Femtosecond multidimensional spectroscopy with multiple repetition-frequency-stabilized lasers: tutorial

Time-resolved multidimensional spectroscopy is an advanced spectroscopic technique that can be used to study the molecular structure and dynamics of chromophores in condensed phases by probing multiple resonances of chemical and biophysical systems. To achieve such a multidimensional measurement, the nonlinear optical response of materials should be measured for varying time delays between multiple optical pulses. Inevitably, the data acquisition time dramatically increases with the number of dimensions and the length of each time-delay scan. Therefore, technical breakthroughs toward improved data acquisition rates and time resolutions have long been sought for more versatile and extensive use of coherent multidimensional spectroscopy. Here, we present a tutorial description of the concepts and methods of coherent multidimensional spectroscopy with multiple repetition-frequency-stabilized lasers. Pulse trains from two lasers with slightly different repetition frequencies enable an asynchronous optical sampling (ASOPS), i.e., an automatic scan of the time delay between the pulses. By combining mechanical delay lines for interferometric pulse-pair generation and signal field detection with an incoherent ASOPS scheme for probing slow population dynamics of chromophores in condensed phases, we show that the dynamic range of time-delay scan and data acquisition speed for coherent multidimensional spectroscopy could be dramatically enhanced. In this tutorial paper, we summarize the current developments in this and related research areas, and provide perspectives on all optically controlled multidimensional spectroscopy with multiple synchronized lasers.