Abstract Optical spectroscopy of a single impurity molecule provides the first truly local probe of host–guest interactions in doped solids. In conventional optical high‐resolution experiments with many molecules only ensemble averages of the microscopic parameters can be obtained. In the single‐molecule regime, the exquisite sensitivity of an individual dopant molecule to both the local environment and to external perturbations has been exploited in recent experiments to reveal a wealth of fascinating novel phenomena such as spectral diffusion in crystals and polymers, optical modification of a single impurity molecule (spectral hole‐burning), local field measurements at the site of a single impurity molecule, and quantum optics in a solid.
Abstract Optical spectroscopy of a single impurity molecule provides the first truly local probe of host–guest interactions in doped solids. In conventional optical high‐resolution experiments with many molecules only ensemble averages of the microscopic parameters can be obtained. In the single‐molecule regime, the exquisite sensitivity of an individual dopant molecule to both the local environment and to external perturbations has been exploited in recent experiments to reveal a wealth of fascinating novel phenomena such as spectral diffusion in crystals and polymers, optical modification of a single impurity molecule (spectral hole‐burning), local field measurements at the site of a single impurity molecule, and quantum optics in a solid.
Optical spectroscopy of single impurity molecules in solids can be used as an exquisitely sensitive probe of the structure and dynamics of the specific local environment around the single molecule (the “nanoenvironment”. Some phenomena that have been studied are the spectral diffusion dynamics of a single molecule, perturbations by external fields, changes in molecular photophysics, shifts in vibrational modes, optical modification of the absorption spectrum, dynamics due to amorphous system physics, and magnetic resonance of a single molecular spin. This demonstrates the vitality of and growing interest in this new field, which may in the future lead to optical storage on the single‐molecule level.
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 shape information 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.
Optical spectroscopy of single impurity molecules in low-temperature solids provides an exquisitely sensitive probe of the structure and dynamics of the specific local environment around the single molecule, unobscured by ensemble averaging. Effects such as spectral diffusion, perturbations by external fields, changes in molecular photophysics, optical switching, dynamics due to amorphous system physics, magnetic resonance of a single molecular spin, and perturbations due to nearby charge transport have all been observed. Because properly chosen single molecules act as two- and three-level quantum systems, a variety of quantum optical effects may be observed, such as photon antibunching, optical Stark effect, and single-photon emission on command. When they carry spin in their ground and excited states, single molecules and other nano-objects open many more experiments in quantum optics and nanophysics, because they are sensitive to magnetic interactions and can serve as readouts for spin systems with long coherence times. As a new class of nanoscale experiments, the ability to optically pump and probe single 2–3-nm-sized molecules in complex systems provides a new window into the local dynamics and heterogeneity of solids.
Abstract In recent years different microscopic and spectroscopic techniques were successfully employed to detect the fluorescence signals of single dye molecules in the condensed phase. In low temperature solids the fluorescence excitation line of a single dopant molecule is a high resolution optical probe of its local nanoenvironment. As inhomogeneous contributions and ensemble averaging are absent, many effects hidden when looking at large populations of molecules become observable. Two important research areas in single molecule spectroscopy deal with the investigation of quantum optical effects and the spectral jumping behaviour of single molecule excitation lines caused by fluctuations of two‐level systems inherent to disordered solids. Rather than delivering a complete presentation, we will focus on some selected examples which emphasize the unique features of spectroscopy with a single absorber.
Experiments with single quantum systems are at the frontier of modern physics. Probing single molecules allows for very precise control of light-matter interaction and for quantum optical studies. Moreover, a single guest molecule in a solid host matrix is a very sensitive probe of local dynamics in its nano-environment. Fluorescence excitation spectroscopy on single molecules is a powerful tool for spectroscopic characterizations at highest resolution, as information can be revealed that is not accessible in experiments on ensembles of molecules. The detection of the fluorescence signal of a single absorber is achieved by exploiting the high spectral selectivity given by the ratio (10 10) of homogeneous to inhomogeneous linewidths of optical transitions at cryogenic temperatures (zero-phonon transitions). Two-photon fluorescence excitation, a fundamental nonlinear optical process, was observed on a single quantum system [1]. Single diphenyl-octatetraene molecules trapped in an n-tetradecane matrix were excited from the 1Ag ground to the lowest excited singlet state by simultaneous absorption of two photons using a single-mode cw laser. Excited molecules were observed through weak one-photon emission resulting from a symmetry breaking induced by the crystal field. The fundamental principles and the experimental technique of single molecule spectroscopy will be introduced briefly and some particular single molecule experiments presented. This work was done in collaboration with T. Plakhotnik, A. Renn and U.P. Wild
As was recently demonstrated1 by using highly efficient fluorescence excitation spcctroscopy with N.A. = 0.98 collection optics, single impurity molecules in both crystals and polymers may now be detected with single-to-noiseratios (SNRs) as high as 30 in a 1 Hz bandwidth at low temperatures. In essence, the solid may be regarded as a trap for the single molecule, which effectively quenches rotation. Our measurements include precise determination of the lifetime limited linewidth and the temperature dependence of the optical dephasing, as well as spectral diffusion (wandering in frequency space due to perturbations by lattice degrees of freedom). For the polymeric host, light-induced changes in resonant frequency of a single molecule can be produced, a process that is normally termed spectral hole-burning when large ensembles of molecules are probed. In recent studies, nonclassical light emission from a single molecule (photon antibunching) has been observed in a fashion analogous to that reported earlier for a single atom in a trap.