Light trapping and absorption optimization in certain thin-film photonic crystal architectures

Abstract

We demonstrate two orders of magnitude enhancement of light absorption in certain thin-film photonic crystal (PC) architectures due to strong resonances arising from parallel interface refraction (PIR). This anomalous type of refraction is acutely negative and usually out of the plane of incidence. Over a wide range of frequencies, light impinging on idealized two-dimensional (2D) thin-film photonic crystals, over a cone of at least 20\ifmmode^\circ\else\textdegree\fi{} in off-normal directions, couples to Bloch modes propagating nearly parallel to the thin-film--to--air interface. For realistic three-dimensional PC films of cubic symmetry, synthesized by photoelectrochemical etching, the PIR effect persists over a spectral range of at least 15% relative to the center frequency and within a cone of 50\ifmmode^\circ\else\textdegree\fi{} of incident angles, normal to the film. This leads to anomalously long optical path lengths and long dwell times before the light beam exits the thin film. This near continuum of high-quality-factor optical resonances, associated with ``transverse optical slow modes'' in a spectral range of high electromagnetic density of states, can be much more effective for trapping and absorbing light than that of the previously reported longitudinal slow-group-velocity effects. The parallel interface refraction effect is general and can be found in specific spectral ranges of both 2D and 3D photonic crystals with cubic or other appropriate symmetries. In the presence of weak optical absorption within the PC backbone, energy conversion enhancement is interpreted using a simple temporal mode-coupling model. It is shown that absorption is optimized when the structural quality factor (in the absence of absorption) of the transverse optical slow modes is comparable to $\ensuremath{\omega}{\ensuremath{\tau}}_{\mathrm{abs}}$, where $\ensuremath{\omega}$ is the optical frequency and ${\ensuremath{\tau}}_{\mathrm{abs}}$ is the absorption time scale of the film material. Quantitative numerical results for light harvesting efficiency are obtained by finite-difference time-domain simulations of the electromagnetic wave field. It is shown that small amounts of fabrication-related structural disorder can introduce additional resonances and broaden existing resonances, thereby improving the overall harvesting of broadband wide-acceptance-angle light.