Metal Film Thickness Research Articles

Manipulating liquid metal flow for creating standalone structures with micro-and nano-scale features in a single step

Standalone structures with periodic surface undulations or ripples can be spontaneously created upon flowing a liquid metal, e.g. Ga, over a metallic film, e.g. Pt, Au, etc, through a complex ‘wetting-reaction’-driven process. Due to the ability of 3-dimensional patterning at the small length scale in a single step, the liquid metal ‘ripple’ flow is a promising non-conventional patterning technique. Herein, we examine the effect of a few process parameters, such as distance away from the liquid reservoir, size of the liquid reservoir, and the geometry, thickness, and width of substrate metal film, on the nature of the ripple flow to produce finer patterns with feature sizes of ≤ 2 μm. The height and the pitch of the pattern decrease with distance from the liquid reservoir and decrease in the reservoir volume. Furthermore, a decrease in the thickness and width of the substrate film also leads to a decrease in the height and pitch of the ripples. Finally, the application of an external electric field also controls the ripple patterns. By optimizing various parameters, standalone ripple structures of Ga with the height and pitch of ≤ 500 nm are created. As potential applications, the ripple patterns with micro-and nano-scopic features are demonstrated to produce a diffraction grating and a die for micro-stamping.

Bistable reflection and beam shifts with excitation of surface plasmons in a saturable absorbing medium.

We investigate the nonlinear reflection of a light beam from a Kretschmann configuration with saturable absorbing medium. The absorption of medium has direct influence on the intrinsic loss of the system, thus affecting the reflectivity and the phase variation when the surface plasmons are resonantly excited. As the incident power changes, the reflectivity can be switched between high and low values and exhibits absorptive optical bistability as a result of the inherent positive feedback by the intensity-dependent saturation effect. The Goos-Hänchen and the Imbert-Fedorov shifts of the reflected beam have the same bistable behavior as the reflectance. The effects of the thickness of metal film and the linear absorption coefficient on the hysteresis loop are analyzed in detail by considering the system losses and the saturated absorption. The bistable reflection and beam shifts may have applications in all-optical devices, such as optical switching.

Tuning the magnetic properties of Fe thin films with RF-sputtered amorphous carbon

RF-sputtered amorphous carbon (a-C) offers a simple and cheap pathway to tune the magnetic properties of transition metal thin films for magnetic memories and different spintronic applications. This paper describes changes in the magnetic properties of iron thin films with a-C overlayers. In as-deposited samples, hybridisation and intermixing at the Fe/a-C interface leads to magnetic softening (Liu et al., 2006) [1], with a reduction in the coercive field (Hc) up to a factor of five for Fe/a-C/Fe trilayers, and a 10–30% lower saturation magnetization as a function of the metal film thickness. After annealing at 500 °C, inter-diffusion and graphitization of the carbon layer results in up to a factor five increased coercivity due to increased pinning as shown via Kerr microscopy. Therefore, RF-sputtered carbon overlayers and post-processing can tune the anisotropy and domain configuration of metallic thin films in a synthesis methodology that is simple, cheap and sustainable.

Surface plasmon resonance (SPR)-based D-shaped photonic crystal fiber polarization filter and refractive index sensor with a hexagonal pore structure

A D-shaped photonic crystal fiber (PCF) polarization filter and sensor with a standard hexagonal arrangement of pores is designed. The effects of geometric parameters, metal film thickness, liquid refractive index, and optical material refractive index on polarization filtering and sensing characteristics are analyzed by the finite element method (FEM). The proposed PCF shows single-polarization filtering in communication window and has ultra-high corresponding bandwidth. At the 1.55 µm communication window band, the confinement loss of the y-polarized core mode is 819.27 dB/cm, while the x-polarized core mode is only 3.45 dB/cm. The crosstalk (CT) reached a maximum of 708.6 dB in the communication bands, and the corresponding bandwidth reaches 1000 µm when the fiber length is 1 mm. Meanwhile, the D-type PCF is widely used in sensing because it avoids filling the gold film and analyte in the internal pores. The change in the thickness of the mental coating from 39 nm to 22 nm provides the proposed PCF with sensing properties. By reducing the refractive index of fiber material, the maximum sensitivity is 10,741 nm/RIU. When the refractive index measurement range of the analyte is specified from 1.33 to 1.36, the average sensitivity reaches 7313 nm/RIU. Good performance enables the proposed compatible filtering and sensing PCF may be used in a wide range of scenarios.

Geometrical scaling law for laser-induced micro-projectile impact testing

Laser-induced micro-projectile impact testing (LIPIT) is a useful experimental method for exploring the dynamic behavior of materials at microscale. It has been a key issue to obtain a scaling law of launch velocity of LIPIT to simplify experimental design and improve experimental configuration. This paper obtains a geometrical scaling law of launch velocity for LIPIT with relative thick metallic film (30–80 µm thick aluminum) using dimensional analysis, experimental measurements, and numerical simulations. Firstly, the dimensional analyses of LIPIT with and without elastomer film configuration are performed, and the dimensionless parameters controlling the launch velocity of the micro-projectile are deduced, from which the geometrical scaling laws of launch velocity for the LIPIT are obtained. Then, the numerical simulation models of the LIPIT are established and validated by LIPIT experimental results, providing a numerical validation for the geometrical scaling laws. In addition, the influences of the dimensionless parameters on the dimensionless launch velocity of micro-projectiles are analyzed by numerical simulations, and the dimensionless formulas for predicting the launch velocities of micro-projectiles in a LIPIT are given, providing an effective method for analyzing and optimizing the LIPIT experiments.

Conversion of a Helical Surface Plasmon Polariton into a Spiral Surface Plasmon Polariton at the Outlet of a Metallic Nanohole.

The conversion of a helical surface plasmon polariton (SPP) creeping out of a circular nanohole in a thick metal (Ag or Au) film into a spiral (Hankel type) SPP outward propagating at the film’s interface is studied theoretically. The dispersion relations of SPPs of various modes in a nanohole, calculated from a transcendental equation, show that the propagation length of an SPP of mode 1 is much larger than the other modes in a specific frequency band, which is dependent on the nanohole size. In this band, the streamlines of the Poynting vector (energy flux) of mode-1 SPP in nanohole exhibit helixes; the surface component of the energy flux is perpendicular to the phase front of the SPP. Numerical results show that, after a helical SPP tunnels through a nanohole, most of the energy flux fans out at the outlet as a dipole radiation. The spatial phase distribution of Ez above the interface indicates that the transmission light carries orbital angular momentum with a topological charge of 1. Additionally, a part of the helical SPP creeping along the edge of an outlet naturally converts into a spiral (Hankel type of order 1) SPP outward propagating at the film’s interface; both SPPs have the same handedness. Moreover, the interferences of multi SPPs generating from two nanoholes and even from a two-dimensional nanohole array are also related to the spiral SPP.

Numerical Simulations of the Molten Metal Droplet Formation in the Electroslag Remelting Process with a Rotating Electrode

Electroslag remelting (ESR) is a well‐known process widely used in the production chain of high‐quality alloys. Herein, numerical investigations of the implementation of a rotating‐electrode (low rotation speed) modification to the flow behavior inside the slag area of a lab‐sized ESR process are shown. Therefore, a coupled simulation approach between a commercial CFD (FVM) and electromagnetics (FEM) software is used. Comparative results for the simulated droplet formation between a stationary and a rotating electrode are shown. The results show a strong effect on the local shaping of the liquid metal film below the electrode and consequently a radial shift of the droplet incidence into the metal pool under electrode rotation. There are no observable positive effects on mean metal film thickness, droplet trajectory, or droplet size, which are suspected as possible explanations for observed material improvements in small‐scale rotating electrode ESR processes. In addition, an overview of current bottlenecks, restrictions, and possible strategies for improved ESR process simulations is given.

Compact Surface Plasmon Resonance IgG Sensor Based on H-Shaped Optical Fiber

A compact surface plasmon resonance sensor based on an H-shaped optical fiber is proposed and demonstrated. The H-shaped optical fiber was fabricated experimentally by using hydrofluoric acid to controllably corrode the polarization-maintaining fiber. A satisfactory distance between the outer surface of the fiber and the core can be achieved, and then the surface plasmon resonance effect can be excited by coating a metal film of appropriate thickness on the surface of the fiber. This technology can realize the preparation of multiple samples at one time, compared to the traditional side-polishing technique. The H-shaped optical fiber obtained from corrosion exhibits a high surface quality and short lengths, down to only a few hundred microns. The effects of the proposed H-shaped optical fiber on spectral properties are induced by process parameters, including fiber remaining thickness, coating thickness and fiber length, and were investigated in detail. The prepared sensor was used for the specific detection of human IgG, and the minimum human IgG concentration that the sensor can distinguish is 3.4 μg/mL. Such a compact surface plasmon resonance fiber sensor has the advantages of an easy fabrication, good consistency and low cost, and is expected to be applied in the specific detection of biomarkers.

Optical Janus Effect in Large Area Multilayer Plasmonic Films

Plasmonic and other nanoparticles have attracted considerable interest for their role in structural coloration. The optical “Janus” effect, where the color of light reflected from a partially transmitting film depends on whether the device is viewed from the substrate or the coating side, is observed using a variety of nanostructured films. Herein, the optical Janus effect produced by homogeneous thin‐film structures comprising only four layers of three different materials with a total thickness less than is demonstrated. An asymmetric Fabry–Perot (FP) nanocavity is formed with a dielectric film bounded by two different metal films of nanoscale thickness. The semitransparent device has a transmitted color that is independent of the viewing direction. A broad color palette is available through the selection of various thicknesses and film materials. In addition to the directional optical effect, the device possesses iridescence properties and can generate images by selective removal of regions of one of the metallic films using simple photolithography. From a manufacturing perspective, this device is scalable and holds significant promise for applications in architecture, producing decorative features, and the development of overt and covert security features.

A Measuring Method of DOM Components Based on Fiber SPR Sensor and ICPSO-BP Neural Network

As the single sensor is not competent for the variation of the total amount and components of DOM in a large range, according to the cross-sensitivity between fiber SPR sensors with different structures, a measuring method of DOM components is proposed by combining the deep learning algorithm with the fiber SPR sensors based on the regulation of metal film thickness. We exploit an improved cooperative particle swarm optimization algorithm (ICPSO) aiming at the problem of particle diversity loss caused by premature convergence of particles which not only considers the optimization information of single particles, the global particles and particles in the groups, but also considers the proportion of shared information. Then, the ICPSO is used to optimize the weights and thresholds of back propagation neural network (BPNN) to establish ICPSO-BP network, so as to construct three classifiers consists of ICPSO-BP (wave length), ICPSO-BP (spectral width), ICPSO-BP (light intensity). By comprehensive training of the resonance wavelength, spectrum width and light intensity of SPR effect for the measured water, five DOM components (tyrosine protein, tryptophan protein, fulvic acid, dissolved microbial metabolites and humic acid) and their concentrations in four water samples, namely, Inner canal (A), Hongze lake (B), Park lake (C) and Campus lake (D), have been effectively predicted. The prediction accuracy is more than 80%, among them, the highest prediction rate of tryptophan protein and its concentration in Hongze lake (B) which can reach 86%. Therefore, the dynamic range of SPR measurement is effectively expanded and better measurement accuracy and sensitivity are maintained, which verifies the feasibility of the proposed method in DOM measuring and provide a new idea for DOM component testing.

Perfect Optical Absorbers by All-Dielectric Photonic Crystal/Metal Heterostructures Due to Optical Tamm State.

In this study, we theoretically and experimentally investigated the perfect optical absorptance of a photonic heterostructure composed of a truncated all-dielectric photonic crystal (PC) and a thick metal film in the visible regions. The three simulated structures could achieve narrow-band perfect optical absorption at wavelengths of 500 nm, 600 nm, and 700 nm, respectively. Based on the measured experimental results, the three experimental structures achieved over 90% absorption at wavelengths of 489 nm, 604 nm, and 675 nm, respectively. The experimental results agreed well with the theoretical values. According to electromagnetic field intensity distributions at the absorption wavelengths, the physical mechanism of perfect absorption was derived from the optical Tamm state (OTS). The structure was simple, and the absorption characteristics were not significantly affected by the thickness of the thick metal layer, which creates convenience in the preparation of the structure. In general, the proposed perfect absorbers have exciting prospects in solar energy, optical sensor technology, and other related fields.

THz pulse train generation through ultrafast development of surface plasmon-polariton modulation instability

Propagation of high-intensity electromagnetic waves in a waveguide structure could initiate nonlinear effects resulting in drastic changes of their spatial and temporal characteristics. We study the modulation instability effect induced by propagation of surface plasmon polaritons in a silver thin-film waveguide. The nonlinear Schrodinger equation for propagating surface plasmon wave is obtained. It is shown numerically that the modulation instability effect can give rise to ultrafast spatial redistribution and longitudinal localization of surface plasmon-polariton wave energy in subwavelength scale. The dependence of plasmon wave dispersion and nonlinear characteristics on metal film thickness is considered. We demonstrate that the use of films with the thickness varying along the waveguide length allows reduction of the generated pulse width and increase of frequency comb bandwidth. The proposed technique is promising for design of ultra-compact (tens of nm) optical generators delivering pulse trains with the repetition rate higher than 1 THz.

Tuning plasmons of metal-coated microsphere arrays towards optimized surface-enhanced spectroscopy

The overall optical response (transmittance, reflectance, and absorbance) of metal film over nanospheres (MFoN) is studied for a wide range of sphere diameters (200 - 1000 nm) and metal film thicknesses (40–200 nm), over the 450 - 2000 nm spectral range. Analyses are performed also in water, with microfluidic surface-enhanced Raman scattering applications in mind. Two main outcomes are the dependence of the plasmonic absorbance band on structural parameters and the behavior in aqueous environment. The parameter ranges for targeting common lasers (633 and 785 nm) are identified. Additionally, for larger sphere size and thicker films, a new absorbance band was identified, exhibiting a multipole-like electric field distribution, different than the dipole-like fields at the main absorption band. It is also shown that the fine morphology of the metal film at the inter-sphere region has a strong impact on reflectance (and absorbance) but not transmittance. The individual roles of the metal particles formed on the substrate or the dielectric sphere array on the overal optical response are discussed. Finally, the role of the metal type (Au, Ag, Cu, Al) is also analyzed.

Plasmonic nanocrystals on polycarbonate substrates for direct and label-free biodetection of Interleukin-6 in bioengineered 3D skeletal muscles

Abstract The development of nanostructured plasmonic biosensors has been widely widespread in the last years, motivated by the potential benefits they can offer in integration, miniaturization, multiplexing opportunities, and enhanced performance label-free biodetection in a wide field of applications. Between them, engineering tissues represent a novel, challenging, and prolific application field for nanostructured plasmonic biosensors considering the previously described benefits and the low levels of secreted biomarkers (≈pM–nM) to detect. Here, we present an integrated plasmonic nanocrystals-based biosensor using high throughput nanostructured polycarbonate substrates. Metallic film thickness and incident angle of light for reflectance measurements were optimized to enhance the detection of antibody–antigen biorecognition events using numerical simulations. We achieved an enhancement in biodetection up to 3× as the incident angle of light decreases, which can be related to shorter evanescent decay lengths. We achieved a high reproducibility between channels with a coefficient of variation below 2% in bulk refractive index measurements, demonstrating a high potential for multiplexed sensing. Finally, biosensing potential was demonstrated by the direct and label-free detection of interleukin-6 biomarker in undiluted cell culture media supernatants from bioengineered 3D skeletal muscle tissues stimulated with different concentrations of endotoxins achieving a limit of detection (LOD) of ≈ 0.03 ng/mL (1.4 pM).

Brightening single-photon emitters by combining an ultrathin metallic antenna and a silicon quasi-BIC antenna

Bright single-photon emitters (SPEs) are fundamental components in many quantum applications. However, it is difficult to simultaneously get large Purcell enhancements and quantum yields in metallic nanostructures because of the huge losses in the metallic nanostructures. Herein, we propose to combine an ultrathin metallic bowtie antenna with a silicon antenna above a metallic substrate to simultaneously get large Purcell enhancements, quantum yields, and collection efficiencies. As a result, the brightness of SPEs in the hybrid nanostructure is greatly increased. Due to the deep subwavelength field confinement (mode size < 10 nm) of surface plasmons in the ultrathin metallic film (thickness < 4 nm), the Purcell enhancement of the metallic bowtie antenna improves by more than 25 times when the metal thickness decreases from 20 nm to 2 nm. In the hybrid nanostructures by combining an ultrathin metallic bowtie antenna with a silicon antenna, the Purcell enhancement (Fp≈2.6 × 106) in the hybrid nanostructures is 63 times greater than those (≤ 4.1 × 104) in the previous metallic and hybrid nanostructures. Because of the reduced ratio of electromagnetic fields in the ultrathin metallic bowtie antenna when the high-index silicon antenna is under the quasi-BIC state, a high quantum yield (QY ≈ 0.70) is obtained. Moreover, the good radiation directivity of the quasi-BIC (bound state in the continuum) mode of the silicon antenna and the reflection of the metallic substrate result in a high collection efficiency (CE ≈ 0.71). Consequently, the overall enhancement factor of brightness of a SPE in the hybrid nanostructure is EF* ≈ Fp × QY × CE ≈ 1.3 × 106, which is 5.6 × 102 times greater than those (EF* ≤ 2.2 × 103) in the previous metallic and hybrid nanostructures.

Synthesis of bulk crystals and thin films of the ferromagnetic MnSb

High-temperature ferromagnets are widely used on a practical level. Based on them, magnetic memory for computers and various types of magnetic field sensors are created. Therefore, bulk ingots and thin-film samples of ferromagnet manganese antimonide (MnSb) with a high Curie point are of great interest, both from the practical and fundamental sides. Manganese antimonide films are obtained in hybrid structures using molecular-beam epitaxy. The thickness of the films does not exceed tens of nanometers. Despite their high sensitivity to magnetic fields, their small thickness prevents them from being used as magnetic field sensors. The aim of this work was to synthesise thick bulk ingots of manganese antimonide crystalsand films with a thickness of ~ 400 nm on sitall and silicon substrates. MnSb crystals were synthesised using the vacuum-ampoule method and identified using XRD, DTA, and microstructural analysis. The results of studies of bulk samples indicated the presence of an insignificant amount of antimony in additionto the MnSb phase. According to the DTA thermogram of the MnSb alloy, a small endothermic effect was observed at 572 °C, which corresponds to the melting of the eutectic on the part of antimony in the Mn-Sb system. Such composition, according to previous studies, guaranteed the production of manganese antimonide with the maximum Curie temperature. A study of the magnetic properties showed that the synthesised MnSb crystals were a soft ferromagnet with the Curie point ~ 587 K. Thin MnSb films were obtained by an original method using separate sequential deposition in a high vacuum of the Mnand Sb metals with their subsequent annealing. To optimise the process of obtaining films with stoichiometric composition, the dependences of the thickness of metal films on the parameters of the deposition process were calculated. The temperature range of annealing at which the metals interact with the formation of ferromagnetic MnSb films was established, the films were identified, and their electrical and magnetic properties were measured

Development of a model for evaluating propagation loss of metal-coated dielectric terahertz waveguides

A simple physical model for evaluating propagation loss of a metal-coated dielectric terahertz (THz) waveguide with different metal film thicknesses was developed for those fabricated by three-dimensional printing and film coating techniques. Our model enables a comprehensive understanding of the propagation loss mechanism and two key values: the critical film thickness to behave like the bulk material and loss in a sufficiently thick film. To develop the model, in addition to reflection at the metal–dielectric interface, the thickness-dependent electrical conductivity of the metal film was considered. The model was validated by an in-house multi-channel Au-coated THz parallel-plate waveguide in the lowest transverse-electric mode. The estimated critical thickness of our Au film was 171–207 nm at 0.72–1.4 THz. Our model clarified the contribution of three loss components to the overall loss: penetration loss, ohmic loss by bulk conductivity, and ohmic loss by a decrease in conductivity due to thin-film effects. Evaluation of loss over a broader frequency range (0.03–3.0 THz), which corresponds to fifth- to sixth-generation mobile network, revealed that the critical thickness decreased by up to 1.0 THz but increased above this range due to the transition of the dominant loss component from penetration loss to ohmic loss by a decrease in conductivity. As all three loss components and the critical thickness depend on film quality, a deposition process to yield high-quality films is necessary for high-performance waveguides. Our model is applicable to various waveguides, including rectangular waveguides, at any frequency and with any metal film.

A narrowband dual-polarization filter based on photonic crystal fiber with Au-Ag double-layer films

A narrowband dual-polarization filter based on photonic crystal fiber (PCF) with Au-Ag double-layer films is proposed. The cladding of PCF is composed of three layers of air holes, and Au-Ag double-layer films are coated on two symmetrical air holes to excite surface plasmon resonance (SPR) effect. The special mechanism of the Au-Ag double-layer films takes advantage of the narrower full width at half maximum (FWHM) of silver. On the other hand, gold is used to protect the silver from oxidation and improve the stability of the filter, and can also enhance SPR effect. The simulation results show that the dual-polarization filter with double-layer metal films has higher resonance absorption and better spectra than that with the single-layer metal film. When the thickness of the double-layer metal films is 22.5 nm (the thickness of silver film is 18 nm), the confinement loss of dual-polarization filter at 1430 nm is achieved to 64.08 dB/cm (y-pol core mode) and at 1530 nm is achieved to 37.26 dB/cm (x-pol core mode). The wavelength range of crosstalk (CT) greater than 20 dB and less than −20 dB are 1410–1450 nm and 1510–1550 nm for the device length of 10 mm, respectively. The CT is achieved 555.07 dB and −321.54 dB at the wavelength of 1430 nm and 1530 nm, respectively. These characteristics show that the filter has excellent performance, and will become a suitable candidate for a narrowband polarization filter.

Electronic Speckle Pattern Interference technique for measuring thickness of metallic nano thin films

Electronic Speckle Pattern Interference (ESPI) is a non-contact optical technique generally used for the measurement of surface deformations of various kinds. With the help of phase retrieval algorithms, phase of the deformed object is calculated in this method which can be then converted into displacement information. This paper demonstrates the use of ESPI method to measure the thickness of some nano-sized metallic thin films. Calculated thickness values are in good agreement with the values measured via Profilometer. The least thickness (z-axis) measured by this method is ~ 52 ± 3.1 nm. The z-axis resolution of the used method is at par with other expensive microscopy techniques like atomic force microscopy. Therefore, a low-cost alternative to expensive microscopy techniques is presented in this paper as far as z-axis is concern. Surface roughness of thin films can also be calculated using retrieved surface profiles and they are also found to be in the nanometer range.

Impact of Fabrication and Operation Errors on Super-Focusing Performance of a Nanoslit-Based Metalens

Metasurfaces with optical manipulation at subwavelength resolution show promises for developing ultrathin and flat optical components, attracting great interest from the optical scientific community. In our recent work, a metalens has demonstrated a special capability of quasi-far-field super-resolution focusing, which comprised a metallic nanoslit array with the incidence of a transverse-electric (TE) polarized light. In this paper, in order to guide practical fabrication and operation of a device, we perform a study on the metalens to analyze the impact of many imperfections on the super-resolution focusing capability. We take fabrication and operation errors into account, including errors in nanoslit width, metal film thickness, operating wavelength, polarization and incident angle of incident light. Numerical results illustrate that the sensitivity of the metalens focusing performance to each error is different. To be specific, the focusing performance of the metalens is considerably susceptible to the error in the incidence angle. Therefore, we not only need to fabricate the metalens device precisely, but also need to ensure that the working conditions agree well with the design, so as to achieve the desired focusing performance. Our research offers a valuable guide for the realization of the super-resolution focusing technology in practice.