Enhancement of vacuum-ultraviolet dispersive-wave emission using gas-filled tapered hollow-core fibers

Abstract

The recent breakthroughs in laser-driving 229Th nuclear transition have created an urgent demand for coherent vacuum-ultraviolet (VUV) sources delivering high spectral brightness at the critical 148.38 nm isomer energy. However, generating sufficient photon flux to overcome the low nuclear excitation probability remains a challenge for compact setups. While resonant dispersive wave emission in gas-filled hollow-core fibers offers a promising route, standard capillaries face a fundamental trade-off: maximizing input coupling requires large core diameters, whereas efficient nonlinear VUV conversion demands the high intensities using small cores. Here, we resolve this conflict using a gas-filled tapered capillary fiber. This architecture utilizes a longitudinally decreasing core diameter to combine a large input aperture with adiabatic field concentration, thereby continuously enhancing the nonlinear interaction. Experimentally, we demonstrate a widely tunable source (135-240 nm) that achieves a twofold efficiency enhancement specifically at the 148.38 nm wavelength compared to uniform geometries. By providing a scalable route to high-flux VUV generation, this work establishes a critical tabletop tool for advancing solid-state nuclear clocks and time-resolved spectroscopy.

Figures (5)

• Figure 1: Simulated spectral evolution with different capillary geometries. Spectral evolution of the experimental 10-fs pulse as a function of fiber length for three capillary configurations. a. Constant 100-µ m-diameter capillary (0.6 m length) pumped with 100 µ J at 3.8 bar He. Purple dashed line indicates the ZDW at 418 nm. Pink dashed line marks the positions of maximum soliton compression at 36 cm (P1). b. Tapered capillary with core diameter decreasing linearly from 160 to 100 µ m (0.6 m length) pumped with 130 µ J at 3.25 bar He. Pink dashed line marks the positions of maximum soliton compression at 53 cm (P2). c. Constant 160-µ m-diameter capillary (1 m length) pumped with 128 µ J at 1.48 bar He. Purple dashed line indicates the ZDW at 418 nm. d. Peak intensity and ZDW evolution. Left axis: Simulated peak intensity (orange, blue, and green curves) versus fiber length for the three cases a, b, c. Right axis: ZDW evolution (purple line) for the tapered capillary. The intersection of P2 with the ZDW curve confirms that the ZDW at the DW emission point is 416 nm, nearly identical to that of the constant-core capillaries.
• Figure 2: Experimental setup. Schematic of the two-stage pulse compression and DW generation system. The input pulse is with 800 nm wavelength, 1.55 mJ energy, 50-fs duration and 1 kHz repetition rate. The hollow-core fiber (HCF) has 400 µ m core diameter and 1 m length. The tapered capillary of 0.6 m length has tapering core diameter from 160 µ m at the input to 100 µ m at the output. In comparison, the constant-core capillaries have 100 µ m core diameter and 0.6 m length, or 160 µ m core diameter and 1 m length, respectively. L1--L2: plano-convex lenses; W1--W3: fused silica windows; W4: magnesium fluoride (MgF$_2$) window; HWP: half-wave plate; M1--M3: silver mirrors; CMs: chirped mirrors.
• Figure 3: Measured dispersive-wave spectra with different capillary geometries. Evolution of the DW spectra as a function of tuning pressure for: a. Constant-core capillary of 100 µ m diameter. b. Tapered-core capillary of 160 µ m to 100 µ m diameter. c. Constant-core capillary of 160 µ m diameter. The DW energies corresponding to central wavelengths of 148 nm, 180 nm, and 240 nm are explicitly highlighted in red.
• Figure 4: Performance comparison. a. Measured DW output energy as a function of wavelength. Blue curve: tapered-core capillary of 160 µ m to 100 µ m diameter. Orange curve: constant-core capillary of 100 µ m diameter. Green curve: constant-core capillary of 160 µ m diameter. b. Measured DW conversion efficiency as a function of wavelength.
• Figure 5: Spectral power analysis at 148.38 nm wavelength. Measured and normalized DW spectrum centered near the 148.38 nm $^{229}$Th transition. Shaded regions indicate the spectral energy retained after applying super-Gaussian filters centered at 148.38 nm with bandwidths of 0.05 nm (blue shading) and 0.1 nm (purple shading). These filtered bandwidths correspond to retained energy fractions of 0.20% and 0.39%, respectively.