Computational Design of Nanophotonic Devices, and The Emperor's New Cloak

ABSTRACT:

Photonic crystal structures that strongly confine and manipulate light on wavelength and sub-wavelength scales have many potential applications in telecommunications, optoelectronics, sensing, and energy conversion.

Most photonic crystal device designs are based on the photonic bandgap effect, whereby perfectly periodic dielectric media are used as base structures for 2D and 3D devices, with defects such as waveguides or resonators built into these otherwise perfect photonic crystals.

However, simple finite element based electromagnetics calculations illustrate the point that structural disorder can in some cases improve device characteristics relative to perfectly periodic photonic crystal designs. Based on this observation, we have developed a nonlinear topology optimization approach for designing improved photonic crystal structures, so that intentionally tailored disorder can be used to enhance device geometries with well-defined objective functions. Our approach uses nonlinear programming techniques and design sensitivity analysis, and an innovative geometry projection method, in combination with finite element electromagnetics calculations. Here the approach is described and demonstrated first for the simple case of a 2D waveguide geometry, and then for the case of a 3D photonic crystal laser device, where the optimized design leads to a significantly enhanced quality factor relative to the conventional photonic crystal design. The experimental realization of the optimized photonic crystal laser device is also discussed. Finally, inspired by the disorder-based photonic crystal device design work, the results of a new study in the area of passive nanophotonics for optical cloaking are presented. In this simple feasibility analysis, a multilayer silicon photonic crystal structure is shown to be potentially useful for guiding light around a circular container. Approximate cloaking can be achieved in the visible spectrum using this strategy, with properties rivaling the results of other proposed methods involving meta-materials.