arXiv subjects

Ki-Yong Kim

Publications and source records attributed to Ki-Yong Kim.

9 recordsLinked to original sources

arXiv2025

Generation and characterization of coherent terahertz radiation from 100-TW laser-wakefield acceleration

We experimentally characterized terahertz (THz) radiation emitted from laser-wakefield acceleration (LWFA) driven at 100-TW laser power. Simultaneous measurements of the laser energy, electron-bunch charge, and THz energy reveal a quadratic dependence of the THz energy on both charge and laser energy. This behavior indicates coherent collective emission in the generation process and provides a useful scaling law for THz output. Microbolometer-based beam profiling shows a relatively large THz beam divergence (~0.2 rad). Single-shot THz interferometry further shows that the emitted THz pulse is sub-picosecond in duration and broadband. Combining the beam-profile and interferometric measurements, the THz spectrum is expected to span approximately 1-20 THz. Together, these results support coherent acceleration radiation as the dominant mechanism for THz generation in 100-TW LWFA.

physics.plasm-ph↗

arXiv2020

Single-shot terahertz spectrometer using a microbolometer camera

We demonstrate a single-shot terahertz spectrometer consisting of a modified Mach-Zehnder interferometer and a microbolometer focal plane array. The spectrometer is simple to use and can measure terahertz field autocorrelations and spectral power with no moving parts and no ultrashort-pulsed laser. It can effectively detect radiation at 10$\sim$40 THz when tested with a thermal source. It can be also used to measure the complex refractive index of a sample material. In principle, it can characterize both laser-based and non-laser-based terahertz sources and potentially cover 1$\sim$10 THz with specially-designed terahertz microbolometers.

physics.ins-det↗

arXiv2020

Multicycle terahertz pulse generation by optical rectification in LiNbO$_3$, LiTaO$_3$, and BBO crystals

We report multicycle, narrowband, terahertz radiation at 14.8 THz produced by phase-matched optical rectification of femtosecond laser pulses in bulk lithium niobate (LiNbO$_3$) crystals. Our experiment and simulation show that the output terahertz energy greatly enhances when the input laser pulse is highly chirped, contrary to a common optical rectification process. We find this abnormal behavior is attributed to a linear electro-optic (or Pockels) effect, in which the laser pulse propagating in LiNbO$_3$ is modulated by the terahertz field it produces, and this in turn drives optical rectification more effectively to produce the terahertz field. This resonant cascading effect can greatly increase terahertz conversion efficiencies when the input laser pulse is properly pre-chirped with additional third order dispersion. We also observe similar multicycle terahertz emission from lithium tantalate (LiTaO$_3$) at 14 THz and barium borate (BBO) at 7 THz, 10.6 THz, and 14.6 THz, all produced by narrowband phase-matched optical rectification.

physics.optics↗

arXiv2020

Generation of 0.7 mJ multicycle 15 THz radiation by phase-matched optical rectification in lithium niobate

We demonstrate efficient multicycle terahertz pulse generation at 14.6 THz from large-area lithium niobate crystals by using high-energy (up to 2 J) femtosecond Ti:sapphire laser pulses. Such terahertz radiation is produced by phase-matched optical rectification in lithium niobate. Experimentally, we achieve maximal terahertz energy of 0.71 mJ with conversion efficiency of $\sim$0.04%. We also find that the effective interaction length for optimal terahertz conversion is fundamentally limited by terahertz absorption, laser pulse stretching by material dispersion, and terahertz-induced nonlinear cascading effects on the driving laser pulse.

physics.optics↗

arXiv2019

Simplified single-shot supercontinuum spectral interferometry

We have experimentally demonstrated a simplified method for performing single-shot supercontinuum spectral interferometry (SSSI) that does not require pre-characterization of the probe pulse. The method, originally proposed by D. T. Vu, D. Jang, and K. Y. Kim, uses a genetic algorithm (GA) and as few as two time-delayed pump-probe shots to retrieve the pump-induced phase shift on the probe [Opt. Express 26, 20572 (2018)]. We show that the GA is able to successfully retrieve the transient modulations on the probe, and that the error in the retrieved modulation decreases dramatically with the number of shots used. In addition, we propose and demonstrate a practical method that allows SSSI to be done with a single pump-probe shot (again, without the need for pre-characterization of the probe). This simplified method can prove to be immensely useful when performing SSSI with a low-repetition-rate laser source.

physics.optics↗

arXiv2018

Single-shot Ultrafast Imaging via Spatiotemporal Division of Femtosecond Laser Pulses

We have developed a single-shot imaging technique that can capture ultrafast events occurring on femtosecond to picosecond time scales. The technique is based on an optical pump-probe method, in which multiple time-delayed femtosecond pulses simultaneously probe a pump-excited sample. Here we use two sets of 2-by-2 mirror arrays for spatial/temporal division and routing of multiple probe pulses. This single-shot scheme is successfully applied to capture femtosecond ionization fronts propagating at the speed of light in air, as well as laser-induced ablation of solid targets.

physics.ins-det↗

arXiv2018

Simplified Chirp Characterization in Single-shot Supercontinuum Spectral Interferometry

Single-shot supercontinuum spectral interferometry (SSSI) is an optical technique that can measure ultrafast transients in the complex index of refraction. This method uses chirped supercontinuum reference/probe pulses that need to be pre-characterized prior to use. Conventionally, the spectral phase (or chirp) of those pulses can be determined from a series of phase or spectral measurements taken at various time delays with respect to a pump-induced modulation. Here we propose a novel method to simplify this process and characterize reference/probe pulses up to the third order dispersion from a minimum of 2 snapshots taken at different pump-probe delays. Alternatively, without any pre-characterization, our method can retrieve both unperturbed and perturbed reference/probe phases, including the pump-induced modulation, from 2 time-delayed snapshots. From numerical simulations, we show that our retrieval algorithm is robust and can achieve high accuracy even with 2 snapshots. Without any apparatus modification, our method can be easily applied to any experiment that uses SSSI.

physics.ins-det↗

arXiv2013

THz generation by optical Cherenkov emission from ionizing two-color laser pulses

Two-color photoionization produces a cycle-averaged current driving broadband, conically emitted THz radiation. We investigate, through simulation, the processes determining the angle of conical emission. We find that the emission angle is determined by an optical Cherenkov effect, where the front velocity of the current source is faster than the THz phase velocity.

physics.optics↗

arXiv2012

Summary report of working group 5: Beam and radiation generation, monitoring, and control

This paper summarizes the activities and presentations of Working Group 5 of the Advanced Accelerator Concepts Workshop held at Annapolis, Maryland in June 2010. Working Group 5 touched on a broad range of topics in the fields of beam and radiation generation and their monitoring and control. These topics were not comprehensively covered in this Workshop, but rather the Working Group concentrated on specific new developments and recent investigations. The Working Group divided its sessions into four broad categories: cathodes and electron guns, radiation generation, beam diagnostics, and beam control and dynamics. This summary is divided into the same structure.

physics.acc-ph↗