TFLN Fabry–Perot Microresonators

• TFLN Fabry–Perot microresonators are integrated photonic devices characterized by high-Q resonance, precise spectral control, and strong nonlinear, piezo-, and electro-optic responses.
• They employ advanced mirror engineering with tapered photonic crystal and Sagnac loop reflectors to enable broadband reflectivity, low scattering losses, and efficient mode confinement.
• Dynamic tuning via electro-optic and thermo-optic mechanisms, combined with state-of-the-art nanofabrication, makes these resonators ideal for nonlinear optics, sensing, and optomechanical applications.

Thin-film lithium niobate (TFLN) Fabry–Perot (FP) microresonators constitute a rapidly advancing class of integrated photonic devices, combining high-Q resonant enhancement, precise spectral and modal engineering, and on-chip compatibility with strong nonlinear, piezo-, and electro-optic functionality. Leveraging straight waveguide architectures, photonic crystal (PhC) or Sagnac-loop-type reflectors, and advanced nanofabrication techniques, these resonators enable new regimes of efficiency, tunability, and scalability for nonlinear optics, ultrafast modulation, sensing, and hybrid optomechanics.

1. Device Architectures and Mirror Engineering

The canonical TFLN FP microresonator consists of a straight, suspended or substrate-supported waveguide section (typical lengths from 100 µm to several mm) bounded by two high-reflectivity mirrors. The mirrors are implemented using either photonic crystal (PhC) reflectors—periodic arrays of holes or corrugations with or without tapering—or via fabrication-tolerant Sagnac loop reflectors (SLRs) constructed from Mach–Zehnder interferometers (MZIs) with looped feedback and adjustable beam splitter ratios (Yang et al., 8 Oct 2025, Hwang et al., 19 May 2025, Qi et al., 29 May 2025).

PhC mirrors: Realized by etching periodic hole arrays into half-etched ridge waveguides (typical lattice constant a ≈ 430–500 nm; unit cell widths w ≈ 1–1.2 µm; hole sizes ≈ 600×350 nm tapering to ≈ 400×233 nm), PhC reflectors generate a wide bandgap in which standing-wave cavity modes are strongly confined. Tapering the PhC—by gradually decreasing lattice period and hole size over several (e.g., n = 10) transition cells—suppresses abrupt index mismatch and reduces scattering at the interface, thereby enhancing spectral flatness and mirror reflectivity (Yang et al., 8 Oct 2025, Hwang et al., 19 May 2025).

SLRs: Sagnac loop reflectors, each formed by an MZI with loop-back arms, provide nearly perfect reflectivity even when the couplers deviate from the ideal 50:50 split. The modularity of this scheme enables robust, broadband, and tunable reflection, tolerant of large beamsplitter ratio variations (15:85 to 85:15) (Qi et al., 29 May 2025). The reflectivity can be actively adjusted by phase tuning the MZIs—via thermo-optic or electro-optic methods—granting dynamic control over the FP cavity’s boundary conditions.

2. Optical Performance: Q Factor, FSR, and Loss

Intrinsic quality factors (Q) in recent TFLN FP microresonators reach 6×10⁵ (loaded, 100 µm cavity, 1530 nm) (Yang et al., 8 Oct 2025), 1.4×10⁶ (intrinsic, PhC design, C-band) (Hwang et al., 19 May 2025), and up to 2×10⁶ for SLR-based designs (Qi et al., 29 May 2025). Such high Q is achieved through a combination of extremely low propagation loss (verified via advanced nanofabrication including optimized dry etching, electron beam lithography, and atomic layer cleaning) and minimized reflection/scattering at mirrors and interfaces.

Loaded and intrinsic Q relate to resonance linewidth (δλ) and finesse (ℱ) as:

$$Q = \frac{\lambda_{res}}{\delta\lambda}, \quad \mathcal{F} = \frac{\pi\sqrt{R}}{1 - R}, \quad \delta\lambda = \frac{\lambda_\mathrm{FSR}}{\mathcal{F}}$$

where R is mirror reflectivity and λ_FSR is the free spectral range, which for a one-dimensional FP of length L and group index n_g is:

$\mathrm{FSR} = \frac{c}{2 n_g (L + \Delta L)}$

with ΔL accounting for field penetration into mirrors or tapers. TFLN geometries support FSRs from several nm (compact, ~100 µm resonators; λ_FSR ≈ 4.8 nm) to <0.1 nm in mm-scale variants. Measured round-trip losses are on the order of 3% for advanced SLR-FP devices; for state-of-the-art TFLN propagation loss values (~3 dB/m), the propagation-limited Q is much higher, establishing interface and mirror loss as the dominant limitation (Qi et al., 29 May 2025).

3. Advanced Tuning: Electro-Optic, Thermo-Optic, and Photonic Crystal Control

One of the central advantages of TFLN FPs is multifunctional tunability. Electro-optic tuning is enabled via the Pockels effect, using lateral or coplanar waveguide electrodes on z-cut (or optimally oriented) TFLN. Electro-optic phase tuning in MZI mirrors achieves V_π as low as 3.5 V over 3.5 mm (V_πL ≈ 3.8 V·cm), supporting high-speed, power-efficient, and full-range tuning of mirror reflectivity and thus the loaded Q and bandwidth (Sayem et al., 6 Sep 2025). The reflectivity as a function of EO-induced phase shift Δφ is:

$$R(\Delta\phi) = \sin^2 \left(\frac{\Delta\phi}{2}\right), \quad \Delta\phi = \frac{\pi V}{V_\pi}$$

Thermo-optic tuning is further enhanced by etching thermal isolation trenches around the heater region, achieving a π-phase shift with just 2.5 mW—an order of magnitude improvement over non-isolated designs—by confining heat and increasing local resistance (Qi et al., 29 May 2025).

PhC reflector geometry, specifically lattice period a and corrugation/taper parameters, governs the position and width of the photonic bandgap. The Bragg condition

$2 n_\mathrm{eff} a = \lambda_\mathrm{center}$

permits linear bandgap scaling over the S-, C-, and L-bands (e.g., 430–440 nm period for 1500–1600 nm operation) without degrading performance or Q (Hwang et al., 19 May 2025). Apodized PhC designs with adiabatic transitions, as compared with uniform period PhC mirrors, significantly reduce interface scattering and preserve loaded Q across the band.

4. Modal Engineering, Nonlinear Effects, and Optomechanics

The standing-wave character and engineered mode confinement in FP geometries yield several critical advantages over conventional curved microresonators:

• Dispersion management: The group velocity dispersion (GVD) is nearly constant in the straight cavity, with FSR decoupled from group index profile, supporting broadband phase matching for nonlinear processes.
• Mitigation of parasitic nonlinearities: The spectral confinement of high-Q modes within the PhC bandgap suppresses parasitic effects (e.g., Raman scattering), as out-of-band photons experience low confinement and weak amplification (Hwang et al., 19 May 2025).
• Enhanced optomechanical and electro-optic overlap: The straight waveguide enables polarization alignment along the strongest r₃₃ tensor axis, optimizing optomechanical cooperativity $C = \frac{4 n_p g_0^2}{\kappa_o \kappa_\mu}$ (where $g_0$ is the vacuum coupling rate) and maximizing field overlap with piezoelectric/electro-optic domains (Yang et al., 8 Oct 2025). Piezoelectric modulation (via on-chip electrodes) of GHz-frequency mechanical modes has been experimentally demonstrated, evidencing the piezo-optomechanical integration capabilities.

5. Fabrication and Integration Strategy

TFLN FP microresonators utilize advanced nanofabrication for both optical and mechanical precision. The process typically involves e-beam lithography (masking with HSQ), dry RIE with Ar gas, and dual-mode etching (half for ridge, full for holes). Suspension by buffered oxide etch and critical point drying creates high-quality, low-loss air-clad waveguides (Yang et al., 8 Oct 2025).

Electrode integration (e.g., 50 nm Au, liftoff patterning adjacent to the waveguide) exploits the suspended architecture for direct piezoelectric and electro-optic access. The modular nature of SLR/MZI reflectors further supports robust circuit scaling, as SLRs tolerate significant variations in MZI splitting ratios, and integrated phase shifters with isolation trenches permit dense, energy-efficient actuation (Qi et al., 29 May 2025).

6. Applications and Prospects

The advances in TFLN FP microresonator technology translate directly into a diverse range of photonic applications:

• Nonlinear photonics: The combination of high-Q, high mode confinement, and strong χ^2, χ^3 enables efficient second-harmonic generation, Kerr combs, and parametric processes.
• Hybrid microwave–optical transduction: High optomechanical coupling and piezoelectric actuation allow development of devices for coherent microwave-to-optical conversion, key to future quantum networks.
• Programmable photonic circuits: Modular, low-loss, and fabrication-tolerant SLR-FP building blocks (with mW-level tuning) support scalable and reconfigurable optical processors and sensors (Qi et al., 29 May 2025).
• Sensing: Spectral selectivity and dual-polarization operation (as implemented in SiN FPs with SLRs) (Xiong et al., 5 Sep 2025) make FP microresonators acutely sensitive to local environmental changes and suitable for label-free detection schemes.

7. Challenges and Future Directions

TFLN FPs face certain challenges: the mechanical fragility of suspended structures, the necessity for precise control in nanoscale tapering, and the integration of metal electrodes without degrading optical performance. Ongoing advances in nanofabrication (e.g., refinement of etching and cleaning steps), mirror design (e.g., optimized taper and apodization), and scalable photonic integration (incorporating EO/TO tuning, complex routing, and low-power operation) are rapidly pushing toward the material Q limit (~163 million) (Zhu et al., 25 Feb 2024) and toward robust complex photonic systems operating across the telecom and visible bands.

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Summary Table: Representative TFLN Fabry–Perot Microresonator Metrics

Device Type	Q (Loaded/Intrinsic)	Mirror Type	Tuning Method	Footprint
PhC-FP (Yang et al., 8 Oct 2025)	6×10⁵	Tapered PhC	Piezo, EO, TO	100 µm cavity
SLR-FP (Qi et al., 29 May 2025)	2×10⁶ (intrinsic)	Sagnac loop	EO/TO phase shifters	1–5 mm cavity
PhC-FP (Hwang et al., 19 May 2025)	1.4×10⁶ (intrinsic)	Apodized PhC	Passive/thermal	200–900 µm cavity

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TFLN Fabry–Perot microresonators synthesize advances in cavity engineering, nonlinear material response, loss minimization, and on-chip actuation, offering high Q, flexible control, and integration readiness for next-generation nonlinear, quantum, and programmable photonic circuits.