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Journal of the Optical Society of America B

Optical spectra and kinetics of single impurity molecules in a polymer: spectral diffusion and persistent spectral hole burning

Th. Basché, W. Patrick Ambrose, W. E. Moerner

Published May 1, 1992• 102 citations

Abstract

With high-efficiency fluorescence excitation techniques optical spectra of single impurity molecules of perylene in a polyethylene matrix can be obtained at 1.5 K. Analysis of such spectra shows a variety of spectral diffusion effects, including fast (<2 s) resonance frequency changes on the 1–100-MHz scale, which lead to a range of apparent linewidths, as well as discontinuous jumps in the resonance frequency of 10–1000 MHz on a longer time scale. In addition, light-induced changes in the resonance frequency of a single molecule (persistent spectral hole burning) have been conclusively observed by showing that the burning time decreases with increased laser power. Surprisingly, hole-burned single molecules often spontaneously return to the original frequency in 1–100 s. Measurements of the burning time for a large number of hole-burning events for the same single molecule yield an exponential burn-time distribution, which is the first direct measurement to our knowledge of the stochastic kinetics of a single molecule. Analysis of the signal-to-noise function appropriate to these experiments gives the conditions under which other systems may permit single-molecule detection: strong absorption, high fluorescence yield, weak bottlenecks in the optical pumping process, and low hole-burning quantum efficiency.

DOI: 10.1364/josab.9.000829

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Recent advances in the optical detection and spectroscopy of single impurity centers in crystals has provided the possibility of obtaining information about defect-solid interactions on a truly local level. For the system composed of pentacene impurity molecules in the crystal p-terphenyl, single molecules have been studied using both absorption1 and fluorescence excitation2 techniques at liquid helium temperatures. The superior signal-to-noise of the latter technique has led to direct observations of the lifetime-limited Lorentzian homogeneous profile of a single pentacene impurity3 as well as the surprising observation of spontaneous spectral diffusion4. Spectral diffusion, or changes in the resonance frequency of an impurity molecule with time as a result of low-energy excitations (two-level system transitions (TLS)),5 is generally expected in amorphous hosts. Indeed, the presence of TLSs in amorphous solids is intimately connected with nonphotochemical mechanisms for persistent spectral hole-burning (PSHB) in amorphous materials6-8. Using perylene impurity molecules in poly(ethylene), we have observed the optical spectra of single molecules in a polymeric host for the first time. At 1.5K, individual perylene molecules show the expected spectral diffusion; moreover, we observe light-induced changes in resonance frequency, i.e., persistent spectral hole-burning, which allows one to envision optical storage on the single-molecule level.

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.

Fluorescence excitation spectra of single terrylene molecules in p-terphenyl crystals were investigated at low temperatures. The line shifts due to hydrostatic pressure and external electric fields were measured, with the electric field strength oriented either parallel or perpendicular to the k vector of the laser light. A linear Stark effect was found for most molecules, indicating local environments of low symmetry. In sites X1 and X3 (and XY, a photoproduct state), spectral hole burning was observed. Whereas the holes are stable in sites X1 and XY, they decay with a time constant of some minutes in X3. Pressure experiments were performed on the stable hole spectra in sites X1 and XY. In weakly doped samples, holes composed of very few molecular lines were generated with halfwidths distinctly narrower than twice the homogeneous linewidth.

The concept of the homogeneous spectral linewidth of an impurity center is more complicated in disordered media such as organic glasses and polymers than it is in crystals. In amorphous media the local environments for impurity centers are continuously changing with time. As a result transition frequencies of individual centers diffuse within the inhomogeneous broadened spectral contour, which is called spectral diffusion. When the timescale for spectral diffusion processes is much longer than the dephasing time, it is possible to distinguish homogeneous dephasing from spectral diffusion. On the other hand, when the timescales of spectral diffusion and dephasing processes are comparable these phenomena become indistinguishable.

The use of persistent spectral hole burning in optical data storage allows an increase of the storage density up to a factor of 1000 compared to the common spatial matrix of optical bits.1 The information is stored by burning at low temperature a sequence of narrow spectral holes in the inhomogeneously broadened optical transition of optical centers, e.g. dye molecules embedded in an amorphous polymer, with a tunable narrow-bandwidth laser. One bit of information is encoded by the presence or absence of a hole at a given location in frequency space. The information is read by observing the absorption or the fluorescence of the storage material using a laser with tunable frequency. One of the problems of spectral hole burning data storage arises from the fact that for addressing the memory locations the laser frequency has to be changed over a large frequency range with high accuracy and speed.

Recent advances in absorption1 and fluorescence2 detection now allow high-quality measurement of the optical absorption spectrum of individual single-impurity molecules in both crystals and polymers at liquid-helium temperatures. Here, the solid may be viewed as a trap for the defect center similar to the well-known electromagnetic single-ion traps, except that Doppler and recoil effects are replaced by (static) inhomogeneous broadening and host-guest interactions. Most importantly, molecular rotation is absent because of steric effects at low temperatures.