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Physical Review A

Ultrafast time-resolved fluorescence spectroscopy

S. H. Lin, Benjamin Fain, C. Y. Yeh

Published Mar 1, 1990• 20 citations
Publisher

Abstract

We present a theory of time-resolved emission spectroscopy in the femtosecond region. It is shown that a femtosecond time-resolved emission spectrum consists of the contributions from the population and coherence of the system. Theoretical expressions for describing femtosecond time-resolved emission molecular spectra based on the use of the adiabatic approximation are derived. Detailed numerical calculations have been carried out to show the effect of the time evolution of the population and coherence on the time-resolved emission spectra.

DOI
10.1103/physreva.41.2718

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We present a theory of time-resolved emission spectroscopy in the femtosecond region. It is shown that a femtosecond time-resolved emission spectrum consists of the contributions from the population and coherence of the system. Theoretical expressions for describing femtosecond time-resolved emission molecular spectra based on the use of the adiabatic approximation are derived. Detailed numerical calculations have been carried out to show the effect of the time evolution of the population and coherence on the time-resolved emission spectra.
We present a fully quantum theory of time-resolved transient absorption spectroscopy, providing a general framework for describing the linear response of a nonequilibrium target to quantum probe fields beyond the semiclassical approximation. We elucidate how quantum correlations between probe-field frequency modes couple to coherences in the target and derive a simple expression for computing the resulting spectroscopic signals. We identify conditions under which quantum light can modify transient absorption spectra beyond the limits achievable with classical fields. Our work establishes a theoretical foundation for exploiting quantum light in ultrafast spectroscopy and suggests new routes for resolving dynamics in matter on timescales shorter than the duration of the probe pulse.
Time - dependent behaviors of molecular coherence in resonant multiphotoni onization spectra of molecules are theoretically investigated . An expression for the transition probability of time-resolved two-photon ionization processes via coherently excited resonant intermediate states is derived in the perturbative density matrix formalism. Model calculations of the probability of ionization are performed to predict appearance of quantum beats in pump-probe two-photon ionization experiments. The quantum beat signals in the time-resolved ionization spectra are compared with those in time-resolved total photon emission spectra.
The resonance secondary emission (RSE) in femtosecond laser excitation is discussed in reference to the motion of the created wave packet moving on the excited state potential surface. The density matrix of emitted light for the multi-intermediate-level system is outlined, from which the emission correlation function is derived. The correlation function is put into the theoretical expression of the time-dependent ‘‘physical spectrum’’ for the Fabry–Perot interferometer (which is used in order to consider temporal and energetic resolution inherent in detection). The compact and practical expressions obtained connect the time- and frequency-resolved spectrum with the time evolution of the wave packet. Numerical results for a displaced harmonic oscillator model indicate that the time- and frequency-resolved spectrum can reveal how the wave packet created by a fs laser pulse travels on the excited potential surface if the response time 1/Γd of the photodetector satisfies the relation that Ω<Γd <∼ the Stokes shift (where Ω is the vibrational frequency). It is shown that the excited state wave function can be split into two terms, the one that adiabatically follows the temporal change in incident light (the adiabatic term) and the one that represents the effect of spectral broadening of light (the Fourier broadening term). It is only the Fourier broadening term that survives after the termination of incident light and reflects the motion of the created wave packet on the excited potential surface. In off-resonance excitation, the adiabatic term produces Raman-like emission and the Fourier broadening one produces fluorescence-like emission. In resonance excitation, these two terms are indistinguishable from each other with respect to emission frequency: for the duration of incident light, the adiabatic term offsets the Fourier broadening one, leading to a slow buildup of intensity in the time- and frequency-resolved spectrum (which is slower than the initial rise of the incident pulse profile).
Time-resolved spectroscopy has long served as a cornerstone technique for investigating the photophysics of materials. However, conventional methods based on intense optical pulses and ensemble measurements often fail to resolve quantum dynamics due to spectral overlap, coherent artifacts, and multiphoton effects. To overcome these limitations, single-photon transient stimulated emission (SP-TSE) has been proposed as a femtosecond spectroscopy technique using quantum light and detection principles. Here, we theoretically demonstrate frequency-resolved SP-TSE. Unlike semiclassical techniques, SP-TSE reveals a strong dependence of the time-resolved spectrum on the frequency-resolving method. With a tunable optical filter, the observed spectrum reflects a quantum double-slit interference in the time-frequency domain. In contrast, interferometric detection produces spectra resembling semiclassical results, but the signal emerges specifically from the second harmonic frequency window due to quantum interference. This framework provides insight into ultrafast quantum processes and suggests a foundation for future applications in quantum spectroscopy and technologies.
Recent advances in single-photon femtosecond spectroscopy have highlighted the power of entangled photons in probing the properties of materials, previously inaccessible through semiclassical spectroscopic approaches. In this study, we theoretically propose a new single-photon-based femtosecond time-resolved spectroscopy technique termed single-photon transient stimulated emission (SP-TSE). SP-TSE not only enables the selective investigation of singly excited superposition states but also harnesses the quantum mechanical nature of photons for the efficient data acquisition of transient responses, thereby supporting the feasibility of experimental realization of SP-TSE. The key aspect of SP-TSE is the selective detection of two-photon states produced through stimulated emission using coincidence counting techniques. Our theoretical framework, supported by numerical simulations, demonstrates the efficacy in capturing the pure decoherence dynamics of a model molecular cavity, highlighting its potential to reveal quantum mechanical properties that are difficult to observe with semiclassical femtosecond time-resolved experiments.
Recent progress in single-photon femtosecond spectroscopy has highlighted the crucial role of entangled photons in exploring material properties beyond the reach of traditional semiclassical approaches. In this work, we introduce a theoretically proposed technique named single-photon transient stimulated emission (SP-TSE), a new method in femtosecond time-resolved spectroscopy. SP-TSE uniquely enables the selective probing of single-excitation superposition states and utilizes the quantum mechanical characteristics of photons for the efficient acquisition of transient response data, enhancing the practicality of SP-TSE experiments. A fundamental feature of SP-TSE is its employment of coincidence counting techniques to selectively detect two-photon states produced by stimulated emission. Our theoretical framework, bolstered by numerical simulations, demonstrates its effectiveness in elucidating the pure decoherence dynamics of a model molecular ensemble, thereby revealing quantum mechanical properties that are otherwise challenging to observe using conventional femtosecond time-resolved experiments.