Optical Resonator Physics

Characterizition optical wave propagation along line defects in two-dimensional arrays of air-holes in free-standing silicon slabs. The fabricated waveguides contain random variations in orientation of the photonic lattice elements which perturb the in-plane translational symmetry. The vertical slab symmetry is also broken by a tilt of the etched sidewalls. We discuss how these lattice imperfections affect out-of-plane scattering losses and introduce a mechanism for high-Q cavity excitation related to polarization mixing.

See "Out-of-plane scattering from vertically asymmetric photonic crystal slab waveguides with in-plane disorder" for the full article.

Introduction

Two-dimensional (2D) photonic crystals (PhCs) in dielectric slabs have attracted considerable interest due to their ability to slow-down and confine light it in small spaces. Slow-light propagation and localization are advantageous for a variety of photonic applications. They enhance the strength of interactions between radiation and matter which can facilitate detection and utilization of nonlinear and quantum effects . PhCbased nanoscale resonators and waveguides are poised to play a crucial role in future miniaturized photonic devices such as nanocavity lasers, single photon sources , compact filters, all-optical switches, delay lines, and refractometric sensors. Structural disorder limits the performance and the capabilities of these devices.

Implications of structural disorder have been very well studied. Most of the work focused on in-plane disorder which breaks the periodicity of the 2D photonic lattice causing the optical waves propagating in intrinsically lossless photonic crystal waveguides (PhCWs) to scatter. Disorder-induced back- and out-of-plane scattering have been shown to be the dominant losses in the important slow-light regime. Interesting disorder-induced localization effects that affect wave propagation have also been reported in this regime in coupled-resonator chains, disordered 1D lattices, and line-defect PhCWs. Vertical (or out-of-plane) asymmetry is another significant but far less studied contributor to the guiding losses. While several loss channels related to polarization conversion have been identified and observed experimentally, the effects of intermodal interactions in dispersive waveguides and their contributions to out-of-plane losses have not yet been sufficiently studied.

In this article we investigate vertical scattering in disordered PhCWs composed of lines of voids in 2D photonic lattices in free-standing silicon slabs. The fabricated PhCs contain enhanced in-plane disorder created by random orientations of non-circular lattice elements, and their out-of-plane symmetry is also perturbed by a sidewall tilt. We study how these structural asymmetries affect propagation of linearly polarized light through the dispersive waveguides. Our study highlights the spectrally narrow enhancement of polarization conversion associated with band edge intermittency due to disorder. Finally, we also introduce a mechanism based on polarization-mixing for coupling light into PhC-based resonators, demonstrating that TE-like high-Q nanocavities in vertically asymmetric PhCWs can be excited with TM-like guided modes.