Photonics

Axel Scherer

Photonics has recently become an attractive alternative to electronics for communications and information processing. Devices that use photons rather than electrons as information carriers can benefit from higher speeds and reduced cross talk between optical channels. Miniaturization of compact optical components such as resonators, waveguides, and interferometers has become very desirable. At the same time, microfabrication has emerged as a powerful technology that enables the construction of sub-100nm structures in a reproducible and controllable manner. The same technology that was driven by the continuing desire to miniaturize electronic components on silicon microchips has now evolved to a precision that allows us to control the flow of photons.

Fully optical chips would deliver the ultimate in speed and would benefit from lower heat and fewer power problems. A significant bottleneck in current networks is the switching from optical to electrical media for processing and routing, and then back from electrical to optical. If this bottleneck can be reworked to smoothly avoid the OEO transformation, tremendous gains can be attained. The need for data transfer and routing with high band-width is also compelling for many military applications, where optical solutions to high-frequency signal processing and routing offer lower power dissipation and higher bandwidth, which produce more robust and compact systems. Size, weight, and immunity to radio-frequency interference are particularly important for mobile communications, sensors, and radar systems.

Recent technological advances in silicon optoelectronics have highlighted the need for inexpensive multiwavelength light sources on silicon photonics. The use of silicon CMOS electronics for the multiplexing, control, and routing of many parallel optical signal channels on a silicon chip may have been an inevitable outcome of the lack of adequate electronic interconnection solutions for next-generation microprocessors. Silicon on insulator (SOI) waveguiding optics may produce much less expensive alternatives to more conventional telecommunications and data communications platforms such as GaAs, InP, and Lithium Niobate (LiNbO₅).

The most important disadvantage of using silicon optoelectronics has been the lack of sufficient gain for signal amplification and efficient on-chip light generation. The vision of an on-chip silicon laser has so far been elusive, although much time and effort have already been invested in various promising silicon light-amplification strategies, such as porous silicon and erbiumdoped silicon waveguides.

Silicon nanophotonic components are so small that components for 1,000-wavelength division multiplexing channels could easily fit in a corner of an electronic chip. Optical nanodevices can now be constructed from the standard electronic semiconductor materials, such as silicon on insulator (SOI), Gallium Arsenide (GaAs), and Indium Phosphide (InP). By combining the need for integrated photonics with the capabilities offered by high-resolution microfabrication, the field of nanophotonics has emerged. Optical devices that have traditionally been constructed of glass and lithium niobate can now be scaled down in silicon, GaAs, or InP. Ultrasmall optical systems can also be integrated, thus realizing for the first time the dream of large-scale, multifunctional, all-optical chips for information processing. Moreover, because nanophotonics devices are constructed from standard electronic materials, such devices can be integrated side by side with electronic components, enabling the construction of hybrid systems of higher complexity.

The emergence of silicon nanophotonic technology in SOI wafers has made a profusion of optical components available on chip and essentially at zero marginal cost. These components include resonators, filters, waveguides, modulators, and (with the availability of germanium) detectors. To these must be added the full functionality of CMOS electronic technology, particularly high-quality transistors.

Thus there would be a great advantage in a mode-locked source that provides steady output at many frequencies simultaneously. In passive optical filtering and routing chips, waveguide losses, insertion losses, and resonator losses all contribute to a deterioration of the input signal. Thus, it is desirable to include gain in such chips so that the routed signal is amplified before coupling out of the routing switch. Good-quality mode locking requires high-Q cavities and a good optical modulator, both functions that are now readily fabricated on an SOI silicon chip. Thus many of the ingredients for highly dense microoptical circuits and for an internal mode-locked light source in silicon SOI are already availableexcept for optical gain. A goal, then, is the large-scale integration of multiwavelength sources within a single integrated chip.

Recently, optical coupling to disk and ring resonators has become an extremely effective way of fabricating add/drop filters. Much of the work has been demonstrated in glass waveguides, both monolithically and through micromechanical coupling of microspheres close to thinned-down optical fibers. An 8x8 crossbar router was recently demonstrated by Little et al. in a planar waveguide geometry in which high-index glass disks were aligned above the waveguide layers. Very high Qs and correspondingly narrow spectral filters have been demonstrated.

The minimum feature size required to couple the resonator disk to the waveguide, approximately 150500nm, can already be obtained with high-resolution lithography, UV lithography, and electron beam lithography. Other lithography techniques, such as embossing, imprinting, and molding, will undoubtedly follow for the lower-cost development of high-resolution single-level lithography.

The photonic crystal (PC) is one of the platforms that can enable the miniaturization of photonic devices and their large-scale integration. These microfabricated periodic nanostructures can be designed to form frequency bands (photonic band gaps) within which the propagation of electromagnetic waves is forbidden, irrespective of the propagation direction. One of the most attractive planar photonic crystal devices is a compact and efficient nanocavity. This is due to the extraordinary feature of Planar Photonic Crystals (PPCs) to localize high electromagnetic fields into very small volumes for a long period of time. Moreover, PC cavities can be engineered to concentrate light in the air, and thus they are natural candidates for the investigation of interaction between light and matter on a nanoscale level. Such nanoscale optical resonators are of interest for a number of applications, of both practical and scientific importance.

Ultrasmall quantities of biochemical reagents can be placed in the air region where field intensity is the strongest, and their (strong) influence on the optical signature of the resonator can be monitored. This can lead to realization of integrated spectroscopy systems (for example, on-chip Raman spectroscopy). PC nanocavities can have high Q factors (>10,000) and can be highly integrated (less than 5mm apart), something that makes them promising candidates for realization of channel drop filters in dense wavelength-division multiplex systems.