Guang-Hoon Kim

Optical vortices produced with a nonspiral phase plate

We present simple methods to produce optical vortices on the axis of beam propagation with nonspiral phase plates. We show that a phase plate that provides linear phase retardation on one half of a laser beam produces optical vortices, which is demonstrated experimentally by use of a thickness-varying glass platelet. We also demonstrate and explain that mixed dislocations of a bent edge dislocation transform into a pair of vortices with opposing topological charges.

An array of phase singularities in a self-defocusing medium

Abstract We report on experimental investigations of nonlinear propagation of an array of first-order phase singularities with the same charges, which is lining up in a straight line, through a self-defocusing medium. The nonlinear rotation of the singularities depends on their density and is not uniform for each singularity pair. We discuss the non-uniform rotation considering singularity–singularity interaction.

Lasing images from two merging ink-doped liquid droplets

Lasing images from two merging ink-doped ethanol droplets (size ~37 microm), which were generated by modulation techniques and pumped by nanosecond laser pulses, were photographed through a microscope. By measuring the size difference of two adjacent droplets from the spectral shifts of lasing peaks, we confirmed that mass transfer occurred in the breakup process of the liquid jet. We observed lasing images from merging droplets in a variety of patterns. Noteworthy among these is the lasing from snowman-shaped droplets that are conjectured to stem from coupled modes with orbits around both droplets.

Observation of Q-spoiling effects on the resonance modes from a noncircularly deformed liquid jet

Spectral changes in fluorescence and lasing spectra were observed from a noncircularly deformed ink-doped ethanol jet, which was induced by a lateral gas flow. The distortion parameter was determined from the analysis of diffraction patterns to be as much as ~10%. Q -spoiling effects were clearly observed in the appearance of high- Q (~10(7)) modes in fluorescence and their disappearance in lasing. From the behavior of resonance modes, we concluded that the effective Q does not decrease so rapidly as predicted by the relation obtained from the ray-optics model. We also found that the signals leak out, with a wide angular spreading, mainly from near the boundary of the major axis, even for a large distortion parameter.

Liquid microdroplet generator with glass orifice

A uniform size liquid microdroplet generator was fabricated with glass orifice. The orifice of the generator was made as small as 5 μm on a Pyrex tube. The size of the uniform ethanol droplets generated by the 5 μm orifice varied from 19 to 14 μm as the driving frequency changed from 150 to 400 kHz. Operation characteristics of this generator were comparable to those of the commercial devices. We also measured the interdroplet separation with the diffraction pattern of the array of droplets in order to get the velocity of droplet stream at various stream positions and driving frequencies. The deceleration of droplet stream was nearly independent of the driving frequency which determines the droplet size and interdroplet separation.

Characterization of TRIUMF dc H− ion sources for enhanced brightness

At TRIUMF, H− ion sources have been characterized on a teststand to improve the understanding of their performance. Measured beam characteristics such as current, emittance, and e∕H− ratio were correlated with source conditions as a function of relevant plasma parameters. Plasma densities, temperatures, and plasma potentials were measured with a Langmuir probe and correlated with beam properties for different confining magnetic fields and different values of arc power and gas pressure. The mechanism of beam extraction was studied by correlating plasma potential profiles with the corresponding plasma electrode bias voltages obtained from optimizing ion source outputs. Experiments with collar biasing and noble gas mixing were unsuccessful because of undesirable plasma potential profiles.

Pulsed current wave shaping with a transmission line by utilizing superposition of a forward and a backward voltage wave for fast capillary Z-pinch discharge

By using a water transmission line, current wave shaping was demonstrated for a fast capillary Z-pinch discharge recombination soft x-ray laser study. The pulsed power system consists of a water capacitor, a gap switch, a transmission line, and a capillary plasma load. A voltage wave initiated at the water capacitor propagates toward the capillary load through the transmission line. Control of the pulse delay that occurred in the transmission line provides the superposition of the forward and the backward voltage waves effectively in order to perform current wave shaping with higher current amplitude and rapid current decay.

Spectral changes of stimulated Raman scattering from modulated water cylinder

The spectral changes of stimulated Raman scattering (SRS) from a micrometer-sized sinusoidally modulated water jet (radius, ∼14.5µm) have been observed. The pumping pulses of a frequency-doubled Q-switched Nd:YAG laser were synchronized to the modulation wave on the water jet. Before conducting SRS experiments, we measured the modulation amplitudes of the water jet from the diffraction patterns. The spectral changes of SRS exhibited the peaks shifts, the narrowing down of the SRS spectral range, and the reduction of the SRS peak. From the estimation of the mode orders of the SRS peaks with the cylinder modes, we found that the cylinder modes are not sufficient to explain the observed SRS peaks at small modulation amplitude, which are thought to be caused by the appearance of new modes in the modulated jet. We have interpreted the spectral changes as the reduction of the effective quality factor in the modulated cylinder.

Photoluminescence fatigue in amorphous carbon (a-C) films prepared by d.c. magnetron sputtering

Abstract We have studied the fatigue effect in unhydrogenated amorphous carbon ( a -C) films prepared by d.c. magnetron sputtering of graphite. At 12 K the fatifued photoluminescence spectra show neither a spectral shape change nor peak shift during illumination. The fatigued photoluminescence intensity at a fixed energy shows a t − b time process during illumination. The self-recovery of the fatigued photoluminescence in the dark requires much more time than hydrogenated amorphous silicon ( a -Si : H) and less than amorphous phosphorus ( a -P). The light induced defects in a -C films are in metastable states and explained to be π defects rather than dangling bonds (σ defects) as in a -Si : H or hydrogenated amorphous carbon ( a -C : H).

Three - dimensional odd ring dark spatial solitons

The propagation properties of three-dimensional dark spatial solitons having odd ring formation is analyzed numerically in the frame of the (1 + 2)-dimensional nonlinear Schrodinger equation and compared with a pair of odd dark solitons. We discuss the experimental excitation condition of an odd ring dark soliton, which is superimposed on a finite-width background beam, with phase masks.