There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Slow light with a remarkably low group velocity is a promising solution for buffering and time-domain processing of optical signals. It also offers the possibility for spatial compression of optical energy and the enhancement of linear and nonlinear optical effects. Photonic-crystal devices are especially attractive for generating slow light, as they are compatible with on-chip integration and room-temperature operation, and can offer wide-bandwidth and dispersion-free propagation. Here the background theory, recent experimental demonstrations and progress towards tunable slow-light structures based on photonic-band engineering are reviewed. Practical issues related to real devices and their applications are also discussed. The unique properties of wide-bandwidth and dispersion-free propagation in photonic-crystal devices have made them a good candidate for slow-light generation. This article gives the background theory of slow light, as well as an overview of recent experimental demonstrations based on photonic-band engineering.

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Slow light in photonic crystals

Recently, strongly modulated photonic crystals, fabricated by the state-of-the-art semiconductor nanofabrication process, have realized various novel optical properties. This paper describes the way in which they differ from other optical media, and clarifies what they can do. In particular, three important issues are considered: light confinement, frequency dispersion and spatial dispersion. First, I describe the latest status and impact of ultra-strong light confinement in a wavelength-cubic volume achieved in photonic crystals. Second, the extreme reduction in the speed of light is reported, which was achieved as a result of frequency dispersion management. Third, strange negative refraction in photonic crystals is introduced, which results from their unique spatial dispersion, and it is clarified how this leads to perfect imaging. The last two sections are devoted to applications of these novel properties. First, I report the fact that strong light confinement and huge light–matter interaction enhancement make strongly modulated photonic crystals promising for on-chip all-optical processing, and present several examples including all-optical switches/memories and optical logics. As a second application, it is shown that the strong light confinement and slow light in strongly modulated photonic crystals enable the adiabatic tuning of light, which leads to various novel ways of controlling light, such as adiabatic frequency conversion, efficient optomechanics systems, photon memories and photons pinning.

https://iopscience.iop.org/article/10.1088/0034-4885/73/9/096501/pdf

Manipulating light with strongly modulated photonic crystals

A system, method and apparatus provide the ability to detect a chemical in an analyte. To detect the chemical, the invention utilizes a laser having an open cavity. A photonic crystal lattice structure having a defect defines a suitable geometry for such a cavity (1000). The analyte is introduced directly into a high optical field of the cavity (1002). Thereafter, the cavity is pumped (1004) and an emission from the laser is used to detect the presence of the chemical (1006) in the analyte.

Photonic crystal laser sources for chemical detection

The efficient delivery of photons from light sources to photonic circuits is central to any fibre-optic or integrated optical system. Coupling light emitters to optical fibres or waveguides determines the photon flux available in, and therefore the performance of, photonic devices used in applications such as optical communication and information processing. Many solutions have been proposed to improve impedance matching in light-emitting diode-to-fibre or photonic-crystal cavity-to-waveguide systems; however, the efficient coupling of integrated light sources into nanophotonic circuits remains a challenge. Here, we propose an optically or electrically driven photonic structure that uses active semiconductor nanowires to light up photonic-crystal waveguides. The photonic crystal is used to either guide or filter out different colours of light as desired. In addition, we report an active nanowire-based optical structure that can generate two different colours of light and then send them in opposite directions. The hybrid nanowire/photonic-crystal waveguide represents a significant advance towards all-optical processing in nanoscale integrated photonic circuits and a new addition to the nanophotonic toolbox.

http://cml.harvard.edu/assets/nphoton.2008.180.pdf

A wavelength-selective photonic-crystal waveguide coupled to a nanowire light source

Straight single-line defect optical waveguides in photonic crystal slabs are designed by the finite difference time-domain method and fabricated into a silicon-on-insulator (SOI) wafer. By employing an airbridge structure, clear light propagation for both polarizations is observed without any leakage along the waveguide. This experimental result is well explained by photonic bands of pure guided modes. Minimum propagation loss is estimated to be 11 dB/mm. This value is lower than that reported so far for three-line-defect waveguides with an SOI slab structure and almost comparable to that for an index confinement waveguide with a rectangular Si core. This propagation loss is dominated by the scattering loss by some irregularities. However, photonic crystal waveguides have the possibility of an essential lower scattering loss than in the index confinement waveguide because of the inhibition of radiation modes by the photonic bandgap.

T. Iwai

Papers

Light propagation characteristics of straight single-line-defect waveguides in photonic crystal slabs fabricated into a silicon-on-insulator substrate