Controlling high-harmonic generation from solids for subdiffraction imaging

Optical microscopy has undergone continuous refinement since its first implementation, yet its resolving power has been fundamentally constrained by the Abbe diffraction limit. While fluorescence-based super-resolution techniques such as STED, PALM, and STORM have successfully surpassed this barrier, they depend on exogenous fluorescent labels. This dependency limits their applicability in fields where label-free operation is essential, such as semiconductor metrology or solid-state physics. This work introduces and develops Harmonic Deactivation Microscopy (HADES), a novel, label-free super-resolution approach based on the controlled modulation of high-harmonic generation (HHG) in solids. Label-free nonlinear microscopy, particularly second- and third-harmonic generation (SHG/THG), provides intrinsic contrast in a sample. However, these modalities typically employ near-infrared excitation, resulting in diffraction-limited resolutions in the micron regime. Parallel to the development of superresolution techniques, strong-field and ultrafast physics have demonstrated the precise control of HHG from solids, which forms the conceptual basis for HADES. HADES adapts the conceptual foundation of STED microscopy to nonlinear harmonic emission. Instead of depleting fluorescence, HADES deactivates harmonic emission via a donut-shaped control pulse. Where STED drives fluorophores to the ground state, HADES reduces HHG efficiency by increasing electron-hole scattering and manipulating the trajectories of electron-hole pairs in a two-color field. Because HHG scales nonlinearly with the fluence of the deactivation beam, suppressing harmonic generation outside a narrow central region produces a significantly reduced point-spread function (PSF). HADES represents a new kind of nonlinear, label-free, ultrafast super-resolution microscopy. Future pathways include increasing deactivation efficiency, mitigating sample damage, exploiting higher harmonic orders, and integrating temporal resolution, providing a path to a fully optical microscope with sub-100 nm resolution. Beyond materials science, semiconductor metrology, and photonics, HADES may become a powerful tool for dynamic imaging in condensed matter, nanofabrication, and biological systems.