Electro-Optically Tunable Barium Titanate Metasurfaces Operating in the Visible

We present the design and fabrication of electro-optically tunable barium titanate (BTO)-based metasurfaces tailored for applications in the visible spectrum. BTO, with its exceptionally high Pockels coefficient (> 800 pm/V) [1], offers a compelling alternative to traditional materials such as liquid crystals and lithium niobate. BTO’s ability to achieve fast, localized and subwavelength electro-optic control makes it particularly attractive for applications that require efficient and high-speed amplitude and phase modulation [2].
Our work focuses on integrating BTO nanoblock resonators into transmissive active metasurfaces that leverage higher-order Mie resonances. These metasurfaces are optimized for operation at a wavelength of 630 nm, 532 nm, and 460 nm, achieving active modulation of the RGB colors for dynamic additive color mixing via the Pockels effect. Electrostatic simulations are utilized to predict the electro-optic response of the BTO material. When coupled with full-wave optical simulations, the results are integrated into a multi-physics model to accurately calculate the electro-optic tuning performance of the metasurface. This approach allows us to achieve a refractive index modulation as high as 0.2 for an applied voltage of 6 V. The phase shift resulting from this modulation exceeds 240°, while maintaining optical efficiencies above 50 %. We further engineer spatial phase gradients across the metasurface to control beam steering, thus enabling the dynamic manipulation of transmitted light.
A key challenge in realizing high-performance electro-optic metasurfaces is the fabrication of defect-free BTO thin films. To address this, we developed a novel fabrication process based on mechanical spalling, which allows for the exfoliation of moderately large (approximately 100 µm × 100 µm), high-quality single-crystal BTO thin films. This technique offers significant advantages over traditional bottom-up methods such as molecular beam epitaxy and pulsed laser deposition, which are often prone to defects like variable grain sizes and stoichiometric deviations. Spalling, on the other hand, enables the efficient exfoliation of thin (100 nm to several microns) films from bulk single-crystal BTO substrates, making them highly suitable for nanophotonic applications.
We successfully demonstrate the transfer of spalled BTO thin films onto a variety of substrates using a PPC/PDMS method, which ensures precise placement and minimal damage to the films during transfer. We characterize the crystallinity and uniformity of the spalled films and measure the electro-optic coefficients using a Teng-Man reflectometry characterization technique.
The combination of our advanced metasurface design and the novel fabrication technique for high-quality BTO thin films demonstrates the significant potential of BTO-based systems in advancing the field of reconfigurable electro-optic metasurface devices.

[1] Abel et al., Nat. Mater. 18, 42–47 (2019).
[2] Thomaschewski et al., Applied Physics Reviews, 9,2, (2022)