Fishnet Structure Research Articles

An improved fishnet three-dimensional metamaterial with multiband left-handed characteristics at terahertz frequencies

We present the design of a multiband left-handed three-dimensional (3D) metamaterial based on improved fishnet structure at terahertz frequencies. The design realizes a three-dimensional material by mechanical stacking of multiple layers. The electromagnetic properties of the metamaterial have been investigated by numerical simulation. The results show that simultaneously negative values of permittivity, permeability and refractive index are found around the frequencies of 0.73, 0.85 and 1.12 THz for the electromagnetic wave normal incidence. The proposed metamaterial with independent polarization and compact effect offers a way to develop THz 3D materials and devices suitable for multifrequencies.

Photonic Bloch Oscillations in Multi-layered Fishnet Structure

This study investigated the time-evolved photonic Bloch oscillations (PBOs) in a multi-layered fishnet structure (MLFS) with both a negative refraction index (NRI) and a positive refraction index (PRI) at different transmission bands. The PBOs in MLFS are generated by the Bragg reflection and total internal reflection of the incident pulse undergoing at the two opposite ends of the MLFS. The amplitude and period of PBOs in MLFS with both effective NRI and PRI are almost independent of the central frequency of the incident pulse.

Complex k band diagrams of 3D metamaterial/photonic crystals

A finite element method (FEM) for solving a complex valued k(ω) vs. ω dispersion curve of a 3D metamaterial/photonic crystal system is presented. This 3D method is a generalization of a previously reported 2D eigenvalue method [Opt. Express 15, 9681 (2007)]. This method is particularly convenient for analyzing periodic systems containing dispersive (e.g., plasmonic) materials, for computing isofrequency surfaces in the k-space, and for calculating the decay length of the evanescent waves. Two specific examples are considered: a photonic crystal comprised of dielectric spheres and a plasmonic fishnet structure. Hybridization and avoided crossings between Mie resonances and propagating modes are numerically demonstrated. Negative index propagation of four electromagnetic modes distinguished by their symmetry is predicted for the plasmonic fishnets. By calculating the isofrequency contours, we also demonstrate that the fishnet structure is a hyperbolic medium.

Fishnet metastructure for efficiency enhancement of a thin film solar cell

We propose embedment of a fishnet metastructure in the back passivation layer for enhancing the efficiency of hydrogenated amorphous silicon (a-Si:H) thin film solar cells. Incident light excites a plasmon resonance that results in frequency dependent effective impedance for the embedding layer so that the input impedance satisfies impedance matching condition. Reflection is very low under this condition. A planar 20 nm-thick fishnet structure is embedded in the back passivation layer of the solar cell to enhance light absorption near the bandgap of a-Si:H. This enhancement remains over a wide range of incident angles. Detailed electromagnetic modeling of the absorption in different layers of the solar cell is performed. Only absorption in the a-Si:H is included in computing the photocurrent generation. 64% of the total absorbed energy at resonance is in the silicon layer and this absorption is uniformly distributed inside the silicon. Based on the enhancement of photocurrent density near the bandgap of a-Si:H, we obtained 14.8% enhancement in total short circuit current at normal incidence and the estimated PV efficiency of the solar cell with the fishnet is 7.43% at normal incidence compared to 6.36% without fishnet. The fishnet can be tuned to provide absorption enhancement at any desired frequency where the intrinsic absorption of the semiconductor is low.

Anomalous retroreflection from strongly absorbing nanoporous semiconductors

Pronounced retroreflection behavior is reported for a fishnet nanoporous strongly absorbing semiconductor material. Retroreflection features a half-cone about 0.35 rad along with diffusive specular reflection for all angles of incidence. Retroreflection is apparent by the naked eye with daylight illumination and exhibits no selectivity with respect to wavelength and polarization of incident light featuring minor depolarization of retroreflected light. The reflectance in the backward direction measures 12% with respect to a white scattering etalon. The phenomenon can be classified neither as coherent backscattering nor as Anderson localization of light. The primary model includes light scattering from strongly absorptive and refractive superwavelength clusters existing within the porous fishnet structure. A reasonable qualitative explanation is based on the fact that strict retroreflection obeys shorter paths inside absorbing medium, whereas all alternative paths will lead to stronger absorption of light.

Loss compensated negative index material at optical wavelengths

AbstractWe present a computational approach, allowing for a self-consistent treatment of three-dimensional (3D) fishnet metamaterial operating at 710nm wavelength coupled to a gain material incorporated into the nanostructure. We show numerically that loss-free negative index material is achievable by incorporating gain material inside the fishnet structure. The effective gain coefficient of the combined fishnet-gain system is much larger than its bulk counterpart and the figure-of-merit (FOM=|Re(n)/Im(n)|) increases dramatically with gain. Transmission, reflection, and absorption data, as well as the retrieved effective parameters, are presented for the fishnet structure with and without gain.

Time domain homogenization of metamaterials

We propose a simple, robust, and versatile time domain approach to homogenize metamaterials, taking into account both frequency and spatial dispersion. The macroscopic electromagnetic response of optical metamaterials, such as the fishnet structure and plasmonic metamaterials with near zero parameters, is characterized in terms of local effective parameters $\ensuremath{\varepsilon}$ and $\ensuremath{\mu}$, under the assumption that the effects of spatial dispersion are weak.

Enhanced lens byεandμnear-zero metamaterial boosted by extraordinary optical transmission

In this paper we report directivity enhancement by a short-focal-length plano-concave lens engineered by stacked subwavelength hole arrays (fishnet-like stack) with an effective negative index of refraction close to zero, $n$ \ensuremath{\rightarrow} 0, that arises from \ensuremath{\varepsilon} and \ensuremath{\mu} near-zero extreme values. The plano-concave lens frequency response shows two enhancement peaks, one at the wavelength corresponding to $n$ $=$ \ensuremath{-}1 and, prominent in this configuration, another unexpected peak when $n$ \ensuremath{\rightarrow} 0 that comes as a result of the similar low values of \ensuremath{\varepsilon} and \ensuremath{\mu}. The frequency-dependent negative refractive index and beam-forming properties of the lens are supported by finite-integration-frequency- and time-domain simulations and experimental results. This near-zero metamaterial lens can find applications in terahertz and even optics since the building block, stacked extraordinary optical transmission layers, has already been reported for those regimes under the name of fishnet structure.

Ultrafast nonlinear optical spectroscopy of a dual-band negative index metamaterial all-optical switching device

We study the nonlinear optical response of a fishnet structure-metamaterial all-optical switching device that exhibits two near-infrared negative-index resonances. We study and compare the nonlinear optical response at both resonances and identify transient spectral features associated with the negative index resonance. We see a significantly stronger response at the longer wavelength resonance, but identical temporal dynamics at both resonances, providing insight into separately engineering the switching time and switching ratio of such a fishnet structure metamaterial all-optical switch. We also numerically reproduce the nonlinear behavior of our device using the Drude conductivity model and a finite integration technique over wide spectral and pump fluence ranges. Thereby, we show that beyond the linear properties of the device, the magnitude of the pump-probe response is completely described by only two material parameters. These results provide insight into engineering various aspects of the nonlinear response of fishnet structure metamaterials.

Low-Loss Multilayered Metamaterial Exhibiting a Negative Index of Refraction at Visible Wavelengths

We experimentally demonstrate a low-loss multilayered metamaterial exhibiting a double-negative refractive index in the visible spectral range. To this end, we exploit a second-order magnetic resonance of the so-called fishnet structure. The low-loss nature of the employed magnetic resonance, together with the effect of the interacting adjacent layers, results in a figure of merit as high as 3.34. A wide spectral range of negative index is achieved, covering the wavelength region between 620 and 806 nm with only two different designs.

Dual-Band Terahertz Left-Handed Metamaterial with Fishnet Structure

We present the design of a dual-band left-handed metamaterial with fishnet structure in the terahertz regime. Its left-handed properties are described by the retrieved effective electromagnetic parameters. We introduce an equivalent circuit which offers a theoretical explanation for the left-handed behavior of the dual-band fishnet metamaterial, and investigate its losses receiving higher figure of merit. The design is beneficial to the development of frequency agile and broadband THz materials and devices. The dual-band fishnet metamaterial can be extended to infrared and optical frequency ranges by regulating the structural parameters.

Negative Refractive Index Metasurfaces for Enhanced Biosensing.

In this paper we review some metasurfaces with negative values of effective refractive index, as scaffolds for a new generation of surface plasmon polariton-based biological or chemical sensors. The electromagnetic properties of a metasurface may be tuned by its full immersion into analyte, or by the adsorption of a thin layer on it, both of which change its properties as a plasmonic guide. We consider various simple forms of plasmonic crystals suitable for this purpose. We start with the basic case of a freestanding, electromagnetically symmetrical plasmonic slab and analyze different ultrathin, multilayer structures, to finally consider some two-dimensional “wallpaper” geometries like split ring resonator arrays and fishnet structures. A part of the text is dedicated to the possibility of multifunctionalization where a metasurface structure is simultaneously utilized both for sensing and for selectivity enhancement. Finally we give an overview of surface-bound intrinsic electromagnetic noise phenomena that limits the ultimate performance of a metasurfaces sensor.

Fabrication of Large Area Fishnet Optical Metamaterial Structures Operational at Near-IR Wavelengths.

In this paper, we demonstrate a fabrication process for large area (2 mm × 2 mm) fishnet metamaterial structures for near IR wavelengths. This process involves: (a) defining a sacrificial Si template structure onto a quartz wafer using deep-UV lithography and a dry etching process (b) deposition of a stack of Au-SiO2-Au layers and (c) a ‘lift-off’ process which removes the sacrificial template structure to yield the fishnet structure. The fabrication steps in this process are compatible with today’s CMOS technology making it eminently well suited for batch fabrication. Also, depending on area of the exposure mask available for patterning the template structure, this fabrication process can potentially lead to optical metamaterials spanning across wafer-size areas.

Self-consistent calculations of loss-compensated fishnet metamaterials

We present a computational approach, allowing for a self-consistent treatment of three-dimensional (3D) fishnet metamaterial coupled to a gain material incorporated into the nanostructure. We show numerically that one can compensate the losses by incorporating gain material inside the fishnet structure. The pump rate needed to compensate the loss is much smaller than the bulk gain and the figure of merit (FOM = |Re(n)/Im(n)|) increases dramatically with the pump rate. Transmission, reflection, and absorption data, as well as the retrieved effective parameters, are presented for the fishnet structure with and without gain material. Kramers-Kronig relations of the effective parameters are in excellent agreement with the retrieved results with gain.

Lumped elements circuit of terahertz fishnet-like arrays with composite dispersion

We have analyzed and characterized a terahertz stacked holes array over a broad transmission bandwidth. In this context, we identify the key contributions of the surface plasmon modes related to the periodicity of array, and of the subcut off transmission through the holes responsible of a ground left-handed dispersion branch. The experimental verification of a multimode electromagnetic behavior is performed by time domain spectroscopy, with a complementary assessment by full-wave analysis. A circuitlike approach appears suitable for the understanding of a zero-gap transition between the left- and right-handed characteristics, opening the way of nonvanishing group velocity in a near-zero index propagation medium. Such a balanced composite operation, pointed out over the past in the dual element transmission line approaches, is thus generalized for a stacked subwavelength holes array, whose operation can be compared to the one of the so-called fishnet structures.

Optical loss compensation in a bulk left-handed metamaterial by the gain in quantum dots

A bulk left-handed metamaterial with fishnet structure is investigated to show the optical loss compensation via surface plasmon amplification with the assistance of the gain medium of PbS quantum dots. Simultaneously negative permittivity and permeability are confirmed at the telecommunication wavelength (1.5 μm) by the retrieval of the effective electromagnetic property. The dependence of enhanced transmission on the gain coefficient, as well as on the propagation layers, demonstrates that ultralow loss is feasible in bulk left-handed metamaterials.

3D materials made of gold using Nanoimprint Lithography

In this work we show large area 3D stacking of gold structures like Split Ring Resonators (SRRs) and fishnet structures using Nanoimprint Lithography (NIL). Two main fabrication processes are used to stack multiple layers of such structures on top of each other: For the stacked fishnet structures a finished double-layer of gold structures is covered with a spacer layer which is planarized using chemical mechanical polishing so that a second layer can be processed on top. For the SRR structures a transfer printing process is used. Here the gold structures are transferred from one substrate onto another substrate. Especially this process is a new and unique possibility to stack gold patterns by transferring gold structures several time on the same substrate and therefore to build up 3D materials in a fast and cost efficient way.

Transmission properties of electromagnetic metamaterials: From split-ring resonator to fishnet structure

We experimentally presented the electromagnetic behavior of transformative magnetic metamaterials: from the early invented split-ring resonator to its improvement, the cut-wire pair, for providing a negative magnetic permeability. By adding the continuous wire to cut-wire pair structure, the left-handed (LH) transmission made by doubly negative permittivity and permeability was demonstrated in combined structure. Interestingly, until the width of cut-wire pair increases to be physically merged with the adjacent continuous wire, in other words, to form the so-called fishnet structure, the LH behavior is still observed. This result indicates that in a broad sense, the essence of the electromagnetic response in the fishnet structure is similar to that in a combined structure. Our experimental results show a good agreement with previous theoretical study.

Holey metal films: From extraordinary transmission to negative-index behavior

In this paper we study the common physical background of the phenomenon of extraordinary optical transmission in holey metal films and the emergence of the negative refractive index behavior in double fishnet structures. Here we provide further evidence that the resonant magnetic response of the latter structures is associated with the excitation of gap surface plasmon modes. The evolution of the optical response with the number of double fishnet layers is also addressed, finding that the effective refractive index reaches a converged value for a moderate number of layers.

Subpicosecond Optical Switching with a Negative Index Metamaterial

We demonstrate a nanoscale, subpicosecond (ps) metamaterial device capable of terabit/second all-optical communication in the near-IR. The 600 fs response, 2 orders of magnitude faster than previously reported, is achieved by accessing a previously unused regime of high-injection level, subpicosecond carrier dynamics in the alpha-Si dielectric layer of the metamaterial. Further, we utilize a previously unrecognized, higher-order, shorter-wavelength negative-index resonance in the fishnet structure, thereby extending device functionality (via structural tuning of device dimensions) over 1.0-2.0 microm. The pump energy required to modulate a single bit is only 3 nJ over our current 700 microm(2) area device and can be easily scaled into the picoJoule regime with smaller cross sectional areas.