Two-dimensional THz spectroscopy of liquids: non-linear vibrational response to a series of THz laser pulses

We propose a non-linear experiment using the non-linear interaction with THz pulses, the generation of which has recently become well established. In the lowest non-linear process, we have two controllable delay times. This is another optical analogue of two-dimensional (2D) NMR as the recently developed 2D Raman spectroscopy. Our model calculation for liquid water demonstrates the striking capability of the proposed technique, clearly distinguishing two types of anharmonicity in the low-frequency modes.