Nanophotonic inspection of deep-subwavelength integrated optoelectronic chips.

Artificial nanostructures with ultrafine and deep-subwavelength features have emerged as a paradigm-shifting platform to advanced light-field management, becoming key building blocks for high-performance integrated optoelectronics and flat optics. However, direct optical inspection of integrated chips remains a missing metrology gap that hinders quick feedback between design and fabrications. Here, we demonstrate that photothermal nonlinear scattering microscopy can be used 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, yielding optical resolving power down to 80 nm (~λ/7). The versatility of this approach has been exemplified by imaging silicon grating couplers and metalens with minimum critical dimensions of 100 nm, as well as central processing unit chip with 45-nm technology, unfolding the long-sought possibility of in situ, nondestructive, high-throughput optical inspection of integrated optoelectronic and nanophotonic chips.