Low Temperature Photon Echo Measurements of Organic Dyes in Thin Polymer Films Richard Metzler ‘06, Eliza Blair ‘07, and Carl Grossman, Department of Physics and Astronomy, Swarthmore College; Abstract The relaxation of optically excited molecules to their ground states is characterized by two quantities: the optical relaxation time T 1 and the optical dephasing time T 2. While T 1 is characteristic of the molecule and tells us something about the transition time to its ground state, T 2 is the time constant corresponding to the decay of the coherent superposition of ground and excited states after optical excitation. Thus, T 2 can be thought of as a measure of the influence of the local environment on a molecule’s optical state and should have a strong dependence on temperature. Measurements at varying temperatures are performed using photon echoes on on bulk samples are used to extract this dependence. Comparison of this data with previous data taken using scanning confocal microscopy should provide insight in ultra-fast molecular processes. 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. Dyes used as samples for the echo experiment were dissolved in thin PMMA (polymer) films. Other dyes in with peak emissions on the red side a sample’s peak absorption were pumped with a YAG laser to produce large dephasing times. Sample Fabrication Procedure: Dissolve polymer and dye in chlorobenzene, filter and drop on sapphire slide. Air dry over night in a clean room as annealing can produce ripples in the film causing unwanted optical artifacts. Absorption spectra of various dyes (right). Photon Echo Theory Echo Data Acknowledgements Thank you to our awesome advisor Carl Grossman, Ed from the Vietnamese Place, the Howard Hughes Medical Institute, and Swarthmore College. Analysis & Conclusions 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). 12 Sept Temperature Dependence The photon echo results from the 3rd order perturbation expansion of the interaction term H I of the total molecular Hamiltonian. H I represents the first incident beam which then interacts with the delayed beam to produce the correlation signal that we measure. Classically, we have induced an oscillating dipole in the sample and are measuring its phase information through use of nonlinear optical processes with a second delayed beam. We approximate the total wavefuction as a sum of two states: the ground state |1>, and a total excited state |2>, which in actuality is grouping of all distinct excited states. From the Schrödinger we obtain the effect on the probability amplitudes of the wavefunction: Through some manipulation we find the inversion and the density of the mixed-state terms where the damped terms are added from phenomenological observation. Solution to these coupled equations to third order and correlation with the delayed beam give us the total intensity with respect to the delay time tau: Of interest are two of the resulting terms from the total intensity. These beams the form of two identically shaped beams of different trajectory delayed by a time 2T 2 - hence the term “photon echo.” It is these which we measure in the lab. Nile Blue’s Dephasing Time vs. temperature