Sub- and super-Poissonian photon statistics of single-molecule fluorescence blinking

An analysis of intermittent fluorescence is presented for a single molecule driven by a continuous-wave laser field. The interruptions of fluorescence are caused by transition of the molecule to a triplet state. A method previously developed to calculate photon distribution for continuous-wave fluorescence is applied to analyze photon statistics of fluorescence blinking. The probability w _N (T) of counting N photons over a time interval T is derived for intermittent fluorescence. The photons counted over relatively short intervals are found to have a sub-Poissonian (narrower than Poisson) distribution. The photon distribution over intervals longer than the mean off time has a complicated form with two maxima; i.e., a super-Poissonian (wider than Poisson) distribution is obtained.     

Statistical model of fluorescence blinking

Blinking of single molecules and nanocrystals is modeled as a two-state renewal process with on (fluorescent) and off (non-fluorescent) states. The on and off-times may have power-law or exponential distributions. A fractional generalization of the exponential function is used to develop a unified treatment of the blinking statistics for both types of distributions. In the framework of the two-state model, an equation for the probability density p(t _on|t) of the total on-time is derived. As applied to power-law blinking, the equation contains derivatives of fractional orders α and β equal to the exponents of the on and off-time power-law distributions, respectively. In the limit case of α = β = 1, the distributions become exponential and the fractional differential equation reduces to an integer order differential equation. Solutions to these equations are expressed in terms of fractional stable distributions. The Poisson transform of p(t _on|t) is the photon number distribution that determines the photon counting statistics. It is shown that the long-time asymptotic behavior of Mandel’s Q parameter follows a power law: M(t) ∝ t ^γ . The function γ(α, β) is defined on the (α, β) plane. An analysis of the relative variance of the total on-time shows that it decays only when α = β = 1 or α < β. Otherwise, relative fluctuations either exhibit asymptotic power-law growth or approach a constant level. Analytical calculations are in good agreement with the results of Monte Carlo simulations.     

NMR investigations of polymer dynamics in a partially filled porous matrix

The interior surface of well-defined porous alumina membranes (Anopore) of 20 nm and 200 nm pore diameter, respectively, was coated with polymer layers generated from solution by the solvent evaporation method. Deposits of poly(dimethyl siloxane) (PDMS) with nominal thicknesses ranging from 0.15 to 4.5 nm --corresponding to submonolayer to multilayer films--were investigated, and were compared to poly(butadiene) (PB) as an example for non-wetting polymers. Molecular weights below and above the critical value were studied since the bulk dynamics of such polymers are known to be qualitatively different. First results of NMR relaxation dispersion experiments on these systems are presented, supplemented by transverse relaxation times and double-quantum measurements obtained from high-field NMR. A systematic decrease of relaxation times at low fields with decreasing polymer amount is found for PDMS, but molecules retain a high degree of mobility irrespective of molecular weight. The relaxation dispersion results are supported by T2 data and 1H residual dipolar coupling (RDC) constants, and are discussed in terms of molecular order and reorientational dynamics.     

Photon distribution function in blinking fluorescence of individual molecules

A photon distribution function w_N(T) for blinking fluorescence with bright on- and dark off-intervals is derived. The function w_N(T) is expressed via few Poissonian functions each of which relates to corresponding exponential process in quantum dynamics of a given individual molecule. The distribution of photons is calculated for short, middle and long time intervals as compared to off-intervals. The distributions are much broader than Poissonian distribution and have rather complicated shape. If time resolution of an experiment does not permit us to see off-interval and, therefore, fluorescence looks like CW emission, the distribution of photons gives a signal about existence of hidden off- intervals in such CW fluorescence.     

Influence of phonon-associated tunneling rate on transport through a single-molecule transistor

We study the electronic transport through a single-molecule transistor (SMT) by considering the phonon-associated tunneling rate. We find that the electron-phonon interaction (EPI) changes the constant conductivities of the leads into a multi-channel structure of single vibration frequency. This interference of the multi-channel tunneling process results in a bias-dependent tunneling rate and obscures the conductance peaks at large bias voltage. The bias-dependent tunneling rate further causes a remarkable conductivity gap between the chemical potential of the leads (n=0) and the first phonon sideband (n=1). These anomalies are consistent with the experimental observations in transport experiments.     

Surface-plasmon-mediated single-molecule fluorescence through a thin metallic film

We demonstrate that fluorescence of single molecules in the nanometric vicinity of a thin gold film can be effectively excited and detected through the film with an epi-illumination scanning confocal microscope. A full theoretical treatment of the fluorescence signal indicates that both excitation and emission are surface-plasmon mediated. Remarkably, the number of photons detectable from chromophores perpendicular to the interface is enhanced by the presence of the metal.     

Electronic resonance and symmetry in single-molecule inelastic electron tunneling

Two families of exact global solutions to the equations of plasma equilibrium are derived. The solutions model astrophysical jets and solar prominences and provide counterexamples to Parker's hypothesis.     

Enhancement and quenching of single-molecule fluorescence

We present an experimental and theoretical study of the fluorescence rate of a single molecule as a function of its distance to a laser-irradiated gold nanoparticle. The local field enhancement leads to an increased excitation rate whereas nonradiative energy transfer to the particle leads to a decrease of the quantum yield (quenching). Because of these competing effects, previous experiments showed either fluorescence enhancement or fluorescence quenching. By varying the distance between molecule and particle we show the first experimental measurement demonstrating the continuous transition from fluorescence enhancement to fluorescence quenching. This transition cannot be explained by treating the particle as a polarizable sphere in the dipole approximation.     

Spin quantum tunneling via entangled states in a dimer of exchange-coupled single-molecule magnets

A new family of supramolecular, antiferromagnetically exchange-coupled dimers of single-molecule magnets (SMMs) has recently been reported. Each SMM acts as a bias on its neighbor, shifting the quantum tunneling resonances of the individual SMMs. Hysteresis loop measurements on a single crystal of SMM dimers have now established quantum tunneling of the magnetization via entangled states of the dimer. This shows that the dimer really does behave as a quantum mechanically coupled dimer, and also allows the measurement of the longitudinal and transverse superexchange coupling constants.     

Statistics of photocounts of blinking fluorescence of quantum dots

The blinking of quantum dots under the action of laser radiation is described based on a model of a binary (two-state) renewal process with on (fluorescent) and off (non fluorescent) states. The T _on and T _off sojourn times in the on and off states are random and power-law distributed with exponents 0 < α < 1 and 0 < β < 1; the averages of the on and off times are infinite. As a consequence of this, the Gaussian statistics is inapplicable and the process is described using a more general statistics. An equation for the density of distribution p(t _on|t) of the total on time during the observation time t is derived that contains derivatives of fractional orders α and β. A solution to this equation is found in terms of fractional stable distributions. The Poisson transform of the density p(t _on|t) leads to the photon counting distribution and determines the fluorescence statistics. It is demonstrated that, if a blinking process with exponents α < β is implemented, then, at fairly long times, the on time will considerably prevail over the off time, i.e., blinking will be suppressed. This behavior is evidenced by the types of distributions of the total fluorescence time, the decay of relative fluctuations, and the Monte Carlo simulated trajectories of the process.     

Fine fluorescence and absorption spectra of hypericin in a polymer matrix at a temperature of 4.2 K

Monochromatic photoexcitation and selective registration in bands of purely electronic S₁↔S₀ transitions resulted in fine fluorescence and absorption (fluorescence excitation) spectra of hypericin in polyvinylbutyral at a temperature of 4.2 K. Vibration analysis of the spectra is done and conclusions are made about the form of many normal vibrations. It is found that the formation of intramolecular hydrogen bonds in the hypericin structure has a substantial effect on the relative position of the electronic vibrational levels of the pigment molecule. Institute of Molecular and Atomic Physics of the National Academy of Sciences of Belarus, 70, F. Skorina ave., Minsk 200072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 4, pp. 539–545, July–August, 1998.     

Enhancement of single-molecule fluorescence detection in subwavelength apertures

We report the experimental proof of molecular count rate enhancement (up to 6.5-fold) and lifetime reduction for single fluorescent molecules diffusing in subwavelength apertures milled in aluminum films. The observed enhancement dependence with the aperture diameter agrees qualitatively with numerical electromagnetic computations of the excitation power density into the aperture volume.     

Real-time observation of single-molecule fluorescence in microdroplet streams

We report real-time observation of fluorescence bursts from individual Rhodamine 6G molecules in streams of microdroplets (peak signal-to-noise ratios, approximately 30) whose trajectories are constrained with a linear electric quadrupole. This approach offers a reasonable dynamic range in droplet size (3- 12-microm diameter) with <1% shot-to-shot size fluctuations and sensitivity comparable with that of droplet levitation techniques with at least 10(3) higher analysis rates. Applications to the study of single-molecule microcavity effects and stimulated emission are discussed.     

Simulation of a stream of photon counts in single-molecule fluorescence fluctuation spectroscopy

We present a simulation model for a stream of photon counts that can be detected in fluorescence fluctuation spectroscopy systems from diffusing molecules undergoing spontaneous transitions between radiative and nonradiative states. Using our model, we can obtain various characteristics of a random stream of photon counts which allows us to test fluorescence fluctuation spectroscopy methods and to predict the results of full-scale experiments.     

Nuclear spin dynamics in the quantum regime of a single-molecule magnet

We show that the nuclear spin dynamics in the single-molecule magnet Mn12-ac below 1 K is governed by quantum tunneling fluctuations of the cluster spins, combined with intercluster nuclear spin diffusion. We also obtain the first experimental proof that-surprisingly-even deep in the quantum regime the nuclear spins remain in good thermal contact with the lattice phonons. We propose a simple model for how T-independent tunneling fluctuations can relax the nuclear polarization to the lattice that may serve as a framework for more sophisticated theories.     

Electron-vibration interaction in single-molecule junctions: from contact to tunneling regimes

Point contact spectroscopy on a H(2)O molecule bridging Pt electrodes reveals a clear crossover between enhancement and reduction of the conductance due to electron-vibration interaction. As single-channel models predict such a crossover at a transmission probability of tau=0.5, we used shot noise measurements to analyze the transmission and observed at least two channels across the junction where the dominant channel has a tau=0.51 +/- 0.01 transmission probability at the crossover conductance, which is consistent with the predictions for single-channel models.     

Influence of deformations of a polymer matrix on the characteristics of dual fluorescence of 3-hydroxyflavone

The possibility of considerably changing the conditions for the proton transfer reaction in 3-hydroxyflavone molecules in polyvinyl alcohol (PVA) polymer matrices by stretching deformations is demonstrated. Samples of this kind are traditionally used to obtain ensembles of fluorophore molecules oriented along a chosen axis and for polarization measurements. The fluorescence spectrum of 3-hydroxyflavone in PVA has two characteristic bands in the violet and green spectral regions, which indicates excited-state proton transfer. Stretching leads to a strong reduction in the violet band, whose contribution in undeformed samples is comparable to the contribution of green fluorescence. Even twofold stretching of PVA films strongly decreases the violet band intensity, which is more pronounced in the case of sixfold stretching. In the latter case, the fluorescence spectrum behavior is very close to the pattern observed in nonpolar and aprotic solvents, in which the violet fluorescence intensity is very low. The data obtained indicate that mechanical stretching allows one to eliminate the main intermolecular factors that slow down the proton transfer between the active groups in PVA.     

Bias-controlled spin memory and spin injector scheme in the tunneling junction with a single-molecule magnet

A bias-controlled spin-filter and spin memory is theoretically proposed, which consists of the junction with a singlemolecule magnet sandwiched between the nonmagnetic and ferromagnetic(FM) leads. By applying different voltage pulses Vwriteacross the junction, the spin direction of the single-molecule magnet can be controlled to be parallel or anti-parallel to the magnetization of the FM lead, and the spin direction of SMM can be "read out" either by the magneto-resistance or by the spin current with another series of small voltage pulses V_(probe). It is shown that the polarization of the spin current is extremely high(up to 100%) and can be manipulated by the full-electric manner. This device scheme can be compatible with current technologies and has potential applications in high-density memory devices.     

界面电子转移对量子点荧光闪烁行为的影响

利用激光扫描共聚焦显微系统分别测量了CdSe/ZnS量子点在SiO2玻片表面、铟锡氧化物(ITO)纳米粒子表面和聚甲基丙烯酸甲酯(PMMA)薄膜表面上的荧光闪烁行为.研究发现,不同界面环境中量子点的亮态发光持续时间的概率密度都服从指数修正的幂律分布P(t)∝t-αexp(-t/μ).与处于SiO2玻片表面的情况相比,在ITO表面上的单量子点具有非常短暂的亮态发光持续时间,而在PMMA表面的单量子点亮态发光持续时间最长.这种荧光闪烁行为的不同主要归因于量子点与三种材料之间的界面电子转移特性.     

Nonclassical photon statistics in single-molecule fluorescence at room temperature

The fluorescence of single terrylene molecules in a crystalline host is investigated at room temperature by scanning confocal optical microscopy. Photon arrival times are analyzed in terms of interphoton time distributions, second order correlation functions, and the variance of the photon number probability distribution. Antibunching at short times and bunching behavior for longer times is observed, associated with sub- and super-Poissonian statistics, respectively. A rate-equation analysis of the molecular level populations indicates an accelerated reverse intersystem crossing.