Quantifying the Coupled Role of Mie Resonance Order and Geometry in Hot-Electron Photodetection

Hot-electron photodetectors (HE-PDs) convert photon-excited hot carriers into photocurrents. Although optical resonances often have multiple orders, how the resonance order influences HE-PD performance remains unclear. Using a coaxial Ag-TiO₂ nanowire as a model system, we investigate the role of Mie resonance order |m| in the Ag-TiO₂ junction. For coaxial nanowires, the Mie resonance order and the device geometry are intrinsically coupled because higher-order Mie resonance necessarily requires a larger TiO₂ core. By fixing the Ag shell thickness at 8 nm and varying only the TiO₂ core radius, we excite pure Mie resonances from the first to the eighth order at an identical wavelength of 900 nm. We find that |m| directly determines the spatial distribution of hot-electron generation and indirectly leads to increasing separation between the generation site and the metal-semiconductor interface with a rise in |m|. Moreover, the internal quantum efficiency of the designed HE-PD decreases under the combined effect of the device size and |m| variation, which is attributed to the increased hot-electron travel distance in the coaxial structure and the reduced transport probability. In contrast, external quantum efficiency, which convolutes photon absorption with internal quantum efficiency, increases with |m| and reaches its maximum at the sixth order because of the highest normalized absorption cross section at |m| = 6. In addition, both the absorption and external quantum efficiency spectra narrow, and their linewidths decrease as the resonance order increases. Overall, our study highlights the critical role of |m| intrinsically coupled with device geometry in HE PD performance, deepening our understanding of resonance order effects in hot-electron devices.