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Angle-Resolved Coherent Optical Wave-Mixing (ARC)

A schematic of the ARC apparatus

Introduction

Coherent optical four-wave mixing has been used widely to obtain information on the time scales of molecular energy transfer. However, the interpretation of these measurements can be challenging, in particular for complex multi-level systems. ARC is a novel nonlinear optical method for which this information is obtained in a single projection image, without the need of post-processing. In addition, ARC can distinguish between energy transfers and coherently coupled transitions in a very intuitive manner. This method was first demonstrated with the B800 and B850 pigments of the light harvesting complex II (LH2) from purple bacterias expressed and purified by Codgell and colleagues at Glasgow Biomedical Research Centre, Glasgow University(Cogdell et. al., Q. Rev. Biophys. 39,227). Photosynthetic systems are remarkably efficient in transporting energy, and analysing their energy transport gives important insight into the underlying quantum effects.
The ARC techniques was invented by Dr. Ian Mercer from the School of Physics at University College Dublin. The first experiment was done at RAL using hollow fibre pulse compression transferred there from this project (Mercer et. al., PRL, 2009, 102 5). In collaboration with Dr. Mercer, an ARC setup has now been established at Imperial College.
The research was covered on BBC News Online

Experimental Setup

Fig. 1: The optical layout of the ARC system also showing the box geometry at the camera plane
The optical layout is shown in Fig1. The laser used for measurements on photosynthetic systems has to be broadband enough excite the B800 and B850 pigments. Here, the output of our few-cylce laser source was used that produces a spectral bandwidth ranging from below 600nm to above 900nm. A diffractive optics (DO) splits up the beam and three first order beams are guided through a telescope that reimages the DO on the sample plane. The relative tilts can be changed by the second set of focusing mirrors. An f-to-f setup was then used to image the angles onto a CCD camera. The use of collimated beams with radius of a few mm allows to use much higher energies and permits detection of the emitted signal with high angular resolution.
The direction of the signal is given by momentum conservation. In the transient grating sequence, where only one beam is delayed with respect to the other two, the map on the camera can be readily interpreted by looking at horizontal and vertical displacements. A vertical displacement results from a difference in the interaction frequencies of beams 1 and 3, corresponding to an energy transfer. This is completely decoupled from a horizontal deviation in signal corresponding to a coherent excitation of two coupled single electron transitions.

Current research

Currently the ARC setup at our labs are being use for strain comparison of two light harvesting molecules -rhodopseudomoas acidophila strain 10050 and rhodopseudomoas palustris 2.1.6. Investigating these will help understand the subtle differences in energy transport in these light harvesting complexes. Keep coming back to see the latest results of this ongoing research.