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We consider the collection efficiency for photons emitted by a single molecule in a solid excited by monochromatic light at low temperature, which depends upon the dipole radiation pattern, the orientation of the emission dipole, total internal reflection, and the numerical aperture of the collecting optics. By using the complete expression for the collection efficiency and varying the collection angle, we have determined the orientation of the dipole of a single terrylene molecule in hexadecane to be almost perpendicular to the substrate. By using saturation data we find that the angle between the optical axis of the collection paraboloid and the emission dipole moment is 16±6°. We estimate that the true saturation intensity of terrylene (for dipole moment parallel to the electric field of the light) is about (80±30) mW/cm 2 and that the ratio of the intersystem crossing rate to the total decay rate of the triplet state is about 3.3±1.2.

The photoluminescence of the negatively charged nitrogen-vacancy (NV-) center displays anomalous saturation behavior at high excitation pulse energies. Where the luminescence is expected to approach a maximum value asymptotically, we have found that it instead drops by as much as a factor of two. In this report, we present evidence that these effects are caused by optical spin depolarization of the NV- center. We show that the presence of an external magnetic field results in a marked decrease in the anomalous character of the NV- centers' luminescence and demonstrate that low-energy pulses applied after a strong depolarizing pulse can repolarize the spin of the center. We also offer a model and derive important parameters describing dynamics in the NV center and its interaction with light, which quantitatively explain all the observations.

Near-field single-molecule spectroscopy was performed on a crystal of pentacene-doped $p$-terphenyl using an Al-coated optical fiber tip with 60 nm diam aperture as a near-field light source. Individual molecules located \ensuremath{\cong}400 nm from the tip had linewidths of 10-20 MHz at 1.8 K. The background fluorescence provided a useful distance-sensing signal, as it increased exponentially during approach with a characteristic length of \ensuremath{\cong}100 nm. Single molecules closest to the tip were identified by Stark shifts.

We report fluorescence excitation spectra of individual impurity molecules in a new system: terrylene in a Shpol'skii matrix of hexadecan, Studies of optical incoherent saturation, fluorescence microscope images, and resonance frequency changes are reported for the 0-0 electronic transition at 1.7 K. For four stable molecules, the low-power optical line width at 1.7 K is 40 +/- 2 MHz full width at half-maximum, the saturation intensity is near 1 W/cm(2), and the peak detected count rate can reach 11000 counts per second. Most single molecules either are stable or require many minutes of irradiation before the resonance frequency changes suddenly, but some show spectral changes on a time scale of seconds. Since higher laser power increases the average rate of frequency shifts, this effect represents a light-driven change in the local environment analogous to nonphotochemical hole-burning in large ensembles. Several two-state molecules have been observed, and a preliminary characterization of this light-driven ''molecular switch'' has been completed. Multistate behavior also occurs. This work demonstrates that Shpol'skii matrices should provide fertile ground for the generation of a large number of new materials in which single-molecule spectra can be obtained.

Using fluorescence excitation spectroscopy single pentacence molecules were studied under a microscope at a temperature of 1.8 K. The microscope has a resolution of 3 μm and allows the location of a single emitting molecule with an accuracy of 0.5 μm, in a total viewing field of 235 μm×175 μm. 133 images have been recorded by increasing the excitation frequency of the light at 592.344 nm in steps of 2 MHz. Each single molecule has a Lorentzian absorption profile with a width of about 14 MHz and can be observed in 10 consecutive images. The “position” of the single molecule is characterized by the x- and y-coordinates in the spatial domain and by its absorption frequency in the spectral domain.

The orientations of the transition dipole moments of single pentacene molecules located in the O-1 and O-2 site of p-terphenyl were investigated. Two distinct orientations were found for most of the molecules. The results are discussed with respect to the crystallographic structure of the p-terphenyl host crystal.

It is shown that the intensity of photoluminescence of nitrogen-vacancy (NV) centers in nanodiamond decreases 4-fold (with a wide spread among nanocrystals) when the surrounding temperature rises from 300 to 670 K. The effect is accompanied by a 2.7-fold decrease in the luminescence lifetime but negligible changes in the shape of the emission spectra. The heating–cooling circle is reversible. The effect is suggested to be practically useful for thermometry with nanometre spatial resolution but also stimulates deeper insight into the photophysics and photochemistry of NV-centers.

We show that the wandering of transition frequencies in colloidal quantum dots does not follow the statistics expected for ordinary diffusive processes. The trajectory of this anomalous spectral diffusion is characterized by a sqrt[t] dependence of the squared deviation. The behavior is reproduced when the electronic states of quantum dots are assumed to interact with environments such as, for example, an ensemble of two-level systems, where the correlation times are distributed according to a power law similar to the one generally attributed to the dot's luminescence intermittency.

Spectral fluctuations observed in single CdSe/ZnS nanocrystals at 5 K are found to be entirely the result of discrete charge hops in the local environment of the nanocrystal, which occur at a rate comparable to the acquisition time of a single spectrum. We show that intervals between discrete spectral hops introduce a correlation between the successive measurements of the emission wavelength of single CdSe nanocrystals. This correlation can be recovered even in the presence of noise, but is shown to be sensitive to the experimental acquisition time, in good agreement with theory. However, we only find correlations for the smaller of the two nanocrystal sizes studied and discuss this in terms of size-dependent time scales correlated with the amount of excess energy dissipated in the nanocrystal due to hot-carrier relaxation. © 2010 The American Physical Society

Nitrogen-vacancy colour centres in diamond can undergo strong, spin-sensitive optical transitions under ambient conditions, which makes them attractive for applications in quantum optics, nanoscale magnetometry and biolabelling. Although nitrogen-vacancy centres have been observed in aggregated detonation nanodiamonds and milled nanodiamonds, they have not been observed in very small isolated nanodiamonds. Here, we report the first direct observation of nitrogen-vacancy centres in discrete 5-nm nanodiamonds at room temperature, including evidence for intermittency in the luminescence (blinking) from the nanodiamonds. We also show that it is possible to control this blinking by modifying the surface of the nanodiamonds.

A study was conducted to investigate the properties of weakly bound clusters of single-digit nanodiamonds (SND) using spectrally and temporally resolved luminescence detection, electron paramagnetic resonance (EPR) spectroscopy, and transmission electron microscopy (SEM). Nitrogen-vacancy (NV) centers were created in diamonds by high-energy proton irradiation followed by thermal annealing. Samples containing equal weights of 55-nm HTHP diamonds and SNDs were uniformly distributed on quartz substrates for luminescence measurements. It was observed that the SNDs exhibited significant luminescence in the red spectral region before irradiation. The emission was blue-shifted and the absence of the zero-phonon lines at 637 and 575 nm indicated that the luminescence did not originate from NV centers as compared with the NV spectrum.

Comparative investigations of intensity distribution in excitation and fluorescence spectra of 1,4-dihydroxyanthraquinone (quinizarin) and 1,4,5,8-tetrahydroxy-9,10-anthraquinone cooled in a supersonic jet and in a crystalline n-octane matrix were made. It is shown that in a jet the intensity distribution is appreciably affected by saturation effects. A special analysis of the effects has been made. It is noted that the investigation of saturation effects in jet spectra may serve as a tool for the lifetime measurements of the molecular excited states. In the absence of saturation, one has defined the constants of Franck-Condon and Herzberg-Teller interactions for the molecules under investigation. It has been established that the differences of intensity distribution in the spectra of molecules in a jet and in n-octane are mainly attributed to the Herzberg-Teller parameter variation.

A general conceptual problem of time-dependent single molecule spectra is discussed theoretically in the framework of recently developed intensity-time-frequency correlation spectroscopy. It is shown that the new method is closely related to a "gedanken" three-pulse photon echo experiment done on an ensemble of identical molecules interacting with statistically identical microscopic environments. The correlation function is an integral transform (under certain conditions a Fourier transform) of the echo amplitude as a function of the delay between the first and the second pulses. [S0163-1829(99)10907-X].

This book reviews recently developed theoretical and numerical approaches to deal with optical and mechanical signals from individual molecules. The character of data generated by single molecules, and more generally by single nano-objects, qualitatively differs from those obtained in conventional experiments on large ensembles of molecules. Fluctuations, randomness and irreproducibility are central to single-molecule measurements, and the specific methods required to extract reliable and statistically relevant information from them are presented here. With contributions mainly from participants of the “Theory, Modeling and Evaluation of Single-Molecule Measurements” workshop held in Leiden, the Netherlands, on April 16–20, 2007, this book is an authoritative compendium on the subject.

We have measured 10 and 20 mT magnetic fields using NV centers in nanodiamonds and have demonstrated that the accuracy of these measurements is about 20 μT. This value is an order of magnitude larger than the precision defined by random fluctuations in the measurements. The presented analysis indicates that the 20 μT systematic errors at such fields are intrinsic to the nanodiamonds. The possible reason for this error is coupling of NV centers with paramagnetic entities in the nanodiamonds such as other NV centers, C13, and N14 nuclei.

We demonstrate a temperature noise floor of 0.3 K Hz(-1/2) and a long-term stability better than 0.6 K (peak-to-peak value) using a single crystal of diamond smaller than 50 nm across and containing about 100 nitrogen-vacancy centres as a temperature sensor. We compare the achieved characteristics to other single-particle sensors and show that it is one of the best ratiometric all-optical nano-probes of temperature to date.

A rigorous numerical solution is presented for plane wave scattering by a photoreceptor cell. The calculated optical intensity distribution exhibits axial oscillations which are explained on the basis of substantial Bessel-wave type radiation mode excitation.

A proposal for using single molecules as nanoprobes capable of detecting the trajectory of an elementary charge is discussed in detail. Presented numerical simulations prove that this singlemolecule technique allows determination of a three-dimensional single-electron displacement within a few seconds with an accurocy better than 0.006 nm. Surprisingly, this significantly exceeds the accuracy with which the probe;, molecule itself can be localized (given the same measuring time by means of single-molecule microscopy. It is also shown that the optimal concentration of probe molecules in the vicinity of:the electron (i.e. the concentration which provides the best accuracy of the inferred electron displacement) is of the order of 10(-5) m.

We report on the observation of the excitation/emission spectrum of a dendrimer-encapsulated gold nanocluster; the synthesis of Au-PAMAM was based on reduction of HAuCl4 x 3 H2O co-dissolved in methanol together with fourth-generation OH-terminated PAMAM.

One- and two-photon excitation (OPE and TPE) spectra of single 1,8-diphenyloctatetraene molecules in n-tetradecane (transition at 444 nm) were recorded at cryogenic temperature. The high-power infrared light used for TPE changed the conditions in the sample so that the number of single molecule lines observed in OPE spectra was approximately twice the number in TPE spectra or OPE spectra with added infrared illumination. Single molecules detectable under OPE and TPE, only under OPE, and only under TPE were observed.

Two-photon fluorescence excitation spectra of single diphenyloctatetraene molecules trapped in an n-tetradecane matrix were measured at cryogenic temperatures. The purely electronic zero-phonon line (transition at 444 nanometers) of these single molecules with a width of about 60 megahertz was excited by a continuous-wave, single-mode laser at 888 nanometers. Even though the two-photon absorption cross section is extremely small, a high photon count rate and low background allowed nonlinear spectroscopy to be extended to the single-molecule level. This experiment also suggests the possibility of two-photon single-molecule scanning microscopy.

▪ Abstract Recent experimental and theoretical studies concerning single-molecule spectroscopy in solids are discussed. Pure quantum effects—such as photon bunching, antibunching, and spectral jumps—and more classical phenomena—such as near-field excitation, saturation, ac/dc Stark shifts, spectral diffusion, two-photon excitation, and customary spectroscopic analysis—are considered. The emphasis of this review is on physical results and their interpretation. This is preceded by a general introduction, where fundamentals of single-molecule spectroscopy are explained.

Spectral shifts of the molecular resonance frequency have been observed since the early days of single molecule spectroscopy (SMS) [4]. We call these effects for concreteness light-induced spectral shifts (LISS) or spontaneous spectral shifts (SSS or S3) to distinguish the two possible reasons for the spectral change. In order to gain a more complete picture of these effects we have undertaken three different approaches.

An unusual two- to threefold decrease in the luminescence intensity of nitrogen-vacancy centers in 30 nm diamonds under increasing pulsed-laser irradiation (up to the level of 0.1 J cm−2) has been measured. The effect showed little dependence on the pulse repetition rate below 1 MHz and was accompanied by insignificant changes in the emission spectrum. The primary cause of the effect was attributed to the heat generated by the laser pulse and the consequent increase of the crystal temperature, which was estimated in the range of 300–400 °C.

Typical disturbances of biological environment such as background scatter and refractive index variations have little effect on the size-dependent scattering property of highly refractive nanocrystals, which are potentially attractive optical labels. We report on what is to our knowledge the first investigation of these scattering optical labels, and their sizing, in particular, by imaging at subvideo frame rates and analyzing samples of diamond nanocrystals deposited on a glass substrate in air and in a matrix of weakly scattering polymer. The brightness of a diffraction-limited spot appears to serve as a reliable measure of the particle size in the Rayleigh scattering limit.

Scanning near-field optical microscopy (SNOM) is an optical microscopy whose resolution is not bound to the diffraction limit. It provides chemical information based upon spectral, polarization and/or fluorescence contrast images. Details as small as 20 nm can be recognized. Photophysical and photochemical effects can be studied with SNOM on a similar scale. This article reviews a good deal of the experimental and theoretical work on SNOM in Switzerland.

In a ring travelling-wave dye laser a parasitic structure in the intracavity absorption spectrum is fully eliminated and a detection limit as low as 10 -10 cm -1 is obtained. The sensitivity dependence of the intracavity laser spectrometer with travelling-wave cavity on the pumping level is also investigated.

We report investigations on one- and two-photon excitation of single molecules of diphenyloctatetraene in n-tetradecane at cryogenic temperatures. Saturation intensities, count rates and linewidths of single molecules are compared with corresponding ensemble values obtained from statistical fine structure investigations. The single-molecule linewidths under one-photon excitation are generally two to three times narrower than in two-photon excitation spectra. This effect is discussed with respect to saturated spectral diffusion induced by the high power 1R light necessary for efficient two-photon absorption.

When a single quantum system (SQS) interacts with a fluctuating environment, ordinary approaches to spectroscopy fail because measured spectra are subject to strong stochastic fluctuations. Strictly speaking, it is also impossible to employ spectral line shape theories based on the fluctuation dissipation theorem for such systems. Such theories apply when light interacts with an ensemble of SQSs or with a SQS but for an infinitely long time. A way out of both experimental and theoretical difficulties is offered by intensity–time–frequency-correlation (ITFC) spectroscopy. In this report a theoretical foundation of the method is explained in detail. Presented computer simulations show advantages of the technique, regions where the old approaches give rise to big errors, and also where they are still valid.

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.

To model the light-guiding properties of a hexagonal array of dielectric cylinders, we have numerically solved Maxwell's equations with the finite-difference time-domain technique. The sizes and refractive indices of the cylinders are representative of those of the outer segments of the cone photoreceptors in the human central retina. In the array, light propagates predominantly as a “slow” mode, with a noticeable contribution of a “fast” mode, with the optical field localized in the intra- and inter-cylinder spaces, respectively. Interference between these modes leads to substantial (up to approximately 60%) axial oscillations in optical power within the cylinders. Our numerical model offered approximate dependence of the optical intensity distribution within the cylinders on their radii and separations.

Fluorescence excitation spectroscopy is used to study vibronic transitions ( S 1 ← S 0 n←0) in terrylene molecules doped in a naphthalene crystal. Evidence for the excitation of individual dye molecules is reported. The average linewidth (FWHM) for transitions to the first and second vibronic levels is, in both cases, 30(4) GHz. Linewidths and vibrational frequencies are not the same for all molecules. A method is presented for establishing a one-to-one correspondence between individual lines in the vibronic and the purely electronic band.

Alight induced frequency shift of the 0-0 line was measured in two-photon excitation spectra of single diphenyloctatetraene molecules doped in a crystal matrix. The shifts were proportional to the laser power with a slope of about 600 MHz/W when the laser beam of about 300 mW power was focused to a diameter of 2 mu m. Significantly, the observed line broadenings were an order of magnitude smaller than the shifts. The effect is ascribed mainly to a ''fast'' energy exchange between a local vibration and thermal phonons created by the third harmonic C-H band absorption in the matrix, and partially to an ac Stark shift.

A new method based on the calculation of autocorrelation functions for spectra measured at a high acquisition rate is developed to study spectral dynamics of single molecules. The technique allows for spectroscopy with time resolutions down to the luminescence lifetime. The method is used to study spectral diffusion in two-photon excitation spectra of diphenyloctatetraene molecules doped in an n-tetradecane crystal matrix. The diffusion is light induced, and is absent in one-photon excitation spectra. It has a "steplike" time behavior, different from gradual diffusion observed in glasses.

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.

A new design of an optical resonator for generation of single-photon pulses is proposed. The resonator is made of a cylindrical or spherical piece of a polymer squeezed between two flat dielectric mirrors. The mode characteristics of this resonator are calculated numerically. The numerical analysis is backed by a physical explanation. The decay time and the mode volume of the fundamental mode are sufficient for achieving more than 96% probability of generating a single-photon in a single-mode. The corresponding requirement for the reflectivity of the mirrors (~99.9%) and the losses in the polymer (100 dB/m) are quite modest. The resonator is suitable for single-photon generation based on optical pumping of a single quantum system such as an organic molecule, a diamond nanocrystal, or a semiconductor quantum dot if they are imbedded in the polymer.

In this work, discrepancies between measured single-molecule linewidth distributions and distributions simulated with the standard tunneling model in the sudden-jump approximation are studied for terrylene and di-tert-butyltenylene dopant molecules in poly(vinylbutyral). The experimental data has been carefully checked for systematic errors. It is shown that the results are in disagreement with the standard model, but that the incorporation of a distribution of molecule-polymer coupling strengths would remove this disagreement. Additionally, the experimental observation of a surprising correlation between the single-molecule line areas and the matrix-molecule interaction strength is presented, and a hypothesis is put forward for its physical origin.

The dependence of the sensitivity of intra-cavity laser spectroscopy (ILS) at the transition stage of generation on the pumping power and spectral width has been investigated experimentally. The time interval of linear growth of the sensitivity is shown to be limited and determined by the excess of the pumping power over the threshold. The combination of physical parameters of the system which determines the value of the effect is found. Possible mechanisms of the phenomenon are discussed. Insufficiency of the generally accepted description of a multimode laser in ILS within the frames of rate equations and the necessity of the inclusion of parametric processes are pointed out.

We built an, interferometer where one of the two slits of a classical Young's setup is replaced by a single molecule embedded in a solid matrix. This enabled direct measurement of the first order coherence of the 0-0 single-molecule emission, which at high excitation powers proves to be split in coherent and incoherent parts. We demonstrate an order of magnitude higher precision in axial localization of single molecules in comparison with that of confocal microscopy. These experiments open a possibility for single-molecule holography. Detection of single molecules with low luminescence quantum yields could be another application of this technique.