Byung-Tae Ahn

Field propagation-induced directionality of carrier-envelope phase-controlled photoemission from nanospheres

Near-fields of non-resonantly laser-excited nanostructures enable strong localization of ultrashort light fields and have opened novel routes to fundamentally modify and control electronic strong-field processes. Harnessing spatiotemporally tunable near-fields for the steering of sub-cycle electron dynamics may enable ultrafast optoelectronic devices and unprecedented control in the generation of attosecond electron and photon pulses. Here we utilize unsupported sub-wavelength dielectric nanospheres to generate near-fields with adjustable structure and study the resulting strong-field dynamics via photoelectron imaging. We demonstrate field propagation-induced tunability of the emission direction of fast recollision electrons up to a regime, where nonlinear charge interaction effects become dominant in the acceleration process. Our analysis supports that the timing of the recollision process remains controllable with attosecond resolution by the carrier-envelope phase, indicating the possibility to expand near-field-mediated control far into the realm of high-field phenomena.

Attosecond nanoscale near-field sampling

The promise of ultrafast light-field-driven electronic nanocircuits has stimulated the development of the new research field of attosecond nanophysics. An essential prerequisite for advancing this new area is the ability to characterize optical near fields from light interaction with nanostructures, with sub-cycle resolution. Here we experimentally demonstrate attosecond near-field retrieval for a tapered gold nanowire. By comparison of the results to those obtained from noble gas experiments and trajectory simulations, the spectral response of the nanotaper near field arising from laser excitation can be extracted.

Real-Time Head Orientation from a Monocular Camera Using Deep Neural Network

We propose an efficient and accurate head orientation estimation algorithm using a monocular camera. Our approach is leveraged by deep neural network and we exploit the architecture in a data regression manner to learn the mapping function between visual appearance and three dimensional head orientation angles. Therefore, in contrast to classification based approaches, our system outputs continuous head orientation. The algorithm uses convolutional filters trained with a large number of augmented head appearances, thus it is user independent and covers large pose variations. Our key observation is that an input image having (32 times 32) resolution is enough to achieve about 3 degrees of mean square error, which can be used for efficient head orientation applications. Therefore, our architecture takes only 1 ms on roughly localized head positions with the aid of GPU. We also propose particle filter based post-processing to enhance stability of the estimation further in video sequences. We compare the performance with the state-of-the-art algorithm which utilizes depth sensor and we validate our head orientation estimator on Internet photos and video.

An Autonomous Driving System for Unknown Environments Using a Unified Map

Recently, there have been significant advances in self-driving cars, which will play key roles in future intelligent transportation systems. In order for these cars to be successfully deployed on real roads, they must be able to autonomously drive along collision-free paths while obeying traffic laws. In contrast to many existing approaches that use prebuilt maps of roads and traffic signals, we propose algorithms and systems using Unified Map built with various onboard sensors to detect obstacles, other cars, traffic signs, and pedestrians. The proposed map contains not only the information on real obstacles nearby but also traffic signs and pedestrians as virtual obstacles. Using this map, the path planner can efficiently find paths free from collisions while obeying traffic laws. The proposed algorithms were implemented on a commercial vehicle and successfully validated in various environments, including the 2012 Hyundai Autonomous Ground Vehicle Competition.

Variations of morphology and electrical property of diamond with doping using diborane in a methane–hydrogen gas mixture

Abstract The shape of isolated diamond particles and the surface morphology of diamond films were investigated at various boron concentrations. The diamond particles and films were deposited using a microwave plasma-assisted chemical vapor deposition method at 70xa0Torr with various diborane concentrations from 0 to 4000xa0ppm in hydrogen–2% methane gas. The shape of isolated particle changed from cubo-octahedron to octahedron via truncated-octahedron as the diborane concentration increased. The growth rate decreased rapidly and then remained constant with further increasing diborane concentration. The changes of particle shape and growth rate were explained by the charge-polarity model of cursor gas, and by the relative growth rate of (100) and (111) facets, which were controlled by the boron concentration. The surface morphology of diamond films varied in accordance with the change of the shape of the diamond particles. The resistivity of the diamond film decreased owing to the increase of carrier concentration as the diborane concentration increased to 500xa0ppm, below which the shape of isolated diamond particles was cubo-octahedral.

Prepulse effect on laser-induced water-window radiation from a liquid nitrogen jet

The authors show the prepulse effect on the conversion efficiency of a visible laser into water-window (λ=2.3–4.4nm) x ray from a liquid nitrogen jet. It is observed that a prepulse of only 2mJ enhances the conversion efficiency by 10–15 times for the main pulse of 15–60mJ at a delay of 3–6ns. The photon flux is ∼1.2×1012photons∕pulsesr at a delay of 4ns for a main pulse of 60mJ with a prepulse of 4–8mJ. It is noticed that the conversion efficiency increases with the delay up to 3ns and is then saturated.

Optimization of laser parameters for the maximum efficiency in the generation of water-window radiation using a liquid nitrogen jet

A laser-produced plasma is a suitable compact x-ray source that can be of broad band or quasimonochromatic with a proper choice of material and filter. To address the maximum conversion efficiency for an efficient, quasimonochromatic source at 2.88 nm (NVI1s2‐1s2p transition) using liquid nitrogen jet for soft x-ray microscopy, the radiation characteristics such as absolute intensity, spectra, and angular distribution have been investigated for different laser pulse durations (picosecond and femtosecond pulses) and laser energies The comparison of conversion efficiencies between picosecond [120 ps full width at half maximum (FWHM)] and femtosecond (40–500fs FWHM) lasers indicates that the picosecond laser would provide better conversion efficiency, which is 1.6% at 2×1013W∕cm2. The investigation shows that the laser intensity for the maximum conversion efficiency scales as Im∝1∕τα, where α=0.9±0.15. This empirical formula is useful to choose the laser parameters properly for a given pulse width.

Appearance-based gaze estimation using kinect

Human gaze tracking has gathered much attention due to its capability to detect intuitive attention. Appearance-based methods can work with a single camera in ordinary conditions to track human gaze. An effective way to generate eye appearances is proposed using the Kinect. The head pose information is obtained from the Kinect after a series of calibrations. The Eye Appearance features are collected through ASM and KLT feature tracker. With 23 training samples, the error is found to be 1.07°. This paper proposes an efficient scheme for gaze tracking using a single Kinect device.

Improved Optical Transmittance of Boron Doped ZnO Thin Films by Low Pressure Chemical Vapor Deposition with Pulse Boron Doping

A pulse doping technique is introduced to obtain boron doped ZnO thin films with high conductivity and improved transmittance. The ZnO thin films were deposited by low pressure chemical vapor deposition and boron was doped into the ZnO thin films by the pulse supply of B2H6 gas. The pulse boron doped ZnO thin films exhibited a well textured surface morphology with large pyramid shape grains and a minimum resistivity about 1.0 × 10−3Ω·cm. These films also had a lower carrier concentration and higher mobility than conventional boron doped ZnO thin film, which has a similar conductivity value. In addition, because of lower carrier concentration and larger surface grains, the pulse boron doped ZnO films exhibited enhanced total transmittance in the near-infrared wavelength range and diffuse transmittance in the overall wavelength range compared to the conventional boron doped ZnO thin film. CIGS solar cells using pulse boron doped ZnO film showed higher efficiency than those using conventional boron doped ZnO film. Based on these results, we suggest a modified low pressure chemical vapor deposition process with pulse boron doping, which is an advantageous technique to deposit doped ZnO thin films with favorable electrical and optical properties for thin film solar cells.

Na Doping Properties of Cu(In,Ga)Se 2 Absorber Layer Using NaF Interlayer on Mo Substrate

In high-efficiency Cu(In,Ga) solar cells, Na is doped into a Cu(In,Ga) light-absorbing layer from sodalime-glass substrate through Mo back-contact layer, resulting in an increase of device performance. However, this supply of sodium is limited when the process temperature is too low or when a substrate does not supply Na. This limitation can be overcome by supplying Na through external doping. For Na doping, an NaF interlayer was deposited on Mo/glass substrate. A Cu(In,Ga) absorber layer was deposited on the NaF interlayer by a three-stage co-evaporation process As the thickness of NaF interlayer increased, smaller grain sizes were obtained. The resistivity of the NaF-doped CIGS film was of the order of indicating that doping was not very effective. However, highest conversion efficiency of 14.2% was obtained when the NaF thickness was 25 nm, suggesting that Na doping using an NaF interlayer is one of the possible methods for external doping.