For applications in microelectronics, an extremely powerful technology platform based on silicon chips has been developed in the recent decades. This is now the basis of complex microprocessors, large memory circuits, and other digital and analog electronics. With the introduction of the silicon-on-insulator technology  it has been demonstrated that photonic functions can be integrated into this technology platform, so that silicon-based photonic integrated circuits became possible. Here, different kinds of optical components can be connected with each other using silicon waveguides . Such circuits could be used e.g. to establish very fast communication between circuit boards, between chips on a board, or even within single chips, e.g. connecting different cores of a microprocessor. There is a strong need for such fast communication links, because the rapid progress of microprocessors may soon be severely limited by the transmission bandwidth capabilities of electronic connections, made e.g. of copper. Optical data transmission allows for much higher data rates and would at the same time eliminate problems resulting from electromagnetic interference. The technology may also be useful for other areas of optical communications, such as fiber to the home.
Silicon photonics can also be considered from the viewpoint of photonics, which is so far normally based on other materials. (Silica = amorphous SiO2 is common in photonics, e.g. in silica fibers, but not elementary silicon.) The implementation of silicon-based photonic devices, maybe even electrically pumped silicon lasers and silicon amplifiers, could possibly lead to much smaller and much cheaper photonic devices, making accessible a range of applications which so far have been impossible already for reasons of too high cost.
Although the possible merits of silicon-based photonics are huge, there are also very substantial challenges for such a technology:
- Having an indirect bandgap, silicon is a very inefficient light emitter. Although various tricks have been developed to get around this, the laser or amplifier performance of silicon-based devices cannot compete with that for other approaches, based on, e.g., gallium arsenide or indium phosphide.
- The bandgap of silicon is also larger than desirable, making it impossible to detect light in the telecom spectral regions around 1.5 and 1.3 μm.
- Silicon has no χ(2) nonlinearity, making it impossible to realize electro-optic modulators with this material.
- The heat dissipated by a laser source on a chip might well be more than is convenient.
- Optical connections often require very precise alignment, which demands improved alignment technologies for efficient mass production.
It is possible to fabricate hybrid devices where the photonic functions are provided by structures made of III–V semiconductors (with a direct bandgap and electro-optic properties), such as indium phosphide, and these are placed on a silicon chip containing the bulk of the electronic components. One class of techniques is based on epitaxial regrowth procedures, which are complicated and often greatly reduce the yield. For that reason, hybrid devices tend to be expensive and are strongly limited in complexity. Another approach is to apply a sophisticated bonding process to combine a silicon chip containing waveguides with an indium phosphide chip providing the optical gain . Still, all-silicon solutions, arising from the “siliconization of photonics”, would probably be more suitable for widespread application.
State of Research
The following paragraphs briefly describe the current state of research concerning basic building blocks of silicon photonics:
- For guiding light in waveguides, silicon is suitable [1, 2]. There are e.g. rib waveguides with oxide cladding, exhibiting propagation losses of well below 1 dB/cm. The transparency range of silicon extends from ≈ 1.1 μm to the far-infrared region. The tight mode confinement allows sharp bends without excessive bend losses. It also enables the use of nonlinearities for certain functions, e.g. amplification via four-wave mixing. Efficient coupling to single-mode fibers, having much larger effective mode areas, is possible with nanotapers .
- For laser light sources and for amplifiers, the indirect bandgap of silicon is hardly usable. Some progress has been achieved with porous silicon and with silicon nanoparticles in silica, but the performance achieved can not compete with that of e.g. indium-phosphide-based devices. However, silicon allows for efficient Raman amplification, because the Raman gain coefficient of silicon is very high and the waveguides confine the mode to a very small area. Although a Raman laser [8, 18, 22] or amplifier  still requires an optical pump source, it can be useful for accessing longer wavelength regions, and possibly even to generate multiple wavelengths . Another approach is to provide the active function in a III–V semiconductor material (see above), which is bonded to a silicon waveguide structure; the evanescent field of the silicon waveguide can then be strong enough for efficient amplification . Also, it is possible to fabricate monolithic structures with germanium grown directly on silicon (Ge-on-Si technology), where doped germanium serves as the laser material .
- Silicon-based optical modulators can be realized with Mach–Zehnder interferometers and phase modulation via a change in carrier density : injecting carriers with an electrode changes the refractive index in one arm of the interferometer, which translates the phase change into a change in power transmission. Another possibility is to use a micro-ring resonator [19, 24]. Transmission bandwidths of multiple gigabits per second can be achieved with such devices. Very compact and energy-efficient devices can also be realized as electroabsorption modulators made with epitaxial germanium on silicon .
- A silicon photodetector (photodiode) is normally sensitive only for light with wavelengths below 1.1 μm, corresponding to the bandgap. Photodetectors for telecommunication wavelengths around 1.5 or 1.3 μm are possible with silicon–germanium alloys (SiGe) . Problems arise from the resulting lattice mismatch, which leads to crystal defects.
It is clear that an enormous amount of work, corresponding to huge capital investments, is still required before silicon photonics can be established as a key technology. However, the potential merits motivate big players such as Intel to pursue this development seriously. If it is successful, it can lead to a very powerful technology with huge benefits for photonics and microelectronics and their applications.
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This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) and software could become very valuable for your business!
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