Miniaturized leaky-wave antenna with backward-to-forward beam scanning and suppressed open stop-band based on substrate-integrated plasmonic waveguide.

This work presents a theoretical design and experimental demonstration of a novel miniaturized leaky-wave antenna (LWA) using composite waveguide based on substrate-integrated plasmonic waveguide (SIPW). The SIPW is designed by embedding hybrid dual spoof surface plasmon polariton (SSPP) structure into a three-layer substrate integrated waveguide (SIW). Due to the slow-wave effect of SIPW, the proposed miniaturized composite waveguide forms slowed phase velocity and decreased lower cutoff frequency. To excite backward-to-forward beam scanning mode and suppress the open stop-band, an asymmetric sinusoidal modulated structure is introduced to the surface of the composite waveguide. The experimental results indicate that the proposed SIPW-based LWA can achieve continuous beam scanning from the backward to the forward direction within the bandwidth of 10.6-13.7 GHz, passing through the broadside at 11.6 GHz.

Experimental realization of a transmissive microwave metasurface for dual vector vortex beams generation.

This work presents a theoretical design and experimental demonstration of a transmissive microwave metasurface for generating dual-vector vortex beams (VVBs). The proposed metasurface consists of an array of pixelated dartboard discretization meta-atoms. By rotating the meta-atoms from 0° to 180°, a Pancharatnam-Barry (P-B) phase covering the full 360° range is achieved, with a transmittance exceeding 90% over the frequency range from 9.7 to 10.2 GHz. The measured results demonstrate that when a linearly polarized microwave normally impinges on the metasurface, the transmitted beams correspond to the dual VVBs with different directions. A good agreement among Poincaré sphere theory, full-wave simulation, and experimental measurement is observed. This proposed transmissive microwave metasurface for VVBs may offer promising applications in communications and radar detection.

Recent Progress in Resonant Acoustic Metasurfaces.

Acoustic metasurfaces, as two-dimensional acoustic metamaterials, are a current research topic for their sub-wavelength thickness and excellent acoustic wave manipulation. They hold significant promise in noise reduction and isolation, cloaking, camouflage, acoustic imaging, and focusing. Resonant structural units are utilized to construct acoustic metasurfaces with the unique advantage of controlling large wavelengths within a small size. In this paper, the recent research progresses of the resonant metasurfaces are reviewed, covering the design mechanisms and advances of structural units, the classification and application of the resonant metasurfaces, and the tunable metasurfaces. Finally, research interest in this field is predicted in future.

Lightweight Machine-Learning Model for Efficient Design of Graphene-Based Microwave Metasurfaces for Versatile Absorption Performance.

Graphene, as a widely used nanomaterial, has shown great flexibility in designing optically transparent microwave metasurfaces with broadband absorption. However, the design of graphene-based microwave metasurfaces relies on cumbersome parameter sweeping as well as the expertise of researchers. In this paper, we propose a machine-learning network which enables the forward prediction of reflection spectra and inverse design of versatile microwave absorbers. Techniques such as the normalization of input and transposed convolution layers are introduced in the machine-learning network to make the model lightweight and efficient. Particularly, the tunable conductivity of graphene enables a new degree in the intelligent design of metasurfaces. The inverse design system based on the optimization method is proposed for the versatile design of microwave absorbers. Representative cases are demonstrated, showing very promising performances on satisfying various absorption requirements. The proposed machine-learning network has significant potential for the intelligent design of graphene-based metasurfaces for various microwave applications.

Modified metasurface Alvarez lens based on the phase compensation in a microwave band.

Alvarez lenses, a kind of passive zoom lenses with reconfigurable focus, have been widely applied in optics but very few at lower frequencies such as in a microwave band, where the phase approximation for Alvarez lenses becomes inaccurate. In this article, we propose a design of a modified Alvarez lens with phase compensation for microwave, which consists of a pair of transmissive metasurfaces with high efficiency. The proposed metasurface consists of miniaturized units with the capability of full 2π phase modulation. We further analyze the phase distribution principle of the Alvarez lens and proposed a phase compensation scheme. The simulation results confirm that the proposed modified Alvarez lens has a very good dynamic focal length with theoretical expectation and can be continuously adjusted from 100 to 200 mm.

Time-Modulated Transmissive Programmable Metasurface for Low Sidelobe Beam Scanning.

Programmable metasurfaces have great potential for the implementation of low-complexity and low-cost phased arrays. Due to the difficulty of multiple-bit phase control, conventional programmable metasurfaces suffer a relatively high sidelobe level (SLL). In this manuscript, a time modulation strategy is introduced in the 1-bit transmissive programmable metasurface for reducing the SLLs of the generated patterns. After the periodic time modulation, harmonics are generated in each reconfigurable unit and the phase of the first-order harmonic can be dynamically controlled by applying different modulation sequences onto the corresponding unit. Through the high-speed modulation of the real-time periodic coding sequences on the metasurface by the programmable bias circuit, the equivalent phase shift accuracy to each metasurface unit can be improved to 6-bit and thus the SLLs of the metasurface could be reduced remarkably. The proposed time-modulated strategy is verified both numerically and experimentally with a transmissive programmable metasurface, which obtains an aperture efficiency over 34% and reduced SLLs of about -20 dB. The proposed design could offer a novel approach of a programmable metasurface framework for radar detection and secure communication applications.

Terahertz meta-biosensor based on high-Q electrical resonance enhanced by the interference of toroidal dipole.

Electrical dipole resonances typically have low Q factor and broad resonant linewidth caused by strong free-space coupling with high radiative loss. Here, we present a strategy for enhancing the Q factor of the electrical resonance via the interference of a toroidal dipole. To validate such a strategy, a metasurface consisting of two resonators is designed that responsible to the electric and toroidal dipoles. According to constructive and destructive hybridizations of the two dipole modes, enhanced and decreased Q factors are found respectively for the two hybrid modes, compared to the one for the conventional electric dipole resonance. As a practical application of such high Q resonance, we further experimentally investigate the sensing performance of the metasurface biosensor by detecting the cell concentration of lung cancer cells (type A549). Moreover, through monitoring both resonance frequency and amplitude variation of the metasurface biosensor, the dielectric permittivity of the lung cancer cells is delicately estimated by the conjoint analysis of both simulated and measured results. Our proposed metasurface paves a promising way for the study of multipole interference in the field of nanophotonics and validates its effectiveness in biomedical sensing.

Graphene-Based Optically Transparent Metasurface Capable of Dual-Polarized Modulation for Electromagnetic Stealth.

Microwave stealth technology with optical transparency is of great significance for solar-powered aircrafts (e.g., satellites or unmanned aerial vehicles) in increasingly complex electromagnetic environments. By coating them with optically transparent absorbing materials or devices, these large-sized solar panels could avoid detection by radar while maintaining highly efficient collection of solar energy. However, conventional microwave-absorbing materials/devices for solar panels suffer from bulky volume and fixed stealth performance that significantly hinders their practicality or multifunctionality. Particularly, dynamic modulation of microwave absorption for dual polarization remains a challenge. In this paper, we propose the design, fabrication, and characterization of an optically transparent and dynamically tunable microwave-absorbing metasurface that enables dual modulations (amplitude and frequency) independently for two orthogonal linearly polarized excitations. The tunability of the proposed metasurface is guaranteed by an elaborately designed anisotropic meta-atom composed of a patterned graphene structure whose electromagnetic responses for different polarizations can be dynamically and independently controlled via bias voltages. The dual tunability in such a graphene-based absorbing metasurface is experimentally measured, which agrees well with those numerical results. We further build an equivalent lumped circuit model to analyze the physical relation between the tunable sheet resistance of graphene and the polarization-independent modulations of the metasurface. Taking into account the advantages of optical transparency and flexibility, the proposed microwave-absorbing metasurface significantly enhances the multitasking stealth performance in complex scenarios and has the potential for advanced solar energy devices.

Switchable metasurface for nearly perfect reflection, transmission, and absorption using PIN diodes.

Active metasurfaces with dynamically switchable functionalities are highly in demands in various practical applications. In this paper, we experimentally present an active metasurface based on PIN diodes which can realize nearly perfect reflection, transmission and absorption in a single design. Such switchable functionalities are accomplished by controlling the PIN diodes integrated in both layers of the metasurface. A transmission line model is employed to further investigate the underlying mechanism of the metasurface. This proposal is confirmed by numerical simulations and experiments. As a novel metasurface with multiple switchable functionalities, our design may find some practical applications such as smart radomes.

Highly sensitive detection of malignant glioma cells using metamaterial-inspired THz biosensor based on electromagnetically induced transparency.

Metamaterial-inspired biosensors have been extensively studied recently years for fast and low-cost THz detection. However, only the variation of the resonance frequency has been closely concerned in such sensors so far, whiles the magnitude variation, which also provide important information of the analyte, has not been sufficiently analyzed. In this paper, by the observation of two degree of variations, we propose a label-free biosensing approach for molecular classification of glioma cells. The metamaterial biosensor consisting of cut wires and split ring resonators are proposed to realize polarization-independent electromagnetic induced transparency (EIT) at THz frequencies. Simulated results show that the EIT-like resonance experiences both resonance frequency and magnitude variations when the properties of analyte change, which is further explained with coupled oscillators model theory. The theoretical sensitivity of the biosensor is evaluated up to 496.01 GHz/RIU. In experiments, two types of glioma cells (mutant and wild-type) are cultured on the biosensor surface. The dependences of frequency shifts and the peak magnitude variations on the cells concentrations for different types give new perspective for molecular classification of glioma cells. The measured results indicate that the mutant and wild-type glioma cells can be distinguished directly by observing both the variations of EIT resonance frequency and magnitude at any cells concentrations without antibody introduction. Our metamaterial-based biosensor shows a great potential in the recognition of molecule types of glioma cells, opening alternative way to sensitive biosensing technology.

Single-layered meta-reflectarray for polarization retention and spin-encrypted phase-encoding.

Broadband communication with high data rates is a dire need for state-of-the-art wireless technologies. For achieving efficient wireless communication (particularly in an indoor environment), the electromagnetic (EM) waves should maintain their state of polarization despite encountering multiple reflections. Metasurfaces provide a unique platform to design subwavelength-featured meta-reflectarrays which enable the desired retention of the polarization state of an EM wave upon reflection. We present a single-layered broadband meta-reflectarray, simultaneously breaking n-fold (n > 2) rotational and mirror symmetry, which exhibits an unprecedented control over the phase, amplitude, and polarization of a reflected EM wave. This unique control enables the retention of polarization state and recording of spin-encrypted information for the reflected EM waves. Such novel multifunctional meta-reflectarray can be crucial to building an indoor setup for high data rate wireless communications. Meanwhile, the meta-array's ability to encode phase information provides an extra degree of freedom to structure and control (via incident spin) the reflected EM beam in the desired way. For the proof of concept, we have experimentally demonstrated a spin-encrypted holographic display which reconstructs the recorded holographic image at an image plane for the left circularly polarized (LCP) illumination and exhibits circular dichroism for the right circularly polarized (RCP) incident waves. The proposed meta-array can find applications in 5G indoor wireless communication, chiral sensing, spin-selective imaging, holography, and encryption.

Optical trapping of single nano-size particles using a plasmonic nanocavity.

Trapping and manipulating micro-size particles using optical tweezers has contributed to many breakthroughs in biology, materials science, and colloidal physics. However, it remains challenging to extend this technique to a few nanometers particles owing to the diffraction limit and the considerable Brownian motion of trapped nanoparticles. In this work, a nanometric optical tweezer is proposed by using a plasmonic nanocavity composed of the closely spaced silver coated fiber tip and gold film. It is found that the radial vector mode can produce a nano-sized near field with the electric-field intensity enhancement factor over 103 through exciting the plasmon gap mode in the nanocavity. By employing the Maxwell stress tensor formalism, we theoretically demonstrate that this nano-sized near field results in a sharp quasi-harmonic potential well, capable of stably trapping 2 nm quantum dots beneath the tip apex with the laser power as low as 3.7 mW. Further analysis reveals that our nanotweezers can stably work in a wide range of particle-to-tip distances, gap sizes, and operation wavelengths. We envision that our proposed nanometric optical tweezers could be compatible with the tip-enhanced Raman spectroscopy to allow simultaneously manipulating and characterizing single nanoparticles as well as nanoparticle interactions with high sensitivity.

Bi-layer metasurface based on Huygens' principle for high gain antenna applications.

A planar isotropic unit cell based on Huygens' principle is presented for achieving transmission phase control. By tailoring overlapping electric and magnetic resonances with geometry of the proposed unit cell, the transmission phase ranging from 0 - 2π is achieved with high transmittance. The proposed unit cell is then employed to design a metasurface lens with center frequency at 9.3 GHz and a square shaped patch antenna is placed at the focal point of the designed lens to perform conversion from spherical wave front of the source antenna to planar wave front. The designed lens antenna is capable to realize pencil beam radiation pattern with a gain of 19.6 dB and side lobe levels less than -15 dB in simulation. To experimentally verify the proposed design, a prototype of the metasurface lens is fabricated and measured. The measurement results well validate the proposed design and its enhanced performance.

Double-arrow metasurface for dual-band and dual-mode polarization conversion.

We present experimentally a double-arrow metasurface for high-efficiently manipulating the polarization states of electromagnetic waves in the dual-band. The metasurface is capable of converting a linearly polarized (LP) incident wave into a circularly polarized (CP) wave or its cross-polarized LP wave at different frequencies. It is numerically shown that in the two bands from 14.08 to 15.71 GHz and from 17.63 to 19.55 GHz the metasurface can convert the LP wave into CP wave, of which the axis ratio is lower than 3 dB. Meanwhile, the proposed metasurface also can convert the LP wave into its cross-polarized LP wave at 13.39 GHz and 20.29 GHz. To validate the theoretical analysis and simulated results, a prototype is fabricated and measured. The experimental results are reasonably consistent with the theoretical and simulated results, which demonstrates that such a metasurface can successfully achieve dual-band and dual-mode polarization conversion.

Reversible optical binding force in a plasmonic heterodimer under radially polarized beam illumination.

We investigated the optical binding force in a plasmonic heterodimer structure consisting of two nano-disks. It is found that when illuminated by a tightly focused radially polarized beam (RPB), the plasmon modes of the two nano-disks are strongly hybridized, forming bonding/antibonding modes. An interesting observation of this setup is that the direction of the optical binding force can be controlled by changing the wavelength of illumination, the location of the dimer, the diameter of the nano-disks, and the dimer gap size. Further analysis yields that the inhomogeneous polarization state of RPB can be utilized to readily control the bonding type of plasmon modes and distribute the underlying local field confined in the gap (the periphery) of the dimer, leading to a positive (negative) optical binding force. Our findings provide a clear strategy to engineer optical binding forces via changes in device geometry and its illumination profile. Thus, we envision a significant role for our device in emerging nanophotonics structures.

Ultrathin Single Layer Metasurfaces with Ultra-Wideband Operation for Both Transmission and Reflection.

Artificially engineered metasurfaces provide extraordinary wave control at the subwavelength scale. However, metasurfaces proposed so far suffer due to limited bandwidths. In this paper, extremely thin metasurfaces made of single metallic layer is experimentally presented for ultra-wideband operation from 9.3 to 32.5 GHz (with a fractional band of 112%), working at both transmission and reflection modes simultaneously. The phase control is achieved by azimuthally rotating the scatterer based on Pancharatnam-Berry phase principle. Nearly uniform efficiency (≈25%), approaching the theoretical limit of the infinitely thin metasurface, is achieved throughout the operation band. Finally, the proposed design is implemented for applications, e.g., the generation of electromagnetic waves carrying orbital angular momentums as well as anomalous reflections and refractions. The metasurfaces are characterized numerically and experimentally and the results are in good agreements.

Characterizing localized surface plasmon resonances using focused radially polarized beam.

We demonstrate a scheme to characterize the localized surface plasmon resonances (LSPRs) of an individual metallic nanorod by employing a focused radially polarized beam (RPB) illumination under normal incidence. The focused RPB has a unique three-dimensional electric field polarization distribution in the focal plane, which can effectively and selectively excite the dipole and multipole plasmon resonances in a metallic nanorod by just moving the nanorod within the focal plane. This performance can be attributed to the mode matching between the excitation electric field of the incident RPB and the LSPRs in a metallic nanorod. Emphatically, in contrast to the commonly used oblique incidence illumination with the linearly polarized light, our proposed scheme is based on the normally incident light illumination and compatible with conventional optical microscopy, which is more scalable for spectroscopic characterization of individual nanostructures.

sp2-sp3-Hybridized Atomic Domains Determine Optical Features of Carbon Dots.

Carbon dots (CDots) are a promising biocompatible nanoscale source of light, yet the origin of their emission remains under debate. Here, we show that all the distinctive optical properties of CDots, including the giant Stokes shift of photoluminescence and the strong dependence of emission color on excitation wavelength, can be explained by the linear optical response of the partially sp2-hybridized carbon domains located on the surface of the CDots' sp3-hybridized amorphous cores. Using a simple quantum chemical approach, we show that the domain hybridization factor determines the localization of electrons and the electronic bandgap inside the domains and analyze how the distribution of this factor affects the emission properties of CDots. Our calculation data fully agree with the experimental optical properties of CDots, confirming the overall theoretical picture underlying the model. It is also demonstrated that fabrication of CDots with large hybridization factors of carbon domains shifts their emission to the red side of the visible spectrum, without a need to modify the size or shape of the CDots. Our theoretical model provides a useful tool for experimentalists and may lead to extending the applications of CDots in biophysics, optoelectronics, and photovoltaics.

Enhanced second-harmonic generation assisted by breathing mode in a multi-resonant plasmonic trimer.

Boosting the nonlinear conversion rate in nanoscale is pivotal for practical applications such as highly sensitive biosensors, extreme ultra-violate light sources, and frequency combs. Here, we theoretically study the enhancement of second-harmonic generation (SHG) in a plasmonic trimer assisted by breathing modes. The geometry of the trimer is fine-tuned to produce strong plasmonic resonances at both the fundamental and SH wavelengths to boost SHG intensity. Moreover, it is found that breathing modes show remarkable ability to augment SHG by increasing the enhancement area. In particular, these breathing modes ensure a substantial spatial mode overlap at the fundamental and SH wavelengths, resulting in further promotion of the SHG conversation rate. We envision that our findings could enable applications in nanoscale frequency converters with high efficiency.

Highly efficient generation of Bessel beams with polarization insensitive metasurfaces.

We present a generic approach for the generation of pseudo non-diffracting Bessel beams using polarization insensitive metasurfaces with high efficiency. Cascaded unit cells, which are fully symmetric, are designed for the complete 2π phase control in the transmission mode. Based on the topological arrangements of such unit cells, two metasurfaces for the generation of zero-order (i.e., single phase profile) and first-order (i.e., merger of two distinct phase profiles) Bessel beams are designed and characterized. Both numerical simulations and experimental measurements are in agreement with each other, confirming the electromagnetic characteristics of the reported Bessel beams. Owing to the isotropy of the unit cells and the rotational symmetry of the arrangements, the proposed metasurfaces are polarization insensitive, providing a promising avenue for achieving such wave manipulations with any linear or circular polarization.