Coherent cancellation of background in four-wave mixing spectroscopy

A technique for the cancellation of background contribution to four-wave mixing is proposed and demonstrated. The technique consists of the destructive interference of the background parametric waves produced by two samples that differ only in one or more properties of interest. This cancellation technique is applicable even when the background itself contains resonances in the frequency range of interest. Cancellation factors of 500 and higher were achieved using a double cell for liquid specimens. This technique is demonstrated using phenol water solution. It is shown that with full background the 1004-cm^-1 phenol line can hardly be observed at 1% phenol concentration whereas the same line is easily observed when the background is cancelled even at 0.1% phenol concentration. This technique can also be used to extract the real and imaginary parts of the nonlinear susceptibility without the use of the Kramers-Kronig relations. We have performed this calculation in the case of 5% phenol solution in water.