Reflection Configuration Research Articles

The Von Neumann Paradox for the Euler Equations

The reflection of a weak shock wave is considered using a shock polar. We present a sufficient condition under which the von Neumann paradox appears for the Euler equations. In an attempt to resolve the von Neumann paradox for the Euler equations, two new types of reflection configuration, one called the von Neumann reflection (vNR) and the other called the Guderley reflection (GR), are observed in numerical calculations. Finally, we obtain that GR is a reasonable configuration and vNR is an unreasonable configuration to resolve the von Neumann paradox.

Broadband chromatic dispersion measurements in higher-order modes selectively excited in optical fibers using a spatial light modulator.

We propose an improvement of the interferometric method used up to now to measure the chromatic dispersion in single mode optical fibers, which enables dispersion measurements in higher-order modes over a wide spectral range. To selectively excite a specific mode, a spatial light modulator was used in the reflective configuration to generate an appropriate phase distribution across an input supercontinuum beam. We demonstrate the feasibility of the proposed approach using chromatic dispersion measurements of the six lowest order spatial modes supported by an optical fiber in the spectral range from 450 to 1600 nm. Moreover, we present the results of numerical simulations that confirm sufficient selectivity of higher-order mode excitation.

Patterning of transition metal dichalcogenides catalyzed by surface plasmons with atomic precision

Summary Plasmon-induced charge transfer has drawn considerable interests and spurred rapid developments of photovoltaics and photocatalysis. Various plasmonic metal/transition metal dichalcogenide (TMD) heterostructures have been developed to promote charge separation. For plasmon-induced catalysis, the transformation of TMDs is rare, and the erosion of TMDs has not yet been reported. Here, we report the etching of two-dimensional TMDs by planar surface plasmon polaritons (SPPs). We found that SPPs can etch various TMDs into desired layers and lateral size through controlling the light power and incident direction. In aqueous media, the strong plasmonic coupling at Au/MoS2 interface generates holes in the valence band of MoS2 that can weaken its interlayer interaction. As a synergistic effect, the plasmonic hot electrons produce oxidizing H2O2 to dissolve MoS2 from the top layers. Our results provide a new perspective for understanding the plasmonic coupling at metal-TMD interfaces and a reliable route toward fabricating well-defined TMDs.

Design and development of fiber optic sensor for measurement of purity of ethanol

Pure alcohol (ethanol) has number of applications in automobile, chemical and pharmaceutical industries. Fiber optic sensor has widespread use due to its non contact type measurement in retro reflective configuration. This paper proposes non contact type refractometric fiber optic sensor for measurement of purity of alcohol manufactured in distilleries. This refractometric based sensor shows variation in the received light intensity with variation in water content of ethanol i.e. purity of alcohol. The sensor is studied and analyzed by performing simulation based on mathematical model developed using ray tracing technique. The sensor is fabricated and tested by preparing samples in the laboratory. The refractive indices are later measured using Abbe’s refractometer having the resolution of 0.001 in white light. Field tests are also carried out at the local distilleries. Experimental results show good agreement with the simulated results as well as with the conventional methods used in industry for measurement of the purity of alcohol. The sensor shows linear variation in the output voltage with the variation in alcohol purity of alcohol from 90% to 99%. The repeatability of the sensor is checked by repeating the experiment 60 times for each value of purity.

Super resolution far field infrared microscopy

We present and compare the performances of label-free super-resolution infrared microscopy based on non-linear properties intrinsic to all vibrational transition in three configurations such as pump-probe, differential absorption, and attenuated total reflection configurations. We then highlight experimental demonstrations achieved in label-free super-resolution chemical imaging that succeed in exploiting this novel concept.

CaAlSiN3:Eu/glass composite film in reflective configuration: A thermally robust and efficient red-emitting color converter with high saturation threshold for high-power high color rendering laser lighting

To achieve high color rendering and proper color temperature, a red color converter is essential for phosphor-converted white lighting devices. CaAlSiN3:Eu2+ (CASN) is a highly suitable red phosphor for white light-emitting diodes. However, it can be hardly used in high-power laser lighting due to poor thermal/chemical performance of the phosphor/silicone resin mixture. A series of all-inorganic CASN-based phosphors (e.g., composite ceramic and phosphor-in-glass) were developed to avoid the use of resin. However, new challenges emerged: none of them showed sufficient luminous efficacy (i.e., >50 lm/W) and adequate saturation-threshold (i.e., >30 W or 10 W/mm2). Here, we report a facile fabrication of CASN/glass composite films using a simple and efficient blade-coating method. Upon 450 nm excitation, the resultant composite film presents a high internal quantum efficiency of ~83%, comparable to that of pristine CASN powder (~90%). When irradiated with a blue laser, the composite film shows a record high luminous efficacy of 82 lm/W. Furthermore, its saturation threshold was investigated in high power and high power density mode, respectively. When measured in high power mode, it shows a high saturation threshold over 29.7 W (1.75 W/mm2), thus achieving a high luminous flux of 1576 lm; when measured in high power density mode, it shows a saturation threshold of ~10.2 W/mm2 (1.13 W). With abovementioned excellent properties, the CASN/glass composite film has great potential for use in high-power and high color rendering laser lighting.

Spatial Dependence of the Dipolar Interaction between Quantum Dots and Organic Molecules Probed by Two-Color Sum-Frequency Generation Spectroscopy

Given the tunability of their optical properties over the UV–Visible–Near IR spectral range, ligand-capped quantum dots (QDs) are employed for the design of optical biosensors with low detection threshold. Thanks to non-linear optical spectroscopies, the absorption properties of QDs are indeed used to selectively enhance the local vibrational response of molecules located in their vicinity. Previous studies led to assume the existence of a vibroelectronic QD–molecule coupling based on dipolar interaction. However, no systematic study on the strength of this coupling has been performed to date. In order to address this issue, we use non-linear optical Two-Color Sum-Frequency Generation (2C-SFG) spectroscopy to probe thick QD layers deposited on calcium fluoride (CaF2) prisms previously functionalized by a self-assembled monolayer of phenyltriethoxysilane (PhTES) molecules. Here, 2C-SFG is performed in Attenuated Total Reflection (ATR) configuration. By comparing the molecular vibrational enhancement measured for QD–ligand coupling and QD–PhTES coupling, we show that the spatial dependence of the QD–molecule interactions (∼1/r3, with r the QD–molecule distance) is in agreement with the hypothesis of a dipole–dipole interaction.

Mechanism of hysteresis in shock wave reflection.

This paper reports on the mechanism of the hysteresis in the transition between regular and Mach shock wave reflections. We disclose that, for a given inflow Mach number, a stable reflection configuration should maintain the minimal dissipation. As the wedge angle varies, the set of the minimal dissipation points forms the valley lines in the dissipation landscape, and these valley lines compose the hysteresis loop. The saddle-nodes, intersections of the ridge line, and the valley lines are actually the transition points. Additionally, the predicted reflection configurations agree well with the experimental and numerical results, validating this theory.

Design and Analysis of Aluminum-Silicon-Graphene Based Plasmonic Device for Biosensing Applications in the Optical Communication Band

This work utilizes the modified Attenuated Total Reflection (ATR) configuration, to detect minute refractive index changes near the sensing surface. In the proposed ATR configuration, the presence of the graphene layer increases the interaction with bio-analyte by adsorbing the biomolecules and the presence of a thin silicon layer helps to enhance the sensitivity of the proposed device. The use of aluminum as the plasmonic metal serves an economical value as well as compatibility with the optoelectronic devices. All the geometrical parameters of the layers over the base index prism are engineered for maximum sensitivity and narrow linewidth in the optical communication band using the transfer matrix method. The stacking of silicon-graphene layers over the thin metal-coated glass prism leads to the maximum sensitivity of 200°/RIU and figure of merit of 95.23 RIU−1 at the wavelength of 1550 nm. To demonstrate the proposed device as a bio-sensor, rodent urine is considered as the analyte under test to detect the changes in the varying concentration of Leptospira bacterium. The proposed plasmonic device opens a new window for the detection of biomolecular interactions in the optical communication band.

Magnetization dynamics in ordered spin structures revealed by diffractive and reflectometry ferromagnetic resonance

Synchrotron radiation based techniques provide unique insight into both the element and time resolved magnetization behavior in magnetic spin systems. Here, we highlight the power of two recent developments, utilizing x-ray scattering techniques to reveal the precessional magnetization dynamics of ordered spin structures in the GHz regime, both in diffraction and reflection configurations. Our recently developed diffraction and reflectometry ferromagnetic resonance (DFMR and RFMR) techniques provide novel ways to explore the dynamics of modern magnetic materials, thereby opening up new pathways for the development of spintronic devices. In this paper we provide an overview of these techniques, and discuss the new understanding they provide into the magnetization dynamics in the chiral magnetic structure in Y-type hexaferrite and the depth dependence to the magnetization dynamics in a [CoFeB/MgO/Ta]4 multilayer.

Resolving Structural Changes of Photoreceptors in Living Escherichia coli via In-cell Infrared Difference Spectroscopy.

Several in-cell spectroscopic techniques have been developed recently to investigate the structure and mechanism of proteins in their native environment. Conditions in vivo differ dramatically from those selected for in vitro experiments. Accordingly, the cellular environment can affect the protein mechanism for example by molecular crowding or binding of small molecules. Fourier transform infrared (FTIR) difference spectroscopy is a well-suited method to study the light-induced structural responses of photoreceptors including changes in cofactor, side chains and secondary structure. Here, we describe a protocol to study the response of cofactor and protein in living E. coli cells via in-cell infrared difference (ICIRD) spectroscopy using the attenuated total reflection (ATR) configuration. Proteins are overexpressed in E. coli, the cells are transferred into saline solution and the copy number per cell is determined using fluorescence spectroscopy. The suspension is centrifuged and the concentrated cells transferred onto the ATR cell inside the FTIR spectrometer. The thermostatted cell is sealed and illuminated from the top with an LED. Intensity spectra are recorded before and after illumination to generate the difference spectrum of the receptor inside the living cell. With ICIRD spectroscopy, structural changes of soluble photoreceptors are resolved in a near-native environment. The approach works in H2O at ambient conditions, is label free, without any limitations in protein size and does not require any purification step. Graphic abstract: In-cell infrared difference spectroscopy on photoreceptors in living E. coli using attenuated total reflection.

Double-walled carbon nanotube film as the active electrode in an electro-optical modulator for the mid-infrared and terahertz regions

The lack of efficient optical components operating with terahertz (THz) radiation is a limiting step in the ongoing large-scale development of this technology in fields such as telecommunication and imaging. In this work, we propose the use of double-walled carbon nanotube (DWCNT) films as the active electrode in THz modulation devices. Using six bounces in an internal total reflection configuration in a silicon waveguide prism, we achieved high attenuation from a 5 nm thin film, reaching up to −ΔT/T=6% at 50 THz, albeit with a slow speed of modulation on the order of minutes. Moreover, this attenuation −ΔT/T attains a value of 20% at 30 THz using a thicker 20 nm DWCNT film. As a consequence of doping, the modulation of a phonon-related Fano resonance is also observed in the mid-infrared, which could be used as a modulable narrow-band optoelectronic filter. Our study provides a sense of the capabilities unlocked by exploiting the optical and electronic properties of carbon nanotubes in the terahertz and infrared regimes.

Visualization of White Matter Fiber Tracts of Brain Tissue Sections With Wide-Field Imaging Mueller Polarimetry.

Identification of white matter fiber tracts of the brain is crucial for delineating the tumor border during neurosurgery. A custom-built Mueller polarimeter was used in reflection configuration for the wide-field imaging of thick sections of fixed human brain and fresh calf brain. The maps of the azimuth of the fast optical axis of linear birefringent medium reconstructed from the experimental Mueller matrix images of the specimen by applying a non-linear data compression algorithm showed a strong correlation with the silver-stained sample histology image, which is the gold standard for ex-vivo brain fiber tract visualization. The polarimetric maps of fresh calf brain tissue demonstrated the same trends in the depolarization, the scalar retardance and the azimuth of the fast optical axis as seen in fixed human brain tissue. Thus, label-free imaging Mueller polarimetry shows promise as an efficient intra-operative modality for the visualization of healthy brain white matter fiber tracts, which could improve the accuracy of tumor border detection and, ultimately, patient outcomes.

Modulation of spontaneous emission near graphene/hBN multilayers

The spontaneous emissions of a quantum emitter near three different samples, namely, graphene-covered hexagonal boron nitride (hBN) bulk (monocell), graphene/hBN/graphene sandwich (sandwich), and graphene/hBN/graphene/hBN (double-cell), are investigated in detail. The spontaneous emission decay rate near the graphene/hBN samples can be increased by several orders of magnitude because of the hyperbolic characteristics of hBN. For the monocell, two sharp peaks appear in the Purcell factor curve, corresponding to the hyperbolic frequency ranges of hBN. The Purcell factor can be actively controlled by the chemical potential of graphene via the coupling of surface plasmons (SPs) supported by graphene and hyperbolic phonon polaritons (HPPs) supported by hBN. As the chemical potential increases, the Purcell factor decreases when the distance between the quantum emitter and the sample is less than 1/1000 transition wavelength. Conversely, the Purcell factor increases when the distance ranges from 1/1000 to 1/100 transition wavelength. The interaction between two quantum emitters in the proximity of different samples in reflective configuration is investigated, and the interaction exhibits an oscillation between superradiant and subradiant states in accordance with the separation between the two emitters. The interaction between quantum emitters can also be controlled freely with the chemical potential of graphene. This work provides a meaningful basis for modulating the spontaneous emission, and could be valuable in expanding the application of new and emerging materials in the field of light–matter interaction.

The Remarkably Strong Arctic Stratospheric Polar Vortex of Winter 2020: Links to Record‐Breaking Arctic Oscillation and Ozone Loss

AbstractThe Northern Hemisphere (NH) polar winter stratosphere of 2019/2020 featured an exceptionally strong and cold stratospheric polar vortex. Wave activity from the troposphere during December–February was unusually low, which allowed the polar vortex to remain relatively undisturbed. Several transient wave pulses nonetheless served to help create a reflective configuration of the stratospheric circulation by disturbing the vortex in the upper stratosphere. Subsequently, multiple downward wave coupling events took place, which aided in dynamically cooling and strengthening the polar vortex. The persistent strength of the stratospheric polar vortex was accompanied by an unprecedentedly positive phase of the Arctic Oscillation in the troposphere during January–March, which was consistent with large portions of observed surface temperature and precipitation anomalies during the season. Similarly, conditions within the strong polar vortex were ripe for allowing substantial ozone loss: The undisturbed vortex was a strong transport barrier, and temperatures were low enough to form polar stratospheric clouds for over 4 months into late March. Total column ozone amounts in the NH polar cap decreased and were the lowest ever observed in the February–April period. The unique confluence of conditions and multiple broken records makes the 2019/2020 winter and early spring a particularly extreme example of two‐way coupling between the troposphere and stratosphere.

Monitoring of the Critical Meniscus of Very Low Liquid Volumes Using an Optical Fiber Sensor

A novel sensing system based on single mode optical fiber in reflective configuration has been developed to measure the critical meniscus height (CMH) of low volumes of liquids, which is then used to calculate the contact angle. The sensing system has been designed especially for very low volumes of liquids (e.g. bio-liquids) and the work has demonstrated that measurements are possible with a minimum liquid volume of 5 μL . The sensing system is based on monitoring the spectral variation induced by the difference in the refractive index regions surrounding the fiber tip, at the air-liquid or liquid-liquid interfaces. From the experiments performed in water, (by immersing and extracting the fiber sensor in the liquid sample), it can be concluded that the CMH forming on the fiber decreases as the temperature increases. The change of temperature (in this experiment from 22 to 60 °) does not influence the CMH of the sample used in the evaluation (P3 mineral oil), giving an indication of its thermal stability. In addition, a fixed fiber was used to measure the variation in the liquid level when another fiber is immersed in the liquid. The error in the liquid level obtained in the work was small, at 0.34 ± 0.04 %. Such a sensor, allowing accurate measurements with very small quantities is especially useful where liquid sample volumes are limited e.g. biologically sourced liquids or specialized, expensive industrial material in the liquid phase.

Terahertz time-domain spectroscopy characterization of nitrocellulose in transmission and reflection configurations

In this work, we report the first terahertz characterization of nitrocellulose, a workhorse energetic material widely employed in many civilian and military applications, especially as a propellant or explosive ingredient. The aim behind this study is to find the main spectral features and to draw the absorption and refractive index profiles of nitrocellulose using a terahertz time-domain spectroscopy setup over the frequency range from 0.3 to 3 THz. We particularly highlight the interest of carrying out combined measurements in transmission and in reflection configurations in order to increase the identification capabilities using the terahertz spectroscopy. Thus, five spectral absorption lines have been clearly observed in transmission mode, namely 0.41, 0.69, 1.01, 2.15 and 2.76 THz. These spectral resonances constitute fingerprints of nitrocellulose. When spreading a few particles of NC on an adhesive ribbon and performing characterization in reflection, the lines at 0.69 and 2.15 THz are still easily observed. These spectral fingerprints allow a simple, easy and highly sensitive remote detection and identification of nitrocellulose even if it is concealed behind barrier materials like papers, textiles and so forth.

Photothermal Porosity Estimation in CFRP by the Time-of-Flight of Virtual Waves

Porosity is an unavoidable defect in carbon fiber reinforced polymers and has noticeable effects on mechanical properties since gas filled voids weaken the epoxy matrix. Pulsed thermography is advantageous because it is a non-contacting, non-destructive and fast photothermal testing method that allows the estimation of material parameters. Using the Virtual Wave Concept for thermography data, ultrasonic evaluation methods are applicable. In this work, the pulse-echo method for Time-of-Flight measurements is used, whereby the determined Time-of-Flight is directly related to the thermal diffusion time of the examined material. We introduce a signal-to-noise dependent approach, the optimum evaluation time, for evaluating only relevant time ranges which contain information of heat diffusion. After the validation of the method for heterogeneous materials, effective medium theories can be used for quantitative porosity estimation from the estimated diffusion time. This model-based approach for porosity estimation delivers more accurate results for transmission and reflection configuration measurements compared to thermographic state-of-the-art methods. The results are validated by X-ray computed tomography reference measurements on a wide range of different porous carbon fiber reinforced plastic specimens with different number of plies and varying porosity contents.

Multiple Fano Resonances in Multilayer Thin Film-Coupled Attenuated Total Reflection Configuration

Multiple Fano resonances (FRs) and plasmon-induced transparency (PIT) are observed and analyzed in one-dimensional photonic crystals (1D-PhCs) coupled surface plasmon polariton (SPP) structure under the attenuated total reflection (ATR) configuration. Coupling between the excited guided modes of 1D-PhCs and SPP mode leads to Fano type resonances which are confirmed by numerical calculation. Magnetic field distributions at the resonances are also presented to verify the existence of multiple FRs. It is demonstrated that the position and the linewidth of the FRs can be engineered by the structural parameters. Moreover, the position of FRs can be obtained at any desired wavelengths in the near-infrared (NIR) range. These results pave a new way for the development of multiband reflectors working in NIR based on FR.

Similarity in Mach stem evolution and termination in unsteady shock-wave reflection

Abstract