Metal Nanorod Arrays Research Articles

Enhanced Broadband Metamaterial Absorber Using Plasmonic Nanorods and Muti-Dielectric Layers Based on ZnO Substrate in the Frequency Range from 100 GHz to 1000 GHz

A broadband thin film plasmonic metamaterial absorber nanostructure that operates in the frequency range from 100 GHz to 1000 GHz is introduced and analyzed in this paper. The structure consists of three layers: a 200 nm thick gold layer that represents the ground plate (back reflector), a dielectric substrate, and an array of metallic nanorods. A parametric study is conducted to optimize the structure based on its absorption property using different materials, gold (Au), aluminum (Al), and combined Au, and Al for the nanorods. The effect of different dielectric substrates on the absorption is examined using silicon dioxide (SiO2), aluminum oxide (Al2O3), titanium dioxide (TiO2), and a combination of these three materials. This was followed by the analysis of the effect of the distribution of Al, and Au nanorods and their dimensions on the absorption. The zinc oxide (ZnO) layer is added as a substrate on top of the Au layer to enhance the absorption in the microwave range. The optimized structure achieved more than 80% absorption in the ranges 100–280 GHz, 530–740 GHz and 800–1000 GHz. The minimum optimized absorption is more than 65% in the range 100 GHz to 1000 GHz.

Significant Near-Field Enhancement over Large Volumes around Metal Nanorods via Strong Coupling of Surface Lattice Resonances and Fabry–Pérot Resonance

Metal nanoparticles supporting plasmons are widely used to enhance electromagnetic fields, resulting in strong light–matter interactions at the nanoscale in a diverse range of applications. Recently, it has been shown that when metal nanorods are periodically arranged with proper lattice periods, surface lattice resonances (SLRs) can be excited and near fields can be greatly enhanced over extended volumes. In this work, we report significant near field enhancement over even larger volumes by placing the metal nanorod array within a Fabry–Pérot (F-P) microcavity. Simulation results show that by taking advantage of strong coupling between the SLR and the photonic F-P resonances, the electric field intensity of the bonding split mode can be enhanced by up to 1935 times, which is about three times of the enhancement of the SLR, and the greatly enhanced field can extend over most of the F-P microcavity. We further show that the F-P resonances of both odd and even orders can strongly couple to the SLR by varying the nanorods position from the middle of the microcavity. We expect that the proposed plasmonic-photonic coupling system will find promising applications in nanolasers, nonlinear optics and sensing.

Extraction and control of permittivity of hyperbolic metamaterials with optical nonlocality.

Metal nanorod arrays exhibit hyperbolic dispersion and optical nonlocality under certain conditions. Therefore, their optical behaviors can hardly be expressed by incident-angle-independent effective permittivity. Here we extract effective permittivity of silver nanorod arrays with diameters of 4 nm, 12 nm, and 20 nm by polarized transmission method in the visible range. The incident angles are chosen from 20° to 60° to study the influence of optical nonlocality on permittivity. We demonstrate how the diameter of the nanorods can control the effective permittivity beyond the effective medium theory. The results suggest that the effective permittivity gradually loses its accuracy as the diameter increases due to the optical nonlocality. Our experiment verifies that ultrathin nanorod arrays can resist the fluctuations caused by changes in incident angle. We also extract k-dependent effective permittivity of nanorods with larger diameters.

Topological Graphene Plasmons in a Plasmonic Realization of the Su–Schrieffer–Heeger Model

Graphene hybrids, made of thin insulators, graphene, and metals can support propagating acoustic plasmons (AGPs). The metal screening modifies the dispersion relation of usual graphene plasmons leading to slowly propagating plasmons, with record confinement of electromagnetic radiation. Here, we show that a graphene monolayer, covered by a thin dielectric material and an array of metallic nanorods can be used as a robust platform to emulate the Su-Schrieffer-Heeger model. We calculate the Zak's phase of the different plasmonic bands to characterise their topology. The system shows bulk-edge correspondence: strongly localized interface states are generated in the domain walls separating arrays in different topological phases. We find signatures of the nontrivial phase which can directly be probed by far-field mid-IR radiation, hence allowing a direct experimental confirmation of graphene topological plasmons. The robust field enhancement, highly localized nature of the interface states, and their gate-tuned frequencies expand the capabilities of AGP-based devices.

Metallic and porous Ti nanorod arrays for visible-IR light absorption and dendrite-free stable lithium-metal batteries

It is still a challenging task to prepare highly porous nanorod arrays of metals formed on substrates for optical and energy storage applications. Herein, we demonstrate the design and synthesis of black color, metallic and highly porous Ti nanorod arrays as novel current collectors for dendrite-free and highly stable Li-metal anodes. The high porosity of metallic nanorod arrays provides numerous heterogeneous nucleation sites and huge contact area and large space for the accommodation of Li metal. The conductive metallic Ti nanorod arrays enhance electrode integration. Effectively, it eliminates formation Li dendrites and demonstrates superior cycling stability over 300 cycles. Additionally, the unique porous structures of the nanorod arrays can decrease the amplitude of forced vibration in narrow space leading to light absorption. Interestingly, the metal is black instead of metallic color. The black metallic nanorod arrays can absorb more than 96% of both visible and infra-red lights. This black color metallic porous nanorod arrays may find additional applications in aerospace, energy, biomedical, defence, and chemical industries.

Numerical investigation of excitonic resonances of few-layer black phosphorus based on field enhancement

The optical absorption characteristic, which is one of the key parameters of two-dimensional black phosphorus when functioning as an optoelectronic semiconductor device, can be affected by the exciton binding energy. Field enhancement achieved by metal nanostructures can be used to adjust the photoelectric properties of low-dimensional materials. Here, we propose a new coupled field enhancement method by using metal nanorod arrays and Fabry–Perot cavity to improve the exciton-dominated optical absorption in few-layer black phosphorus. Our simulation results indicate that the exciton-dominated optical absorption can be improved to 84% (increased by 5.8-fold), while achieving a polarization-dependent optical absorption ratio of more than 30. Meanwhile, full width at half maximum of the field-enhanced formant and the polarization ratio can be improved by optimizing the geometric parameters of metal nanostructures. Our results reveal that coupled field enhancement of few-layer black phosphorus could provide a potential new scheme for tunable or polarization-sensitive optoelectronic devices.

Fabrication of metal nanorod arrays using anodic porous alumina mask with elliptical apertures prepared by lattice conversion process

Highly ordered Au nanorod arrays were fabricated using anodic porous alumina with elliptical nanoholes as an evaporation mask and applied as a substrate for measurements of surface-enhanced Raman scattering (SERS) spectra. Shape of nanoholes in anodic porous alumina obtained by self-ordering anodization process is usually circular. In the present study, elliptical nanoholes ordered in rectangular lattice were successfully fabricated by lattice conversion process using naturally ordered anodic porous alumina. The Au nanorod arrays were fabricated by thermal evaporation using the obtained anodic porous alumina. The SERS activity of the Au nanorods were higher than that of a circular Au nanodots.

Highly stable and reproducible Au nanorod arrays for near-infrared optofluidic SERS sensor

Surface-enhanced Raman spectroscopy (SERS) is a sensitive vibrational spectroscopy technique that can enable fast and non-destructive detection of trace molecules. SERS substrates are critical for the advancement of the SERS application. By incorporating SERS substrates into microfluidic devices, the function of microfluidic devices can be extended, and an efficient on-site trace analysis platform with powerful sensing capabilities can be realized. In this paper, we report the fabrication of a rapid and sensitive optofluidic SERS device using a unique Au nanorod array (AuNRA) with a plasmon resonance frequency in the near IR region. The highly stable and reproducible AuNRA were fabricated by a facile dynamic oblique angle deposition technique. A typical spectrum of 4,4-bipyridine (BPY) with enhanced peaks was observed within a few seconds after the injection of an aqueous solution BPY. Time-course measurements revealed an outstandingly quick response of SERS in this system. Using the AuNRA microfluidic device, approximately 2 × 10−12mol molecules were enough to produce detectable SERS signals, and BPY was detected down to a concentration of 15.6 × 10−9 g/mL. This work demonstrates rapid and sensitive chemical sensing using an optofluidic device equipped with a unique noble metal nanorod array.

Surface plasmon resonance sensor based on core-shell metal nanorods

A surface plasmon resonance (SPR) sensor based on core-shell metal nanorod arrays for ultra-sensitive refractive index sensing applications is proposed using three-dimensional finite element method. Simulation results show that the absorptance (A) of the proposed structure can be reached A=90.0% which is about 3.5 times enhancement compared to its solid counterpart (A=25.57%). Result shows that the sensitivity can be obtained as high as 700.00 nm/RIU (RIU is the refractive index unit). The proposed SPR sensor could be a desirable candidate for applications in nanophotonic devices.

Plasmonic perfect absorber based on metal nanorod arrays connected with veins

We propose a plasmonic perfect absorber (PPA) based on metal nanorod arrays with the connected veins for ultra-sensitive refractive index (RI) sensing applications in the near-infrared region. The physical origin is explained through the absorptance/reflectance spectra, distribution of electric/magnetic field intensity, surface charge density, height and gap distance. The designed PPA can constitute a cavity resonance center and serve as a plasmonic sensor. There is a huge difference of absorptance and reflectance between the case with and without veins due to the vein effect has a dominant influence on plasmon resonance in the proposed PPA. Simulation results show that the absorptance (A) of the proposed PPA is nearly perfect (A = 99.67%) which is about 1332.49 times enhancement compared to its counterpart without the connected veins (A = 0.075%). The calculated sensitivity, figure of merit and quality factor can be achieved up to 800.00 nm/RIU (RIU is the refractive index unit), 12.17 (RIU−1) and 14.78, respectively.

Tunable metasurface refractive index plasmonic nano-sensor utilizing an ITO thin layer in the near-infrared region.

In this paper, a refractive index plasmonic nano-sensor with an array of metallic nano-rods has been designed in the near-infrared region. The tunable sensor is based on a perfect absorber, and an indium tin oxide (ITO) layer has been utilized as an active metasurface to improve sensor performance. By applying the voltage to the ITO layer, a perfect absorption has been obtained. Also, the sensitivity rises from 625 to 1125 nm/RIU (refractive index unit), and the figure of merit is 10 times higher. Moreover, the sensor application in the fields of medicine and hematology has been shown. The proposed sensor can be used to diagnose and determine anemia and malaria parasites.

Broadband Light Harvesting for Highly Efficient Hot-Electron Application Based on Conformal Metallic Nanorod Arrays

The utilization of nonradiative decay of surface plasmons (SPs) in the form of hot electrons in metallic nanostructures has a great potential for applications in photovoltaics, photocatalysis, photodetection, and surface imaging. Unfortunately, the metallic nanostructures usually support only narrowband plasmon resonances; moreover, the hot-electron thermalization loss during the transport to Schottky interface and the confined momentum space for hot-electron injection into semiconductor together result in an inefficient internal quantum process. In this study, we propose and experimentally demonstrate a broadband super absorber based on the metallic nanorod arrays (NRs). Optically, the average absorption across the entire visible band is up to 0.8, which is over 16-fold enhancement of the planar reference. Electrically, the hot electrons are controlled to be preferentially generated near Schottky interface within the mean free path, relieving the severe hot-electron thermalization loss. Moreover, the thr...

Effective Radiative Properties of Tilted Metallic Nanorod Arrays Considering Polarization Coupling

With the advent of new nanomanufacturing techniques has come the rise of the field of nanophotonics and an increased need to determine optical properties of novel structures. Commercial software packages are able to estimate the behavior, but require large resources and heavy computational time. By combining coordinate transforms and Effective Medium Theory (EMT), an effective relative permittivity tensor is defined and further exploited to calculate the polarization-coupled Fresnel coefficients through Maxwell’s equations. A uniaxial simplification is made to show the case of tilted nanorod arrays. To demonstrate the flexibility of this system, the interfacial reflectance has been calculated for both s- and p-polarizations as well as the coupled case with the volume filling fractions of f = 0.10 and 0.30 for silver (Ag) and titanium (Ti) nanorods, and a scenario of a Ag nanorod array with polymethyl methacrylate (PMMA) as the surrounding medium. The exact results computed by the finite-difference time-domain method justify the validity of EMT with polarization coupling taken into account. The effects of incidence angle and azimuthal angle on reflectance are also discussed. The relatively simple nature of this approach allows for fast estimations of the optical properties of various nanostructures.

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.

Photo-induced-heat localization on nanostructured metallic glasses

Materials with large photo-thermal energy conversion efficiency are essential for renewable energy applications. Photo-excitation is an effective approach to generate controlled and localized heat at relatively low excitation optical powers. However, lateral heat diffusion to the surrounding illuminated areas accompanied by low photo-thermal energy conversion efficiency remains a challenge for metallic surfaces. Surface nanoengineering has proven to be a successful approach to further absorption and heat generation. Here, we show that pronounced spatial heat localization and high temperatures can be achieved with arrays of amorphous metallic glass nanorods under infrared optical illumination. Thermography measurements revealed marked temperature contrast between illuminated and non-illuminated areas even under low optical power excitation conditions. This attribute allowed for generating legible photo-induced thermal patterns on textured metallic glass surfaces.

Optically saturated and unsaturated collective resonances of flat metallic nanoantenna arrays

We study collective optical properties of arrays of flat gold nanoantennas, demonstrating they can support optically saturated and unsaturated plasmonic lattice modes when the incident light is polarized along their short axes. The saturated mode is nearly immune to the variation of the refractive index of the environment, while the unsaturated mode undergoes a large red shift without degradation as the refractive index increases. Our results show that when the incident light becomes polarized along the long axes of the nanoantennas, an increase of the refractive index of the superstrate leads to the formation of secondary plasmon peaks. These peaks are spectrally narrow and can detect variations of the ambient refractive index with a sensitivity of up to 620 nm/RIU (refractive index unit). The results suggest that the periodic arrays of flat metallic nanostructures can support hybridization of their multipolar plasmonic resonances with diffraction orders with distinct similarities and differences compared to those seen in cases of arrays of metallic nanorods.

Orthogonal Chemical Modification of Template-Synthesized Nanostructures with DNA.

Very few chemical strategies for the selective functionalization of nanostructures have been developed despite their potential for controlling high-order assembly processes. We report a novel approach for the selective chemical functionalization and localized assembly of one-dimensional nanostructures (rods), based upon the systematic activation (DNA functionalization) and passivation (self-assembled monolayers) of specific surface sites through the use of orthogonal chemical reactions on electrochemically grown metal nanorod arrays in porous anodic aluminum oxide templates. The ability to orthogonally functionalize the ends or the side of a nanorod, as well as the gaps between two rods, with different DNA strands allows one to synthesize nanostructure assemblies that would be difficult to realize any other way and that could ultimately be utilized for making a wide variety of device architectures.

Biological sensing and control of emission dynamics of quantum dot bioconjugates using arrays of long metallic nanorods

We study biological sensing using plasmonic and photonic-plasmonic resonances of arrays of long metallic nanorods and analyze the impact of these resonances on emission dynamics of quantum dot bioconjugates. We demonstrate that the localized surface plasmon resonances (LSPRs) and plasmonic lattice modes of such arrays can be used to detect a single self-assembled monolayer of alkanethiol at the visible (550 nm) and near infrared (770 nm) range with well resolved shifts. We study the adsorption of streptavidin-quantum dot conjugates to this monolayer, demonstrating that formation of nearly two dimensional arrays of quantum dots with limited emission blinking can lead to extra well-resolved wavelength shifts in these modes. Using spectrally-resolved lifetime measurements, we study the emission dynamics of such quantum dot bioconjugates within their monodispersed size distribution. We show that, despite their close vicinity to the nanorods, the rate of energy transfer from these quantum dots to nanorods is rather weak, while the plasmon field enhancement can be strong. Our results reveal that the nanorods present a strong wavelength or size-dependent non-radiative decay channel to the quantum dot bioconjugates.

Describing the dispersion of plasmonic nanorod arrays via coupling of elementary excitations

Metamaterials such as metal-dielectric multilayers and cylindrical nanowires are well known, for instance because of their hyperbolic dispersion. Here we examine in detail the mode characteristics in an array of square and rectangular metallic nanorods. In particular we propose a method to describe the dispersion via the coupling of specific elementary excitations. Apparently, these fundamental modes depend on the size and shape of the nanorods, and on the particular symmetry of the Bloch modes. Specifically, we show that arrays of relatively small square nanorods are associated with coupling of single rod modes. In contrast, large nanorod arrays correspond with a basic structure consisting of four metallic corners. In the medium size case, the nature of the elementary excitation depends on the frequency range and Bloch mode symmetry. Finally, we study rectangular nanorods, which turn out to derive from a basic geometry with two semi-infinite rods. The analysis method is thus useful for a better comprehension of many other types of metamaterials.

Wettability manipulation of magnetic transition metal nanorod arrays by X-ray irradiation

Wettability manipulation of glancing angle deposited Fe/Co/Ni nanorod arrays was realized by X-ray irradiation in ultra-high vacuum chamber. Reversible transition was also purchased by alternating ethanol immersion and X-ray irradiation. Alkyl group adsorption–desorption mechanism and corresponding morphology dependence of wettability manipulation were revealed.