Third order difference frequency generation of mid-infrared light

We study Four Wave Mixing (FWM) generated based on a mid-infrared pulse and a near infrared pulse. The mid infrared light is near resonant with vibrational modes of molecule, and it can create a coherence between vibrational states. The near infrared light will probe the coherence and result in FWM based on third order nonlinearity through different pathways. One pathway is a third-order Sum Frequency Generation (tSFG) and the other is a third-order Difference Frequency Generation (tDFG). We report experimental investigation of a time resolved tDFG generated from plastic materials such as mixed beads and a thin Low-Density Polyethylene (LDPE) film. We compare results of the tDFG with that of the tSFG in terms of their intensities and phase matching conditions. Our results show that a vibrational spectroscopy combing the tDFG and the tSFG can be versatile tools in studying of physical chemistry, dynamics of complicated molecular system, bioimaging and so on.