takashiyagi

Dr. Takashi Yagi

Research History; the parentheses indicate reference numbers in the publication list

1975–1985 Space and Aurora Research

I started my career as a researcher in optical aeronomy and space physics for my M.Sc. program (Univ. Niigata, Japan 1995). The research was on the ground-based observations of the hydrogen Balmer alpha line in the daytime Geo-corona using a large throughput, high resolution spectrometer. The temperature and atomic hydrogen content in the Earth’s exosphere were evaluated from the observed line intensities. The radiant flux of the Hα spectral line reaching the ground was estimated using the Radiative Transfer Model. I extended my research work to earn a Ph.D. (La Trobe Univ., Melbourne Australia, 1979), specializing in the ground-based Doppler measurement of wind and temperature of the neutral atmosphere in the nighttime upper ionosphere at the mid-latitudes of the Earth. To measure the spectral line profile at the wavelength of 630 nm of the atomic oxygen emitted in the thermosphere at an altitude of ~250 km, a Fabry–Perot Interferometer (FPI, φ = 150 mm) with a large aperture was installed at Beveridge field station at a distance of 50 km from the city of Melbourne. A periscope and a gas-scanning unit for the FPI were controlled by a microprocessor, and the spectral data were recorded on a magnetic tape [5, 6]. The entire system of the computer-controlled FPI was developed in the workshop of the physics department at La Trobe Univ. The wind and temperature deduced from the spectral line indicated the dynamic motion of the neutral atmosphere interacting with the plasma in the ionospheric F-layer. The observed wind data indicated the effect of the dragging force by  drifting ions. The electric field was generated in the dawn-to-dusk direction by the accumulated electronic charge at the sheath of the magnetosphere at a magnetically quiet time [1,2]. It was exciting to see the strong wind with a velocity of over 500 m/s, which blew from the Antarctic ionosphere due to the Auroral Joule Heating [4]. During the magnetically quiet time in equinox, periodic oscillations in the wind with a period and amplitude of 40 min and ~50 m/s, respectively, were routinely observed. Their origin was attributed to the atmospheric Internal Gravity Waves [3].

My research life at the physics department of La Trobe U., with my colleagues, was quite exciting, and my resident life at Glen College, filled with students from around the world, was rich with friendship.

After completing my PhD, I was involved in a project on infrared spectroscopy for molecular kinetics of OH radicals in the mesosphere in the arctic region of the Earth at the Geophysical Institute of the University of Alaska (1984). The project aimed at clarifying the thermal properties of the Earth’s mesosphere and their influence on the dynamics of the upper atmosphere, since OH molecules were considered a huge thermal reservoir [7]. The spectroscopic observation required a wavelength scan speed of ~ms with LN2-cooled InSb, PbS, and CdTe detectors. Hence, high-speed imaging spectroscopy in the infrared spectral region was desired. “Nature was sparkling” could be an adequate description of life in Alaska.

 

1983–2016 Laser Physics and Laser Matter Interaction:

Along with my interest in the atmospheric optical phenomena, I was quite captivated with ultra-fast spectroscopy. Opportunity existed at the Atmospheric Science Department State of University of New York at Albany (1983), where I did my first research on the ultra-fast plasma spectroscopy, ~1 ns of time resolution with a streak camera. The purpose of the research was to clarify the triggering mechanism of sparks and natural lightning. The ultrafast optical technique enabled the measurement of time-resolved spectra of the 1^st and 2^nd positive bands of N₂ molecules from the sparks. It determined the evolution of the temperature and density of electrons and ions in the streamer track. It propagated at 1/3 of the light speed, and rapidly grew to the spark breakdown plasma. I was amazed by the multitude of scientific phenomena involved in such a small spark of discharge in the gas. The variety of the scientific phenomena in the small spark led me to the geophysical scale of lightning. However, I did not recognize the spectacular phenomena “Sprite” even when I was in Fairbanks later. The city of Albany was surrounded by mountain ranges, neighboring the state of Vermont, where the landscape before winter had a pleasant autumn color.

As a natural extension of the ultra-fast spectroscopy, I was granted a chance to conduct extensive research and development of an Excimer Laser at the IRI Laser Laboratory (Japan, 1987). The research was supported by a large-scale project on the excimer laser and ion beam development by the Advanced Material Processing and Machining Technology Research Association, which started in 1987 and continued until 1995, sponsored by the Japanese Government (AMMTRA project by NEDO). My mission was to develop a diagnostics technology for laser beams, beam cleanup with a scalable laser power, and laser beam modification for various applications. The laser beam diagnostics with a high frame-rate CCD; charge coupled device, was capable of capturing the coherence property and mode structure of a high repetition-rate excimer laser. These measurements indicated that the pulse-by-pulse stability and long-term variation of the laser characteristics were due to the contaminating agents in the laser gas [8-11, 14, 15]. For the scalability of the laser power with high laser beam quality, the laser beams and laser beam clean up were combined coherently using Optical Phase Conjugation based on the Stimulated Brillouin Scattering in liquid and gas at a pulse repetition rate of 250 Hz [12, 13, 17, 29].

Laser beam modification was often required in various applications of the excimer laser. Thin film coating on the solid surface by laser ablation required a flat top beam profile to avoid micro droplets covering the coated area. We managed to produce a square beam profile with a segmented mirror, serving as the thin-film coating of a high Tc superconductor [30].

The excimer laser was known to produce high temperature plasma, emitting Extreme Ultraviolet Radiation, from the solid target. A shorter pulse was considered favorable to increase the radiation efficiency. For this purpose, we reduced the laser pulse duration of 25 ns to ~500ps using Stimulated Thermal (Entropy) Scattering by focusing the laser pulse on the liquid surface. The laser pulse shortened by this method was phase-conjugated, and high fidelity was obtained. The interesting phenomena involving the backward-scattered STS pulse and the incoherent light scattering at the surface are not clarified until today [40]. Further pulse shortening was obtained by seeding the 3^rd harmonics of the Ti:Sapphire Femtosecond Laser pulse with a 248-nm wavelength to the KrF excimer laser amplifier. It produced an optical pulse with an energy and duration of 3mJ and 300 fs, respectively [27, 28, 34]. The femtosecond excimer laser pulse generated by this method was used to produce x-rays and applied to the micromachining and triggering long spark discharge [32]. The laser system to produce the seed pulse consisted of a mode-locked Ti:Sapphire laser oscillator, a homemade CPA system using a regenerative amplifier, a 3^rd harmonic converter, and a pulse compressor with a 248-nm wavelength. By this time we obtained fundamental understanding and engineering know-hows on the high power femtosecond laser system based on the Ti:Sapphire laser material [23-25, 37-39]

The generation of new wavelengths was required for the applications. Nonlinear optics, such as Stimulated Raman Scattering (SRS) and Harmonic Generation, was examined in this project to generate new wavelengths from the excimer laser. Owing to the significantly high conversion efficiency, we focused our efforts on SRS. Owing to the parametric process in SRS, we generated over 30 different new wavelengths simultaneously in the mixed molecular gas. As the pump laser for the SRS was a KrF excimer laser operated at 250 Hz, we had to circulate the high-pressure H₂ gas as a Raman active medium, while cooling by liquid nitrogen to improve the Raman conversion efficiency. Finally, we completed a LN₂-cooled gas circulating Raman Shifter to generate Anti-Stokes lines in the deep UV spectral region. [16, 18-22, 35]

The excimer laser was considered suitable for generating soft x-rays from the laser-produced plasma due to the wavelength and high peak power of the UV laser. It was indicated that the emission of the x-ray was significantly enhanced by applying a DC electric field to the plasma [26, 31]. The detailed mechanism of this enhancement was not known at that time; however, we constructed a high voltage generator, a Co-Axial Marks Bank, which was triggered by the excimer laser pulse for efficient generation of x-rays [33, 41, 43]. The Marks bank was synchronized very precisely with the plasma produced by the excimer laser, resulting in remarkably enhanced x-ray radiation. We finally constructed a module with an output voltage of 1MV, resulting in hard x-rays with enough energy to pass through 10-mm steel. A prototype module of a 200-keV x-ray generator triggered using the excimer laser or the 4^th harmonics of the Nd:YAG laser was released to the industry as a trial product. The high voltage pulse generated by this device had a duration of 50 ns and a timing jitter of 2ns, and the estimated photon yield was ~10¹². The emission mechanism of the x-ray was finally identified in the succeeding project. The laser-created plasma emits strong UV and EUV rays, which produce photoelectrons from the vacuum chamber wall. The electrons are accelerated and focused on the tip of the pin electrode where the plasma is formed, resulting in x-ray radiation. The interaction mechanism of the pulsed electron beam using the high-density plasma and its effects on the x-ray radiation are still unknown.

The project concluded with rich scientific and engineering aspects. I appreciate the significant contributions of colleagues, overseas post-doctoral fellows, visiting scientists, and a number of graduate students from universities.

The results of the AMMTRA project were further improved when I participated in successive NEDO projects after I joined Tokai Univ. (1997). In the Femtosecond Technology Project (1998), a high-power regenerative amplifier of the Ti:Sapphire as the front end of the femtosecond laser system was successfully constructed, producing ~6 mJ of pulse energy at a 786-nm wavelength with a pulse duration of ~100 fs [42, 47]. A dual passing laser pulse stretcher and compressor were invented to obtain large stretching ratio and efficient compression of the stretched pulse for the high peak power laser pulse [46]. As a next generation of the femtosecond laser, we developed a Yb:YAG laser with a SESAM used as the cavity mirror. We finally reached the stable mode-locked oscillation after a number of trials. We also investigated the performance of the laser oscillation with a long cavity of 5m [unpublished], with Yb:YAG and semiconductor laser mediums for the environmental study. With this cavity, mode-locking was not achieved with the Yb:YAG owing to the insufficient internal power for the SESAM, whereas stable laser oscillation was achieved in a normal cavity condition. The mode-locking operation with the gain medium of the semiconductor laser in the 10 m long cavity was tried using current modulation; however, the laser was unstable due to the coherent collapse problem [unpublished]. Some other types of lasers were incompletely developed: a micro-chip laser [44] and Whispering gallery mode oscillation in the tiny disc dye laser in collaboration with private companies, and relaxation oscillation to achieve a short pulse laser diode, with a 50 ps pulse duration. These lasers were mostly developed by graduate students. Development of the lasers was a great challenge and fun.

In the Advanced Photon Processing and Measurement Technologies Project (1999), a pulsed x-ray formed by the photo-electron beam generated by the laser plasma EUV was utilized as the Debye–Sheller x-ray source to observe the lattice constant of steel. We used the x-ray device that was developed in a previous national project with some modifications. This work was conducted in collaboration with the national laboratory and industry sector. We managed to observe a single shot Debye–Sheller diffraction ring pattern of the turbofan spinning at ~10,000 rev/min. The results showed the transient change in the lattice constant of the metallic fan owing to the stretched lattice structure caused by the centrifugal force due to rotation [46, 48].

At present, my research interest is strongly directed towards the interaction of the femtosecond laser pulses with the solid-state matter. Since clean-cut and drilling of the solid surface of micron meter scales are industrially important, we have investigated the quality of machining of the solid surface by the femtosecond laser. In our previous study about laser drilling, which was later done in collaboration with a private company, we formed a hole with a 12μm diameter and a sharp opening-edge through a silicon plate with a 750μm thickness, in a second [50]. We also indicated that the debris accumulating around the opening of the hole was the synthesized product of the ablated matter with the interacting environmental gas [49, 54]. From this work, we realized that irradiating the laser beam on a sample in vacuum was indispensable for the study of the interaction of laser and a solid, without being contaminated by air.

For the machining, which involved further reduction of size to sub-micrometer levels, we require special optics to focus the laser beam on the sample from outside the vacuum chamber, and full understanding of the physical mechanism of the material removal in molecular scale [52]. I developed an optical system to meet this requirement, including Conical-Axicons as parts, to form a spot with a ~1µm diameter at a working distance of 40 mm. It formed a Bessel beam pattern with a Bessel zone, 150μm in length; it worked as, in other words, an amazingly large focal depth. Owing to the long working distance, damage to the optics by the ablation gas and debris was avoided [53, 54]. The understanding the decomposition mechanism of the crystalline solid following the optical excitation is not well established. The ionic motion after the laser excitation, affected by the ionic interaction potential needs to be further clarified, experimentally and theoretically. We utilize “White Continuum” optical pulses generated from a short photonic crystal fiber to investigate the energy band structure of the solid surface as a diagnostic tool to observe the ultrafast surface restructuring processes [51], and nonlinear optical phenomena at the solid surface to monitor the time-varying crystalline symmetry.

The submicron machining quality is also affected by the surface plasmon-polariton resulting from the coupling of the free electrons and the laser light, and the phonon excitation due to the lattice dynamics. These elementary processes leave “a foot print” that is clearly visible in the excited zone of micron size on the sample surface. The ablated hole was covered with periodic wavy patterns, as the laser polarization was linear, while the circular polarization formed a round hole with a 600 nm diameter, without deterioration [56]. The ablated patterns formed by the various polarization distributions are periodically aligned as linear or vortex trenches. The origin of these patterns needs to be investigated further. The axicon optical system is installed in a compact unit and shaped for higher stability, ready for industrial use.

Apart from these surface phenomena, the laser ablation process produces large number of spherical particles with diameters of ~50 nm with a perfect crystalline structure; nano-particles. The origin and crystallization process of these particles are not fully understood [55]. We are seeking an industrial application of these results.

Following this successful formation of the Bessel beam, my current interest is in understanding the collective motion of free electrons in a solid interacting with the ultra-short laser pulse, and its effect on further reduced size of Nanometer-Machining. This will be followed by the application of laser micromachining to producing a Terra Hertz Radiation Emitter.

Most of the research activities in Japan were supported by government projects and the industrial sector. It was a pleasure to work with skillful graduate students in overcoming a number of difficulties in pursuing these research projects.

 

Teaching record 2016:

Mathematics for Physics for 1^st year,

text http://www.sp.u-tokai.ac.jp/~yagi/index.html, in Japanese

Physics Experiment for 1^st year

Electricity and Magnetism for 2^nd year.

Optics and Lasers for 3^rd year,

text http://www.sp.u-tokai.ac.jp/~yagi/index.html, in Japanese

Contents: geometrical optics and Fermat’s principle, light as an electromagnetic wave, theory of diffraction, quantum theory of dispersion, nonlinear optics, theory of a laser, quantization of the EM field.

Physics of “Laser and Matter Interactions” for the graduate course.