Non-Markovian Unified Theory of Dephasing in Two-Photon and Multiphoton Processes
A general formalism was developed recently towards the microscopic calculation of dephasing phenomena in multiphoton processes.1–3 We present here the exact solution of a stochastic model for dephasing in two photon processes. Our solution accounts in a unified way for a variety of dephasing processes in the material system (collisional, phonon induced, density fluctuations in condensed phases, etc.) as well as dephasing induced by phase fluctuations of the radiation fields. The key quantity in our theory is a four point correlation function of the dipole operator F(τ1,τ2,τ3). We derive a general expression for F which is not restricted to the Markovian limit and show how the various types of dephasing correspond to special cases of F. Both time-resolved and frequency-resolved experiments are examined. We further present an approximate expression for fluorescence line shapes in a strong field and for the nonlinear susceptibility X(3), obtained using the factorization approximation.2 The latter approximation allows us to express multiphoton line shapes in terms of single photon line shape functions, for which a microscopic theory is readily available.
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