Ji Sang Yahng

Nonlinear enhancement of femtosecond laser ablation efficiency by hybridization with nanosecond laser.

Synchronization of femtosecond laser with nanosecond (~250 ns) laser results in a large enhancement in laser ablation efficiency of the Si wafer 12 times more than that with an independent laser exposure. Transient changes in the status of target material due to the proceeding nanosecond laser increase the femtosecond laser ablation efficiency.

High-speed terahertz reflection three-dimensional imaging using beam steering

High-speed terahertz (THz) reflection three-dimensional (3D) imaging is demonstrated using electronically-controlled optical sampling (ECOPS) and beam steering. ECOPS measurement is used for scanning an axial range of 7.8 mm in free space at 1 kHz scan rate while a transverse range of 100 × 100 mm(2) is scanned using beam steering instead of moving an imaging target. Telecentric f-θ lenses with axial and non-axial symmetry have been developed for beam steering. It is experimentally demonstrated that the non-axially symmetric lens has better characteristics than the axially symmetric lens. The total scan time depends on the number of points in a transverse range. For example, it takes 40 s for 200 × 200 points and 10 s for 100 × 100 points. To demonstrate the application of the imaging technique to nondestructive testing, THz 3D tomographic images of a glass fiber reinforced polymer sample with artificial internal defects have been acquired using the lenses for comparison.

High-speed broadband frequency sweep of continuous-wave terahertz radiation.

We present a new technical implementation of a high-speed broadband frequency sweep of continuous-wave terahertz (THz) radiation. THz frequency sweeping with a kHz sweep rate and a THz sweep range is implemented using THz photomixing in which an optical beat source consists of a wavelength-swept laser and a distributed feedback laser diode. During the frequency sweep, frequency-domain THz interferograms are measured using the coherent homodyne detection employing signal averaging for noise reduction, which can give time-of-flight information via fast Fourier transform. Multiple reflections in a Si wafer and the thickness of the wafer are measured to demonstrate the potential of this method for fast THz tomography and thickness measurement.

High-speed frequency-domain terahertz coherence tomography

High-speed frequency-domain terahertz (THz) coherence tomography is demonstrated using frequency sweeping of continuous-wave THz radiation and beam steering. THz frequency sweeping with a kHz sweep rate and a THz sweep range is implemented using THz photomixing in which an optical beat source consists of a wavelength-swept laser and a distributed feedback laser diode. During the frequency sweep, frequency-domain THz interferograms are measured using the coherent homodyne detection employing signal averaging for noise reduction, which are used as axial scan data via fast Fourier transform. Axial scan data for 100×100 points can be acquired in 100 s while scanning a transverse range of 100×100 mm2 using a THz beam scanner comprised of a two-dimensional galvanometer scanner and a telecentric f-θ lens.

Temperature dependence of ultrafast laser ablation efficiency of crystalline silicon

Ultrafast laser ablation of crystalline silicon was investigated as a function of temperature. The ablation efficiency is slightly enhanced with an apparent decrease in ablation threshold and surface roughness at a high substrate temperature.

Application of Ultrafast Laser Optoperforation for Plant Pollen Walls and Endothelial Cell Membranes

-fertilization and numerous other clinical areas (Nelson & Berm, 1989; Senz & Miiller, 1989). Furthermore, it is also a useful tool for studying basic cellular and biochemical processes. Fluence, energy per unit area, is one of the most important and critical aspects of the laser treatment (Sidhu et al., 2009). Depending upon the laser fluence, either transient or permanent changes can be induced in various cellular compartments including cell walls, plasma membranes, and even organelles with high resolution under physiological environment. For instance, Higashiyama et al. performed a selective ablation of a target compartment of the embryo sac with ultraviolet (UV) pulsed laser to investigate the contribution of each gametophytic cell to pollen tube attraction and found that the synergic cells adjacent to the egg cell plays an important role in attracting the pollen tube (Higashiyama et al., 2001). Laser pulses with a sufficient spatial resolution can introduce a physical gate in either a membrane or a cell wall under physiological environment such that one could deliver a foreign material into the cell and bring about selective manipulation of subcellular components without demolishing the cell’s integrity. It has been reported that in Japonica rice (

Increasing femtosecond laser processing efficiency by hybridization with nanosecond laser

The temporal coupling of femtosecond and nanosecond laser induces a remarkable increase in the processing efficiency 12 times more than that with an independent laser exposure. When femtosecond laser arrives before nanosecond laser, the dependence of the ablation efficiency on the time delay between the femtosecond and nanosecond laser pulses is very resemble to nanosecond laser traces. When femtosecond laser arrives after nanosecond laser, however, we observed an apparent delayed decaying component with a time constant of several hundreds of nanosecond in the ablation efficiency curve. Based on the current observation, we have explained the rather large enhancement in femtosecond laser ablation efficiency with synchronization between femtosecond and nanosecond laser in terms of silicon surface metallization due to the proceeding nanosecond laser. Such a progress in femtosecond laser micro processing makes it possible to maximize the processing speed and reduce the processing threshold energy. The current findings prominently reduce a various high order nonlinear effects which are frequently confronted when we focus high-power femtosecond laser pulses on the target under atmospheric conditions.