Record nonlinear conversion efficiency in the production of high spectral purity vacuum ultraviolet laser at 148 nm

Technical Summary: Record Nonlinear Conversion Efficiency in 148 nm VUV Laser Production

Problem Statement
The development of Thorium-229 (Th-229) nuclear clocks and high-precision metrology requires intense, narrow-linewidth vacuum ultraviolet (VUV) lasers at approximately 148 nm. While recent experiments have demonstrated resonant excitation of the Th-229 nuclear transition, existing VUV generation methods face significant limitations regarding power scalability, system complexity, and conversion efficiency. Current approaches include:

• High-Harmonic Generation (HHG): Requires extreme peak intensities and suffers from low efficiency and broad spectral spans.
• Four-Wave Mixing (FWM) in atomic vapors: While capable of continuous-wave (CW) operation, it necessitates high-temperature vapor cells and multi-watt driving lasers, complicating compact and field-deployable implementations.
• Random Quasi-Phase-Matched (RQPM) Crystals: Previous solid-state solutions using randomly quasi-phase-matched strontium tetraborate (SBO) have shown promise but are limited by poorly defined microstructures, resulting in lower conversion yields.

The primary challenge is identifying a nonlinear crystal that combines VUV transparency (extending below 150 nm), high second-order nonlinearity ($\chi^{(2)}$), and the capability for quasi-phase-matching (QPM) to enable efficient, scalable, and compact VUV sources.

Methodology
The authors demonstrate a VUV frequency comb generation scheme based on cascaded frequency doubling (Third-Wave Mixing, TWM) of a 2400 nm optical frequency comb. The experimental setup involves a four-stage up-conversion process:

1. Driving Source: An ultrafast polycrystalline Cr:ZnS frequency comb (fundamental wavelength $\lambda \approx 2.4$ µm, pulse width $<24$ fs, repetition rate 80 MHz).
2. First Stage (2f and 4f): The fundamental comb undergoes two consecutive frequency doublings in a single 0.5 mm thick PPLN crystal containing two distinct QPM gratings ($f \to 2f \to 4f$).
3. Second Stage (8f): The resulting 4f signal is frequency doubled in a 0.5 mm thick BBO crystal ($4f \to 8f$), producing light in the UVB range (~300 nm).
4. Final Stage (16f): The 8f signal is focused into a proprietary, bulk-grown, spatially uniform QPM crystal (designated "Xtal-3") developed by IPG Photonics. This crystal is optimized for the $8f \to 16f$ frequency doubling process, targeting the 148 nm wavelength.

The experiments were conducted in an airtight chamber filled with argon gas (1–10 mbar) to prevent absorption of the VUV light by ambient air. VUV power and spectral characteristics were measured using a photomultiplier tube (PMT), a VUV-enhanced silicon photodiode, and an aberration-corrected VUV spectrometer equipped with a CCD sensor.

Key Contributions

• Novel Material: The introduction of a proprietary QPM crystalline material (Xtal-3) that uniquely combines VUV transparency down to 130 nm, high $\chi^{(2)}$ nonlinearity, and optimized QPM structures for the 148 nm regime.
• Efficient Cascaded Conversion: Successful demonstration of a 16th harmonic generation ($f \to 2f \to 4f \to 8f \to 16f$) from a 2.4 µm comb to 148 nm using a chain of nonlinear crystals.
• Record Efficiency: Achievement of a nonlinear conversion efficiency that is an order of magnitude higher than previously reported methods for high-spectral-purity VUV generation.

Results

• Pulsed Performance: Using a 2.5 W average power driving laser (with 60 mW of 8f power incident on the final crystal), the system generated 40–50 µW of average VUV power at 148 nm.
  • This corresponds to a conversion efficiency of approximately $7 \times 10^{-4}$ for the $1 \text{ PHz} \to 2 \text{ PHz}$ frequency doubling step.
  • The VUV output is a frequency comb with 80 MHz mode spacing and a spectral bandwidth of ~5 THz (centered at ~147 nm).
  • The power per comb mode is approximately 1 nW, which is sufficient for initial Th-229 spectroscopy.
• CW Extrapolation: By analyzing the dependence of VUV power on pulse duration (stretching the 8f pulses), the authors extrapolated the performance to a continuous-wave regime.
  • For a 1 W CW input power, the estimated VUV power is approximately 60 nW.
  • This estimated CW efficiency is roughly 60 times higher than that achieved with RQPM-SBO crystals and comparable to FWM techniques, but with a simpler solid-state architecture.
• Scalability: The authors note that the system is inherently power-scalable. Using longer nonlinear crystals (10–20 mm) or enhancement cavities could potentially increase VUV power to the milliwatt level.

Significance
The paper establishes a scalable, compact, and reliable route to generating high-spectral-purity VUV light via direct frequency doubling in solid-state QPM materials. The demonstrated conversion efficiency represents a record for this class of nonlinear processes. These results provide a practical foundation for developing robust, transportable nuclear clock lasers and advancing applications in precision metrology, quantum optics, and materials science. The work addresses the critical need for intense, narrow-linewidth VUV sources required to fully exploit the potential of the Th-229 nuclear clock transition.