Weijie Luo

Effects of chemical fluctuations on microstructures and properties of multiferroic BiFeO3 thin films

BiFeO3 films have been grown on SrTiO3 (001) substrates by pulsed laser deposition. It was found that oxygen partial pressure is crucial to phase purity, surface morphology, and surface chemistry. Single-phase BFO films were obtained at 1Pa O2, while Bi2O3 appeared in the films deposited at 0.01Pa as confirmed by x-ray diffractions. It was revealed that Fe2+ and metallic Bi exist in the films fabricated at 0.01Pa by x-ray photoelectron spectroscopy investigation. Owing to Fe2+ in the samples deposited at 0.01Pa, the saturation magnetization is much larger than the ones fabricated at 1Pa. A well-saturated ferroelectric hysteresis loop with a polarization of 23.6μC∕cm2 was observed in the single-phase samples. In contrast, the films deposited at 0.01Pa exhibited poor ferroelectric properties.

Tunable microwave metasurfaces for high-performance operations: dispersion compensation and dynamical switch

Controlling the phase distributions on metasurfaces leads to fascinating effects such as anomalous light refraction/reflection, flat-lens focusing, and optics-vortex generation. However, metasurfaces realized so far largely reply on passive resonant meta-atoms, whose intrinsic dispersions limit such passive meta-devices’ performances at frequencies other than the target one. Here, based on tunable meta-atoms with varactor diodes involved, we establish a scheme to resolve these issues for microwave metasurfaces, in which the dispersive response of each meta-atom is precisely controlled by an external voltage imparted on the diode. We experimentally demonstrate two effects utilizing our scheme. First, we show that a tunable gradient metasurface exhibits single-mode high-efficiency operation within a wide frequency band, while its passive counterpart only works at a single frequency but exhibits deteriorated performances at other frequencies. Second, we demonstrate that the functionality of our metasurface can be dynamically switched from a specular reflector to a surface-wave convertor. Our approach paves the road to achieve dispersion-corrected and switchable manipulations of electromagnetic waves.

Aberration-free and functionality-switchable meta-lenses based on tunable metasurfaces

Constructing a meta-lens with tunable meta-atoms with varactor diodes incorporated, we can precisely control the phase profile of the meta-lens by varying the external voltages imparted on the diodes, such that the dispersion-induced phase distortions at off-working frequencies can be rectified and the functionality of the meta-lens can be dynamically changed. As an illustration, we design and fabricate a tunable meta-lens in the microwave regime and employ both experiments and numerical simulations to demonstrate the aberration-free and dynamically switchable focusing performances of the meta-lens. Our approach paves the road to achieve dispersion-corrected and switchable manipulations of electromagnetic waves in the microwave regime.

High-efficiency chirality-modulated spoof surface plasmon meta-coupler

Efficiently exciting surface plasmon polaritons (SPP) is highly desired in many photonic applications, but most approaches (such as prism and grating couplers) cannot control flexibly their SPP excitation directions. While Pancharatnam-Berry (PB) metasurfaces were recently proposed to achieve direction-controllable SPP excitations, such scheme suffers from low-efficiency issue due to both direct reflections at the coupler surface and the mode mismatch between the coupler and the guiding-out plasmonic structure. In this article, we solve these issues via imposing two criterions to guide design both the metasurface and the plasmonic metal, based on which a direction-controllable SPP excitation with very high efficiency can be realized. As a proof of concept, we designed/fabricated a realistic device working in the microwave regime, and performed both near-field and far-field measurements to demonstrate that it can achieve an spoof SPP conversion efficiency ~78%, much higher than previous devices. Full-wave simulations are in good agreement with experiments, showing that the efficiency can be further pushed to 92% with optimized designs. Our findings can stimulate spoof SPP-related applications, particularly can help enhance the spin-dependent light-matter interactions in low frequency regime.

High-efficiency generation of Bessel beams with transmissive metasurfaces

Circularly polarized Bessel beams (BBs) are important in biomolecule-sensing-related applications, but the available generators are too bulky in size and/or exhibit low efficiencies. Here, we design and fabricate ultra-thin ( ∼λ/6) transmissive Pancharatnam-Berry metasurfaces and perform near-field scanning measurements to show that they can generate circularly polarized BBs within a frequency window of 10.7–12.3 GHz. We experimentally demonstrate that the generated BBs exhibit a self-healing effect, illustrating their non-diffraction characteristics. Finally, we employ far-field measurements to demonstrate that the working efficiency of our devices can reach 91%, while the simulated efficiency reaches 92%. All experimental results are in perfect agreement with full-wave simulations.

Recent advances on metasurfaces

We briefly summarize our recent efforts in employing meta-surfaces to control electromagnetic waves, including realizing high-efficiency photonic spin-hall effect and surface-plasmon couplers, and controlling phases with graphene-based meta-surfaces.