Fluctuations and single molecules

Abstract

The fluorescence of single molecules coupled to a thermal bath is studied both experimentally and theoretically. The effect of different fluctuations on the coherence properties of resonance fluorescence is considered first. Coherence is measured in an interference experiment where a single molecule is used as a light source. A standard approach based on the optical Bloch equations apparently provides quite an accurate description of the interference experiment. Systems with long correlation times (where spectra are time dependent on any timescale) are considered next. It is shown that intensity—time—frequency correlation spectroscopy, which provides both high signal‐to‐noise ratio and high time resolution, is very suitable for such a case. The Bloch equations are further tested in an experiment where the shape of an excitation spectral line of a single molecule is accurately measured over six orders of magnitude of the exciting laser power. Significant deviations from the predictions of the Bloch equations are found. The role of critical parameters—the correlation time of the bath, the Rabi oscillation period, and the coupling constant between the bath and the molecule—is discussed. The paper also includes a short general introduction to the methodology of single‐molecule studies.

Line “narrowing” in strong optical fields

“…Two additional parameters (background and the overall spectral shift) have been used for every spectrum. The residual errors were close to the ultimate limit set by the shot noise [17].…”

“…To get the dynamics of rðtÞ; a two-time correlation function g ð2Þ ðt; t þ tÞ can be measured which gives a normalized probability to detect a photon at time t þ t given that a photon was detected at time t [5]. For a number of reasons it is much easier to measure the line shape [17]. For example, measurement of g ð2Þ ðt; t þ tÞ at a large O requires long accumulation times because one has to accumulate enough photons separated by a time interval smaller than 1=O: Most photons are separated by a time interval on the order of 2T 1 =ðDZÞ; where DE10 À3 and Zo1; therefore photons with a time separation shorter than 1=O make a very small fraction of the total number of photons detected by the detector.…”

Single‐Molecule Dynamic Triangulation

“…These narrow spectral lines are very sensitive to any changes in external conditions, which include variations in temperature, electric field and pressure. [8,10,11] This paper focuses on detection of charge transport in nanostructures and analyzes the ultimate accuracy with which the path of an elementary charge can be detected by using single molecules as sensors. Herein, the theoretical analysis of such single-molecule dynamic triangulation (SMDT) of a single electron is based on numerical simulations and experimentally measured essential characteristics of the probe molecules.…”