Andrei V. Naumov

Levy Statistics for Random Single-Molecule Line Shapes in a Glass

We demonstrate that the statistical behavior of random line shapes of single tetra-tert-butylterrylene chromophores embedded in an amorphous polyisobutylene matrix at T=2 K is described by Lévy statistics as predicted theoretically by Barkai, Silbey, and Zumofen [Phys. Rev. Lett. 84, 5339 (2000)]. This behavior is a manifestation of the long-range interaction between two-level systems in the glass and the single molecule. A universal amplitude ratio is investigated, which shows that the standard tunneling model assumptions are compatible with the experimental data.

Moments of single-molecule spectra in low-temperature glasses: Measurements and model calculations

Single-molecule spectroscopy of isolated chromophores in solids yields detailed information about the matrix on a microscopic level. In most studies so far, single-molecule (SM) spectra have been characterized by their linewidths. We demonstrate that in a doped amorphous polymer, in which SM excitation spectra generally have a complicated shape, the shapes of SM lines are more fully characterized by their moments than by the usually used linewidth. We present the measured and simulated distributions of the first four moments and of the linewidth for low-temperature spectra of single tetra-tert-butylterrylene molecules embedded in an amorphous poly(isobutylene) matrix. The simulations are based on the standard tunneling model of glasses with appropriate modifications and on a theory of SM line shapes in a disordered host, which was recently derived by Geva and Skinner [J. Phys. Chem. B 101, 8920 (1997)]. The comparative analysis of the measured and simulated distributions allowed us to evaluate the minimal...

Ortho-Dichlorobenzene Doped with Terrylene—a Highly Photo-Stable Single-Molecule System Promising for Photonics Applications

The study of a new dye-matrix system-quickly frozen ortho-dichlorobenzene weakly doped with terrylene--via single-molecule (SM) spectroscopy is presented. The spectral and photo-physical properties, dynamics, and temperature broadening of SM spectra at low temperatures are discussed. The data reveal a broad inhomogeneous distribution, which indicates a high degree of matrix inhomogeneities, but at the same time, huge fluorescence emission rates and extraordinary SM spectral and photochemical stability with almost complete absence of blinking and bleaching. These unusual properties render the new system a promising candidate for applications in photonics, for example, for delivering single photons on demand.

Dynamics of a doped polymer at temperatures where the two-level system model of glasses fails: Study by single-molecule spectroscopy

We investigated the spectra of a large number of single tetra-tert-butylterrylene molecules embedded in an amorphous polyisobutylene matrix and analyzed the distributions of their linewidths (widths of single spectral peaks). The measurements were performed at 2, 4.5, and 7 K. This is a temperature region, where the standard two-level system (TLS) model of low-temperature glasses begins to fail. At T=2 K the temporal behavior (history of frequency jumps) of most of the measured spectra and their linewidth distributions were found to be consistent with the TLS model. At higher temperatures the main features of individual spectra (number of spectral peaks, temperature variation of peak widths, ratio of intensities of different peaks, etc.) still appear consistent with the predictions of this model. An increase of temperature leads mainly to the broadening of spectral peaks. A detailed analysis of the linewidth distributions reveals deviations from a standard TLS model at T=4.5 and 7 K. This difference is at...

Study of Vibronic Interactions in Impurity Centers by Conjugate Fluorescence and Absorption Spectra with a Poorly Resolved Vibrational Structure

The conjugate fluorescence and fluorescence excitation spectra of recently synthesized substituted arylpolyene (C6H5-[CH=CH]2-C6H4-NH2) are studied in solid n-octane at a temperature of 4.2 K. The spectra exhibit a weakly pronounced vibrational structure. A method of determination of the vibronic interaction parameters responsible for the shape of the spectra is developed. The method is based on the modeling of the spectra by series of vibronic bands, each of which consists of a narrow zero-phonon line and a broad phonon wing (phonon sideband). This makes it possible to calculate the fluorescence and fluorescence excitation spectra with the weakly pronounced vibrational structure and compare them with the measured spectra. The deviations from the mirror symmetry between the measured fluorescence and fluorescence excitation conjugate spectra are explained by the combined effect of the Franck-Condon and Herzberg-Teller interactions. The parameters of these interactions are determined.

Dispersion of the local parameters of quasilocalized low-frequency vibrational modes in a low-temperature glass : Direct observation via single-molecule spectroscopy

Spectra of single tetra-tert-butylterrylene chromophore molecules embedded in an amorphous polyisobutylene matrix as microprobes were recorded. The individual temperature dependences of the spectral linewidths for the same single molecules (SMs) in a broad temperature interval (1.6 < T < 40 K) have been measured. This enabled us to separate the contributions of tunneling two-level systems and quasilocalized low-frequency vibrational modes (LFMs) to the observed linewidths. The analysis of the T dependences yields the values of LFM frequencies and SM-LFM coupling constants for the LFMs in the local environment of a given chromophore. Pronounced distributions of the observed parameters of LFMs were found. This result can be regarded as the first direct experimental proof of the localized nature of LFMs in glasses.

Low-Temperature Dynamics of Amorphous Polymers and Evolution over Time of Spectra of Single Impurity Molecules: I. Experiment

The optical dynamics of a doped amorphous system, tetra-tert-butylterrylene in amorphous polyisobutylene, has been experimentally studied by the spectra of single impurity molecules measured at temperatures of 2, 4.5, 7, and 15 K. The study of the temporal evolution of the fluorescence excitation spectra of the molecules under consideration made it possible to unambiguously establish the individual identity of the spectra of particular molecules and to analyze their multiplet structure. Repeated scanning of a selected spectral range with subsequent summation of the data made it possible to considerably reduce the errors that arise upon single scanning of the spectra of single molecules. The majority of the spectral trails detected were in agreement with the model of two-level systems. Jumps of spectral lines due to transitions in such systems were observed at all temperatures.

Low-temperature dynamics of amorphous polymers and evolution over time of spectra of single impurity molecules: II. Model calculations and analysis of results

The results of a statistical analysis of the spectra of single molecules of tetra-tert-butylterrylene in amorphous polyisobutylene at temperatures of 2, 4.5, and 7 K are presented. Model calculations of such spectra for this system are performed in the context of the stochastic theory of the spectra of single molecules in lowtemperature glasses. Analysis of the multiplet structure of the experimental and model spectra made it possible to obtain data about the minimal distance between impurity chromophore molecules and two-level systems and about the distribution parameters of their coupling constant. The interaction of a chromophore with quasilocal low-frequency vibrational modes of the matrix was found to influence the structure and parameters of the spectra observed. The model calculations performed showed that the specific structure of the spectra of single molecules at low temperatures is determined by the interactions with a small number of nearby two-level systems.

Spectrally resolved analysis of fluorescence blinking of single dye molecules in polymers at low temperatures

We present a method for the spectrally resolved analysis of fluorescence blinking of single quantum emitters. It is based on the well-known technique of repeated recording of single-molecule (SM) fluorescence excitation spectra. The potential of our approach is presented for the example of single tetra-tert-butylterrylene molecules in an amorphous polymer matrix (polyisobutylene), which exhibit fluorescence blinking at cryogenic temperatures. Measuring the spectral dependence of the blinking statistics improves the possibility to clarify the microscopic nature of the dark state(s) of the emitters. We demonstrate how the blinking statistics can be definitely attributed to conformational changes in the local environment of a SM and how the parameters of the corresponding elementary excitations can be measured. The analysis of the blinking statistics as a function of the optical excitation frequency allows us to discriminate between photo-induced and spontaneous transitions into a dark state.

Order-dependent structure of high harmonic wavefronts

The physics of high harmonics has led to the generation of attosecond pulses and to trains of attosecond pulses. Measurements that confirm the pulse duration are all performed in the far field. All pulse duration measurements tacitly assume that both the beams wavefront and intensity profile are independent of frequency. However, if one or both are frequency dependent, then the retrieved pulse duration depends on the location where the measurement is made. We measure that each harmonic is very close to a Gaussian, but we also find that both the intensity profile and the beam wavefront depend significantly on the harmonic order. Thus, our findings mean that the pulse duration will depend on where the pulse is observed. Measurement of spectrally resolved wavefronts along with temporal characterization at one single point in the beam would enable complete space-time reconstruction of attosecond pulses. Future attosecond science experiments need not be restricted to spatially averaged observables. Our approach paves the way to recovery of the single molecule response to the strong field.