Photonics is the science and technology of generating and controlling the flow of photons. It is persistently in the scientific limelight; one can read of it daily in the popular and technical science press, and it receives enormous levels of funding. Why all the fuss? Because photonics promises to underpin imminent technological revolutions in signal processing, communications, computing, astrophysics, biology, and even medicine.

The photonic crystal is the working currency of photonics. It is a regular structure that offers propagating photons a periodic variation in refractive index. That periodicity can range in size from centimeters for some applications, down to tens of nanometers for others. Maxwell’s equations coupled with the periodic boundary conditions imposed by the crystalline structure predict that the propagation of electromagnetic radiation through a photonic crystal will be inhibited if the periodicity has a length scale similar to the radiation’s wavelength. And herein lies the technological allure:...

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