San Diego State University has received US Patent No. 8,268,551, "Sensitive sensing based on optical nonlinear wave mixing." The patent claims techniques for using four-wave mixing in microarrays with sample cells of biological or chemical materials. In one implementation, a DNA microarray is placed in an optical degenerate four-wave mixing system operating at an optical wavelength within an absorption spectral range of the DNA cells to generate a DFWM signal in one DNA cell. The DFWM signal is collected and measured.