David C. Arnett

Optical dephasing on femtosecond time scales: Direct measurement and calculation from solvent spectral densities

The connection between dephasing of optical coherence and the measured spectral density of the pure solvent is made through measurements and calculations of photon echo signals. 2‐pulse photon echo measurements of a cyanine dye in polar solvents are presented. Signals are recorded for both phase matched directions enabling accurate determination of the echo signal time shift. Echo signals are calculated by two approaches that employ the response function description of nonlinear spectroscopy; (i) a single Brownian oscillator line shape model, and (ii) the line shape obtained using the solvent spectral density. The strongly overdamped Brownian oscillator model incorporates only a single adjustable parameter while the experimental data present two fitting constraints. The second model incorporates the measured solvent spectral density. Both give very good agreement with the experimental results. The significance of the second method lies in this being a new approach to calculate nonlinear spectroscopic sign...

Time-gated photon echo spectroscopy in liquids

Abstract Optical dephasing of a cyanine dye chromophore in solution is investigated by time-gated photon echo spectroscopy in a three-pulse scattering geometry. The third-order polarization response is gated by up-conversion with a reference pulse in a nonlinear optical crystal. Time gating establishes two controllable periods of system evolution in an optical coherence. The measurements are compared with numerical simulations employing the measured OKE spectrum of solvent (butanol, DMSO) fluctuations. The time-shift of the time-gated echo signals for several gating forming pulse time delays agrees with simulation while the narrower simulated widths suggest a frequency-dependent solute-solvent interaction and the consideration of chromophore vibronic transitions.

The solvent spectral density and vibrational multimode approach to optical dephasing: Two-pulse photon echo response

A rigorous theoretical connection between the polarizability spectral density obtained from optical Kerr effect (OKE) measurement with the correlation function describing solvent‐induced optical dephasing detected in photon echo measurements is given. The experimentally obtained spectral density has a more reasonable physical basis than model correlation function descriptions of solvent fluctuations. The experimental OKE spectrum is demonstrated to provide a natural description of solvent motions that modulate the chromophore electronic states in the case of weak induced‐dipolar interactions. The chromophore optically active vibrational modes are obtained from pump‐probe spectra and are employed in the calculation of echo signals. It is found that the fast decays of the two‐pulse echo signals result from both solvent and solute intramolecular motions while the echo peak shifts are dominated by the solvent intermolecular modes.

Femtosecond-pulse cavity-dumped solid-state oscillator design and application to ultrafast microscopy

The construction, modeling, and performance characteristics of a new resonator design for ultrafast cavity-dumped oscillators are presented. An acousto-optic Bragg cell was incorporated at the end of the longer arm of a Ti:sapphire oscillator rather than in the shorter arm as in several recent studies. The new arrangement improves the pulse intensity stability of the oscillator and significantly reduces the effort required in construction. The experimental findings are supported by comparison of the stability regions of the laser cavities based on the two different designs. To demonstrate the potential of cavity-dumped oscillators for spatially resolved ultrafast spectroscopy studies, the pulse duration is characterized at the focal plane of two achromatic high-N.A. oil-immersion objectives with different amounts of flat-field correction. Transform-limited pulse widths as short as 15 fs are obtained. To our knowledge, this is the shortest pulse duration measured with true high-N.A. (N.A. > 1) focusing conditions.

Optical pump-terahertz probe spectroscopy utilizing a cavity-dumped oscillator-driven terahertz spectrometer

A terahertz spectrometer capable of steady-state and time-resolved measurements over the 0.1-3.5-THz spectral region has been built. This spectrometer routinely produces and detects terahertz pulses that exhibit signal-to-noise ratios (SNRs) greater than 6000 in the time domain and a spectral noise door of magnitude 2.7/spl times/10/sup -4/. Hence, the spectrometer achieves nearly four decades of dynamic range in the frequency domain. Two pulse generation processes are observed to give rise to the measured terahertz pulse. High-quality optical pump-terahertz probe data on GaAs samples are presented, demonstrating the applicability of this spectrometer to the study of optically induced dynamical processes. Non-Drude relaxation behavior is observed in the transient terahertz spectra.

Tunable excitation source for coherent Raman spectroscopy based on a single fiber laser

We demonstrate a wavelength tunable optical excitation source for coherent Raman scattering (CRS) spectroscopy based on a single femtosecond fiber laser. Electrically controlled wavelength tuning of Stokes optical pulses was achieved with soliton self frequency shift in an optical fiber, and linear frequency chirping was applied to both the pump and the Stokes waves to significantly improve the spectral resolution. The coherent anti-Stokes Raman scattering (CARS) spectrum of cyclohexane was measured and vibrational resonant Raman peaks separated by 70u2009cm(-1) were clearly resolved. Single laser-based tunable excitation may greatly simplify CRS measurements and extend the practicality of CRS microscopy.

Fluorescence quenching studies of structure and dynamics in calmodulin–eNOS complexes

Activation of endothelial nitric oxide synthase (eNOS) by calmodulin (CaM) facilitates formation of a sequence of conformational states that is not well understood. Fluorescence decays of fluorescently labeled CaM bound to eNOS reveal four distinct conformational states and single‐molecule fluorescence trajectories show multiple fluorescence states with transitions between states occurring on time scales of milliseconds to seconds. A model is proposed relating fluorescence quenching states to enzyme conformations. Specifically, we propose that the most highly quenched state corresponds to CaM docked to an oxygenase domain of the enzyme. In single‐molecule trajectories, this state occurs with time lags consistent with the oxygenase activity of the enzyme.

Ultrafast studies of exciton dynamics in light-harvesting dimers

The electronic characteristics and rapid dynamics associated with bacteriochlorophyll dimers in photosynthetic systems are investigated using novel ultrafast anisotropy techniques. The excitonic structure of isolated subunits (B820) from the core (LH-1) light harvesting complex of Rs. Rubrum and the reassociated complex (B873) is revealed in coherent anisotropy responses following impulsive excitation. For B820, the oscillatory anisotropy responses indicate excitonic splitting frequencies ranging from 370 cm-1 to 490 cm-1 indicating significant inhomogeneity in the excitonic spectrum. The complete set of wavelength dependent anisotropy result is analyzed to reveal the sources of inhomogeneity for B820; correlated distributions of dimer energetic parameters are necessary to reproduce the results. The coherent response of reassociated B873 complexes exhibit multiple frequencies, revealing the extended excitonic structure of the aggregates. In other experiments, the electronic properties of the photosynthetic reaction center and the rapid electronic dynamics prior to charge separation are investigated by both two color anisotropy. A variety of excitation and detection conditions provide the first room temperature characterization of the excitonic structure of the special pair, and a detailed description of rapid energy transfer to and within the special pair. The results presented here stress the importance of delocalized excitation in both the light harvesting antenna complexes and the photosynthetic reaction center.

Charge-Transfer Dynamics in Blue Copper Proteins: Experiment and Simulation

Pump-probe and classical dynamics simulation results for electron transfer in blue copper proteins are reported.

Optical Coherence and Anisotropy Studies of the First Events in Photosynthesis

Femtosecond optical coherence and anisotropy measurements are reported for the photosynthetic reaction center (Rb. Sph) revealing rapid electronic dephasing of both P* and B*, and novel polarization responses.