Polarization-insensitive high-numerical-aperture metalens for wide-field super-resolution imaging.

The development of super-oscillatory lens (SOL) offers opportunities to realize far-field label-free super-resolution microscopy. Most microscopes based on a high numerical aperture (NA) SOL operate in the point-by-point scanning mode, resulting in a slow imaging speed. Here, we propose a high-NA metalens operating in the single-shot wide-field mode to achieve real-time super-resolution imaging. An optimization model based on the exhaustion algorithm and angular spectrum (AS) theory is developed for metalens design. We numerically demonstrate that the optimized metalens with an NA of 0.8 realizes the imaging resolution (imaging pixel size) about 0.85 times the Rayleigh criterion. The metalens can achieve super-resolution imaging of an object with over 200 pixels, which is one order of magnitude higher than the unoptimized metalens. Our method provides an avenue toward single-shot far-field label-free super-resolution imaging for applications such as real-time imaging of living cells and temporally moving particles.

Spatial Information Lasing Enabled by Full-k-Space Bound States in the Continuum.

Optical amplification and massive information transfer in modern physics depend on stimulated radiation. However, regardless of traditional macroscopic lasers or emerging micro- and nanolasers, the information modulations are generally outside the lasing cavities. On the other hand, bound states in the continuum (BICs) with inherently enormous Q factors are limited to zero-dimensional singularities in momentum space. Here, we propose the concept of spatial information lasing, whose lasing information entropy can be correspondingly controlled by near-field Bragg coupling of guided modes. This concept is verified in gain-loss metamaterials supporting full-k-space BICs with both flexible manipulations and strong confinement of light fields. The counterintuitive high-dimensional BICs exist in a continuous energy band, which provide a versatile platform to precisely control each lasing Fourier component and, thus, can directly convey rich spatial information on the compact size. Single-mode operation achieved in our scheme ensures consistent and stable lasing information. Our findings can be expanded to different wave systems and open new scenarios in informational coherent amplification and high-Q physical frameworks for both classical and quantum applications.

Polarity-Tunable Field Effect Phototransistors.

Field-effect phototransistors feature gate voltage modulation, allowing dynamic performance control and significant signal amplification. A field-effect phototransistor can be designed to be inherently either unipolar or ambipolar in its response. However, conventionally, once a field-effect phototransistor has been fabricated, its polarity cannot be changed. Herein, a polarity-tunable field-effect phototransistor based on a graphene/ultrathin Al2O3/Si structure is demonstrated. Light can modulate the gating effect of the device and change the transfer characteristic curve from unipolar to ambipolar. This photoswitching in turn produces a significantly improved photocurrent signal. The introduction of an ultrathin Al2O3 interlayer also enables the phototransistor to achieve a responsivity in excess of 105 A/W, a 3 dB bandwidth of 100 kHz, a gain-bandwidth product of 9.14 × 1010 s-1, and a specific detectivity of 1.91 × 1013 Jones. This device architecture enables the gain-bandwidth trade-off in current field-effect phototransistors to be overcome, demonstrating the feasibility of simultaneous high-gain and fast-response photodetection.

Realization of maximum optical intrinsic chirality with bilayer polyatomic metasurfaces.

Optical chirality plays a key role in optical biosensing and spin-selective optical field manipulation. However, the maximum optical intrinsic chirality, which is represented by near-unity circular dichroism (CD), is yet to be achieved in a wide bandwidth range based on nanostructures. Here, we utilize dielectric bilayer polyatomic metasurfaces to realize the maximum optical intrinsic chirality over a wide bandwidth range. The CD efficiency of the two designed metasurfaces with opposite chirality is 99.9% at 1350 nm and over 98% from 1340 nm to 1361 nm. Our work provides a straightforward and powerful method for the realization of maximum optical intrinsic chirality, which has great potential in spin-selective optical wave manipulation.

Tunable dual-band and high-quality-factor perfect absorption based on VO2-assisted metasurfaces.

Perfect absorbers with high quality factors (Q-factors) are of great practical significance for optical filtering and sensing. Moreover, tunable multiwavelength absorbers provide a multitude of possibilities for realizing multispectral light intensity manipulation and optical switches. In this study, we demonstrate the use of vanadium dioxide (VO2)-assisted metasurfaces for tunable dual-band and high-quality-factor perfect absorption in the mid-infrared region. In addition, we discuss the potential applications of these metasurfaces in reflective intensity manipulation and optical switching. The Q-factors of the dual-band perfect absorption in the proposed metasurfaces are greater than 1000, which can be attributed to the low radiative loss induced by the guided-mode resonances and low intrinsic loss from the constituent materials. By utilizing the insulator-metal transition in VO2, we further proved that a continuous tuning of the reflectance with a large modulation depth (31.8 dB) can be realized in the designed metasurface accompanied by a dual-channel switching effect. The proposed VO2-assisted metasurfaces have potential applications in dynamic and multifunctional optical devices, such as tunable multiband filters, mid-infrared biochemical sensors, optical switches, and optical modulators.

Multi-band on-chip photonic spin Hall effect and selective excitation of whispering gallery modes with metasurface-integrated microcavity.

We propose an approach to realize a multi-band on-chip photonic spin Hall effect and selective excitation of whispering gallery modes (WGMs) by integrating metasurfaces with microcavities. Free-space circularly polarized light with opposite spin angular momentum can effectively excite WGMs with opposite propagation directions at fixed wavelengths. Moreover, the different WGMs with different propagation directions and polarizations can be selectively excited by manipulating the number of antennas. We demonstrate that the optical properties (i.e., coupling efficiency, peak positions, and peak widths) of the proposed metasurface-integrated microcavities can be easily tailored by adjusting different geometric parameters. This study enables the realization of chiral microcavities with exciting novel functionalities, which may provide a further step in the development of photonic integrated circuits, optical sensing, and chiral optics.

Batch production of uniform graphene films via controlling gas-phase dynamics in confined space.

Batch production of continuous and uniform graphene films is critical for the application of graphene. Chemical vapor deposition (CVD) has shown great promise for mass producing high-quality graphene films. However, the critical factors affected the uniformity of graphene films during the batch production need to be further studied. Herein, we propose a method for batch production of uniform graphene films by controlling the gaseous carbon source to be uniformly distributed near the substrate surface. By designing the growth space of graphene into a rectangular channel structure, we adjusted the velocity of feedstock gas flow to be uniformly distributed in the channel, which is critical for uniform graphene growth. The monolayer graphene film grown inside the rectangular channel structure shows high uniformity with average sheet resistance of 345 Ω sq-1 without doping. The experimental and simulation results show that the placement of the substrates during batch growth of graphene films will greatly affect the distribution of gas-phase dynamics near the substrate surface and the growth process of graphene. Uniform graphene films with large-scale can be prepared in batches by adjusting the distribution of gas-phase dynamics.

Vortical Reflection and Spiraling Fermi Arcs with Weyl Metamaterials.

Scatterings and transport in Weyl semimetals have caught growing attention in condensed matter physics, with observables including chiral zero modes and the associated magnetoresistance and chiral magnetic effects. Measurement of electrical conductance is usually performed in these studies, which, however, cannot resolve the momentum of electrons, preventing direct observation of the phase singularities in scattering matrix associated with Weyl point. Here we experimentally demonstrate a helical phase distribution in the angle (momentum) resolved scattering matrix of electromagnetic waves in a photonic Weyl metamaterial. It further leads to spiraling Fermi arcs in an air gap sandwiched between a Weyl metamaterial and a metal plate. Benefiting from the alignment-free feature of angular vortical reflection, our findings establish a new platform in manipulating optical angular momenta with photonic Weyl systems.

Ultrahighly Saturated Structural Colors Enhanced by Multipolar-Modulated Metasurfaces.

Colors with high saturation are of prime significance for display and imaging devices. So far, structural colors arising from all-dielectric metasurfaces, particularly amorphous silicon and titanium oxide, have exceeded the gamut of standard RGB (sRGB) space. However, the excitation of higher-order modes for dielectric materials hinders the further increase of saturation. Here, to address the challenge, we propose a new design strategy of multipolar-modulated metasurfaces with multi-dielectric stacked layers to realize the deep modulation of multipolar modes. Index matching between layers can suppress the multipolar modes at nonresonant wavelength, resulting in the dramatic enhancement in the monochromaticity of reflection spectra. Ultrahigh-saturation colors ranging from 70% to 90% with full hue have been theoretically and experimentally obtained. The huge gamut space can be realized in an unprecedented way, taking up 171% sRGB space, 127% Adobe RGB space, and 57% CIE space. More interestingly, the coverage for Recommendation 2020 (Rec. 2020) space, which almost has not been successfully realized so far, can reach 90%. We anticipate that the proposed multipolar-modulated metasurfaces are promising for the enlargement of the color range for high-end and advanced display applications.

Giant spin-selective asymmetric transmission in multipolar-modulated metasurfaces.

Spin-selective manipulation of optical waves is widely utilized in various optical techniques and plays a key role in modern nanophotonics. While numerous efficient approaches have been applied in metasurfaces to realize spin-selective manipulation of optical waves, the implementation of giant spin-selective asymmetric transmission remains a challenge. Here, we propose an all-dielectric metasurface to realize giant tri-band spin-selective asymmetric transmission in the near infrared regime. The proposed giant spin-selective asymmetric transmission is attributed to the excitation of overlapping multipolar resonances in the dielectric elliptic cylinders, which can be well manipulated by changing the structure parameters. This research demonstrates the great potential of all-dielectric metasurfaces for spin-selective transmission manipulation, which provide helpful insights and intriguing possibilities for applications in information optics, quantum optics, optical sensing, and imaging.

Hybrid graphene heterojunction photodetector with high infrared responsivity through barrier tailoring.

The graphene/Si heterojunction is attractive for high gain and broadband photodetection through photogating effect. However, the photoresponsivity in these devices are still limited to under 1 A W-1 if no narrowband absorption-enhanced nanostructures were used. In this paper, the effects of barriers on photoresponse are systematically studied at 1550 nm wavelength. Different barrier heights are obtained through selection of substrates, graphene doping and electrical tuning. Lower barrier height for graphene side and higher barrier height for silicon side are found to be beneficial for better infrared photoresponse. Through Polyetherimide doping of graphene and back-gated electrical modulation, the responsivity finally reached 5.71 A W-1, which to our knowledge is among the best results for graphene-based infrared photodetectors with graphene adopted as a light-absorption material. It is found that the thermionic emission efficiency of indirect transition in graphene is related to the difference in emissioin barrier height, and the lifetime of photoinduced carriers in the channel can be enhanced by built-in potential. These results lay the foundation for the photodetection applicatioins of graphene/Si heterojunction in the longer-wavelength infrared region.

Conductivity mapping of graphene on polymeric films by terahertz time-domain spectroscopy.

Fast inline characterization of the electrical properties of graphene on polymeric substrates is an essential requirement for quality control in industrial graphene production. Here we show that it is possible to measure the sheet conductivity of graphene on polymer films by terahertz time-domain spectroscopy (THz-TDS) when all internally reflected echoes in the substrate are taken into consideration. The conductivity measured by THz-TDS is comparable to values obtained from four point probe measurements. THz-TDS maps of 25x30 cm2 area graphene films were recorded and the DC conductivity and carrier scattering time were extracted from the measurements. Additionally, the THz-TDS conductivity maps highlight tears and holes in the graphene film, which are not easily visible by optical inspection.

High-quality-factor multiple Fano resonances for refractive index sensing.

We design and numerically analyze a high-quality (Q)-factor, high modulation depth, multiple Fano resonance device based on periodical asymmetric paired bars in the near-infrared regime. There are four sharp Fano peaks arising from the interference between subradiant modes and the magnetic dipole resonance mode that can be easily tailored by adjusting different geometric parameters. The maximal Q-factor can exceed 105 in magnitude, and the modulation depths ΔT can reach nearly 100%. Combining the narrow resonance line-widths with strong near-field confinement, we demonstrate an optical refractive index sensor with a sensitivity of 370 nm/RIU and a figure of merit of 2846. This study may provide a further step in sensing, lasing, and nonlinear optics.

Fano-resonance-based mode-matching hybrid metasurface for enhanced second-harmonic generation.

Plasmonic nanostructures have been considered as potential candidates for enhancing the nonlinear upconversion rate at nanoscale levels due to their strong near-field enhancement. Here, we propose a Fano-resonance-based mode-matching hybrid metasurface that combines the advantages of Fano resonances and mode-matching for boosting second-harmonic conversion. A confined and strong near-field intensity is generated by gold nanoantennas within the volume of polycrystalline zinc sulfide nanoparticles, thus resulting in a larger effective second-harmonic coefficient. The combination of the abovementioned features allows for the realization of a second-harmonic generation (SHG) conversion efficiency of 5.55×10-8, and the SHG signal is twice that obtained with dipole hybrid metasurfaces. Our designed metasurface may pave the way for optimizing nonlinear light-matter interactions at the nanoscale.

Tunable dual-band asymmetric transmission for circularly polarized waves with graphene planar chiral metasurfaces.

The asymmetric transmission effect has attracted great interest due to its wide modern optical applications. In this Letter, we present the underlying theory, the design specifications, and the simulated demonstration of tunable dual-band asymmetric transmission for circularly polarized waves with a graphene planar chiral metasurface. The spectral position of the asymmetric peak is linearly dependent on the Fermi energy and can be controlled by changing the Fermi energy. The success of tunable dual-band asymmetric transmission can be attributed to the enantiomerically sensitive plasmonic excitations of the graphene metasurface. This work offers a further step in developing tunable asymmetric transmission of circularly polarized waves for applications in detectors and other polarization-sensitive electromagnetic devices.

Realization of broadband cross-polarization conversion in transmission mode in the terahertz region using a single-layer metasurface.

We present the design specifications and in-depth analysis of a terahertz (THz) broadband cross-polarization converter composed of a single-layer metasurface. This device can convert linearly polarized light into its cross-polarization in transmission mode. Different from other polarization conversion devices, this effect results from the suppression and enhancement for different electric components. The broadband characteristic is also achieved by specific partial symmetries designed in the structure. The proposed polarization converter can aid in the development of novel plasmonic polarization devices, and can help to overcome certain limitations of the customary designs that have been proposed thus far.

Generation of vector beams with arbitrary spatial variation of phase and linear polarization using plasmonic metasurfaces.

A novel method is proposed to generate vector beams with arbitrary spatial variation of phase and linear polarization at the nanoscale using compact plasmonic metasurfaces with rectangular nanoapertures. The physical mechanism underlying the simultaneous control of light polarization and phase is explained. Vector beams with different spiral phasefronts are obtained by manipulating the local orientation and geometric parameters of the metasurfaces. In addition, radially and azimuthally polarized vector beams and double-mode vector beams are achieved through completely compensating for the Berry phase, which provides additional degrees of freedom for beam manipulation.

Fully interferometric controllable anomalous refraction efficiency using cross modulation with plasmonic metasurfaces.

We present a method of fully interferometric, controllable anomalous refraction efficiency by introducing cross-modulated incident light based on plasmonic metasurfaces. Theoretical analyses and numerical simulations indicate that the anomalous and ordinary refracted beams generated from two opposite-helicity incident beams and following the generalized Snell's law will have a superposition for certain incident angles, and the anomalous refraction efficiency can be dynamically controlled by changing the relative phase of the incident sources. As the incident wavelength nears the resonant wavelength of the plasmonic metasurfaces, two equal-amplitude incident beams with opposite helicity can be used to control the anomalous refraction efficiency. Otherwise, two unequal-amplitude incident beams with opposite helicity can be used to fully control the anomalous refraction efficiency. This Letter may offer a further step in the development of controllable anomalous refraction.

Co-enhancing and -confining the electric and magnetic fields of the broken-nanoring and the composite nanoring by azimuthally polarized excitation.

We present a novel broken-nanoring, which can realize strongly localized confinement and highly enhancement for both electric and magnetic fields at two resonant modes excited by normal incident azimuthally polarized light. Two resonant modes of the broken-nanoring are formed by different resonant mechanisms as different resonant lengths. The physical model for two resonant modes is also proposed to explain the mechanisms of the electromagnetic enhancement. The enhancement of the electric and magnetic fields can be further improved by adding a nanoring at the outside of the broken-nanoring to form a composite nanoring, which can freely tune or easily merge the resonant modes of the solitary broken-nanoring while keeping larger enhancement of the electric and magnetic fields.

Dynamically tunable plasmonically induced transparency by planar hybrid metamaterial.

We design and numerically analyze a dynamically tunable, plasmonically induced transparency (PIT) planar hybrid metamaterial (MM) in a near-infrared regime, which combines the near-field coupling effect into dynamic MM. The embedded position of tunable material in dynamic MM is optimized. Thermal-tunable VO(2) stripes are filled in the cut-out slots as components of a plasmonic system, which dramatically improve the dynamic modulation depth of the PIT. We also present a four-level plasmonic system to quantitatively analyze the dynamically tunable PIT device. This work may offer a further step in the design of the tunable PIT effect.