Metal Film Thickness Research Articles

Chirality parameter sensing based on surface plasmon resonance D-type photonic crystal fiber sensors.

We report a method to sense a surrounding chiral drug based on D-type single-mode photonic crystal fiber (PCF) sensors in this paper. The electromagnetic theory of surface plasmon resonance on metal-chiral drug structure is derived. The wave equation containing constitutive relations of a chiral drug is given and integrated into the finite element method to compute the effective refractive index, confinement loss, and plasmon resonance wavelength for a D-type PCF sensor immersed in the chiral drug. The effects of the chirality parameter on resonance behaviors are displayed. The wavelength sensitivities of the chirality parameter for the sensor changing with different kinds of metal film layers, side-polished depth, and thickness of metal film layer are calculated. The wavelength sensitivity can reach a maximum of 17,580 nm/chirality as the refractive index and chirality parameter of the drug are 1.36 and 0.08, respectively. Furthermore, simultaneous dual-parameter detection of the chirality parameter and refractive index is realized by using two different D-type PCF sensors with gold and silver metal film layers, respectively. This study may provide sufficient guidelines to the field of biochemical sensing.

On efficient asymptotic modelling of thin films on thermally conductive substrates

Abstract

Localized surface plasmon resonance in deep ultraviolet region below 200\xa0nm using a nanohemisphere on mirror structure

Localized surface plasmon resonance (LSPR) was performed in the deep ultraviolet (UVC) region with Al nanohemisphere structures fabricated by means of a simple method using a combination of vapor deposition, sputtering, and thermal annealing without top-down nanofabrication technology such as electron beam lithography. The LSPR in the UV region was obtained and tuned by the initial metal film thickness, annealing temperature, and dielectric spacer layer thickness. Moreover, we achieved a flexible tuning of the LSPR in a much deeper UVC region below 200 nm using a nanohemisphere on a mirror (NHoM) structure. NHoM is a structure in which a metal nanohemisphere is formed on a metal substrate that is interposed with an Al2O3 thin film layer. In the experimental validation, Al and Ga were used for the metal hemispheres. The LSPR spectrum of the NHoM structures was split into two peaks, and the peak intensities were enhanced and sharpened. The shorter branch of the LSPR peak appeared in the UVC region below 200 nm. Both the peak intensities and linewidth were flexibly tuned by the spacer thickness. This structure can contribute to new developments in the field of deep UV plasmonics.

Self-referenced plasmonic biosensor with gold grating on thin gold film

Surface plasmon resonance sensors based on grating coupling exhibiting two resonant wavelengths are less observed because usually thick metal films are used. In this paper, we propose gold gratings on a gold film with two narrow reflection dips. One is sensitive to the refractive index (RI) of the cover media with a sensitivity of 720 nm/RIU. However, the other is sensitive to the RI of the substrate, and hence, it can be used as a reference dip. This self-referenced sensor significantly reduces the error contributions of the light intensity fluctuations and temperature variation. Moreover, the electric field is stronger with less decay length than that of gold gratings without gold film. Therefore, gold gratings with a gold film are more suitable to be a biosensor. Besides, we also investigate the structure parameters (the thickness of the gold film, the thickness and width of gold gratings, and the RI of the substrate) on the reflection spectra.

Temperature and humidity sensor based on a double D-shaped optical fiber with incorporated toluene and polyethylene

A double D-shaped optical fiber for simultaneous temperature and relative humidity (RH) sensing is described. The flat surfaces of the D-shaped optical fiber are coated with toluene as a thermally sensitive material and polyethylene as a humidity-sensitive material. Using the finite element method, the effects of the metal film thickness and polished depth of the D-shaped optical fiber on the sensing performance were analyzed and the optimal structural parameters were obtained. The temperature and humidity sensing properties were studied and show that temperature and humidity sensitivities of 1.02 nm/°C and 0.79 nm/% RH were obtained from 25 °C to 65 °C and 30% RH and 70% RH with maximum linearities of 0.996 and 0.997. The proposed optical fiber sensor demonstrates potential applications for dual-parameter detection.

Metalizadora Construída em Laboratório: Deposição de Filme de Alumínio sobre Substrato de Vidro e Posterior Anodização para Obtenção de Óxido de Alumínio

In this work, the assembly of a low-cost metallizer is presented, based on physical vapor deposition, to obtain metallic aluminum film deposited over a glass substrate. Later, the Al film is anodized and converted into a porous nanostructured oxide film. The metallic film thickness was measured by profilometry, and the sample position inside the chamber was evaluated. Samples positioned at the center of the sample holder showed considerable thickness and best homogeneity compared to those samples positioned at the edges of the sample holder. A thick metallic film of Al (6 to 7 µm) was deposited over the glass substrate for subsequent anodizing treatment. Mild anodization allowed the total conversion of metallic Al after 7000 s, producing a transparent anodic aluminum oxide (AAO) film adhered to the glass substrate. The AAO morphology was investigated by SEM. Some factors have been listed to describe the low regularity and homogeneity of nanopores in the outer layer of AAO, such as internal defects, compaction of deposited Al film layers and the glass substrate roughness.

Graphene-based electrically controlled terahertz polarization switching between a quarter-wave plate and half-wave plate

We theoretically present a high-efficiency switchable reflective terahertz polarization converter composed of a periodic array of rectangular-shaped metal-dielectric-graphene sandwich structure on a dielectric substrate supported by a thick metallic film. Graphene sheet together with the rectangular-shaped metal patch provides tunable anisotropic hybrid magnetic plasmon resonance to obtain tunable phase delay of 90° and 180°, corresponding to a quarter-wave plate (QWP) and half-wave plate (HWP), respectively. Results of numerical simulations indicate that the proposed structure can switch functions between a QWP and HWP at a certain frequency simply by adjusting the Fermi energy of graphene. Both the QWP and HWP have high energy conversion efficiency, respectively 83% and 90% at 15.96THz, and high polarization conversion ratio closed to 1.

Improved Mechanical Properties and Corrosion Resistance of Mg-Based Bulk Metallic Glass Composite by Coating with Zr-Based Metallic Glass Thin Film

Mg-based bulk metallic glass (BMG) and its composite (BMGC) can be excellent candidates as lightweight structure materials, but lack of anti-corrosion ability may restrict their application. In order to enhance the natural weak point of Mg-based BMGC, a 200-nm thick Zr-based metallic glass thin film (MGTF) ((Zr53Cu30Ni9Al8)99.5Si0.5) was applied and its mechanical properties as well as its corrosion resistance were appraised. The results of a 3-point bending test revealed that the flexural strength of the Mg-based BMGC with 200-nm thick Zr-based MGTF coating can be greatly enhanced from 180 to 254 MPa. We propose that the Zr-based MGTF coating can help to cover any small defects of a substrate surface, provide a protecting layer to prevent stress concentration, and cease crack initiation from the specimen surface during bending tests. Moreover, the results of anti-corrosion behavior analysis revealed a similar trend between the Mg-based BMG, Mg-based BMGC, and Mg-based BMGC with Zr-based MGTF coating in 0.9 wt.% sodium chloride solution. The readings show a positive effect with the Zr-based MGTF coating. Therefore, the 200-nm thick Zr-based MGTF coating is a promising solution to provide protection for both mechanical and anti-corrosion behaviors of Mg-based BMGC and reinforce its capability as structure material in island environments.

Optimum design of the surface plasmon resonance sensor based on polymethyl methacrylate fiber

A novel type of polymethyl methacrylate (PMMA) fiber refractive index sensor based on surface plasmon resonance (SPR) is proposed in this work, which is applied to measure the refractive index of liquid. The performance is investigated by the SPR theory and the structural properties of the side-polished D-type fiber. In order to obtain the optimized parameters of SPR sensor, the effects of metal film material, the metal film thickness and the length of the sensing zone on spectral characteristics of the sensor are studied. The fiber SPR sensor is manufactured and applied to the preparation experiment. The change of refractive index of glycerol solution is detected and analyzed at a constant temperature of 20 ​°C. Experimental results indicate that the proposed fiber SPR sensor exhibits refractive index sensitivity up to 3328.1 nm/RIU in the range of 1.334–1.388, which can be utilized as the alternative material for bio-sensing studies.

Strain-Mediated High Conductivity in Ultrathin Antiferromagnetic Metallic Nitrides.

Strain engineering provides the ability to control the ground states and associated phase transition in epitaxial films. However, the systematic study of the intrinsic character and strain dependency in transition-metal nitrides remains challenging due to the difficulty in fabricating stoichiometric and high-quality films. Here the observation of an electronic state transition in highly crystalline antiferromagnetic CrN films with strain and reduced dimensionality is reported. By shrinking the film thickness to a critical value of ≈30 unit cells, a profound conductivity reduction accompanied by unexpected volume expansion is observed in CrN films. The electrical conductivity is observed surprisingly when the CrN layer is as thin as a single unit cell thick, which is far below the critical thickness of most metallic films. It is found that the metallicity of an ultrathin CrN film recovers from insulating behavior upon the removal of the as-grown strain by the fabrication of freestanding nitride films. Both first-principles calculations and linear dichroism measurements reveal that the strain-mediated orbital splitting effectively customizes the relatively small bandgap at the Fermi level, leading to an exotic phase transition in CrN. The ability to achieve highly conductive nitride ultrathin films by harnessing strain-control over competing phases can be used for utilizing their exceptional characteristics.

Particle-in-cell simulation of surface plasmon polaritons excited by external introduction of electron density

Plasmonic devices work at visible and near-infrared frequencies, where a large number of bound electrons (polarization) in metals are excited. However, the existing particle-in-cell (PIC) software cannot take into account these bound electrons. In this paper, a new PIC simulation method is developed to study plasmonic devices, which has the ability to model both free electrons and bound electrons in metals. In this study, surface plasmon polaritons (SPPs) are excited by depositing excess electrons at one end of the metal film of thickness 140 nm (surrounded by air), thus initiating nonequilibrium in the electron density to start the oscillations. The spectrum, mode size, and propagation length of SPPs are calculated to confirm the electrical excitation of SPPs and characterize their properties. The excited SPPs are of a broad range of frequencies. Unlike SPPs excited by inelastic electron tunneling, the spectrum of SPPs excited in this way is not determined by eVbias but only by the properties of metals and dielectrics, and therefore the excited SPPs are intrinsic. The mode sizes of the intrinsic SPPs in the Ag–air geometry are 30 nm in Ag and 120 nm in air. The propagation length is ∼730 nm. The excited SPPs are converted into radiation modes by a sub-wavelength slit etched on the back side of the film to enable the detection of the radiation modes.

Influence of Adsorbed Metal Atoms on Light Absorption by a Fused Silica Surface

Optical properties of submonolayer metal (silver, gold, and indium) films obtained by laser ablation on a fused silica surface have been investigated. Extinction and absorption spectra of silica and substrates with deposited films and difference spectra are obtained. The minimum values of the effective metal-film thicknesses, at which islands begin to nucleate and localized plasmon resonances manifest themselves, are determined. It is shown that the absorption of the silica substrate–metal film complex exceeds the sum of absorptions by the silica surface and metal atoms. A relationship between this phenomenon and charge transfer from metal to defects on the substrate surface is revealed.

Demonstration of > 2\u03c0 reflection phase range in optical metasurfaces based on detuned gap-surface plasmon resonators

Plasmonic metasurfaces, representing arrays of gap-surface plasmon (GSP) resonators and consisting of arrays of metal nanobricks atop thin dielectric layers supported by thick metal films, constitute an important subclass of optical metasurfaces operating in reflection and enabling the realization of numerous, diverse and multiple, functionalities. The available phase variation range is however limited to being <! 2pi, a circumstance that complicates the metasurface design for functionalities requiring slowly varying phases over the whole range of 2pi, e.g., in holographic applications. The available phase range also determines the wavelength bandwidth of metasurfaces operating with linearly polarized fields due to the propagation (size-dependent) nature of the reflection phase. We suggest an approach to extend the phase range and bandwidth limitations in the GSP-based metasurfaces by incorporating a pair of detuned GSP resonators into a metasurface elementary unit cell. With detailed simulations related to those for conventional single-resonator metasurfaces and proof-of-concept experiments, we demonstrate that the detuned-resonator GSP metasurfaces designed for beam steering at {900},,hbox {nm} wavelength exhibit the extended reflection phase and operation bandwidth. We believe that the considered detuned-resonator GSP metasurfaces can advantageously be exploited in applications requiring the design of arbitrary phase gradients and/or broadband operation with linearly polarized fields.

Large-Scale Synthesis of Highly Uniform Silicon Nanowire Arrays Using Metal-Assisted Chemical Etching.

The combination of metal-assisted chemical etching (MACE) with colloidal lithography has emerged as a simple and cost-effective approach to nanostructure silicon. It is especially efficient at synthesizing Si micro- and nanowire arrays using a catalytic metal mesh, which sinks into the silicon substrate during the etching process. The approach provides a precise control over the array geometry, without requiring expensive nanopatterning techniques. Although MACE is a high-throughput solution-based approach, achieving large-scale homogeneity can be challenging because of the instability of the metal catalyst when the experimental parameters are not set appropriately. Such instabilities can lead to metal film fracture, significantly damaging the substrate and thus compromising the nanowire array quality. Here, we report on the critical parameters that influence the stability of the metal catalyst layer for achieving large-scale homogeneous MACE: etchant composition, metal film thickness, adhesion layer thickness, nanowire diameter and pitch, metal film coverage, Si/Au/etchant interface length, and crystalline quality of the colloidal template (grain size and defects). Our results investigate the origin of the catalyst film fracture and reveal that MACE experiments should be optimized for each Si wire array geometry by keeping the etch rate below a certain threshold. We show that the Si/Au/etchant interface length also affects the etch rate and should thus be considered when optimizing the MACE experimental parameters. Finally, our results demonstrate that colloidal templates with small grain sizes (i.e., <100 μm2) can yield significant problems during the pattern transfer because of a high density of defects at the grain boundaries that negatively affects the metal film stability. As such, this work provides guidelines for the large-scale synthesis of Si micro- and nanowire arrays via MACE, relevant for both new and experienced researchers working with MACE.

All-metallic metasurfaces towards high-performance magneto-plasmonic sensing devices

Magneto-plasmonic sensors based on surface plasmon resonance have been studied considerably in recent years, as they feature high sensitivity and ultrahigh resolution. However, the majority of such investigations focus on prism-based sandwich architectures that not only impede the miniaturization of devices but also have a weak transverse magneto-optical Kerr effect (TMOKE) in magnitude. Herein, we theoretically demonstrate a magneto-plasmonic sensor composed of Au/Co bilayer nanodisk arrays on top of optically thick metallic films, which supports a narrow surface plasmon resonance (SPR) with a bandwidth of 7 nm and allows for refractive index sensitivities as high as 717 nm/RIU. Thanks to the high-quality SPR mode, a Fano-like TMOKE spectrum with a subnanometer bandwidth can be achieved in the proposed structure, thereby giving rise to ultrahigh sensing of merit values as large as 7000 in water. Moreover, we demonstrate a large TMOKE magnitude that exceeds 0.6. The value is 1 order of magnitude larger than that of magneto-plasmonic sensors reported. We also demonstrate that the behavior of TMOKE spectra can be controlled by tuning the geometrical parameters of the device including the diameter and thickness of nanodisk arrays. This work provides a promising route for designing magneto-plasmonic sensors based on metasurfaces or metamaterials.

An optical method for determination of the mass thickness of thin gold films with arbitrary morphology

The mass thickness of thin metal films is an important parameter determining the film structure, however a robust routine method for its determination is still missing. In this work, we have developed a simple optical method for determining the mass thickness of gold films with arbitrary morphology on transparent substrates in the range 1–20 nm. The method is based on film spectrophotometry in the far-UV range when the light attenuation is due to interaction with core electrons and thus the influence of the film morphology is negligible. The calibration was performed at a wavelength of 200 nm for gold films of known thickness and a good agreement with the Beer–Lambert law was obtained. The feasibility of the method was demonstrated with gold films of various morphologies produced on quartz substrates by pulsed laser deposition.

Metastructure-inspired ultraviolet and blue light filter

Currently, many studies are being conducted to apply optical metamaterials to the transmittance control of visible light and near infrared wavelengths. In this study, a metastructure was fabricated with periodic metal holes that can reduce the transmittance of only ultraviolet (UV) and blue light in the wavelength band from UV to visible light. The connection structure of the metal-hole-patterns, with a hole size of tens to hundreds of μm, and the crystallinity of the metal could be controlled through the three-step process of polystyrene (PS) nanosphere array formation, using the air–water interface method, and size adjustment of the PS nanosphere, through the etching process, and through the adjustment of the metal film thickness. The glass substrate, with an Al-hole-patterned metastructure (an Al-hole-pattern size of ∼245 nm and an Al thickness of ∼60 nm), showed a low transmittance of 33% and 28% in the wavelength bands of 350 nm and 470 nm, respectively, and a high transmittance of 67% and 57% in the ranges of 540 nm and 670 nm, respectively, owing to the metacharacteristics manifested in periodically connected metal patterning structures. This Al-hole-patterned metastructure is expected to be applicable to various optical devices that require a reduction in the transmittance of only the UV and blue light wavelength band.

FPCB Masked One-Step Etching Large Aperture Mirror for LiDAR

This paper presents a Flexible Printed Circuit Board (FPCB) masked one-step etching large aperture mirror for Light Detection and Ranging (LiDAR). A FPCB structure is bonded on a thin silicon wafer ( $50\sim 200~\mu \text{m}$ thick) coated with a 100 nm thick metal film and one-step of DRIE is used to etch the wafer with the FPCB structure as the mask and the metal film as the stop layer. The simple fabrication process and 0.1 mm (instead of $ needed for microfabrication) resolution photolithography lead to a very low cost of a few dollars. Copper coils embedded in the FPCB structure are attached to the backside of the large aperture mirror plate, instead of beside the mirror plate as the case in MEMS magnetic mirrors, to generate a high magnetic force and achieve a relatively high frequency considering the large aperture and high thickness. Modeling and prototyping of two designs, Mirror_A and Mirror_B, are presented to verify the novel mirror technology. LiDAR applications of the novel mirror technology are demonstrated and tested. Achieved performances are: Mirror_A, aperture $12 \times 12$ mm, oscillation frequency of 510 Hz, field of view (FOV) of 30° and radius of curvature (ROC) of 10 m; Mirror_B aperture $24 \times 24$ mm, oscillation frequency of 160 Hz, FOV of 20° and ROC of 10 m. [2020-0085]

Electro-Optical Performance Study of 4H-SiC/Pd Schottky UV Photodetector Array for Space Applications

4H-SiC Schottky photodetector array was fabricated by pulsed dc sputter deposition of palladium thin film as UV semitransparent Schottky contact on active pixel. Results of electro-optical characterization of these devices as a function of temperature varying from 33 to 393 K are presented. A peak spectral responsivity of 122 mAW−1 at 280 nm is observed. Further, spatial response uniformity for a 4 mm $\times4$ mm detector, scanned by UV spot of $100~\mu \text{m}$ diameter, is within ±5%, whereas the crosstalk between the pixels in $4\times 2$ detector array with a pixel size of $850\,\,\mu \text{m}\,\,\times 850\,\,\mu \text{m}$ is within 1.3%. Capacitance at near zero bias is around 11 pF and 4.79 pF at a reverse bias of 30 V. From the ${I}$ – ${V}$ characteristics, the calculated Schottky barrier height is 1.37 eV for 70 A thick palladium metal film on 4H-SiC as against the theoretical value of 1.96 eV. Operating temperature studies on detector UV response at 300 nm have shown that the photo-current response at 113 K falls to half of its peak value at ambient. The responsivity of the detectors at 248-nm wavelength is almost constant over a wide temperature range of 33 to 393 K.

High-fidelity fabrication of plasmonic nanoholes array via ion-beam planarization for extraordinary transmission applications

Plasmonic nanoholes are a kind of important building blocks for practical plasmonic applications due to their unique properties supporting extraordinary transmission and strong field enhancement. However, the reliable fabrication of plasmonic nanoholes with high-fidelity geometry, especially in thick metallic film, is still challenging with the current wet lift-off or focused-ion beam process. In this work, we demonstrate a process combining negative-tone electron-beam lithography, metal sputtering and flood ion-beam planarization to reliably fabricate plasmonic nanoholes. Compared to existing lift-off process, it has the advantages of 100% yield, uniform geometry and smooth structural edges. In contrast to focused-ion beam process, the current approach has apparently higher throughput and structural fidelity. Plasmonic nanoholes with different geometries, materials and sizes are reliably fabricated to demonstrate the advantages of this approach. The extraordinary transmission properties and their potential for refractive-index sensing are also demonstrated. This lithography-deposition-planarization strategy can also be extended to fabricate other optical structures such as optical metasurfaces and X-ray zone-plates.