Plasmonic Metamaterials Research Articles

Enhancing ultra-wideband THz fingerprint sensing of unpatterned 2D carbon-based nanomaterials

THz molecular fingerprint sensing is a promising non-destructive method to accurately detect ultra-thin carbon-based materials in the nanoscale. Due to their extremely low THz absorption, plasmonic metamaterials or all-dielectric metasurfaces have been adopted to enhance the light-matter interaction for detection. However, they cause considerable parasitic losses or complicated material processing on a patterned surface. Here, we propose a lithography-free all-dielectric sensor to enhance THz absorption via an evanescent wave, which can lead to high detecting performance by a coupled mode. In view of the molecular broadband features, we use a thickness-multiplexed scheme to boost the detection of fingerprint significantly. The enhancing factor for the minimum fluctuation of fingerprint feature points is up to 534. Our method drastically enhances the broadband fingerprint intensity of the ultra-thin nanoscale layer and make it comparable to that of a 700-times thick sample layer, measured with a regular approach. Our study paves the way for broadband THz fingerprint sensing of trace-amount analytes and will inspire many burgeoning THz detection applications on 2D or ultra-thin carbon-based nanomaterials.

Plasmonic anapole states of active metamolecules

Anapole states, accompanied by strong suppression of light scattering, have attracted extensive attention in recent years due to their supreme performance in enhancing both linear and nonlinear optical effects. Although both low- and high-order anapole states are observed in the dielectric particles with high refractive index, so far few studies have touched on the topic of plasmonic anapole states. Here we demonstrate theoretically and numerically that the ideal plasmonic anapole states (strong suppression of electric dipole scattering) can be achieved in metallic metamolecules via increasing the coupling strength between Cartesian electric dipole and toroidal dipole moments of the system. The increasing coupling is based on compensation of ohmic losses in a plasmon system by introducing of a gain material, the influence of which is well described by the extended coupled oscillator model. Due to suppression of dipole radiation losses, the excitation of anapole states in plasmonic systems can result in enhancement of the near fields in subwavelength spatial regions outside of nanoparticles. That is especially important for developments of nonlinear nanophotonic and plasmonic devices and active functional metamaterials, which provide facilities for strong light energy concentration at the nanoscale. Development of the considered anapole effect with increase of metamolecule components is discussed.

Independently tunable double Fano-like resonances arising from the interference coupling of localized surface plasmons with waveguide modes

Research on the classical analogy of asymmetric Fano resonances has gained momentum in recent years, owing to the steep dispersion of the Fano-like profile in classical systems, such as plasmonic nanostructures and metamaterials, which has numerous applications. We studied the double Fano-like resonances arising in a waveguide structure made of a periodic array of gold nanospheres placed on the top surface of an indium tin oxide (ITO) film, which is deposited on a silica substrate. These resonances emanate from the interference coupling between the dipolar surface plasmon mode of the individual gold nanospheres and the zero-order transverse electric (TE) and transverse magnetic (TM) waveguide modes within the ITO film. Increasing the array period of the gold nanospheres or thickness of the ITO film shifted the resonance peaks towards the low-frequency side corresponding to the excitation frequencies of the zero-order TE and TM waveguide modes, which were red-shifted. Additionally, the double Fano-like resonance profiles were approximated using an analytical Fano interference model.

Revisiting the Optical Dispersion of Aluminum‐Doped Zinc Oxide: New Perspectives for Plasmonics and Metamaterials

Plasmonics and Metamaterials In article number 2000086, Jost Adam, Alireza Shabani, and co-workers demonstrate the potential of aluminum-doped zinc oxide (AZO) for future plasmonic metamaterials. Broadband optical dispersion data for varying dopant concentrations are extracted from first-principles calculations and verified against experimental data. AZO-based split-ring resonator metamaterials reveal increased metallic characteristics and plasmonic performance with rising aluminum concentration.

Angle-insensitive plasmonic nanorod metamaterial-based band-pass optical filters

We demonstrate, experimentally and theoretically, a new class of angle-insensitive band-pass optical filters that utilize anisotropy of plasmonic nanorod metamaterials, in both ε ≃ -1 and epsilon-near-infinity regimes, to minimize dependence of optical path on the incident angle. The operating wavelength and bandwidth of the filter can be engineered by controlling the geometry of the metamaterial. Experimental results are in agreement with full wave numerical and analytical solutions of the Maxwell's equations. Theoretical simulations show that performance of the systems can be further improved by replacing metallic mirrors with dielectric stacks.

Label-Free Ultrasensitive Detection of Abnormal Chiral Metabolites in Diabetes.

Homochirality is necessary for normal biochemical processes in humans. Abnormal amounts of chiral molecules in biofluids have been found in patients with diabetes. However, the detailed analysis of diabetes-related abnormal chirality in biofluids and its potential use for clinical applications have been hindered by the difficulty in detecting and monitoring the chiral changes in biofluids, due to their low molar mass and trace concentrations. Herein, we demonstrate the label-free detection of chiral molecules using only 10 μL with 107-fold enhancement in sensitivity compared with traditional plasmonic chiral metamaterials. The ultrahigh sensitivity and low sample consumption were enabled by microbubble-induced rapid accumulation of biomolecules on plasmonic chiral sensors. We have applied our technique on mouse and human urine samples, uncovering the previously undetectable diabetes-induced abnormal dextrorotatory shift in chirality of urine metabolites. Furthermore, the accumulation-assisted plasmonic chiral sensing achieved a diagnostic accuracy of 84% on clinical urine samples from human patients. With the ultrahigh sensitivity, ultralow sample consumption, and fast response, our technique will benefit diabetes research and could be developed as point-of-care devices for first-line noninvasive screening and prognosis of prediabetes or diabetes and its complications.

Mode Engineering in Large Arrays of Coupled Plasmonic–Dielectric Nanoantennas

AbstractStrong electromagnetic field confinement and enhancement can be readily achieved in plasmonic nanoantennas, however, this is considerably more difficult to realize over large areas, which is essential for many applications. Here, dispersion engineering in plasmonic metamaterials is applied to successfully develop and demonstrate a coupled array of plasmonic–dielectric nanoantennas offering an ultrahigh density of electromagnetic hot spots (1011 cm−2) over macroscopic, centimeter scale areas. The hetero‐metamaterial is formed by a highly ordered array of vertically standing plasmonic dipolar antennas with a ZnO gap and fabricated using a scalable electrodeposition technique. It supports a complex modal structure, including guided, surface and gap modes, which offers rich opportunities, frequently beyond the local effective medium theory, with optical properties that can be easily controlled and defined at the fabrication stage. This metamaterial platform can be used in a wide variety of applications, including hot‐electron generation, nanoscale light sources, sensors, as well as nonlinear and memristive devices.

Wide range variation of resonance wavelength of GaZnO plasmonic metamaterials grown by molecular beam epitaxy with slight modification of Zn effusion cell temperatures

The innovative growth condition of heavily Ga-doped ZnO (GaZnO) plasmonic metamaterials is reported in this article. GaZnO epilayers are deposited by molecular beam epitaxy (MBE) with small variation of Zn effusion cell temperatures. The variation of Zn effusion cells temperatures are 330, 335, and 340 °C corresponding to Ga doping concentration 6.8, 6.3, and 3.2 at%, respectively, which are obtained from the measurements of energy-dispersive X-ray spectroscopy (EDX). Temperatures of Ga effusion cell and sapphire substrate are fixed at 735 and 250 °C in the growth. Hall effect measurements exhibiting free electron concentration of all samples approach 1021 cm−3. The lowest strain/stress (0.08%/0.34 GPa), the largest crystalline size (47.5 nm), the highest Hall mobility (30.1 cm2/Vs), and the lowest material loss of GaZnO epilayers are observed in Ga doping concentration 6.3 at% sample. Statistical analysis of root mean square (RMS) surface roughness declines from 3.1 to 1.74 nm in the atomic force microscope (AFM) measurements when the doping is increased from 3.2 to 6.8 at%. Wide range variation of plasmonic resonance wavelength from 1333 to 1515 nm with the raise of Ga doping concentration is acquired through the analysis of spectroscopic ellipsometry measurements. Slight variations of Zn effusion cell temperatures in the range of 330–340 °C bring about efficient and drastic change of Ga doping concentration as well as wide range variation of plasmonic resonance wavelength (1333–1515 nm) for the applications in fiber-optic communication.

Subwavelength optical localization with toroidal excitations in plasmonic and Mie metamaterials

AbstractSince the performance of electronic circuits is becoming rather limited in face of intensively increasing of amount of information and related operations, all‐optical processing offers a promising strategy for future information system. It would benefit a great deal if the all‐optical processing could be implemented within the developed electronic chips of nanoscale structures. In that it is highly desirable to break the diffraction limit of light for achieving effective light manipulations with deep subwavelength structures compatible with the state‐of‐the‐art nanofabrication processes. It is of fundamental importance to get subwavelength optical localization, that is, squeeze light wave into subwavelength space for achieving freely manipulating of light fields. This review summarizes the development in realizing subwavelength optical localization by exciting toroidal mode in photonic metamaterials. The toroidal excitations in plasmonic metamaterials and Mie resonant metamaterials, in 3D structures and planar metamaterials, with single or few layers in spectral regime from microwave to optical frequencies are surveyed. Based on the discussion on the configurations of toroidal excitations, the recent development on toroidal‐related optical scattering control actively manipulates the toroidal excitations, and promising applications are further investigated and highlighted.image

Extremely high Q-factor terahertz metasurface using reconstructive coherent mode resonance.

High Q-factor resonance has a pivotal role in wide applications for manipulating electromagnetic waves. However, high Q-factor resonance, especially in the terahertz (THz) regime, has been a challenge faced by plasmonic metamaterials due to the inherent ohmic and radiation losses. Here, we theoretically present a unique metasurface scheme to produce extremely high Q-factor Fano resonance of the reconstructive coherent mode in the THz regime. The THz metasurface is composed of periodically arranged vertical symmetric split ring resonators (SRRs), which can produce perfect reconstructive coherent coupling effect in the sense that dipole radiation is destructively suppressed. Under the polarized electric field perpendicular to SRR gap, the surface currents are out of phase for an individual SRR, leading to the cancellation of net dipole moment. The reconstructive coherent mode resonance can occur between each SRR and its neighboring SRRs, accompanied by destructive interference of the scattered fields of each SRR. This is due to the coupling between the localized resonance of individual particles and the Rayleigh anomaly of the array. The proposed metasurface can significantly suppress far-field radiation and perform an extremely high Q-factor beyond 104 level with large modulation depth in the THz region, which pushes the advancement of THz high Q-factor resonance. The extremely high Q-factor of reconstructive coherent mode is tunable by adjusting the geometry parameters. The design strategy is useful to develop ultra-sensitive sensors, narrow-band filters and strong interaction of field-matter in the THz regime.

Broadband Enhanced Chirality with Tunable Response in Hybrid Plasmonic Helical Metamaterials

AbstractDesigning broadband enhanced chirality is of strong interest to the emerging fields of chiral chemistry and sensing, or to control the spin orbital momentum of photons in recently introduced nanophotonic chiral quantum and classical optical applications. However, chiral light‐matter interactions have an extremely weak nature, are difficult to control and enhance, and cannot be made tunable or broadband. In addition, planar ultrathin nanophotonic structures to achieve strong, broadband, and tunable chirality at the technologically important visible to ultraviolet spectrum still remain elusive. Here, these important problems are tackled by experimentally demonstrating and theoretically verifying spectrally tunable, extremely large, and broadband chiroptical response by nanohelical metamaterials. The reported new designs of all‐dielectric and dielectric‐metallic (hybrid) plasmonic metamaterials permit the largest and broadest ever measured chiral Kuhn's dissymmetry factor achieved by a large‐scale nanophotonic structure. In addition, the strong circular dichroism of the presented bottom‐up fabricated optical metamaterials can be tuned by varying their dimensions and proportions between their dielectric and plasmonic helical subsections. The currently demonstrated ultrathin optical metamaterials are expected to provide a substantial boost to the developing field of chiroptics leading to significantly enhanced and broadband chiral light‐matter interactions at the nanoscale.

Revisiting the Optical Dispersion of Aluminum‐Doped Zinc Oxide: New Perspectives for Plasmonics and Metamaterials

Due to the high rate of optical losses and the extensive usage of noble metals, alternative plasmonic materials with maximum tunability and low loss are desired for future plasmonic and metamaterial devices and applications. Herein, the potential of aluminum‐doped zinc oxide (AZO), one of the most prominent members of the transparent conducting oxide family, is demonstrated, for its applicability in plasmonic metamaterials. Using first‐principles density functional theory, combined with optical calculations, AZO‐based, plasmonic split‐ring resonators (SRRs) as model examples are showcased. The results match with experimental reports for the optical dielectric functions of pure and 2.08% Al‐doped zinc oxide (ZnO), if the Hubbard model to the local density approximation is applied. The broadband optical dispersion data for varying dopant concentrations (0%, 2.08%, and 6.25%) are extracted and provided. The subsequent optical response analyses show the existence of pronounced plasmons and inductor–capacitor modes in Al‐doped ZnO SRRs and an enhancement in metallic characteristics and plasmonic performance of AZO upon increasing Al concentration. The findings predict AZO as a low‐loss plasmonic material with promising capability for enhancing future optoelectronics applications. The method introduces a new, versatile approach to design future optical materials of arbitrary geometry.

Ultra-wideband perfect reflection and tunneling by all-dielectric metamaterials.

All-dielectric metamaterials are a promising low-loss alternative to plasmonic metamaterials for near-infrared perfect reflection, but the working bandwidth is still limited. Here we propose an ultra-wideband all-dielectric metamaterial perfect reflector that has a compact structure consisting of the subwavelength high-index grating, connection layer, and multilayer stack. Such a perfect reflector combines the advantages of quarter-wave design and resonant leaky mode, and covers an extremely wide wavelength range from 966 to 2203 nm under the normal incidence of transverse magnetic wave. By engineering the connection layer, the reflection band can be split with an ultra-narrowband tunneling of light transmission. These achievements demonstrate the promising potential of all-dielectric metamaterials as ultra-wideband reflectors for extensive applications in optical devices and systems.

Spoof Localized Surface Plasmons for Sensing Applications

AbstractLocalized surface plasmons (LSPs) are localized oscillations of free electrons in metal nanoparticles at optical frequencies. Confined mode profiles and near‐field enhancements make LSPs ultrasensitive to the dielectric environment, making them good candidates as sensors. The concept and applications have been generalized to spoof LSPs in microwave and terahertz frequencies, via plasmonic metamaterials composed of subwavelength corrugations. Herein, the basic physics, sensing prototypes, detection schemes, and state‐of‐the‐art progress are broadly reviewed from optical LSP sensing to microwave spoof LSP sensing. While optical LSPs exhibit localized sensitivity enhancement with high attenuation, spoof LSPs in microwave and terahertz frequencies combine the characteristics of deep‐subwavelength confinement and sensitivity enhancement of optical LSPs with low loss, high quality factor, multipole modes, and on‐chip detection of optical microcavities. Meanwhile, advances in printed circuits, integrated circuits, wireless communications, wearable devices, and Internet of things have endowed microwave sensing with a solid technical foundation and promising prospects. Applications in liquid sensing, gas sensing, and wearable sensing are demonstrated. Discussions are extended to electromagnetic sensing throughout the wave spectra, with concerns about key supporting technologies. The prospect of microwave sensing is emphatically investigated, specifically on leveraging the advantages of plasmonic enhancement.

Ultrasensitive plasmon enhanced Raman scattering detection of nucleolin using nanochannels of 3D hybrid plasmonic metamaterial

Detection of cancer biomarker is of great significance in cancer diagnostics. In this work, we propose an ultrasensitive and in situ method for plasmon enhanced Raman scattering (PERS) detection of nucleolin (NCL) using a 3D hybrid plasmonic metamaterial (PM). In this aptasensor, thiolated complementary DNA (cDNA) immobilized on PM can hybridize with Rox-labeled NCL-binding aptamer (AS1411-Rox) to form a rigid double-stranded DNA (dsDNA). When NCL passes through the PM nanochannels under a transmembrane voltage bias, it interacts with AS1411-Rox to form G-quadruplexes (G4-AS1411-Rox), resulting in the release of AS1411-Rox from the nanochannels surface and the decrease in PERS signal of the reporter Rox. This change in PERS signals can be recorded in situ without the interference of external environment. With the help of the enrichment function of nanochannel, the present method is able to achieve fast NCL detection within 10 min with a detection limit as low as 71 pM. Furthermore, our method shows excellent specificity, reversibility, uniformity (relative standard deviation of ~6.86%) and reproducibility (~6.65%), providing a new platform for reliable cancer auxiliary diagnosis and drug screening.

Self-Assembled BaTiO3-AuxAg1-x Low-Loss Hybrid Plasmonic Metamaterials with an Ordered "Nano-Domino-like" Microstructure.

Metallic plasmonic hybrid nanostructures have attracted enormous research interest due to the combined physical properties coming from different material components and the broad range of applications in nanophotonic and electronic devices. However, the high loss and narrow range of property tunability of the metallic hybrid materials have limited their practical applications. Here, a metallic alloy-based self-assembled plasmonic hybrid nanostructure, i.e., a BaTiO3-AuxAg1-x (BTO) vertically aligned nanocomposite, has been integrated by a templated growth method for low-loss plasmonic systems. Comprehensive microstructural characterizations including high-resolution scanning transmission electron microscopy (HRSTEM), energy-dispersive X-ray spectroscopy (EDS), and three-dimensional (3D) electron tomography demonstrate the formation of an ordered "nano-domino-like" morphology with Au0.4Ag0.6 nanopillars as cylindrical cores and BTO as square shells. By comparing with the BTO-Au hybrid thin film, the BTO-Au0.4Ag0.6 alloyed film exhibits much broader plasmon resonance, hyperbolic dispersion, low-loss, and thermally robust features in the UV-vis-NIR wavelength region. This study provides a feasible platform for a complex alloyed plasmonic hybrid material design with low-loss and highly tunable optical properties toward all-optical integrated devices.

Interaction between plasmonic silver nanorod arrays and nanosecond pulsed laser

Plasmonic metamaterials have recently received increasing attention due to their favorable optical and electrical properties. The physics of the interaction between plasmonic metamaterials and lasers are rich and open opportunities for applications, such as sensing, imaging and photovoltaics. Here, we investigate the mechanism of the interaction between Ag nanorod arrays and nanosecond (ns) pulsed laser. The experimental results show that ns laser pulses significantly improve the crystallinity of the Ag nanorod arrays. Meanwhile, the laser evaporates some of the Ag into the air. This work paves the way for future high-performance plasmonic meta-devices.

Physics of Anomalous Transient Light Transmission Through Subwavelength Metallic Slit

The dynamics of transient light transmission through a subwavelength slit can be intricate. Their complicated and diverse wave functions, such as beat-like, and their underlying novel physics are illuminated using a concept of step phasors and phase delay without involving nonlinearity. Step represents the change in amplitude for any non-steady function. Phase delay caused by roundtrip reflection within the slit is also critical. The two can trigger complex combinations of constructive and destructive interferences to manifest diverse consequences, which are frequently observed yet less understood. In addition to step broadening and alternative amplitude change, sinusoidal fields can be packaged together to form various wave packets, while a phase shift accumulates until steady state. Besides shedding light into new physics, our step phasor concept can also be generalized to study any form of incident light profiles and to clarify intriguing phenomena involving plasmonic metamaterials frequently occurring in nanophotonic simulations and experiments.

Wavelength-Tunable Purcell Effect in a Surface Plasmon Polariton State on a Metamaterial Edge

Layered metal-dielectric structures are easy-to-fabricate one-dimensional plasmonic metamaterials. Using the effective media approach, we consider surface plasmons that appear on an interface between such metamaterial and a dielectric. We have shown that two types of surface plasmon states appear on an interface. The frequency of the peak in the local density of states of these states is dependent on the fraction of metal in the metamaterial. By reducing the fraction, the frequency can be shifted towards longer wavelengths. There the losses of light in metal are reduced, which, as we have shown, leads to the increase in peak value of the Purcell factor.

Temperature Effect of Plasmonic Circular Dichroism in Dynamic Oligomers of AuNR@Ag Nanorods Driven by Cysteine: The Role of Surface Atom Migration

AbstractAchieving huge chiroptical responses is one of the driving forces for the applications of plasmonic metamaterials. Heat‐assisted symmetry breaking can amplify plasmonic circular dichroism (PCD) signals in Au nanorod (AuNR) oligomers; however, the PCD responses disappear on Ag‐exposed surface (AuNR@Ag core‐shell nanorods) at high temperature. Thus, the inherent reason of different chiral response between Au‐ and Ag‐surface nanorods oligomers is explored, and the chiral mechanism is further cognized. The enhanced mobility of surface Ag atoms at elevated temperatures leads to the damage of cysteine chiral networks on Ag surface. It therefore causes great reduction of chiral drive forces, thus restricting the PCD temperature amplification effect to a lower annealing temperature, compared to their Au‐exposed counterparts. In addition, the PCD signals of the AuNR@Ag oligomers show a pH‐dependent on‐off behavior, substantiating the dominant role of chiral network. In addition, density functional theory simulation and relevant experiments reveal facet‐dependent chiral response of Ag and Au oligomers. Then, Au–Ag alloy shells can suppress Ag atoms migration; the chiral network and PCD temperature amplification effect can exist in high temperature. The findings reveal the importance of understanding interface ligands, plasmonic nanoparticles, and their interactions in the rational design of chiral assemblies composed of plasmonic nano‐units.