Nonlinear compression of naturally down-chirped superradiance pulses from a free-electron laser oscillator by thick germanium plates.

Naturally down-chirped superradiance pulses, with mirco-pulse energy, peak wavelength, and micropulse duration of 40 µJ, 8.7 µm, and 5.1 optical cycles, respectively, emitted from a free-electron laser (FEL) oscillator were nonlinearly compressed down to 3.7 optical cycles using a 30-mm-thick Ge plate. The peak power enhancement owing to nonlinear compression was found to be 40%. The achieved peak power and pulse duration were comparable to those of recently developed high-intensity and few-cycle long-wavelength infrared sources based on solid-state lasers. FEL oscillators operating in the superradiance regime can serve as unique tools for studying strong-field physics in long-wavelength infrared regions.

Dissociation process of polyalanine aggregates by free electron laser irradiation.

Polyalanine (polyA) disease-causative proteins with an expansion of alanine repeats can be aggregated. Although curative treatments for polyA diseases have not been explored, the dissociation of polyA aggregates likely reduces the cytotoxicity of polyA. Mid-infrared free electron laser (FEL) successfully dissociated multiple aggregates. However, whether the FEL dissociates polyA aggregates like other aggregates has not been tested. Here, we show that FEL at 6.1 µm experimentally weakened the extent of aggregation of a peptide with 13 alanine repeats (13A), and the irradiated 13A exerted lesser cytotoxicity to neuron-like cells than non-irradiated 13A. Then, we applied molecular dynamics (MD) simulation to follow the dissociation process by FEL. We successfully observed how the intermolecular ß-sheet of polyA aggregates was dissociated and separated into monomers with helix structures upon FEL irradiation. After the dissociation by FEL, water molecules inhibited the reformation of polyA aggregates. We recently verified the same dissociation process using FEL-treated amyloid-ß aggregates. Thus, a common mechanism underlies the dissociation of different protein aggregates that cause different diseases, polyA disease and Alzheimer's disease. However, MD simulation indicated that polyA aggregates are less easily dissociated than amyloid-ß aggregates and require longer laser irradiation due to hydrophobic alanine repeats.

Full characterization of superradiant pulses generated from a free-electron laser oscillator.

The detailed structure of superradiant pulses generated from a free-electron laser (FEL) oscillator was experimentally revealed for the first time. Owing to the phase retrieval with a combination of linear and nonlinear autocorrelation measurements, we successfully reconstructed the temporal waveform of an FEL pulse including its phase variation. The waveform clearly exhibits the features of a superradiant pulse, the main pulse followed by a train of sub-pulses with π-phase jumps, reflecting the physics of light-matter resonant interaction. From numerical simulations, the train of sub-pulses was found to originate from repeated formation and deformation of microbunches accompanied with a temporal slippage of the electrons and light field, a process quite different from coherent many-body Rabi oscillations observed in superradiance from atomic systems.

Crystallinity in periodic nanostructure surface on Si substrates induced by near- and mid-infrared femtosecond laser irradiation.

Laser-induced periodic surface structure (LIPSS), which has a period smaller than the laser wavelength, is expected to become a potential technique for fine surface processing. We report the microscopic and macroscopic observations of the crystallinity of LIPSSs, where the characteristics such as defects generation and residual strain were analyzed, respectively. The LIPSSs were formed on a Si substrate using two different femtosecond pulses from Ti:Sapphire laser with near-infrared wavelength (0.8 µm) and free-electron laser (FEL) with mid-infrared wavelength (11.4 µm). The photon energies of the former and latter lasers used here are higher and lower than the Si bandgap energies, respectively. These LIPSSs exhibit different crystalline states, where LIPSS induced by Ti:Sapphire laser show residual strain while having a stable crystallinity; in contrast, FEL-LIPSS generates defects without residual strain. This multiple analysis (microscopic and macroscopic observations) provides such previously-unknown structural characteristics with high spatial resolution. To obtain LIPSS with suitable properties and characteristics based on each application it is paramount to identify the laser sources that can achieve such properties. Therefore, identifying the structural information of the LIPSS generated by each specific laser is of great importance.

Degradation of Lignin by Infrared Free Electron Laser.

Lignin monomers have attracted attention as functional materials for various industrial uses. However, it is challenging to obtain these monomers by degrading polymerized lignin due to the rigid ether linkage between the aromatic rings. Here, we propose a novel approach based on molecular vibrational excitation using infrared free electron laser (IR-FEL) for the degradation of lignin. The IR-FEL is an accelerator-based pico-second pulse laser, and commercially available powdered lignin was irradiated by the IR-FEL under atmospheric conditions. Synchrotron-radiation infrared microspectroscopy analysis showed that the absorption intensities at 1050 cm-1, 1140 cm-1, and 3400 cm-1 were largely decreased alongside decolorization. Electrospray ionization mass chromatography analysis showed that coumaryl alcohol was more abundant and a mass peak corresponding to hydrated coniferyl alcohol was detected after irradiation at 2.9 µm (νO-H) compared to the original lignin. Interestingly, a mass peak corresponding to vanillic acid appeared after irradiation at 7.1 µm (νC=C and νC-C), which was supported by our two-dimensional nuclear magnetic resonance spectroscopy analysis. Therefore, it seems that partial depolymerization of lignin can be induced by IR-FEL irradiation in a wavelength-dependent manner.

Enhancement of Oxidation of Silicon Carbide Originating from Stacking Faults Formed by Mode-Selective Phonon Excitation Using a Mid-Infrared Free Electron Laser.

Silicon carbide (SiC) is a promising material for wide applications due to its excellent material properties including high physical and chemical stability as well as great electronic properties of a wide bandgap. The high stability, however, makes its surface processing difficult. Especially, electrochemical processing is not well-established because of low electrochemical reactivity. Here, we show that selective phonon excitation by a mid-infrared free electron laser (MIR-FEL) enhances the anodic reactions. The selective excitation of two different vibration modes of the Si-C bond induces two different stacking faults, which act as a current path. As an application, we discovered that MIR-FEL irradiation enables Pt electroless deposition. This work reveals the interactions among phonons, lattice defects, and electrochemical reactions, encouraging further development of not only electrochemical surface processing but also a new application of MIR-FEL.

Study of the origin of the complex beam profile of a hole-coupled free electron laser oscillator.

Hole coupling is a way to extract laser beams from an optical cavity through a hole on a cavity mirror. Hole coupling is often used in free electron laser oscillators and inherently causes a non-Gaussian beam profile at user stations, which are more than 10 m apart from the coupling hole. The laser beam extracted from a coupling hole has a bright central region (Airy disk) surrounded by a series of concentric rings (Airy pattern). The existence of an Airy pattern results in a non-Gaussian beam profile. It was demonstrated that the beam profile can be changed from a non-Gaussian to a nearly Gaussian distribution by removing the Airy pattern in the experiments and physical optics calculations.

Application of mid-infrared free-electron laser for structural analysis of biological materials.

A mid-infrared free-electron laser (MIR-FEL) is a synchrotron-radiation-based femto- to pico-second pulse laser. It has unique characteristics such as variable wavelengths in the infrared region and an intense pulse energy. So far, MIR-FELs have been utilized to perform multi-photon absorption reactions against various gas molecules and protein aggregates in physical chemistry and biomedical fields. However, the applicability of MIR-FELs for the structural analysis of solid materials is not well recognized in the analytical field. In the current study, an MIR-FEL is applied for the first time to analyse the internal structure of biological materials by using fossilized inks from cephalopods as the model sample. Two kinds of fossilized inks that were collected from different strata were irradiated at the dry state by tuning the oscillation wavelengths of the MIR-FEL to the phosphoryl stretching mode of hydroxyapatite (9.6 µm) and to the carbonyl stretching mode of melanin (5.8 µm), and the subsequent structural changes in those materials were observed by using infrared microscopy and far-infrared spectroscopy. The structural variation of these biological fossils is discussed based on the infrared-absorption spectral changes that were enhanced by the MIR-FEL irradiation, and the potential use of MIR-FELs for the structural evaluation of biomaterials is suggested.

Photo-Modification of Melanin by a Mid-infrared Free-electron Laser.

Melanin is rigidly constructed by several nitrogen-containing aromatic rings, and its excess accumulation in skin tissue is closely associated with melanosis. Although visible lasers (wavelength: 600-1000 nm) are conventionally used for the photo-thermolysis of melanocyte, several pigmented nevi are difficult to be treated. Here, we propose an alternate method for targeting the molecular structure of melanin using an infrared free-electron laser (FEL) tuned to 5.8 µm that corresponds to the stretching vibrational mode of carboxylate group. A drastic morphological change on the black-colored surface of melanin powder was observed after the pulse irradiation with power energy of 500 mJ cm-2 , and the minimum irradiation time for damage to the morphology was 1.4 s. Analyses by mass spectroscopy, infrared spectroscopy, and 13 C-nuclear magnetic resonance implied that a pyrrole group was removed by the FEL irradiation. In addition, the FEL irradiation dispersed almost all of the melanoma cells from a culture solution without any influence on other ingredients in the medium, and one-cell analysis by infrared microscopy showed that the structure of melanoma could be substantially damaged by the irradiation. This study proposes the potency of intense mid-infrared laser as novel alternative way to reduce melanin.

Real-time observation of structural change in semi-crystalline polymer films through mid-IR transmission spectroscopy: determination of instantaneous film temperature during recrystallization.

We report the real-time observation of entire structural change in dye-doped semi-crystalline polymer (polyethylene) films through mid-IR transmission spectroscopy. The laser-heated dye molecules heat the polymer film through thermal diffusion, and accordingly the polymer film undergoes the structural change from the crystalline to amorphous structures, which is followed by the reverse structural change, namely recrystallization, during the natural cooling. By tuning the mid-IR probe pulse to one of the few structure-sensitive vibrational modes and varying the time delay between the pump and probe pulses we can monitor the structural change of the polymer film and time-varying film temperature during recrystallization through the transmission change of the resonant mid-IR probe pulse with the time-resolution of sub-µs.

Pulse duration and wavelength stability measurements of a midinfrared free-electron laser.

We report the pulse duration and wavelength stability measurements of a midinfrared free-electron laser (FEL) where the wavelength fluctuation may not be negligible. The technique we employ is a fringe-resolved autocorrelation (FRAC) method that has good sensitivity on not only the pulse duration and the chirp but also the wavelength stability. By the simple manipulation of experimental FRAC signals, we can obtain the pulse duration even if the amounts of the chirp and the wavelength stability are not known in advance, which is further used to estimate the wavelength stability. Through this procedure we find that the pulse duration of the Kyoto University FEL at 12 µm is about 0.58 ps without any notable chirp, and the wavelength stability is about 1.3%. We also carry out separate experiments for intensity autocorrelation and sum-frequency mixing. The difference we find for pulse duration and wavelength stability by the different measurements is attributed to the different operation conditions of FEL.

Single-shot spectra of temporally selected micropulses from a mid-infrared free-electron laser by upconversion.

We demonstrate the measurement of single-shot spectra of temporally selected micropulses from a mid-infrared free-electron laser (FEL) by upconversion. We achieve the upconversion of FEL pulses at 11 µm using externally synchronized Nd:YAG or microchip laser pulses at 1064 nm to produce sum-frequency mixing signals at 970 nm, which are detected by a compact CCD spectrometer without an intensifier. Our experimental system is very cost-effective, and allows us to obtain the laser spectra of selected micropulses at any temporal position within a single macropulse from an oscillator-type FEL.