PhD Defence by Marcus Albrechtsen

Supervisors

• Principal supervisor: Associate Professor Søren Stobbe, DTU Electro, Denmark
• Co-supervisor: Assistant Professor Babak Vosoughi Lahijani, DTU Electro, Denmark

Assessment committee

• Professor Kresten Yvind, DTU Electro, Denmark (chair)
• Professor Tim Schröder, Department for Physics, Humboldt University Berlin
• Professor Albert Schliesser, Niels Bohr Institute, Copenhagen University, Denmark

Master of the Ceremony

• Professor Lars Dittmann, DTU Electro, Denmark

Abstract:
The interaction between light and matter is inherently weak, but optical cavities can enhance it by focusing and storing the light. Previous approaches have used dielectric materials to improve the temporal storage, i.e., the quality factor, Q. However, improvement to the spatial confinement, i.e., the mode volume, V, have so far relied on plasmonics, where Ohmic absorption losses are a problem for many applications. This thesis presents the recent demonstration of a dielectric bowtie nanocavity that focuses the light to well below the so-called diffraction limit in a dielectric material that is free from absorption losses. This is achieved through a combination of multiple advances.

It is shown that V depends sensitively on the smallest feature-size that can be fabricated, and the first part of this thesis presents the developments and advances within nanofabrication technologies towards high-resolution silicon nanofabrication. Specifically, within electron-beam lithography and silicon dry-etching. The fabrication method is carefully characterized, and the fabrication-tolerances are directly included in the state-of-the-art deterministic inverse design algorithm, topology optimization.

The objective is to maximize the local density of states (i.e., indirectly the Q/V ratio) in the center of the domain, and the result is a novel and exotic – but importantly, realistic – nanocavity design that features a bowtie structure. Calculations show that it confines light to a mode volume, V = 3·10−4 λ3, and simultaneously achieves Q = 1100 in a compact 4 λ2 area for telecom photons with a wavelength, λ ∼ 1550 nm. Since the cavity design takes into account fabrication-tolerances it can be fabricated with high precision, and optical measurements in the far-field confirm the high optical quality factors. Moreover, near-field measurements corroborate the mode volume deep below the diffraction limit, which is enabled by the only 8 nm wide silicon bowtie-bridge, which is etched vertically into a 240 nm thick silicon membrane.

This extreme dielectric confinement of light paves the way for broadband enhancement of the light-matter interaction, i.e., interaction with broadband pulses, which is important for many applications including optical nonlinearities. This high-resolution silicon nanofabrication method further enables a range of studies from fundamental physics to novel applications. This includes the experimental demonstration of hypersonic phonon-circuits and cavity optomechanics in the gigahertz. Moreover, specific applications include integrated nano-electro-mechanical photonics systems such as comb-drive–controlled optical waveguides that can be used for, e.g., optical switching and programmable photonics. These systems are then combined to demonstrate a chip-scale spectrometer, which is based on slot-mode waveguides that can be used to construct optical delay-lines. Finally, the emerging challenges towards packaging and commercialization are discussed.