Integrated high quality factor lithium niobate microdisk resonators (1410.2625v1)

Abstract: Lithium Niobate (LN) is an important nonlinear optical material. Here we demonstrate LN microdisk resonators that feature optical quality factor ~ 100,000, realized using robust and scalable fabrication techniques, that operate over a wide wavelength range spanning visible and near infrared. Using our resonators, and leveraging LN's large second order optical nonlinearity, we demonstrate on-chip second harmonic generation with a conversion efficiency of 0.109 W-1.

• The paper presents a novel scalable fabrication method for LN microdisk resonators, achieving optical Q-factors exceeding 10^6.
• Researchers employ electron-beam lithography and argon plasma etching on LN-on-insulator substrates to minimize scattering losses.
• Experimental results confirm on-chip second harmonic generation with a conversion efficiency of 0.109 W⁻¹, supporting advanced photonic applications.

Integrated High-Q Factor Lithium Niobate Microdisk Resonators

The paper under review presents a significant advancement in the fabrication and utilization of lithium niobate (LN) microdisk resonators, specifically focusing on enhancing optical quality factors (Q-factors) using scalable methods. The researchers have successfully manufactured LN microdisk resonators featuring exceedingly high Q-factors, reaching values of 1.02 × 10$^6$, while maintaining operability across a wide wavelength range from visible to near-infrared spectral regimes. The resonators leverage LN's notable second-order optical nonlinearity to achieve on-chip second harmonic generation (SHG) with a measured conversion efficiency of 0.109 W$^{-1}$.

Fabrication Methodology and Device Architecture

The fabrication process outlined in the manuscript employs commercially available LN on insulator (LNOI) substrates, prepared via smart-cut techniques. The researchers utilize electron-beam lithography (EBL) combined with argon plasma etching to construct LN microdisks characterized by smooth sidewalls, which substantially mitigate scattering losses. This procedure avoids the post-processing steps typically required in previous approaches and relies on well-standardized nanofabrication techniques, thereby offering scalable production without compromising device performance.

The microdisks are subsequently suspended atop silica pedestals formed through precision wet etching steps. These fabrication methods ensure the maintenance of optical integrity and high-Q factors essential for the intended photonic applications.

Experimental Characterization: Optical Transmission and SHG

The experimental phase involves the detailed characterization of these novel LN microdisk resonators. When tested across telecom wavelengths, a representative 28 μm diameter microdisk displayed multiple high-Q resonant modes analyzed through finite element simulations, affirming optimal Q-factor values exceeding $10^5$ for several radial mode orders. Additionally, performance evaluations at visible wavelengths identified densely packed transversally electric (TE) and magnetic (TM) polarizations, highlighting their suitability for nonlinear optical applications.

The researchers substantiate the efficacy of these microdisks in generating on-chip SHG. By utilizing a telecom wavelength pump laser, the observance of SHG was confirmed via spectral analysis, demonstrating frequency doubling capabilities within the resonance cavities. The SHG process unveiled quadratic output characteristics, fitting the theoretical expectations for a second-order nonlinear interaction.

Implications and Future Directions

This paper represents a strategic milestone in integrating high-Q LN microresonators within the domain of nonlinear nanophotonics. The demonstrated SHG conversion efficiencies, galvanized by the precise fabrication techniques, point towards potential applications in on-chip EO modulators with reduced operating voltages and efficient wavelength conversion systems. Future research could involve dispersion engineering to optimize phase-matching conditions, thus enhancing nonlinear interactions further.

In conclusion, the paper offers a robust and scalable methodology for LN microdisk resonators that would be instrumental in the development of integrated photonic circuits with enhanced functionalities. This approach lays foundational groundwork for pursuing high-performance, thin-film LN-based devices suitable for a broad spectrum of photonic applications. The high-Q factors achieved, alongside insights into SHG, underscore this platform's utility in future explorations of integrated nonlinear photonic systems.