PhD defence by Ali Nawaz Babar

Abstract

The key goal in nanophotonics is to enhance light-matter interactions, which can be achieved using optical nanocavities that concentrate and retain light. Recently, a new regime in dielectrics emerged with the introduction of bowtie nanocavities that possess mode volumes below the diffraction limit, which was before limited to plasmonics. The smaller the bowtie dimension, the smaller the mode volume; therefore, nanofabrication limits the spatial confinement in dielectric nanocavities.

This thesis experimentally demonstrates waveguide-coupled topology-optimized bowtie cavities with nanometer-scale dimensions. Nanocavity is designed with an ultra-small footprint, high on-resonance transmission, broad bandwidth, and a bowtie width of 10 nm that allows light confinement well below the diffraction limit with a mode volume of 9.6×10−4 λ3.

The nanocavities are experimentally realized by performing extreme-resolution electron-beam lithography and using a hardmask reactive-ion etching process. Minor improvements in the nanofabrication process can further enhance the light-matter interactions; however, achieving sub-nanometer scale to atomic-scale dimensions is impossible, with aspect ratios exceeding 100 using current lithography technology. Apart from top-down nanopatterning, bottom-up self-assembly is the alternate method for making nanostructures and devices. In this thesis, instabilities provided by the ubiquitous surface forces, including Casimir-van der Waals interactions, are utilized to deterministically self-assemble and self-align suspended silicon nanostructures with void features well below the length scales possible with conventional lithography and etching.

The method is remarkably robust, and the threshold for self-assembly depends on the device geometry parameters, which are investigated using thousands of devices. To demonstrate this concept, nanocavities that are otherwise impossible to fabricate are realized: waveguide-coupled high-Q silicon photonic cavities that confine photons to 2 nm air gaps with an aspect ratio of 100, corresponding to mode volumes more than 100 times below the diffraction limit. This work constitutes the first steps towards a new generation of nanofabrication technology that combines the atomic-scale dimensions enabled by self-assembly with the scalability of planar semiconductors.

Optical interfacing with fiber arrays is required for device packaging and for realizing applications with a suspended silicon photonics platform, e.g., programmable photonics and neuromorphic computing. This thesis demonstrates fabrication process optimization that allows the integration of optical fiber arrays with a silicon photonic directional coupler based on a nanoelectro-mechanical system.

Supervisors

• Principal supervisor: Professor Søren Stobbe, DTU Electro, Denmark
• Co-supervisor: Professor Jesper Mørk, DTU Electro, DenmarkCo-supervisor: Senior Researcher Yi Yu, DTU Electro, Denmark

Evaluation Board

• Senior Researcher Elizaveta Semenova, DTU Electro, Denmark
• Professor Tim Schröder, Humboldt University, Germany
• Group Leader Yonder Berencén, Helmholtz-Zentrum Dresden-Rossendorf, Germany

Master of the Ceremony

• Senior researcher Philip T. Kristensen, DTU Electro, Denmark