Self-assembled Photonic Cavities with Atomic-Scale Confinement
Researchers have developed a method for self-assembling photonic cavities with atomic-scale confinement, enabling extreme light confinement and opening up new possibilities for nanophotonic devices. The study, conducted by scientists at the Technical University of Denmark, combines surface forces and conventional lithography and etching techniques to create suspended silicon nanostructures with void features well below the diffraction limit. The researchers demonstrated the fabrication of waveguide-coupled high-Q silicon photonic cavities that confine telecom photons to 2 nm air gaps with an aspect ratio of 100, resulting in mode volumes more than 100 times below the diffraction limit. Scanning transmission electron microscopy measurements confirmed the sub-nanometer dimensions of the devices. The use of self-assembly in combination with planar semiconductor technology could lead to the development of a new generation of nanophotonic devices that combine atomic-scale confinement with the scalability of planar semiconductors.
Deterministic Self-Assembly
Most self-assembly methods rely on liquid suspensions, but the researchers developed a method that uses surface forces to induce controlled, deterministic collapses and self-align suspended silicon nanostructures. By mapping the surface-force instability as a function of geometry, they were able to design and fabricate platforms that collapse or remain stable depending on the initial gap distance and the spring constant. The collapse threshold provides design rules for realizing suspended silicon devices with high-aspect-ratio gaps and allows for the fabrication of nanostructures with few- or sub-nanometer dimensions.
The collapse experiment was implemented on a silicon-on-insulator platform with suspended silicon platforms of various widths and initial gap distances. The experiments successfully demonstrated the deterministic self-assembly of the platforms, with collapses occurring at gaps smaller than the critical gap determined by the spring constant and platform width.
Self-Assembly of Atomic-Scale Cavities
To illustrate the potential of their method, the researchers fabricated photonic nanobeam cavities with atomic-scale confinement. The cavities consisted of two spring-suspended nanobeams separated by a gap, with a bowtie structure in the center. The nanobeam cavities were designed to have 2 nm air voids at the bowtie centers, which were impossible to fabricate using conventional lithography and etching techniques. However, by introducing an offset during the fabrication process, the researchers were able to achieve controlled collapses and self-assemble the atomic-scale cavities with sub-nanometer gaps.
High-resolution scanning transmission electron microscopy (STEM) measurements confirmed the sub-nanometer dimensions of the self-assembled cavities. The STEM images showed the presence of 2 nm air voids bounded by amorphous silicon oxide, with the thickness of the oxide layer estimated to be 2-2.5 nm. The self-assembled cavities exhibited resonant modes with high quality factors (Q) and nanoscale confinement.
Integration with Photonic Circuits
To demonstrate the scalability and integration potential of their method, the researchers also integrated the self-assembled cavities with photonic circuits. They designed and fabricated waveguide-to-waveguide couplers with broadband transmission windows that allowed for efficient transmission of light across a 100 nm air trench. The couplers were fabricated in two halves, which self-assembled during the fabrication process along with the nanobeam cavities.
The integrated devices were characterized using transmission measurements, which showed the resonant scattering spectra of the self-assembled cavities and the transmission spectra of the photonic circuits. The measurements confirmed the successful integration of the self-assembled cavities with the circuits and demonstrated the transmission of light through the cavity devices. The cavity devices exhibited resonant modes with high Q-factors, indicating efficient light confinement and interaction with the surrounding waveguide architecture.
The successful integration of self-assembled nanobeam cavities with photonic circuits demonstrates the potential for scalable and self-aligned integration of bottom-up self-assembled devices with top-down planar technology. This opens up new possibilities for the development of nanophotonic devices with extreme field confinement and enhanced light-matter interaction.
