PHOEBUS

2DES is the elective tool for the experimental studies of this project, since it measures energy transfers and electronic couplings and coherences in multi-chromophore systems. Thanks to its main advantage of providing simultaneously high temporal and spectral resolution, it allows to disentangle spectrally congested features. In 2DES three consecutive incoming pulses impinge on a sample. This interaction creates a nonlinear polarization that emits a field from the sample. The emitted field can be fully resolved in amplitude and phase when it interferes with a fourth pulse (local oscillator). By Fourier-transforming signal processing one can retrieve a 2D spectrum as a function of excitation and detection frequency. The  correlation 2D maps, function of excitation↔detection axes, allows to detect the energy flow dynamics,  electronic couplings and coherences between excited states.

2DES is technically highly demanding, since it requires interferometric stability between pulse pairs, which need to be locked to within a small fraction of their carrier wavelengths. In 2DES experiment performed in the so-called photon eco configuration,  the three pulses propagate along different directions and the signal is emitted along a fourth direction, defined by the phase-matching condition. The local oscillator interferes with the emitted polarization in the same direction. 

Two-color 2DES experiments can be necessary to detect couplings between chromophores with widely separated transition energies. Due to difficulties in using the photon echo configuration for the two-color experiments, a partially collinear pump-probe geometry can be implemented, involving the use of pulse shaper or similar devices that guarantee high interferometric stability.