Center for Relativistic Laser ScienceExplore the interaction between ultra-intense light and matter

We, the attosecond science group of CoReLS, are investigating ultrafast phenomena in nature. In particular, we are interested in studying the ultrafast dynamics of electrons and ions under extreme conditions. These are ultrafast transition dynamics in atoms and molecules, chemical reactions, electron dynamics in nanostructures, and the relativistic motion of particles in a plasma. The ultimate goal of our research is to understand ultrafast processes and find methods to control them. Furthermore, we would like to seek new applications from these studies.

The development of a new light source is an essential task to pursue these research objects. It can be used as a tool for time-resolved studies such as the pump-probe experiment. At the same time, the emission generated from a target provides critical information on its dynamics and structure. Attosecond x-ray pulses are beneficial due to their extremely short duration. High-order harmonics generated in gas has been widely used to produce attosecond x-ray pulses since the 1990s. Its low generation efficiency, however, limits its applications.

On the other hand, high harmonic radiation emitted from a relativistically driven plasma can overcome this issue. The generation efficiency of relativistic high harmonic generation is at least one order of magnitude higher than high harmonic generation in gas.  Since it requires a high power laser, it could have been studied only by high power laser facilities, and there are still many research opportunities.

Figure. Animation for relativistic oscillating mirror model that explains the generation mechanism of relativistic high harmonic generation. The animation is made by Yong Jin Kwon.

When an intense laser pulse with an intensity higher than 10¹⁸ W/cm² is irradiated on the surface of a solid target, an overdense plasma is produced. Here, ‘overdence’ means that the plasma density is so high that the incident laser can be reflected at the critical density layer. At the same time, the electrons in the plasma are strongly driven by the laser field. The electrons are driven back and forth with the velocity close to the speed of light in the relativistic regime. The reflection thus occurs at the relativistically moving surface. The frequency of the incident laser pulse is up-shifted due to the Doppler effect. Since the Doppler effect occurs periodically in every optical cycle, the reflected laser pulse forms harmonic pulses that contain multiple harmonics. The harmonic radiation contains a broad range of wavelengths from visible to x-rays. According to theoretical research, these harmonics are phase-locked. Thus, an intense attosecond light pulse can be obtained when the harmonic pulse is properly filtered. These attosecond pulses will be used as a light source for applications such as time-resolved imaging experiments.

Figure. Result of a part-in-cell simulation. The incident laser field is reflected on the plasma surface. The reflection position moves along with the laser field. The PIC simulation was performed by Dr. Ki Hong Pae and Hyun Kim.

The Center for relativistic laser science (CoReLS) operates ultra-high power lasers with output powers of 150 TW and 4 PW. When these laser pulses are tightly focused, the peak intensities may reach intensities of 10²⁰ W/cm² and 10²³ W/cm². It is thus possible to obtain high-order harmonics generated from a solid surface in the relativistic regime. Since the maximum photon energy scales with the intensity of a driving laser pulse, we expect the generation of intense attosecond x-ray pulses. The focused intensity of the 4 PW laser at CoReLS reached the highest in the world. Using the ultrahigh power lasers, we can produce an intense attosecond x-ray pulse and characterize an intense attosecond x-ray pulse.