Photonic Crystal Devices for Wavelength-Division-Multiplexing and Optical Modulation

This paper reviews photonic crystal (PC) based demultiplexers, and briefly reports our latest experimental achievement in ultra-compact, power-efficient silicon photonic crystal waveguide (PCW) modulators. We review the modeling techniques for photonic crystal superprism devices, which utilize anomalous refraction on a photonic crystal surface for wavelength demultiplexing. The finite difference time domain method tends to be time consuming for the superprism devices as such devices demand fine spatial grids to resolve fine wavelength difference. Other theoretical methods suffer a variety of other drawbacks. A general, efficient PC refraction theory that can handle any surface orientation is needed for scientific research and device design. We review a rigorous PC refraction theory that we recently developed for these needs. Essentially, the refraction problem can be rigorously solved by computing the electromagnetic field in only a single cell on the surface. A new concept, surface-orientation-dependent eigenmode degeneracy, is introduced to explain certain subtle effect that occurs when the surface orientation undergoes a slight change. In addition, the transmission of a Gaussian beam or other realistic beam profiles is discussed. A complete theoretical framework of the photonic crystal refraction and transmission has thus been established. The theory has been applied to design a high channel-count dense WDM demultiplexer with 3dB or lower losses. Lastly, a silicon PCW Mach-Zehnder modulator with an 80-micron interaction length is reported. The slow group velocity in PCWs is exploited to enhance the modulation efficiency and reduce the peak drive current to 0.15mA at a modulation depth over 90%.