Loss-favored ultrasensitive refractive index sensor based on directional scattering from a single all-dielectric nanosphere

Abstract

Losses are generally undesired in most nanophotonic devices except for some applications such as photothermal conversion because high losses can result in reduced resonance quality. A lot of efforts have thus been made to achieve high-quality resonances for refractive index (RI) sensing applications. Here, we propose and demonstrate a new and counterintuitive sensing mechanism that leverages a single high-loss nanosphere as an effective optical RI sensor. We recognize that the scattering intensity of a high-loss all-dielectric nanosphere is more sensitive to the environmental RI variations than a low-loss one, which provides a unique means to put loss into advantages for RI sensing. Furthermore, directional scattering arises in a high-index (high-n) nanosphere leading to alternately dominant scattering in the forward and backward direction. At a certain wavelength (denoted as λ_FB) equal scattering intensity is achieved for both directions and the spectral value of λ_FB is highly sensitive to the environmental RI change. With this sensing mechanism, scattering intensity changes of all-dielectric nanospheres can be converted into wavelength shifts. Theoretically, the sensitivity of an individual Te nanosphere (150 nm) reaches 919 nm per RIU, surpassing the performance of Si and Ge nanospheres with the same size. With nanosecond laser ablation in liquid method, we mass-produced high-loss Te nanoparticles and experimentally demonstrated a RI sensitivity of 484 nm per RIU in the visible range on a single-particle level. This work provides an alternative platform for highly sensitive RI sensing using individual all-dielectric nanospheres.