Tightly focused laser scanning excites displacement resonance

We study the scattering behavior of silicon nanoblocks in various displacements with respect to the optical axis of a tightly focused linearly polarized Gaussian beam. Experimentally, the laser scanning image of a single nanoblock deviates significantly from coherent image convolution. Theoretically, with exact Cartesian multipole decomposition, the results are explained through generation of high-order multipoles at large focus displacement and multipole interference. Surprisingly, due to the high-order multipoles, the efficiency of photothermal nonlinearity and Raman scattering are better with displaced focus. Our result extends Mie theory with displaced tight focus, opening up new opportunities in nanoscale light-matter interactions.