Significance Femtosecond laser technology plays an important role in the study of ultrafast dynamics of light-induced reactions. Many ultrafast spectroscopy techniques, such as transient absorption spectroscopy, ultrafast Raman spectroscopy, and ultrafast photoelectron spectroscopy/imaging, are widely used in scientific research in the fields of physics, chemistry, biology, and materials science. At present, the laser wavelength range produced by mature commercial femtosecond lasers is mainly limited to infrared, visible, and ultraviolet (UV) bands. When the absorption spectrum or ionization energy of a sample is in the vacuum ultraviolet (VUV) band below the wavelength of 200 nm (6 eV), commercial femtosecond laser pulses are insufficient for achieving single-photon excitation/ionization of a sample. The two-photon or multiphoton absorption of long-wavelength lasers leads to low excitation/ionization efficiency compared with the single-photon process. In the past two decades, the technology for developing a miniaturized tabletop femtosecond VUV laser source using commercial femtosecond lasers (such as Ti: sapphire laser) in laboratories has advanced rapidly. This review briefly introduces four-wave mixing (FWM) techniques, which are widely used in a tabletop femtosecond VUV laser source. This work mainly focuses on the development of FWM in gas-filled hollow fibers and filaments. Progress An early femtosecond VUV laser system was capable of producing tunable femtosecond VUV pulses by two-photon near-resonant four-wave difference-frequency mixing in argon (Fig. 3). To generate VUV pulses using the four-wave difference-frequency mixing scheme, high-intensity femtosecond laser pulses are required as a driving source. Moreover, only a part of the incident spectrum can contribute effectively to the frequency-mixing process, thereby leading to spectrally narrowed and temporally lengthened VUV pulses. Therefore, this near-resonant requirement limits the phase-matching bandwidth, tunability, pulse width, and conversion efficiency. FWM is achieved by converting the frequency of ultrashort-pulse Ti: sapphire laser pulses from visible light into deep UV light using a hollow-fiber geometry (Fig. 4). Collinear phase matching using off-resonant X-(3) processes in a hollow fiber to generate VUV light is more efficient and generates broader bandwidths than past schemes. It is confirmed that the conversion efficiency can be significantly improved by exciting higher-order transverse modes and coating the inner surface of the hollow fiber with aluminum. Another method for producing ultrafast VUV pulses is developing FWM in a filament (Fig. 6, Fig. 7, and Fig. 9). Typically, considerably more energy-driving pulses can be used in a filament than in a hollow fiber. Furthermore, the alignment of two laser beams, e.g., the third harmonic and fundamental of a Ti: sapphire laser, in a gas cell is considerably easier than in a narrow hollow fiber. Intensity clamping and mode-cleaning effects of filamentation provide stable and spatially clean output pulses. In contrast to the above-mentioned FWM schemes, VUV pulses with remarkable high pulse energies can be generated via a third-harmonic generation process (Fig. 10). However, the conversion efficiency of the high-harmonic generation process from fundamental radiation is low, and it is desirable to avoid such a significant loss of VUV pulse energy. A short-wavelength driving laser eases this difficulty. To acquire tunable femtosecond VUV pulses, the use of optical parametric amplifier (OPA) or noncollinear optical parametric amplifier (NOPA) systems is considered (Fig. 11). Although it is convenient to produce continuously tunable femtosecond pulses using OPA or NOPA systems, the wavelength range covered by continuously tunable tabletop VUV lasers is limited. Conclusions and Prospects In the past two decades, the development of tabletop femtosecond VUV laser sources has made great progress. The demand for developing femtosecond VUV laser sources is increasing in tandem with the advancement of scientific research and application. In the future, it is critical to improve the frequency up-conversion efficiency of the high-harmonic generation/FWM process and continuously investigate the development and application of new nonlinear media.
