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We demonstrated sub-micrometer processing of two kinds of thin films, polymethyl methacrylate (PMMA) and metal nano-particle resist, by focusing high-order harmonics of near-IR femtosecond laser pulses in the extreme ultraviolet (XUV) wavelength region (27.2 – 34.3 nm) on the thin film samples using an ellipsoidal focusing mirror. The ablation threshold fluences for the PMMA sample and the metal nano-particle resist per XUV pulse obtained by the accumulation of 200 XUV pulses were determined to be 0.24 mJ/cm2 and 0.10 mJ/cm2, respectively. The diameters (FWHM) of a hole created by the ablation on the PMMA film at the focus were 0.67 and 0.44 μm along the horizontal and vertical directions, respectively. The fluence dependence of Raman microscope spectra of the processed holes on the PMMA sample showed that the chemical modification, in which C=C double bonds are formed associated with the scission of the PMMA polymer chains, is achieved by the irradiation of the XUV pulses.

We have studied the generation of the broadband THz pulse using a compact linear accelerator. The THz pulse is generated by control of an electron beam angle to Cherenkov radiation angle. In addition, we have succeeded in producing a quasi-monochromatic THz pulse by the spatially modulated electron beam by passing through a slit. This work aims to develop a detection system to elucidate the spectrum of the quasi-monochromatic THz pulse. To detect it stably in a noisy radiation environment, the stability of probe laser system for Electro Optic sampling and timing synchronization system are important. In this conference, the generation method of each THz pulses, the results of development of detection system, and future prospect will be reported., Proceedings of the 13th International Particle Accelerator Conference, IPAC2022, Bangkok, Thailand

We have been studying terahertz (THz) generation via Cherenkov radiation with high-quality electron beams from a photocathode rf (radio frequency) gun. In our early studies, we have succeeded in the generation of coherent Cherenkov radiation by controlling the tilt of the electron beam using an rf-deflector. For further enhancement, we are planning to stack the THz pulses in an optical cavity. Multi-bunch operation of the rf-gun will generate electron beams with a repetition rate of 119 MHz, and THz pulses as well. These pulses will be accumulated in the cavity for up to 150 pulses. In this conference, we report the design study of the enhancement cavity and discuss the performance of the THz source., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

THz waves, located between microwaves and light waves, have transparency, directionality and fingerprint spectrum of specific materials. Therefore, they are expected to be useful for various applications. We have been studying THz waves generation via Cherenkov radiation with electron beams from a photocathode rf-gun. In our early studies, we have succeeded in the generation of coherent Cherenkov radiation by tilted electron beams using an rf-deflector. Furthermore, we have generated quasi-monochromatic THz waves by spatially modulated electron beams and have succeeded in its measurement by bandpass filters. This study aims to obtain the THz wave form in time domain by electro-optic (EO) sampling, which is an useful detection system for obtaining the information of the electric field and the phase simultaneously with high S/N. In this conference, we report about our probe laser system, results of the time-domain spectroscopy measurement of THz waves by EO sampling, and future prospects., Proceedings of the 12th International Particle Accelerator Conference, IPAC2021, Campinas, SP, Brazil

平成29年度衝撃波シンポジウム (2018年3月7日-9日. 東北大学片平さくらホール ）仙台市, 宮城県, (2018年3月7日-9日. 宇宙航空研究開発機構角田宇宙センター ）角田市, 宮城県, Symposium on Shock Waves in Japan, FY2017 (March 7-9, 2018. Sakura Hall), Sendai, Miyagi, (March 7-9, 2018. Kakuda Space Center, Japan Aerospace Exploration Agency (JAXA)(KSPC)), Kakuda, Miyagi, 著者人数: 13名, 資料番号: SA6000119010, レポート番号: 1A2-3

We have been studying a coherent Cherenkov radiation by using tilted electron bunches. Bunch tilting can enhance the radiation power about 10 times due to the wavefront matching of radiations. Recently, we investigated that this technique can produce high peak power THz pulses with sufficient pulse energy. Resulting pulse energy was more than 30 nJ/pulse and peak power was about 10 kW. Introducing the oscillator cavity with two concave mirrors, it would be possible to achieve lasing using tilted electron bunches. In the calculation, 1 uJ/micro-pulse and 100 uJ/macro-pulse broadband THz pulses are expected to be achieved, which are powerful THz sources compared with the existing THz FELs. In this conference, we will report the experimental results of coherent Cherenkov radiation, calculated results toward lasing and future prospectives., Proceedings of the 38th Int. Free Electron Laser Conf., FEL2017, Santa Fe, NM, USA

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 analyzed by use of the power meter and time domain spectroscopy(TDS). At this conference, the generation of THz Cherenkov radiation from different target mediums and the measurement results will be reported along with future perspectives., Proceedings of the 9th Int. Particle Accelerator Conf., IPAC2018, Vancouver, BC, Canada

THz frequency is a special electromagnetic wave which is categorized between a radio wave and a light wave. It can pass through the various materials like a radio wave and can be transported with optical components like a light wave. Thus, it's suitable for imaging application of materials. At Waseda University, it's possible to generate a high-quality electron beam using Cs-Te photocathode RF-Gun and the electron beam is applied to several application researches. As an application of this electron beam, we generate a coherent Cherenkov radiation, and succeed in observing a high power THz light. The successful results of high power THz radiation encourage us to perform the THz imaging with transmission and reflection imaging using some materials, cross-section imaging using a simple material. On studying the THz imaging, it is necessary to clarify the spatial resolution. So, we tried to evaluate the spatial resolution in our device. Furthermore, our target is to get the three-dimensional THz images. We will introduce the CT technique in order to obtain the clear cross-section image. In this conference, we report the recent results of the THz imaging and future prospective., Proceedings of the North American Particle Accelerator Conf., NAPAC2016, Chicago, IL, USA

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., Proceedings of the 8th Int. Particle Accelerator Conf., IPAC2017, Copenhagen, Denmark

We have been developing a compact accelerator based a laser photocathode rf electron gun at Waseda University. Low emittance and short bunched electron beam can be generated from the gun. Also, the rf transverse deflecting cavity was developed for the bunch length measurement. We performed an experiment for generating a coherent Cherenkov radiation using bunch tilting. The rf transverse deflector can give a tilt for the electron bunch, and the tilt angle was set to the Cherenkov radiating angle which determined by the target refractive index. We successfully demonstrated a coherent Cherenkov radiation and the characterization of the radiation. The principle of coherent Cherenkov radiation generation, the experimental results and future prospective will be presented at the conference., Proceedings of the 7th Int. Particle Accelerator Conf., IPAC2016, Busan, Korea

At Waseda University, we have been studying a high quality electron beam generation and its application experiments with Cs-Te photocathode RF-Gun. We have already succeeded in generating a stable high-charged single-bunch electron beam. To generate more intense electron beam, we designed a multi-bunch electron linac and developed the multi-pulse UV laser which irradiates to the cathode. The target values of the number of electron bunch and bunch charges are 100 bunches/train and 800 pC/bunch, respectively. In addition, we adopted the method of the amplitude modulation of the incident RF pulse to the S-band klystron in order to compensate the energy difference in each bunch because of the slow rise time of acceleration voltage in cavity and beam loading effect in the accelerating structure. In this conference, we will report design properties of our multi-bunch electron linac, the results of the multi-bunch electron beam diagnosis and the energy difference compensation using the RF amplitude modulation method., Proceedings of the 1st International Particle Accelerator Conference, IPAC2010, Kyoto, Japan