Two Dimensional Electronic Spectroscopy

The techniques of multidimensional Fourier transform spectroscopy have revolutionized nuclear magnetic resonance (NMR), making it an invaluable tool for determining high resolution structures of complicated biomolecules. NMR can also study dynamics, but on limited time scales. Two dimensional electronic spectroscopy (2DES) is the optical analog of the simplest multidimensional NMR experiment. In this technique, an optical "pump" pulse excites the sample, setting its charges in motion at their natural oscillation frequency. Depending on what this frequency is, a second "pump" pulse, arriving a delay t1 later, may enhance or suppress the initial oscillation. After a second delay t2 , during which the sample can relax, a third "probe" pulse excites the sample and the emitted "echo" is recorded as a function of time t3.

Fourier transforming with respect to the t1 and t3 variables yields a 2D spectrum at each time delay t2. In analogy to 2D NMR, the 2D spectrum reveals the coupling between electronic transitions, which appear as cross-peaks. The underlying lineshapes are also revealed in the 2D spectrum, free from inhomogeneous broadening. Broadly tunable pump and probe pulses give access to a wide range of transitions and their couplings. In addition, 2DES is a very general spectroscopy: all other third order nonlinear spectroscopies are a subset of 2DES.

2DES is an exciting new method for studying energy and charge flow in biological systems. The electronic spectra of many biological systems are difficult to study with linear spectroscopic techniques, which cannot distinguish between spectral features that arise from heterogeneous environments, inhomogeneous broadening, and coupling between electronic transitions. 2DES promises to clarify many of these confounding factors, allowing us to better understand the underlying design mechanisms employed by nature in energy and charge transfer processes.

One of the major obstacles to performing 2DES is attaining phase sensitive detection of the signal field, which requires nanometer-scale stability in the optical paths used in the experiment. Our diffractive-optics-based approach can solve this problem (read about it here).