Perforated Metal Films Research Articles

Broadband optical transmission through two-dimensional hole arrays

We suggest that a metal film perforated with two-dimensional (2D) hole arrays can support broadband optical transmission. This phenomenon is based on the impedance matching between the perforated metal film and incoming light waves at oblique incidence. Theoretical formulas for the effective impedance of the hole arrays and Brewster angle of broadband transmission have been derived in the infrared regime, taking account of the field penetration effect on the hole walls. With the numerical simulations, the predicted optical effect has also been demonstrated. Compared with the narrow transmission band due to surface-plasmon polariton resonance, our results present a non-resonant, broadband, and robust transmission channel for the 2D hole arrays.

Light-Trapping Electrode for the Efficiency Enhancement of Bifacial Perovskite Solar Cells.

Antireflection and light-trapping coatings are important parts of photovoltaic architectures, which enable the reduction of parasitic optical losses, and therefore increase the power conversion efficiency (PCE). Here, we propose a novel approach to enhance the efficiency of perovskite solar cells using a light-trapping electrode (LTE) with non-reciprocal optical transmission, consisting of a perforated metal film covered with a densely packed array of nanospheres. Our LTE combines charge collection and light trapping, and it can replace classical transparent conducting oxides (TCOs) such as ITO or FTO, providing better optical transmission and conductivity. One of the most promising applications of our original LTE is the optimization of efficient bifacial perovskite solar cells. We demonstrate that with our LTE, the short-circuit current density and fill factor are improved for both front and back illumination of the solar cells. Thus, we observe an 11% improvement in the light absorption for the monofacial PSCs, and a 15% for the bifacial PSCs. The best theoretical results of efficiency for our PSCs are 27.9% (monofacial) and 33.4% (bifacial). Our study opens new prospects for the further efficiency enhancement for perovskite solar cells.

Transparent absorber composed of two stacked ultrathin metal films perforated with small holes.

A transparent absorber refers to the device which can absorb light strongly within a narrow frequency range but transmit light efficiently outside that range. Because of the contradiction between absorption and transmission, however, the performances of the transparent absorbers are usually compromised. In this work, we propose a transparent absorber based on a sandwiched metal-insulator-metal (MIM) structure, i.e., two perforated ultrathin metal films separated by a central dielectric layer. This structure has the advantage that the narrow-band absorption can be greatly enhanced because of the cooperation of surface-plasmon polariton (SPP) mode and multiple reflections in the dielectric cavity. Moreover, the ultrathin thickness of the stacked metal films enables high transmission when the wavelength of incident light deviates from the SPP resonance. A semi-analytical Fabry-Perot model has been employed to describe the optical properties, which agrees well with the simulation. The dependence of optical properties on the structural parameters has also been studied systematically. In addition, by covering the transparent absorber with an antireflection layer, highly efficient absorption of red (∼87% @ 629 nm), green (∼89% @ 524 nm), or blue (∼68% @ 472 nm) light and high transmission (∼80%) in the transparent region have been suggested. With its excellent visible-wavelength selective absorption, polarization independence, high angle-tolerance, and structural simplicity, the proposed MIM transparent absorber may have potential applications in the display technology and other smart scenarios.

Extraordinary optical transmission in silicon nanoholes

In this work, for the first time, a study was conducted of the existence of Extraordinary Optical Transmission (EOT) in Silicon (Si) thin films with subwavelength holes array and high excess carrier concentration. Typically EOT is studied in opaque perforated metal films. Using Si would bring EOT and its many applications to the silicon photonics realm and the mid-IR range. Since Si thin film is a semi-transparent film in mid-IR, a generalization was proposed of the normalized transmission metric used in literature for EOT studies in opaque films. The plasma dispersion effect was introduced into the studied perforated Si film through either doping or carriers’ generation. Careful consideration for the differences in optical response modeling in both cases was given. Full-wave simulation and analysis showed an enhanced transmission when using Si with excess carriers, mimicking the enhancement reported in perforated metallic films. EOT was found in the mid-IR instead of the visible range which is the case in metallic films. The case of Si with generated excess carriers showed a mid-IR EOT peak reaching 157% around 6.68 µm, while the phosphorus-doped Si case showed a transmission enhancement of 152% around 8.6 µm. The effect of varying the holes’ dimensions and generated carriers’ concentration on the transmission was studied. The analogy of the relation between the fundamental mode cutoff and the EOT peak wavelength in the case of Si to the case of metal such as silver was studied and verified. The perforated Si thin film transmission sensitivity for a change in the refractive index of the holes and surroundings material was investigated. Also, a study of the device potential in sensing the hole and surroundings materials that have almost the same refractive index yet with different absorption fingerprints was performed as well.

Coupling between subwavelength nano-slit lattice modes and metal-insulator-graphene cavity modes: a semi-analytical model

We present a semi-analytical model of the resonance phenomena occurring in a hybrid system made of a 1D array of periodic subwavelength slits deposited on an insulator/graphene layer. We show that the spectral response of this hybrid system can be fully explained by a simple semi-analytical model based on weak and strong couplings between two elementary sub-systems. The first elementary sub-system consists of a 1D array of periodic subwavelength slits viewed as a homogeneous medium. In this medium lives a metal-insulator-metal lattice mode interacting with surface and cavity plasmon modes. A weak coupling with surface plasmon modes on both faces of the perforated metal film leads to a broadband spectrum while a strong coupling between this first sub-system and a second one made of a graphene-insulator-metal gap leads to a narrow band spectrum. We provide a semi-analytical model based on these two interactions thus allowing efficient access of the full spectrum of the hybrid system.

Modal analysis of metallic screen with finite conductivity perforated by array of subwavelength rectangular flared holes.

An array of flared rectangular holes pierced through a conducting screen is treated herein by a rigorous full-wave modal analysis using the moment method entailing Green's functions for rectangular cavities and planar multilayer structures in the spectral domain as well as classical Floquet theorem and the mode-matching technique. In this way, flared holes with arbitrary taper profile that may each even be composed of different dielectric sections and which perforated metal films that may be sandwiched between multiple layers of dielectric slabs on both sides is herein treated. The eclectic permutations of geometrical, structural, and material attributes thus afforded by this generic topology facilitate correspondingly diverse investigations that may prove pivotal to the success of future explorations in search for new breakthrough discoveries and innovations in the subject of extraordinary transmission through subwavelength hole arrays, to which the herein-analyzed configuration is central. Oblique angles of incidence for both principal polarizations and metal losses incurred by imperfect conducting screens are also investigated in this work, all constituting crucial aspects that may often be neglected.

Tuning of Plasmonic Resonances in the Near Infrared Spectrum Using a Double Coaxial Aperture Array

Plasmonic filters are excellent candidates for spectral filters in the near infrared spectrum. Such filters require only a single nanoscale thickness perforated metal film that can be tailored to tune the location of the transmission maximum. Here, we demonstrate using computational methods a double coaxial aperture array (DAA) in a hexagonal geometry that can be tuned to produce plasmon resonances accompanied by strong transmission in the near-infrared spectrum and at telecommunication wavelengths with a peak wavelength insensitive to angle of incidence. The DAA consists of two concentric annular apertures and radial distance between the two apertures is varied to tune the resonant wavelength. The presented geometry finds potential applications in modulators for telecommunication, spectroscopy, solar cells, high-resolution CMOS image sensors, chemical sensors, and biosensors.

Single mode approach with versatile surface wave phase correction for the extraordinary optical transmission comprehension of 1D period nano-slits arrays

A simple semi-analytical model for the comprehension of the extraordinary optical transmission of a 1D array of periodic subwavelength slit is proposed. In this single mode model, the mono layer of the perforated metal film is considered as a homogeneous medium. Therefore, the electromagnetic response of this structure to a plane wave excitation is equivalent to that of a slab with homogeneous equivalent permittivity. A versatile phase correction is added to this model in order to handle the contribution of surface waves in the EOT phenomenon. The proposed model leads to a cavity-like dispersion relation that allows accurate prediction of the resonance frequencies of the 1D structure.

Wide-Band and High-Efficiency 90° Polarization Rotator Based on Tri-Layered Perforated Metal Films

Tri-layered metal films perforated with rectangle hole arrays of different orientations and separated by glass/air spacers have been studied theoretically and experimentally. The devices can rotate the polarization of the incident light by 90° with high efficiency and wide bandwidth in the near-infrared and microwave regions. The high-efficiency and wide bandwidth polarization rotation can be ascribed to the near-field coupling regime rather than the Fabry–Perot resonance according to the investigation of the difference between the two regimes.

Corrugated plasmonic cavity for enhanced intersubband photodetection

We study the optical properties of a corrugated plasmonic cavity consisting of a perforated metal film and a flat metal sheet separated by a semiconductor spacer. Corrugation enhances dramatically the coupling between the propagating surface plasmon and the Fabry-Perot mode and induces Rabi-like splitting forming bright bonding and dark anti-bonding modes. The anti-bonding mode exhibits considerably higher volume-averaged field enhancement factors (∼16.5 for E-field and ∼14.1 for Ez-component) than its bonding counterpart as well as a very high polarization conversion ratio (∼85.5%) from transverse electric to transverse magnetic waves. These characteristics make the corrugation induced anti-bonding mode particularly suitable for semiconductor quantum well intersubband photodetectors. Our work may provide a general guideline to the design of metamaterial-coupled intersubband hybrid devices for practical applications.

Fano resonances in a photonic crystal covered with a perforated gold film and its application to bio-sensing

Optical properties of the photonic crystal covered with a perforated metal film were investigated and the existence of the Fano-type resonances was shown. The Fano resonances originate from the interaction between the optical Tamm state and the waveguide modes of the photonic crystal. It manifests itself as a narrow dip in a broad peak in the transmission spectrum related to the optical Tamm state. The design of a sensor based on this Fano resonance that is sensitive to the change of the environment refractive index is suggested.

Shape Control of Periodic Metallic Nanostructures for Transparent Conductive Films

The transmittance limitations of metallic nanomeshes with hexagonally ordered perforations that are typically prepared by conventional nanosphere lithography (NSL) were demonstrated. Specially, the optoelectronic properties of perforated metallic films with continuously varying morphologies, ranging from spherical apertures to egg-shaped apertures, zigzag wires and approximate straight wires to metallic crosswire grids, are investigated, shedding light on the dependence of the optoelectronic properties on subtle changes of the film morphology especially on its periodicity. Further, to exploit both the high transmittance of these nanowires and the low sheet resistance of the nanomesh, we have developed a novel transparent metal electrode composed of two overlapping nanowire arrays via a stepwise ARNSL method. This sophisticated electrode has an increased optoelectronic performance with a sheet resistance of 100 ω sq and an average optical transmittance of 80% and is hopeful to be further improved. The stepwise ARNSL method addresses the challenges of fabricating intercrossed pattern from parallelly aligned conductive wires arrays, which are significant in constructing nanowire devices. These findings provide useful insights in the design and fabrication of unique 2D metallic electrodes for potential high-performance transparent electrode application in the transparent electronics.

Role of dielectric properties in terahertz field transmission

We compare the field transmission characteristics of a freestanding perforated metal film (as a conductor) and a polymethylmethacrylate–graphite composite film (as an absorber) in the terahertz frequency range. The role of dielectric properties of the materials and the contribution of surface waves toward enhanced transmission with periodic and random hole arrays are discussed. Periodic subwavelength hole arrays in metal films do support enhanced terahertz field transmission whereas random arrays do not. In contrast, neither periodic nor random arrays of subwavelength holes punctured in dielectric absorbers support such transmission. Notably, even a dielectric absorber with large holes, which is sufficiently larger than subwavelength holes, can result in features in transmission due to the shape resonance, but the effect is very small.

Polymer blend lithography for metal films: large-area patterning with over 1 billion holes/inch(2).

Polymer blend lithography (PBL) is a spin-coating-based technique that makes use of the purely lateral phase separation between two immiscible polymers to fabricate large area nanoscale patterns. In our earlier work (Huang et al. 2012), PBL was demonstrated for the fabrication of patterned self-assembled monolayers. Here, we report a new method based on the technique of polymer blend lithography that allows for the fabrication of metal island arrays or perforated metal films on the nanometer scale, the metal PBL. As the polymer blend system in this work, a mixture of polystyrene (PS) and poly(methyl methacrylate) (PMMA), dissolved in methyl ethyl ketone (MEK) is used. This system forms a purely lateral structure on the substrate at controlled humidity, which means that PS droplets are formed in a PMMA matrix, whereby both phases have direct contact both to the substrate and to the air interface. Therefore, a subsequent selective dissolution of either the PS or PMMA component leaves behind a nanostructured film which can be used as a lithographic mask. We use this lithographic mask for the fabrication of metal patterns by thermal evaporation of the metal, followed by a lift-off process. As a consequence, the resulting metal nanostructure is an exact replica of the pattern of the selectively removed polymer (either a perforated metal film or metal islands). The minimum diameter of these holes or metal islands demonstrated here is about 50 nm. Au, Pd, Cu, Cr and Al templates were fabricated in this work by metal PBL. The wavelength-selective optical transmission spectra due to the localized surface plasmonic effect of the holes in perforated Al films were investigated and compared to the respective hole diameter histograms.

Synthesis of free-standing, curved Si nanowires through mechanical failure of a catalyst during metal assisted chemical etching

The fabrication of orderly arrays of free-standing, curved Si nanowires over large areas (1 cm × 1 cm) was demonstrated by means of interference lithography and intentional mechanical failure of a perforated Au catalyst during metal assisted chemical etching. Photoresist microgrooves were deposited on the perforated Au film to cause uneven etching which resulted in the build-up of bending stresses in the Au film to the point of catastrophic failure. By considering the initial positions of the holes in the perforated Au film relative to the photoresist constraints, the precise location of the fracture can be predicted using simple beam mechanics. Therefore, the type of curved nanowires obtained can be designed with a high degree of reliability and control. Four distinct types of nanowire arrangements were demonstrated for this study.

Forward and backward unidirectional scattering from plasmonic coupled wires

We analyze the resonant electromagnetic response of sub-wavelength plasmonic dimers formed by two silver strips separated by a thin dielectric spacer and embedded in a uniform dielectric media. We demonstrate that the off-resonant electric and resonant, geometric shape-leveraged, magnetic polarizabilities of the dimer element can be designed to have close absolute values in a certain spectral range, resulting in a predominantly unidirectional scattering of the incident field due to pronounced magneto-electric interference. Switching between forward and backward directionality can be achieved with a single element by changing the excitation wavelength, with the scattering direction defined by the relative phases of the polarizabilities. We extend the analysis to some periodic configurations, including the specific case of a perforated metal film, and discuss the differences between the observed unidirectional scattering and the extraordinary transmission effect. The unidirectional response can be preserved and enhanced with periodic arrays of dimers and can find applications in nanoantenna devices, integrated optic circuits, sensors with nanoparticles, photovoltaic systems, or perfect absorbers; while the option of switching between forward and backward unidirectional scattering may create interesting possibilities for manipulating optical pressure forces.

Enhanced optical transmission by V-shaped nanoslit in metal film

In this paper, we reveal that the enhanced transmission through a perforated metal film can be further boosted up by a V-shaped nanoslit, which consists of two connected oblique slits. The maximum transmission at resonance can be enhanced significantly by 71.5% in comparison with the corresponding vertical slit with the same exit width. The value and position of transmission resonance peak strongly depend on the apex angle of the V-shaped slit. The optimum apex angle, at which the transmission is maximal, is sensitive to the slit width. Such phenomena can be well explained by a concrete picture in which the incident wave drives free electrons on the slit walls. Moreover, we also simply analyze the splitting of the transmission peak in the symmetry broken V-shaped slit, originating from the resonances of different parts of the V-shaped slit. We expect that our findings will be used to design the nanoscale light sources based on the metal nanoslit structures.

Enhanced refractive index sensitivity of elevated short-range ordered nanohole arrays in optically thin plasmonic Au films

A simple development of the colloidal lithography technique is demonstrated for fabrication of perforated plasmonic metal films elevated above the substrate surface. The bulk refractive index sensitivity of short-range ordered nanohole arrays in 20 nm thick Au films exhibits an increase of up to 37% due to reduction of substrate effect caused by lifting with a 40 nm silica layer. Analysis of the local electric field distribution suggests that the sensitivity increase is due to revealing of the enhanced field near the holes.

Enhanced broad-band extraordinary optical transmission through subwavelength perforated metallic films on strongly polarizable substrates

We demonstrate through simulations and experiments that a perforated metallic film, with subwavelength perforation dimensions and spacing, deposited on a substrate with a sufficiently large dielectric constant, can develop a broad-band frequency window where the transmittance of light into the substrate becomes essentially equal to that in the film absence. We show that the location of this broad-band extraordinary optical transmission window can be engineered in a wide frequency range (from IR to UV), by varying the geometry and the material of the perforated film as well as the dielectric constant of the substrate. This effect could be useful in the development of transparent conducting electrodes for various photonic and photovoltaic devices.

Hybridized effects of plasmonic quadrupolar and dipolar resonances on the perforated planar metallic film

We have designed a detuned double-L-shaped nanohole arrays in the metallic film, on which the hybridized plasmonic quadrupolar and dipolar resonance modes can be excited by the incident light, respectively. Compared with the conventional plasmonic resonance modes, the antibonding quadrupolar resonance mode has a higher quality factor due to its lower radiative loss. A quality factor of 38 (theoretical prediction exceeding 100) is obtained in the experiment.