Sungho Jeong

Explosive change in crater properties during high power nanosecond laser ablation of silicon

Mass removed from single crystal silicon samples by high irradiance (1×109 to 1×1011 W/cm2) single pulse laser ablation was studied by measuring the resulting crater morphology with a white light interferometric microscope. The craters show a strong nonlinear change in both the volume and depth when the laser irradiance is less than or greater than ≈2.2×1010 W/cm2. Time-resolved shadowgraph images of the ablated silicon plume were obtained over this irradiance range. The images show that the increase in crater volume and depth at the threshold of 2.2×1010 W/cm2 is accompanied by large size droplets leaving the silicon surface, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of the layer heated to approximately the critical temperature. The model includes transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency). In this case, the estimated thickness of the superheated layer at a delay time of 200–300 ns shows a close agreemen...

Evidence for phase-explosion and generation of large particles during high power nanosecond laser ablation of silicon

The craters resulting from high-irradiance (1×109–1×1011 W/cm2) single-pulse laser ablation of single-crystal silicon show a dramatic increase in volume at a threshold irradiance of 2.2×1010 W/CM2. Time-resolved shadowgraph images show ejection of large particulates from the sample above this threshold irradiance, with a time delay ∼300 ns. A numerical model was used to estimate the thickness of a superheated layer near the critical state. Considering the transformation of liquid metal into liquid dielectric near the critical state (i.e., induced transparency), the calculated thickness of the superheated layer at a delay time of 200–300 ns agreed with the measured crater depths. This agreement suggests that induced transparency promotes the formation of a deep superheated layer, and explosive boiling within this layer leads to particulate ejection from the sample.

Shock wave and material vapour plume propagation during excimer laser ablation of aluminium samples

A probe beam deflection technique was utilized to measure the propagation of a shock wave and material vapour plume generated during excimer laser ablation of aluminium samples. The measured transit time of the laser-induced shock wave was compared with the prediction based on an ideal blast-wave model, using the Sedov-Taylor solution. The prediction of the incident laser energy converted into the laser-induced gasdynamic flow utilizing this blast-wave model overestimated the efficiency, even under conditions when the measured shock-wave velocity follows the correct model relation. The propagation of material vapour was measured from the deflection of the probe beam at later times. The propagation velocity of material vapour ranged from 20-40 m s-1 with a greater velocity near the target surface.

Propagation of the shock wave generated from excimer laser heating of aluminum targets in comparison with ideal blast wave theory

Abstract Propagation of the shock wave generated during pulsed laser heating of aluminum targets was measured utilizing a probe beam deflection technique. The transit time of the laser-generated shock wave was compared with that predicted from the Sedov–Taylor solution for an ideal spherical blast wave. It was found that the most important parameters for the laser-generated shock wave to be consistent with the theoretically predicted propagation are the ambient pressure and the laser beam spot size. The prediction for laser energy conversion into the laser-induced vapor flow using the Sedov–Taylor solution overestimated the energy coupling efficiency, indicating a difference between a laser-induced gas-dynamic flow and an ideal blast wave.

Numerical modeling of pulsed laser evaporation of aluminum targets

A one dimensional thermal model was used to investigate the pulsed laser evaporation of aluminum targets. The gasdynamic flow of the vapor was predicted by solving one dimensional compressible flow equations for an inviscid fluid. The variation of the threshold laser fluence for target evaporation with respect to target surface reflectivity was investigated. The threshold laser fluence predicted by a thermal evaporation model was larger than the values obtained experimentally. The effects of the ambient pressure and the laser fluence on the local Mach number of the vapor at the edge of the Knudsen layer were studied. The structure of the gasdynamic flow including the shock wave and the contact surface was shown in the density profiles.

Fabrication and evaluation of a copper flat micro heat pipe working under adverse-gravity orientation

The fabrication of a prototype flat micro heat pipe (FMHP) of a size appropriate for mobile electronics and its performance test results are reported. To ensure reliable operation under repeated thermal loads and to enhance the heat transport capacity, copper is selected as the packaging material considering its high thermal conductivity and good strength. The wick structure of the FMHP consists of fan-shaped microgrooves with a width and depth of about 100 and 200 µm, respectively. The fabrication of microgrooves was done using a laser micromachining technique and water was used as the working fluid. Fan-shaped microgrooves were found to induce a greater capillary pressure than triangular microgrooves of a similar size. Subsequent test results confirmed that despite its small size, 56 mm (L) × 8 mm (W) × 1.5 mm (H), the FMHP had a high heat transport capacity; the maximum heat transfer rate was 8 W under stable operation conditions and 13 W at the dryout point. In addition, the FMHP worked under adverse-gravity conditions with little change in cooling capacity, a key advantage for application in modern mobile electronics.

Design and characterization of a micromachined inchworm motor with thermoelastic linkage actuators

A new micromachined inchworm motor has been designed and fabricated for micro assembly applications. In order to implement inchworm motions, two thermoelastic actuators are contrived to have five-linkage mechanism with two-dimensional motions in tangential and normal directions. The thermoelastic actuators consist of two amplification bars and two coupling bars with four hinge springs. A forked tip, located on the apex of the linkage, is used to fit the teeth of shuttle mass for inchworm operation. The thermal expansion of the active bars generates the displacement of the actuator, which is then transformed into a bending of the active hinges to be finally amplified by the amplification bar. The inchworm actuator progressed by the designed steps of 5 /spl mu/m and latched up by the teeth. The estimated driving force was 50 /spl mu/N with less than 0.2 /spl mu/m tolerance.

Subdermal Flexible Solar Cell Arrays for Powering Medical Electronic Implants

A subdermally implantable flexible photovoltatic (IPV) device is proposed for supplying sustainable electric power to in vivo medical implants. Electric properties of the implanted IPV device are characterized in live animal models. Feasibility of this strategy is demonstrated by operating a flexible pacemaker with the subdermal IPV device which generates DC electric power of ≈647 μW under the skin.

The rapid growth of vertically aligned carbon nanotubes using laser heating

Growth of densely packed vertically aligned carbon nanotubes (VA-CNTs) using laser-induced chemical vapor deposition with visible laser (lambda = 532 nm) irradiation at room temperature is reported. Using a multiple-catalyst layer (Fe/Al/Cr) on quartz as the substrate and an acetylene-hydrogen mixture as the precursor gas, VA-CNT pillars with 60 microm height and 4 microm diameter were grown at a high rate of around 1 microm s(-1) with good reproducibility. It is demonstrated that the fabrication of uniform pillar arrays of VA-CNTs can be achieved with a single irradiation for each pillar using LCVD with no annealing or preprocessing of the substrate. Here, laser fast heating is considered the primary mechanism facilitating the growth of VA-CNT pillars. Field emission characteristics of an array of VA-CNT pillars were then examined to investigate their potential application in vacuum electronic devices.

LASER HEATING OF A CAVITY VERSUS A PLANE SURFACE FOR METAL TARGETS UTILIZING PHOTOTHERMAL DEFLECTION MEASUREMENTS

The effects of a cylindrical cavity in a metal surface on the energy coupling of a laser beam with the solid were investigated by using a photothermal deflection technique. The photothermal deflection of a probe beam over the cavity was measured while the bottom of the cavity was heated with a Nd–YAG laser with a wavelength of 1064 nm. Cavities in three different materials and with two different aspect ratios were used for the experiment. Temperature distributions in the solid and the surrounding air were computed numerically and used to calculate photothermal deflections for cavity heating and for plane surface heating. Reflection of the heating laser beam inside the cavity increased the photothermal deflection amplitude significantly with larger increases for materials with larger thermal diffusivity. The computed photothermal deflections agreed more closely with the experimental results when reflection of the heating laser beam inside the cavity was included in the numerical model. The overall energy c...