Tailored Spectro-Temporal Dynamics in Hot-Carrier Plasmonics

Plasmonic metamaterials and metasurfaces can serve as a versatile arena for the investigation and utilization of optically excited, or plasmonically induced, energetic carriers. This is particularly intriguing when the decay pathways of hot carriers are rationally engineered with purposeful selections of the constituent materials and geometrical symmetries. The generation, transport, and relaxation of hot carriers also provide a novel route to active and nonlinear optical effects with ultrafast response rates.<br/><br/>Ultrafast optical switching in plasmonic platforms relies on the third-order Kerr nonlinearity, which is tightly linked to the dynamics of hot carriers in nanostructured metals. Although extensively utilized, a fundamental understanding on the dependence of the switching dynamics upon optical resonances has often been overlooked. Here we employ all-optical control of resonance bands in a hybrid photonic-plasmonic crystal as an empowering technique for probing the resonance-dependent switching dynamics upon hot carrier formation. Differential optical spectral measurements reveal an enhanced switching performance near the anti-crossing point arising from strong coupling between local and nonlocal resonance modes. Furthermore, entangled with hot-carrier dynamics, the nonlinear correspondence between optical resonances and refractive index change results in tailorable dispersion of recovery speeds which can notably deviate from the characteristic lifetime of hot carriers.<br/><br/>We further exploit hot-carrier dynamics to facilitate the creation of handedness-selectable transient chirality in achiral optical metasurfaces. As much as chiral metasurfaces are significant in stereochemistry and polarization control, tunable chiroptical response is important for their dynamic counterparts. A single metasurface device with invertible chiral states can selectively harness or manipulate both handedness of circularly polarized light upon demand, where in fact chiral inversion in molecules is an active research field. Tactics for chirality switching can be classified into geometry modification and refractive index tuning. However, these generally confront slow modulation speed or restrained refractive index tuning effects in the visible regime with forbidden ‘true’ inversion. Here, we reconfigure the ‘optical’ geometry through inhomogeneous spatiotemporal distribution of hot carriers as a breakthrough, transforming a plasmonic achiral metasurface into an ultrafast transient chiral medium with near-perfectly-invertible handedness in the visible. The photoinduced chirality relaxes through the fast spatial diffusion process of electron temperature compared to electron-phonon relaxation, empowering hot-carrier-based devices to be particularly suitable for ultrafast chiroptics.<br/><br/>Our comprehensive understanding provides new protocols for optically characterizing hot-carrier dynamics and optimizing all-optical light modulation and chirality manipulation for operations across the visible spectrum.