| Summary: | A nonlinear absorber in which the excited state absorption is larger than the ground state can undergo a process called reverse saturable absorption (RSA). 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 non-radiative 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 10^16~17 W/cm2. 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 extreme state of matters, as well as affording the potential to regulate ultrafast XFEL pulses.
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