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

Abstract

Electro-optic modulators from non-linear χ (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 λ 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 ∆λ res =11 nm, surpassing its linewidth. We contrast the properties of quasi-BIC modulators by studying also guided mode resonances that we tune by ∆λ res =20 nm. Our approach showcases the potential for ultrathin GHz-speed free-space electro-optic modulators.

Tunable Metasurface Using Thin-Film Lithium Niobate in the Telecom Regime

“…The experimental results correspond very well to the numerical simulations and the analytic prediction. Recent works on EO optical tuning, including tunable polymers 59 and organic layers such as JRD1:PMMA 60,61 on metasurfaces, exhibit a high spectral shift of Δλ = 11 nm but use a higher applied voltage, which yields merely…”

Space and Time Modulations of Light with Metasurfaces: Recent Progress and Future Prospects

“…However, material losses and structural imperfections significantly lower the experimental q-BIC quality factor and lead to an only 1.46× larger modulation with respect to the homogeneous film. Another example of active spectral tuning of q-BIC states is realized using silicon resonators covered by a polymer with dispersed EO-molecules (active JRD1/PMMA layer) . It employs second-order nonlinear materials exhibiting a linear EO effect as a surrounding for meta-atoms supporting high-quality factor resonances designed by symmetry breaking.…”

“…Patterning EO materials such as lithium niobate require specific etching techniques (such as dry Ar + etching) and hard unconventional resists . Apart from etching solid EO materials, coating polymers with organic electro-optic molecules on a nanostructured silicon platform appears even less technologically challenging. Coating metasurfaces with EO materials might nevertheless complicate the pixel-by-pixel addressing issue.…”

“…Results demonstrate the transmission amplitude modulation with a speed of up to 5 GHz. Reprinted with permission from ref . Copyright 2021 Federico Capasso.…”