The ultrafast motions of electrons and nuclei in atoms and molecules play a crucial role in describing their interaction with matter and light. These motions occur over timescales ranging from picoseconds for nuclei to attoseconds for electrons. In many applications, ultrashort pulses are necessary to initiate and investigate specific motions of electrons and nuclei through a pump-probe scheme. Femtosecond pulses in the violet and ultraviolet spectral range are essential for a wide range of applications in fields such as physics, chemistry, biology, and materials science. Time-resolved violet and ultraviolet spectroscopy with sub-hundred femtosecond pulses provides access to processes involving nuclear motions in chemical reactions and biological phenomena. Even shorter pulses are highly desired for observing and controlling fast electron and nuclear motions in the sub-10 fs regime. However, the violet and ultraviolet pulse duration in low-order harmonic generation is limited by the bandwidths of the near-infrared fundamental pulses, which generally support pulse durations of only tens of femtoseconds.

In a study published in High Power Laser Science and Engineering, vol. 12, Issue 2 (Xinhua Xie, Yi Hung, Yunpei Deng, Adrian L. Cavalieri, Andrius Baltuška, Steven L. Johnson. Generation of millijoule-level sub-5 fs violet laser pulses[J]. High Power Laser Science and Engineering, 2024, 12(2): 02000e16), researchers from Paul Scherrer Institute in Switzerland and TU Wien in Austria showcase a compact, robust, and pulse energy-scalable method for generating intense and broadband violet pulses in the sub-5 fs regime, representing a significant advancement in ultrafast laser technology. This methodology opens new avenues for advances in nonlinear optics and ultrafast spectroscopy.

Since the pulse duration of a laser pulse is limited by its spectrum bandwidth, it is essential to broaden the spectrum, generating new colors, of the pulse to achieve a shorter pulse duration. The experiment utilized a thin beta barium borate (BBO) crystal on a fused silica glass substrate as the nonlinear interaction medium. By combining second harmonic generation in the BBO crystal with self-phase modulation in the glass substrate, millijoule-level broadband violet pulses were efficiently generated from a single optical component. The BBO crystal converts near-infrared laser light to violet light through frequency doubling, and the generated strong violet laser induces self-phase modulation in the glass substrate, effectively generating multicolored light around the violet spectrum region. The multicolored spectrum covered a range of ultraviolet (down to 310 nm), violet, blue, cyan, and green (up to 550 nm) with an impressive full width at half maximum bandwidth of 65 nm. Subsequently, the researchers compressed the colorful broadband beam to an unprecedented duration of 4.8 femtoseconds with a pulse energy of 0.64 millijoules, or 5 femtoseconds with a pulse energy of 1.05 millijoules, using a chirped mirror compressor.

Graphic description: A focused near-infrared laser beam propagates through a BBO crystal and its glass substate. Second harmonic generation (SHG) occurs in the BBO crystal. Self-phase modulation of the violet second harmonic beam in the glass significant broadens the spectrum, generating new colors to cover a spectral range from ultraviolet to visible.

Lead author Dr. Xinhua Xie emphasized the importance of their findings, stating, "Our study paves the way for generating intense violet and ultraviolet broadband pulses in the sub-5 fs regime, which holds great potential for various applications in nonlinear physics. Our all-solid setup offers several advantages, including its compact size, robustness, and scalability in terms of pulse energy."

The paper is poised to inspire further research and innovation in the field of ultrafast laser technology, driving advancements in fundamental science and technological applications alike.