Beat-frequency-resolved two-dimensional electronic spectroscopy: disentangling vibrational coherences in artificial fluorescent proteins with sub-10-fs visible laser pulses

We perform a beat-frequency-resolved analysis for two-dimensional electronic spectroscopy using a high-speed and stable 2D electronic spectrometer and few-cycle visible laser pulses to disentangle the vibrational coherences in an artificial fluorescent protein. We develop a highly stable ultrashort light source that generates 5.3-fs visible pulses with a pulse energy of 4.7 uJ at a repetition rate of 10 kHz using multi-plate pulse compression and laser filamentation in a gas cell. The above-5.3-fs laser pulses together with a high-speed multichannel detector enable us to measure a series of 2D electronic spectra, which are resolved in terms of beat frequency related to vibrational coherence. We successfully extract the discrete vibrational peaks behind the inhomogeneous broadening in the absorption spectra and the vibrational quantum beats of the excited electronic state behind the strong stationary signal in the typical 2D electronic spectra

Characterization of attosecond XUV pulses utilizing a broadband UV~VUV pumping

We propose a simple scheme to characterize attosecond extreme ultraviolet (XUV) pulses. A broadband ultraviolet (UV) ∼ vacuum ultraviolet (VUV) pump pulse creates a coherent superposition of atomic bound states, from which photoionization takes place by the time-delayed attosecond XUV probe pulse. Information on the spectral phase of the XUV pulse can be extracted from the phase offset of the interference beating in the photoelectron spectra using a standard SPIDER (spectral phase interferometry for direct electric-field reconstruction) algorithm. We further discuss the influence of the chirp and polychromaticity of the pump pulse, and show that they do not spoil the reconstruction process. Since our scheme is applicable for various simple atoms such as H, He, and Cs, etc., and capable of characterizing attosecond XUV pulses with a pulse duration of a few hundred attoseconds or even less, it can be an alternative technique to characterize attosecond XUV pulses. Specific numerical examples are presented for the H atom utilizing the 2p and 3p states

Reconstruction of attosecond pulses using two-color pumping

We propose a two-color pumping scheme to characterize attosecond extreme ultraviolet (XUV) pulses. The fundamental and its second harmonic of a femtosecond Ti:sapphire laser create a coherent superposition of the 4p and 5p states of K, and we retrieve the spectral phase of the XUV pulse from the phase offset of the photoelectron signal as a function of time delay after the pump pulse. The scheme is technically simple and efficient to characterize ∼100 as pulses

Transient-reflection spectroscopy of laser-plasma formation on solid and liquid surfaces

We investigate ultrafast dynamics of laser-plasma formation by measuring transient reflection from a PM formed by an intense laser pulse. Transient reflectivity measurement enables us to visualize spatial dynamics of ablating surfaces. The spallation in excited fused silica (FS) and organic polymers is clearly identified from a temporal oscillation lasting for longer than 500 ps with a period of several tens of picoseconds. The surface expansion and densification in the outer region of an ablating hole are also suggested by the spatial dependence of the transient reflectivity measurement. Using UV and VUV pulses, we measure the increase in reflectivity of the excited FS and water liquid sheet surfaces. On the basis of the Drude model, it is suggested that the electron density increases to the level of 10^{22} cm^{-3}. The spectrum measurement of the transient reflection provides more exact time-dependent reflectivity including phase through the cross-correlation type frequency-resolve optical gating analysis.4th Asian symposium on Intense Laser Science (ASILS) online semina

Mixing time of homogeneous/heterogeneous solutions in a micro-mixer with free impinging jets

We have developed a micro-mixer based on a free impinging liquid-sheet jet technique. We identified a mixing position where two different solutions mixed uniformly and evaluated a corresponding mixing time in the liquid-sheet jets with homogeneous combination (water and water) and heterogeneous combination (ethanol and water). Both combinations could produce the liquid-sheet jet with a length of 3-4 mm, which corresponds to a time-range of longer than 100 μs. By observing a quenching reaction of N-Acetyl-L-tryptophan amide by N-Bromosuccinimide, the mixing times were evaluated to be 36 μs for the homogeneous combination (H2O/H2O), and 46 μs for the heterogeneous combination (C2H5OH/H2O). To clarify the mixing mechanism in the liquid-sheet jet, the theoretical mixing times were calculated by two different models assuming laminar and turbulence flows. Simulations based on molecular diffusion across a well-defined interface for the laminar flow showed a large discrepancy with the experimental results. The calculated mixing times based on energy dissipation in the turbulence flow are in great agreement with the observed mixing times for both H2O/H2O and C2H5OH/H2O combinations. These results indicate that turbulent mixing is a dominant mixing mechanism in the liquid-sheet jet, and that no clear interface is formed between H2O solutions and between C2H5OH and H2O solutions. The liquid-sheet jet technique provides a windowless and ultra-thin target, ideal for applications with X-ray or intense laser pulses, and would be useful to investigate intermediates in mixing-driven chemical reactions such as an oxidation in solution and a folding reaction of proteins proceeding in a microsecond time scale