Thick Gold Film Research Articles

A label-free thrombin photoelectrochemical aptasensor based on structure-switching in G-quadruplexes

In this work, an Au/GaN photoelectrode was prepared by sputtering a 30.0-nm–thick gold film on an n-type gallium nitride (GaN) surface. Guanine (G) was used as a hole trap in photoinduced charge separation due to its lowest oxidation potential and because it could be easily oxidized among the five common nucleobases. Guanosine-rich DNA sequences can form stable secondary structures known as G-quadruplexes. The efficiency of hole transport was significantly improved after G-quadruplexes formation. The thrombin aptamer rich in guanine could be modified on the electrode surface through Au–S bonds, forming stable G-quadruplexes in the presence of target thrombin. Due to the presence of the G-quadruplexes, the electron-hole recombination rate of the system decreased and the photocurrent increased. Under optimized conditions, high specificity, selectivity, and linearity could be achieved in the concentration range of 0.100–10.0 nm, and the limit of detection was approximately 20.0 pM. Thus, the detection results of human serum thrombin were satisfactory.

Gold Micropetals Self‐Assembled by Shadow‐Sphere Lithography for Optofluidic Control

AbstractThe self‐assembly of gold micropetals and microfluidic control using their photothermal properties are demonstrated experimentally in this work. Gold micropetals are fabricated on a glass substrate using the shadow‐sphere lithography technique. Each micropetal consists of two areas with thin gold films of different thicknesses resulting in an inhomogeneous optical absorption distribution. The photothermal property of the micropetal results in the generation of a water vapor microbubble on the micropetal when the micropetal is immersed in degassed water and irradiated by a continuous wave laser. By applying a temperature gradient to the microbubble, Marangoni flow in the direction parallel to the substrate is successfully induced. The direction of the flow varies with the combination of the optical absorption distribution in the micropetal and the laser power. This variation can be explained by the position of the bubble on the micropetal and the resulting temperature gradient around the bubble. Controlling the position at which the bubble is generated on the micropetal is found to be important for controlling the flow direction. This technique allows microfluids to be pumped through the irradiation of light with a simple intensity distribution on spatially arranged micropetals.

Design and Numerical Analysis of a Single-Polarization Filter Based on PCF with Plasmonic Layers of Gold and Indium Tin Oxide

This paper presents a single-polarization filter based on PCF with plasmonic layers of gold and indium tin oxide (ITO). The plasmonic materials are metallic gold and ITO coated on the inner walls of two extra-large vertically arranged air holes. The resonance effect is triggered by guided modes propagating through the silica core and coupling to the coating areas. The finite element method is used to analyze the properties of the filter for the two fundamental orthogonal polarizations. A filtering effect is achieved in the communication window by optimizing the structural factors as well as gold film and ITO deposition thicknesses. When the filter is 1 mm long, the obtained filtering effect is 1319.689 dB/cm for the y-polarization and 31.881 dB/cm for the x-polarization, thus efficiently attenuating the y-component at a communication window of 1.15 μm. With a filtering bandwidth of 602 nm, the proposed filter shows superior characteristics compared with previously reported results. Applications of the proposed plasmonic PCF-based filter can be found in polarization-maintaining and polarization-suppressing systems for optical sensing and broadband transmission.

Experimental and numerical studies on micro-bumps without melting of gold films with different thicknesses induced by ultrafast laser

The micro-bumps of metal films induced by ultrafast laser have potential applications in various fields, suchas biotechnology and energy, etc. The mechanism of micro-bump deformation of the gold film irradiated by single ultrafast laser pulse is controversial. In this study, the experimental research of single ultrafast laser pulse irradiating gold films with different thicknesses from 20 nm to 200 nm is carried out, with the laser fluence varying from 1 to 50 J/cm 2. The micro-bumps without melting are observed by the Scanning Electron Microscope (SEM) and different separation thresholds can be obtained. The separation threshold of gold films ranges from 5.1 to 43.5 J/cm 2 and it increases with the thickness of gold film increasing. The heights of micro-bumps are measured by Atomic Force Microscope (AFM). One-dimension (1D) Molecular dynamic (MD) simulations are carried out to analyze the temperature and stress evolutions. Large-scale two-dimension (2D) MD simulations are furthermore performed to obtain the micro-bump structure. MD simulations showing a good agreement with experiments, and the essential mechanism responsible for micro-bump without melting is clearly analyzed. It is also analyzed that the heights of micro-bumps are related to the laser fluences and radii of spot.

Optimization of nanoparticle yield for biomedical applications at femto-, pico- and nanosecond laser ablation of thin gold films in water

The paper considers the generation of colloidal gold nanoparticles by laser ablation of thin gold films of variable thickness in deionized water. The effect of laser fluence, pulsewidth, exposure, and film thickness on the generation of nanoparticles, its productivity and ergonomicity, particle dispersion and size is studied along with the related ablative mass loss from the films.

Scotch-tape surface wrinkling based thin-film material properties extraction

This paper presents a new simple method to measure thin film material properties using scotch-tape surface wrinkling. Thin metal films have been deposited on polymer substrates by e-beam evaporation. After patterned by photolithography and wet etching, long and narrow thin metal layers have been transferred by peel-off onto the scotch tape. The effect of velocity of scotch tape peel-off on the metal film transfer from polymer substrates to the scotch tape has been investigated. After metal transfer, metal film wrinkling patterns have been established on the scotch tape due to mechanical properties mismatch between the two materials. The wrinkling patterns have been characterized in term of amplitude and wavelength and they are compared with finite element method buckling simulation results for material properties extraction. By consequence, elastic moduli of 300 nm thick gold film and a multilayer of 30 nm Ti on 300 nm Au have been found 147 GPa and 885 GPa based on the measured wavelength of the wrinkling films.

Characteristics of a rectangular channel sensor based on evanescent field enhanced photonic crystal fiber

A miniaturized, rectangular-channel photonic crystal fiber (RC-PCF) high-sensitivity sensing scheme is proposed for high-refractive-index liquid analysis. The designed PCF is polished and features an open rectangular channel in the upper-middle part of the fiber cross-section, which facilitates the efficient flow of the sample liquid. The design utilizes the merit of photonic crystal array and the operability of external sensing. Gold is chosen as the plasma material because of its chemical stability and biocompatibility in a liquid environment. To obtain high sensitivity, the effect of the sample liquid height in the channel on the sensing performance is investigated. Numerical results show that the maximum wavelength sensitivity and amplitude sensitivity of the sensor is 28,000 nm/RIU (refractive index unit) and − 1266.5 R I U − 1 with a liquid analyte height of 100% of the channel height. The maximum wavelength sensitivity increases from 28,000 to 42,000 nm/RIU when the gold film thickness decreases from 30 nm to 20 nm. Due to the sensor’s simple structure and high sensitivity, it will have potential applications in the detection of high-refractive-index samples in biology, medicine, and chemistry.

Unique reflection from birefringent uncoated and gold-coated InP nanowire crystal arrays.

We demonstrate unique reflective properties of light from bare and gold-coated InP nanowire (NW) photonic crystal arrays. The undoped wurtzite InP nanowire arrays are grown by selective area epitaxy and coated with a 12-nm thick Al2O3 film to suppress atmospheric oxidation. A nominally 10-nm thick gold film is deposited around the NWs to investigate plasmonic effects. The reflectance spectra show pronounced Fabry-Perot oscillations, which are shifted for p- and s-polarized light due to a strong intrinsic birefringence in the NW arrays. Gold-coating of the NW array leads to a significant increase of the reflectance by a factor of two to three compared to the uncoated array, which is partially attributed to a plasmon resonance of the gold caps on top of the NWs and to a plasmonic antenna effect for p-polarized light. These interpretations are supported by finite-difference-time-domain simulations. Our experiments and simulations indicate that NW arrays can be used to design micrometer-sized polarizers, analyzers, and mirrors which are important optical elements in optoelectronic integrated circuits.

Rapid formation methods of arrays of randomly distributed Au and Ag nanoparticles, their morphologies and optical characteristics

By the method of rapid radiation heating (at a speed of 20-25 K/s) of Au and Ag films with a thickness of 4-35 nm to temperatures of 573-693 K in air and in the process of vacuum deposition of silver on heated (up to 700 K at a heating rate of 10 K/s ) glass substrates formed Au and Ag NPs arrays with nanoparticle sizes from several tens to hundreds of nanometers, the position λSPR of which is in the range of 520-597 nm for Au NPs and 424-509 nm for Ag NPs. It is established that the average size of nanoparticles depends on the thickness of gold and silver films and the annealing temperature. The results testify that glass substrates with arrays of randomly distributed gold NPs can be used as effective SERS-substrates for the investigation of Raman spectra of nanosized (50-100 nm) chalcogenide films.

A Broadband Gold-Coated Photonic Crystal Fiber Polarization Filter with a High Loss Ratio of Both Polarizations at 1550 and 1310 nm

A new kind of gold-coated hexagonal photonic crystal fiber polarization filter is designed in this paper. The filtering properties can be adjusted through varying the structural parameters. With the 25.60 nm gold film thickness, the losses of the respective modes of Y and X-polarized core mode at 1550 nm are 1024.84 and 0.12 dB/cm with the loss ratio of 8540.33 between two polarizations. However, the losses of Y and X-polarized core mode at 1310 nm are 682.14 and 0.03 dB/cm, and the loss ratio is 22,738 with the gold film thickness of 55.30 nm. That indicates that the proposed filter has a higher loss ratio. Moreover, the crosstalk value with the fiber length of 200 μm at 1550 and 1310 nm are 178.01 and 118.49 dB, respectively. The bandwidths with crosstalk value greater than 20 dB are 640 and 180 nm. The designed polarization filter represents good filtering characteristics and allows great fabrication tolerances. Therefore, the designed hexagonal filter can be well applied in the domain of optical fiber communication.

Surface plasmon resonance (SPR) based temperature and magnetic field sensor in a dual-core D-shaped photonic crystal fiber (PCF)

A high sensitivity surface plasmon resonance (SPR) based temperature and magnetic field sensor in a dual-core D-shaped photonic crystal fiber (PCF) with two types of holes is reported. The flat surface of the dual-core D-shaped PCF is coated with ethanol as the temperature-sensitive material. A large diameter hole coated with a gold ring is filled with a magnetic fluid as a magnetic field-sensitive material. Using the finite element method (FEM), the electric field distribution mode is simulated, and the loss spectra of the temperature sensing channel (channel I) and magnetic field sensing channel (channel II) are calculated. The influence of the types of metal films and rings, large hole diameter, small hole diameter, and gold film and gold ring thicknesses on the sensor performance are discussed. The results showed that the reported sensor is able to measure temperature and magnetic field independently and simultaneously due to its dual channels. The influence of temperature on magnetic field measurements is reduced by reducing the hole diameter. The results show that the temperature sensitivity is 1.151 nm/°C and the correlation coefficient is 0.9954 from 20 to 70 °C. The magnetic field intensity sensitivity is 0.0779 nm/Oe from 25 to 200 Oe and 0.0212 nm/Oe from 200 to 275 Oe with correlation coefficients of 0.9937 and 0.9989 across these ranges. This high sensitivity D-shaped PCF sensor will promote the development of high-performance dual-parameter devices.

Simple structure dual-core photonic crystal fiber polarization beam splitter covering the O + E + S + C + L + U band based on the surface plasmon resonance effect

In this paper, a simple structure dual-core photonic crystal fiber (SS-DC-PCF) polarization beam splitter (PBS) based on the surface plasmon resonance (SPR) effect and symmetric dual-core coupling mode theory is proposed. For the proposed SS-DC-PCF-1 PBS, the coupling lengths (CLs) and coupling length ratio (CLR) are analyzed, and the variations of the normalized output powers with the propagation length are investigated for the chosen wavelengths 1.434, 1.451, and 1.469 µm. The extinction ratio in cores A and B are compared at the three splitting lengths (SLs) 170, 173, and 176 µm. When the optimal SL is 176 µm, the splitting bandwidth (SB) is 255 nm (1.437–1.692 µm), and the insertion loss (IL) is less than 0.043 dB. The regulations of the CLs and CLR with the change of the structure parameters are analyzed. For the proposed SS-DC-PCF-2 PBS, the optimal SL is 232 µm, the SB is 201 nm (1.249–1.450 µm), and the IL is less than 0.042 dB. Finally, it is demonstrated that the total SB of the proposed SS-DC-PCF-1 PBS and SS-DC-PCF-2 PBS is 443 nm (1.249–1.692 µm), which can cover theO+E+S+C+L+Uband, even if the gold film thickness changes by±1nm. The proposed SS-DC-PCF PBS has excellent performances, such as ultrashort SL, ultrawide SB, ultralow IL, and good error-tolerance rate. It will have important applications in all-fiber optical systems.

Surface Plasmon Resonance Sensor Based on Double-Sided Polished Microstructured Optical Fiber With Hollow Core

A surface plasmon resonance sensor based on double-sided polished microstructured optical fiber with hollow core is put forward for refractive index sensing. Two gold films parallel to each other attached to the polished surface act as microfluidic sensing channels for the analyte. The artificially introduced air hole can facilitate the phase matching between the core mode and the plasmon mode. The sensitivities of the proposed sensor are investigated by the wavelength, amplitude and phase interrogation methods when the analyte refractive index increases from 1.33 to 1.34. In contrast to the D-shaped design, the double-sided polished structure demonstrates narrower resonance spectral width and greater phase sensitivity. Moreover, the numerical results indicate that the proposed sensor shows a good stability in the fabrication tolerances of ±5% of the thickness of gold film and the depth of polishing, respectively.

Effects of gold film thickness and annealing time on diameter of silicon nanowires grown by VLS process

In this paper, silicon nanowires were prepared by chemical vapor deposition (CVD) in the presence of gold catalyst. We discussed the effects of gold film thickness and annealing time on droplet diameter and silicon nanowire diameter. The study revealed that the diameter of silicon nanowires increases with the increase of the thickness of the gold film. With the increase of annealing time, the number of alloy droplets decreases, and the diameter increases. The average diameter of silicon nanowires decreases first and then increases. The results provide useful information for long-time annealing of silicon nanowires.

Electrical Tuning of Fresnel Lens in Reflection

Optical metasurfaces have been extensively investigated, demonstrating diverse and multiple functionalities with complete control over the transmitted and reflected fields. Most optical metasurfaces are however static, with only a few configurations offering (rather limited) electrical control, thereby jeopardizing their application prospects in emerging flat optics technologies. Here, we suggest an approach to realize electrically tunable optical metasurfaces, demonstrating dynamic Fresnel lens focusing. The active Fresnel lens (AFL) exploits the electro-optic Pockels effect in a 300-nm-thick lithium niobate layer sandwiched between a continuous thick and nanostructured gold film serving as electrodes. We fabricate and characterize the AFL, focusing 800-900 nm radiation at the distance of 40 $\mathrm{\mu}$m with the focusing efficiency of 15 % and demonstrating the modulation depth of 1.5 % with the driving voltage of $\pm 10$ V within the bandwidth of $\sim\! 4$ MHz. We believe that the electro-optic metasurface concept introduced is useful for designing dynamic flat optics components.

Numerical simulation and parameters estimation of the time fractional dual-phase-lag heat conduction in femtosecond laser heating

In this paper, considering the heat flux and the temperature gradient approximated by the fractional Taylor's series expansions with different orders in dual-phase-lag (DPL) model, we propose the time fractional DPL heat conduction model for thin gold films heating by femtosecond laser pulses. The solutions of the model are investigated analytically and numerically. Firstly, the semi-analytical solution is presented by the Laplace transform method and the Fourier cosine transform method. With the L1 approximation of the Caputo fractional derivative, the finite difference scheme is derived to describe the effects of femtosecond laser pulses on the temperature distributions of the gold films. The effectiveness of the numerical algorithm is examined by the comparison with the semi-analytical solution. And then based on the numerical algorithm and the experimental datasets of femtosecond laser pulses heating on gold films of various thicknesses, the estimations of the model parameters are considered by the modified hybrid Nelder–Mead simplex search and particle swarm optimization (MH-NMSS-PSO). Using the optimal values of the parameters estimation, we verify the consistencies between the semi-analytical solutions of the model and the datasets. It proves the accuracy of the time fractional DPL heat conduction model in characterizing the temperature distributions of thin gold films. Finally, the varieties of temperature distribution with time and position are considered and then the effects of the model parameters on the temperature distribution are also discussed.

Side-Polished D-Type Fiber SPR Sensor for RI Sensing With Temperature Compensation

We propose and demonstrate a dual-channel surface plasmon resonance (SPR) sensor based on the side-polished D-type optical fiber, which is available for simultaneous measurement of refractive index (RI) and temperature. The single mode fiber (SMF) is side-polished to remove the fiber cladding and plated a layer of gold film on the polished surface to excite SPR effect. The RI sensing characters are researched by simulation and experiment. The sensitivity increases from 1543 nm/RIU to 6300 nm/RIU as RI increasing from 1.333 to 1.410. Besides, the resonance wavelength red-shifts about 40 nm as the gold film thickness increasing from 40 nm to 60 nm. Based on the above, we design the temperature compensated RI sensor by modifying the side-polished D-type fiber. One half of the side-polished region is applied for RI sensing and the other half is additionally coated a layer of polydimethylsiloxane (PDMS), which is high sensitive to temperature variation, for temperature sensing. In addition, the resonance wavelengths of two sensing channels can be controlled by adjusting the film thickness to avoid crosstalk. In the range from 1.333 to 1.390, the RI sensitivity reaches as high as 2260.1 nm/RIU. In the range from 20 °C to 60 °C, the temperature sensitivity reaches as high as -2.41 nm/°C. Finally, by temperature compensation, the RI measurement accuracy can be greatly improved.

Multifunctional polymer-assisted spontaneous transformation of thin gold films into nanoparticles

In this document, we report an efficient synthesis of well-defined, soluble, high molecular weight, film-forming aromatic multifunctional homopolymers bearing two distinct functional groups per repeating unit. The polymers were obtained in nearly quantitative yields by a one-pot, room-temperature, non-stoichiometric superacid-catalyzed step-polymerization of derivatives of pyruvic acid with aromatic hydrocarbons. It was found that thin gold films (2–12 nm) sputtered onto the surface of a multifunctional homopolymer (PTBC) containing carboxy- and bromomethyl- functionalities in every repeating unit undergo spontaneous, solvent-free, room-temperature, quantitative transformation into nanoparticles. For the first time, the key factors affecting film-to-particle transformation were revealed. Internal factors are dependent on the polymer structure, its homogeneity, and the composition of the polymer/metal interface, while external factors include gold-film thickness, temperature and ageing atmosphere. By varying the functional-group combinations and aromatic fragments, a set of multifunctional homopolymers has been synthesized and it was found that the presence in the homopolymer of a repeating unit of two phenylene rings and two functional groups – bromomethyl and carbonyl – attached to the same carbon backbone atom (forming a so-called “Reactive site”) is critically important for film-particle transformation. The process is accelerated with increasing temperature, proceeds in ambient air, and is halted in a vacuum. The average grain size of the nanoparticles increased from 3 to 18 nm over a week at room temperature and from 5 to 27 nm over the same period at 40 °C. The size and shape of the nanoparticles are modulated by controlling the thickness of the gold film, the temperature and the ambient atmosphere. The transformation represents a very simple, one-step, environmentally-friendly process that is not dependent on any other chemical reagents, requires no previous temperature or high-energy processing, and can easily be scaled up to fabricate high-purity, fully accessible polymer-(film- or nanofiber-) supported gold nanoparticles of long-term stability.

A High‐Sensitivity Refractive Index Sensor Based on an External Gold Coating of a Square‐Lattice Photonic Crystal Fiber

A high‐sensitivity square‐lattice photonic crystal fiber (PCF) refractive index (RI) sensor based on an external gold coating is proposed. This design eliminates the limitations of selective coating and liquid penetration in the micrometer holes, and avoids the side polishing of D‐type fibers. The effects of air hole diameter and gold film thickness on the sensing performance are studied in detail using the full vector finite element method (FV‐FEM). In addition, the sensing performance of the gold‐coated PCF under the optimized parameters is compared with that of PCF with the same air hole diameter and a gold grating. The results show that the proposed PCF has a higher structural tolerance to the air hole diameter, which improves the manufacturing feasibility. Moreover, the gold‐coated PCF has a maximum sensitivity of 31 000 nm RIU−1 and a resolution of 3.23 × 10−6 RIU in the RI measurement range of 1.34–1.49. The maximum sensitivity of the gold‐grating PCF with lower loss is 30 000 nm RIU−1 in the RI detection range of 1.38–1.46, and the resolution is 3.33 × 10−6 RIU. Low loss is beneficial for extending the length of the fiber, which is convenient for practical implementation. Thus, it has potential applications in biomolecular and biochemical analysis.

Thin-film lithium niobate-on-insulator (LNOI) shear horizontal surface acoustic wave resonators

This work investigates the design methodology to obtain large electromechanical coupling factor () and high quality factor (Q) of shear-horizontal surface acoustic wave (SH-SAW) resonators based on the thin-film lithium niobate-on-insulator (LNOI) technology. The guided SH wave can be excited through interdigital transducers and propagate at the very surface of the material stackings. Such a guided SH wave in LN/SiO2 double layer structure is expected to offer high by confining the elastic strain energy in the piezoelectric thin film. To capture the optimum design window for high-performance LNOI SH-SAW devices, the impact of electrode material and its thickness on the dispersive characteristics are intensively investigated by finite element method (FEM). In this study, various one-port resonators with wavelengths from 2.8 μm to 8 μm were fabricated on a LNOI wafer with LN and SiO2 thickness of 0.7 and 2 μm, respectively. The 100 nm thick gold film was chosen as the electrode of the devices, which demonstrate a similar dispersive behavior to the FEM simulation with small discrepancy. Among the measurement results over several tested samples, a high- of 25.5% and Q of 960 was recorded at a resonance frequency of 581 MHz (FOM = = 245), revealing great potential for the application of wide-band frequency selection in telecommunications.