Integrated optical and mechanical resonators for evanescent field sensing

• Author / Creator

  Doolin, Callum

• Nanoscale optical and mechanical resonators store energy in a way characterized by a sharp resonance frequency, and through interaction with their surroundings offer a path to the next generation of sensitive measurement tools. In this thesis we investigate a particular geometry of nanofabricated devices-that of monolithically fabricated optical microdisks and nanomechanical resonators, in which the optical microdisk operates as a high-gain amplifier of the mechanical resonator's position.

  We began the study with nanoscale silicon microdisks and cantilevers fabricated with a commercial photolithography process for silicon photonics, and used the optomechanical interaction between the cantilevers and optical microdisks to demonstrate readout of the mechanical motion to the fm Hz^-0.5 precision level. This approach has enabled thermally limited readout of forces on the cantilever to 130 +- 40 aN Hz^-0.5 at room temperature, optimized by their nanometer-sized geometry and femtogram-scale masses. We then explored the possibility of using these cantilevers for fundamental quantum measurements of phonon number, and although we concluded the cantilever measurement lacked the necessary characteristics, we developed a framework for characterizing the type of optomechanical coupling exhibited by an optomechanical device.

  Continuing on resonator development, we switched to fabricating similar geometry optomechanical devices from silicon nitride, an insulating material used in semiconductor fabrication, known to enable a high quality factor mechanical resonator geometry termed nanostrings. Using a fiber-waveguide coupling technique we were able to optomechanically measure picogram-scale nanostring devices down to temperatures below 1 K, finding mechanical quality factors of 10^6, while exhibiting less optically-induced heating than similar silicon devices. While the optical microdisks enable high-precision readout of mechanical motion, they more generally measure refractive index changes. Using aqueously submerged silicon nitride microdisks, we were able to measure LiCl induced refractive index changes down to the 10^-6 level.

  Finally, we carry out a discussion and review on the subject of digital signal processing. Although appearing unrelated, the techniques covered in Chapter 7 underline every single experimental result covered in this thesis. With an understanding of digital signals, flexible and well adapted measurement protocols can be constructed without being stuck relying on the output of fixed-pipeline measurement tools.

• Subjects / Keywords

  • effective mass
  • discrete fourier transform
  • nanofabrication
  • silicon nitride
  • microdisk
  • whispering-gallery mode
  • optical
  • resonator
  • power spectral density
  • filter
  • chirp drive
  • mechanics
  • quadratic phase
  • digital signals
  • nonlinear
  • optics
  • tapered fiber
  • force sensing
  • sensing
  • cryogenic
  • nanostring
  • low-temperature
  • waveguide
  • evanescent field
  • mechanical

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-n5dw-9z96

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.