Developing two-dimensional fluorescence excitation spectroscopy: experimental approaches and first insights

Two-dimensional fluorescence excitation spectroscopy (2D-FLEX) was introduced recently as a novel ultrafast spectroscopic technique focusing on excited state dynamics. 2D-FLEX is an extension of broadband-detected time-gated fluorescence experiments, upgraded by double pulse excitation. A Fourier transformation over the pulse-pair delay retrieves the excitation frequency dependence of the fluorescence signal. This is especially advantageous for studying ultrafast excited state relaxation and reorganization processes in excitonically coupled systems. This work describes two experimental implementations of 2D-FLEX using fluorescence upconversion and non-collinear optical parametric amplifier (NOPA) technology. We compare and discuss both approaches, outlining their advantages and limitations. We present first results from two-photon excitation spectroscopy measurement conducted with 10 fs pulses.