Mirroring effect
White light enters the waveguide, propagating down the strip of silicon. The selected wavelength is trapped in the gap between the fourth and fifth holes, undergoing internal reflection -- a "mirroring effect." As it bounces back and forth, the light builds up optical power and emerges from the other end of the waveguide as a strong signal. A very narrow spectrum of light centered at 1.54 µm emerges. The device thus can transmit light at a chosen frequency or act as a switch, picking out one frequency from a number of incoming wavelengths.
The researchers said their project demonstrates that multiple reflections caused by the very high index of refraction contrast in the silicon device prevent light from propagating over a wide band of wavelengths. They also noted that the device operates between 1.3 and 1.7 µm, the wavelengths used in fiber optic communications.