Off-Resonance Enhancement of the Refractive Index via Indirect Optical Transitions

At resonance, the optical response of a homogeneous medium to incident light is governed by temporal dispersion, which induces changes in the refractive index (RI). However, recent experimental results reveal RI enhancements exceeding the fundamental limit of 5 in spatially confined systems. Herein, we study the off-resonance enhancement of RI in sub-5 nm gold nanoparticles using electronic light scattering (ELS). We discuss that the RI driven by the distribution and dynamics of excited-state charge carriers can be locally boosted through the breaking of spatial symmetry in matter. These findings are critical for optically probing RI variations in spatially confined media using ELS.