Enhanced Electro-Optic Modulation in Resonant Metasurfaces of Lithium Niobate

Abstract

In display technologies or data processing, planar and subwavelength free-space components suited for flat photonic devices are needed. Metasurfaces, which shape the optical wavefront within hundreds of nanometers, can provide a solution for thin and portable photonic devices, e.g. as CMOS-compatible modules. While conventional electro-optic modulators are inconvenient to operate in free space configurations, its principle can largely be applied to the development of active metasurfaces with the prospect of modulation speeds up to the GHz region. Here, we use this concept to realize fast and continuous modulation of light at low voltage and MHz speed with a lithium niobate metasurface tuned by the linear electro-optic effect. Furthermore, we exploit the resonance in the visible to enhance the modulation of the transmitted light by two orders of magnitude, namely by a factor of 80, compared to the unstructured substrate. This proof-ofconcept work is a first important step towards the use of lithium niobate metasurfaces for free space modulation.

Fabrication of lithium niobate metasurfaces via a combination of FIB and ICP-RIE

“…The footprint of the metasurface array was 20 μm × 20 μm. The SEM image in the right panel demonstrates the cross section of the LN nano-ridge, which is an isosceles trapezoid with a top width of 211 nm and a bottom width of 275 nm, hence a slant angle of about 82°, which is larger than that of previously reported metasurfaces [20,23] , implying the high fabrication quality of the samples.…”

“…Furthermore, as a typical artificial nanostructured material, the metasurfaces that are formed by engineered subwavelength building blocks known as meta-atoms have demonstrated the ability to control optical waves with unprecedented flexibility, providing a new promising solution for shrinking optical devices down to subwavelength dimensions. In the recent years, studies of LN metasurfaces have captured a growing interest, and various intriguing optical phenomena have been demonstrated, including structural color [16] , engineerable second harmonic generation [17][18][19][20][21] , enhanced electro-optic modulations [22][23][24] , and generation of entangled photon pairs [25] .…”

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

“…Several current attempts to apply Pockels effects focused on using EO materials as resonant units, including the recently demonstrated metasurface made of lithium niobate crystal cylindrical posts. 103 Nanostructuring the lithium niobate thin film leads to lower-order multipolar resonances (in particular, resonance dominated by the electric dipole mode). By placing this structured surface between the electrodes (Figure 3a) and applying the AC driving signal, the resonance position is shifted, leading to a significant transmission modulation.…”

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

“…The demonstrated switching speeds typically reach a few kilohertz to a few megahertz. As an outstanding candidate, the electro-optic effect 34 – 37 is compatible with high-speed modulation, but the current active metasurface features low modulation efficiencies. This is linked to the sub-wavelength size of typical metasurface elements, leading to an interaction region only a few hundred nanometers long, commensurate with the thickness of flat optics.…”