Polarization-angle-scanning 2DIR spectroscopy of coupled anharmonic oscillators: A polarization null angle method

Two-dimensional (2D) optical spectroscopy based on stimulated photon echo geometry requires multiple ultrashort pulses of which spatiotemporal properties and optical phases can be precisely controlled. Also, it is possible to change the incident beam polarization directions defined in a laboratory frame. Here, we introduce the polarization-angle-scanning (PAS) 2D spectroscopy and show that the diagonal and cross-peak amplitudes in the 2D spectrum can be arbitrarily modulated by spatially controlling the beam polarization directions. For a coupled anharmonic oscillator system, we specifically demonstrate that either diagonal or cross-peaks in the measured 2DIR spectra can be selectively eliminated and show that such polarization angles provide direct information on the relative angles between coupled transition dipoles and thereby on intricate details of molecular structures. We thus anticipate that the present PAS 2D optical spectroscopy can be a useful experimental method enabling us to probe structural evolutions of nonequilibrium state molecules by monitoring the time-dependent changes of the relative transition dipole directions.