Observation of Size-dependent Optical Modes and Confinement in Fin Cavities using Angle-Resolved Cathodoluminescence

Abstract

We observe novel optical fringes and emission coherence in laterally formed semiconductor fins using wavelength- and angle-resolved cathodoluminescence (WARCL). These findings demonstrate that cavity mode and emission confinement are directly governed by cavity geometry including cavity dimensions, facet angles, and orientation. In flat fins narrowing the width from 3000 nm to 200 nm sharpens fringe intensity, indicating stronger mode confinement and increased coherence. Finite-difference time-domain (FDTD) simulations confirm that the observed fringes originate from internal reflections among the fin facets, consistent with a Fabry-Pérot (FP) cavity model where the cavity length determines the fringe spacing. FDTD also reveals a transition from multi-mode to single-longitudinal-mode behavior in 200-nm fins, consistent with experimentally observed fringe narrowing. Increasing fin length from 10 μm to 30 μm compresses fringe spacing via longer round-trip paths. Conversely, tilting the fin end-facet from 90° to 45° reduces the effective light path length, increasing the incoherent optical emission. Remarkably, this loss can be offset by adjusting the fin length-to-height ratio. Results suggest WARCL is a robust in-situ method for direct characterization of individual dielectric nanocavities, mapping their modal profiles and light directionality as a function of geometry for custom design of nanoscale optical sources and waveguides.