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Greater Than One Billion Optical Q Factor for On-Chip Microresonators


Wu, Lue Leo (2024) Greater Than One Billion Optical Q Factor for On-Chip Microresonators. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/n1cn-tn34.


This thesis is focused on making ultra-high-Q optical microresonators on silicon chips based on design and constructing ultra-low-loss optical waveguides (with losses around 20 dB/km), their fabrication process development, and device applications in on-chip nonlinear optics, including frequency combs, low-noise microwave generation, and narrow-linewidth lasers.

First, using thermally grown oxide (thermal silica) and wedge microresonator structure, a record Q factor exceeding 1.1 billion is achieved. Then, the limitations of the Q-factor due to surface roughness scattering loss and OH absorption loss are investigated and identified. Absorption limited Q-factor of 8 billion mainly attributed to OH ions is measured. To further explore the potential of thick thermal silica as under cladding material, wedge resonator fabricated in 25-µm-thick thermal silica achieves a Q-factor of over 60 million, along with a sixfold improvement in thermal stability and a 5 billion absorption-limited Q-factor. Subsequently, low noise microwave signal generation is demonstrated using these devices in a fully optical packaged form, operating soliton microcomb to generate beatnote microwave signals. Noise limitations arising from dispersive waves induced by distinct transverse modes are identified. Additionally, a low-fundamental-linewidth microcavity Brillouin laser is demonstrated, benefiting from device high Q-factor. The noise limits stemming from thermal refractive fluctuation at low offset frequencies and laser output power at high offset frequencies are identified. To improve device integration level, an engineered reduction of interface scattering using TM mode enables a demonstration of 700 million Q factor in a fully-integrated high-aspect-ratio thin SiN platform fabricated in a CMOS foundry. To add one more thing, room temperature soliton microcomb generation is demonstrated for the first time in high-Q AlGaAs microresonators.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:optical microresonator; Q-factor; optical microcavity; nanofabrication; thermal silica; photonic device fabrication; laser; microcomb; nonlinear optics; integrated photonics; soliton
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Applied Physics
Minor Option:Applied Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Vahala, Kerry J.
Thesis Committee:
  • Faraon, Andrei (chair)
  • Vahala, Kerry J.
  • Painter, Oskar J.
  • Marandi, Alireza
Defense Date:11 September 2023
Non-Caltech Author Email:leoluewu (AT)
Record Number:CaltechTHESIS:06042024-003837900
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for ch.2. adapted for ch.3. adapted for ch.5. adapted for ch.6. adapted for ch.8.
Wu, Lue Leo0000-0002-7503-7057
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16493
Deposited By: Lue Wu
Deposited On:06 Jun 2024 23:10
Last Modified:08 Jul 2024 19:07

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