Figure 2: Photonic filters
(a) This photonic waveguide formed from a thin silicon-nitride membrane
contains a triangular lattice of air holes separated by 300 nm. The air gap
below the waveguide leads to stronger photon confinement. (b) A plot of
transmission versus wavelength for the device shows that it has a photonic band
gap between 725 nm and 825 nm if the electric field associated with the
electromagnetic wave is perpendicular to the direction in which the light wave
propagates (red). In contrast, the light can pass freely through the material
when the magnetic field is perpendicular to the direction of travel (blue). In
this case, the photonic band gap is said to be incomplete. (c) A photonic
crystal made from a piece of silicon approximately 1 x 1.5 cm viewed from
above. Laser radiation with a wavelength that lies inside the band-gap range
enters the structure from the left and is completely blocked. (d) Light
at longer wavelengths outside the band gap can pass through the structure.