Self-assembled photonic cavities with atomic-scale confinement

Abstract

Despite tremendous progress in research on self-assembled nanotechnological building blocks, such as macromolecules1, nanowires2 and two-dimensional materials3, synthetic self-assembly methods that bridge the nanoscopic to macroscopic dimensions remain unscalable and inferior to biological self-assembly. By contrast, planar semiconductor technology has had an immense technological impact, owing to its inherent scalability, yet it seems unable to reach the atomic dimensions enabled by self-assembly. Here, we use surface forces, including Casimir–van der Waals interactions4, to deterministically self-assemble and self-align suspended silicon nanostructures with void features well below the length scales possible with conventional lithography and etching5, despite using only conventional lithography and etching. The method is remarkably robust and the threshold for self-assembly depends monotonically on all the governing parameters across thousands of measured devices. We illustrate the potential of these concepts by fabricating nanostructures that are impossible to make with any other known method: waveguide-coupled high-Q silicon photonic cavities6,7 that confine telecom photons to 2 nm air gaps with an aspect ratio of 100, corresponding to mode volumes more than 100 times below the diffraction limit. Scanning transmission electron microscopy measurements confirm the ability to build devices with sub-nanometre dimensions. Our work constitutes the first steps towards a new generation of fabrication technology that combines the atomic dimensions enabled by self-assembly with the scalability of planar semiconductors.

Singular and Nonsingular Transitions in the Infrared Plasmons of Nearly Touching Nanocube Dimers

“…Unfortunately, we could not find any narrow-gap nontouching face-to-face dimer, presumably because of the strong van der Waals attraction that pushes the faces toward collapse, as recently discussed in dielectric gaps with an exquisite degree of control. 47 Nonetheless, our level of confidence in the theory is supported by the excellent agreement that we find between the theory and the experiment for a wide range of nanocube configurations, including dissimilar dimers under touching and nearly touching conditions (Figures S6−S11 in Supporting Information), with geometrical parameters extracted from HAADF micrographs and dielectric properties modeled through the local dielectric function in eq 1. It should be noted that quantum confinement and nonlocal effects become relevant at small separations in the range of the Fermi wavelength of the material (e.g., ≲1 nm in noble metals and F-ITO), leading to strong plasmon damping and frequency shifts 48−53 as well as emerging modes involving charge transfer via tunneling across the gap, 51,52 but we do not consider such small distances in the present work.…”

“…Also, experimental measurements (Figure a–c) confirm the absence of a low-energy feature for the touching configuration. Unfortunately, we could not find any narrow-gap nontouching face-to-face dimer, presumably because of the strong van der Waals attraction that pushes the faces toward collapse, as recently discussed in dielectric gaps with an exquisite degree of control . Nonetheless, our level of confidence in the theory is supported by the excellent agreement that we find between the theory and the experiment for a wide range of nanocube configurations, including dissimilar dimers under touching and nearly touching conditions (Figures S6–S11 in Supporting Information), with geometrical parameters extracted from HAADF micrographs and dielectric properties modeled through the local dielectric function in eq .…”