High-quality-factor Si₃N₄ microresonators towards integrated nonlinear and quantum light sources

Abstract

<test> Photonic integrated circuit (PIC) is an excellent platform for nonlinear frequency conversion due to its superior scalability and the capability of integrating linear optics components. Silicon nitride (SiN), with low intrinsic linear and nonlinear two-photon absorption losses in the telecommunications window, shows its advantage as a potential material. In this thesis, we explore high quality factor Si₃N₄ microresonators towards integrated nonlinear and quantum light sources. We develop a stress-released stoichiometric silicon nitride (Si₃N₄) fabrication process for dispersion-engineered integrated silicon photonics. To relax the high tensile stress of a thick Si₃N₄ film grown by low-pressure chemical vapor deposition (LPCVD) process, we introduce a dense stress-release pattern prior to the Si₃N₄ film deposition. The pattern is applied either on the lower-cladding oxide layer, or onto a thin Si₃N₄ film ( &lt; 400 nm before the cracks start to generation). Our pattern helps minimize crack formation by releasing the stress of the film along high-symmetry periodic modulation directions and helps stop cracks from propagating. We demonstrate crack-free Si₃N₄ films of up to~950nm-thick in a single deposition run on a 4” silicon wafer. Our Si₃N₄ photonic platform enables dispersion-engineered, waveguide-coupled microring and microdisk resonators, with cavity sizes of up to a millimeter. We characterize and analyze the linear properties through the transmission spectrum measurement. Our 115μm-radius microring exhibits an intrinsic quality (Q)-factor of ~2.0×10⁶ for the TM₀₀ mode and our 575μm-radius microdisk demonstrates an intrinsic Q of ~4.0×10⁶ for TM modes in 1550nm wavelengths. We study the simulated nonlinear frequency conversion from our fabricated high-Q microresonators. By pumping at the high-Q modes, we observe optical parametric oscillation (OPO). We achieve a threshold power of ~20 mW inside the coupled waveguide on microring resonators with a loaded Q-factor of ~1×10⁶ in TE polarization. We study the quantum light sources based on the spontaneous four-wave mixing (SFWM) process. we demonstrate a high-rate and high-purity photon-pair source through pump-degenerated SFWM using Si₃N₄ whispering-gallery-mode (WGM) microrings. Our 61.5μm-radius and 8μm-wide microrings, with a typical loaded Q-factor of ~1×10⁶, demonstrate a photon-pair generation rate (PGR) of ~1.03 MHz/mW², with spectral brightness of ~5×10⁶ pairs/s/mW²/GHz that is comparable with the state-of-the-art. We study the heralded single-photon property with the conditional self-correlation measurement, which reveals a low conditional self-correlation g_H⁽²⁾(0) of 0.008 ± 0.003. We explore the radial order degree of freedom in WGM microrings for photon-pair generations. Our 119μm-radius and 8μm-wide microrings demonstrate PGRs from tens to hundreds of kHz/mW² for the five lowest-radial-order TM modes. </test>

Date of Award	2021
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Andrew Wing On POON (Supervisor)