Spatiotemporal photothermal modulation microscopy (SPM²) for high-sensitivity deep-subwavelength defect inspection

The weak scattering and overwhelming background of periodic structures fundamentally hinder the inspection of subwavelength defects embedded in dense nanopatterns. Herein, we introduce an actively tunable photothermal modulation scheme that leverages the temperature-dependent resonance shifts of silicon nanostructures to engineer their far-field scattering signatures. Localised optical heating induces a redshift in the underlying resonances, producing a strongly nonlinear change in both the defect and background scattering. This modification amplifies defect-induced perturbations and suppresses background contributions, substantially enhancing the inspection sensitivity for deep-subwavelength defects. A coupled optical-thermal model quantitatively captures the temperature rise and transient thermal evolution and predicts the resonance modulation achievable under the given pump conditions. This study establishes reversible, non-destructive photothermal resonance modulation as a general mechanism for dynamically engineering optical contrast in patterned media, offering a pathway towards high-sensitivity wafer inspection and tunable nanophotonic sensing.