
[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.
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.
[6][35][36] In 2006, Intel Senior Vice President - and future CEO - Pat Gelsinger stated that, "Today, optics is a niche technology.
[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.
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.
The technology does not directly replace existing cables in that it requires a separate circuit board to interconvert electrical and optical signals.
[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.
Construction can be greatly simplified by fabricating the optical and electronic parts on the same chip, rather than having them spread across multiple components.
[51] Silicon microphotonics can potentially increase the Internet's bandwidth capacity by providing micro-scale, ultra low power 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.
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.
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.
[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.
[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.
[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.
