Richard S. Lepkowicz

Excited-state absorption dynamics in polymethine dyes detected by polarization-resolved pump–probe measurements

Polarization-resolved excitation-probe measurements are performed for a new series of polymethine dyes in several solvents and a polyurethane acrylate elastopolymer. We describe our experimental studies and give an analysis of the nature of the rotational motions of excited molecules and orientation of the excited-state transitions relative to transitions from the ground state.

Femtosecond-to-nanosecond nonlinear spectroscopy of polymethine molecules

The linear and nonlinear optical properties of a series of polymethine molecules are investigated to study the effects of molecular structure and the host environment on overall nonlinear absorption performance. The linear characterization includes measuring the solvatochromic shifts between absorption and fluorescence peaks and studying the excited-state orientational diffusion kinetics. The nonlinear characterization involves measuring the excited-state absorption spectra with a femtosecond white-light-continuum pump-probe technique and performing Z scans and nonlinear transmission measurements from the picosecond to the nanosecond time regimes. The results of these experiments allow us to develop an energy-level structure for the polymethines, which accurately predicts nonlinear absorption properties from the picosecond to the nanosecond time regimes. From this model we are able to identify the key molecular parameters for improved nonlinear absorption.

Picosecond absorption anisotropy of polymethine and squarylium dyes in liquid and polymeric media

Abstract Time-resolved excitation-probe polarization measurements are performed for polymethine and squarylium dyes in ethanol and an elastopolymer of polyurethane acrylate (PUA). These molecules exhibit strong excited-state absorption in the visible, which results in reverse saturable absorption (RSA). In pump–probe experiments, we observe a strong angular dependence of the RSA decay kinetics upon variation of the angle between pump and probe polarizations. The difference in absorption anisotropy kinetics in ethanol and PUA is detected and analyzed. Anisotropy decay curves in ethanol follow a single exponential decay leading to complete depolarization of the excited state. We also observe complete depolarization in PUA, in which case the anisotropy decay follows a double exponential behavior. Possible rotations in the PUA polymeric matrix are connected with the existence of local microcavities of free volume. We believe that the fast decay component is connected with the rotation of molecular fragments and the slower decay component is connected with the rotation of entire molecules in local microcavities, which is possible because of the elasticity of the polymeric material.

Femtosecond to nanosecond characterization of the excited-state properties of polymethine molecules

Saturation limits excited-state absorption (ESA) in many organic molecules. We studied polymethines using continuum spectroscopy to determine ESA spectra, two-color pump-probe anisotropy for transition dipole moment orientations, picosecond and nanosecond z-scans and nonlinear transmission measurements.

Orientation of transition moments in excited-state absorption of polymethine and squarylium dyes

Summary form only given. Over the past several years our efforts have been directed toward a systematic investigation of nonlinear properties of a new series of polymethine dyes (PDs) and squarylium dyes (SDs) in liquid solutions and polymeric media. These studies led to the development of several new dyes showing strong excited-state absorption (ESA) in the visible region with the ratios of excited-to ground-state absorption cross-sections up to 200 at 532 nm. One of these dyes (labeled PD2338) chemical structure is shown along with its singlet energy-level diagram. The study of the excited state transition moments will lead to a deeper understanding of the nature of ESA in organic molecules and will aid in the quantum-chemical calculations for the prediction of their nonlinear properties. To measure the orientation of the transition moments we use a pump-probe geometry in which the probe polarization is fixed and the pump polarization can be rotated using a halfwave plate. In a pump-probe experiment an intense pump beam induces a nonlinear response in the medium that is then sensed by the weak probe beam. This experimental procedure may be used to determine population decay kinetics and rotational relaxation times. These decay times can be measured by conducting three measurements for the pump polarization-parallel, perpendicular, and at the magic angle (57.3 degrees) to the probe. The results for PD2338, using the second harmonic of a picosecond (25 ps FWHM) Nd:YAG laser are shown.

Excited state dynamics in polymethine molecules detected by polarization-resolved pump-probe measurements

Excited-state absorption of polymethine dyes in various media is measured using polarization-resolved, picosecond and femtosecond pump-probe techniques. These measurements yield information about decay and reorientation times, solvent effects, and orientation of excited-state transition moments.