Publications associated with High Power Laser Group

Observation of Reverse Saturable Absorption of an X-ray Laser.

Physical review letters 119 (2017) 075002-

BI Cho, MS Cho, M Kim, H-K Chung, B Barbrel, K Engelhorn, T Burian, J Chalupský, O Ciricosta, GL Dakovski, V Hájková, M Holmes, L Juha, J Krzywinski, RW Lee, CH Nam, DS Rackstraw, S Toleikis, JJ Turner, SM Vinko, JS Wark, U Zastrau, PA Heimann

A nonlinear absorber in which the excited state absorption is larger than the ground state can undergo a process called reverse saturable absorption. It is a well-known phenomenon in laser physics in the optical regime, but is more difficult to generate in the x-ray regime, where fast nonradiative core electron transitions typically dominate the population kinetics during light matter interactions. Here, we report the first observation of decreasing x-ray transmission in a solid target pumped by intense x-ray free electron laser pulses. The measurement has been made below the K-absorption edge of aluminum, and the x-ray intensity ranges are 10^{16} -10^{17}  W/cm^{2}. It has been confirmed by collisional radiative population kinetic calculations, underscoring the fast spectral modulation of the x-ray pulses and charge states relevant to the absorption and transmission of x-ray photons. The processes shown through detailed simulations are consistent with reverse saturable absorption, which would be the first observation of this phenomena in the x-ray regime. These light matter interactions provide a unique opportunity to investigate optical transport properties in the extreme state of matters, as well as affording the potential to regulate ultrafast x-ray free-electron laser pulses.

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