We have developed a highly sensitive molecular sensor based on the heterodyne-detected third order nonlinear optical measurement with metal thin-film gratings. Pumped by an ultra-violet laser, the stripe pattern of the grating is transferred to the sample solution. The properties of the solution are detected via probe beam diffraction output from this special modulation. Time dependent signals profiles in nitrobenzene/2-propanol solution are thus obtained. The signal intensities as well as the concentrations of solute are proportional to the induced third order nonlinear electric susceptibility of solution. This technique can even detect the properties of those molecules and chemical compounds which are colorless and non-fluorescent. The merits of this method, such as simplicity, the high sensitivity, and the high speed, should be suitable for the molecular sensor of the microfluidic devices.
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