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.
A thin metal electrode tip instantly changes its shape into a sphere or a needlelike shape in a single electrical discharge of
high current. These changes occur within several hundred microseconds. To observe these high-speed phenomena in a
single discharge, an imaging system using a high-speed video camera and a high repetition rate pulse laser was
constructed. A nanosecond laser, the wavelength of which was 532 nm, was used as the illuminating source of a newly
developed high-speed video camera, HPV-1. The time resolution of our system was determined by the laser pulse width
and was about 80 nanoseconds. The system can take one hundred pictures at 16- or 64-microsecond intervals in a single
discharge event. A band-pass filter at 532 nm was placed in front of the camera to block the emission of the discharge
arc at other wavelengths. Therefore, clear images of the electrode were recorded even during the discharge. If the laser
was not used, only images of plasma during discharge and thermal radiation from the electrode after discharge were
observed. These results demonstrate that the combination of a high repetition rate and a short pulse laser with a high
speed video camera provides a unique and powerful method for high speed imaging.
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.
Interaction between an intense laser pulse and a material is a key issue to understand and control photo-induced processes. It is, however, rather difficult to study the interaction in detail because it occurs in a very narrow area and a very short period. In addition, it is usually accompanied by plasma formation, which causes difficulties for the most of observation and measurement techniques. We have developed a time-resolved imaging technique which uses a pulse
laser as a probe light and an ICCD camera as a recording devise, and successfully observed the dynamical change of the spatial distribution of laser induced transient stress fields by both shadowgraph and photoelasticity images using transparent materials. Recently, we have found that photoelasticity imaging of epoxy-resin observed under water provide clear images which allow us the semi-quantitative estimation of the magnitude of laser induced stress. We can make the semi-quantitative estimation of the laser-induced stress by comparing images to those obtained for designated pulse energies. Effect of surface roughness is examined. The effect of the confinement of laser ablation by water is shown semi-quantitatively. The interaction of laser to the metal surfaces is examined by using epoxy samples the surfaces of which are coated by several metals. Transient stress induced in inside of the bulk materials by focusing the laser at inside of transparent materials are dynamically observed and compared with permanent damages induced by the laser irradiation.
We have investigated about machining of glass plates by a UV laser, a nanosecond FHG Nd:YAG laser. Laser wavelength was 266 nm and pulse width was about 4 - 6 ns. Repetition rate of the laser pulse was up to 30Hz. Borosilicate glass plate of 1 mm thick was irradiated by laser pulse in air. An X-Y axis precision trnaslation stage was used to change the laser irradiation spot on the glass surface. The spot size of focusing beam at the glass surface was varied by changing the distance between the glass and a focal lens. Focal position of the focal lens was determined by measuring burn patterns on thermo-sensitive paper as a function of distance between the lens and the paper. When the glass plate was placed at the focal point, radial cracks occurred on the surface. When the glass plate was moved toward the lens by 0.7 mm, ablation still took place and provided good quality groove on the surface with no significant cracks. The cross section of the groove was conical for slower scanning speed while it became shallow dip with rather flat bottom for faster scan speed. The grove depth increased linearly as the laser fluence increased.
We report on dynamical observation about interaction of the fundamental radiation from a Q-switched Nd:YAG laser to the surface of PMMA observed by an imaging system with nanosecond time resolution, with special emphasis on stress waves inside the sample. We observed stress wave which propagated into the material and a pressure wave propagated into the atmosphere by shadowgraph and photoelastic methods. In the shadowgraph images, a shadow corresponded to the longitudinal elastic wave was clearly observed in the bulk. In photoelasticity images, another contrast which should correspond to the transverse wave was also observed. The contrast of the photoelasticity images depended on laser pulse energy. The contrast and thus the amount of absorbed energy were affected by the surface roughness: roughness greater than 1 micrometer caused complete absorption of laser energy at the surface. Based on this result, we try to examine the effects of incidence angle of the laser to coupling between laser radiations and the material.
Holes and grooves of a few tens of micrometers have been fabricated on glass plates by femtosecond laser pulses in air. Shapes and surface morphology of the fabricated structures have been studied in detail by making their replicas. Details of the internal shape of the strictures formed have been studied by making replicas of them. Development of the shape of drilled hole with the increase in irradiated pulse number shows some distinguishable stages. Smaller number of pulses make channel at the center of rather flat bottom. With the increase of pulse number, the channel changed into conical, funnel-like shape as the depth increased. Shape of the groove depends on the scanning speed, i.e., number of irradiated pulses on the same position. The groove show little cracking or beaking with clear edges. Some amount of debris deposited along the groove but these debris were easily removed by smear. Inner surface of the groove has coarser morphology than that of the hole. Depth of the grove was shallower than that of the hole, even though the number of irradiated pulse was estimated to be equal.
We have applied Fast Fourier Transform analysis to optical and acoustic emissions during bead-on-plate welding with CO2 and YAG lasers in our previous study. Some characteristic peaks in their frequency spectra were correlated with the bead profile. In this study, we have done butt welding of mild steel and stainless steel with 4 kW CW YAG laser. The optical and acoustic emissions during the welding were measured and the frequency spectra were analyzed. The major frequency peaks of the acoustic emission appeared in the range of 1 - 4 kHz and 6 - 7 kHz, although there were some additional peaks up to 40 kHz. These strong peaks appeared always in deep penetration mode welding. However, these peaks became small when the penetration depth became shallow by increasing the welding speed. We have shown that the acoustic spectrum of deep penetration mode welding is different from that of shallow penetration mode welding. The deference of the frequency spectrum should be useful in the development of a feedback system for control of weld quality.
Holes of few tens of micrometers have been drilled through a glass plate by femtosecond laser pulses. A femtosecond laser (Clark-MXR, CPA 2001) which delivered 775 nm, 150 fs pulses at 1 kHz was used to machine holes through glass plates of about 150 micrometer thick. Laser pulses were focused by a plano-convex lens on to the glass plate. An electric shutter controlled number of irradiating laser pulse. Entrances of the drilled hole were crack-free but shell-like cracks appeared at exit end of them. Shapes of the drilled holes were examined in detail by taking replicas of them using silicon-rubber based material. Small debris scattered around the drilled hole and its deposition range depended on the laser peak power. Development of the hole with increasing pulse numbers showed several distinct stages. The first few tens of pulses drilled a shallow, flat bottom hole with a deeper channel at the center of a flat bottom. Then, the flat bottom and the channel emerged into a deep, funnel shaped hole with tapered wall. The hole became deeper and finally went through to the rear side at certain pulse numbers, which depended on pulse energy and focusing conditions. Position of the focus relative to the glass surface affected the shape of drilled holes. This means that the shape of the drilled hole is very sensitive to laser intensity distribution with a well defined threshold and would suggests the possibility of drilling designed-shape holes with proper diffractive optics for modifying the intensity distribution of the laser light.
Laser ablation of transparent materials is induced by non- linear absorption and some laser-induced damages are introduced in the bulk as well as on the surface. This process is used in laser marking and other applications such as refractive index modification of optical materials and 3- D data storage. We have observed the laser ablation dynamics in inside of bulk transparent materials by nanosecond time- resolved imaging technique. Output of fundamental radiation (1064nm) from a Q-switched Nd:YAG laser was focused at the inside of bulk PMMA and soda glass. Second harmonic radiation (532nm) from the same laser was used as illuminating light and images were taken by a CCD camera with a band-pass filter at 532 nm. Series of images were taken at different intervals between the fundamental and the second harmonic light, which was controlled by the optical delay line. In observation at longer intervals than 50ns, another laser was used as illuminating source. When the laser was focused at inside of the bulk PMMA, damages occurred simultaneously at several independent points without the ablation at the surface. They located along laser incident axis. Propagation of shock waves, which started from these points, was clearly observed in the bulk. In the glass, absorbing point in the bulk formed a continuous line and its end-point moved from inside outward to the surface along the laser beam. Laser induced damages (cracks) continued to develop until some microseconds after laser pulse in PMMA.
When we focus high power laser pulses on a surface or inside of transparent materials, non-linear absorption of the laser radiation occurs and some laser-induced damages are introduced. This process is used in laser marking of transparent materials. We have made dynamical observations of the process by the stroboscopic imaging system which has a few nanoseconds time resolution. Output of fundamental radiation (1064nm) from a Q-switched Nd:YAG laser was focused at the surface or inside of PMMA and soda glass blocks. Second harmonic radiation (532nm) form the same laser was used as illuminating light and images were taken by a CCD camera with a band-pass filter of 532 nm. Series of images were taken at different intervals between fundamental and second harmonic light, which was controlled by optical delay line. In observation at longer intervals than 50ns, another laser was used as illuminating score. Non-linear absorption in PMMA occurs at several points simultaneously and propagation of shock waves started from these points is clearly observed. Non-linear absorptions in soda glass occurs from inside outward along the laser beam. Laser- induced damages (cracks) continue to develop until some hundreds milliseconds after laser pulse. Effects of pulse energy and roughness of the target surface were examined.
Effects of irradiating number of pulses of Nd:YAG laser in laser ablation of metals in air have been studied by both photoacoustic and fast-imaging techniques. Photacoustic detection technique using piezoelectric polymer film revealed the change of coupling among laser radiation, ablated matter, plasma and the target as a function of the laser fluence. Nanosecond imaging technique, where the second harmonic radiation from the same laser was used as illuminating light pulse, showed surface phenomena during and immediately after the ablating laser pulse. Photacoustic signal intensity as a function of laser fluence was measured at constant pulse energy. It was constant at low fluence, started to increase with fluence at certain threshold, reached a peak and then decreased gradually with increasing fluence. Shapes of the functions were similar but the threshold fluence and the fluence at the peak increased with irradiating pulse number. Imaging observation revealed that a surface layer and/or absorbed contaminants was ablated by initial few pulses and that material ablation occurred at higher fluence than the surface layer. Ablation of the surface layer caused shock wave and terminated less than 10 pulses while aluminum ablation was accompanied with bright plume and shock wave and affected only slightly by preceding pulses.
We have developed a nanosecond imaging technique, where fundamental radiation of a Nd:YAG laser was used as ablating beam while the second harmonic radiation from the same laser was used as illuminating light pulse, to study surface phenomena during and immediately after the ablating laser pulse. Using this system, we observed the ablation phenomena of metals in air and found that, at fluence higher than about 10 J/cm2, there appeared jet-like plasma growing towards incident laser beam at velocities of as high as 105 ms1 in addition to the laser induced plume. The jet grew during the laser pulse and when the pulse terminated, its rapid growth stopped. In this paper, we investigated the effects of laser pulse shape on this jet- like plasma. We found that appearance of the jet became later and its grown continued longer as pulse width increased. Growth speed of the jet depended on the pulse shape, even though the jet grew up to similar height at the end of the pulse. Our results showed that, even though a laser parameter in laser ablation was usually represented as the fluence, the controlling parameter for the jet growth was temporal change of laser power.
Laser ablation of some metals in gases induced by nanosecond Nd:YAG laser has been studied by both photoacoustic and fast imaging techniques. Photoacoustic technique using piezoelectric polymer film revealed the change of coupling among laser radiation, ablated matter, plasma and target as a function of the laser fluence. Nanosecond imaging technique showed surface phenomena during and immediately after the ablating laser pulse. Photoacoustic signal intensity as a function of laser fluence was measured at constant pulse energy. It is constant at low fluence, starts to increase with fluence at certain threshold, reaches the maximum and then decreases gradually with increasing fluence. At fluence higher than about 7 J/cm2, there appeared jet-like plasma growing toward incident laser beam at velocities of as high as 105 ms-1 in addition to the laser induced plume. The jet grew during the laser pulse and when the pulse terminated, its rapid growth stopped. At lower fluence, laser induced plasma expanding at about 104 ms-1 was observed. The growth speed of the jet-like plasma depended on laser fluence and gas atmosphere but did not change for different metals.
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