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 µJ 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 incoherent population background in the typical 2D electronic spectra.

Plane photoacoustic wave generation in liquid water using irradiation of terahertz pulses.

We demonstrate photoacoustic wave propagation with a plane wavefront in liquid water using a terahertz (THz) laser pulse. The THz light can effectively generate the photoacoustic wave in water because of strong absorption via a stretching vibration mode of the hydrogen bonding network. The excitation of a large-area water surface irradiated by loosely focused THz light produces a plane photoacoustic wave. This is in contrast with conventional methods using absorbers or plasma generation using near-infrared laser light. The photoacoustic wave generation and plane wave propagation are observed using a system with a THz free-electron laser and shadowgraph imaging. The plane photoacoustic wave is generated by incident THz light with a small radiant exposure of < 1 mJ/cm2 and delivered 600 times deeper than the penetration depth of THz light for water. The THz-light-induced plane photoacoustic wave offers great advantages to non-invasive operations for industrial and biological applications as demonstrated in our previous report (Yamazaki et al. in Sci Rep 10:9008, 2020).

High-speed terahertz color imaging using a 100 kHz line scan camera.

We develop a high-speed two-dimensional (2D) terahertz (THz) color imaging system for practical applications. This system performs THz time-domain spectroscopy (THz-TDS) measurements in one-dimensional (1D) space simultaneously to reduce the spatial scan from 2D to 1D and obtains the 2D THz color image in which the spectral data is possessed in each pixel. We realize measurements on the image with 750 × 1000 pixels (13 mm × 25 mm) with the spatial resolution of 1.5 mm within 10 seconds. This is two orders of magnitude faster than conventional THz color imaging methods. High-speed 2D THz color imaging will be used in non-destructive and non-invasive inspections of industrial products and biological tissues in the future.

Photon Energy-Dependent Ultrafast Photoinduced Terahertz Response in a Microcrystalline Film of CH3NH3PbBr3.

Time-resolved terahertz (THz) spectroscopy is applied for a microcrystalline film of methylammonium lead bromide perovskite, CH3NH3PbBr3, to observe the carrier dynamics around the band edge. The ultrafast response of the transmitted THz electric field amplitude after carrier generation is modeled with a biexponential curve with ∼5 and 180 ps time constants, which are ascribed to Auger and electron-hole recombination processes, respectively. From the pump photon energy dependence of the time evolution of the THz electric field amplitude, it is shown that the bound exciton states and free interband excited carrier states show a clearly different temporal response. These measurements support the idea that the bound excitons generated in CH3NH3PbBr3 remain as stable excitons even at room temperature (RT). This is in clear contrast to the cases in CH3NH3PbI3 in which the excitons and band-edge free carriers are interchangeable at RT.

Propagation of THz irradiation energy through aqueous layers: Demolition of actin filaments in living cells.

The effect of terahertz (THz) radiation on deep tissues of human body has been considered negligible due to strong absorption by water molecules. However, we observed that the energy of THz pulses transmits a millimeter thick in the aqueous solution, possibly as a shockwave, and demolishes actin filaments. Collapse of actin filament induced by THz irradiation was also observed in the living cells under an aqueous medium. We also confirmed that the viability of the cell was not affected under the exposure of THz pulses. The potential of THz waves as an invasive method to alter protein structure in the living cells is demonstrated.

Plasma-mirror frequency-resolved optical gating using a liquid-sheet jet in ultraviolet region.

We demonstrate plasma-mirror frequency-resolved optical gating (PM-FROG) using a liquid-sheet jet of water and characterize a waveform of a high-repetition laser pulse (1 kHz) in the ultraviolet (UV) region. The measured PM-FROG trace is reconstructed by the least-squares generalized projections algorithm. The retrieved UV spectral phase is consistent with that by self-diffraction FROG. The complex reflection coefficient of a plasma mirror is discussed in terms of the free electron density dependence. The PM-FROG technique is applicable to not only the pulse characterization, but also the investigation of ionization dynamics of a liquid.

High-Speed Terahertz Waveform Measurement for Intense Terahertz Light Using 100-kHz Yb-Doped Fiber Laser.

We demonstrate a high-speed terahertz (THz) waveform measurement system for intense THz light with a scan rate of 100 Hz. To realize the high scan rate, a loudspeaker vibrating at 50 Hz is employed to scan the delay time between THz light and electro-optic sampling light. Because the fast scan system requires a high data sampling rate, we develop an Yb-doped fiber laser with a repetition rate of 100 kHz optimized for effective THz light generation with the output electric field of 1 kV/cm. The present system drastically reduces the measurement time of the THz waveform from several minutes to 10 ms.

Yb:YAG thin-disk chirped pulse amplification laser system for intense terahertz pulse generation.

We have developed a 1 kHz repetition picosecond laser system dedicated for intense terahertz (THz) pulse generation. The system comprises a chirped pulse amplification laser equipped with a Yb:YAG thin-disk amplifier. At room temperature, the Yb:YAG thin-disk regenerative amplifier provides pulses having energy of over 10 mJ and spectral bandwidth of 1.2 nm. The pulse duration achieved after passage through a diffraction grating pair compressor was 1.3 ps. By employing this picosecond laser as a pump source, THz pulses having a peak frequency of 0.3 THz and 4 µJ of energy were generated by means of optical rectification in an Mg-doped LiNbO3 crystal.

Liquid-sheet jets for terahertz spectroscopy.

We investigated liquid-sheet jets with controllable thickness for application to terahertz (THz) spectroscopy. Slit-type and colliding-jet nozzles were used to generate optically flat liquid jets. The thickness of the liquid sheet was determined precisely by spectral interference and THz time-domain-spectroscopy methods. By adjusting the collision angle of the colliding-jet nozzle, we could control the thickness of the liquid sheet from 50 to 120 µm.

Contact grating device with Fabry-Perot resonator for effective terahertz light generation.

A novel design for a contact grating device with an incorporated Fabry-Perot resonator is proposed for high-power terahertz (THz) light generation. We deposited a multilayer consisting of Ta(2)O(5) and Al(2)O(3) on a magnesium-doped stoichiometric LiNbO(3) substrate and fabricated grating grooves on the outermost layer. The multilayer was designed such that conditions for a Fabry-Perot resonator were satisfied for light diffracted by the grating. Consequently, the fraction of light transmitted into the LiNbO(3) substrate, i.e., the diffraction efficiency, was enhanced by the resonator. The diffraction efficiency of the fabricated device was 71%, which is close to the calculated value of 78% from the optimized design. THz light generation was also demonstrated with the contact grating device. The THz output of 0.41 µJ was obtained using near-infrared pump light of 2.7 mJ.

Infrared spectroscopy of chloromethyl radical in solid parahydrogen and its nuclear spin conversion.

We present high-resolution infrared absorption spectra of chloromethyl radical produced by in situ UV photolysis of chloroiodomethane isolated in solid parahydrogen. The radicals were stable over a few days in solid parahydrogen kept at 3.6 K. Analysis of the rotation-vibration spectra revealed that the radical exhibited quantized one-dimensional rotational motion about the C-Cl bond, while the ortho and para nuclear spin species were still clearly distinguishable in the spectra. Temporal change of the spectra indicated that the nuclear spin conversion between the ortho and para nuclear spin species of the radical in solid parahydrogen occurred in a time scale of a few hours at 3.6 K. It was also found that the nuclear spin conversion became significantly slower in a higher concentration of chloroiodomethane.

Development of high-efficiency etalons with an optical shutter for terahertz laser pulses.

A high-efficiency etalon operated in the terahertz (THz) frequency region has been proposed to generate a THz pulse train. To achieve high-conversion efficiency to the pulse train, an optical shutter is employed in this etalon. The etalon is composed of a silicon (Si) plate as an input coupler and an indium-tin-oxide (ITO)-coated glass plate as an output coupler. After THz light is introduced into the etalon through the Si plate, the optical shutter pulse irradiates the Si surface to generate a photoconductive layer that acts as a highly reflective mirror for THz light. A THz pulse train and its comb-shaped spectrum have been realized by the use of the proposed etalon with the optical shutter. A finesse F of 9.04 was achieved at the free spectral range of 75 GHz in this etalon.

Terahertz tomography of a photo-induced carrier based on pump-probe spectroscopy using counterpropagation geometry.

A novel technique for the terahertz (THz) tomography of a photo-induced carrier that is based on optical-pump THz-probe time-resolved reflection spectroscopy using counterpropagation geometry of the pump and probe pulses has been proposed. Transient reflection due to the photo-induced carrier provides information about the physical properties and spatial distribution separately. We have experimentally demonstrated this method using a silicon wafer. The obtained complex reflection can be reproduced by the exact solution of Maxwell's equations, assuming an exponential distribution of the photo-induced carrier density.

Laser induced fluorescence spectroscopy of tetracene with large Ar, Ne, and H2 clusters in superfluid He nanodroplets.

Clusters of tetracene molecules with different numbers of attached (Ar)(N), (Ne)(N) and (H(2))(N) particles (N = 1-2000) are assembled inside superfluid He nanodroplets and studied via laser-induced fluorescence. The frequency shift of the fluorescence spectrum of the tetracene molecules is studied as a function of cluster size and pickup order of tetracene and cluster species. For (Ar)(N) and (Ne)(N) clusters, our results indicate that the tetracene molecules reside inside the clusters when tetracene is captured by the He nanodroplet before the cluster species; conversely, the tetracene molecules stay on the surface of the clusters when tetracene is captured after the cluster species. In the case of (H(2))(N) clusters, however, tetracene molecules reside inside the (H(2))(N) clusters irrespective of the pickup order. We conclude that (Ar)(N) and (Ne)(N) clusters are rigid at T = 0.38 K, while (H(2))(N) clusters of up to N = 2000 remain fluxional at the same temperature. The results may also indicate the occurrence of heterogeneous nucleation of the (H(2))(N) clusters, which is induced by the interaction with tetracene chromophore molecules.

Cross-correlation frequency-resolved optical gating for mid-infrared femtosecond laser pulses by an AgGaGeS(4) crystal.

We introduce an AgGaGeS(4) crystal for sum-frequency generation in cross-correlation frequency-resolved optical gating (XFROG) to characterize mid-IR femtosecond laser pulses between the wavelengths of 3 microm and 11 microm. The performance of the AgGaGeS(4) crystal was examined by comparing it with a LiNbO(3) crystal, which has been frequently used in the near- to mid-IR region. We can obtain XFROG images by the AgGaGeS(4) crystal with efficiency 30 times greater than LiNbO(3) at the wavelength of 5 microm.

Photoelectron kinetic energy dependence in near threshold ionization of NO from A state studied by time-resolved photoelectron imaging.

Photoelectron angular distributions in the laboratory frame (LF-PADs) from the A((2)sigma(+)) state of NO molecule were measured by femtosecond time-resolved photoelectron imaging with (1 + 1(')) resonance enhanced multiphoton ionization via the A state. High-precision measurements of the anisotropy parameters of LF-PADs were performed for the photoelectron kinetic energy from 0.03 to 1.05 eV as a function of the pump-probe delay time. The revival feature of the rotational wave packet on the A state was clearly observed in the time dependence of the photoelectron anisotropy parameters. By approximating the phase shifts of the photoelectron partial waves by the quantum defects in the high-lying Rydberg states using the multichannel quantum defect theory, the energy-dependent photoionization transition dipole moments were determined, for the first time, from time-dependent LF-PADs measured by time-resolved photoelectron spectroscopy.