Determination of nonlinear absorption and scattering in a single plasmonic nanostructure using X-scan technique

Nonlinear nano-plasmonics opens up many exciting opportunities, such as nano-laser, nano-antenna, nano-modulator, etc. A highly desirable tool in the field of nonlinear nano-plasmonics is to characterize nonlinearity of optical absorption and scattering in single nanostructures. Currently, the most widely used method to quantify optical nonlinearity is z-scan, which can derive real and imaginary parts of permittivity through translating a thin sample across a laser focus. However, z-scan typically works with thin films, and thus acquires nonlinear responses from ensemble of nanostructures, not a single one. In this work, we present an X-scan technique, which is based on laser scanning microscopy equipped with forward and backward detectors. The two-channel detection allows simultaneous quantification of nonlinear behaviours of scattering, absorption, as well as total attenuation, from a single nanostructure. At low excitation intensity, both scattering and absorption responses are linear, thus confirming the linearity of detection system. At high excitation intensity, we found that the nonlinear response can be derived directly from the point spread function of X-scan images. Surprisingly high level of nonlinearities in both scattering and absorption are unravelled simultaneously for the first time. Our study not only provides a novel method for characterizing single-nanostructure nonlinearity, but also reports exceptionally large plasmonic nonlinearities.