Ki-Yong Kim

Terahertz emission from ultrafast ionizing air in symmetry-broken laser fields

A transient photocurrent model is developed to explain coherent terahertz emission from air irradiated by a symmetry-broken laser field composed of the fundamental and its second harmonic laser pulses. When the total laser field is asymmetric across individual optical cycles, a nonvanishing electron current surge can arise during optical field ionization of air, emitting a terahertz electromagnetic pulse. Terahertz power scalability is also investigated, and with optical pump energy of tens of millijoules per pulse, peak terahertz field strengths in excess of 150 kV/cm are routinely produced.

Single-shot supercontinuum spectral interferometry

We have developed a single-shot spectral interferometer using the supercontinuum pulse generated by self-focusing in atmospheric pressure air. The diagnostic can be used to measure ultrafast refractive index transients either in a direct frequency-to-time mapping mode or in a full Fourier transform mode. In the direct mapping mode, temporal resolution is shown to be strongly restricted by the pulse chirp. In the transform mode, the ultimate temporal resolution is limited by the supercontinuum pulse bandwidth and the maximum pump-induced phase shift.

Generation of coherent terahertz radiation in ultrafast laser-gas interactionsa)

The generation of intense terahertz radiation in ultrafast laser-gas interactions is studied on a basis of transient electron current model. When an ultrashort pulse laser’s fundamental and its second harmonic fields are mixed to ionize a gas, a nonvanishing, directional photoelectron current can be produced, which simultaneously emits terahertz radiation in the far field. Here, the generation mechanism is examined with an analytic derivation and numerical simulations, in which tunneling ionization and subsequent electron motion in the combined laser field play a key role. In the simulations, three types of laser-gas interactions are considered: (i) mixing the fundamental and its second harmonic fields, (ii) mixing nonharmonic, two-color fields, and (iii) focusing single-color, few-cycle pulses. In these interactions, terahertz generation and other nonlinear effects driven by the transient current are investigated. In particular, anticorrelation between terahertz and second (or third) harmonic generation ...

Intense terahertz generation in two-color laser filamentation: energy scaling with terawatt laser systems

We investigate high-power terahertz (THz) generation in two-color laser filamentation using terawatt (TW) lasers including a 0.5?TW, 1?kHz system, as well as 2 and 30?TW systems both operating at 10?Hz. With these lasers, we study the macroscopic effect in filamentation that governs THz output energy yields and radiation profiles in the far field. We also characterize the radiation spectra at a broad range of frequencies covering radio?micro-waves to infrared frequencies. In particular, our 1?kHz THz source can provide high-energy (>1??J), high average power (>1?mW), intense (>1?MV?cm?1) and broadband (0.01?60?THz) THz radiation via two-color filamentation in air. Based on our scaling law, an ?30?TW laser can produce >0.1?mJ of THz radiation with multi-gigawatt peak power in ?1.5?m long filamentation.

Measurement of the average size and density of clusters in a gas jet

We present an all-optical method for measurement of the average size and density of clusters produced by high-pressure gas jet flow. The technique employs Rayleigh scattering imaging combined with interferometry. The cluster size and density in a gas jet under the irradiation of intense laser fields plays a critical role in the laser–cluster coupling dynamics.

Single-shot terahertz pulse characterization via two-dimensional electro-optic imaging with dual echelons

A single-shot measurement of terahertz electromagnetic pulses is implemented using two-dimensional electro-optic imaging with dual echelon optics. The reported embodiment produces sequentially delayed multiprobe beamlets, routinely providing a time window of >10 ps with ~25 fs temporal step sizes. Because of its simplicity and robustness, the technique is ideally suited for real-time ultrashort relativistic electron bunch characterization.

High-Power Broadband Terahertz Generation via Two-Color Photoionization in Gases

We review high-energy, broadband terahertz (THz) generation in two-color laser-produced gaseous plasma. We first describe our microscopic plasma current model for directional plasma current and far-field THz radiation generation. Experimental results for THz yield dependence on laser energy, optical phase difference, gas species, and gas pressure are presented. We also describe ultrabroadband THz generation and detection in our experiments and numerical simulations. Finally, we discuss 2-D plasma currents for THz polarization control and macroscopic phase-matched THz generation.

Generation of strong terahertz fields exceeding 8 MV/cm at 1 kHz and real-time beam profiling

We demonstrate high-field (>8 MV/cm) terahertz generation at a high-repetition-rate (1 kHz) via two-color laser filamentation. Here, we use a cryogenically cooled femtosecond laser amplifier capable of producing 30 fs, 15 mJ pulses at 1 kHz as a driver, along with a combination of a thin dual-wavelength half-waveplate and a Brewster-angled silicon window to enhance terahertz generation and transmission. We also introduce a cost-effective, uncooled microbolometer camera for real-time terahertz beam profiling with two different modes.

Algorithm for high-resolution single-shot THz measurement using in-line spectral interferometry with chirped pulses

Z. P. Jiang and X. C. Zhang demonstrated a single-shot THz diagnostic based on spectral encoding of a chirped optical probe pulse [Appl. Phys. Lett. 72, 1945 (1998)]. This technique is thought to have an inherent uncertainty principle-imposed temporal resolution limitations. In this letter, we describe a method to recover the THz field without distortions, surpassing previous resolution limitations. Our approach is based on interpreting the spectral encoding experiment as in-line spectral interferometry, analogous to Gabor’s in-line spatial holography [D. Gabor, Nature (London) 161, 777 (1948)]. We recover the THz field from the interferogram and the characterized probe by using Tikhonov regularization combined with lower and upper triangular decomposition.

Single-shot, interferometric, high-resolution, terahertz field diagnostic

We present a single-shot, high-temporal-resolution terahertz diagnostic capable of measuring free-space far-infrared electromagnetic fields in time and space. We show that by using a chirped probe electro-optic sampling technique, in combination with a recently described interferometric retrieval algorithm [Appl. Phys. Lett. 87, 211109 (2005)], the diagnostic can provide transform-limited temporal resolution, mainly limited by the spectral bandwidth of the optical probe pulse, regardless of its chirp.