CW Vacuum Ultraviolet Laser

• Continuous-wave VUV lasers are coherent light sources operating in the 100–200 nm range using nonlinear processes like SHG and four-wave mixing.
• They overcome material and phase-matching challenges through resonantly enhanced techniques and advanced quasi-phase matching for ultranarrow linewidths.
• These lasers enable high-precision applications such as nuclear clocks, quantum metrology, and deep-UV spectroscopy by delivering stable, tunable, sub-Hz linewidth outputs.

Continuous-wave (CW) vacuum ultraviolet (VUV) lasers are photonic sources emitting coherent radiation in the 100–200 nm spectral interval with continuous temporal operation. These lasers address fundamental experimental needs in atomic, molecular, nuclear, and quantum optical physics, including precision spectroscopy, quantum information processing, and the development of optical nuclear clocks. Generating CW VUV light is technologically challenging due to material restrictions on nonlinear conversion, stringent phase-matching requirements, strong absorption below 200 nm, and the necessity for ultranarrow linewidths often well below 1 kHz. Recent advances—especially resonantly enhanced four-wave mixing (FWM) in vapor-phase nonlinear media—have facilitated the creation of intense, ultranarrow-linewidth, tunable CW VUV sources that fulfill the requirements of next-generation quantum and nuclear experiments.

1. Principles of CW VUV Laser Generation

CW VUV lasers exploit nonlinear frequency conversion mechanisms due to the lack of direct gain media at VUV wavelengths. The two dominant approaches are cascaded second-harmonic generation (SHG) in nonlinear crystals and resonantly enhanced four-wave mixing in atomic vapors or gases:

• Cascaded SHG: Frequency quadrupling is achieved via two sequential SHG stages. For example, a Ti:sapphire laser at 854.4 nm is doubled to 427.2 nm (in LBO), and then to 213.6 nm (in BBO) using cavity-enhanced SHG. The overall output power is given by the product of successive stage efficiencies and is highly sensitive to phase matching, crystal walk-off, and thermal effects (Möller et al., 10 Jun 2025).
• Four-Wave Mixing (FWM) in Vapors: Coherent CW VUV radiation at 148.4 nm is generated by mixing three laser beams in cadmium vapor—two photons at 375 nm and one at 710 nm. This nonlinear process is dramatically boosted by exploiting two-photon atomic resonances, which enhance the third-order susceptibility, $|\chi^{(3)}|$. The output power is proportional to $$P_4 \propto P_1^2\cdot P_3 \cdot |\chi^{(3)}|^2 \cdot G$$, where $G$ is a phase-matching factor, and the linewidth reflects the coherence of the input CW lasers and the resonant enhancement (Xiao et al., 25 Jul 2025, Xiao et al., 24 Jun 2024).
• SHG with Random Quasi-Phase Matching: CW VUV generation at 148.4 nm can be achieved by SHG in randomly poled strontium tetraborate (SBO), where random domain orientation provides an effective quasi-phase-matching without periodic poling. The conversion efficiency scales linearly with crystal length, $P_\text{SHG} \propto L$, and quadratically with fundamental power, $P_\text{SHG}\propto (P_\text{fundamental})^2$ (Lal et al., 23 Jul 2025).

2. Characteristic Performance Metrics and Conversion Schemes

Characteristics of modern CW VUV lasers include tunability, output power, linewidth, and operational stability:

Method	Wavelength (nm)	Output Power	Linewidth
FWM (Cd vapor)	148.4	100 nW	< 100 Hz (sub-Hz)
Quadrupled Ti:sapph	213	130 mW	$\ll$ 1 MHz
SHG in SBO	148.4	$\sim$1.3 nW	Hz–kHz (projected)

FWM in cadmium vapor achieves sub-Hz linewidth (<0.08 Hz FWHM), five orders-of-magnitude narrower than previous lasers below 190 nm. Its output is broadly tunable (at least 140–175 nm) by adjusting the input wavelengths. Quadrupled Ti:sapphire systems offer high output power in the deep UV (e.g., 213 nm, 130 mW) with multihour stability and moderate tunability. SHG in SBO is currently limited by quasi-phase-matching domain randomness, yielding nW VUV powers, but with potential for MHz–kHz linewidth and improved power spectral densities upon stabilization (Xiao et al., 25 Jul 2025, Möller et al., 10 Jun 2025, Lal et al., 23 Jul 2025).

3. Nonlinear Optical Processes and Phase Coherence

The efficiency of VUV generation via nonlinear mixing is governed by atomic or crystal properties, pump beam quality, and phase coherence/phase-matching:

• Resonant Enhancement: FWM processes are resonantly enhanced by tuning two input photon frequencies to a two-photon resonance in the nonlinear medium, resulting in large values of $|\chi^{(3)}|$ (Xiao et al., 24 Jun 2024, Xiao et al., 25 Jul 2025). The output power is described by

$$P_4 = \frac{9}{4}\frac{\omega_1 \omega_2 \omega_3 \omega_4}{\pi^2 \epsilon_0^2 c^6} \frac{1}{b^2 (\Delta k_a)^2} |\chi_a^{(3)}|^2 P_1 P_2 P_3\, G(bN\Delta k_a)$$

where $b$ is the confocal parameter, $\Delta k_a$ is the per-atom wavevector mismatch, $N$ is number density, and $G$ is a phase-matching function maximized when $bN\Delta k_a = -4$.

• Phase Noise Multiplication: For VUV generation via FWM, the phase noise in fundamental lasers is carried into the VUV output as $$\Delta\phi_{\text{VUV}} = 2\Delta\phi_{375} + \Delta\phi_{710}$$, thus phase-stabilizing input lasers is essential for sub-Hz VUV linewidths (Xiao et al., 25 Jul 2025).
• Random Quasi-Phase Matching: In SBO crystals, locally random domain orientation yields an effective quasi-phase matching for SHG: the output power increases only linearly with crystal length. Efficient VUV generation requires tight beam focusing, multi-axis crystal positioning, and careful thermal management (Lal et al., 23 Jul 2025).

4. Applications: Nuclear Clocks, Quantum Metrology, Spectroscopy

The availability of intense, ultranarrow-linewidth, tunable CW VUV lasers enables several key scientific advances:

• 229Th Nuclear Clock: The exceptionally low-energy isomer transition in $^{229}$Th at 148.4 nm is uniquely accessible to VUV lasers (Xiao et al., 25 Jul 2025, Xiao et al., 24 Jun 2024, Lal et al., 23 Jul 2025). Intense, sub-Hz linewidth lasers enable coherent Rabi manipulation of the nuclear transition, foundational for high-precision nuclear optical clocks with fractional frequency uncertainty potentially below $10^{-19}$.
• Quantum Metrology and Nuclear Quantum Optics: VUV lasers allow for precision tests of fundamental constants, search for new physics, and realization of quantum control protocols in nuclear and atomic systems.
• Rydberg-Ion Quantum Computing: CW VUV sources make feasible direct excitation of Rydberg states of trapped ions (e.g., Al⁺), removing dephasing associated with two-photon schemes and enabling high-fidelity quantum logic (Xiao et al., 25 Jul 2025).
• High-Resolution VUV Spectroscopy and Material Physics: Applications include angle-resolved photoemission spectroscopy (ARPES), surface science, and studies of quantum many-body systems, benefitting from the high energy resolution and excellent coherence of modern VUV lasers.

5. Experimental Innovations and Linewidth Control

Key technical challenges in VUV laser operation include phase noise management, nonlinear medium selection, and output stability:

• Phase Noise Characterization: The spatially resolved homodyne technique is developed to place sub-Hz limits on VUV phase noise and linewidth. Two independently generated VUV beams are interfered with a small crossing angle, and CCD-based fringe displacement analyses extract the phase evolution and Allan deviation, confirming $\lesssim0.08$ Hz FWHM linewidth (Xiao et al., 25 Jul 2025).
• Thermal and Material Management: SBO crystal-based SHG systems require precise temperature stabilization (via Peltier modules) and operation in high-purity N₂ to avoid VUV power drift and maintain stable quasi-phase matching (Lal et al., 23 Jul 2025). In FWM systems, vapor cell temperature, beam focusing, buffer gas composition, and vapor density are tightly controlled for optimal conversion efficiency and phase matching (Xiao et al., 25 Jul 2025).
• Broad Tunability: Tuning input beam wavelengths and exploiting different atomic/vapor resonances in FWM permit access to a wide swath of the VUV spectrum. The operational range is set by accessible input lasers, atomic transitions in the vapor, and phase-matching conditions.

6. Comparison to Previous and Pulsed VUV Sources

Prior VUV generation relied on pulsed four-wave mixing in noble gases, frequency tripling in Xe or Kr, or high-harmonic generation in solids or noble gases. These methods were limited either by broad linewidths (>GHz), low power spectral density, or narrow tunability (Lal et al., 23 Jul 2025, Gray et al., 2020). CW FWM in Cd vapor now represents a five-orders-of-magnitude leap in linewidth control (from GHz to sub-100 Hz) below 190 nm (Xiao et al., 25 Jul 2025). SHG in random quasi-phase-matched crystals demonstrates viable CW VUV generation but with weaker output power; efficiency may be improved with engineered domain structures. Unlike pulsed lasers, CW VUV sources offer steady-state operation essential for velocity-insensitive spectroscopy, clock applications, and quantum control.

7. Future Directions and Technical Challenges

Anticipated progress includes:

• Engineering patterned nonlinear crystals for improved quasi-phase matching and output, enabling reproducible high-efficiency SHG at VUV wavelengths (Lal et al., 23 Jul 2025).
• Further reduction of phase noise below the sub-Hz regime in FWM systems for ultimate clock precision.
• Scaling VUV output power by increasing fundamental CW laser powers, optimizing phase-matching geometries, and using external enhancement cavities (Xiao et al., 24 Jun 2024, Xiao et al., 25 Jul 2025).
• Extension of VUV sources to shorter wavelengths (<130 nm), limited by window material transmission, nonlinear medium transparency, and increased reabsorption.
• Integration of CW VUV lasers into new application areas: quantum information processing, ultrafast spectroscopy (by combining CW sources with ultrashort pulses), and nanolithography.

• For ultranarrow-linewidth, tunable CW VUV generation via resonantly enhanced FWM in Cd vapor: (Xiao et al., 25 Jul 2025, Xiao et al., 24 Jun 2024).
• For high-power continuous-wave quadrupled Ti:sapph at 213 nm using cavity-enhanced SHG: (Möller et al., 10 Jun 2025).
• For SHG in randomly quasi-phase matched SrB₄O₇ crystals at 148.4 nm: (Lal et al., 23 Jul 2025).
• For comparison of pulsed VUV sources and pressure-broadened absorption limitations: (Gray et al., 2020).
• For practical continuous-wave deep-UV designs based on quadrupled Yb fiber-amplified systems: (Burkley et al., 2018, Shaw et al., 2021).

These developments collectively mark the emergence of robust, high-coherence, tunable, and intense CW VUV laser sources addressing fundamental challenges in nuclear, atomic, and quantum optical science.