Jennifer P. Ogilvie

Vibronic coherence in oxygenic photosynthesis

Photosynthesis powers life on our planet. The basic photosynthetic architecture consists of antenna complexes that harvest solar energy and reaction centres that convert the energy into stable separated charge. In oxygenic photosynthesis, the initial charge separation occurs in the photosystem II reaction centre, the only known natural enzyme that uses solar energy to split water. Both energy transfer and charge separation in photosynthesis are rapid events with high quantum efficiencies. In recent nonlinear spectroscopic experiments, long-lived coherences have been observed in photosynthetic antenna complexes, and theoretical work suggests that they reflect underlying electronic-vibrational resonances, which may play a functional role in enhancing energy transfer. Here, we report the observation of coherent dynamics persisting on a picosecond timescale at 77 K in the photosystem II reaction centre using two-dimensional electronic spectroscopy. Supporting simulations suggest that the coherences are of a mixed electronic-vibrational (vibronic) nature and may enhance the rate of charge separation in oxygenic photosynthesis.

Use of coherent control for selective two-photon fluorescence microscopy in live organisms

We demonstrate selective fluorescence We demonstrate selective fluorescence excitation of specific molecular species in live organisms by using coherent control of two-photon excitation. We have acquired quasi-simultaneous images in live fluorescently-labeled Drosophila embryos by rapid switching between appropriate pulse shapes. Linear combinations of these images demonstrate that a high degree of fluorophore selectivity is attainable through phase-shaping. Broadband phase-shaped excitation opens up new possibilities for single-laser, multiplex, in-vivo fluorescence microscopy.

Two-color two-dimensional Fourier transform electronic spectroscopy with a pulse-shaper.

We report two-color two-dimensional Fourier transform electronic spectroscopy obtained using an acousto-optic pulse-shaper in a pump-probe geometry. The two-color setup will facilitate the study of energy transfer between electronic transitions that are widely separated in energy. We demonstrate the method at visible wavelengths on the laser dye LDS750 in acetonitrile. We discuss phase-cycling and polarization schemes to optimize the signal-to-noise ratio in the pump-probe geometry. We also demonstrate that phase-cycling can be used to separate rephasing and nonrephasing signal components.

Using coherence to enhance function in chemical and biophysical systems

Coherence phenomena arise from interference, or the addition, of wave-like amplitudes with fixed phase differences. Although coherence has been shown to yield transformative ways for improving function, advances have been confined to pristine matter and coherence was considered fragile. However, recent evidence of coherence in chemical and biological systems suggests that the phenomena are robust and can survive in the face of disorder and noise. Here we survey the state of recent discoveries, present viewpoints that suggest that coherence can be used in complex chemical systems, and discuss the role of coherence as a design element in realizing function.

Fourier-transform coherent anti-Stokes Raman scattering microscopy

We report a novel Fourier-transform-based implementation of coherent anti-Stokes Raman scattering (CARS) microscopy. The method employs a single femtosecond laser source and a Michelson interferometer to create two pulse replicas that are fed into a scanning multiphoton microscope. By varying the time delay between the pulses, we time-resolve the CARS signal, permitting easy removal of the nonresonant background while providing high resolution, spectrally resolved images of CARS modes over the laser bandwidth (approximately 1500 cm(-1)). We demonstrate the method by imaging polystyrene beads in solvent.

Experimental implementations of two-dimensional fourier transform electronic spectroscopy.

Two-dimensional electronic spectroscopy (2DES) reveals connections between an optical excitation at a given frequency and the signals it creates over a wide range of frequencies. These connections, manifested as cross-peak locations and their lineshapes, reflect the underlying electronic and vibrational structure of the system under study. How these spectroscopic signatures evolve in time reveals the system dynamics and provides a detailed picture of coherent and incoherent processes. 2DES is rapidly maturing and has already found numerous applications, including studies of photosynthetic energy transfer and photochemical reactions and many-body interactions in nanostructured materials. Many systems of interest contain electronic transitions spanning the ultraviolet to the near infrared and beyond. Most 2DES measurements to date have explored a relatively small frequency range. We discuss the challenges of implementing 2DES and compare and contrast different approaches in terms of their information content, ease of implementation, and potential for broadband measurements.

Probing Photosynthetic Energy and Charge Transfer with Two-Dimensional Electronic Spectroscopy.

Two-dimensional electronic spectroscopy (2DES) has emerged as a powerful method for elucidating the structure-function relationship in photosynthetic systems. In this Perspective, we discuss features of two-dimensional spectroscopy that make it highly suited to address questions about the underlying electronic structure that guides energy- and charge-transfer processes in light-harvesting materials. We briefly describe a pulse-shaping-based implementation of two-dimensional spectroscopy that is making the method widely accessible to problems spanning frequency regimes from the ultraviolet to the mid-infrared. We illustrate the utility of 2DES in the context of our recent studies of the primary energy-transfer and charge separation events in the photosystem II reaction center, discussing remaining challenges and speculating about exciting future directions for the field of multidimensional spectroscopy.

Chapter 5 Multidimensional Electronic and Vibrational Spectroscopy: An Ultrafast Probe of Molecular Relaxation and Reaction Dynamics

Abstract Multidimensional optical spectroscopy in the visible and infrared is a rapidly developing technique enabling direct observation of complex dynamics of molecules in complex environments such as liquids and proteins. Measuring the correlation between excited and detected frequencies with sub‐picosecond resolution has enabled the resolution of long‐standing problems such as energy transfer in photosynthesis and the life‐sustaining structural rearrangements of liquid water. This chapter aims to provide a bridge between the concepts familiar in the AMO physics community and how those ideas and experimental methods are applied to condensed phase molecular spectroscopy. We outline the technical challenges of these powerful methods while considering a few examples of experiments that showcase the unique perspective offered by 2D electronic and vibrational spectroscopy.

Fourier transform measurement of two-photon excitation spectra: applications to microscopy and optimal control.

We report a novel Fourier transform method for measuring two-photon excitation spectra. We demonstrate this method using simple dye molecules and discuss its applications in two-photon fluorescence microscopy and optimal control. This method facilitates an intuitive interpretation of recent control experiments in terms of tuning the nonlinear spectrum of the exciting laser source.

Two-dimensional electronic spectroscopy with a continuum probe

We report 2D Fourier transform electronic spectroscopy obtained in the pump-probe geometry using a continuum probe. An acousto-optic pulse shaper placed in the pump arm of a standard pump-continuum probe experiment permits 2D spectroscopy that probes a broad spectral range. We demonstrate the method on a simple dye system exhibiting vibrational wavepacket dynamics that modulate the peak shapes of the 2D spectra. The broad spectral range of the continuum probe allows us to observe vibronic cross peaks in the 2D spectra.