Hwang Woon Lee

Radiation pressure acceleration of protons to 93 MeV with circularly polarized petawatt laser pulses

The radiation pressure acceleration (RPA) of charged particles has been a challenging task in laser-driven proton/ion acceleration due to its stringent requirements in laser and target conditions. The realization of radiation-pressure-driven proton acceleration requires irradiating ultrathin targets with an ultrahigh contrast and ultraintense laser pulses. We report the generation of 93-MeV proton beams achieved by applying 800-nm 30-fs circularly polarized laser pulses with an intensity of 6.1×1020 W/cm2 to 15-nm-thick polymer targets. The radiation pressure acceleration was confirmed from the obtained optimal target thickness, quadratic energy scaling, polarization dependence, and three-dimensional particle-in-cell simulations. We expect this clear demonstration of RPA to facilitate the realization of laser-driven proton/ion sources delivering energetic and short-pulse particle beams for novel applications.

Development of foam-based layered targets for laser-driven ion beam production

We report on the development of foam-based double-layer targets (DLTs) for laser-driven ion acceleration. Foam layers with a density of a few mg cm−3 and controlled thickness in the 8–36 μm range were grown on μm-thick Al foils by pulsed laser deposition (PLD). The DLTs were experimentally investigated by varying the pulse intensity, laser polarisation and target properties. Comparing DLTs with simple Al foils, we observed a systematic enhancement of the maximum and average energies and number of accelerated ions. Maximum energies up to 30 MeV for protons and 130 MeV for C6+ ions were detected. Dedicated three-dimensional particle-in-cell (3D-PIC) simulations were performed considering both uniform and cluster-assembled foams to interpret the effect of the foam nanostructure on the acceleration process.

Stable multi-GeV electron accelerator driven by waveform-controlled PW laser pulses

The achievable energy and the stability of accelerated electron beams have been the most critical issues in laser wakefield acceleration. As laser propagation, plasma wave formation and electron acceleration are highly nonlinear processes, the laser wakefield acceleration (LWFA) is extremely sensitive to initial experimental conditions. We propose a simple and elegant waveform control method for the LWFA process to enhance the performance of a laser electron accelerator by applying a fully optical and programmable technique to control the chirp of PW laser pulses. We found sensitive dependence of energy and stability of electron beams on the spectral phase of laser pulses and obtained stable 2-GeV electron beams from a 1-cm gas cell of helium. The waveform control technique for LWFA would prompt practical applications of centimeter-scale GeV-electron accelerators to a compact radiation sources in the x-ray and γ-ray regions.

Autocorrelation-subtracted Fourier transform holography method for large specimen imaging

We developed a variation of Fourier transform holography (FTH) method to record larger objects than those tolerable in conventional FTH. This method eliminates the separation condition of FTH by removing the autocorrelation signal, thus allowing three-fold larger specimens than those previously used in FTH under the same illumination conditions. We experimentally demonstrated this FTH variation, using a table-top Ag X-ray laser at 13.9 nm, with a sample violating the separation constraint. The portion of the object image hidden behind its autocorrelation in the FTH image was recovered by subtracting an independently measured autocorrelation signal of the object.

Improved Maker-fringe analysis for estimation of second-order nonlinearities of arrayed ZnO nanorods

Second harmonic Maker-fringe signals, measured for determination of second-order nonlinear coefficients of arrayed ZnO nanorods, were analyzed with an improved method. In this analysis, multiple reflections and interferences of fundamental and second harmonic waves were taken into account within ZnO nanorod layer, which thickness approaches the wavelength of the incident beam.

Development of ultrashort x-ray/gamma-ray sources using ultrahigh power lasers (Conference Presentation)

Short-pulse x-ray/gamma-ray sources have become indispensable light sources for investigating material science, bio technology, and photo-nuclear physics. In past decades, rapid advancement of high intensity laser technology led extensive progresses in the field of radiation sources based on laser-plasma interactions - x-ray lasers, betatron radiation and Compton gamma-rays. Ever since the installation of a 100-TW laser in 2006, we have pursued the development of ultrashort x-ray/gamma-ray radiations, such as x-ray lasers, relativistic high-order harmonics, betatron radiation and all-optical Compton gamma-rays. With the construction of two PW Ti:Sapphire laser beamlines having peak powers of 1.0 PW and 1.5 PW in 2010 and 2012, respectively [1], we have investigated the generation of multi-GeV electron beams [2] and MeV betatron radiations. We plan to carry out the Compton backscattering to generate MeV gamma-rays from the interaction of a GeV electron beam and a PW laser beam. Here, we present the recent progress in the development of ultrashort x-ray/gamma-ray radiation sources based on laser plasma interactions and the plan for developing Compton gamma-ray sources driven by the PW lasers. In addition, we will present the applications of laser-plasma x-ray lasers to x-ray holography and coherent diffraction imaging. [references] 1. J. H. Sung, S. K. Lee, T. J. Yu, T. M. Jeong, and J. Lee, Opt. Lett. 35, 3021 (2010). 2. H. T. Kim, K. H. Pae, H. J. Cha, I J. Kim, T. J. Yu, J. H. Sung, S. K. Lee, T. M. Jeong, J. Lee, Phys. Rev. Lett. 111, 165002 (2013).

Optical parametric chirped pulse amplifier for a 4 PW laser front-end

A two-stage non-collinear optical parametric chirped pulse amplifier (OPCPA), using type-I BBO nonlinear crystal, has been developed as a pre-amplifier of the 4 PW laser at CoReLS.

0.1 Hz 4.0 PW Ti:Sapphire laser at CoReLS

0.1-Hz 4.0-PW laser is being developed for research on relativistic laser-matter interactions. The upgrade will be achieved by increasing the laser energy of the current 1.5-PW laser to 80 J and decreasing the pulse duration to 20 fs.

Ultrafast optical response of a new metal organic complex-polymer composite film

A new Cu-based metal organic complex incorporated into poly (methyl mathacrylate) film exhibits large and fast offresonant optical nonlinearity in the 800 to 1550 nm wavelength range. A large χ⁽³⁾ value of 1.8×10⁻¹⁰ esu is estimated.