Decoupling Optical and Thermal Responses: Thermo-optical Nonlinearities Unlock MHz Transmission Modulation in Dielectric Metasurfaces

TL;DR

This paper demonstrates that thermo-optical nonlinearities in dielectric metasurfaces can be decoupled to achieve fast, reversible, and large amplitude optical modulation at MHz speeds, surpassing traditional thermal response limits.

Contribution

It introduces a method to decouple thermal and optical response times in metasurfaces, enabling MHz-speed optical modulation using thermo-optical nonlinearities.

Findings

Achieved 85% transmission modulation amplitude.

Demonstrated optical modulation at 100 kHz, twice the laser modulation speed.

Decoupled thermal and optical timescales, with optical response up to 7 times faster.

Abstract

Thermo-optical nonlinearities (TONL) in metasurfaces enable dynamic control of optical properties like transmission, reflection, and absorption through external stimuli such as laser irradiation or temperature. As slow thermal dynamics of extended systems are expected to limit modulation speeds ultimately, research has primarily focused on steady-state effects. In this study, we investigate photo-driven TONL in amorphous silicon (a-Si) metasurfaces both under steady-state and, most importantly, dynamic conditions (50 kHz modulation) using a 488 nm continuous-wave pump laser. First, we show that a non-monotonic change in the steady-state transmission occurs at wavelengths longer than the electric-dipole resonance (800 nm). In particular, at 815 nm transmission first decreases by 30% and then increases by 30% as the laser intensity is raised to 5 mW/{\mu}m2. Next, we demonstrate that TONL decouple the thermal and optical characteristic times, the latter being up to 7 times shorter in the tested conditions (i.e {\tau}opt =0.5 {\mu}s vs {\tau}th =3.5 {\mu}s). Most remarkably, we experimentally demonstrate that combining these two effects enables optical modulation at twice the speed (100 kHz) of the excitation laser modulation. We finally show how to achieve all-optical transmission modulation at MHz speeds with large amplitudes (85%). Overall, these results show that photo-driven TONL produce large and fully reversible transmission modulation in dielectric metasurfaces with fast and adjustable speeds. Therefore, they open completely new opportunities toward exploiting TONL in dynamically reconfigurable systems, from optical switching to wavefront manipulation.