Abstract

1. Photon Echo and Single Molecule Fluorescence Measurements of Organic Dyes in Thin Polymer Films Robin Smith ‘03 and Carl Grossman, Department of Physics and Astronomy, Swarthmore College François Treussart and Jean-François Roch, Laboratoire de Photonique Quantique et Moléculaire, Ecole Normale Supérieure de Cachan, France Abstract Optical dephasing is the decay of the coherent superposition of ground and excited molecular states after optical excitation. Comparison of dephasing times in single-molecule and bulk samples as a function of temperature gives insight into molecular processes that occur on ultrashort time scales. Scanning confocal microscopy can be used to directly observe fluorescence light emission from single organic dye molecules in thin polymer films. Fluorescence intensity is plotted as a function of emission wavelength; the reciprocal of the width of the resulting spectral profile gives the dephasing time. Photon antibunching exhibited by the photon-pair correlation function is monitored to confirm that single molecule emission spectra are obtained. Photon echoes are used to measure dephasing times in bulk samples. Scattered light intensity is recorded as a function of time delay between excitation and probe light pulses. The time delay is twice the dephasing time. In the experiments we performed, dephasing times of bulk Rhodamine-640 approach 10 fs at room temperature, in agreement with single molecule studies.

2. Dye Selection and Sample Fabrication Dyes were chosen for their known bulk absorption and emission wavelength ranges and molecular structure. The following dyes were studied in the SMF experiment: Rhodamine-640, Nile Blue, Disperse Red 1, Disperse Red 11, Nile Red, Rhodamine-101, Styryl-7, Bodipy, and Cyanine in PMMA (polymer) films. The first four dyes were used in the photon echo experiment. Sample Fabrication Procedure: Dissolve polymer and dye in chlorobenzene, spin-coat solution onto microscope slide, and anneal in drying oven. Thin-film samples were fabricated in these clean room facilities at ENS Cachan (right). Photon Echo Experiment * The experimental technique uses two-beam, time-delayed degenerate four wave mixing (DFWM) with incoherent light. * Dephasing lifetimes were measured directly as a function of temperature. A plot of Scattered Intensity vs. Time Delay for Rhodamine-640 at 40 K is shown at left below. Peak shift is twice the dephasing time (T. Kobayashi et al, Applied Physics B, 47, 107, 1988). This peak shift of 46.5 fs gives a dephasing time of ~23 fs. Dephasing times approach ~10 fs for extrapolation to room temperature for R-640 and Nile Blue (middle and right below, respectively).

3. SMF Experiment: Scanning Confocal Microscopy This schematic depicts our single molecule fluorescence experimental setup. Green argon ion laser light was fed into a confocal microscope. A dilute spin-coated slide of thickness 5-nm with trace amounts of organic dye dissolved in PMMA polymer in chlorobenzene was placed on the piezoelectric stage (PZT). The polymer was tested for residual background fluorescence, which was low compared signal. We looked for molecules by first adjusting the z-position height of the stage to find the layer of molecules and then running xy-lateral scans through a Labview interface to find single molecules. We used 565 and 532 nm dichroic mirrors (DM) to direct excitation light toward the sample and permit transmission only of longer-wavelength fluorescence. We focused light from the sample onto a pinhole (PH), the confocal part. And a notch filter (NF) again cut out the laser excitation wavelength. Correlation measurements (coincidence count detection) were performed using a Hanbury-Brown-Twiss interferometry setup: a beam splitter (BS) divided and directed fluorescence emission toward two avalanche pthotodiodes (APD’s). Not shown here is the second beam splitter (between NF and BS) that sent fluorescence emission to a Spex spectrophotometer (a thermoelectrically cooled CCD camera) for spectrum acquisition. Photon Antibunching The photon-pair correlation function is the normalized distribution of photon pairs separated by time delay tau. For a single emitter, one photon is emitted at a time. Before a second photon can be emitted, the molecule must be excited again. Therefore, the probability of photons being simultaneously detected by both APD’s is small. So we expect antibunching: the correlation between detected photons as a function of time delay to dip near zero for zero time delay between APD detection events.

4. Time Intensity Plots and Single-Molecule Emission Spectrum Fluorescence of several molecules R-640 was being detected. Fluorescence of two R-640 molecules was detected: when the first bleached (stopped fluorescing), the remaining molecule blinked. This emission spectrum for R-640 was fit to a Lorentzian lineshape function. The reciprocal of the FWHM gives the dephasing time. Peak Wavelength + Dephasing Time Distributions Nile Blue: highly dispersed emission wavelength peaks, short dephasing time, low photostability Rhodamine-640: narrow peak emission wavelength distribution, short dephasing time, high fluorescence yield, high photostability

5. Conclusions * Single-molecule photostability and fluorescence yield are related to the width of the peak emission wavelength distribution. Possible causes: Molecule - environment interaction Effect of local environment variation on molecular configuration Homogeneous broadening Spectral Diffusion (molecule motion) Observed effects: Photostability Fluorescence Yield Peak Emission Wavelength Distribution Dephasing Time * Single NB and R-640 molecules have dephasing times on the order of 10 fs at room temperature. * Dephasing time is inversely related to temperature for bulk samples of NB and R-640 from 10 to 150 K. * Ultrashort dephasing times for bulk samples by extrapolation agree with new single-molecule results making spectral diffusion unlikely and suggesting phonon-assisted variations in local environment (homogeneous broadening). Single terrylene molecule in p-terphenyl. Future Directions * Measure bulk sample dephasing times at temperatures above 150 K. * Perform photon echo measurements on Nile Red since it exhibits high single-molecule photostability. * Investigate systematically the photostability of dyes: consider possible effects on bleaching and blinking including dye-polymer phonon interactions, excitation laser power, and oxygen-gas environment. Scattered Intensity vs. Time Delay for Nile Blue at 310 K

6. * Perform single molecule fluorescence measurements at low temperatures to better under stand single-molecule stability. * Control single-molecule orientation using Langmuir-Blodgett film deposition. * Study other systems as possible single-photon sources…including Nitrogen-Vacancy color centers in diamond. Acknowledgements Thank you to my awesome advisor Carl Grossman,François Treussart and Jean-François Roch for collaboration on SMF, Peter Collings, Romain Alléaume, Elliot Reed ‘03, Daniel Sproul ‘03, Benjamin Bau, and Steve Palmer. Funding: American Chemical Society, Howard Hughes Medical Institute, and Swarthmore College