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Perfect Channel Drop Filters
An important device for optical communications and in many other applications is a channel-drop filter. Given a collection of signals propagating down a waveguide (called the bus waveguide), a channel-drop filter picks out one small wavelength range (channel) and reroutes (drops) it into another waveguide (called the drop waveguide).
For an example of how this is useful, imagine an optical telephone line carrying a number of conversations simultaneously in different wavelength bands (i.e. using wavelength division multiplexing). Each conversation needs to be picked out of the line and routed to its destination, and to separate a conversation you need a channel-drop filter.
It turns out that by using photonic crystals, one can construct a perfect channel drop filter--that is, one which reroutes the desired channel into the drop waveguide with 100% transfer efficiency (i.e. no losses, reflection, or crosstalk), while leaving all other channels in the bus waveguide propagating unperturbed.
For a demonstration of how this works, we have put together a couple of animations (in QuickTime format) showing a perfect channel-drop filter in operation. Both of these animations are for two-dimensional systems, but the same principle works in three dimensions.
Here, we see a signal of the (tunable) drop wavelength propagating down one waveguide that is rerouted into a second waveguide by a channel-drop filter in the center. The filter consists of a hexapole defect in the photonic crystal, and the channel is dropped in the backwards direction. The crystal itself consists of a square lattice of dielectric columns.