Young Uk Jeong

Terahertz transmission and sheet conductivity of randomly stacked multi-layer graphene

We investigate transmission characteristics and sheet conductivity of mono- to multi-layer graphene deposited on quartz in the terahertz (THz) frequency region. The free carrier absorption and Fabry-Perot interference between graphene layers give rise to nonlinear decrease of THz transmission from 76.7% to 27% for mono- to 12-layer graphene. These phenomena are well explained with a modified theoretical model based on Drude conductivity. The optical sheet conductivity of multi-layer graphene, made by layer-by-layer random stacking of high-quality mono-layer graphene, at 1 THz exhibits two orders of magnitude higher values than the universal optical conductivity due to intraband transition of intrinsic graphene.

Highly nonlinear organic crystal OHQ-T for efficient ultra-broadband terahertz wave generation beyond 10 THz.

We report on efficient generation of ultra-broadband terahertz (THz) waves via optical rectification in a novel nonlinear organic crystal with acentric core structure, i.e. 2-(4-hydroxystyryl)-1-methylquinolinium 4-methylbenzenesulfonate (OHQ-T), which possesses an ideal molecular structure leading to a maximized nonlinear optical response for near-infrared-pumped THz wave generation. By systematic studies on wavelength-dependent phase-matching conditions in OHQ-T crystals of different thicknesses we are able to generate coherent THz waves with a high peak-to-peak electric field amplitude of up to 650 kV/cm and an upper cut-off frequency beyond 10 THz. High optical-to-THz conversion efficiency of 0.31% is achieved by efficient index matching with a selective pumping at 1300 nm.

Laser driven terahertz generation in hot plasma with step density profile

An analytical formalism of terahertz (THz) radiation generation by beating of two lasers in a hot plasma with step density profile is developed. The lasers propagate obliquely to plasma surface normal, and the nonlinearity arises through the ponderomotive force. The THz is emitted in the specular reflection direction, and the yield is enhanced due to coupling with the Langmuir wave when the plasma frequency is close to THz frequency. The power conversion efficiency maximizes at an optimum angle of incidence.

Simple method for the temporal characterization of amplified spontaneous emission in femtosecond terawatt Ti:sapphire lasers

We have analyzed the temporal characteristics of amplified spontaneous emission (ASE) in femtosecond terawatt Ti:sapphire lasers by using a simple method based on fast photodiodes. Instead of measuring ASE directly with fast photodiodes, we created a narrow gap in the spectrum of seed pulses and, after amplification, detected the pure ASE signal through the gap by using a fast photodiode covered with a bandpass filter with high transmission at the gap. Because the detected ASE signal was completely separated from amplified main pulses, preceding and even trailing ASEs could be characterized quantitatively in a single-shot measurement. We believe that our method is a good alternative or a complement to conventional methods for ASE measurements.

Diffraction-Limited High-Power Single-Cycle Terahertz Pulse Generation in Prism-Cut LiNbO 3 for Precise Terahertz Applications

We report the generation of 3.3-mW single-cycle terahertz (THz) pulses at 1-kHz repetition rate via optical rectification in MgO-doped prism-cut stoichiometric LiNbO3. Efficient pulse-front tilting of 800-nm pulses was realized by an optimized single-lens focusing scheme for radially-symmetric propagation of THz beams. In this geometry, nearly-diffraction-limited THz Gaussian beams with electric field strength as high as 350 kV/cm were generated. The pump-to-THz energy conversion efficiency of

Generation of a quasi-monoenergetic high energy proton beam from a vacuum-sandwiched double layer target irradiated by an ultraintense laser pulse

1.36{\times}10^{-3}

Magnetron driven classical microtron as an injector for a wide band tunable compact far infrared free electron laser

and the extremely high signal-to-noise ratio of ~1:15000 achieved are among the best results for 1-kHz single-cycle terahertz pulse generation ever demonstrated in room temperature operation.

Quasi-monoenergetic proton beam from a proton-layer embedded metal foil irradiated by an intense laser pulse

An acceleration mechanism to generate a high energy proton beam with a narrow energy spread in the laser-induced plasma acceleration of a proton beam is proposed; this mechanism employs two thin foils separated by a narrow vacuum gap. Instead of a thin sheath field at the plasma surfaces, it utilizes an electrostatic field formed in the bulk of the plasma. From a one-dimensional fluid analysis, it has been found that with an appropriate target thickness, protons on the front surface of the second layer can be fed into the plasma, in which the protons are accelerated by an electrostatic field built into the bulk of the plasma. This leads to a proton beam with higher energy and a narrower energy spread than those accelerated at the rear surface of the second layer. The acceleration mechanism is also verified by a two-dimensional particle-in-cell simulation. With a 27-fs long and 2×1019 W/cm2 intense laser pulse, a proton beam with an 18-MeV peak energy and a 35% energy spread is generated. The peak energy i...

Control of terahertz nonlinear transmission with electrically gated graphene metadevices

A compact 12 turn classical microtron driven by a 2.8 GHz magnetron has been improved for use as an injector of a compact wide-band far infrared (FIR) free electron laser (FEL). The microtron provides an accelerated beam current of up to 70 mA by total energy of up to 7.2 MeV in a 5.5 /spl mu/s duration macro pulse. The frequency of the magnetron was stabilized in the range of (3-4)/spl times/10/sup -5/ by the frequency pulling of the magnetron with the reflected wave from the accelerating cavity. The measured energy spread and emittances in vertical and horizontal components of the electron beam were less than 0.4%, 1.5 mm/spl middot/mrad and 3.5 mm/spl middot/mrad, respectively. A measured temporal deviation of the bunch repetition rate of less than 120 kHz could be obtained by optimization of the microtron operating conditions for the FEL. With a total energy of electrons and macro pulse current of 7 MeV and 45 mA respectively stable FIR lasing could be obtained in the wavelength range from 97 to 150 /spl mu/m by changing the undulator K-parameter. The accelerating cavity moving system was upgraded to increase the variable range of the electron beam energy from 4.3 to 7 MeV, which provides wide tuning range of the FIR FEL wavelength up to 300 /spl mu/m.

Generation of High-Current Photoelectrons Using a Subpicosecond Ultraviolet Laser

A target structure, ion-layer embedded foil (ILEF) is proposed for producing a quasi-monoenergetic proton beam by utilizing a bulk electrostatic field, which is generated by irradiating the target with an ultra-intense laser pulse, inside the plasma. Compared with the case of a single metal foil in which the proton layer is initially present on the surface, in the ILEF target, the proton layer is initially located inside a metal foil. A two-dimensional particle-in-cell (PIC) simulation shows that the target generates a proton beam with a narrow energy spread. With a laser intensity of 2 × 1019 W/cm2, a 22-MeV proton beam with an energy spread of 8% at the full-width-half-maximum (FWHM) is obtained when the proton layer is located at 0.4 μm inside the rear surface of a 2.4 μm-thick copper foil. When the proton layer moves toward the front side, a proton beam with a flat-top energy distribution ranging from 15 MeV to 35 MeV is obtained. Further, with a higher laser intensity of 1021 W/cm2, a proton beam wit...