Changes In Optical Reflectivity Research Articles

Measurement of Surface Diffusion at Electrochemical Interfaces Using in Situ Linear Optical Diffraction

Surface diffusion processes play an important role in many reactions at electrode surfaces and are of relevance to electrocatalysis, electrodeposition, and many other areas. Such processes have therefore been extensively studied on solids under vacuum conditions with a variety of different methods. In sharp contrast, only few studies exist to measure surface diffusion in electrochemical environment due to the lack of suitable methodology. We aim on partially filling this gap by introducing the all-optical in situ LOD methodology, which allows to measure surface diffusion rates over a wide range and thus overcomes limitations of established methods that are mostly restricted to slow diffusion rates. The method is based on the generation of a periodic spatial modulation of adsorbates using two interfering laser pulses. The subsequent equilibration of this pattern and thus the diffusion rate of adsorbates is probed by the linear optical diffraction (LOD) signal from a second laser.First proof-of-principle measurements were performed on sulfur diffusion on the Pt(111) electrode. For this purpose, the optical and electrochemical properties of this systems were investigated first, including studies of the optical reflectance change induced by chemisorbed sulfur (Sad) and investigations of the Sad oxidation mechanism. The subsequent studies of Sad surface diffusion revealed potential and coverage dependent diffusion rates which were at least an order of magnitude faster than under vacuum conditions. Supported by numerical simulations, the implications of strong adsorbate-adsorbate interaction combined with different grating modulation depths to the temporal evolution of the LOD signal were demonstrated and data accuracy and pitfalls of the method were discussed.Kattwinkel and Magnussen, ACS Meas. Sci. Au, in press (2023), https://doi.org/10.1021/acsmeasuresciau.2c00066Kattwinkel and Magnussen, Electrochimica Acta 434, 141297 (2022), https://doi.org/10.1016/j.electacta.2022.141297

Spectral analysis of amplitude and phase echoes in picosecond ultrasonics for strain pulse shape determination

We introduce a spectral analysis method in picosecond ultrasonics to derive strain pulse shapes in an opaque sample with known optical properties. The method makes use of both the amplitude and phase of optical transient relative reflectance changes obtained, for example, by interferometry. We demonstrate this method through numerical simulation and by analysis of experimental results for a chromium film.

Plasmonic Mode Analysis on Asymmetric ITO/Au/ITO Hyperbolic Metamaterials Using Prism-Coupled Spectroscopic Ellipsometry

Various trilayers (ITO-Au-ITO) of a constant total thickness of 43 nm, with different bottom and top ITO film thicknesses, have been prepared by magnetron sputtering. The effects of the relative position of the inserted gold layer on the trilayers’ plasmonic properties have been systemically investigated. To study their optical reflectance changes caused by the surface plasmon resonance, total internal reflection ellipsometry (TIRE) has been employed. Two reflectance minima, corresponding to the pseudo-Brewster (PB) and bulk plasmon polariton (BPP), are observed from the reflectance spectra. The impacts of the light incident angles on the behaviors of these modes have been monitored. The measured reflectivity spectra match well with the simulated reflectivity spectra based on the Fresnel formalism. For the ITOt/Au/ITOb = 5/3/35 sample, where the Au layer is positioned closest to the sampling liquid, the BPP mode becomes the sharpest (based on the amplitude and half-width) as compared to other samples, indicating that the closer the ITOt/Au interface is to the sampling liquid, the stronger the observable plasmonic resonance effect is. Furthermore, it is found that the dip position of this BPP mode is independent of the angle of incidence but is sensitive to the refractive index of the sampling liquid. Our results show that the dip position shifts to a shorter wavelength with increasing refractive index of the sampling liquid. This means that the trilayer can act as a refractive index sensor. By linear regression, the obtained sensing accuracy is up to 552 nm/RIU. This facilitates the potential application of this trilayer structure as a surface plasmon-enhanced refractive index sensor.

Low temperature deposition of vanadium dioxide on III–V semiconductors and integration on mid-infrared quantum cascade lasers

We demonstrate low temperature deposition conditions for vanadium dioxide (VO2) phase change material by pulsed laser deposition, which are compatible with III–V semiconductors heterostructures typically used in optoelectronic applications. The characterizations of the VO2 coated thin films grown on GaAs show a 50% change in optical reflectivity in the mid-infrared range and a variation of electric conductivity of two orders of magnitude between the insulating (low temperature) and the metallic (high temperature) states. The transition temperature is estimated around 68 °C (341 K). We also study the functionalization of mid-infrared quantum cascade lasers (QCLs) (operating at wavelengths λ ∼ 7–8 μm) with VO2 layers, in view of engineering the laser emission properties with an integrated VO2 layer. We demonstrate QCLs that integrate a VO2 layer on the surface that interacts with the guided laser mode. A maximum operating temperature of 61 °C (334 K) has been measured.

Optical reflectance studies on the oxidation of chemisorbed sulfur at the Pt(111) electrode

The oxidation of chemisorbed sulfur on Pt(111) was studied in 0.1 M H2SO4 using combined electrochemical and in situ optical reflectance. The adsorbed sulfur adlayer, prepared by immersion of the Pt electrode in Na2S solution, was successively removed by potential cycling between 0.05 and 1.22 V vs. RHE. Correlation of the electrochemical data with the parallel measured changes in optical reflectance provides insight into the intermediate steps of the sulfur oxidation process. In addition to features related to sulfur oxidation and Pt oxide formation and reduction, two transient cathodic current peaks at rather negative potentials (0.19 and 0.41 V) are observed the first cycles, which can be assigned to intermediates of the sulfur oxidation process. The sulfur coverage oxidized in each cycle is initially large but then decreases rapidly, becoming negligibly small after about 10 cycles. Furthermore, a nonlinear change of the reflectance with decreasing sulfur coverage is observed, which suggests changes in the nature of the S-Pt bond with coverage.

Direct observation of ultrafast carrier coupling dynamics in monolayer graphene/metal system

• Femtosecond laser pump-probe technique measures the reflectivity changes of graphene/copper system. • A model assuming phonon-phonon coupling at graphene/copper interface is built and validated by the experiment. • Establish a new research paradigm to directly study the carrier coupling dynamics between 2D materials and substrates. For promising graphene-based devices, graphene/metal contacts are inevitable. When they operate at high frequencies, it is essential to explore the ultrafast carrier dynamics between graphene and metal, which has been rarely studied. Owing to the special monolayer graphene/copper structure we designed, that is, the graphene is grown on copper, we have directly observed the ultrafast carrier dynamics between monolayer graphene and copper by using femtosecond laser pump-probe technique for the first time. The measured optical reflectivity changes of graphene/copper system are significantly different from that of bare copper, which is attributed to the ultrafast carrier dynamics between graphene and copper, and graphene optical properties. A model considering carriers coupling in graphene and copper, including electron and phonon, has been built and validated by the experiment. Present study provides direct observation of the ultrafast carrier dynamics between graphene and metal, which is of great significance for graphene-based device applications.

Acetic acid detection in photovoltaic modules during ultraviolet irradiation and damp-heat combined tests

In this study, we evaluated the acetic acid generation in photovoltaic (PV) modules during an accelerated reliability test that combines ultraviolet (UV) irradiation and damp-heat (DH) using tin film sensors. We employed a tin film to detect acetic acid through the change in optical reflectance due to the chemical reaction from metallic tin to transparent tin tetraacetate. The relative reflectance of the tin film sensors laminated in a PV module was measured and used as an indicator of acetic acid generated during the reliability test. Consequently, we achieved non-destructive detection of a small amount of acetic acid generated during the UV irradiation stage of the UV+DH combined test with high spatial and temporal resolution using tin film sensors.

Embedding Quaternary V1–x–ySrxWyO2 into Multilayer Systems to Enhance Its Thermochromic Properties for Smart Glass Applications

Smart window glazing technology possesses a great potential for reducing the overall energy demand in the building sector. Thin films based on thermochromic vanadium dioxide (VO2) are ideally suited as functional layers of thermochromic smart windows. VO2 itself exhibits a significant change in optical transparency and reflectivity in the infrared spectrum. However, the optimization of VO2-based thin films for application as intelligent window glazing is still ongoing. The improvement is rather tedious because the key parameters set by the application such as transmittance modulation ΔT2500, luminous transmittance Tlum, solar transmittance modulation ΔTsol, and transition temperature ϑc affect each other. Codoping of VO2 with strontium (Sr) and tungsten (W) drives the parameters close to the desired range. However, the requirements set for the use as smart window coating cannot be fully met. Here, we show by a systematic comparison of simulations and experiments that embedding a layer of thermochromic V1–x–ySrxWyO2 between an optimized TiO2 buffer and a TiO2 antireflection layer already adds additional degrees of freedom to simultaneously fulfill all smart window requirements for the optical parameters ΔT2500, Tsol, Tlum, and ΔTsol. In particular, the luminous transmittance Tlum is improved by not less than ≈10% in such a trilayer structure.

Chip-scale optical airflow sensor

Airflow sensors are an essential component in a wide range of industrial, biomedical, and environmental applications. The development of compact devices with a fast response and wide measurement range capable of in situ airflow monitoring is highly desirable. Herein, we report a miniaturized optical airflow sensor based on a GaN chip with a flexible PDMS membrane. The compact GaN chip is responsible for light emission and photodetection. The PDMS membrane fabricated using a droplet-based molding process can effectively transform the airflow stimuli into optical reflectance changes that can be monitored by an on-chip photodetector. Without the use of external components for light coupling, the proposed sensor adopting the novel integration scheme is capable of detecting airflow rates of up to 53.5 ms−1 and exhibits a fast response time of 12 ms, holding great promise for diverse practical applications. The potential use in monitoring human breathing is also demonstrated.

Theoretical and experimental research of femtosecond laser processing fused silica

The ablation threshold, depth and crater shape of fused silica for femtosecond laser processing are investigated theoretically and experimentally. Based on tracking the spatiotemporal distribution of the free electron density, free electron temperature, and laser intensity, the electron dynamics as well as the transient optical and thermophysical properties of femtosecond laser irradiated fused silica are quantitatively determined. The numerical model is validated by comparing the calculated threshold fluence, depth and crater shape of ablation with the experimentally measured ones at a wavelength of 800 nm. The free electron relaxation time at different laser fluences and pulse durations throughout the photoionization process and impact ionization process are probed. In the present work, the findings are as follows. 1) The electron relaxation time significantly affects the material optical properties and femtosecond laser energy absorption. The optical properties change dramatically. The fused silica becomes opaque for the case of laser irradiation with fluence higher than the ablation threshold. Moreover, the transition from electron-phonon collision to electron-ion collision accompanies with the femtosecond laser ablation of fused silica. 2) By using the proposed model, the experimentally observed saturation of ablation depth at high laser fluence is elucidated by the significant change of optical reflectivity and absorption coefficient. Both the results of theoretical simulation and experimental observation indicate that laser fluence has a strong influence on the shape of the ablation crater. The ablation volume increases sharply with the increase of laser fluence for femtosecond laser irradiation, compared with that for picosecond laser irradiation. 3) With the increase of femtosecond laser fluence, the ablation depth removal efficiency and ablation efficiency are both saturated, followed by slight decrements. The peak of ablation depth removal efficiency peak occurs at the femtosecond laser fluence close to 1.4 times of the ablation threshold. While the accuracy is slightly low due to the higher sensitivity of the ablation characteristics (ablation crater depth and ablation volume) to the shorter femtosecond laser pulse. For the femtosecond laser fluence higher than 3.5 times of the ablation threshold, good repeatability over a very wide fluence range can achieve accurate processing results, because a more consistent flat-bottom ablation profile tends to appear. However, the heat-affected zone leads the processing quality to degrade, compared with the scenario of femtosecond laser fluence close to the ablation threshold.

Acoustic mapping by picosecond ultrasonics for elastic property measurement: Experimental demonstration on a TRISO fuel compact

Picosecond ultrasonics has been demonstrated on a tristructural isotropic (TRISO) fuel compact to measure the elastic properties of each compact layer. This technique utilizes an ultrashort pump laser pulse to excite vibrations in a gold transducer film covering the surface of each component and a second probe laser pulse to record the resulting acoustic strain induced change in optical reflectance. From the damping of this film vibration, the acoustic reflection coefficient, which couples the elastic properties of the transducer film and the sample, can be obtained, enabling a calculation of the sample's acoustic velocity and elastic modulus. Results obtained from this method are consistent with known values of elastic moduli, namely that the SiC coating is the stiffest component of the compact while the carbonaceous matrix is the most compliant. Nanoindentation was conducted as a benchmark technique on the same sample and shows satisfactory agreement with the results of picosecond ultrasonics. Compared to other methods like nanoindentation, picosecond ultrasonics is multimodal with a capability of measuring several key properties simultaneously and has potentials to be coupled into optical fibers for remote sensing. Thus, these demonstration measurements reveal the methodology to be a promising candidate for in-situ and high-throughput optical characterizations of nuclear materials.

Electrochemical Impedance Imaging on Conductive Surfaces

Electrochemical impedance spectroscopy (EIS) is a powerful tool to measure and quantify the system impedance. However, EIS only provides an average result from the entire electrode surface. Here, we demonstrated a reflection impedance microscope (RIM) that allows us to image and quantify the localized impedance on conductive surfaces. The RIM is based on the sensitive dependence between the materials' optical properties, such as permittivity, and their local surface charge densities. The localized charge density variations introduced by the impedance measurements will lead to optical reflectivity changes on electrode surfaces. Our experiments demonstrated that reflectivity modulations are linearly proportional to the surface charge density on the electrode and the measurements show good agreement with the simple free electron gas model. The localized impedance distribution was successfully extracted from the reflectivity measurements together with the Randles equivalent circuit model. In addition, RIM is used to quantify the impedance on different conductive surfaces, such as indium tin oxide, gold film, and stainless steel electrodes. A polydimethylsiloxane-patterned electrode surface was used to demonstrate the impedance imaging capability of RIM. In the end, a single-cell impedance imaging was obtained by RIM.

A Comparison of Characteristics of Periodic Surface Micro/Nano Structures Generated Via Single Laser Beam Direct Writing and Particle Lens Array Parallel Beam Processing

Abstract Changing material surface micro/nanostructures using laser beam texturing is a valuable approach in wide applications such as control of cell/bacterial adhesion and proliferation, solar cells and optical metamaterials. Here, we report a comparison of the characteristics of surface micro/nanostructures produced using single beam laser direct writing and particle lens array parallel laser beam patterning. A Nd:YVO4 nanosecond pulsed laser at the wavelength of 532 nm was used in the laser direct writing method to texture the stainless steel surface submerged in water and in air with different scanning patterns. Changes in surface morphology, wettability, surface chemistry, and optical reflectivity were analyzed. In the particle lens array method, an excimer nanosecond laser at 248 nm wavelength was adopted to produce surface patterns on GeSbTe (GST) film coated on a polycarbonate substrate by splitting and focusing a single laser beam into millions of parallel breams. Single beam laser direct writing shows that the surface of high roughness and oxygen percentage content presented high wettability and low reflectivity characteristics. However, the controllability of the type of surface micro/nanopatterns is limited. The parallel laser beam processing using particle lens array allows rapid production of user designed periodic surface patterns at nanoscale overcoming the optical diffraction limit with a high degree of controllability. Controlling the uniformity of the particle lens array is a challenge.

User-Interactive Thermotherapeutic Electronic Skin Based on Stretchable Thermochromic Strain Sensor.

User‐interactive electronic skin (e‐skin) with a distinguishable output has enormous potential for human–machine interfaces and healthcare applications. Despite advances in user‐interactive e‐skins, advances in visual user‐interactive therapeutic e‐skins remain rare. Here, a user‐interactive thermotherapeutic device is reported that is fabricated by combining thermochromic composites and stretchable strain sensors consisting of strain‐responsive silver nanowire networks on surface energy‐patterned microwrinkles. Both the color and heat of the device are easily controlled through electrical resistance variation induced by applied mechanical strain. The resulting monolithic device exhibits substantial changes in optical reflectance and temperature with durability, rapid response, high stretchability, and linear sensitivity. The approach enables a low‐expertise route to fabricating dynamic interactive thermotherapeutic e‐skins that can be used to effectively rehabilitate injured connective tissues as well as to prevent skin burns by simultaneously accommodating stretching, providing heat, and exhibiting a color change.

Vanadium dioxide thin films synthesized using low thermal budget atmospheric oxidation

Vanadium dioxide is a complex oxide material, which shows large resistivity and optical reflectance change while transitioning from the insulator to metal phase at ~68 °C. In this work, we use a modified atmospheric thermal oxidation method to oxidize RF-sputtered Vanadium films. Structural, surface-morphology and phase-transition properties of the oxidized films as a function of oxidation duration are presented. Phase-pure VO2 films are obtained by oxidizing ~130 nm Vanadium films in short oxidation duration of ~30 s. Compared to previous reports on VO2 synthesis using atmospheric oxidation of Vanadium films of similar thickness, we obtain a reduction in oxidation duration by more than one order. Synthesized VO2 thin film shows resistance switching of ~3 orders of magnitude. We demonstrate optical reflectance switching in long-wave infrared wavelengths in VO2 films synthesized using atmospheric oxidation of Vanadium. The extracted refractive index of VO2 in the insulating and in the metallic phase is in good agreement with VO2 synthesized using other methods. The considerable reduction in oxidation time of VO2 synthesis while retaining good resistance and optical switching properties will help in integration of VO2 in limited thermal budget processes, enabling further applications of this phase-transition material.

Electrotunable Nanoplasmonics for Amplified Surface Enhanced Raman Spectroscopy.

Tuning the properties of optical metamaterials in real time is one of the grand challenges of photonics. Being able to do so will enable a class of adaptive photonic materials for use in applications such as surface enhanced Raman spectroscopy and reflectors/absorbers. One strategy to achieving this goal is based on the electrovariable self-assembly and disassembly of two-dimensional nanoparticle arrays at a metal |liquid interface. As expected, the structure results in plasmonic coupling between NPs in the array but perhaps as importantly between the array and the metal surface. In such a system, the density of the nanoparticle array can be reversiblycontrolled by the variation of electrode potential. Theory suggests that due to a collective plasmon-coupling effect less than 1 V variation of electrode potential can give rise to a dramatic simultaneous change in optical reflectivity from ∼93% to ∼1% and the amplification of the SERS signal by up to 5 orders of magnitude. Thisis experimentallydemonstrated using a platform based on the voltage-controlled assembly of 40 nm Au-nanoparticle arrays at a TiN/Ag electrode in contact with an aqueous electrolyte. We show that all the physics underpinning the behavior of this platform works precisely as suggested by the proposed theory, setting the electrochemical nanoplasmonics as a promising direction in photonics research.

OPTICAL COHERENCE TOMOGRAPHY LEAKAGE IN NEOVASCULAR AGE-RELATED MACULAR DEGENERATION: Identification of Choroidal Neovascularization Activity by Location and Quantification of Abnormal Fluid Under Anti-Vascular Endothelial Growth Factor Therapy.

To test optical coherence tomography leakage in the identification and quantification of choroidal neovascularization-related fluid, its change after anti-vascular endothelial growth factor therapy in neovascular age-related macular degeneration eyes and its relation to functional outcome. Prospective analysis of a cohort of neovascular age-related macular degeneration cases treated with 2.0-mg intravitreal aflibercept. Eyes included were analyzed before, 1-week, and 1-month after one injection. Best-corrected visual acuity was assessed using Early Treatment Diabetic Retinopathy Study method. Optical coherence tomography leakage maps depicting low optical reflectivity (LOR) sites were acquired with OCT Cirrus AngioPlex (Zeiss, Dublin, CA). The LOR area ratio was correlated to retinal thickness and best-corrected visual acuity. Optical coherence tomography angiography was simultaneously performed. Twenty-two eyes of 18 patients with neovascular age-related macular degeneration were included. The LOR ratio of the full retina scan and retinal pigment epithelium-Bruch layer decreased from baseline to Month 1 (P < 0.05). Changes in retinal thickness and LOR ratio were positively correlated (P < 0.05). Best-corrected visual acuity change correlated with the outer segment layer LOR change (rho = -0.53, P = 0.014), and LOR was inferior in better responders (P = 0.021). Optical coherence tomography leakage identified eyes with recurrent fluid in the external layers. Optical coherence tomography leakage identified and quantified the fluid related to choroidal neovascularization activity. Low optical reflectivity change in the outer segment layer correlates with functional outcome and increasing LOR in the external layers may be a marker of early recurrence. Combining optical coherence tomography angiography and optical coherence tomography leakage allows both for choroidal neovascularization morphology and activity analysis.

Temperature effects on magnetic, electrical and optical properties of CoFeAlSi Heusler alloy thin films

Quaternary Co2FeAl0.5Si0.5 Heusler alloy thin films have attracted special attention for the application in magnetoelectronic devices. Co2FeAl0.5Si0.5 alloy thin films are fabricated by magnetron sputtering at room temperature. The temperature effects on the magnetic, electrical and optical properties in sputtered Co2FeAl0.5Si0.5 alloy films are investigated. Thermomagnetic measurements reveal increased magnetization arising from enhanced crystalline ferromagnetic ordering. Anomalous increase in electrical resistivity with temperature is probed in the alloy film, accompanied with significant change in optical reflectance. The large variations in electrical and optical signals in the alloy film can be attributed to the transition of atomic orderings, confirmed by X-ray diffraction measurements. The activation barrier for the transformation of atomic orderings is determined to be 1.22 eV, showing good thermal stability of the Heusler alloy thin film.

Single-shot arrival timing diagnostics for a soft X-ray free-electron laser beamline at SACLA.

Arrival timing diagnostics performed at a soft X-ray free-electron laser (FEL) beamline of SACLA are described. Intense soft X-ray FEL pulses with one-dimensional focusing efficiently induce transient changes of optical reflectivity on the surface of GaAs. The arrival timing between soft X-ray FEL and optical laser pulses was successfully measured as a spatial position of the reflectivity change. The temporal resolution evaluated from the imaging system reaches ∼10 fs. This method requires only a small portion of the incident pulse energy, which enables the simultaneous operation of the arrival timing diagnostics and experiments by introducing a wavefront-splitting scheme.

Magnetic and thermal properties of amorphous TbFeCo alloy films

Amorphous TbFeCo material with perpendicular magnetic anisotropy is currently attracting more attention for potential applications in spintronic devices and logic memories. We systematically investigate magnetic, structural, thermal, optical and electrical properties of TbFeCo alloy films. It shows out-of-plane easy axis of the films turns into in-plane orientation after annealing. Significant increase in saturation magnetization in the temperature range between 400 and 450°C is revealed by thermomagnetic measurements. The occurrence of crystallization and oxidation at high temperatures is confirmed by X-ray diffraction measurements. Pronounced changes in optical reflectance and sheet resistance are observed with temperature, in line with structural relaxation and change. The activation barriers for crystallization and oxidation are determined to be 1.01eV and 0.83eV, respectively, for FeCo-rich and Tb-rich samples. Better thermal stability against crystallization and oxidation is demonstrated in the FeCo-rich sample than the Tb-rich type. Our results provide some useful information for the alloy used in device fabrication.