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Martensitic crystal structures are usually obtained by rapid thermal quenching of certain alloys, which induces stress and subsequent shear deformation. Here, we demonstrate that it is also possible to intentionally excite a suitable transverse acoustic phonon mode to induce a local shear deformation. We irradiate the surface of a partially stabilized zirconia plate with intense terahertz pulses and verify martensitic transformation from the tetragonal to the monoclinic phases by Raman spectroscopy and the observed destructive spallation of the zirconia microcrystals. We calculate the phonon modes in tetragonal zirconia and determine the effective channel that triggers the transformation. These modes can be excited via the Klemens process. Since terahertz pulses can induce a specific local shear deformation beyond thermal equilibrium, they can be used to elucidate phase transformation mechanisms with dynamical approaches. Terahertz-induced martensitic transformation is considered to be useful for material strengthening and shape memory ceramics.

The control of magnetic domain in correlated spin systems by optical pulses has been gaining importance in spintronics and related fields. In particular, ultrafast control of magnetic domain is one of the top areas of interest. Herein we demonstrate the magnetic domain control in ErFeO3 single crystal, a weak ferromagnet, induced by high-intensity terahertz (THz) pulse excitation from a free-electron laser. THz pulses with a central frequency of 4 THz and pulse energy in the order of 10 mJ were used. The microscopic images of the magnetic domain inversion induced by the THz irradiation were recorded by the Faraday-rotation method using a He–Ne probe laser. The experimental results of the THz pulse irradiation at room temperature and at low temperature around the spin reorientation phase transition are presented. We show that the THz pulse excitation can expand the minor magnetic domain toward the direction of the THz beam spot. The magnetic domain inversion is caused by the combination of depinning of the magnetic domain due to local heating and the entropic force owing to the thermal gradient induced by THz irradiation.

Modification of structure and morphology of polymers has attracted great interest, because their physical properties such as mechanical strength and thermal property strongly depend on their structure and morphology. In particular, the structural change associated with crystallization drastically modifies their physical properties. In this work, we report on the intense-terahertz-wave-induced crystallization of poly(lactic) acid with the terahertz free electron laser (THz-FEL). The moderate THz-FEL irradiation causes the α-phase crystallization in the PLA thin film. This result indicates that the THz-FEL is one of the promising methods to control the morphology and structure of polymeric materials.

Reflection type measurements were performed on narrow-gap semiconductors such as InSb and InAs bulk samples for harmonic generation using intense terahertz pulses from free electron laser. We successfully measured the characteristic azimuthal angle dependence of the third harmonic generation (THG). The results show cos3θ and cos6θ dependence at different polarization layouts for InAs(111).

Reflection type measurements for harmonic generation using intense terahertz pulses from free electron laser for InSb and InAs were performed. We successfully observed the spectra third harmonic generation (THG) and 5th harmonic generation (5HG) from the semiconductor surfaces.

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. 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

The influence of the terahertz (THz) radiation for deep tissues has been neglected due to its strong absorption by water molecules on the surface of the skin. In this report, we found for the first time the demolishing effect of the THz pulsed radiation on actin filaments located in aqueous solution with a few millimeters in depth. This result demonstrates that the THz photon energy propagates in water with several millimeters in depth, possibly as shockwave, and affected actin filaments. Thus, it is implicated that THz waves have the potential to manipulate functions of proteins and cells in deep tissues.

We performed ferromagnetic domain control in Erbium orthoferrite by excitation of the terahertz free electron laser. The magnetic domain structure, which was monitored by faraday microscopy, changes permanently by the irradiation of high-intensity terahertz pulses near the domain edges. The mechanism of the change of the magnetic domain structure was discussed and explained by the combination of the depinning effect and the entropic force induced by the excitation.

Understanding the interaction between intense terahertz (THz) electromagnetic fields and spin systems has been gaining importance in modern spintronics research as a unique pathway to realize ultrafast macroscopic magnetization control. In this work, we used intense THz pulses with pulse energies in the order of 10 mJ/pulse generated from the terahertz free electron laser (THz-FEL) to irradiate the ferromagnetic domains of ErFeO3 single crystal. It was found that the domain shape can be locally reconfigured by irradiating the THz − FEL pulses near the domain boundary. Observed domain reconfiguration mechanism can be phenomenologically understood by the combination of depinning effect and the entropic force due to local thermal gradient exerted by terahertz irradiation. Our finding opens up a new possibility of realizing thermal-spin effects at THz frequency ranges by using THz-FEL pulses.

With rapid advances occurring in terahertz (THz) radiation generation techniques, the interaction between matter and intense THz fields has become an important research topic. Among different types of THz radiation sources, the free electron laser (FEL) is a promising experimental tool that is expected to pave the way for new forms of material processing, control of phase transitions, and serve as a test bench for extreme operating conditions in high-speed small-size electrical and magnetic devices through the exploitation of strong THz electrical and magnetic fields without the presence of interband electronic excitation. In the current work, we irradiated Ge2Sb2Te5 phase change memory material with intense THz pulse trains from an FEL and observed THz-induced surface changes due to damage as a precursor to ablation and the formation of fine surface undulations whose spatial period is comparable to or slightly smaller than the wavelength of the excitation THz pulses in the material. The formation of undulations as well as the fact that no significant thermal effect was observed below the volume expansion threshold suggests that THz-induced effects mainly contributed to the observed changes. To the best of our knowledge, this is the first experimental observation of THz-induced undulations (so-called “LIPSS”), which are of potential importance for laser material processing.

A higher intensity THz beam is generated with a free-electron laser (FEL) based on the L-band (1.3 GHz) electron linac at Osaka University by increasing the electron bunch charge four times higher from 1 nC in the conventional mode (the 108 MHz mode) to 4 nC in the new mode (the 27 MHz mode). This is realized by expanding the electron bunch intervals four times longer with the grid pulser of the electron gun, generating a series of 5 ns pulses at a repetition frequency of 27 MHz (the 48th subharmonic of 1.3 GHz), whereas the beam loading of the linac remains unchanged. The energy of an FEL macropulse comprised of many micropulses is measured in the 27 MHz mode to be approximately 28.5 mJ at a maximum of around 65 μ m or 4.6 THz compared with the maximum macropulse energy of 13 mJ in the 108 MHz mode. The maximum micropulse energy in the 27 MHz mode is approximately 260 μ J using 110 micropulses, which is estimated by a macropulse duration of 4 μ s and micropulse intervals of 36.9 ns. The micropulse energy is nearly 10 times higher than the micropulse energies in the conventional mode and in other FELs in the same wavelength or frequency region.

Using intense beam with frequency 4 THz from Free Electron Laser to pump the InSb crystal, we obtained strong third harmonic generation (THG) with efficiency up to $4 \mathrm{x}10^{-4}$ at room temperature. The THG can originate from the nonlinear third order electric current due to non-parabolicity of the band structure in InSb. InSb yields significantly higher THG signal compared with InAs whose shape of energy band structure is more parabolic. The THG signal increases up to 2.5 times at 370 K due to the increase of the free carrier.

A grid pulser for a thermionic-cathode electron gun for an L-band electron linac with an RF frequency of 1.3 GHz was developed in an effort to increase the power of a terahertz (THz) free-electron laser (FEL) based on the linac. The grid pulser can generate a train of electron pulses with a 5-ns duration at intervals of 36.9 ns or at a repetition frequency of 27 MHz, which is the 48th sub-harmonic of the RF frequency, and with a peak current of up to 2.4 A or higher, which is four times higher than the current that can be obtained with the conventional grid pulser. In combination with the sub-harmonic buncher (SHB) system, whose fundamental frequency is 108 MHz (the 12th sub-harmonic of the RF frequency), an electron beam that comprises electron bunches separated by 36.9-ns intervals for a period of 8 μs can be successfully accelerated using the linac to an energy of 15 MeV under the condition that the average beam current or, correspondingly, the beam loading in the acceleration tube of the linac is the same as that obtained using the conventional grid pulser. The time-resolved energy spectrum indicates that the energy is constant over the latter 6 μs and that the energy spread is 1.2% (FWHM); this performance satisfies the requirements for the FEL. The bunch charge measured at the end of the FEL beamline is 4 nC, which is four times higher than that obtained using the conventional grid pulser.

We developed a solid-state switch with static induction thyristors for the klystron modulator of the L-band electron linear accelerator (linac) at the Institute of Scientific and Industrial Research, Osaka University. This switch is designed to have maximum specifications of a holding voltage of 25 kV and a current of 6 kA at the repetition frequency of 10 Hz for forced air cooling. The turn-on time of the switch was measured with a matched resistor to be 270 ns, which is sufficiently fast for the klystron modulator. The switch is retrofitted in the modulator to generate 1.3 GHz RF pulses with durations of either 4 or 8 μs using a 30 MW klystron, and the linac is successfully operated under maximum conditions. This finding demonstrates that the switch can be used as a high-power switch for the modulator. Pulse-to-pulse variations of the klystron voltage are measured to be less than 0.015%, and those of RF power and phase are lower than 0.15% and 0.1°, respectively. These values are significantly smaller than those obtained with a thyratron; hence, the stability of the main RF system is improved. The solid-state switch has been used in normal operation of the linac for more than a year without any serious trouble. Thus, we confirmed the switch’s robustness and long-term reliability.

We report time-of-flight mass spectrometry under the irradiation of the terahertz free electron laser pulses with different excitation frequencies to clarify the novel desorption caused by the strong excitation resonance with the THz modes in hydrogen-bonded solids.

Field–sensitive nonlinear phenomena in semiconductors beyond the conventional perturbation theory have been investigated recently using extremely intense terahertz pulses. This year spring, we reported the anomalous luminescence under the irradiation of the THz free electron laser (THz-FEL) pulses[1]. The results, strong excitation frequency- and power-dependences of the luminescence from ZnO nanoparticles and surrounding nitrogen molecules, reflected the kinetic energy and trajectory of electrons outside the nanoparticles, which provided a new excitation path. However, one THz-FEL macropulse consists of about 100 FEL micropulses separated at 37 ns intervals (27 MHz repetition), which complicates the luminescence via undesired charge build-up on the nanoparticles.

In this presentation, we succeed in altering polymer morphology using terahertz (THz) wave irradiation. Pulsed THz waves generated by a free electron laser (FEL) were irradiated to a poly(3-hydroxybutylate) (PHB) / chloroform solution during solvent casting crystallization. Morphological observation shows the formation of micrometer-sized crystals via the THz wave irradiation. Further, a 10–20% increase in crystallinity is observed through analysis of the infrared (IR) absorption spectra.

We investigated the soft desorption mechanism of organic molecules under irradiation of intense THz free electron laser pulses resonant for intermolecular motions in the solids by time-of-flight mass spectrometry and time-resolved luminescence measurements.

We report anomalous luminescence in ZnO nanocrystals under the nonresonant THz free electron laser excitation. The strong excitation-frequency and power dependences of the time-resolved luminescence measurement show that the electrons emitted via field electron emission excite the nitrogen molecules in the air and nanoparticles selectively.

As terahertz (THz) frequencies correspond to those of the intermolecular vibrational modes in a polymer, intense THz wave irradiation affects the macromolecular polymorph, which determines the polymer properties and functions. THz photon energy is quite low compared to the covalent bond energy; therefore, conformational changes can be induced “softly,” without damaging the chemical structures. Here, we irradiate a poly(3-hydroxybutylate) (PHB) / chloroform solution during solvent casting crystallization using a THz wave generated by a free electron laser (FEL). Morphological observation shows the formation of micrometer-sized crystals in response to the THz wave irradiation. Further, a 10−20% increase in crystallinity is observed through analysis of the infrared (IR) absorption spectra. The peak power density of the irradiating THz wave is 40 MW/cm2, which is significantly lower than the typical laser intensities used for material manipulation. We demonstrate for the first time that the THz wave effectively induces the intermolecular rearrangement of polymer macromolecules.

The authors developed terahertz (THz) imager which incorporates 320x240 focal plane array (FPA) with enhanced sensitivity in sub-THz region (ca. 0.5 THz). The imager includes functions such as external-trigger imaging, lock-in imaging, beam profiling and so on. The function of the external-trigger imaging is mainly described in this paper, which was verified in combination of the THz imager with the pulsed THz free electron laser (THz-FEL) developed by Osaka University. The THz-FEL emits THz radiation in a wavelength range of 25 - 150 μm at repetition rates of 2.5, 3.3, 5.0 and 10 pulses per second. The external trigger pulse for the THz imager was generated with a pulse generator, using brightening pulse for THz-FEL. A series of pulses emitted by the THz-FEL at 86 μm were introduced to the THz imager and Joule meter via beam splitter, so that the output signal of THz imager was normalized with the output of the Joule meter and the stability of the THz radiation from FEL was also monitored. The normalized output signals of THz imager (digits/μJ) obtained at the repetition rates mentioned above were found consistent with one another. The timing-relation of the external trigger pulse to the brightening pulse was varied and the influence of the timing-relation on beam pattern is presented. These experimental results verify that the external trigger imaging function operates correctly.

We report on luminescence in ZnO nanocrystals under intense THz free electron laser excitation. The spectrum of the THz-induced luminescence is drastically changed for the excitation frequency due to field electron emission by avalanche breakdown.

We have developed a new feed-forward control system to precisely control the amplitude and phase of the pulsed RF power in an electron linear accelerator (linac) to make the accelerating field constant. Fast variations and ripples in the amplitude and the phase in RF pulses of some-microsecond duration are compensated for by applying anti-variation control signals to a variable attenuator and a phase shifter in the low-level system. The system makes use of the overdrive technique, which is commonly used in analog circuits, to speed up the slow response of the analog phase shifter, while the control signals are digitally processed; thus, the method is a hybrid of analog and digital techniques. Peak-to-peak variations in the amplitude and phase are reduced by these means from 11.6% to 0.4% and from 6.1° to 0.3°, respectively, in 7.6-μs-long RF pulses for the L-band electron linac at Osaka University.

Polymer electrolyte fuel cells (PEMFC) have emerged as portable power source for future transportation and home based systems. Over the past years perfluorosulfonated cation exchange membranes have been used as solid polymer electrolyte in fuel cells, however, humidity control and water management have remained major issues in the area of stack development. Local drying that results in the changes in the local conductivity behavior and physical failure of the membrane is one of the key issues to be addressed in detail. It is therefore necessary to estimate these changes precisely to evaluate the membrane behavior during the fuel cell operation. Positron annihilation spectroscopy offers a very sensitive probe to the determination of polymer free-volume as well as electronic state of the polymer. In the present study we studied Nafion 117 with varying temperature and humidity using positron annihilation spectroscopy. The ortho-positronium lifetime (o-Ps, i.e., τ3) which gives the size of the free-volume holes inside the polymer was determined. It was found that the o-Ps lifetime increases for relative humidity between 40% and 60% and thereafter it decreases in the temperature range of 62–95 °C. When the inversion point at which the o-Ps lifetime is the maximum was plotted with actual vapor pressure, it was observed that it has a logarithmic behavior with the ordinate intercept of 1 ns. This indicates that at higher temperature the membrane has larger free-volume holes; and also at higher temperature a larger vapor pressure is essential to saturate the membrane. Above this point the clusters expand and the free-volume hole size decreases. This also indicates the optimum vapor in the cell to be maintained during the fuel cell operation. The studies also indicated that there is a correlation between o-Ps lifetime and the hydrogen crossover current density under the similar temperature and humidity condition.

High-power extreme ultra-violet (EUV) sources are required for next generation semiconductor lithography. We start to develop a compact EUV source in the spectral range 13–14 nm, which is based on a laser Compton scattering between a 7 MeV micro-bucnhed electron beam and a high-intensity CO 2 laser pulse. The electron beam extracted from a DC photocathode gun is micro-bunched using a laser modulation techinque with the Compton wavelength at a harmonic of the seeding laser before the main laser Compton scattering for EUV generation. A considerating scheme for the compact EUV source based on the laser Compton scattering with micro-bunched electron beam and the analytical study of micro-bunch generation are described in this paper.

Free electron laser (FEL) and self-amplified spontaneous emission (SASE) are being developed in the far-infrared region using the L-band electron linac at the Institute of Scientific and Industrial Research (ISIR), Osaka University. The L-band linac was recently remodeled extensively not only for higher operational stability and reproducibility but also for high power operation of FEL. After commissioning of the linac, we first began SASE experiment with a newly-developed strong-focusing wiggler. Recently we began FEL experiment and obtained lasing with the high peak power at 70 μm again after a long break.

The proton conductivity in Polymer Electrolyte Membrane in Fuel Cells (PEMFC) is affected by the size of free spaces and hydrogen bond at the hydrophilic group. These parameters are influenced by the environmental conditions. We tried to correlate annihilation process of positron and proton conductivity by using Age-Momentum Correlation (AMOC) measurement. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The thermal degradation of Nafion membrane was analyzed with age-momentum correlation (AMOC) measurement and four-electrode AC impedance measurement. In the heated sample, the decrease in proton conductivity was observed. The lifetimes and corresponding relative intensities showed fairly good agreement between heated and nonheated samples within the experimental error. In the analysis of the photopeak of annihilation γ-ray, on the other hand, the difference between those two kinds of samples was observed, and this difference was found to be caused by the annihilation of free positrons with low energy electrons by using AMOC method. The decrease in proton conductivity was caused by the low energy electrons, namely sulfonic radicals.

Both 640×480 and 320×240 terahertz (THz) imagers were developed whose sensitivity were improved in sub-THz region by a factor of 10. The imagers include functions such as external-trigger imaging, lock-in imaging, beam profiling and so forth. The function of the external-trigger imaging was verified, using the pulsed THz free electron laser developed by Osaka University.

Uncooled microbolometer-type 640x480 and 320x240 Terahertz (THz) focal plane arrays (FPAs) with enhanced sensitivity in sub-THz region are developed, and incorporated into 640x480 and 320x240 cameras, respectively. The pixel in the THz-FPA has such a structure that an area sensitive to electromagnetic wave is suspended above read-out integrated circuit (ROIC). A thin metallic layer is formed on the top of the sensitive area, while a thick metallic layer is formed on the surface of ROIC. The structure composed of the thin metallic layer and the thick metallic layer behaves as an optical cavity. The THz-FPAs reported in this paper have a modified pixel structure which has several times longer optical-cavity length than NEC’s previous pixel does, by forming a thick SiN layer on the ROIC. The extended optical-cavity structure is favorable for detecting electromagnetic wave with lower frequency. Consequently, the Minimum Detectable Power per pixel (MDP) is improved ten times in sub-THz region, especially 0.5-0.6 THz. This paper presents spectral frequency dependences of MDP values for THz-FPA with the modified pixel structure and THz-FPA with the previous pixel structure, using THz free electron laser (FEL) developed by Osaka University. The modification of pixel structure extends high sensitivity region to lower frequency region, such as sub-THz region, and the wider spectral coverage of THz camera surely expands its applicability

A picosecond stroboscopic pulse radiolysis system which consists of a femtosecond laser and a picosecond L-band linear accelerator (linac) is demonstrated. A newly installed femtosecond laser system operated at 960 Hz can cover a wide region of wavelength ranging from

We propose a novel concept to make a low-error wiggler to be used for self-amplified spontaneous emission. Magnetization errors of permanent magnet blocks can be reduced by using a pair or quartet of magnet pieces quarried out of a single raw block of permanent magnet. The concept has been tested experimentally using magnet blocks for a trial model of the edge-focus wiggler, which can produce a focusing field as high as 1 T/m in the wiggler. The errors in the magnetization of magnet blocks were considerably reduced. This was calculated with the vertical magnetization axis with which it is easier to measure three components of the magnetic field, and the measured values are reliable. It has been experimentally confirmed that the concept works well.

We are pursing operational stability of the RF linac with a method of system analysis. The RF phase and the power for the pre-buncher have been measured, together with environmental parameters including temperatures of the main components, the room temperatures and the AC line voltage. The measured data have been analyzed with the autoregressive model, and the feedback structure of the system has been derived from noise contribution ratios and impulse responses analysis. It was found that the AC line voltage and the temperatures of the klystron room significantly affected operational stability of the linac.

We are developing a new type of wiggler with a strong focusing force, named the edge-focusing wiggler. It is a Halbach-type wiggler made with permanent magnet blocks with an edge angle φ. An application is given for SASE in the shorter wavelength region. The characteristics of the wiggler are studied in detail using a three-dimensional magnetic field calculation program with the magnetic charge method.

The nonlinear harmonic generation of self-amplified spontaneous emission (SASE) has been studied in the far-infrared region. Angular distributions of the second harmonic and the third harmonic as well as the fundamental of SASE have been measured. The third harmonic radiation is emitted on the electron beam axis, while the second harmonic radiation is emitted slightly off-axis in the horizontal direction. It seems that there is competition in development of radiation between the fundamental and the other higher harmonics.

In order to measure the longitudinal phase-space profile and transverse slice emittances of the electron beam, we are developing a compact measurement system that consists of a Cherenkov radiator and a streak camera. The Cherenkov radiator with the aerogel has been installed on the downstream side of the bending magnet in the beam transport, and the preliminary profile image of the electron beam in the longitudinal phase-space has been obtained.

Image reconstruction method for non-synchronous THz signals was developed for a combination of THz Free Electron Laser (THz-FEL) developed by Osaka University with THz imager. The method employs a slight time-difference between repetition period of THz macro-pulse from THz-FEL and a plurality of frames for THz imager, so that image can be reconstructed out of a predetermined number of time-sequential frames. This method was applied to THz-FEL and other pulsed THz source, and found very effective. Thermal time constants of pixels in 320x240 microbolometer array were also evaluated with this method, using quantum cascade laser as a THz source.

We are conducting experiments to generate self-amplified spontaneous emission (SASE) in the far-infrared region and to measure its characteristics, using a single bunch electron beam accelerated with the L-band linac at ISIR, Osaka University. We have measured the wavelength spectra of SASE in the wavelength region between 80 and 180 μm using a grating monochromator and a Ge : Ga detector. The measured spectral widths of the fundamental peak are slightly larger than those predicted by the one-dimensional model. We have also observed the second harmonic peak of SASE.

Computer simulation studies are currently underway for the proposed FEL amplifier experiment to be carried out using the existing 38 MeV, L-band linac based FIR FEL system at the Institute of Scientific and Industrial Research (ISIR), Osaka University. This paper describes the time-dependent and time-independent simulation results using Genesis code. Dependence of FEL amplifier characteristics are investigated such as the power spectrum and profile, frequency spectrum and FEL bunching parameter on parameters like the beam current, input power and interaction length. The operating current regime and input seed laser powers required for the optimum amplification are given.

The Siam Photon Project promoted at the Siam Photon Laboratory is progressing slowly but steadily. The construction of the laboratory building was completed and the installation of the accelerator complex, that is the modified SORTEC machines from Tsukuba, is now under way. The first stage of the synchrotron assembly has been completed recently. The installation of the injector linac is about to be started. The construction of the additional magnet system, the new machine control system and the new vacuum chambers have been completed. The construction of the first beam line with photoemission experimental stations as well as that for the electron beam monitor will start soon. The design work for an undulator will also start soon. Some problems found in the case of the utilization of a second hand system are described.

We have recently developed a pulsed slow positron beam to be used for the characterization of polymer thin films. It consists of the sections of stretching and pulsing of slow positrons from an electron linac. The positron lifetime spectra measured using this system had a time resolution of 1.4 ns. The details of the system are described.

We are conducting FEL experiments using the infrared FEL system based on the L-band electron linac at ISIR, Osaka University, to extend the wavelength region toward the longer-wavelength side and to study FEL dynamics in the long wavelength region. The FEL wavelength was extended to 150 μm, which is the longest wavelength so far obtained with FELs based on RF linacs. We observed a phenomenon that FEL oscillation was obtained at two different optical cavity lengths but not in between them.

We are conducting experiments of self-amplified spontaneous emission (SASE) in the far-infrared region at ISIR, Osaka University by using a high brightness single-bunch beam from the L-band linac. As the first step, characteristics of the SASE-FEL are estimated using the one-dimensional analytic formulas and the measured characteristics of the electron beam. The calculated FEL parameter is 0.02 and the gain length is 0.14 m at an electron energy of 13 MeV and K =1.472. Recently, we observed a strong optical signal radiated from a single-bunch beam passing through the wiggler for the existing infrared FEL system.