In the realm of ultrafast optics, ultrashort laser pulses in the ultraviolet (UV) spectrum play a pivotal role in investigating the ultrafast dynamics of electrons at atomic and molecular scales. To achieve superior temporal resolution, high-energy UV pulses with durations less than 5 femtoseconds (fs) are particularly crucial for the study of chemical reactions, molecular biological phenomena, and surface reactions of materials. Traditional methods for generating UV pulses are often constrained by the initial spectral bandwidth. While low-order harmonic generation can produce ultrafast pulses in the UV range, their pulse widths typically measure in the tens of femtoseconds. This article showcases the generation of sub-5 fs UV pulses at the millijoule level through an innovative combination of second harmonic generation (SHG), spectral broadening, and dispersion compensation techniques.^[1]

The experimental setup is depicted in Figure 1. The primary light source is a titanium-sapphire laser with a central wavelength of 800 nm, pulse energy of 20 mJ (adjustable via a beam splitter), repetition rate of 100 Hz, pulse width of 30 fs, and beam diameter of 18 mm. During the experiment, the beam is loosely focused through a lens with a focal length of 750 mm, then passes through a nonlinear medium comprising a 50 μm thick BBO crystal and a 2 mm thick fused silica substrate to sequentially undergo SHG and self-phase modulation (SPM). The resultant second harmonic and fundamental frequency light are subsequently separated by a beam splitter, with the second harmonic (UV) light collimated by a silver concave mirror and dispersion-compensated using chirped mirrors.

Figure 1: Schematic representation of the experimental setup^[1]

SHG and SPM are the cornerstone nonlinear processes in this experiment, with SHG facilitated by the BBO crystal and SPM occurring in the fused silica substrate. The experimental results reveal that as the nonlinear medium approaches the focal point, both the intensity and bandwidth of the second harmonic light increase significantly. When the BBO crystal is positioned 600 mm from the focus, the spectral width of the second harmonic light expands to 65 nm, nearly tripling the initial fundamental frequency bandwidth. By adjusting the position of the BBO crystal, the bandwidth of the second harmonic light can be effectively tuned to achieve the desired spectral width.

Figure 2: (a) Spectral broadening at varying BBO positions, (b) intensity and bandwidth of second harmonic light at different BBO positions; (c) output spectra of second harmonic light at three distinct positions.^[1]

The authors delve deeper into two potential mechanisms behind the spectral broadening of the second harmonic light. Experiments indicate that the broadening predominantly occurs in the fused silica, with the BBO crystal itself not significantly expanding the spectrum of the second harmonic light. The authors then analyze the dispersion compensation, energy scalability, and output stability of the pulses. Chirped mirrors and wedges are employed to precisely compensate for dispersion, with the compressed pulse measuring 4.8 fs (Fourier Transform Limit of 4.2 fs), a pulse energy of 0.64 mJ, and a compression efficiency of approximately 56%. By adjusting the beam diameter of the incident light, the pulse energy can be scaled up while maintaining a constant energy density. When the input laser energy is increased to 16 mJ, the compressed pulse energy reaches 1.05 mJ with a pulse width still maintained at 5 fs, showcasing the robust energy scalability of the method. Finally, the authors assess the stability of the compressed second harmonic light, recording an energy variation of 1.0% (RMS) and pointing stability of 2.1 μrad and 7.5 μrad, respectively.

Figure 3: Measurement of compressed UV pulse width^[1]

This article presents the generation of sub-5 fs UV ultrafast pulses at the millijoule level using a compact all-solid-state free-space setup that synergizes SHG and SPM. The experimental approach exhibits excellent scalability in pulse energy and output stability. The produced second harmonic light boasts a substantial spectral width (65 nm) and supports ultrashort pulse durations (4.8 fs), positioning it as a promising candidate for applications in nonlinear optics, ultrafast spectroscopy, and attosecond science. Reference: [1]X. Xie, Y. Hung, Y. Deng, A. L. Cavalieri, A. Baltuška, and S. L. Johnson, "Generation of millijoule-level sub-5 fs violet laser pulses," High Power Laser Science and Engineering 12(e16) (2024).