Engineering light at the sub-wavelength scale using silicon photonics

As a result of the evolution semiconductor fabrication tools and methods over several decades, it now possible to routinely design and make optical devices with features comparable to or smaller than the wavelength of the light that propagates through these structures. This paper will review some silicon optical structures with critical features at these extremely short length scales. For example it becomes possible to create segmented waveguide structures with optical properties that can be tuned continuously between those of the cladding and waveguide core, using lithographic patterning rather than varying etch depth. Using thin high index contrast waveguides and the correct polarization, the optical electric field profiles can be shaped to maximize the coupling to molecular monolayers or cladding layers with specific functionality. Examples are given from our recent work on optical biosensors chips which employ grating couplers made by sub-wavelength digital patterning, and use waveguides optimized for coupling to molecular monolayers.