Introduction
A research team led by Prof. Meng Pang and Associate Prof. Zhiyuan Huang at the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, in collaboration with Prof. Junqiu Liu’s team at the International Quantum Academy, has achieved a major advance in ultrafast vacuum-ultraviolet (VUV) laser science. The team proposed and experimentally demonstrated a synergistic optimization strategy based on gas-filled tapered hollow-core fibers. This approach overcomes the fundamental trade-off in conventional hollow-core fibers between input coupling efficiency and nonlinear conversion efficiency, enabling highly efficient generation of high-flux VUV dispersive waves at the key wavelength of 148.38 nm.
The work, titled “Enhancement of vacuum-ultraviolet dispersive-wave emission using gas-filled tapered hollow-core fibers,” was published as a Letter in Physical Review Applied. In recognition of its significance for solid-state nuclear clock research, the paper was selected as an Editors’ Suggestion and featured on the journal homepage.
Research Background
Ultrafast VUV sources, with their high temporal resolution and broad spectral coverage, are powerful tools for exploring fundamental light-matter interactions. Recently, growing attention has focused on using VUV lasers to drive the 229Th nuclear transition, whose isomer energy is approximately 8.36 eV, corresponding to a wavelength of about 148.38 nm. However, the extremely low excitation probability of this nuclear transition places stringent demands on the photon flux of the VUV source.
Among tabletop approaches, resonant dispersive-wave (DW) emission in gas-filled hollow-core fibers is considered one of the most promising routes for VUV generation. Yet conventional constant-diameter capillaries suffer from an inherent physical trade-off. A larger core diameter supports efficient coupling of high-energy pump pulses, but its weaker nonlinearity tends to red-shift the DW emission, making access to the VUV region difficult. By contrast, a smaller core provides the high intensity and strong nonlinearity required for VUV conversion, but fundamentally limits coupling efficiency and energy handling. This long-standing contradiction has constrained efficient scaling of compact VUV sources toward shorter wavelengths.
Research Highlights
To address this bottleneck, the team introduced a tapered hollow-core fiber with a core diameter that gradually decreases along the propagation direction. The fiber has an entrance core diameter of 160 μm, allowing efficient coupling of high-energy pump pulses. As the pulse propagates, the core adiabatically narrows to 100 μm, progressively concentrating the optical field and enhancing the waveguide nonlinearity.
A key advantage of this design is that it decouples the ionization dynamics at the fiber entrance from those at the pulse-compression point. The large input aperture keeps the intensity below the ionization threshold at the entrance, thereby avoiding the coupling failure caused by plasma defocusing in traditional small-core fibers under strong pumping. As a result, the system maintains a robust coupling efficiency of about 70%. At the same time, the decreasing core diameter postpones strong ionization until the self-compression stage near the fiber end. There, the generated plasma induces a soliton self-frequency blueshift, increasing spectral overlap with the VUV resonance and further boosting the conversion efficiency.
Experimental Validation
The team systematically compared fibers with different geometries. The tapered capillary showed outstanding performance over a broad tuning range from 135 to 240 nm. Most importantly, at the critical wavelength of 148.38 nm required for driving the 229Th nuclear clock transition, the tapered fiber generated 1.7 μJ of VUV dispersive-wave energy from an 84 μJ pump pulse, corresponding to a conversion efficiency of about 2.0%.
By comparison, a conventional 100 μm constant-core fiber required 194 μJ of pump energy to produce only 0.9 μJ of VUV output, with a conversion efficiency of just about 0.46%. These results clearly show that the tapered architecture nearly doubles the output energy while improving the conversion efficiency by a factor of 4.3.
Enhanced VUV Performance of the Tapered Hollow-Core Fiber
Summary and Outlook
This work breaks the energy-scaling limit of constant-core fibers both experimentally and theoretically, and provides a robust new architectural route toward high-flux VUV sources. More importantly, this highly efficient single-pass conversion platform opens a clear path toward VUV optical frequency combs.
By replacing the current kilohertz pump laser with a phase-stabilized infrared frequency comb operating at megahertz repetition rates, the platform could potentially generate a VUV frequency comb at the critical 148.38 nm wavelength. Combining high photon flux with exceptional coherence, such a source would substantially improve the signal-to-noise ratio and long-term stability of nuclear-transition detection, and could become a key enabling technology for future solid-state nuclear clocks.
Publication Information
The co-first authors of the paper are Yinuo Zhao, Donghan Liu, and Baoqi Shi. The corresponding authors are Associate Prof. Zhiyuan Huang, Prof. Meng Pang, and Prof. Junqiu Liu. This work was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Science, the Shanghai Science and Technology Plan Project Funding, the Shanghai Municipal Science and Technology Major Project, the Shenzhen Science and Technology Program, the Shenzhen-Hong Kong Cooperation Zone for Technology and Innovation, and the Quantum Science and Technology--National Science and Technology Major Project.
Enhancement of vacuum-ultraviolet dispersive-wave emission using gas-filled tapered hollow-core fibers, Physical Review Applied 25, L041001 (2026). Original link: https://doi.org/10.1103/ym25-kz9p
