Achromatic frequency doubling of supercontinuum pulses for transient absorption spectroscopy

Clean hundred-µJ-level sub-6-fs blue pulses generated with helium-assisted solid thin plates

In this work, 85 µJ, 5.5 fs pulses spanning 350-500 nm with 96% energy concentrated on the main pulse are generated by pulse compression using a helium-assisted, two-stage solid thin plate apparatus. To the best of our knowledge, these are the highest energy sub-6 fs blue pulses obtained to date. Furthermore, during the spectral broadening process, we observe that solid thin plates are much more easily damaged by blue pulses in a vacuum than in a gas-filled environment at the same field intensity. Helium, with the highest ionization energy and extremely low material dispersion, is adopted to create a gas-filled environment. Thus, the damage to solid thin plates is eliminated, and high-energy, clean pulses can be obtained with only two commercially available chirped mirrors in a chamber. Furthermore, the excellent output power stability of 0.39% root mean square (rms) fluctuations over 1 h is maintained. We believe that few-cycle blue pulses at the hundred-µJ level can open the door to numerous new ultrafast and strong-field applications in this spectral region.

Transient absorption spectroscopy based on uncompressed hollow core fiber white light proves pre-association between a radical ion photocatalyst and substrate

We present a hollow-core fiber (HCF) based transient absorption experiment, with capabilities beyond common titanium:sapphire based setups. By spectral filtering of the HCF spectrum, we provide pump pulses centered at 425 nm with several hundred nJ of pulse energy at the sample position. By employing the red edge of the HCF output for seeding CaF₂, we obtain smooth probing spectra in the range between 320 and 900 nm. We demonstrate the capabilities of our experiment by following the ultrafast relaxation dynamics of a radical cationic photocatalyst to prove its pre-association with an arene substrate, a phenomenon that was not detectable previously by steady-state spectroscopic techniques. The detected preassembly rationalizes the successful participation of radical ionic photocatalysts in single electron transfer reactions, a notion that has been subject to controversy in recent years.

Generation of 5.2 fs, energy scalable blue pulses

In this Letter, ultrashort blue pulses spanning 350-500 nm are generated by combining the broadband frequency doubling technology with the two-stage multiplate continuum (MPC) generation scheme. We prepare relatively broadband input pulses and use a two-stage configuration for MPC generation, allowing us to employ thinner and less solid plates for further spectral broadening. Therefore, the deteriorations of the spectral phase, energy conversion efficiency, and beam quality, which occur more easily for 400 nm pulses, are effectively suppressed. After fine dispersion management, we obtain clean 5.2 fs blue pulses with a root-mean-square energy stability of 0.69% over one hour and excellent beam quality. Furthermore, lower than 8% energy loss during the spectral broadening process at each stage is achieved. The overall optimized performances and energy scalability of this blue pulse, as well as the possibility of further compressing the pulse duration, are likely to motivate more strong-field research with sub-cycle time resolution in this extended wavelength range.