Breaking Abbe's diffraction limit with harmonic deactivation microscopy

Nonlinear optical microscopy provides elegant means for label-free imaging of biological samples and condensed matter systems. The widespread areas of application could even be increased if resolution was improved, which the famous Abbe diffraction limit now restrains. Super-resolution techniques can break the diffraction limit but most rely on fluorescent labeling. This makes them incompatible with (sub)femtosecond temporal resolution and applications that demand the absence of labeling. Here, we introduce harmonic deactivation microscopy (HADES) for breaking the diffraction limit in nonfluorescent samples. By controlling the harmonic generation process on the quantum level with a second donut-shaped pulse, we confine the third-harmonic generation to three times below the original focus size of a scanning microscope. We demonstrate that resolution improvement by deactivation is more efficient for higher harmonic orders and only limited by the maximum applicable deactivation-pulse fluence. This provides a route toward sub-100-nanometer resolution in a regular nonlinear microscope.