Introduction

Electronics and photonics share many attributes. In particular, both these areas of technology deal with information, so their size is not determined by some physical quantity that has to be received, stored, manipulated, and transmitted. Both electronics and photonics can therefore potentially benefit from miniaturization, which leads to lower cost of production, transportation, installation, and maintenance. In comparison to electronics, however, photonic devices are significantly more challenging to miniaturize. The reason is that photons, being bosons, interact only weakly with matter and are therefore difficult to store and manipulate. Even optical detectors require relatively large volumes to effectively absorb light, and larger still are other types of photonic devices that change some aspect of the photons without absorbing them.

One way to ameliorate this situation is to use coherence to increase the interaction between photons and matter. Photonic crystals, with their periodic variations in the dielectric constant, do exactly that. In a photonic crystal, relatively weak reflections or scattering from a periodic array add in-phase to create strong reflections, which in turn can set up optical resonances, or modes, in the photonic crystal. It follows that the functional output of a photonic crystal device relies on interference between the incoming field, the coherent reflections, and the resonant modes.

This qualitative description points to the usefulness of photonic crystals in miniaturized and tunable devices. By enhancing photon-matter interaction, photonic crystals enable optical functions to be performed in smaller volumes than devices that rely on traditional optical principles. This is true across the spectrum of optical devices, including lenses, mirrors, detectors, waveguides, modulators, resonators, and lasers. In fact there are very few, if any, optical functions that cannot be implemented and miniaturized through the aid of photonic crystals.

In addition, photonic crystals offer a number of mechanisms for tuning:

1. The refractive index of the crystal lattice and/or the unit cells can be changed by the plasma effect, the thermo-optic effect, or by the electro-optic effect.

2. The refractive index of the medium surrounding the photonic crystal can be tuned through the same effects or by immersing the photonic crystal in a liquid crystal or other tunable medium.

3. The boundary conditions of the photonic crystal can be altered to tune its properties.

4. The structure of the photonic crystal itself, its lattice or its unit cells, can be mechanically stressed or displaced.