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We design and fabricate a grating coupler for interfacing suspended silicon photonic membranes with free-space optics while being compatible with single-step lithography and etching in 220 nm silicon device layers. The grating coupler design simultaneously and explicitly targets both high transmission into a silicon waveguide and low reflection back into the waveguide by means of a combination of a two-dimensional shape-optimization step followed by a three-dimensional parameterized extrusion. The designed coupler has a transmission of −6.6 dB (21.8 %), a 3 dB bandwidth of 75 nm, and a reflection of −27 dB (0.2 %). We experimentally validate the design by fabricating and optically characterizing a set of devices that allow the subtraction of all other sources of transmission losses as well as the inference of back-reflections from Fabry-Pérot fringes, and we measure a transmission of 19 % ± 2 %, a bandwidth of 65 nm and a reflection of 1.0 % ± 0.8 %.

Optical nanocavities confine and store light, which is essential to increase the interaction between photons and electrons in semiconductor devices, enabling, e.g., lasers and emerging quantum technologies. 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 limit. 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 nanofabrication, 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 optimization. Our cavities are defined within a compact device footprint of 4 lambda^2 and exhibit mode volumes down to V = 3E-4 lambda^3 with wavelengths in the lambda = 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 surface. 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 few- and single-photon nonlinearities over electronics-photonics integration to biosensing.

Photonic nanocavities achieve tight temporal and spatial confinement of light through the quality factor, Q, and the mode volume, V, respectively. This results in local enhancements of the electric field, E, which is central to a number of applications requiring enhanced light-matter interaction [1], such as nonlinearities [2] or efficient optical interconnects [3]. Previously, it was believed that the mode volume in dielectrics was bound by the diffraction limit [4], and therefore field enhancements were achieved by large quality factors [5]. With the recent discovery of dielectric bowtie cavities, however, mode volumes deep below the diffraction limit are possible in devices with nanometer-scale features [2,6]. Such features, in turn, pose challenges to the resolution of fabrication at the deep nanoscale. Here we investigate the importance of precise pattern design and the effects of rasterization (shot-filling) in electron-beam lithography when pushing the resolution limit. We consider a novel nanocavity design obtained by inverse design using tolerance-constrained topology optimization [6] in which the local density of optical states (LDOS) is optimized at the very center of a silicon cavity to have Q = 1100, V = 0.08 (λ/2n)3, and λ = 1551 nm. To illustrate the importance of shot-filling for high-resolution electron-beam lithography, we first consider the test structure shown in Fig. 1. The contours between material boundaries are used to define a set of polygons as shown in Fig. 1a. Individual patterns are well separated to be isolated from long-range proximity effects [7]. Figures 1b and d show the fracturing of the polygon as well as the discretization into individual shots separated by a pitch, p, and exposed with a uniform dose density, D. This means that the impinging charge dose of each shot (in coulomb) is q = p2D. Electron-scattering through the material broadens the point-like exposures along with the other process steps, here development and etching, to yield an effective deposited dose density, Deff, shown on the grayscale map in Figs. 1c and e [7-8]. The regions that receive an effective dose density greater than the dose to clear, D0, will be developed as indicated by the green contour. Figure 1c, shows visible line-edge roughness caused by the coarse discretization and poor dose uniformity, which, for a cavity, can cause substantial optical loss through scattering (reduction in Q) [5], while the finer discretization in Fig. 1e produces much smoother edges and a higher fidelity in the pattern transfer due to the finer discretization. Figure 2 shows three nanocavities fabricated with the same process on the same chip where the current, and therefore pitch, is varied. Already when the pitch is increased from 1 nm to 3.5 nm, several of the small features cannot be resolved, and with p = 6 nm the central part becomes disconnected, thus charging up under SEM inspection. We will report on our latest progress towards realizing structures with extreme confinement of light.