Real-time TDDFT simulation of nonlinear effects in the absorption of intense soft X-rays in solid-state systems

Synchrotron based X-ray spectroscopies are well described within linear response theory. The interaction with matter, of intense X-ray pulses from modern X-ray free electron lasers, can extend into the nonliner and non-perturbative regimes and requires theoretical descriptions that go beyond linear response. In this study the velocity gauge real-time TDDFT approach is employed to investigate nonlinear effects in the interaction of intense few femtoscond X-ray pulses with solid-state materials. In particular the role of light-induced transparency and stimulated emission in modulating the absorption signatures of XFEL pulses at light element K-edges and transition metal L-edges is explored through prototypical simulations on Silicon Carbide and metallic Cobalt. Absorption changes driven by nonlinear interactions are contrasted against those induced by lattice and electronic heating effects. Additionally, TDDFT results are compared to predictions from phenomenological optical bloch equation simulations.