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Annual Meeting Optical Society of America

Fundamental laser spectroscopy of molecules in solids: statistical fine structure

T. P. Carter, L. Kador, W. E. Moerner

Published Jan 1, 1988• 0 citations
Publisher

Abstract

Recently, we reported the first observation of the statistical fine structure (SFS) on an inhomogeneously broadened optical absorption line, the 0-0 origin transition of pentacene molecules in p-terphenyl crystals at liquid helium temperatures.1 SFS is a time-independent spectral structure resulting from number fluctuations in the spectral density of absorbers that we have detected directly using laser frequency modulation (FM) techniques. In our samples where the number of molecules per homogeneous linewidth N¯ H is large, a zero-background technique such as FM spectroscopy is optimal for detection of SFS because only the deviations of the absorption coefficient from the background value produce FM signals. In addition, for extremely low signal levels, we use double modulation with a Stark field to remove the effects of residual amplitude modulation in the modulator. Our goal is to describe the systematics and analysis of SFS spectra at a fixed low temperature, 1.6 K. For example, we show that the rms value of the SFS grows as the square root of N¯ H . More important, by computing the autocorrelation of the measured frozen noise spectra, the homogeneous linewidth can be determined to good accuracy without requiring spectral hole-burning or coherent transients. Since the source for SFS is fundamental statistical fluctuations in the absorber spectral density, SFS should occur for all inhomogeneously broadened lines; furthermore, the effect imposes a fundamental limit on the detectability of shallow spectral features in solids.

DOI
10.1364/oam.1988.thr3

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Recently, we reported the first observation of the statistical fine structure (SFS) on an inhomogeneously broadened optical absorption line, the 0-0 origin transition of pentacene molecules in p-terphenyl crystals at liquid helium temperatures.1 SFS is a time-independent spectral structure resulting from number fluctuations in the spectral density of absorbers that we have detected directly using laser frequency modulation (FM) techniques. In our samples where the number of molecules per homogeneous linewidth N¯ H is large, a zero-background technique such as FM spectroscopy is optimal for detection of SFS because only the deviations of the absorption coefficient from the background value produce FM signals. In addition, for extremely low signal levels, we use double modulation with a Stark field to remove the effects of residual amplitude modulation in the modulator. Our goal is to describe the systematics and analysis of SFS spectra at a fixed low temperature, 1.6 K. For example, we show that the rms value of the SFS grows as the square root of N¯ H . More important, by computing the autocorrelation of the measured frozen noise spectra, the homogeneous linewidth can be determined to good accuracy without requiring spectral hole-burning or coherent transients. Since the source for SFS is fundamental statistical fluctuations in the absorber spectral density, SFS should occur for all inhomogeneously broadened lines; furthermore, the effect imposes a fundamental limit on the detectability of shallow spectral features in solids.
By using laser FM spectroscopy, we have observed statistical fine structure (SFS) in the inhomogeneously broadened zero‐phonon S1←S0 (0–0) absorption of pentacene molecules in crystals of p‐terphenyl at liquid helium temperatures. SFS results from variations in the spectral density of absorbers with optical wavelength due to statistics, illustrating that the inhomogeneous line profile is not a simple, smooth Gaussian function as has been previously assumed but rather contains significant fine structure. Theoretical analysis of the SFS spectra using autocorrelation techniques can be used to estimate the underlying value of the homogeneous linewidth. This new observation not only provides a novel way to study the statistics of inhomogeneous broadening, but it also allows estimation of the homogeneous width without requiring spectral hole‐burning or coherent transient techniques.
Using laser frequency-modulation spectroscopy, we have observed statistical fine structure (SFS) in the inhomogeneously broadened optical absorption of pentacene in ppp-terphenyl at liquid-helium temperatures. SFS is time-independent structure on the inhomogeneous line profile caused by statistical variations in the spectral density of absorbers in each frequency interval. The size of the SFS varies as the square root of the number of centers, and the autocorrelation of the SFS yields an estimate of the homogeneous linewidth without the requirement of spectral hole burning or coherent transients.
Using two different double-modulation techniques, we have observed the optical-absorption spectrum of single dopant molecules of pentacene in a p-terphenyl host crystal at liquid-helium temperatures. To achieve this, frequency-modulation spectroscopy was combined either with Stark or ultrasonic modulation to remove interfering background signals from residual amplitude modulation, and the number of molecules in resonance was reduced to one by operating in the wings of the inhomogeneous line. Triplet bottleneck saturation appears to be suppressed in the single-molecule regime.
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.
Single-molecule detection and spectroscopy in solids has the potential for direct observation of the microscopic physics and chemistry of defects without multicenter averaging. Initial observations of the optical absorption spectra of a single pentacene molecule in p-terphenyl crystals achieved signal-to-noise ratios (SNR's) of 3-5 in a 1-Hz bandwidth and required intensities at the sample above the saturation intensity. Careful noise analysis of our detectors showed that little improvement in SNR could be expected with an absorption technique unless simple and reliable sources of squeezed light were available. Recent advances in fluorescence- excitation spectroscopy have removed this SNR limitation.3
The study of optical relaxation processes in solids at low temperature has progressed tremendously in the last 20 years due to the development of coherent optical techniques. The latter would not have been possible without the advent of narrow-band tunable lasers, on the one hand, and lasers with very short pulses (picoseconds, femtosecoods) on the other hand. Such coherent techniques are necessary because spectral line shapes of solids doped with guest molecules are seldom determined by dynamical interactions but principally by strain or structural disorder. This gives rise to inhomogeneous broadening, rlOh, which in crystalline hosts at low temperature varies between ' 0.1 and 10 cm -I, whereas in glasses it may amount to � 100-500 em-I. One of the techniques to achieve narrowing of spectral lines is site­ selection spectroscopy (1-3). A sub-ensemble of molecules within thc inhomogeneously broadened absorption band is selectively excited by mcans of a narrow-band laser, and the fluorescence or phosphoresccnce signal is detected with a high-resolution monochromator and/or a Fabry­ Perot interferometer. Fluorescence line-narrowing ( I) is a special case of this technique (for a review see 3). The intrinsic or homogeneous spectral linewidth, r hom, which yields information on the relaxation processes of the excited state, is given by the effective optical dephasing time, T2: