Nanometer-scale photon confinement inside dielectrics

Abstract

Optical nanocavities confine and store light1–3, which is essential to increase the interaction between photons and electrons in semiconductor devices, enabling, e.g., lasers and emerging quantum technologies4–6. While temporal confinement has improved by orders of magnitude over the past decades, spatial confinement inside dielectrics was until recently believed to be bounded at the diffraction limit7,8. The conception of dielectric bowtie cavities (DBCs) shows a path to photon confinement inside semiconductors with mode volumes bound only by the constraints of materials and nanofabrication9–15, but theory was so far misguided by inconsistent definitions of the mode volume and experimental progress has been impeded by steep nanofabrication requirements. Here we demonstrate nanometer-scale photon confinement inside 8 nm silicon DBCs with an aspect ratio of 30, inversely designed by fabrication-constrained topology optimization13,16. Our cavities are defined within a compact device footprint of 4λ2 and exhibit mode volumes down to V = 3×10−4 λ3 with wavelengths in the λ = 1550 nm telecom band. This corresponds to field localization deep below the diffraction limit in a single hotspot inside the dielectric. A crucial insight underpinning our work is the identification of the critical role of lightning-rod effects at the surface17–19. They invalidate the common definition of the mode volume, which is prone to gauge meretricious surface effects or numerical artefacts rather than robust confinement inside the dielectric. We use near-field optical measurements to corroborate the photon confinement to a single nanometer-scale hotspot. Our work enables new CMOS-compatible device concepts ranging from fewand single-photon nonlinearities12 over electronics-photonics integration20 to biosensing21. A wealth of mechanisms can be exploited for building nanocavities, including distributed Bragg reflection1–3,22, total internal reflection23, Fano resonances24, plasmonic resonances25, topological confinement26, and bound states in the continuum27. Common to existing approaches is that neither of them allow optical mode volumes, V , in the deep subwavelength regime unless introducing absorption losses3,8. The underlying principle of DBCs is local field enhancements due to the electromagnetic boundary conditions across material interfaces9–13,28–30. They demand that the tangential component of the electric field, E, and the normal component of the displacement field, D = εE, are continuous. This implies that a semiconductor bridge surrounded by void features, cf. Figs. 1a and b, can confine light inside the material, which is crucial to enhance the interaction with embedded