Assoc. Prof. Jiahao Yan

Jinan University, China

Title：

The near-field photothermal effect and the far-field Kerker effect of silicon-based Mie resonators

Abstract：

Nano-resonators made from high-refractive-index dielectric materials are called Mie resonators because of the strong Mie resonances in visible or near-infrared ranges. The unique magnetic dipole resonances make dielectric Mie resonators different from plasmonic counterparts and exhibit different performances in nanophotonic applications. Therefore, it is important to understand the basic optical properties of dielectric Mie resonators in both near-field and far-field. In this work, we have studied the near-field photothermal effect and the far-field Kerker effect thoroughly based on the typical Mie resonator: silicon nano-resonators (nanoparticles or nanostripes).

For the near-field study, to realize sensitive photothermal measurement of Si nanoparticles, we combined the Mie resonances of Si nanoparticles and the phase change of vanadium dioxide (VO2) film to obtain the thermally controlled dark-field scattering. Through adding tungsten disulfide (WS2) flakes between VO2 films and Si nanoparticles and adjusting the thickness of WS2 from monolayer to bulk (>30nm), the slab modes in WS2 and VO2 films gradually modify the Mie resonances of Si nanoparticles. Specifically, Si nanoparticles on thick WS2 (>30nm) can produce Fabry-Perot (F-P) mode assisted Mie resonances, which convert the conventional intensity change of scattering spectrum under two states of VO2 to obvious wavelength shift. The new type thermally controlled scattering greatly enhances the sensitivity on temperature sensing. If the surrounding of Si nanoparticles is pre-heated to the starting point of the phase change region of VO2 (60oC), additional temperature increase because of the light-induced photothermal effects of Si nanoparticles will affect the phase transition of VO2 and the final dark-field scattering spectrum measured from the Si nanoparticles. In experiment, we first demonstrated different local heating performances of Si nanoparticles with different aggregation states. A thermal resolution less than 1 oC was realized.

For the far-field study, we observed the directional PL manipulation of WS2 monolayers based on the Kerker condition of Si nanostripes. Kerker condition was first proposed by M. Kerker et al. to study the zero backward or forward scattering arising from the interference between electric and magnetic dipole modes based on the Mie coefficients. This theory can be expanded to study the directional scattering from dielectric nanostructures with arbitrary shapes containing multipole electric and magnetic resonant modes. Moreover, Kerker condition brings new mechanism on the coupling between 2D excitons and dielectric nanostructures. However, it still remains unclear if we can realize significant PL enhancements based on pure far-field Kerker effects. Therefore, it is very important to study the Mie-exciton coupling and obtain strong PL enhancements based on pure Kerker effects. In this work, we proposed a Si nanostripe with a 135 nm top oxide layer. Hybrid structures were fabricated through placing monolayer and bilayer WS2 on Si nanostripes with different widths. The presence of oxide spacer blocks the near-field enhancements and heating effects of Si nanostripes and helps obtain the pure Kerker effect PL enhancements. In experiments, the measured PL enhancement factors are comparable to the best enhancement performance of single dielectric nanostructures. Moreover, the pure far-field effect greatly inhibited the Joule heating which is inevitable for plasmonic platforms. In theory, dipole sources were used to simulate the excitonic emission from WS2 layers, and we found the Kerker conditions and top/bottom radiation ratios change a lot with widths of Si nanostripes and the thickness of oxide spacer. Through optimizing the Kerker condition, we observed strong PL enhancements without near-field enhancements and large Q. The proposed

new characterizations on the near-field and far-field optical properties of Mie resonators will greatly inspire the design of all-dielectric nanophotonic and optoelectronic devices in the future.