Some exact properties of the nonequilibrium response function for transient photoabsorption

Abstract

The physical interpretation of time-resolved photoabsorption experiments is not as straightforward as for the more conventional photoabsorption experiments conducted on equilibrium systems. In fact, the relation between the transient photoabsorption spectrum and the properties of the examined sample can be rather intricate since the former is a complicated functional of both the driving pump and the feeble probe fields. In this work we critically review the derivation of the time-resolved photoabsorption spectrum in terms of the nonequilibrium dipole response function χ and assess its domain of validity. We then analyze χ in detail and discuss a few exact properties useful to interpret the transient spectrum during (overlapping regime) and after (nonoverlapping regime) the action of the pump. The nonoverlapping regime is the simplest to address. The absorption energies are indeed independent of the delay between the pump and probe pulses and hence the transient spectrum can change only by a rearrangement of the spectral weights. We give a close expression of these spectral weights in two limiting cases (ultrashort and everlasting monochromatic probes) and highlight their strong dependence on coherence and probe-envelope. In the overlapping regime we obtain a Lehmann-like representation of χ in terms of light-dressed states and provide a unifying framework of various well known effects in pump-driven systems. We also show the emergence of spectral sub-structures due to the finite duration of the pump pulse.

Floquet analysis of excitations in materials

“…The dipole response function of any (closed) system out of equilibrium can be expressed in terms of timeevolution operators as [20]…”

Origin of space-separated charges in photoexcited organic heterojunctions on ultrafast time scales

“…In a pump-probe experiment, the sample is firstly irradiated by an energetic pump pulse and the resulting excited (nonequilibrium) state of the sample is consequently examined using a second, weaker, probe pulse, whose time delay with respect to the pump pulse can be tuned. [50][51][52] Our theoretical approach to a pump-probe experiment considers the interaction with the pump pulse as desribed in Sec. II B and Ref.…”

“…The corresponding nonequilibrium dipole-dipole retarded correlation function is then used to calculate pump-probe signals. 52,53 To study pump-probe experiments, we extended our two-band lattice semiconductor model including more single-electron (single-hole) energy levels per site. Multiple single-electron (single-hole) levels on each site should be dipole-coupled among themselves in order to enable probe-induced dipole transitions between various exciton states.…”

Studies of spuriously shifting resonances in time-dependent density functional theory

“…The non-equilibrium response function plays a crucial role in analyzing non-equilibrium dynamics, as has been recently highlighted in theoretical formulations of timeresolved photoabsorption spectroscopy [37,38]. In fact the language of pump-probe dynamics is very useful for establishing the exact condition, although the relevance of the condition is not at all restricted to such a setup.…”

“…In Section II we review the generalized non-equilibrium linear response formalism for TDDFT, introduced in Refs. [25,37], pointing out differences from the standard ground state linear response. We discuss the difficulty of defining a pole structure for the non-equilibrium KS response function around an arbitrary state.…”