Rie Tanabe

Laser-induced shock process in under-liquid regime studied by time-resolved photoelasticity imaging technique

Stress enhancement in laser-induced shock process by plasma-confining effect of liquid overlay was demonstrated visually and its dependence on liquid layer thickness was studied. Time-resolved photoelasticity imaging technique in bright-field mode was used to observe the stress wave in solid phase and the shock wave, plasma, and cavitation bubble in the liquid phase simultaneously. From the photoelastic images, intensity of the laser-induced stress wave (LSW) inside a solid was evaluated semi-quantitatively. We prove that LSW is weaker with thinner liquid layer. To achieve the same effect with bulk liquid, the liquid film needs to be thicker than a threshold value.

Time-resolved observation of the change of electrode shape in single discharge: Rapid self-forming of a thin electrode

A very thin, needlelike electrode can be formed at the tip of thin tungsten electrode by a single discharge with rather high discharge current. We have demonstrated some practical applicability of such very thin electrode. Understanding the formation mechanism of such electrodes is an important issue to utilize the phenomenon to practical applications. We should see what happens on the electrode during the discharge. In this study, the change of electrode shape at tip part of a thin metal electrode induced by single discharge with a large current was observed dynamically by a stroboscopic imaging system with high time resolution. In the imaging system, the second harmonics of a Q-switched neodyum: yttrium-aluminum-garnet laser is used for lighting of charge-coupled device camera and it can take a photo with time resolution of about 10ns. Electrode materials were tungsten and copper of 100μm diameter and the discharge current was 30A with 400μs duration. Both the positive and negative polarities of electro...

Effect of liquid-sheet thickness on detection sensitivity for laser-induced breakdown spectroscopy of aqueous solution

For aqueous-solution-based elemental analysis, we used a thin liquid sheet (μm-scale thickness) in laser-induced breakdown spectroscopy with nanosecond laser pulses. Laser-induced plasma is emitted by focusing a pulsed Nd:YAG laser (1064 nm) on a 5- to 80-μm-thick liquid sheet in air. To optimize the conditions for detecting elements, we studied how the signal-to-background ratio (SBR) for Hα Balmer and Na-neutral emission lines depends on the liquid-sheet thickness. The SBR of the Hα Balmer and Na-neutral lines was maximized for a sheet thickness of ~20 μm at the laser energy of 100 mJ. The hydrodynamics of liquid flow induced by the laser pulse was analyzed by laser flash shadowgraph imaging. Time-resolved observation of the hydrodynamics and plasma emission suggests that the dependence of the SBR on the liquid-sheet thickness is correlated with the volume of flowing liquid that interacts with the laser pulses.

Influences of Focusing Conditions on Dynamics of Laser Ablation at a Solid--Liquid Interface

Influences of focusing conditions relative to a sample surface on shock processes in under-liquid laser-induced ablation were studied using a custom-designed time-resolved photoelasticity imaging technique. A nanosecond laser pulse was focused onto the surface of epoxy-resin blocks immersed in liquids. When the surface was located close to the focal point, the breakdown that occurred on the solid–liquid interface resulted in reduced laser-induced stress. At positions above the focal point, the stress-wave intensity was mainly determined by laser pulse energy, but not by fluence. Impurities in liquids initiated breakdowns at multiple sites along the light path and reduced stress-wave intensity.

Mechanism of dynamic plasma motion in internal modification of glass by fs-laser pulses at high pulse repetition rate

Evolution of free-electron density in internal modification of glass by fs-laser pulses at high pulse repetition rates is simulated based on rate equation model, which is coupled with thermal conduction model in order to incorporate the effect of thermal ionization. Model shows that highly absorbing small plasma generated near the geometrical focus moves toward the laser source periodically to cover the region, which is much larger than focus volume. The simulated results agree qualitatively with dynamic motion of plasma produced in internal modification of borosilicate glass by fs-laser pulses at 1 MHz through the observation using high-speed video camera. The paper also reveals the physical mechanism of the internal modification of glass when heat accumulation is significant.

Trepanning drilling of microholes on cemented tungsten carbide using femtosecond laser pulses

Microholes of 100–300 µm diameters were trepanned on cemented tungsten carbide plates of 0.5 and 1 mm thicknesses by means of femtosecond laser pulses using a galvano-scanner with an f-θ lens of 80 mm focal length. It was found that the focus position of the laser beam relative to the sample surface affects the drilled hole shape and drilling speed. Focus on the surface produced a less tapered hole and a higher machining speed than focus 0.5 mm away from the surface in either direction. A low scan speed of the laser beam of 0.035 mm/s produced straighter, less tapered, and more symmetric holes than a high scan speed of 15 mm/s. However, the high scan speed resulted in a higher machining speed. A circular polarized beam resulted in round holes, whereas a linearly polarized beam produced distortions in the shape of the holes. It was also found that laser-induced periodic surface structures with a period of 300 nm were formed on the entire side surface of the holes drilled using circularly polarized beam. Th...

Fabrication of OLED display by an ultrashort laser: selective patterning of thin metal electrode

Organic light emitting diode (OLED) is now in practical use and also a subject of active research and development. In industrial production of OLED displays, one of the key technologies is patterning of electrodes, especially a metal cathode, which is usually made on a thin layer of organic electro-luminescence (OEL) compounds. Difficulties in machining of the OLED come from the fact that the OLED has multi-layered structures consisted from very thin layers of different materials, one of which is a highly heat- and chemical-sensitive organic material. The typical OLED sample has indium tin oxide (ITO) electrode of about 150 nm thick at the bottom. The organic electro-luminescence material of less than 200 nm is deposited on it and the top is aluminum electrode of 100 to 150 nm thickness. We have constructed a fabrication system of the OLED by using an ultrashort fiber laser in the patterning of aluminum electrode and fabricated a display panel successfully. The system has several advantages comparing to other methods currently used. To investigate the process in detail, we have constructed two ultra-fast photography systems, with either sub-picoseconds or nanoseconds time resolution, and carried out the time-resolved observation of the process. It is found that the underlying layer affects much to the machinability of the top metal layer. The ITO layer seems to enhance the machining efficiencies of the aluminum electrode: the ablated spot size becomes larger for that on ITO, even though the laser pulse energy is kept constant.

Micromachining through silicon substrates by ultrafast laser at 1552 nm

Micromachining of silicon (Si) using lasers is attracting much interest in the field of modern microfabrications. Because Si absorbs majority of industrial lasers in near-IR to uv region, laser processing of Si is usually performed using linear absorption processes of the laser radiation. Micromachining by non-linear absorption processes induced by short pulse lasers has many unique features and has been applied to transparent materials in many fields. The application of these non-linear processes to the micromachining of Si would be very advantageous. In this study, ultrafast pulses from a fiber-based infrared laser at 1552.5 nm were irradiated through silicon substrates such that they were focused at the front or the rare surfaces of the substrates under an infrared microscope. Focused outputs of the laser made sharp and deep trenches on the front surface. When the laser was focused on a gold (Au) film on the surface of the second substrate placed at the back of the first Si substrate, the Au film was ablated by irradiation through the Si substrate. This was applied successfully to an up-conversion of the frequency of crystal oscillator in a Si package. When the focus was placed at rear surface of a Si substrate and aberration of the laser light caused by large refractive index of Si was compensated, an Au film on the rear surface was ablated and deposited on another substrate placed at the back of the substrate.Micromachining of silicon (Si) using lasers is attracting much interest in the field of modern microfabrications. Because Si absorbs majority of industrial lasers in near-IR to uv region, laser processing of Si is usually performed using linear absorption processes of the laser radiation. Micromachining by non-linear absorption processes induced by short pulse lasers has many unique features and has been applied to transparent materials in many fields. The application of these non-linear processes to the micromachining of Si would be very advantageous. In this study, ultrafast pulses from a fiber-based infrared laser at 1552.5 nm were irradiated through silicon substrates such that they were focused at the front or the rare surfaces of the substrates under an infrared microscope. Focused outputs of the laser made sharp and deep trenches on the front surface. When the laser was focused on a gold (Au) film on the surface of the second substrate placed at the back of the first Si substrate, the Au film was a...

Up-conversion of crystal oscillator frequency in silicon package by near infrared, ultrashort laser

Using an ultrashort pulse laser, photon energy of which is smaller than the band gap energy of silicon, machining of substances located at back of a silicon plate should be achievable. To realize this possibility, machining of a silicon substrate as well as machining of gold film on it was carried out using femtosecond laser pulses, wavelength of which lay between 1.5 to 2.5 μm. It is demonstrated that the rare surface of the silicon substrate and the gold film placed at the back of the silicon substrate can be machined with no detectable change on its front surface. Frequency adjustment of crystal oscillator sealed in a silicon package is tried and up-conversion of the frequency is achieved by removing small amount of thin gold film on the crystal with irradiation of 1.5 μm laser pulses through the silicon lid.

Visualization of High-speed Phenomena Using a Short-pulse Laser Illumination

レーザー加工や放電加工は,短時間で高密度の光パルス を微小領域に集光照射することにより,または,電極と被 加工物の間に高電圧を印加して極間に電流を流すことによ り,被加工物を短時間に加熱,溶融,蒸発させることで除 去し,加工が進展する.これらの加工は,局所的に短時間 で形態変化が生じる高速加工現象であり,加工パラメータ の調整により加工形態は大きく変化する.また,高密度の エネルギー加工であるため,加工時には多くの場合,プラ ズマ生成による強い発光が伴い,肉眼や一般的な観察手段 では,これらの現象を直接観察することは困難である.そ のため,加工過程や加工原理の追究は,多数の加工結果か らの推定やシミュレーションによる解析によらざるを得な い.しかし,これらの方法による加工現象の解析では,解 明できない点も多く残されており,実際の現象を把握する ことが重要である.そのためには,時間的な変化と空間的 な変化とを同時に,高い分解能で,時間分解観察する必要 がある. レーザーは,従来,切断,溶接,穴あけ,マーキングな どの加工を行う装置として利用されるのが一般的であった が,現象を撮影するためのカメラの照明光源としての利用 も可能である.特に,高速な変化を伴う加工現象の研究で は,大きな威力を発揮しつつある.照明光源としてレーザ ーを用いた可視化の研究は,いつから始まったのか十分に は調べていないが,日本では 30 年以上前でもすでに取り 入れられており,例えば,ディーゼル噴霧により発生する 衝撃波や,爆破に伴う応力波の挙動の可視化が行われてい る.しかし,パルスレーザーはランプ等に比べ高価で あったことや,カメラ等の他の機器との同期を取ることが 容易ではなく,一般的な手法とはなっていない.そこで本 稿では,レーザー加工や放電加工など加工時に発光が伴う 高速度現象を,高分解能で可視化するためのレーザー利用 について,その撮影手法や特色を紹介する.