M. Hosaka

Observation of intracavity Compton backscattering of the UVSOR free electron laser

Abstract High-energy gamma rays produced by intracavity Compton backscattering of a free electron laser (FEL) on the UVSOR storage ring have been observed. In the experiment, the storage ring was operated at an electron energy of 600 MeV. The FEL wavelengths were 466 and 270 nm, and then maximum photon energies were calculated to be 14.6 and 25.2 MeV, respectively. Spectra of the photons measured by using a large NaI scintillation detector were found to be consistent with a theoretical calculation.

Chirality Emergence in Thin Solid Films of Amino Acids by Polarized Light from Synchrotron Radiation and Free Electron Laser

One of the most attractive hypothesis for the origin of homochirality in terrestrial bioorganic compounds is that a kind of “chiral impulse” as an asymmetric excitation source induced asymmetric reactions on the surfaces of such materials such as meteorites or interstellar dusts prior to the existence of terrestrial life (Cosmic Scenario). To experimentally introduce chiral structure into racemic films of amino acids (alanine, phenylalanine, isovaline, etc.), we irradiated them with linearly polarized light (LPL) from synchrotron radiation and circularly polarized light (CPL) from a free electron laser. After the irradiation, we evaluated optical anisotropy by measuring the circular dichroism (CD) spectra and verified that new Cotton peaks appeared at almost the same peak position as those of the corresponding non-racemic amino acid films. With LPL irradiation, two-dimensional anisotropic structure expressed as linear dichroism and/or linear birefringence was introduced into the racemic films. With CPL irradiation, the signs of the Cotton peaks exhibit symmetrical structure corresponding to the direction of CPL rotation. This indicates that some kinds of chiral structure were introduced into the racemic film. The CD spectra after CPL irradiation suggest the chiral structure should be derived from not only preferential photolysis but also from photolysis-induced molecular structural change. These results suggest that circularly polarized light sources in space could be associated with the origin of terrestrial homochirality; that is, they would be effective asymmetric exciting sources introducing chiral structures into bio-organic molecules or complex organic compounds.

Intense Terahertz Synchrotron Radiation by Laser Bunch Slicing at UVSOR-II Electron Storage Ring

A laser bunch slicing system has been constructed at the UVSOR-II electron storage ring to produce a submillimeter-sized microstructure on an electron bunch. As its first result, terahertz coherent synchrotron radiation (THz CSR) was successfully produced by the dense structure of the electron bunch. Its intensity is higher by four or five orders of magnitude than that of normal synchrotron radiation. The intensity is proportional to the square of the peak current of the electron bunch, as expected for coherent emission. It is also demonstrated that the spectral shape of the THz CSR can be controlled by changing the slicing laser pulse duration.

SAMRAI: A novel variably polarized angle-resolved photoemission beamline in the VUV region at UVSOR-II

A novel variably polarized angle-resolved photoemission spectroscopy beamline in the vacuum-ultraviolet (VUV) region has been installed at the UVSOR-II 750 MeV synchrotron light source. The beamline is equipped with a 3 m long APPLE-II type undulator with horizontally/vertically linear and right/left circular polarizations, a 10 m Wadsworth type monochromator covering a photon energy range of 6-43 eV, and a 200 mm radius hemispherical photoelectron analyzer with an electron lens of a +/-18 degrees acceptance angle. Due to the low emittance of the UVSOR-II storage ring, the light source is regarded as an entrance slit, and the undulator light is directly led to a grating by two plane mirrors in the monochromator while maintaining a balance between high-energy resolution and high photon flux. The energy resolving power (hnu/Deltahnu) and photon flux of the monochromator are typically 1 x 10(4) and 10(12) photons/s, respectively, with a 100 microm exit slit. The beamline is used for angle-resolved photoemission spectroscopy with an energy resolution of a few meV covering the UV-to-VUV energy range.

Measurements of threshold photoemission magnetic dichroism using ultraviolet lasers and a photoelastic modulator

A magneto-optical method based on valence band photoemission by laser excitation is described. Total photoexcited electron dichroism is detected using wave plates or a photoelastic modulator. Compared to the direct current method using wave plates, a modulation technique assisted by a photoelastic modulator has an advantage to improve the signal-to-noise ratio of magnetic dichroism by a factor of 10. The magnetic circular and linear dichroism can be investigated with this technique. An application to magnetic domain imaging using photoemission electron microscope is also demonstrated.

From the operation of an SRFEL to a user facility

Storage ring free-electron lasers (SRFELs) likely have a potentiality for scientific application as a unique light source because of the good coherence and temporal feature in addition to variable wavelength. At the UVSOR, the performance of the SRFEL has been improved aiming at users applications. Recently, an experiment using SRFEL combined with synchrotron radiation (SR) was begun. As the first experiment, the double-resonant excitation of Xe has been investigated by using SR and SRFEL as pump and probe lights, respectively. The relevance of making use of SRFEL for the pump/probe experiment is demonstrated.

Testing for qualification of a helical optical klystron for UV storage-ring free electron lasers

Abstract A new helical optical klystron, UNKO-3, for a UV-free electron laser has been installed and commissioned on the UVSOR storage ring, Okazaki. Lasings at a visible wavelength region around 470 nm and a UV region down to 260 nm were obtained in test experiments. It was found that the mirror degradation rate with radiation from a helical mode was several tens of times slower than that with a planar mode.

Observation of Intense Bursts of Terahertz Synchrotron Radiation at UVSOR-II

We have detected very intense bursts of terahertz synchrotron radiation at the UVSOR-II electron storage ring operated in single bunch mode. The bursts were observed in the wavelength range from 0.2 to 3.0 mm by using a liquid-helium-cooled InSb hot-electron bolometer. The typical duration and interval of the bursts were about 200 µs and 10–15 ms, respectively. Each burst shows the quasi-periodic structure of about 30 µs. The peak intensity of the bursts was about 10000 times larger than that of ordinary synchrotron radiation in the same wavelength region. The extremely high intensity strongly suggests that the bursts are coherent synchrotron radiation, although the radiation wavelength was much shorter than the electron bunch length.

Angular Momentum of Twisted Radiation from an Electron in Spiral Motion

We theoretically demonstrate for the first time that a single free electron in circular or spiral motion emits twisted photons carrying well-defined orbital angular momentum along the axis of the electron circulation, in adding to spin angular momentum. We show that, when the electron velocity is relativistic, the radiation field contains harmonic components and the photons of lth harmonic carry lℏ total angular momentum for each. This work indicates that twisted photons are naturally emitted by free electrons and are more ubiquitous in laboratories and in nature than ever thought.

High-Speed Y–Ba–Cu–O Direct Detection System for Monitoring Picosecond THz Pulses

A high-speed YBa₂Cu₃O_7-δ direct detection system was developed to monitor terahertz picosecond pulses in the time domain. High-T_C superconducting thin-film YBa₂Cu₃O_7-δ microbridges with critical temperatures of T_C = 85 K were embedded into a planar log-spiral antenna to couple the broadband terahertz radiation (0.1 -2 THz) of several picosecond pulsed sources. The YBa₂Cu₃O_7-δ detectors were installed in a liquid nitrogen cryostat equipped with 18 GHz effective bandwidth readout electronics. THz pulses generated at the electron storage rings ANKA and UVSOR-II have been resolved with a temporal resolution of 30 ps (full width at half maximum) limited by the readout electronics bandwidth. Beam dynamic effects of bursting coherent synchrotron radiation were successfully monitored.