  Artificial nanostructures with ultrafine and deep-subwavelength feature sizes
have emerged as a paradigm-shifting platform to advanced light field
management, becoming a key building block for high-performance integrated
optoelectronics and flat optics. However, direct optical inspection of such
integrated chips with densely packed complex and small features remains a
missing metrology gap that hinders quick feedback between design and
fabrications. Here, we demonstrate that photothermal nonlinear scattering
microscopy can be utilized for direct imaging and resolving of integrated
optoelectronic chips beyond the diffraction limit. We reveal that the inherent
coupling among deep-subwavelength nanostructures supporting leaky resonances
allows for the pronounced heating effect to access reversible nonlinear
modulations of the confocal reflection intensity, leading to optical resolving
power down to 80 nm (~lambda/7). The versatility of this approach has been
exemplified by direct imaging of silicon grating couplers and metalens with a
minimum critical dimension of 100 nm, as well as central processing unit (CPU)
chip with 45 nm technology, unfolding the long-sought possibility of in-situ,
non-destructive, high-throughput optical inspection of integrated
optoelectronic chips and nanophotonic chips.
