Abstract Optical metasurfaces, planar subwavelength nanoantenna arrays with the singular ability to sculpt wavefront in almost arbitrary manners, are poised to become a powerful tool enabling compact and high-performance optics with novel functionalities. A particularly intriguing research direction within this field is active metasurfaces, whose optical response can be dynamically tuned postfabrication, thus allowing a plurality of applications unattainable with traditional bulk optics. Designing reconfigurable optics based on active metasurfaces is, however, presented with a unique challenge, since the optical quality of the devices must be optimized at multiple optical states. In this article, we provide a critical review on the active meta-optics design principles and algorithms that are applied across structural hierarchies ranging from single meta-atoms to full meta-optical devices. The discussed approaches are illustrated by specific examples of reconfigurable metasurfaces based on optical phase-change materials.

/pdf/design-for-quality-reconfigurable-flat-optics-based-on-2mj4aorx30.pdf

Design for quality: reconfigurable flat optics based on active metasurfaces

/pdf/bound-states-in-the-continuum-in-resonant-nanostructures-an-1ddtdo5xqr.pdf

Bound states in the continuum in resonant nanostructures: an overview of engineered materials for tailored applications

All-dielectric metasurfaces have attracted attention for highly efficient visible light manipulation. So far, however, they are mostly passive devices, while those allowing dynamic control remain a challenge. A highly efficient tuning mechanism is immersing the metasurface in a birefringent liquid crystal (LC), whose refractive index can be electrically controlled. Here, an all-dielectric tunable metasurface is demonstrated based on this concept, operating at visible frequencies and based on TiO2 nanodisks embedded in a thin LC layer. Small driving voltages from 3~5 V are sufficient to tune the metasurface resonances, with an associated transmission modulation of more than 65%. The metasurface optical responses, including the observed electric and magnetic dipole resonance shifts as well as the interfacial anchoring effect of the LC induced by the presence of the nanostructures, are systematically discussed. The dynamic tuning observed in the transmission spectra can pave the way to dynamically tunable metasurface devices for efficient visible light modulation applications.

/pdf/efficient-visible-light-modulation-based-on-electrically-3hs3vaiepv.pdf

Efficient visible light modulation based on electrically tunable all dielectric metasurfaces embedded in thin-layer nematic liquid crystals

Abstract In this paper, a dual-band reflective meta-hologram is designed providing two distinct information channels whose field intensity distributions can be independently manipulated at the same time. The proposed pure-phase meta-hologram is composed of several frequency-dispersive coding meta-atoms possessing each of 2-bit digital statuses of “00”, “01”, “10”, and “11” at either the lower (X-band) or the higher (Ku-band) frequency band. Relying on the weighted Gerchberg-Saxton phase retrieval algorithm, different illustrative examples have been provided to theoretically inspect the dual-band performance of our coding meta-hologram. Numerical simulations validate the proposed frequency multiplexing meta-holography with the ability to project two different high-quality images with low cross-talk on two X-band and Ku-band near-field channels located at distinct pre-determined distances from the metasurface plane. As proof of concept, two meta-hologram samples are fabricated, and the experimental results corroborate well the numerical simulations and theoretical predictions. The designed meta-hologram features all fascinating advantages of the coding metasurfaces while its performance overcomes that of previous studies due to providing two information channels rather than the conventional single-channel holography. The frequency multiplexing acquired by the proposed bi-spectral coding meta-hologram may provide great opportunities in a variety of applications, such as data storage and information processing.

/pdf/frequency-multiplexed-pure-phase-microwave-meta-holograms-3e844ddnf0.pdf

Frequency-multiplexed pure-phase microwave meta-holograms using bi-spectral 2-bit coding metasurfaces

Tailored nanostructures provide at-will control over the properties of light, with applications in imaging and spectroscopy. Active photonics can further open new avenues in remote monitoring, virtual or augmented reality and time-resolved sensing. Nanomaterials with χ(2) nonlinearities achieve highest switching speeds. Current demonstrations typically require a trade-off: they either rely on traditional χ(2) materials, which have low non-linearities, or on application-specific quantum well heterostructures that exhibit a high χ(2) in a narrow band. Here, we show that a thin film of organic electro-optic molecules JRD1 in polymethylmethacrylate combines desired merits for active free-space optics: broadband record-high nonlinearity (10-100 times higher than traditional materials at wavelengths 1100-1600 nm), a custom-tailored nonlinear tensor at the nanoscale, and engineered optical and electronic responses. We demonstrate a tuning of optical resonances by Δλ = 11 nm at DC voltages and a modulation of the transmitted intensity up to 40%, at speeds up to 50 MHz. We realize 2 × 2 single- and 1 × 5 multi-color spatial light modulators. We demonstrate their potential for imaging and remote sensing. The compatibility with compact laser diodes, the achieved millimeter size and the low power consumption are further key features for laser ranging or reconfigurable optics. Spatial light modulators (SLM) provide tailored light fields for many applications. Here, the authors present an SLM device based on an organic electro-optic material that manipulates the properties of individual pixels by electronic signals at speeds up to 50 MHz.

/pdf/electro-optic-spatial-light-modulator-from-an-engineered-5a1j6scknw.pdf

Electro-optic spatial light modulator from an engineered organic layer.

Electrically tunable active metasurfaces are attracting great interest as a possible means to realize two-dimensional pixelated surface-normal modulators for wide applications, such as high-speed imaging, optical interconnects, and photonic switching. In this work, we fabricate a metallic metasurface embedded with an electro-optic (EO) polymer and experimentally demonstrate the electrical tuning of its reflectance property. Unlike the previously demonstrated surface-normal modulators based on the EO polymer, we utilize the Fabry-Perot resonance of a metal-insulator-metal mode to trap the incident light inside a thin EO polymer layer to enhance the modulation efficiency. By applying voltage between the top and bottom layers of Au, we observe a clear spectral shift in the plasmonic resonance as well as a 5-MHz dynamic modulation at 1630-nm wavelength.

Electrical tuning of metal-insulator-metal metasurface with electro-optic polymer

Electrically tunable metasurfaces have gained special interest as they can realize ultrathin surface-normal modulators in planar geometries. In this paper, we demonstrate a novel metasurface modulator based on electro-optic (EO) polymer that utilizes bimodal resonance inside a metallic subwavelength grating to increase the modulation efficiency. When two metal-insulator-metal (MIM) resonant modes are excited simultaneously inside the grating, they couple strongly to generate a sharp dip in the reflected spectrum. As a result, efficient intensity modulation with 15-dB extinction ratio can be obtained at the resonant wavelength under a small refractive index change of 8.5 × 10-3, corresponding to modulation voltage of less than 10 V. Due to the low parasitic capacitance of EO polymer and high conductivity of metallic gratings which is also used as the electrodes, the RC bandwidth of the device should easily exceed 100 GHz, potentially applicable to high-speed surface-normal modulators.

Active metasurface modulator with electro-optic polymer using bimodal plasmonic resonance.

Boron-doped Carbon Nanoparticles for Identification and Tracing of Microplastics in “Turn-on” Fluorescence Mode

A dual-emission porphyrin coordination polymer for ratiometric fluorescent sensing of pH and copper ion

A surface-normal optical modulator using electro-optic polymer embedded inside a 570-nm-thick silicon high-contrast-grating resonator is fabricated. With a voltage applied to silicon grating, we obtain $> 20\%$ intensity modulation of the transmitted light at 30 MHz. © 2019 The Author(s)

Jiaqi Zhang

Papers

Electrical tuning of metal-insulator-metal metasurface with electro-optic polymer

Active metasurface modulator with electro-optic polymer using bimodal plasmonic resonance.

Boron-doped Carbon Nanoparticles for Identification and Tracing of Microplastics in “Turn-on” Fluorescence Mode

A dual-emission porphyrin coordination polymer for ratiometric fluorescent sensing of pH and copper ion

Electro-optic polymer surface-normal modulator using silicon high-contrast grating resonator