[Paper Review] Whispering gallery mode microresonator for nonlinear optics

Yihang Li, Xuefeng Jiang+2 more|arXiv (Cornell University)|Sep 13, 2018

Advanced Fiber Laser Technologies254 references20 citations

TL;DR

This review synthesizes recent advances in whispering gallery mode (WGM) microresonators for nonlinear optics, highlighting their ultrahigh Q-factors and small mode volumes that enable strong light-matter interactions. It details fundamental nonlinear effects—Kerr, Pockels, harmonic generation, four-wave mixing, and stimulated scattering—alongside thermal and PT-symmetric nonlinearities, cascaded processes like frequency comb generation, and applications in microlasers and non-reciprocal devices.

ABSTRACT

Whispering gallery mode (WGM) microresonators, benefitting from the ultrahigh quality (Q) factors and small mode volumes, could considerably enhance the light-matter interaction, making it an ideal platform for studying a broad range of nonlinear optical effects. In this review, the progress of optical nonlinear effects in WGM microresonators is comprehensively summarized. First, several basic nonlinear effects in WGM microresonator are reviewed, including not only Pockels effect and Kerr effect, but also harmonic generations, four-wave mixing and stimulated optical scattering effects. Apart from that, nonlinearity induced by thermal effect and in PT-symmetric systems are also discussed. Furthermore, multistep nonlinear optical effects by cascading several nonlinear effects are reviewed, including frequency comb generations. Several selected applications of optical nonlinearity in WGM resonators are finally introduced, such as narrow-linewidth microlasers, nonlinearity induced non-reciprocity and frequency combs.

Motivation & Objective

• To systematically review the state-of-the-art in nonlinear optical effects within whispering gallery mode (WGM) microresonators.
• To analyze the role of ultrahigh Q-factors and small mode volumes in enhancing light-matter interactions for nonlinear optics.
• To explore both fundamental and cascaded nonlinear processes, including four-wave mixing and frequency comb generation.
• To examine emerging nonlinearities induced by thermal effects and in PT-symmetric systems.
• To highlight practical applications such as narrow-linewidth microlasers, non-reciprocity, and frequency combs.

Proposed method

• Compilation and synthesis of experimental and theoretical results from recent literature on WGM microresonators.
• Categorization of nonlinear optical effects into intrinsic (Kerr, Pockels, harmonic generation) and extrinsic (thermal, PT-symmetric) mechanisms.
• Analysis of cascaded nonlinear processes, particularly four-wave mixing leading to frequency comb generation.
• Use of coupled-mode theory and nonlinear Schrödinger-type equations to model mode interactions and nonlinear dynamics.
• Incorporation of thermally induced nonlinearity models and parity-time (PT) symmetric Hamiltonians to describe non-Hermitian optical systems.
• Systematic presentation of device configurations, material platforms (e.g., silica, chalcogenide, silicon nitride), and experimental realizations.

Experimental results

Research questions

• RQ1 How do ultrahigh Q-factors and small mode volumes in WGM microresonators enhance nonlinear optical effects?
• RQ2 What are the dominant intrinsic nonlinear mechanisms—Kerr, Pockels, harmonic generation, four-wave mixing—observed in WGM resonators?
• RQ3 How do thermal effects and PT-symmetric systems contribute to novel forms of optical nonlinearity in these systems?
• RQ4 What role do cascaded nonlinear processes play in enabling frequency comb generation in WGM microresonators?
• RQ5 What are the most promising applications of nonlinear WGM microresonators in integrated photonics and optical sensing?

Key findings

• WGM microresonators achieve Q-factors exceeding 10^9, enabling strong light-matter interaction and low-threshold nonlinear effects.
• Four-wave mixing in WGM resonators has demonstrated sideband generation with conversion efficiencies exceeding 10% in optimized systems.
• Frequency comb generation via cascaded four-wave mixing has been experimentally realized in microtoroid and microdisk resonators with repetition rates up to tens of GHz.
• Thermal nonlinearity in WGM resonators leads to self-pulsating dynamics and bistability, enabling all-optical switching and memory effects.
• PT-symmetric WGM systems exhibit unidirectional lasing and enhanced sensitivity in optical sensing due to exceptional points in the spectrum.
• Nonlinearity-induced non-reciprocity has been demonstrated in WGM resonators, enabling optical diodes with isolation ratios exceeding 30 dB in proof-of-concept devices.