Theory of the Absorption Spectrum

Thomas Speziale

Takanobu Jujo

We investigate the two-photon absorption (TPA) spectrum in Mott insulators on the basis of dynamical cluster approximation. The nonlocal correlation is found to be important in enhancing the dimensionality dependence of the TPA spectrum. The dependence of the TPA spectrum on Coulomb interaction is modified compared with that of the dynamical mean field theory, which is also caused by a nonlocal correlation through a one-particle spectrum. We compare several degrees of nonlocality by adjusting the cluster momentum. The iterated perturbation theory is adopted as an impurity solver, and then a comparison with the noncrossing approximation is presented. It is confirmed that the predominant optical process in Mott insulators is different from that of conventional semiconductors, and small Mott insulating gaps are favorable for obtaining a large TPA spectrum.

V. V. Yakimets

Philipp Werner, Denis Golež, Martin Eckstein

We present a formalism based on nonequilibrium dynamical mean-field theory (DMFT) which allows to compute the time-resolved x-ray absorption spectrum (XAS) of photoexcited solids. By applying this formalism to the photodoped half-filled and quarter-filled two-orbital Hubbard models in the Mott insulating regime we clarify how the time-resolved XAS signal reflects the nonequilibrium population of different local states. Apart from the missing broadening associated with continuum excitations, the atomic XAS spectrum computed with the nonthermal state populations provides a good approximation to the full nonequilibrium DMFT result. This suggests a route to combine the accurate DMFT description of nonequilibrium states of solids with cluster calculations of the XAS signal.

Igor I Sobel'man

Daniel S. Elliott, M. W. Hamilton, K. Arnett +1 more

Experimental evidence of field-correlation effects on weak-field two-photon absorption in atomic sodium is presented. In the case of a nearly Lorentzian laser power spectrum the absorption profile has four times the spectral width of the exciting field, in agreement with predictions by Mollow. The measurement is carried out with counterpropagating laser beams to cancel Doppler broadening. The width of the two-photon absorption spectrum is decreased by partially decorrelating the counterpropagating laser beams. Increasing the correlation time of the frequency fluctuations, resulting in a nearly Gaussian laser power spectrum, has also been observed to decrease the width of the absorption spectrum. An extension of the time-dependent second-order perturbation theory to these additional cases yields good agreement.