We experimentally demonstrate terahertz (THz) nonlinear transmission spectroscopy of the amino acids microcrystals using intense mono-cycle THz pulse. Absorptions of intermolecular vibration mode are saturated with incident electric field amplitude. This is due to the rearrangements of population distributed in an anharmonic intermolecular vibration potential. This will open a new THz pump probe spectroscopy in complex organic materials for mode assignments.
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