DESCRIPTION ON 'PLASMA' FOR NON-SPECIALISTS
Plasma can be considered simply as a 'gas' of ionized particles. That is, the plasma is a collection of many electrons and ions which are not bound to each other. The big difference from usual gas (or fluid) is that the plasma is strongly subseptible to electromagnetic fields as it is composed of charged particles. The response of the plasma particles is not random, but is collective, and also nonlinear. Those collective and nonlinear behavior is the basis of many interesting applications and natural phenomena.
|Figure: High density shock in real and phase spaces|
Recently, for the first time, we revealed that a 'Circularly Polarized' short pulse can yield a high density shock more effectively than the conventional linearly polarized pulse, via the 'relativistic transparent heating' of electrons. This work has been published in Phys. Rev. E in 2015.
Among many types of THz radiation sources, laser-plasma-based ones have strong advantages in field strength (~ GV/m), high power (~ MW), and high frequency (>1 THz). On the other hand, the laser equipment is not that portable, and THz wave from laser-plasma is typically a wide-band, short pulse. Thus it is most suitable for scientific purposes such as THz-pump and laser-probe experiments for phonon excitation, or lattice control in superconductor.
|Figure: Strong, monochromatic THz pulse from collision of counter-propagating short pulses in a magnetized plasma|
In CPL, we devised a new scheme to generate a strong, and much more monochromatic THz pulses than any conventional source. Such a 'monochromatic' but still strong THz pulse is expected to open a new regime of pump-probe experiments, and other chemical and physical applications. The idea is related with the selectively enhanced pumping (SEP), which has been identified for the first time in CPL, and is still under progress. The key idea is more or less confidential, but a piece of basic idea can be found in our recent publication in New J. Physics.
The modern technology of high power lasers originated from the chirped-pulse-amplification (CPA) technique. Generally the amplification of the laser pulse is limited by the threshold of material damage, where the material here means the amplification medium to obtain the population inversion. The idea of CPA is stretching the short pulse longitudinally, after which the pulse amplitude is significantly lowered. The low amplitude long pulse is amplified under the damage threshold, then is recompressed to eventually obtain a high peak power, ultra-short laser pulse. In the process of stretch, the frequency is naturally chirped. For the stretch and recompression, gratings are used. For higher power, larger gratins are required, which is technologically challenging, difficult to handle, and most of all, very expensive.
The idea of Raman backward amplification (RBA) is using the plasma as a replacement of 'recompression' stage in CPA technique. As the plasma is already 'broken down' material, no worry for the additional damage. The stretched long pulse is, after amplification, sent to a plasma. A small seed pulse, usually detached from the main pulse as a small fraction, is sent to the same plasma in counter-direction. In this situation, the energy of the long pulse can be transferred to the seed pulse, resulting in a highly amplified, ultra-short pulse. This process can be considered as 'recompression' of the long pule.
Conventionally the RBA has been modelled by three-wave equations. But the plasma has been known to be not just a laminar fluid, so the three-wave model could not predict the results exactly at high-amplitude cases. I successfully created the two-wave-one-kinetic simulation code, called 'aPIC', under the collaboration with Prof. J. Wurtele (UC Berkeley) and Dr. G. Penn (LBNL). (M.S. Hur et al., PoP 2004). Soon after that I found the way to model the kinetic effects quantitatively by envelope-kinetic equation of the plasma wave (M.S. Hur et al., PRL 2005). Presently I'm working on improving the previous model equations and codes with Dr. S. Yoffe in Prof. D. Jaroszynski's group at Univ. of Strathclyde.