Terrylene Molecules Research Articles

Single Photon Light Sources

In the past, generation of single photons for subsequent use in physical or physicochemical experiments was difficult. In the last few years the situation has changed dramatically. Now, such sources based on a number of different techniques are available. Two of them produce antibunching photons by controlled recombination of electrons and holes in semiconductor heterojunctions. A second type of source is based on indium-arsenide quantum dots in high Q factor microcavities. Two further sources use single dibenz-anthanthrene or single terrylene molecules. While all but one sources have to be operated at temperatures close to zero Kelvin, the terrylene source works at room temperature and provides up to 10 billion photons before photodecay, i.e. it can be operated for hours. When the latter is operated at zero Kelvin, it is even more stable and can be used in a microscope to generate complete images. The present review compares and summarizes properties of these single optical photon sources and, in addition, tries to present an argument, why single photons cannot be particle like objects but must have a spatial extension of at least several tens of nanometers.

Anomalous Fluorescence of Terrylene in Neon Matrix

Fluorescence and fluorescence excitation spectra of terrylene deposited in Ne and Ar matrixes at 7 K have been studied. The (0,0) fluorescence band of terrylene in Ne matrixes is shifted by 330 cm-1 to low energy with respect to the (0,0) fluorescence excitation band observed at 18990 cm-1. Furthermore, the vibronic structures of both spectra are different. In contrary to this observation, the fluorescence and fluorescence excitation spectra of terrylene in Ar matrixes have a common origin at 18312 cm-1 and demonstrate mirror symmetry. Semiempirical AM1 calculations combined with the analysis of the vibronic structure of the spectra suggest that terrylene molecules in the excited S1 state are subject to relaxation. This relaxation is possible in soft Ne but it is frozen in rigid Ar matrix.

Spectral diffusion in polyethylene: Single-molecule studies performed between 30 mK and 1.8 K

Linewidth distributions for single terrylene molecules in polyethylene have been measured in the temperature range from 30 mK to 1.83 K. The temperature dependence of the average linewidth is linear over the full temperature range. Linewidth distributions were simulated using the tunneling two-level system model and compared to the data in order to extract the lifetime-limited linewidth distributions and the distributions of coupling strengths between the probe molecules and the two-level systems. Differences of the average lifetime and coupling strengths were observed for samples produced with different sample-preparation techniques.

Non-lorentzian single-molecule line shape: pseudolocal phonons and coherence transfer

The excitation line shape of a single terrylene molecule in a naphthalene crystal has been investigated. In addition to the conventional Lorentzian, it consists of a dispersive component in the core region and a sideband. This is due to a pseudolocal phonon caused by the substitution of a host molecule with the chromophore. When the pseudolocal phonon is excited, the resonance frequency of the chromophore slightly changes, resulting in the appearance of a second, quasiresonant transition. Coherence transfer between these two optical transitions causes the deviation from the purely Lorentzian line shape.

The anomalous Stark effect of single terrylene molecules in p-terphenyl crystals

Single terrylene molecules embedded in a p-terphenyl crystal behave like well-defined optical switches. Recent measurements of the shift of their optical lines under an applied electric field (Stark effect) have revealed surprising anomalies: unexpectedly large and widely spread differences of polarizability between the ground and excited states, even in crystals of excellent quality. Some molecules even had a greater polarizability in the ground than in the excited state. We propose to explain these results by a symmetry breaking in the supermolecule (including the terrylene molecule and the first shells of p-terphenyl neighbours), leading to two distorted conformations with opposite dipole moments. Tunnelling between these conformations gives rise to a quadratic Stark effect with a large effective polarizability. The model is internally consistent, and provides reasonable orders of magnitude for the tunnel barrier. We argue that this tunnelling involves correlated movements of groups of atoms, 1 nm apart, and therefore the system is particularly sensitive to small residual strains in otherwise perfect crystals.

Triplet-exciton dynamics in solid naphthalene investigated by single molecule spectroscopy

The dynamics of triplet excitons in naphthalene crystals are investigated using single terrylene molecules as local probes. Triplet excitations in the host matrix are generated by Xe-lamp illumination. The triplet excitations induce shifts in the resonance frequencies of the individual probe molecules. These excitation-induced frequency shifts fluctuate and relax on a time scale of seconds subsequent to the matrix excitation. For the description of the frequency shifts two models are applied. One is based on localized excitations and is similar to the sudden-jump model. The other is based on mobile excitations that are harvested by an energy funnel of trap states in the vicinity of the probe molecule. The analysis indicates that the frequency fluctuations result from triplet excitations migrating within the funnel.

Zero–phonon lines of single molecules in polyethylene down to millikelvin temperatures

Linewidth distributions for single terrylene molecules in polyethylene have been measured in the temperature range from 30 mK to 1.83 K. The temperature dependence of the average linewidth is best described by a linear relationship over the full temperature range. At 30 mK, the linewidth distribution has a full-width at half-maximum of ∼18.6 MHz and an average linewidth of 42.8(6) MHz.

Statistics of a single terrylene molecule in hexadecane

In this work, frequency trajectories for a single terrylene molecule in n-hexadecane undergoing light-induced spectral diffusion are studied in detail. The molecule was scanned over 5000 times at three different laser powers and jumped between 6 sites. Two mechanisms for the jumping are suggested and compared to the data statistically. The linewidths are also analyzed in detail. In addition, it is revealed that a correlation between the linewidth and the line area is always present in noisy data when both parameters are found using a nonlinear least-squares fit.

Dynamical theory for two-photon correlators: spectral line jumps and spectral diffusion

A relation between jumping spectral lines of a single molecule and temporal behavior of the two-photon correlators is discussed. Temporal behavior of the optical bands and two-photon correlator for a single molecule coupled to one and two TLSs is considered within the frame of a dynamical theory. The theory is employed to treat experimental data for terrylene molecule in polyethylene

On the role of spectral diffusion in single-molecule spectroscopy

This paper reports on measurements of fluorescence excitation spectra of single impurity molecules of terrylene in n-decane at T=1.7 K. Spectra measured within the same spectral interval but at consecutive instants of time exhibit zero-phonon (ZP) lines of single impurity molecules of several species, differing in the behavior of the line shape and frequency in time. On the one hand, one observes stable ZP lines, well approximated with a Lorentzian. On the other, one sees spectral features with a profile varying from one spectrum to another, with only individual fragments of such a profile allowing the Lorentzian approximation; such features are interpreted assuming the presence of unstable impurity molecules, the ZP lines of which display small (a few tens of MHz) spectral jumps with a time interval of about 10 s. Such molecules exhibit a substantial decrease in the spectral jump frequency within a measurement period of the order of 5000 s, which is attributed to a decrease in the contribution due to spectral diffusion resulting from sample structure relaxation.

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.

Absorption spectroscopy on single molecules in solids

Absorption signals of single terrylene molecules in n-hexadecane and naphthalene crystals were recorded at liquid-helium temperatures. The method is based upon rf Stark effect modulation in the megahertz range. The electric rf field strength was applied by means of interdigitating electrodes with 18 μm spacing. Signal-to-noise ratios better than 10 were obtained with approximately 300 ms integration time. The measured line shapes depend on the relative contributions of the linear and the quadratic Stark shift.

The distribution of line widths of single probe molecules in a crystalline host at milliKelvin temperatures

The line width distributions for single terrylene molecules in a naphthalene crystal have been measured at temperatures down to 30 mK. The line width distribution becomes narrower with decreasing temperature, and has a full-width at half-maximum of approximately 4.3(13) MHz at 30 mK and an average line width of 42.7(3) MHz.

Imaging and spectroscopy of terrylene molecules isolated in vapor-deposited n-alkane matrices

The matrix isolation technique was applied for producing very thin samples of terrylene doped n-alkane solids directly on the surface of a microscope objective. Fluorescence microscopy allowed the study of the spectroscopic properties of many single terrylene molecules in parallel. From images recorded sequentially at different laser excitation frequencies, the spectral line profiles of corresponding zero-phonon absorption lines were measured. For the two matrices n-decane and n-hexane the line width distributions were determined. Under similar excitation conditions, the individual terrylene molecules were `burned out' much faster in n-hexane than in n-decane matrices.

Are the spectral trails of single molecules consistent with the standard two-level system model of glasses at low temperatures?

The spectral trail of a single molecule is a 2D plot of many repeated spectra displayed as a function of time. Here, we present an analysis of the spectral trails of some 70 terrylene molecules in polymers. We compare the features of individual trails to the standard model of two-level systems (TLSs) in glasses at low temperatures, and we find a good overall agreement for a majority of molecules. However, more than 30% of the molecular trails cannot be reconciled with the standard model. For example, we found three-level systems, evidence for a strong coupling between some TLSs, and spectral drifts. Whereas our data confirm that the standard TLS model describes large ensembles of TLSs quite satisfactorily, this may result only from a cancellation of the deviations. Therefore, the application of the model to explain the spectral diffusion of individual molecules must be questioned in each particular case.

Terrylene in p-terphenyl: single-molecule experiments at room temperature

This paper reports on fluorescence microscopy and spectroscopy of single terrylene molecules embedded in p-terphenyl at room temperature. The basic photophysical parameters of this crystalline host/guest system such as photostability, single-molecule emission rates and the properties of dispersed fluorescence spectra are reported. The stability of the single-molecule signals allowed the direct observation of singlet–triplet quantum jumps at room temperature, by means of which the underlying intersystem crossing rates can be investigated. The results of earlier investigations at liquid-helium temperatures are reviewed briefly where appropriate. The properties of terrylene in p-terphenyl are also compared to those of other host/guest systems at room temperature.

A single molecule as a probe of optical intensity distribution

Single terrylene molecules embedded in microscopic p-terphenyl crystals are identified with the technique of fluorescence excitation spectroscopy. By use of the architecture of a scanning-probe microscope at T = 1.4 K , a single molecule is scanned through an excitation laser beam while the fluorescence signal is recorded. In this manner we have mapped the intensity distribution in a one-dimensional optical standing wave, demonstrating the potential of a single molecule as a nanometric probe. We discuss future experiments aimed at combining the high spatial and spectral sensitivity of a single molecule.

Laser scanning microscope for low temperature single molecule and microscale spectroscopy based on gradient index optics

A scanning optical microscope for low temperature imaging and spectroscopy with a gradient index rod-shaped microlens as an objective lens is presented. The solid immersion effect enhances the resolution to 310 nm of the full-width at half-maximum at the wavelength of 545 nm. Laser scanning mechanism located outside an optical cryostat enables to achieve large scanning ranges independent of temperature. The performance is demonstrated at 1.6 K on single molecules of terrylene in a dodecane crystal and on molecular J aggregates in thin polymer films.

High photo-stability of single molecules in an organic crystal at room temperature observed by scanning confocal optical microscopy

Single molecules have been observed in a molecular crystal at room temperature using scanning confocal fluorescence microscopy. Individual terrylene molecules in a p-terphenyl crystal show high photo-stability with a corresponding ensemble-averaged photo-destruction quantum efficiency lower than 1.2 × 10−8. On average, with a detection efficiency of 6.5%, a stable fluorescence signal of 10−5 photons s−1 could be detected from single molecules during 1 min of continuous photo-excitation. Both irreversible and reversible abrupt fluorescence intensity jumps to the background have been observed. The experimental data indicate that diffusing quenchers at low concentration in the crystal contribute to fluorescence bleaching of single molecules. All molecular fluorescence signals follow a typical power saturation law with a mean saturation count rate of 4.3 × 105 photons s−1. The large photo-stability allows for long illumination times and high emission rates of single molecules trapped in a crystal at room temperature. A potential application as single organic quantum light sources under ambient conditions is conceivable.

Detecting Magnetic Resonance through Quantum Jumps of Single Molecules

The fluorescence of a single molecule exhibits light and dark periods which reveal the jumping back and forth between the singlet and triplet manifold. In this paper we demonstrate how microwave-induced changes to the distribution of the triplet-residence times can be used to detect magnetic-resonance transitions between triplet sublevels and to determine their kinetics. By synchronizing resonant microwave pulses with the quantum jumps of a terrylene molecule it proved possible to record transient-nutation signals with a high contrast.