Fishnet Metamaterials Research Articles

Substrate-thickness dependence of negative-index metamaterials at optical frequencies

Optical metamaterials have attracted intensive attention in recent years for their broad applications in superlenses, electromagnetic cloaking, and bio-sensing. Negative refractive index (NRI) metal–dielectric–metal fishnet metamaterials (MMs) are typically used for beyond-diffraction-limit imaging. However, there are few reports about the substrate-thickness dependence of NRI, which strongly affects the practical application. In our study, it is demonstrated that the membrane-based NRI MMs with a more negative index work better than the bulk substrate-based counterparts. In addition, a regular periodic vibration of NRI with the thickness of the membrane substrate was theoretically studied. The destructive interference of the thin film can explain this phenomenon. Furthermore, the proposed explanation was further proved by substituting the dielectric spacer with a larger permittivity. Therefore, an NRI structure on a membrane substrate with constructive interference can be a good choice in ultra-compact photoelectronic devices. This study can be a guide to the practical application of ultracompact NRI devices.

Realization of Broadband Negative Refractive Index in Terahertz Band by Multilayer Fishnet Metamaterial Approach

Realization of Broadband Negative Refractive Index in Terahertz Band by Multilayer Fishnet Metamaterial Approach

The epsilon-mu-near-zero (EMNZ) device realized by a novel structure of air gap ring at microwave frequency

The epsilon-and-mu-near-zero (EMNZ) medium, as a kind of zero-refractive-index material, has broad application perspectives. In general, there are three main methods to realize the EMNZ phenomenon based on conventional metamaterials such as fishnet metamaterials, Dirac-like cone-based metamaterials, and doped ENZ mediums in the full frequency range. However, the above EMNZ metamaterials have complex structures and stringent process requirements, and the development of mature engineering applications outside the laboratory is still a long way off. Herein, a novel, simple and stable air gap ring compatible with the EMNZ medium based on the doping method has been proposed to mitigate the above constraints. The proposed structure is composed of a set of nested metal boxes and an air gap ring formed between the nested metal boxes. By changing the size of the cross-sectional area of the perfect electric conductor (PEC) doped into an ENZ medium to manipulate the air gap ring in the nested metal boxes, the effective magnetic permeability of the whole structure can be adjusted. Finally, the equivalent EMNZ medium characterized by a simple structure can be realized in the microwave band. Thus, the proposed novel air gap ring device exhibits promising potential for broad applications such as frequency selection, impedance regulation, super-coupling, and power splitting.

Fast Fabrication of Fishnet Optical Metamaterial Based on Femtosecond Laser Induced Stress Break Technique.

To speed up the fabrication of optical metamaterials by making use of the fast speed advantage of femtosecond laser preparation, a metamaterial appropriate for femtosecond laser processing was designed, and the interaction between femtosecond laser and metal-dielectric-metal fishnet stacks was investigated in detail. Two kinds of processing mechanisms, thermal melting and stress break, were revealed during the fabrication. The thermal melting process, dominated by the interaction of femtosecond laser with metals, makes the upper and lower metal layers adhere to each other, which leads to the magnetic resonance impossible. The stress break process, dominated by the interaction of femtosecond laser with dielectrics, can keep the upper and lower metal coatings isolated. Fishnet optical metamaterial was fabricated by femtosecond laser-induced stress break technique, using back side ablation, high numerical aperture and super-Gaussian beam. The resolution and speed can reach 500 nm, and 100 units/s, respectively. Spectrophotometer measurement results proved that the magnetic resonances were found in the fishnet nanostructure. The theoretical refractive index of the metamaterial on a glass substrate reached −0.12 at the wavelength of 3225 nm. It proved that femtosecond laser-induced stress break was a good and fast tool during the fabrication of optical metamaterials.

Determination of effective parameters of fishnet metamaterials with vortex based interferometry.

Metamaterials are artificially engineered structures that have unique properties not usually found in natural materials, such as negative refractive index. Conventional interferometry or ellipsometry is generally used for characterizing the optical properties of metamaterials. Here, we report an alternative optical vortex based interferometric approach for the characterization of the effective parameters of optical metamaterials by directly measuring the transmission and reflection phase shifts from metamaterials according to the rotation of vortex spiral interference pattern. The fishnet metamaterials possessing positive, zero and negative refractive indices are characterized with the vortex based interferometry to precisely determine the complex values of effective permittivity, permeability, and refractive index. Our results will pave the way for the advancement of new spectroscopic and interferometric techniques to characterize optical metamaterials, metasurfaces, and nanostructured thin films in general.

Polarization effects in metallic plates perforated with periodic arrays of subwavelength circular holes

The enhanced transmission phenomenon (ET) discovered by Ebbesen et al has recently been a topic of the intensive research, leading to such a branch of research as fishnet metamaterials. An application of the metamaterials is polarization sensitive devices. Polarization sensitivity for ET frequency selective structures, as for classical ones, is usually associated with the hole shapes being polarization sensitive. This paper discusses a new phenomenon of polarization sensitive extraordinary transmission through a thick screen perforated with polarization independent subwavelength circular holes. The possibility to control the polarization state of the waves transmitted through the screen by manipulation periodical cell dimensions only is shown, while hole shapes are polarization independent. A linear plane wave can be rotated through an arbitrary angle, and converted to a circular or elliptical one, when passing through the screen in the vicinity of Floquet-wave grazing points. Therefore, one can realize a simple and compact polarization device at the expense of reducing bandwidth. The origin of the phenomenon is in a plane wave coupling with different types of symmetry eigenmodes leading to polarization sensitive enhanced transmission. When an incident plane wave couples to orthogonal symmetrical and antisymmetrical eigenmodes with close eigenfrequencies, the screen can behave like a half-wave plate. For the case of several closely spaced eigenmodes jointly contributing to transmitted wave, the effect of enhanced polarization conversion can be observed for the screen to behave like a quarter-wave plate.

Numerical and experimental verification of transmission enhancement through double fishnet structure metamaterials

Numerical and experimental verification of transmission enhancement through double fishnet structure metamaterials

Retrieving effective material parameters of metamaterials characterized by nonlocal constitutive relations

The parameter retrieval is a procedure in which effective material properties are assigned to a given metamaterial. A widely used technique bases on the inversion of reflection and transmission from a metamaterial slab. Thus far, local constitutive relations have been frequently considered in this retrieval procedure to describe the metamaterial at the effective level. This, however, is insufficient. The retrieved local material properties frequently fail to predict reliably the optical response from the slab in situations that deviate from those that have been considered in the retrieval, e.g. when illuminating the slab at a different incidence angle. To significantly improve the situation, we describe here a parameter retrieval, also based on the inversion of reflection and transmission from a slab, that describes the metamaterial at the effective level with nonlocal constitutive relations. We retrieve the effective material parameters at the example of a fishnet metamaterial. We demonstrate that the nonlocal constitutive relation can describe the optical response much better than local constitutive relation would do. Our approach is widely applicable to a large class of metamaterials.

Tunable index metamaterials made by bottom-up approaches.

Despite the exciting optical properties metamaterials exhibit, their implementation in technology is being hampered nowadays by the inherent losses of their metal constituents and the expensive and low-throughput procedures used. As an alternative, we present a new design of double fishnet metamaterials that can be easily realized combining two inexpensive and up-scalable techniques: nanosphere lithography and metallic electrodeposition. A monolayer of polystyrene spheres is used as a template for the infiltration of two symmetric gold layers separated by an air gap. The effective refractive index of the metamaterial can be easily tuned by the appropriate choice of the diameter of the spheres and the gap width between the metallic layers, varying its value from positive to negative. The good agreement between optical measurements and finite-difference time-domain simulations confirms the success of our process. Fishnet metamaterials with refractive index going from 1.5 until −1.0 in the near infrared range are demonstrated and the key parameters for these architectures provided.

Plasmonic resonances in sub-terahertz fishnet metamaterial based on complementary hexagonal resonator

Exploration of the perfect combination between metamaterials (MTMs) and plasmonics has paved the way of the new innovations in electromagnetic and communication engineering, especially in sub-THz spectroscopy. In order to contribute to such developments, plasmonic resonances in sub-THz fishnet MTM on the basis of complementary hexagonal resonator structure is investigated in this paper. The design, in which method of removing hexagonal structure is applied, has been performed based on finite simulation technique. Five distinct plasmonic resonances are determined at some of the specific frequency spots where interactions between electron plasmas and incident electromagnetic waves can be observed. The proposed material can be employed in several subtopics of electromagnetic engineering such as high pass filters and antenna applications. Furthermore, thanks to the THz wave confinement property of this plasmonic structure, incident THz waves can be controlled efficiently and therefore, the proposed MTM can be tailored at THz spectroscopy and imaging, localization of the THz wave and energy, sensors and photonics applications, effectively.

Absorber and sensor applications of complimentary H-shaped fishnet metamaterial for sub-terahertz frequency region

The progress in the technology of manufacturing new materials increases the usage of Terahertz (THz) region, which is considered as the last explored region of electromagnetic spectrum. In biological, chemical, sensing and detecting applications THz region is started to be used widely. We proposed a unique fishnet metamaterial (MTM) absorber and metasensor that can be used to detect and characterize new materials which are produced for THz region. First of all, the reflection and absorption characteristics of fishnet MTM were analyzed. Perfect absorption was achieved by 97.3% absorption rate and with assisting from electric field and surface current distributions we explained the electromagnetic theory behind the frequency resonances. For sensory application, the material that is going to be detected was added as a single and double overlayer (OL) to proposed fishnet MTM and fishnet MTM absorber sensor. The shifts in the resonant frequencies were observed for the change in permittivity and thicknesses of the OL materials. From the analysis of resonant shifts it is proved that the proposed structure shows high sensing ability in THz region.

Phase Reversal Technique Applied to Fishnet Metalenses

In this work, the fishnet metamaterial is applied to several converging metalenses by combining the zoning, reference phase, and phase reversal techniques. First, the zoning and reference phase techniques are implemented in several metalenses at 55 GHz (λ0=5.45 mm) with a short focal length of 1.5 λ0. Then, the phase reversal technique is applied to these metalenses by switching from a concave to a convex profile in order to change the phase distribution inside of them. The designs are evaluated both numerically and experimentally demonstrating that chromatic dispersion (the shift of the position of the focus at different frequencies) is reduced when using the phase-reversed profiles. It is shown how the position of the focus remains at the same location within a relatively broadband frequency range of ~4% around the design frequency without affecting the overall behaviour of the metalenses. The best performance is achieved with the design that combines both reference phase and phase reversal techniques, with an experimental position of the focus of 1.75 λ0, reduced side lobes, and a power enhancement of 6.5 dB. The metalenses designed here may find applications in situations where a wideband response and low side lobes are required because of the reduced chromatic aberrations of the focus.

An ultra-broadband and lightweight fishnet-like absorber in microwave region

Fishnet metamaterials have shown their excellent absorption at terahertz and optical frequencies. Here, we experimentally demonstrate an ultra-broadband and lightweight microwave absorber by fabricating a conventional magnetic absorbing material into a fishnet-like structure. Compared with the pure absorbing material, the broadest absorption bandwidth (less than −10 dB) of the fishnet-like absorber is extended from 7.4–11 GHz to 4.8–20.7 GHz. Meanwhile, the high surface density, which is a important shortcoming for the magnetic absorbing materials, has been reduced clearly because a large part of magnetic microwave absorption material has been removed. The ultra-broadband absorption is a result of the structure-induced strong magnetic resonances and electric confinement excited by the magnetic loop, which made the effective impedance of our structure matched well with the free space within the absorption bandwidth. Our results may provide a method for further improving the performance of the pure microwave absorbing materials.

Mimicking Electromagnetic Wave Coupling in Tokamak Plasma with Fishnet Metamaterials

This paper reports a fishnet hyperbolic metamaterial that mimics the electromagnetic properties of magnetically confined plasma. These electromagnetic properties are strongly anisotropic and different from any conventional material, therefore cannot be mimicked by bulk materials. The structure is made of a stack of thin copper grids spaced by Rohacell foam. We numerically and experimentally show that this kind of structuration matches well the properties of a homogeneous plasma. This solution breaks a long-lasting bottleneck and will accelerate the development of high-frequency heating systems to be used in nuclear fusion.

Regulation of Thermal Radiation Characteristics in Isotope Batteries Based on Fishnet Metamaterials

With the deepening of deep space exploration, traditional photovoltaic cells are difficult to satisfy the needs of day and night alternation and deep space exploration. The development of isotope battery is becoming increasingly important. In this paper, a fishnet metamaterial is applied in the isotope battery to regulate the radiant energy of the heat source, achieving high transmission in the near-infrared bands, and high reflection in the far-infrared band. The radiation process with spectral regulation is simulated by FLUENT commercial software. The results indicate that the temperature distribution of the isotope battery can be effectively changed by the spectral regulation of the fishnet metamaterial. The regulated radiant energy can increase the temperature of the heat source and reduce the temperature of the infrared photovoltaic cell. The results of this paper can provide potential application in the field of military and aerospace.

Thermally and optically tunable sub-terahertz superconducting fishnet metamaterial

In this paper, a novel fishnet metamaterial structure is designed and analyzed under different material combinations and under different active controlling techniques. The results indicate that, the proposed fishnet metamaterial has a single resonance with double negativity at 0.39 THz when quartz substrate and aluminum is utilized in the design. Moreover, when the metallic parts are replaced with YBCO, the proposed design also exhibits double negativity with a stronger resonance and can be used as a switch between the double negative and single negative modes if the temperature is altered. In addition to these, when substrate (quartz) is replaced with MgO, the resonance shifts from 0.39 THz to 0.26 THz and shows double negativity. Moreover, switching properties under illumination can also be obtained when the silicon is utilized in the design (MgO-YBCO combination). According to these results, it is found that, in the case that the conductivity of silicon exceeds a certain value, the character of the resonance changes from double negative to the single negative mode.

Closed-form formulae of effective parameters of hyperbolic metamaterial made by stacked hole-array layers working at terahertz or microwave radiation

A metamaterial made by stacked hole-array layers known as a fishnet metamaterial behaves as a hyperbolic metamaterial at wavelength much longer than hole-array period. However, the analytical formulae of effective parameters of a fishnet metamaterial have not been reported hindering the design of deep-subwavelength imaging devices using this structure. We report the new closed-form formulae of effective parameters comprising anisotropic dispersion relation of a fishnet metamaterial working at terahertz or microwave frequency. These effective parameters of a fishnet metamaterial are consistent with those obtained by quasi-full solutions using known effective parameters of a hole-array layer working at frequency below its spoof plasma frequency with the superlattice period much smaller than the hole-array period. We also theoretically demonstrate the deep-subwavelength focusing at {\lambda}/83 using the composite structure of a slit-array layer and a fishnet metamaterial. It is found that the focused intensity inside a fishnet metamaterial is several times larger than that without the fishnet metamaterial, but the transmitted intensity is still restricted by large-wavevector difference in air and a fishnet metamaterial. Our effective parameters may aid the next-generation deep-subwavelength imaging devices working at terahertz or microwave radiation.

Freestanding optical negative-index metamaterials of green light.

A freestanding, multilayered fishnet metamaterial is reported to experimentally exhibit a negative refractive index in the green-light spectral range. The realization of a negative refractive index at such a high frequency range mainly originates from low-loss magnetic resonance and interactions between the neighboring functional layers. Based on a good agreement between the numerically simulated and experimentally measured transmittance and reflectance spectra, a single negative refractive index of -0.76 with a figure-of-merit of 0.5 is achieved for the metamaterial at the wavelength of 532nm.

Terahertz band-pass filters based on fishnet metamaterials fabricated on free-standing [formula omitted] membrane

Terahertz (THz) band-pass filters constructed by metal–SiNx–metal hollow fishnet metamaterials were designed and fabricated. Results reveal that the as-fabricated single-layer filter responds at 3.75 THz with a high transmission of 88.1% and broad full width at half maximum of 2.79 THz. For such filters, a free-standing SiNx membrane is utilized specially as both dielectric and supporting materials, by which the insertion loss widely existed in those filters fabricated on rigid substrates can be efficiently avoided. Moreover, the critical parameters for determining the responses of the filters were systematically investigated. In our approach, multi-layer filters can be easily fabricated just by stacking the metamaterials layer by layer, and thus the spectral transmission, bandwidth, and out-of-band rejection can be rationally adjusted. This work describes a promising approach to design and fabricate THz band-pass filters based on substrate-free fishnet metamaterials.

Phase accumulation tracking algorithm for effective index retrieval of fishnet metamaterials and other resonant guided wave networks

A phase accumulation tracking (PAT) algorithm is proposed and demonstrated for the retrieval of the effective index of fishnet metamaterials (FMMs) in order to avoid the multi-branch uncertainty problem. This algorithm tracks the phase and amplitude of the dominant propagation mode across the FMM slab. The suggested PAT algorithm applies to resonant guided wave networks having only one mode that carries the light between the two slab ends, where the FMM is one example of this metamaterials sub-class. The effective index is a net effect of positive and negative accumulated phase in the alternating FMM metal and dielectric layers, with a negative effective index occurring when negative phase accumulation dominates.