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 , we have investigated the generation of multi-GeV electron beams  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.
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).
A PW Ti:Sapphire laser with 30-J energy and 30-fs pulse duration has been developed at GIST and applied to generate
x-rays and energetic charged particles. We present the status and plan of developing ultrashort x-ray sources and their
applications. We successfully demonstrated x-ray lasers and their applications to high-resolution imaging. In addition,
we plan to generate high flux x-ray/gamma-ray sources using the PW laser.
We have developed a 0.1-Hz-repetition-rate, 30-fs, 1.5-PW Ti:sapphire laser system for the research on high field physics. In this paper, we describe the design and output performance of the PW Ti:sapphire laser and its applications in the generation of relativistic high order harmonic generation and the acceleration of charged particles (protons and electrons). In the experiment on relativistic harmonic generation, the harmonic order dramatically extended up to 164<sup>th</sup> that corresponds to 4.9 nm in wavelength, and the dramatic extension was explained by the oscillatory flying mirror model. Recently, we could accelerate protons up to 45 MeV from a 10-nm polymer target and show the change in the acceleration mechanism from target normal sheath acceleration to radiation pressure acceleration. The femtosecond high power laser system is a good candidate for developing a compact electron accelerator as well. The generation of multi-GeV electron beam was observed from an injection scheme when a PW laser pulse was focused by a long focal length spherical mirror.
The enhancement of laser-driven proton acceleration mechanism in TNSA regime has been demonstrated through the use of advanced nanostructured thin foils. The presence of a monolayer of polystyrene nanospheres on the target frontside has drastically enhanced the absorption of the incident laser beam, leading to a consequent increase in the maximum proton beam energy and total laser conversion efficiency. The experimental measurements have been carried out at the 100 TW and 1 PW laser systems available at the APRI-GIST facility. Experimental results and comparison with particle-in-cell numerical simulations are presented and discussed.
High-harmonic-seeded x-ray laser became an important issue in x-ray laser development due to the possibility to obtain a
highly coherent and polarized soft x-ray source. We performed theoretical investigations into amplification of high
harmonic pulses in an x-ray lasing medium by using a model based on Maxwell-Bloch equations. From the theoretical
works, we analyze characteristics of energy extraction and temporal profile of output pulse. In addition, preliminary
experimental results and ongoing experiments related the harmonic-seeded x-ray lasers are reported.
The wavefront aberrations in the 100 TW laser pulses are measured and corrected to improve the intensity distribution of
the focal spot. Before correcting wavefront aberration of the laser pulses, the laser pulses have higher-order aberrations
such as coma, trefoil, and spherical aberration as well as defocus and astigmatism. The wavefront aberrations in the laser
pulses are corrected by the deformable mirror. The dynamic and static corrections are tested with the deformable mirror.
When correcting wavefront aberrations with the deformable mirror, the focal spot having a 1.2 times spot size of the
diffraction-limited focal spot is observed.
A novel wavelength swept broadband source based on an ultrashort pulse laser and an external tunable filter was
proposed for application of frequency domain-optical coherence tomography (FD-OCT). The laser beam coupled into
the single mode fiber, which provided 0.5-nm instantaneous spectral linewidth with 1-mW average output power, was
tuned from 740 nm to 850 at a 1 kHz repetition rate. The system with an axial resolution of 5 &mgr;m performed OCT
imaging of air-gap between glass plates proving potential about the application of pulse laser source to FD-OCT
system. The proposed swept source scheme could be applied for the implementation of ultra-high resolution FD-OCT
system based on a supercontinuum source with an ultra-short pulse laser and a high nonlinear optical fiber.