Control and scaling of nonlinear emission for super-resolution microscopy

Super-resolution microscopy—imaging below the Abbe diffraction limit—was a resounding success in the development of optical far-field microscopy and revolutionized bioimaging. Yet most super-resolution techniques are based on fluorescence microscopy. However, fluorescent labels impose restrictions on implementing this technique in some fields of science. Label-free techniques such as third-harmonic generation (THG) microscopy are an alternative, but only provide limited resolution due to their infrared driver. In this work, we show how to optimize the point-spread function (PSF) of THG microscopes, first for a single-color harmonic driving laser, and then in a two-color field. For a single-color driver, a low input intensity leads to the smallest PSF. This occurs because an effective nonlinear order close to the order of the emitted harmonic leads to the maximum possible spot size reduction of the harmonic emission profile with respect to the incident intensity profile of the driver. In the two-color case, we utilize a second light pulse with a donut-shaped fluence profile in focus, which is generated by introducing orbital angular momentum. We show that this second pulse shrinks the PSF below the diffraction limit. Although there is no principal limit to this shrinkage, meaning that there is no principal limit to resolution in our approach, the current implementation is practically limited by sample damage. Currently, this promises a factor of 4 reduction of the PSF and a concomitant improvement of the resolution by a factor of 4 in a coherent harmonic microscopy imaging system. These findings open the pathway to implement super-resolution techniques in a broader scientific and industrial application, i.e., in condensed-matter physics and for semiconductor wafer metrology.

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