Yoshitaka Taira

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.

Photon-induced positron annihilation lifetime spectroscopy using ultrashort laser-Compton-scattered gamma-ray pulses

High-energy ultrashort gamma-ray pulses can be generated via laser Compton scattering with 90° collisions at the UVSOR-II electron storage ring. As an applied study of ultrashort gamma-ray pulses, a new photon-induced positron annihilation lifetime spectroscopy approach has been developed. Ultrashort gamma-ray pulses with a maximum energy of 6.6 MeV and pulse width of 2.2 ps created positrons throughout bulk lead via pair production. Annihilation gamma rays were detected by a BaF2 scintillator mounted on a photomultiplier tube. A positron lifetime spectrum was obtained by measuring the time difference between the RF frequency of the electron storage ring and the detection time of the annihilation gamma rays. We calculated the response of the BaF2 scintillator and the time jitter caused by the variation in the total path length of the ultrashort gamma-ray pulses, annihilation gamma rays, and scintillation light using a Monte Carlo simulation code. The positron lifetime for bulk lead was successfully measured.

Evaluation of Coherent Terahertz Radiation Generated from Tilted Electron Beams Aiming for Higher Light Intensity

When a target medium is irradiated by electron beams travelling at relativistic speed, terahertz(THz) radiation is produced by Cherenkov radiation. THz radiation is released at an angle to the direction of travel of the electron beams, and the coherence of the radiation can be improved by tilting the electron beams to match this angle, resulting in higher light intensity. The Cherenkov angle differs according to the refraction index of the target medium. At Waseda University, the generation of high-quality electron beams by a Cs-Te Photocathode RF-gun and its applications are being researched. By utilizing the RF-Deflector, the tilt angle of the electron beam can be controlled to achieve coherent THz radiation. To gain higher light intensity, the use of Silicon and Aerogel as a target medium was challenged and compared to the conventional medium TOPAS. The THz radiation produced from the three target mediums were analysed by use of the power meter and time domain spectroscopy(TDS). At the conference, the generation of THz Cherenkov radiation from different target mediums and the measurement results will be reported along with future perspectives.

Investigation of the Coherent Cherenkov Radiation Using Tilted Electron Bunch

Cherenkov radiation can be produced when the velocity of the charged particles are faster than the light in some medium. We investigated the coherent Cherenkov radiation using electron bunch tilting for matching the wave front of the Cherenkov radiation. The electron bunch was tilted by using rf transverse deflecting cavity. We tested several materials for the Cherenkov target which has enough transmittance at the wavelength of THz region. As a result, high peak power THz was achieved using this novel technique. We will report the principle of this technique, the experimental results and future prospects at the conference.

Status and Prospects of Coherent Light Source Developments at UVSOR‐II

We are developing coherent light sources at the UVSOR‐II electron storage ring. We have developed a resonator type free electron laser in the visible to the deep UV range, coherent harmonic generation source in VUV range and coherent synchrotron radiation source in the terahertz range. A new five year plan has been started from FY2008, where a new 4m straight section will be created by moving the beam injection point, a new optical klystron type undulator will be installed and dedicated beam‐lines will be constructed. Great advances on the coherent light source developments at the new straight section are expected.

Observation of Saturation Effect on Vacuum Ultraviolet Coherent Harmonic Generation at UVSOR-II

We have observed coherent harmonic generation in a deeply saturated regime. We measured the intensities of coherent harmonic (CH) up to the ninth harmonic of a Ti:sapphire laser in a wide range of peak laser powers. The CH intensities increase with the peak laser power and reach a maximum. The maximum occurs at a lower peak laser power for the higher CHs. Beyond this maximum, the CH intensity oscillates as the peak laser power is increased. A one-dimensional model explains these results in term of the formation of a double-peak structure in the longitudinal electron distribution.