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.

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.

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.

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.

Degradative solvent extraction of biomass using petroleum based solvents.

The aim of this study was to examine the possibility to use two petroleum based solvents, kerosene and a distillate rich in benzene (A150), as practical solvents for the degradative solvent extraction at 350 °C in reference to 1-MN. It was found that the thermal degradation behavior of two biomass samples, a rice straw and Leucaena, in the three solvents was rather similar and that only the distribution of Soluble, Deposit, and Residue was affected by the difference of solubility of the solvents. Preparation of solvent treated biomass (STB) using the three solvents gave the yields close to the sum of the yields of Soluble, Deposit, and Residue. It was judged that A150 may be used to preparing Soluble and that Kerosene can well be used to prepare STB.

Spectral modulation of higher harmonic spontaneous emission from an optical klystron.

Higher harmonics of spontaneous emission from an optical klystron have been observed. The modulation factor of the spontaneous emission spectrum for the higher harmonics can be described by considering the observation system. When the dispersive gap of the optical klystron was fixed, the microstructure interval of the spontaneous emission spectrum at a certain resonant wavelength became narrower as the order of the higher harmonic became larger. Some unique characteristics of the higher harmonics have been clarified, and these studies are likely to contribute to the development of free-electron lasers using higher harmonics of an optical klystron in the shorter-wavelengths region.

The immediate impact of the Fukushima Daiichi accident on local property values.

The Fukushima Daiichi accident released huge amounts of radioactive material over a wide area. We can appreciate the geographical extent of radioactive contamination from the information published online by the Japanese government. Historically, this is an unprecedented situation, which allows "natural experimentation" to estimate the causal effects of radioactive contamination on our society. This study focused on property value losses caused by the accident and analyzed changes in land appraisals around the Fukushima Daiichi plant from July 2010 to July 2011 within the framework of hedonic approach. Thus, we estimated the short-run impact of the contamination or the change in marginal value of proximity to the plant. The results suggest that the appraisals significantly and monotonically depreciated with increasing contamination levels. However, there was no evidence to suggest changes in the marginal value of proximity to the plant. A comparison between the appraisals and transaction prices indicates that this result could be interpreted as an underestimate of actual property value losses.

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.

Nondestructive identification of isotopes using nuclear resonance fluorescence.

Nondestructive identification of heavy isotopes concealed in a thick iron box has been demonstrated by using nuclear resonance fluorescence. A quasi-monochromatic photon beam produced by the collision of laser quanta with high energy electrons was used for resonant excitation of nuclear levels in (206)Pb and (208)Pb. By measuring the resonant γ rays emitted from (206)Pb and (208)Pb, each of these isotopes were clearly identified. The ratio of the effective thickness, i.e., concentration distribution, of these isotopes was deduced from the relative intensities of the measured nuclear resonance fluorescence strengths.

Nuclear power-related facilities and neighboring land price: a case study on the Mutsu-Ogawara region, Japan.

From the perspective of risk, nuclear-power-related facilities (NPRFs) are often regarded as locally undesirable land use. However, construction of NPRFs contributes to social infrastructural improvement and job creation in the host communities. This raises a question: How large are these positive and negative effects? To approach this question from an economic viewpoint, we estimated the hedonic land price function for the Mutsu-Ogawara region of Japan from 1976 to 2004 and analyzed year-by-year fluctuations in land prices around the NPRFs located there. Land prices increased gradually in the neighborhood of the nuclear fuel cycle facilities (NFCFs) in Rokkasho Village, except for some falling (i) from 1982 to 1983 (the first official announcement of the project of construction came in 1983), (ii) from 1987 to 1988 (in 1988, the construction began and opposition movements against the project reached their peak), and (iii) from 1998 to 1999 (the pilot carry-in of spent fuels into the reprocessing plant began in 1998). Land prices around the Higashidori Nuclear Power Plant decreased during the period 1981-1982, when the Tohoku Electric Power Corp. and Tokyo Electric Power Corp. announced their joint construction plan. On the other hand, we obtained some results, even though not significant, indicating that land prices around Ohminato and Sekinehama harbors changed with the arrival and departure of the nuclear ship Mutsu, which suffered a radiation leak in 1974.

Nondestructive detection of hidden chemical compounds with laser Compton-scattering gamma rays.

A nondestructive assay method for measuring a shielded chemical compound has been proposed. The chemical compound is measured by using a nuclear resonance fluorescence (NRF) measurement technique with an energy tunable laser Compton-scattering (LCS) gamma-ray source. This method has an advantage that hidden materials can be detected through heavy shields such as iron plates of a thickness of several centimeters. A detection of a chemical compound of melamine, C(3)H(6)N(6), shielded by 15-mm-thick iron and 4-mm-thick lead plates is demonstrated. The NRF gamma-rays of (12)C and (14)N of the melamine are measured by using the LCS gamma-rays of the energies of up to 5.0 MeV. The observed ratio ((12)C/(14)N)(exp)=0.39+/-0.12 is consistent with (C/N)(melamine)=0.5.