Milliwatt-level UV generation using sidewall poled lithium niobate

1. Problem Statement

Integrated coherent ultraviolet (UV) light sources are critical for applications such as ion-based quantum computing, optical clocks, and advanced microscopy. However, generating UV light on-chip faces significant challenges:

• Material Limitations: Semiconductor laser diodes in the UV range are scarce, lack tunability, and have broad linewidths.
• Nonlinear Conversion Barriers: While Thin-Film Lithium Niobate (TFLN) offers high nonlinearity and UV transparency, previous attempts to generate UV via Second Harmonic Generation (SHG) in TFLN have been limited to microwatt-level powers (e.g., ~30 µW).
• Specific Technical Bottlenecks:
  • High Propagation Loss: Previous TFLN waveguides suffered from high losses (~7.6 dB/cm) due to sidewall scattering and fabrication imperfections.
  • Incomplete Domain Inversion: Standard "pole-after-etch" methods typically place electrodes only on the etched slab, failing to invert the entire waveguide cross-section (specifically the unetched ridge). This results in low effective nonlinearity (often <20% of theoretical maximum).
  • Duty Cycle Instability: Achieving the optimal 50% poling duty cycle over centimeter-long waveguides with small poling periods (µm-scale) has been difficult due to sensitivity to film thickness variations and poling field fluctuations.

2. Methodology

The authors propose a novel Sidewall Poled Lithium Niobate (SPLN) approach to overcome these limitations.

• Waveguide Design:

  • Geometry: A 600 nm thick TFLN film with a 500 nm etch depth, creating a ridge waveguide with a 1.75 µm top width and an 80 nm slab.
  • Optimization: The width was chosen to minimize phase-matching sensitivity to fabrication variations (specifically width and film thickness) while maintaining high optical confinement and low sidewall scattering.
  • Adapted Poling: The poling period was calculated to vary along the waveguide (1.799 µm to 1.814 µm) to compensate for measured film thickness and etch depth variations.

• Sidewall Poling Technique:

  • Electrode Placement: Unlike traditional methods, metal electrodes are patterned directly onto the sidewalls of the waveguide ridge, extending from the slab up the side of the ridge.
  • Mechanism: This placement ensures the electric field penetrates the entire cross-section (ridge and slab), enabling 100% domain inversion across the waveguide.
  • Precision: Electrodes are aligned with <50 nm tolerance using electron-beam lithography.
  • Iterative Poling: Poling is performed iteratively with high-resolution Second Harmonic (SH) microscopy to verify the duty cycle, targeting a precise 50% duty cycle.

• Fabrication:

  • The process involves optical/e-beam lithography, reactive ion etching, and a lift-off process for Cr/Au electrodes.
  • Post-poling, electrodes are removed to minimize optical absorption, and facets are cleaved/etched for coupling.

3. Key Contributions

• Novel Poling Architecture: Introduction of the sidewall poling method that achieves complete cross-sectional domain inversion, a feat previously unattainable with standard pole-after-etch techniques.
• Record-Low Loss: Demonstration of TFLN waveguides with propagation losses as low as 2.3 dB/cm at 405 nm (UV) and 0.6 dB/cm at 780 nm (visible).
• High-Efficiency Conversion: Achievement of a normalized conversion efficiency of 5050 %W⁻¹cm⁻², the highest reported for TFLN UV generation.
• Milliwatt-Level Output: The first demonstration of on-chip UV generation exceeding the milliwatt threshold in TFLN.

4. Experimental Results

• UV Power Generation:
  • Generated 4.2 mW of on-chip UV power at 390.6 nm (upconverted from 780 nm pump).
  • This represents an improvement of more than two orders of magnitude over the state-of-the-art (previous record: ~30 µW).
• Efficiency Metrics:
  • Normalized Efficiency: 5050 %W⁻¹cm⁻² (undepleted pump regime).
  • Absolute Efficiency: 24.5% at ~10 mW input power.
• Propagation Losses:
  • Measured 2.3 dB/cm at 405 nm and 0.6 dB/cm at 780 nm.
  • Comparison with narrower (0.55 µm) waveguides showed significantly higher losses (5.4 dB/cm), validating the design choice of wider waveguides to reduce sidewall scattering.
• Phase Matching:
  • Observed a narrow (~15 pm) sinc²-shaped phase-matching peak, indicating uniform phase matching along the 1.5 cm waveguide length.
  • Demonstrated thermal tunability of 30 pm/°C over a 2.3 nm range.
• Nonlinear Loss Analysis:
  • At high powers (>3 mW), efficiency saturation was observed, attributed to nonlinear losses (Two-Photon Absorption).
  • Modeling suggests that while literature values for TPA explain data up to ~5 mW, further investigation into TFLN material properties at these intensities is required.

5. Significance and Impact

• Practical Viability: Reaching the milliwatt regime makes integrated TFLN UV sources viable for demanding applications like quantum computing (ion trapping), optical atomic clocks, and super-resolution microscopy, which previously required bulky bulk-crystal setups or lacked sufficient power.
• Scalability and Reproducibility: The sidewall poling method is robust against voltage variations and fabrication tolerances, offering a scalable path for mass-producing high-quality integrated UV sources.
• Wavelength Versatility: The platform can be adapted for various UV wavelengths (down to the LN bandgap of ~350 nm) simply by adjusting the poling period, without changing the fundamental chip architecture.
• Linewidth Preservation: By upconverting narrow-linewidth visible lasers, this approach promises fully integrated UV sources with ultra-narrow linewidths, overcoming current limitations in direct UV laser diodes.

In conclusion, this work resolves the long-standing trade-off between high nonlinearity and low loss in TFLN UV generation, establishing a new benchmark for integrated photonic light sources.