GRadient-Assisted Photon Echo Spectroscopy (GRAPES) is a method to circumvent slow point-by-point sampling based on gradient encoding. The τ delay, which is normally scanned by a mechanical delay line (or pulse shaper), is replaced by tilting the wave fronts of pulses 1 and 2. This pulse tilting scheme samples all the τ delays in parallel by spreading them over a spatial coordinate of the sample. GRAPES has been used to study a wide variety of systems from atomic vapors with exceptionally narrow transitions (< 0.3 cm-1) to light-harvesting proteins with broad transitions (> 50 cm-1), demonstrating the versatility of the method.
One major advantage of GRAPES is that it can be employed with unstable light sources such as supercontinuum (SC) light or sources not necessarily frequency locked over the measurement period. Phase fluctuations do not affect the measurement in the same way they do for PBP sampling schemes. Using tilted wave fronts allows spatiotemporal encoding of the various time delays within the 2D PES pulse sequence. Using a non-collinear arrangement isolates the signal field spatially owing to phase matching Finally, a 2D spectrum is generated by Fourier transformation with respect to the spatially encoded τ domain for each waiting time, T. The power of GRAPES is readily apparent for a complex cyanine dye molecule in solution. The linear absorption spectrum is broad and featureless, while the 2D spectrum is highly structured, directly mapping the vibronic structure of the molecule and its ultrafast dynamics on the excited state potential surface. The quantum signatures of the molecule are entirely unique, unlike the linear spectrum. Because of the spatial encoding of GRAPES, the entire 2D spectrum can be captured in one laser shot, a reduction of approximately 3 orders of magnitude over mechanical time sampling. Remarkably, this comes at no sacrifice in signal, and, in fact, an increase in SNR because of the dramatically reduced noise in the Fourier domain.
Two variants of GRAPES – spectral spatial interferometric (SSI) and WHITE GRAPES – were recently developed in our group. SSI GRAPES results in higher spectral resolution and increased throughput for higher signal-to-noise. WHITE GRAPES employees single- or few-cycle supercontinuum (SC) light sources, achieving dramatically higher bandwidth (> 5000 cm-1) and temporal resolution (< 6 fs).
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B. Spokoyny, C. J. Koh, and E. Harel, Stable and high-power few cycle supercontinuum for 2D ultrabroadband electronic spectroscopy, Opt Lett, 40 (6), 1014-1017 (2015). PDF
B. Spokoyny, and E. Harel, Mapping the Vibronic Structure of a Molecule by Few-cycle Continuum Two-Dimensional Spectroscopy in a Single Pulse,J Phys Chem Lett, 5 (16), 2808–2814 (2014). PDF