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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.

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Bragg reflectors (BRFs) are essential elements in optical telecommunication and sensing applications. Their miniaturization down to the sub-micron scale has been achieved in silicon-on-insulator chips. However, their tunability is limited only to thermal tuning. In order to achieve a faster and more practical tunability operation, here we report on electro-optically tunable BRFs with ∼14  dB signal filtering on a lithium-niobate-on-insulator platform, while keeping sub-micron cross-sections. Due to the lithium niobate electro-optic properties and the chosen electrodes configuration, a Bragg tunability coefficient of 23.37±0.55  pm/V is achieved, which enhances ∼33 times the tunability performance of state-of-the-art BRFs.

Three dimensional (3D) nonlinear photonic crystals are one of the potential candidates for enabling photonic circuits. Most of the current top‐down fabrication approaches for such crystals are time consuming and suffer from small surface areas and limitations to specific substrates. Herein, a bottom‐up fabrication technique (UV‐soft nanoimprint lithography) together with solution processing of barium titanate (BaTiO3) nanoparticles is used to create large‐scale 3D nonlinear woodpile photonic crystals. Opposed to other 3D photonic crystals, the woodpile structure can be fabricated layer by layer. This property makes it suitable for roll‐to‐roll fabrication, which can be easily adapted in industry. The prepared photonic crystals with up to eight layers have surface areas of 1.0 cm2. The linear and nonlinear optical behavior of these 3D woodpile structures is characterized. Their transmission stop band lies around 757 nm and a relative linear minimum transmission of 48.4% for a four‐layer sample is obtained. Images of diffraction patterns together with polarization‐dependent second harmonic generation measurements prove a cubic photonic crystal structure in the linear and nonlinear regime. In addition to second‐order nonlinearity, BaTiO3 exhibits electro‐optic, elasto‐optic, and thermo‐optic effects. Therefore, it is especially suited for future active photonic crystal applications.