Thermo-Optical Nonlinearity of Metallic Nanoparticle(s)

The thermal effect is known to be one of the strongest mechanisms of optical nonlinearity but is usually avoided under ultrafast illumination. In this work, quite different from many previous studies in the ultrafast region, we study the thermo-optical nonlinearity of a single metal nanoparticle and many-nanoparticle composite under continuous-wave illumination. We show [Gurwich & Sivan, Phys. Rev. E, 2017; Sivan & Chu, Nanophotonics, 2017; Un & Sivan, Phys. Rev. Mater., 2020] that the thermo-optical nonlinearity of single metal nanoparticle systems strongly depends on the illumination wavelength and the nanoparticle size. The results of the single-nanoparticle system are then used to study the thermo-optical nonlinearity of many-nanoparticle composites. We show that different from the case of a single nanoparticle, the thermo-optical nonlinearity of the composite is strongly sensitive to the thermal conductivity of the host material only. Since photo-thermal effects were shown [Dubi et al., Chem. Sci., 2020] to be responsible for observations of faster chemical reactions, our results can be used to interpret correctly the differences in chemical reaction enhancements originating from the thermo-optical nonlinearity at different illumination intensities.