Single Impurity Molecules Research Articles

Spin-selective low temperature spectroscopy on single molecules with a triplet-triplet optical transition: Application to the NV defect center in diamond

The spin-selective photokinetics of a single matrix-isolated impurity molecule with a triplet-triplet optical transition, T0–T1, is considered and the manifestations of the photokinetics in the fluorescence excitation spectra and intensity autocorrelation functions g(2)(τ) of the molecule undergoing narrow-band optical excitation is studied to resolve the fine structure of the transition. The rates of intersystem crossings (ISCs) T1→S→T0 to and from a nonradiating singlet state S of the molecule and the rate of population relaxation among the ground (T0) state sublevels can be obtained from the spectra and g(2)(τ) using the analytical expressions obtained. New experiments on an individual NV defect center in nanocrystals of diamond, where, for the first time, the fine structure of its triplet-triplet 3A-3E zero-phonon optical transition (~637 nm) at 1.4 K was resolved, are interpreted. It is concluded that the rate of the ISC transition from the mS=0 sublevel of the excited 3E state to the singlet 1A state (~1 kHz) is much slower than the rates from the mS=±1 substates, while the rates of ISC transitions to different mS substates of the ground 3A state are close to each other (~1 Hz). As a result, only the optical transition between mS=0 sublevels in the 3A-3E manifold contributes strongly to the fluorescence. The experimentally observed double-exponential decay of the g(2)(τ) function is explained by the two pathways available to the center for it to leave the S state: (i) the S→ T0(mS)=0) transition and (ii) the S→T0(mS=±1) transitions followed by the slow spin-lattice relaxation T0(mS=±1)→T0(mS=0) (rate ~0.1 Hz). The work is important for studies where the NV center is used as a single photon source or for quantum information processing.

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.

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.

Localized helium excitations in4HeN-benzene clusters

We compute ground and excited state properties of small helium clusters 4He_N containing a single benzene impurity molecule. Ground-state structures and energies are obtained for N=1,2,3,14 from importance-sampled, rigid-body diffusion Monte Carlo (DMC). Excited state energies due to helium vibrational motion near the molecule surface are evaluated using the projection operator, imaginary time spectral evolution (POITSE) method. We find excitation energies of up to ~23 K above the ground state. These states all possess vibrational character of helium atoms in a highly anisotropic potential due to the aromatic molecule, and can be categorized in terms of localized and collective vibrational modes. These results appear to provide precursors for a transition from localized to collective helium excitations at molecular nanosubstrates of increasing size. We discuss the implications of these results for analysis of anomalous spectral features in recent spectroscopic studies of large aromatic molecules in helium clusters.

Spontaneous emission rates of a single impurity molecule in a uniaxial host crystal

A theory of the dependence of the spontaneous emission rate of a single impurity molecule on the orientation of the transition dipole moment vector in a birefringent host crystal is developed. The dependences of the density of the emitted photon final states (expressed through the phase and the ray velocities of the photons) and the local and the average electric field strengths on the orientation of the wave vector of the photon in the host crystal and on the principal refractive indices are taken into account. In a uniaxial crystal we have calculated how the spontaneous emission rate depends on the ordinary and the extraordinary principal refractive indices and on the angle between the transition dipole moment vector and the optical axis.

Purely electronic zero-phonon line as the foundation stone for high resolution matrix spectroscopy, single impurity molecule spectroscopy, persistent spectral hole burning

A few examples of progress of studies and applications of purely electronic zero-phonon line are considered: new experimental values of the narrowest line, femtosecond time-and-space-domain holography and the Taffoli gate by means of it; promises of quantum computing compared to what already done in holography.

Saturation spectroscopy of vibronic transitions in single molecules

A novel technique of saturation spectroscopy is applied to study the individual vibronic spectra of single impurity molecules embedded in a low-temperature solid matrix. Homogeneous linewidths and vibrational frequencies for two lower vibronic levels of the excited electronic state S 1 are determined for several terrylene single molecules in a naphthalene crystal, which indicates a noticeable distribution of these parameters. A certain correlation can be pointed out between vibrational frequency and a purely electronic transition frequency of a terrylene molecule.

X-Ray Studies of Structural Changes of Impurity-Helium Solids

We have performed studies of the formation and evolution of samples of Im-Helium (Im=Ne, D2, N2) solids obtained by allowing a mixed beam of helium and impurity molecules to fall onto the surface of superfluid helium. Clusters consisting of single impurity molecules and/or small aggregates of the molecules surrounded by layers of solid helium agglomerate to form porous solids. These solids have been studied by x-ray diffraction techniques. As the solids were heated above their formation temperature of T=1.5 K, the small aggregates or clusters of impurities tended to form larger clusters, which x-ray diffraction revealed to be nanocrystallites ranging in size from 3 to 6 nm.

Spontaneous emission rates of single impurity molecules in birefringent crystals

We have developed a theory of the dependence of the spontaneous emission rate and, consequently, the radiative line width of a single impurity molecule on the orientation of the transition dipole moment vector in birefringent biaxial host crystal. The calculations for seven biaxial host crystals (naphthalene, anthracene, terphenyl, phenanthrene, fluorine, chrysene, and diphenyl) show that the spontaneous emission rate depends strongly on the orientation of the transition dipole moment vector with respect to the principal axes of the host crystal and has only a weak dependence on the chemistry of the host.

On the theory of ultranarrow pure-electronic zero-phonon lines

A pure-electronic zero-phonon line, which represents an optical analog of the Mossbauer γ-resonance line, is a cornerstone of three remarkable areas of spectroscopy and optics of impurity solids, namely, ultrahigh-resolution spectroscopy, persistent hole-burning spectroscopy, and spectroscopy of single impurity molecules. In recent experiments made by the photon echo technique, a zero-phonon line width as small as 78 Hz was achieved (with the Q-factor, i.e., the ratio of the transition frequency to the line width being 2×1014:78≈ 1012), which is several tenfold lower than the frequency shift of the vibronic spectrum caused by the momentum of the absorbed or emitted photon of optical range. We estimate the position, intensity, and width of zero-phonon lines under conditions when these characteristics, as well as properties of the phonon wing, are determined not only by Stokes losses but also by the photon recoil effect. It is shown that the latter effect, within the approximation used, does not lead to strong and easily detectable phenomena. They, however, take place, and their study is becoming more topical with improving accuracy and sophistication of the experimental technique (in the photon echo configurations, etc.).

Influence of the Optical Anisotropy of a Crystalline Host on the Radiative Line Width of a Single Impurity Molecule

Influence of the Optical Anisotropy of a Crystalline Host on the Radiative Line Width of a Single Impurity Molecule

Influence of the Optical Anisotropy of a Crystalline Host on the Radiative Line Width of a Single Impurity Molecule

Influence of the Optical Anisotropy of a Crystalline Host on the Radiative Line Width of a Single Impurity Molecule

Influence of the Optical Anisotropy of a Crystalline Host on the Radiative Line Width of a Single Impurity Molecule

Influence of the Optical Anisotropy of a Crystalline Host on the Radiative Line Width of a Single Impurity Molecule

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.

Stochastic dynamics of a single impurity molecule from the viewpoint of continuous measurement theory

Stochastic dynamics of a laser-driven four-level system serving as a model for a single guest chromophore molecule in an amorphous polymer host is studied. The dynamics of ``slow'' transitions (spectral jumps), accompanied by instantaneous changes in the fluorescence frequency, is simulated on the basis of continuous measurement theory. It is shown that there is an opportunity to control the statistics of ``bright'' and ``dark'' periods by using laser fields to drive molecules having different tunneling rates for the ground and excited electronic states. The effect of population cycling in a four-level system and related effects of local heating or cooling of the environment are discussed.

Single-molecule nanophotonics in solids

Single-molecule nanophotonics refers to optical studies of the statics and dynamics of single, individual molecules in condensed matter. Optical spectroscopy of single guest impurity molecules in solids provides an exquisitely sensitive probe of the structure and dynamics of the specific local environment around the single molecule (the condensed matter ‘nanoenvironment’). Measurement of the second-order correlation function of the emitted light g (2) shows strong photon antibunching, proving that exactly one molecule is in resonance. Other interesting effects such as two-level-system-induced spectral shifting and AC Stark effects can also be observed at the single molecule level.

A normal-mode study of a polymer glass containing a chromophore impurity

We examine the vibrational density of states of atomistic models of polypropylene glasses containing a single impurity molecule of s-tetrazine. We discuss existing methods and develop new ones to achieve significant data reduction and navigate through the complex spectrum of the normal modes of the glass. By calculating the participation ratio, the distribution of the kinetic energy of each mode on the atomic coordinates, and a mode-proximity index to the solute it is possible to identify impurity-related, polymer-related, and mixed modes and assess their relative contributions to the vibrational density of states. Activation of specific modes using molecular dynamics allows the observation of anharmonicities in the doped glass, even at very low temperatures.

Probing nanoenvironments in solids and quantum optics using individual impurity molecules

Optical spectroscopy of single guest impurity molecules in solids provides an exquisitely sensitive probe of the structure and dynamics of the specific local environment around the single molecule (the condensed matter “nanoenvironment”). Measurement of the second-order correlation function of the emitted light g (2) shows strong photon antibunching, proving that exactly one molecule is in resonance, and that quantum optical experiments with single molecules in solids are now a reality.

Structure and Dynamics in Solids As Probed by Optical Spectroscopy

Optical spectroscopy provides a useful tool for probing the structure and dynamics of solids. Over the years, optical methods have evolved from absorption spectroscopy to more complex methods such as spectral hole burning, photon echoes, and luminescence line narrowing. The absorption spectrum provides information about inhomogeneous broadening, while the latter approaches provide information about the systematics of the homogeneous line shapeinformation that is often obscured by inhomogeneous broadening. When external perturbations such as strain or electric fields are applied, details of the local site can be obtained. Recently, the ultimate limit of optical spectroscopy has been attained, in which the spectral properties of individual single impurity molecules in a solid can be measured, with all ensemble averaging removed. This novel regime has provided additional, previously unobtainable, information on a highly local scale, such as the direct observation of the spectral shifting of a single molecule produced by configurational transitions of the host. It is to be expected that further surprises await us in this new frontier of single-molecule spectroscopy in solids.

Zero-phonon lines as the cornerstone of single impurity molecule spectroscopy and persistent spectral hole burning data storage and processing

Zero-phonon lines as the cornerstone of single impurity molecule spectroscopy and persistent spectral hole burning data storage and processing