Gigahertz free-space electro-optic modulators based on Mie resonances

TL;DR

This paper demonstrates ultrathin free-space electro-optic modulators operating at gigahertz speeds using Mie resonances and hybrid silicon-organic nanostructures, enabling efficient modulation of free-space light with high speed and tunability.

Contribution

The authors introduce a novel free-space electro-optic modulator design based on Mie resonances and quasi-BICs, achieving high efficiency and GHz operation in an ultrathin form factor.

Findings

Achieved high-speed modulation up to 5 GHz.

Demonstrated resonant frequency shift of 11 nm surpassing linewidth.

Integrated hybrid silicon-organic nanostructures with low loss Q=550.

Abstract

Electro-optic modulators from non-linear $\chi^{(2)}$ materials are essential for sensing, metrology and telecommunications because they link the optical domain with the microwave domain. At present, most geometries are suited for fiber applications. In contrast, architectures that modulate directly free-space light at gigahertz (GHz) speeds have remained very challenging, despite their dire need for active free-space optics, in diffractive computing or for optoelectronic feedback to free-space emitters. They are typically bulky or suffer from much reduced interaction lengths. Here, we employ an ultrathin array of sub-wavelength Mie resonators that support quasi bound states in the continuum (BIC) as a key mechanism to demonstrate electro-optic modulation of free-space light with high efficiency at GHz speeds. Our geometry relies on hybrid silicon-organic nanostructures that feature low loss ($Q = $ 550 at $\lambda_{res} = 1594$ nm) while being integrated with GHz-compatible coplanar waveguides. We maximize the electro-optic effect by using high-performance electro-optic molecules (whose electro-optic tensor we engineer in-device to exploit $r_{33} = 100$ pm/V) and by nanoscale optimization of the optical modes. We demonstrate both DC tuning and high speed modulation up to 5~GHz ($f_{EO,-3 dB} =3$ GHz) and shift the resonant frequency of the quasi-BIC by $\Delta\lambda_{res}=$11 nm, surpassing its linewidth. We contrast the properties of quasi-BIC modulators by studying also guided mode resonances that we tune by $\Delta\lambda_{res}=$20 nm. Our approach showcases the potential for ultrathin GHz-speed free-space electro-optic modulators.