Dipole Resonance Mode Research Articles

Decoupled multi‐input multi‐output antennas with a common dipole for wideband 5G laptop computers

AbstractTwo decoupled antennas, comprising two feeding monopoles and one dipole shared by the monopoles, for the fifth‐generation (5G) multi‐input multi‐output (MIMO) applications are proposed. The feeding monopoles are placed on both sides of the coupled‐fed, common dipole. First, to operate in the 3.3–5.0 GHz 5G wideband, two resonant modes at about 3.6 and 4.7 GHz, contributed to by the common dipole and the feeding monopoles respectively, are utilized. Then, by introducing distributed capacitance loaded on the dipole center, the one‐wavelength dipole resonant mode can be lowered to oppose the counter‐phased half‐wavelength mode currents on the half dipole, resulting in self decoupling around 3.6 GHz. The mutual coupling for the common dipole is reduced by about 19 dB in this study. The design shows a small size of 35 mm × 5 mm (0.38‐wavelength × 0.05‐wavelength at 3.3 GHz) and can be a promising candidate for narrow‐bezel 5G laptop computers.

Plasmonic Enhancement Mechanism of Template‐Based Synthesized Au@TiO2 Nanodiscs

AbstractThe localized plasmon resonance enhancement mechanisms were investigated as a function to increase the shell thickness of the prepared hexagonal nanodiscs arrays of Au@TiO2 core‐shell. The bare Au hexagonal nanodiscs array exhibited multiple plasmon resonance modes in the ultraviolet (UV) to near‐infrared (NIR) region. Au@TiO2 nanodiscs configuration present three different enhancement mechanisms with increasing TiO2 shell thickness. First, a strengthened plasmon‐induced resonance energy transfer (PIRET) for higher plasmonic resonance mode in the UV region. Second, redshifted and broadened direct electron transfer (DET) processes for the plasmonic dipole resonance mode from the Vis to the NIR region. Third, an increased hybridization retardation effect for the higher plasmonic mode in the Vis region. By using a facile and cost‐effective technique (nonlithographic route) to fabricate highly ordered core‐shell nanodiscs arrays (Au@TiO2 hexagonal nanodiscs) on a large area of Si substrate (>cm2) via an ultrathin alumina membrane (UTAM), this technique provides a perfect shadow mask to fabricate Au nanodiscs arrays; furthermore, the shadow effect of UTAM pores offers enough space to cover Au nanodiscs by the TiO2. These plasmonic core‐shell nanodiscs reveal a highly promising pathway to discovering new enhancement phenomena that can be applied in diverse applications such as plasmon‐enhanced energy conversion, biosensing, and surface‐enhanced vibrational spectroscopy.

Disorder-immune metasurfaces with constituents exhibiting the anapole mode

Common optical metasurfaces are two-dimensional functional devices composed of periodically arranged subwavelength constituents. Here, we achieved the positional-disorder-immune metasurfaces composed of core–shell cylinders which successively exhibit the magnetic dipole (MD) resonant, non-radiating anapole, and electric dipole (ED) resonant modes when their outer radii are fixed and the inner radii change continuously in a range. The performances of the metasurfaces under a periodically structural design are not degraded even when the positions of the cylinders are subjected to random and considerable displacements. The positional-disorder-immunity is due to the weak non-local effect of the metasurfaces. Because the multiple scattering among cylinders is weak and insensitive to the spacing among the cylinders around the ED and MD resonant modes and vanishing irrespective of the spacing at the non-radiating anapole mode, the reflection properties including the reflection phase and reflectivity of the metasurfaces are insensitive to the spacing between neighboring cylinders for this entire variation range of the inner radius. Our findings can have important implications in understanding the underlying mechanism of the positional-disorder-immunity and provide a unique approach to achieve metasurfaces with various performances robust against large positional disorders. We expect the present work to open a door for the various applications of the metasurfaces in some harsh and unstable environments.

A thermally tunable terahertz three-dimensional perfect metamaterial absorber for temperature sensing application

We present a three-dimensional (3D) perfect metamaterial absorber (PMA) for temperature sensing application in terahertz region. The PMA consists of a 3D metal resonator structure array and a continuous metal film separated by an indium antimonide (InSb) layer. The numerical simulations demonstrate that the PMA can achieve perfect absorption (about 99.9%) with the high [Formula: see text]-factor of about 18.8 at 2.323 THz when the temperature is 300 K (room temperature). Further simulation results indicate that this terahertz PMA is polarization-insensitive and wide-angle for both transverse electric (TE) and transverse magnetic (TM) waves. The electric field and surface current distributions of the unit-cell structure indicate that the perfect absorption is originated from the excitation of the fundamental magnetic and electric dipole resonance mode. Since the permittivity of the InSb is sensitive to the external temperature, the resonance absorption frequency of the PMA can be dynamically adjusted. The temperature sensitivity of the PMA is about 15.24 GHz/K, which may have potential prospects in temperature sensing and detection.

Ultra high-sensitivity and tunable dual-band perfect absorber as a plasmonic sensor

In this paper, we demonstrate a dual-band tunable absorber coupled with a nanoscale metal-dielectric-metal (MDM) structure for sensing applications in the near-infrared spectral region. This structure exhibits a dipole resonance mode in absorbance and reflectance spectra which results in the enhancement of absorbance over a wide range of incident angles for TE polarization. Using a numerical and analytical study, the performance parameters of the structure including sensitivity (SS), the figure of merit (FoM) and quality factor (Q) are investigated by changing the incident polarization, geometrical parameters, filling dielectric and plasmonic metasurface material. Moreover, we study the dependence of the sensitivity as a function of plasmonic metasurface shape to demonstrate a better response compared with other methods. Results show that, in terms of the refractive index unit (RIU), an ultra-high sensitivity and tunable sensor can be designed with a maximum sensitivity of 1240.8 nm/RIU for a refractive index change of Δn = 0.0458. In the optimum design of the proposed dual-band absorber, a Q-factor and FoM equal to 123.45 and 44.5 are obtained. Furthermore, the proposed structure can be utilized for controlling the light propagation. By considering silver as a plasmonic metasurface, a slow down factor as high as 680 is obtained. Our work will be applied to future sensors capable of ultra-high sensitivity.

Wireless Reconfigurable RF Detector Array for Focal and Multiregional Signal Enhancement.

Wirelessly Amplified NMR Detectors (WAND) can utilize wireless pumping power to amplify MRI signals in situ for sensitivity enhancement of deep-lying tissues that are difficult to access by conventional surface coils. To reconfigure between selective and simultaneous activation in a multielement array, each WAND has a dipole resonance mode for MR signal acquisition and two butterfly modes that support counter-rotating current circulation. Because detectors in the same row share the same lower butterfly frequency but different higher butterfly frequency, a pumping signal at the sum frequency of the dipole mode and the higher butterfly mode can selectively activate individual resonators, leading to 4-fold sensitivity gain over passive coupling. Meanwhile, a pumping signal at the sum frequency of the dipole mode and the lower butterfly mode can simultaneously activate multiple resonators in the same row, leading to 3-fold sensitivity gain over passive coupling. When multiple rows of detectors are parallelly aligned, each row has a unique lower butterfly frequency for consecutive activation during the acquisition interval of the others. This wireless detector array can be embedded beneath a headpost that is normally required for multi-modal brain imaging, enabling easy reconfiguration between focal imaging of individual vessels and multiregional mapping of brain connectivity.

Optical resonance enhanced Cs activated nano-structured Ag photocathode

Metallic photocathodes have drawn attention due to their outstanding performances of ultrafast photoelectric response and long operational lifetime. However, due to their high work function and the large number of scattering events, metallic photocathodes typically are driven by ultraviolet laser pulses and characterized by low intrinsic quantum efficiency (QE). In this work, a new type of Mie-type silver (Ag) nano-sphere resonant structure fabricated on an Ag/ITO composite substrate is used to enhance the photocathode QE, where Mie scattering resonance is used to enhance the local density of optical state and then to improve the light absorption and electron transporting efficiency in Ag nano-spheres. The cesium (Cs) activation layer is also used to lower the electron work function and then to excite photoemission in the visible waveband for Ag photocathode. The optical characteristics of Ag nano-sphere arrays are analyzed by using finite difference time domain method. For the investigated Ag nano-sphere array, theoretical results show that Mie-type electric dipole resonance modes can be obtained over the 400–600 nm waveband by adjusting the sphere diameter, and the large resonance-enhanced absorption can be achieved in nanospheres at the resonance wavelength. The Ag nano-spheres are fabricated on the Ag/ITO substrate by magnetron sputtering and annealing process, then the Cs activation layer is deposited on surface, and finally QE is measured in an ultra-high vacuum test apparatus. Experimental results show that over 0.35% of QE is obtained for Ag nano-sphere particle (with a diameter of 150 nm) at a wavelength of 425 nm, and the wavelength positions of QE maxima are in agreement with Mie resonance for corresponding geometry predicted from the computational model. Given these unique optoelectronic properties, Ag nanophotonic resonance structured photocathodes represent a very promising alternative to photocathodes with flat surfaces that are widely used in many applications today.

基于双重共振响应的太赫兹柔性可拉伸超表面（特邀）

Active control of terahertz wave characteristics in stretchable devices is essential for advanced terahertz applications involving large mechanical deformation or stretching. Here, a dual band terahertz active control device based on different mechanisms was designed and fabricated by combining metal metasurface with elastic film polydimethylsiloxane. Based on the deformation mismatch between metal and elastic film under tension, the double band modulation effect was realized by using the periodically sensitive cross structure metasurface. Under 36% deformation, the dipole mode and the lattice resonance mode were exploited to experimentally achieve dual-band modulation with a modulation depth of 90% and a modulation depth of 78% at 1.26 THz and 2.41 THz, respectively. The operating frequency through the lattice mode had a large dynamic range, which could be tuned from 2.41 THz to 1.85 THz. Since the mechanisms of the electric dipole resonance mode and the periodic lattice resonance mode were independent from each other, the two resonance frequencies were designed independently, which allowed the frequency interval of the dual-band modulation to be geometrically adjustable. The stretchable metasurface presented in this paper is simple to prepare, and has the advantages of large intensity modulation depth and wide frequency tuning range. It can be used not only in active control of terahertz wave, but also in passive displacement sensing.

High-Q terahertz Fano-like resonance effect based on robust metamaterial resonator

It is well known that Fano effect usually requires the introduction of asymmetric (or breaking) in their structure designs. The smaller the degree of breaking, the more obvious the Fano effect. However, the small degree of breaking demands extremely precise manufacturing capabilities and levels. This paper proposes a novel robust metallic array structure that can achieve a Fano-like effect with a high [Formula: see text]-factor as 73, consisting of a square patch and a closed-ring resonator. This distinct Fano-like resonance originates from the interactions of quadrupole resonance mode and dipole resonance mode. Moreover, the effect of the size and conductivity of the square patch on the Fano effect are discussed, and the sensing performance of this structure is also discussed. These results will pave the way for designing practical sensors and optical switches in the terahertz frequency range.

Single-particle Correlation Study: Polarization-dependent Differential Interference Contrast Imaging of Two-dimensional Gold Nanoplates

Questions surrounding the optical properties of two-dimensional (2D) triangular single gold nanoplates (AuNPs) remain largely unanswered. Herein, a scanning-electron microscopy-correlated single-particle study was conducted to identify polarization-dependent optical properties of AuNPs under dark-field (DF) and differential interference contrast (DIC) microscopy. AuNPs with an aspect ratio of ∼3 showed a single broad DF scattering spectrum without separation of the two dipole and quadrupole resonance modes present in 2D AuNPs. Polarization-sensitive interference properties of the individual AuNPs were revealed through periodic changes in the intensities and types of DIC images obtained. A dipole resonance mode was found to mainly contribute to the polarization-sensitive interference properties of AuNPs. Furthermore, DIC polarization anisotropy allowed us to track the real-time orientation of a dipole resonance mode of a AuNP rotating on a live cell membrane.

Broadband coherent hyperspectral near-field imaging of plasmonic nanostructures.

We develop a coherent hyperspectral near-field microscope using a combined nano-Fourier Transform Infra-Red (FTIR) spectroscope and a scattering Scanning Near-field Optical Microscope (s-SNOM) illuminated by an ultra-broadband few-cycle femtosecond source, spanning a spectrum from 660 to 1050 nm. Using this spatio-spectral approach, we resolve hyperspectral near-field response of a single plasmonic nano-antennas over 450 nm bandwidth with a spatial resolution of 40 nm and a spectral resolution of 50 cm-1. In particular, we identify the electric near-field spatial distribution of the dipole resonant mode of various nano-antennas and observe, in accordance with previous theoretical reports, that those are spectrally red-shifted from their far-field response. Moreover, we are able to spectrally and spatially differentiate the near-field distribution of the dipole and quadrupole modes at the single nanoparticle level. Being coherent and short-pulsed, our technique opens the path for optical ultrafast characterization and control of light-matter interaction at the nanoscale.

Fano resonance in asymmetric gold nano-dimers with square and rectangular sections

The Fano resonances of the asymmetric dimers of gold nanowires with square and rectangular sections are investigated by using the finite-difference time-domain (FDTD) method. It is found that the Fano resonance peak can be switched on and off by laying or removing the rectangular section nanowire aside the square section one. There is only one dipole resonance mode of a single gold nanowire with a square section on the dielectric substrate, and it redshifts obviously accompanied with the FWHM being broadened along with the side width of the nanowire increasing. A Fano resonance appears when another gold nanowire with the rectangular section is laid aside the one with a square section. The peak value and FWHM of the Fano resonance mode increase obviously with the distance between the two nanowires getting larger. Meanwhile, it can be modulated by the height of the rectangular section nanowire. In addition, they can be regulated by the width and rotating angle of the rectangular section nanowire, but the peak value stays the same. The mechanisms for these behaviors are associated with the interaction of the superradiant and subradiant modes, and the corresponding electric field distributions are plotted to verify this. It is expected that the results will be useful for the design of wavelength biosensing and other new optical devices.

Reconfigurable optical forces induced by tunable mode interference in gold core-silicon shell nanoparticles

The effects of resonant mode interference on optical forces acting on gold core-silicon shell nanoparticles are theoretically investigated with the multipolar expansion method based on the Mie scattering theory. It is found that the total optical radiation force and its two components, the incident force and the recoil force, can be tuned flexibly by engineering the interference interaction among electric, magnetic, and anapole modes. The recoil force acting on the core-shell nanoparticles can be enhanced up to 17 pN compared with the pure silicon nanoparticles with the same size as that of the core-shell nanoparticles when the magnetic dipole resonant mode totally interferes with the electric dipole resonant mode. In addition, the incident force can also be improved to 25 pN by suppressing the interference between the electric dipole and the magnetic dipole resonances. More importantly, the maximum optical radiation force is not dominated by the strongest resonant scattering mode of the hybrid nanostructure due to the modes’ interference induced giant negative recoil forces. We hope our results not only improve the optical trapping and manipulation of core-shell nanoparticles but also help to understand the underlying physical mechanism regarding the tunable optical radiation forces induced by the tunable interference among different resonant modes in core-shell nanoparticles.

Reshaping the Second-Order Polar Response of Hybrid Metal-Dielectric Nanodimers.

We combine the field confinement of plasmonics with the flexibility of multiple Mie resonances by bottom-up assembly of hybrid metal-dielectric nanodimers. We investigate the electromagnetic coupling between nanoparticles in heterodimers consisting of gold and barium titanate (BaTiO3 or BTO) nanoparticles through nonlinear second-harmonic spectroscopy and polarimetry. The overlap of the localized surface plasmon resonant dipole mode of the gold nanoparticle with the dipole and higher-order Mie resonant modes in the BTO nanoparticle lead to the formation of hybridized modes in the visible spectral range. We employ the pick-and-place technique to construct the hybrid nanodimers with controlled diameters by positioning the nanoparticles of different types next to each other under a scanning electron microscope. Through linear scattering spectroscopy, we observe the formation of hybrid modes in the nanodimers. We show that the modes can be directly accessed by measuring the dependence of the second-harmonic generation (SHG) signal on the polarization and wavelength of the pump. We reveal both experimentally and theoretically that the hybridization of plasmonic and Mie-resonant modes leads to a strong reshaping of the SHG polarization dependence in the nanodimers, which depends on the pump wavelength. We compare the SHG signal of each hybrid nanodimer with the SHG signal of single BTO nanoparticles to estimate the enhancement factor due to the resonant mode coupling within the nanodimers. We report up to 2 orders of magnitude for the SHG signal enhancement compared with isolated BTO nanoparticles.

Low loss photonic nanocavity via dark magnetic dipole resonant mode near metal

The dielectric-semiconductor-dielectric-metal 4 layered structure is a well-established configuration to support TM hybrid plasmonic modes, which have demonstrated significant advantages over pure photonic modes in structures without metal to achieve low loss resonant cavities at sub-diffraction limited volumes. The photonic modes with TE characteristics supported by the same 4 layered structure, on the other hand, are less studied. Here we show that a low loss photonic mode with TE01 characteristics exists in the dielectric-semiconductor-dielectric-metal 4 layered structure if a truncated cylindrical disk is chosen as the semiconductor core. This mode exhibits the lowest cavity loss among all resonant modes, including both pure photonic and hybrid plasmonic modes, at cavity radius <150 nm and within the wavelength range 620 nm to 685 nm, with a footprint ~0.83 (λ/2neff)2, physical size ~0.47 (λ/2neff)3 and a mode volume down to 0.3 (λ/2neff)3. The low cavity loss of this TE01 mode is attributed to its substantially reduced radiation loss to the far field by the creation of image charges through the metal response. Because of the low mode penetration in the metal, this photonic mode show equally low cavity loss near industry relevant metals such as Cu. Our study demonstrates an alternative to hybrid plamonic modes and metallo-dielectric modes to achieve low loss cavities with extremely small footprints.

Elucidating the contribution of dipole resonance mode to polarization-dependent optical properties in single triangular gold nanoplates

We synthesized triangular gold nanoplates (AuNPs) through a one-pot seedless growth method and characterized their optical properties under dark-field (DF) microscopy at the single particle level. We experimentally demonstrated that the dipole resonance is not completely separated from the quadrupole resonance for single AuNPs with an aspect ratio of ∼5. We further used a defocused orientation and position imaging technique to visualize the spatial scattering field distributions from dipole and quadrupole modes. Their optical properties were mainly dominated by the dipole resonance, which resulted in the polarization-dependent, periodic DF defocused images and intensities of single AuNPs.

Efficient microparticle trapping with plasmonic annular apertures arrays

In this work, we demonstrate trapping of microparticles using plasmonic tweezers based on arrays of annular apertures. The transmission spectra and the electric-field distribution are simulated to calibrate the arrays. Theoretically, we observe sharp peaks in the transmission spectra for dipole resonance modes and these are red-shifted as the size of the annular aperture is reduced. We also expect an absorption peak at approximately 1115 m for the localised plasmon resonance. Using a laser frequency between the two resonances, multiple plasmonic hot spots are created and used to trap and transport micron and submicron particles. Experimentally, we demonstrate trapping of individual 0.5 μm and 1 μm polystyrene particles and the feasibility of particle transportation over the surface of the annular apertures using less than 1.5 mW μm−2 incident laser intensity at 980 nm.

Theoretical design of twelve-band infrared metamaterial perfect absorber by combining the dipole, quadrupole, and octopole plasmon resonance modes of four different ring-strip resonators.

Multiband metamaterial perfect absorbers (MPAs) have promising applications in many fields like microbolometers, infrared detection, biosensing, and thermal emitters. In general, the single resonator can only excite a fundamental mode and achieve single absorption band. The multiband MPA can be achieved by combining several different sized resonators together. However, it's still challenging to design the MPA with absorption bands of more than four and average absorptivity of more than 90% due to the interaction between differently sized resonators. In this paper, three absorption bands are successfully achieved with average absorptivity up to 98.5% only utilizing single one our designed ring-strip resonator, which can simultaneously excite a fundamental electric dipole mode, a higher-order electric quadrupole mode, and a higher-order electric octopole mode. As the biosensor, the sensing performance of the higher-order modes is higher than the fundamental modes. Then we try to increase the absorption bands by combining different sized ring-strip resonators together and make the average absorptivity above 90% by optimizing the geometry parameters. A six-band MPA is achieved by combining two different sized ring-strip resonators with average absorptivity up to 98.8%, which can excite two dipole modes, two quadrupole modes, and two octopole modes. A twelve-band MPA is achieved by combining four different sized ring-strip resonators with average absorptivity up to 93.7%, which can excite four dipole modes, four quadrupole modes, and four octopole modes.

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 370nm/RIU and a figure of merit of 2846. This study may provide a further step in sensing, lasing, and nonlinear optics.

Dual-channel narrowband polarization absorber with high field enhancement and refractive index sensitivity based on a nanorod array

In this paper, we present a dual-channel narrowband polarization absorber based on a metal-dielectric-metal structure, which consists of a top metallic nanorod array, a metal substrate, and an ultrathin middle dielectric spacer. The proposed structure can achieve high absorptance above 96% in a wide angular range of incidence around ±20° at two remarkable absorption peaks for transverse magnetic polarization under normal incidence. Most significantly, the extremely highly confined enhancement of electromagnetic fields between Au film and nanorods has been observed by employing numerical simulation based on a finite element method, which is up to 110 times compared with the incident electric field. The underlying physics mechanism of a strong gap plasmon resonance is analyzed, and it is primarily attributed to simultaneous excitation of multiple localized electric dipole and magnetic dipole resonance modes in this film-coupled nanorods system. Additionally, we also investigate the dependence of dual resonance peaks on structural parameters as well as their sensitivities to the refractive index of media surrounding nanorods. The wavelength modulation and intensity modulation are also shown simultaneously. This structure is near-perfect absorbing, plasmonic refractive index sensing, and surface-enhanced Raman spectroscopy, all rolled into one. It will have great significance and potential in developing new miniaturized multifunctional photonics devices and their high integrations.