Silicon Photonics

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 [3] 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 [1]. 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 optical materials. (Fused silica = amorphous SiO₂ is common in photonics, e.g. in silica fibers, but much less 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.

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.

Technological Challenges

Although the possible merits of silicon-based photonics are huge, there are also very substantial challenges for such a technology:

• Having an indirect band gap, 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 band gap 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 <$\chi^{(2)}$> 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.

It is possible to fabricate hybrid devices where the photonic functions are provided by structures made of III–V semiconductors (with a direct band gap of suitable width 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. 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 [17]. Here, precise alignment between the two chips is required.

Both technical approaches, leading to silicon hybrid devices, tend to be expensive and are strongly limited in complexity. Therefore, all-silicon solutions, arising from the “siliconization of photonics”, would be more suitable for widespread application. This is also tried for a range of mid-infrared applications such as biochemical sensors.

State of Research

The following paragraphs briefly describe the current state of research concerning basic building blocks of silicon photonics:

Guiding Light

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 [7].

Laser Sources and Amplifiers

For laser light sources and for amplifiers, the indirect band gap 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.

On the other hand, 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, 19, 23] or amplifier [22] still requires an optical pump source, it can be useful for accessing longer wavelength regions, and possibly even to generate multiple wavelengths [23].

Another available amplification mechanism is parametric amplification based on the <$\chi^{(3)}$> nonlinearity of silicon [18]. SOI channel waveguides provide good field confinement and thus substantial broadband parametric gain, at least when applying relatively intense (multi-watt) pump pulses.

Still 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 [17]. 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 [26].

If the realization of a laser source on a silicon chip is not feasible, one may use an external fiber-coupled laser diode and couple the light to the silicon chip with a single-mode fiber, using a nanotaper [7].

Modulation of Light

Silicon-based optical modulators can be realized with Mach–Zehnder interferometers and phase modulation via a change in carrier density [12]: 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 [20, 25]. 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 [24].

Photodetection

A silicon photodetector (photodiode) is normally sensitive only for light with wavelengths below 1.1 μm, corresponding to the band gap. Photodetectors for telecommunication wavelengths around 1.5 or 1.3 μm are possible with silicon–germanium alloys (SiGe) [13]. Problems arise from the resulting lattice mismatch, which leads to crystal defects.

A new kind of silicon-based photodetector is the plasmonic internal-photoemission detector (PIPED). It is based on plasmonic effects at a metal–semiconductor–metal (MSM) interface, which forms an optical waveguide: light is absorbed in a metal barrier, generates surface plasmon polaritons (SPP); those generate “hot electrons” in the metal, which then can tunnel through the semiconductor barrier when a voltage is applied between the electrodes. Such photodetectors can be made very small and can have a very high detection bandwidth for light e.g. at 1550 nm wavelength [29].

Terahertz Silicon Photonics

Silicon technology can not only be used for working with light, but also with terahertz radiation. For example, one can realize plasmonic internal-photoemission detectors (PIPEDs) on the silicon photonics platform, which can be used as terahertz detectors [30].

More to Learn

Encyclopedia articles:

• photonics
• photonic integrated circuits
• integrated optics
• optoelectronics
• channel waveguides