Active Metasurface Based on TiO₂/BTO Heterostructure

Metasurfaces have significantly advanced in recent years, offering exceptional control over the amplitude, phase, polarization, and spectral properties of electromagnetic waves. However, passive metasurfaces cannot be adjusted post-fabrication, driving the quest for reconfigurable metasurfaces with enhanced tunability. Barium titanate (BTO), known for its high electro-optic Pockels coefficient, faces challenges in thin-film form due to costly growth processes and limited thickness at the desired crystal orientation. In this work, we developed an active heterostructure comprising a 2D titanium dioxide (TiO₂) metasurface with interdigitated electrodes atop a high-quality a-oriented BTO layer. The BTO, with a thickness of 300 nm, was deposited using RF sputtering. The metasurface and interdigitated electrodes were fabricated with high precision using atomic layer deposition (ALD) and lift-off process, respectively. Our experimental results demonstrate guided mode resonances (GMR) confined within the BTO thin film, producing a high-quality factor dip in transmission mode around 1.56 μm. We further illustrate the active tunability of the metasurface through two mechanisms: the angle of incidence and electric bias. By changing the angle of incidence, we achieved resonance splitting and continuous shifting at a high rate. Additionally, the precise spectral position of this resonance is modulated via the electro-optic Pockels effect at low voltage, tuning the transmission of the metasurface around a wavelength of 1.56 μm. Our successful demonstration of a tunable metasurface based on TiO₂/BTO heterostructures suggests a promising path toward low-cost, large-scale free-space optical modulators. This work bridges the gap between the unique properties of metasurfaces and their active tunability, with potential applications in sensing, adaptive displays, spatial light modulators for advanced imaging, free-space communication, and beam steering.