• Proceedings of the Korean Society of Precision Engineering Conference
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• 2006.05a
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• pp.669-670
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• 2006

Two-photon absorption coefficient of a series of dyes in polymer thin films was determined by measuring the femtosecond laser ablation threshold. The threshold value of polymeric thin films decreased gradually when the dopant increased. The two-photon absorption coefficient of the dye molecules dispersed in the polymer film was estimated by using the theoretical relationship between the ablation threshold of the blended polymeric thin films and the dye concentration. The relative values of two-photon absorption cross-section are in good agreement with those measured in solution. On the other hand, the absolute values are smaller than the latter.

• Journal of the Korean institute of surface engineering
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• v.49 no.1
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• pp.20-25
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• 2016

Femtosecond laser ablation of ultrathin polymer films on quartz glass using laser pulses of 100 fs and centered at ${\lambda}=400nm$ wavelength has been investigated for nanometer precision thin film patterning. Single-shot ablation craters on films of various thicknesses have been examined by atomic force microscopy, and beam spot diameters and ablation threshold fluences have been determined by square diameter-regression technique. The ablation thresholds of polymer film are about 1.5 times smaller than that of quartz substrate, which results in patterning crater arrays without damaging the substrate. In particular, at a $1/e^2$ laser spot diameter of $0.86{\mu}m$, the smallest craters of 150-nm diameter are fabricated on 15-nm thick film. The ablation thresholds are not influenced by the film thickness, but diameters of the ablated crater are bigger on thicker films than on thinner films. The ablation efficiency is also influenced by the laser beam spot size, following a $w_{0q}{^{-0.45}}$ dependence.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2006.05a
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• pp.657-658
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• 2006

Although there are many numerical models to simulate fs laser ablation of metals, no model can analyze the ablation phenomena over a wide range of fluence. In this work, a numerical code for simulating the fs laser ablation phenomena of metals has been developed. The two temperature model is employed to predict the ablation rate and the crater shape of metals using phase explosion mechanism in the relatively high fluence regime. Also, the ultrashort thermoelastic model is used for the low fluence regime to account for spallation of the sample by high strain rate. It has been demonstrated that the thermoelastic stress generated within the sample can exceed the yield stress of the material even near the threshold fluence. Numerical computation results are compared with the experiment for Cu and Ni and show good agreement. Discussions are made on the hydrodynamic model considering phase change and hydrodynamic flow.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.10a
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• pp.887-890
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• 1997

Recently micromachining using DPSSL(Diode Pumped Solid State Laser) with 3rd harmonic wavelength is actively studied in laser machining area. Micromachining using DPSSL have outstanding advantages as UV source comparing with excimer laser in various aspect such a maintenance cost, maskless machining, high repetition rate and so on. In this study micro-drilling of PCB type substrate which consists of Cu-PI-Cu layer was performed using DPSS Nd:YAG laser(355nm, wavelength) in vector scanning method. Experimental and numerical method(Matlab simulation, FEM) are used to optimize process parameter and control machining depth. The man mechanism of this process is laser ablation. It is known that there is large gap between energy threshold of copper and that of PI. Matlab simulation considering energy threshold of material is performed to effect of duplication of pulse and FEM thermal analysis is used to predict the ablation depth of copper. This study could be widely used in various laser micromachining including via hole microdrilling of PCB, and micromachining of semiconductor components, medical parts and printer nozzle and so on.

• Journal of the korean academy of Pediatric Dentistry
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• v.23 no.4
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• pp.954-967
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• 1996

Laser application to modify healthy permanent dentin to improve microhardness and caries resistence has been previously reported but the physical modification and ablation thresholds of carious and sclerotic dentin has yet to be identified. This study determined the energy density required by modify (physical modification threshold, PMT) and remove (ablation threshold, AT) infected carious, affected and selerotic dentin compared to healthy permanent dentin. $1{\pm}0.25mm$ thick dentin sections(n=272) from extracted human teeth were used. Smear layer was removed 0.5M EDTA for 2 minutes. Utilizing three pulsed fiberopitc delivered contact lasers with different emission wavelengths($1.06{\mu}m$=Nd : YAG, $2.10{\mu}m$=Ho : YAG and $2.94{\mu}mEr$ : YAG). The energy density($J/cm^2$) was incrementally increased and the resulting tissue interaction classified on a scale from 0-6. A minimum of 5 repetitions/energy density were completed. Light microscopy(10-25X) was used to verify the physical modification(scale=3) and ablation thresholds(scale=4) of the various forms of dentin and the data were analyzed by logistic regression at the 95 % confidence interval. PMT and AT by the laser and the dentin types were: PMT and AT was lower in infected dentin than in sound dentin for all lasers. PMT and AT induced by Nd : YAG>Ho : YAG>Er : YAG for all forms of dentin. Microhardness was increased in sound dentin at PMT. Morphology of crater examined by light microscopy showed Nd : YAG was safe and effective for removing carious dentin and Er: YAG was effective for removing sound dentin. The PMT and AT for YAG lasers are different as a function of dentin type which may be utilized for selective modification and removal of dentin.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2006.05a
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• pp.487-488
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• 2006

Picosecond (ps) laser micro-machining has emerged as an attractive method of fabricating high-precision microstructures, especially in metals. In this paper, a metallic mold for diffraction gratings is fabricated with a mode-locked 12 Ps $Nd:YVO_4$ laser. Laser pulses with a wavelength of 355nm are irradiated on the surface of NOK 80, a mold material, to generate line patterns. In order to minimize the line width, laser power is set just above the ablation threshold of NOK 80. Results show that the spectrum from the fabricated mold is good enough for some industrial application.

• Laser Solutions
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• v.9 no.3
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• pp.1-5
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• 2006

Diffraction grating is the optical device which has periodic pattern. Decorative logotypes is the one of application of diffraction grating. In this paper diffraction grating for decorative logotype is fabricated by dot pattern in stead of line pattern. A metallic mold for diffraction gratings is fabricated with a mode-locked 12 ps Nd:YVO4 laser. Laser pulses with a wavelength of 355nm are irradiated on the surface of NOK 80, a mold material, to generate dot patterns. In order to minimize the dot diameter, laser power is set just above the ablation threshold of NOK 80. Results show that the spectrum from the fabricated mold is good enough for some industrial application

• Proceedings of the Korean Society of Laser Processing Conference
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• 2006.11a
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• pp.114-117
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• 2006

Diffraction grating is the optical device which has periodic pattern. Decorative logotypes is the one of application of diffraction grating. In this paper diffraction grating for decorative logotype is fabricated by dot pattern in stead of line pattern. A metallic mold for diffraction gratings is fabricated with a mode-locked 12 ps $Nd:YVO_4$ laser. Laser pulses with a wavelength of 355nm are irradiated on the surface of NOK 80, a mold material, to generate dot patterns. In order to minimize the dot diameter, laser power is set just above the ablation threshold of NOK 80. Results show that the spectrum from the fabricated mold is good enough for some industrial application.

• Transactions on Electrical and Electronic Materials
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• v.1 no.4
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• pp.20-24
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• 2000

This study aimed to develop ITO(Indium Tim Oxide) tin films ablation with a pulsed type KrF excimer laser required for the electrode patterning application in flat panel display into small geometry on a large substrate are. The threshold fluence for ablating ITO on glass substrate is about 0.1 J/㎠. And its value is much smaller than that using 3 .sup rd/ harmonic Nd:YAG laser. Through the optical microscope measurement the surface color of the ablated ITO is changed into dark brown due to increase of surface roughness and transformation of chemical composition by the laser light. The laser-irradiated regions were all found to be electrically isolating from the original surroundings. The XPS analysis showed that the relative surface concentration of Sn and In was essentially unchanged (In:Sn=5:1)after irradiating the KrF excimer laser. Using Al foil made by 2$\^$nd/ harmonic Na:YAG laser, the various ITO patterning is carried out.

• Journal of Photoscience
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• v.10 no.1
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• pp.97-104
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• 2003

The excitation wavelength dependence of laser ablation dynamics of an azobenzene-containing urethane-urea copolymer film was investigated by measuring the laser fluence dependence of etch depth, transient absorbance change at each excitation wavelength, and transient absorption spectra. Moreover expansion/contraction dynamics was studied by applying nanosecond time-resolved interferometry. The threshold was determined at several excitation wavelengths from etch depth measurement, while time-integrated absorbance was obtained under excitation conditions. The photon energy required to remove the topmost of surface layer of the film did not .depend on excitation wavelength, and the penetration depth of excitation pulse dominated the etch depth. When the excitation wavelength was longer than 500 nm, permanent swelling was clearly observed but not for shorter wavelength excitation. In the latter case, photoisomerization occurred during excitation and the following photoreduction may play an important role. On the basis of the observations made in this study, a photochemical and photothermal mechanisms can explain mostly the short and long wavelength excitation results, respectively.