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Journal of the Physical Society of Japan

A Note on the Analytical Treatment of a Localized Electron-Phonon System Including the Change of Frequency

Masashi Ban, Toshihico Arimitsu

Published Mar 1, 1984• 6 citations
Publisher

Abstract

The localized electron-phonon model, which was proposed by Takagahara, Hanamura and Kubo (THK), is extended by including the change of frequency and is treated analytically to obtain the response function and the photon echo signal. Our treatment also gives an analytical expression for the integrand of the transient emission spectrum of the second order optical processes in a localized electron-phonon system which was treated by THK in terms of the eigenvalue problem of the relaxation operator.

DOI
10.1143/jpsj.53.939

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The localized electron-phonon model, which was proposed by Takagahara, Hanamura and Kubo (THK), is extended by including the change of frequency and is treated analytically to obtain the response function and the photon echo signal. Our treatment also gives an analytical expression for the integrand of the transient emission spectrum of the second order optical processes in a localized electron-phonon system which was treated by THK in terms of the eigenvalue problem of the relaxation operator.
A general formula of the time-resolved optical spectrum for transient resonant light scattering is derived systematically by means of the operator algebra within the formalism of nonequilibrium thermo field dynamics (NETFD). The formula is applied to an analytically solvable model of the localized electron and phonon system, i.e., second-order light scattering due to the optically active three electronic states whose intermediate state is dynamically modulated by the phonon interaction mode. The derivation of the analytical expression for a four-point function, needed to obtain the spectrum, is performed with the help of the algebraic manipulations in NETFD, which showed the advantage of its methodology with respect to nonequilibrium transient phenomena. The three-dimensional profiles of the Raman and luminescence components in the time-resolved spectrum in the model are presented to show how the stochastic character comes out in the fast modulation limit, and how the dynamical behavior of the phonon system causing the modulation of the intermediate electronic state comes out in the slow modulation limit.
The quantum mechanical description of the transient resonance Raman scattering including both the excitation and observation processes is presented. To obtain the general formula for the actually observable transient emission spectrum, the following way of observation is proposed; the use of a spectroscope for wave transformation and a photon-counter behind it for particle transformation. On its basis, the transient emission spectrum and its average over the duration time of photon-counting are explicitly defined especially for systems small compared with the spacial extent of an incident pulse. It will be shown that the transient emission spectrum conforms to the minimal case of the Heisenberg uncertainty relation through the restriction for the emission time coming from the Fourier transform of the transfer function of the spectroscope. The theory is applied to a strongly coupled localized electron-phonon system. It will be shown that what aspects of the correlation between absorption and emission are disclosed depends strongly on both the ways of excitation and observation.
We calculate the photon echo decay for a strongly coupled electron–phonon system, without making unrealistic assumptions about excitation pulse durations. We find that asymptotically, the photon echo decay is exponential, and we provide a formal expression for the decay rate 1/T2 which is independent of pulse duration. In contrast to the usual results, our expression for 1/T2 is nonperturbative in the electron–phonon interaction. We calculate the dependences of the echo amplitude on pulse power and duration, finding that the strong electron–phonon coupling modifies the simple two level system result. We also focus on the relationship between the zero phonon line shape in the homogeneous optical absorption spectrum, and the photon echo decay rate. We find that the zero phonon line shape is Lorentzian, with width 1/πT2. We show that the usual optical Bloch equation approach to analyzing time-domain experiments, which normally is only applicable if the electron–phonon coupling is weak, can be extended to the case of strong coupling. These strong coupling Bloch equations are adequate for analyzing the time dependence, but not the pulse power and duration dependences, of time-domain experiments. Our results indicate that within the Condon approximation, electron–phonon coupling cannot explain the very low temperature anomaly of recent photon echo experiments on impurities in molecular crystals.
Electromagnetically coupled two-level systems play a central role in several condensed matter components being considered for quantum information processing applications. If the states couple to phonon excitations, their electromagnetic response is altered via phonon-assisted transitions and lifetime broadening. The former has been treated extensively for a number of specific two-level systems (e.g., excitons in artificial quantum dots, localized states associated with impurities or defects, etc.), but the latter has received less attention. Here we study a microscopic model of the dipole transition of a two-level system under the influence of both diagonal and nondiagonal interactions with a bath of phonons. Our results capture both the influence of the frequency distribution of phonons on the relative spectral weight of the zero-phonon transition and the phonon sidebands, and the broadening of the zero-phonon line due to nondiagonal electron-phonon coupling. We use a formalism that includes non-Markovian effects related to the feedback mechanism between the two-level system and the phonon bath. For simplified forms of the phonon spectral functions we provide analytical expressions up to second order in the coupling strength that demonstrate the importance of including both forms of electron-phonon coupling in studies of these systems. Our formalism can be generalized to higher orders of coupling and for realistic phonon spectral functions.
The optical absorption spectrum in an analytically solvable model of a localized-electron and phonons is investigated using the operator algebra in NonEquilibrium Thermo Field Dynamics (NETFD).The position and the curvature of the adiabatic potentials for the ground and for the excited electronic states are assumed to differ from each other.The structures of the absorption spectra are investigated for various situations including the cases of slow modulation and of fast one.The method of NETFD is explained in order to be self-contained in solving the problem.
We introduce an almost analytical method to simulate 2D electronic spectra as a double Fourier transform of the the non-linear response function (NRF) corresponding to a particular optical pulse sequence. We employ a unitary transformation to represent the total system Hamiltonian in a stationary basis that allows us to separate contributions from decoherence and phonon-mediated population relaxation to the NRF. Previously, one of us demonstrated the use of an analytic, cumulant expansion approach to calculate the decoherence term. Here, we extend this idea to obtain an accurate expression for the population relaxation term, a significant improvement over standard quantum master equation-based approximations. We numerically demonstrate the accuracy of our method by computing the photon echo spectrum of a two-level system coupled to a thermal bath, and we highlight the mechanistic insights obtained from our simulation.