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.
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.
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 164th 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.
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.
We have demonstrated bandwidth control and reshaping of second harmonic (SH) curve in a periodically poled Ti:LiNbO3
(Ti:PPLN) waveguide ( period=16.6 um) by using a temperature-gradient-control technique and a local-temperature-control technique.
We have achieved more than 13 nm second harmonic phase-matching bandwidth and several useful shapes of SH curve such as almost ideal sinc function, and double peaks in a 74 mm long Ti:PPLN waveguide that has pre-chirped SH curve in room temperature.
Ultrafast lasers have many applications mainly due to its two properties, the ultrashort pulse width and the ultrahigh intensity. Because the former is the main cause of the latter, it is very important to exactly measure the pulse width of the ultrafast laser. Currently, there are several different kinds of experimental methods to measure the ultrashort pulse width. Among those systems for this measurement, the autocorrelator using the second harmonic generation (SHG) is by far the most simple and basic method. This type of autocorrelators usually uses inorganic crystals, such as BBO, as the SHG medium. The thinner medium is necessary for analyzing the shorter laser pulses. However, the polishing process which is necessary for obtaining the optically good surfaces makes it difficult to reduce the thickness of medium as desired. We present an autocorrelator system which overcomes these shortcomings. Our system is based on the SHG using organic polymer. Polymers can be easily prepared in the form of thin film on the strong substrate through the process of spin casting. Thickness less than 1 m can be obtained without difficulties. Furthermore, due to its high nonlinearity, thin film of polymer can produce the bright second harmonic light. Polyurea was used as the second harmonic generation material of the autocorrelator because it has the pretty good transparency. An autocorrelator system based on the 397nm-thick poled polyurea thin film has been developed and used to measure the pulse width of a home-made Ti:sapphire laser oscillator. Then, the system was compared with that based on a 100 μm-thick BBO crystal, which is widely used. The pulse width of laser beam was measured to be 9.8 fs with the former. The value is believed to be more accurate than that of 7.2 fs measured with the latter.