Linear Electro-Optic Effect in 2D Ferroelectric for Electrically Tunable Metalens.

The advent of 2D ferroelectrics, characterized by their spontaneous polarization states in layer-by-layer domains without the limitation of a finite size effect, brings enormous promise for applications in integrated optoelectronic devices. Comparing with semiconductor/insulator devices, ferroelectric devices show natural advantages such as non-volatility, low energy consumption and high response speed. Several 2D ferroelectric materials have been reported, however, the device implementation particularly for optoelectronic application remains largely hypothetical. Here, the linear electro-optic effect in 2D ferroelectrics is discovered and electrically tunable 2D ferroelectric metalens is demonstrated. The linear electric-field modulation of light is verified in 2D ferroelectric CuInP2S6. The in-plane phase retardation can be continuously tuned by a transverse DC electric field, yielding an effective electro-optic coefficient rc of 20.28 pm V-1. The CuInP2S6 crystal exhibits birefringence with the fast axis oriented along its (010) plane. The 2D ferroelectric Fresnel metalens shows efficacious focusing ability with an electrical modulation efficiency of the focusing exceeding 34%. The theoretical analysis uncovers the origin of the birefringence and unveil its ultralow light absorption across a wide wavelength range in this non-excitonic system. The van der Waals ferroelectrics enable room-temperature electrical modulation of light and offer the freedom of heterogeneous integration with silicon and another material system for highly compact and tunable photonics and metaoptics.

Magnifying lens designed by optical conformal mapping.

We proposed an alternative method to design a magnifying lens by optical conformal mapping. Different from previous hyperlens or superlens, the proposed lens needs no materials with negative or anisotropic refractive index. The lens has better photonic transporting efficiency than conventional a solid immersion lens due to impedance matching. The proposed lenses have many other advantages, such as broadband, low loss, and no need to redesign the sizes and material parameters when another magnifying ratio is required. Both numerical simulations and experimental demonstrations are implemented to verify the performance of the lens.

Angle-insensitive narrowband optical absorption based on high-Q localized resonance.

Strong optical absorption can be achieved easily based on an array of subwavelength localized resonators. The absorption bandwidth is typically wide since subwavelength metallic resonators are limited by a low quality factor (Q) due to their large material loss and so do dielectric counterparts owing to their weak photon binding. Here, an angle-insensitive narrowband optical absorber is suggested, which consists of subwavelength dielectric cavities buried inside a metal. Within each cavity, a special resonant mode of high Q can be supported, which is absorbed slowly by the metal walls as the electric field is concentrated at the cavity center and leaks slowly into the free space due to the blocking of the top metal film covering the cavities. Such a mode is excited to trap the incident wave in the optical absorption. When low-loss silver is used, one can obtain ultra-narrowband absorption with Q up to 487. At lower optical frequencies, the metal film needs to be punctured so that the incident wave can couple into the cavities effectively. The suggested absorption method may find its promising prospect in thermal radiation, photonic detection, optical sensing, and so on.