Reflection Geometry Research Articles

Daylighting of road tunnels through external ground-based light-pipes and complex reflective geometry

The transfer of solar light to the interior of tunnels to complement electrical lighting during daytime, contributes to save energy and decrease the number of installed projectors. The solutions traditionally implemented, based on light-pipes hanging from the vault, seemed reasonable because the leakages of light-pipes can emit luminous flux like projectors, but had the disadvantage of needing higher clearance gauges with the consequent costs in drilling, building materials, works and maintenance. Furthermore, these hanging light-pipes cannot be installed in working tunnels if they were not foreseen in the initial project. In this article, a new concept of light injection and distribution in tunnels is proposed. It consists of the coupling of three main elements: collectors, light-pipes, and one reflecting vault whose design is optimized to ensure the photometric requirements on the pavement. The proposed system can collect more than 1.5 Mlm of solar flux with not excessively large collectors installed on the ground of the road shoulder, and project them to the vault that finally distributes the light towards the pavement. With this flux, average illuminances of 3478 lx and uniformities of 0.73 can be achieved, which means pavement luminance around 350–400 cd/m2, in good agreement with the requirements during daytime. The savings can be higher than 40 %. Besides these savings, this system can be easily implemented in existing tunnels. The proposal and some estimations are discussed.

Probing the bulk plasmon continuum of layered materials through electron energy loss spectroscopy in a reflection geometry

A periodic arrangement of 2D conducting planes is known to host a (bulk) plasmon dispersion that interpolates between the typical, gapped behavior of 3D metals and a gapless, acoustic regime as a function of the out-of-plane wavevector. The semi-infinite system -- the configuration relevant to Electron Energy Loss Spectroscopy (EELS) in a reflection geometry, as in High Resolution EELS (HREELS) -- is known to host a surface plasmon that ceases to propagate below a cutoff wavevector. As the f-sum rule requires a finite response whether or not there exist sharp excitations, we demonstrate that what remains in the surface loss function -- the material response probed by HREELS -- is the contribution from the (bulk) plasmon of the infinite system. In particular, we provide a one-to-one mapping between the plasmon continuum and the spectral weight in the surface loss function. In light of this result, we suggest that HREELS be considered a long wavelength probe of the plasmon continuum in layered materials.

Chiral surface spin textures in Cu2OSeO3 unveiled by soft X-ray scattering in specular reflection geometry

ABSTRACT Resonant elastic soft X-ray magnetic scattering (XRMS) is a powerful tool to explore long-periodic spin textures in single crystals. However, due to the limited momentum transfer range imposed by long wavelengths of photons in the soft x-ray region, Bragg diffraction is restricted to crystals with the large lattice parameters. Alternatively, small-angle X-ray scattering has been involved in the soft energy X-ray range which, however, brings in difficulties with the sample preparation that involves focused ion beam milling to thin down the crystal to below a few hundred nm thickness. We show how to circumvent these restrictions using XRMS in specular reflection from a sub-nanometer smooth crystal surface. The method allows observing diffraction peaks from the helical and conical spin modulations at the surface of a Cu 2 OSeO 3 single crystal and probing their corresponding chirality as contributions to the dichroic scattered intensity. The results suggest a promising way to carry out XRMS studies on a plethora of noncentrosymmetric systems hitherto unexplored with soft X-rays due to the absence of the commensurate Bragg peaks in the available momentum transfer range.

Justification of the Photoplethysmography Sensor Configuration by Monte Carlo Modeling of the Pulse Waveform

Photoplethysmography (PPG) is an optical technique for detection of blood volume changes in the microvascular bed of a biological tissue. Many aspects of the PPG signal formation are still unclear. In particular, it is not known how the shape of a registered PPG signal depends on the geometry of tissue illumination. The aim of this study is to model the PPG waveform using the Monte Carlo (MC) method. For this, we developed a three-layer optical model of the skin in a reflectance geometry and verified it experimentally for different wavelengths (660, 810, and 940 nm) and source-detector distances (from 4 to 10 mm). The MC simulation results showed that the PPG waveform depends on the source-detector distance. The most pronounced diastolic wave is observed at the distance of 6 mm for the wavelength of 810 nm. The results obtained can be used for the development of reflectance PPG sensors.

A straightforward optical alignment protocol for STM-based single molecule spectroscopy

A light-accessed scanning tunneling microscope (STM) is a powerful spectroscopic tool that enables chemical analysis at the single molecular level, but it requires highly precise optical alignments to pinpoint the nano-scale tunneling gap, leaving experimental challenges. Here we present straightforward procedures to align the optical setup for STM-luminescence and STM-based tip-enhanced Raman spectroscopy (TERS) performed with a reflection geometry in an ultrahigh vacuum chamber. Observing real-space images of the metal tip apex through a spectrograph set to the zeroth-order diffraction enables “in situ” optimization of the detection path and introduction of the excitation light of TERS to the nanogap. The best spatial overlap with the nanogap can be achieved by monitoring plasmon-enhanced, low-frequency inelastic scattering of the metal. This protocol allows us to overcome such difficulties in STM-based spectroscopy and facilitates physicochemical study of single adsorbates on nontransparent substrates.

Development and performance comparison of a modified glazed CPC hybrid solar collector coupled with a bifacial PVT receiver

Innovative concentrating PVT solar collector concepts based on a CPC geometry concept were developed to outperform the asymmetric Solarus CPVT collector and therefore decrease the energy/performance gap between CPVT and PV/ST solar collectors.The updated reflector geometry proved to be the most suitable reflector geometry for CPVTs, where the electrical peak efficiency per gross area reached 10.6%, which is +16.5%rel higher than the electrical peak efficiency of the Solarus CPVT. Optical efficiencies of η0 = 62.3% and η0 = 61.8% for CPC 1 and CPC 2 have been achieved, respectively.A PV module (0.5 m2) combined with an ST solar collector (0.5 m2) system to be able to deliver the same overall energy yield as the newly developed CPVT collector (1 m2) requires on average +0.02 m2 (at 45 °C), −0.06 m2 (at 55 °C) and −0.15 m2 (at 65 °C) of installed area, for a wide range of latitudes.A CPC-PVT system to increase its competitiveness requires a material cost reduction and at the same time an increased overall efficiency. Nevertheless, the energy/performance gap between a system composed of PV + ST technologies and a CPC-PVT decreased significantly.

Exploiting total internal reflection geometry for deep broadband terahertz modulation using a GaAs Schottky diode with integrated subwavelength metal microslits.

We developed a GaAs Schottky diode with integrated periodic subwavelength metal microslits with total internal reflection (TIR) geometry to achieve deep broadband THz modulation at high frequency with low insertion loss. The non-resonant electric field enhancement effect in the subwavelength microslits intensifies the evanescent wave in TIR, which increases broadband absorbance of THz light signals by free carriers in the GaAs Schottky diode. Devices with various microslit spatial periods and gap widths were fabricated and measured. Among the devices, that with a microslit period of 10 µm and gap width of 2 µm produced ∼70% modulation depth at frequencies of 0.2 to 1.2 THz, while in the range of 0.25 to 0.4 THz, ∼90% modulation depth was achieved. By encapsulating the device in high refractive index material, ∼100% modulation depth was achieved in the range of 0.4 to 0.6 THz, the 3 dB bandwidth operational frequency was ∼160 kHz, and the insertion loss introduced by the device was less than 8 dB, which is much lower than existing metasurface-based THz modulators. In general, our first-generation device has improved modulation depth, operational bandwidth, insertion loss, and operational frequency. Optimization of the metal microslits, TIR geometry, and doped layer could further improve the performance of our design.

Encoder-decoder deep learning network for simultaneous reconstruction of fluorescence yield and lifetime distributions.

A time-domain fluorescence molecular tomography in reflective geometry (TD-rFMT) has been proposed to circumvent the penetration limit and reconstruct fluorescence distribution within a 2.5-cm depth regardless of the object size. In this paper, an end-to-end encoder-decoder network is proposed to further enhance the reconstruction performance of TD-rFMT. The network reconstructs both the fluorescence yield and lifetime distributions directly from the time-resolved fluorescent signals. According to the properties of TD-rFMT, proper noise was added to the simulation training data and a customized loss function was adopted for self-supervised and supervised joint training. Simulations and phantom experiments demonstrate that the proposed network can significantly improve the spatial resolution, positioning accuracy, and accuracy of lifetime values.

Anthropomorphic Polydimethylsiloxane silicone-based phantom for Diffuse Optical Imaging

This work presents a method for constructing phantoms suitable for diffuse optical mammography. They are based on Polydimethylsiloxane silicones, with the characteristic of being anthropomorphic, and having similar mechanical and optical properties as a real breast. These phantoms are useful for testing the performance of diffuse optical imaging devices in the near infrared, both in transmittance and reflectance geometries, since they can be constructed containing inclusions, to simulate breast tumors. An alternative component to be used as scattering agent, that is easier to handle than traditional scattering agents, is also studied.The optical properties of the phantoms were tested varying the concentration of scattering and absorbing agents, while their mechanical properties were modified by adding a silicone fluid to the basic mixture.Finally, the phantoms were tested by Diffuse Optical Imaging experiments, and these images were compared to the ones obtained by conventional ultrasound techniques.Results show that the constructed anthropomorphic phantoms properly reproduce the optical and mechanical characteristics of human breasts, and are suitable to be used in Diffuse Optical Imaging.

Mineralogical analysis of Brazilian Portland cements by the Rietveld method with emphasis on polymorphs M1 and M3 of alite

Eight samples of Portland cement and a clinker provided by the Brazilian Association of Portland Cement were analysed with different laboratory diffractometers and a synchrotron instrument to determine the statistical variability in the determination of the mass percentage of the main crystalline phases. Five laboratories participated in the experiment. Data collection was performed by each laboratory following its own internal procedures for a standard Rietveld analysis of mineral phases. Both Cu and Mo radiations were used. Reflection geometries—with and without sample rotation—and transmission geometries were also used. The synchrotron diffraction pattern was acquired from a rotating capillary and a wavelength of 0.41290Å. Analysis of all diffraction patterns was performed with the help of TOPAS Academic v. 6 with the specific purpose of determining the proportions of polymorphs M1 and M3 of alite, since their ratio must be taken into account for the subsequent development of the mechanical properties of concrete.

Optical design optimization for improved lamp-reflector units in high-flux solar simulators.

High flux solar simulators are artificial solar facilities developed to imitate the on-sun operations of concentrating solar power technologies but under a well-controlled lab-scale environment. We report the optical enhancement of different high flux solar simulators for solar thermal and thermochemical applications. The solar simulator enhancement is numerically conducted by optimizing the geometry of ellipsoidal reflectors at focal lengths of 1600, 1800, and 2000 mm. The Monte Carlo ray-tracing technique is employed to evaluate the optical performance of different reflector designs. The typical seven-lamp solar simulator arrangement in hexagonal configuration is modeled to analyze the optical performance at different focal lengths. In addition, different xenon arc lamps are modeled with rated powers of 3000, 4000, 4500, and 5000 W for assessing the radiative flux characteristics of the proposed solar simulators. After the optimization, theoretical results show that peak fluxes and radiative powers of 7.2-14.3MW/m2 and 5.06-10.4 kW, respectively, can be achieved with the proposed designs of solar simulators for the different rated powers. Compared with a commercial reflector, theoretical peak flux and power can be improved up to 36% and 17.9%, respectively, with the proper combination of lamp-reflector units. We provide design alternatives to select a more suitable light source at low-rated powers (≤5000W) and different focal lengths of the reflector, which simplifies the complexity of the design and improves the performance of solar simulators.

Ultrafast Carrier-Coupled Interlayer Contraction, Coherent Intralayer Motions, and Phonon Thermalization Dynamics of Black Phosphorus.

Black phosphorus (bP) exhibits highly anisotropic properties and dynamical behavior that are unique even among two-dimensional and van der Waals (vdW) layered materials. Here, we show that an interlayer lattice contraction and concerted, symmetric intralayer vibrations occur concurrently within few picoseconds following the photoinjection and relaxation of carriers, using ultrafast electron diffraction in the reflection geometry to probe the out-of-plane motions. A strong coupling between the photocarriers and bP's puckered structure, with the alignment of the electronic band structure, is at work for such directional atomic motions without a photoinduced phase transition. Three temporal regimes can be identified for the phonon thermalization dynamics where a quasi-equilibrium without anisotropy is reached in about 50 ps, followed by propagation of coherent acoustic phonons and heat diffusion into the bulk. The early time out-of-plane dynamics reported here have important implications for single- and few-layer bP and other vdW materials with strong electronic-lattice correlations.

Exploring the interrelated effects of soil background, canopy structure and sun-observer geometry on canopy photochemical reflectance index

Photochemical reflectance index (PRI) is an intriguing avenue for monitoring photosynthetic light use efficiency (LUE), which is a crucial physiological variable and the primary source of uncertainty in gross primary production (GPP) prediction. The correlations between PRI and LUE induced by the energy-dependent xanthophyll cycle are overall convincing at the leaf scale. However, their relationship becomes complicated and less reliable at the canopy scale due to several confounding factors, such as soil background, canopy structure and sun-observer geometry. A fundamental understanding of the interrelated effects of these factors is still missing. In this study, the roles of soil reflectance, canopy structure and sun-observer geometry were examined by investigating radiative transfer processes that determine canopy PRI. Theoretical analysis shows that the deviation between canopy- and leaf-level PRI is mainly attributed to soil background, namely the fraction of sunlit soil in the scene and the discrepancy between leaf and soil reflectance. Canopy structure and sun-observer geometry are not the direct drivers of variations in canopy PRI. They affect canopy PRI by altering the contribution of soil to canopy reflectance. Furthermore, several numerical experiments based on 1D and 3D radiative transfer models, which compare PRI of canopies with black soil (i.e., soil reflectance is zero) and non-black background, were conducted to confirm and validate the decisive role of soil background in explaining the deviation between canopy and leaf PRI. Multi-angular canopy PRI from PROBA-CHRIS data was employed to examine the angular dependency of canopy PRI changing with vegetation coverage. The numerical experiments demonstrate that canpy PRI differs from leaf PRI as large as 150% when canopy leaf area index (LAI) or the fraction of vegetation coverage is small. Canopy PRI is almost identical to soil PRI (i.e., PRI computed from soil reflectance) when the contribution of soil background to canopy reflectance is large. In contrast, when the soil reflectance is zero or LAI is greater than four, canopy PRI is almost identical to leaf PRI and independent of canopy structure and sun-observer geometry. Nonetheless, when leaf and soil reflectance are similar, canopy PRI is always close to leaf PRI regardless canopy structure and sun-observer geometry. The analysis of multi-angular PRI reveals that the angular dependency decreases with vegetation coverage due to weaker soil background effect. This study narrows down the causes of variation in canopy PRI to the fraction of observed soil background and soil reflectance. It suggests that estimating and accounting for the soil contribution to canopy PRI are necessary to mitigate angular and canopy structural effects on canopy PRI and eventually extract leaf physiological information from canopy PRI.

A Comparative Feasibility Study for Transcranial Extracorporeal Shock Wave Therapy.

The potential beneficial regenerative and stimulatory extracorporeal shock wave therapy (ESWT) applications to the central nervous system have garnered interest in recent years. Treatment zones for these indications are acoustically shielded by bones, which heavily impact generated sound fields. We present the results of high-resolution tissue-realistic simulations, comparing the viability of different ESWT applicators in their use for transcranial applications. The performances of electrohydraulic, electromagnetic, and piezoelectric transducers for key reflector geometries are compared. Based on density information obtained from CT imaging of the head, we utilized the non-linear wave propagation toolset Matlab k-Wave to obtain spatial therapeutic sound field geometries and waveforms. In order to understand the reliability of results on the appropriate modeling of the skull, three different bone attenuation models were compared. We find that all currently clinically ESWT applicator technologies show significant retention of peak pressures and energies past the bone barrier. Electromagnetic transducers maintain a significantly higher energy flux density compared to other technologies while low focusing strength piezoelectric applicators have the weakest transmissions. Attenuation estimates provide insights into sound field degradation and energy losses, indicating that effective transcranial therapies can readily be attained with current applicators. Furthermore, the presented approach will allow for future targeted in silico development and the design of applicators and therapy plans to ultimately improve therapeutic outcomes.

X-ray third-order nonlinear diffraction in the asymmetric reflection geometry.

X-ray third-order nonlinear asymmetrical diffraction has three independent parameters: the asymmetry angle, the incident wave intensity and the deviation from the exact Bragg orientation. In contrast to the linear case, in the nonlinear case the total reflection region does not exist for all intensity values and asymmetry angles. Theoretical consideration leads to analytical conditions of the total reflection region, and the analysis can be carried out by a graphical method. An exact solution in the total reflection region is found. The numerical solutions of the third-order nonlinear diffraction allow one to find the reflection curves for a fixed asymmetry angle or for a fixed intensity. For very large or very small asymmetry factors the third-order nonlinear effects can be observed for beams with very low intensities.

Radar Cross Section Prediction Using Iterative Physical Optics With Physical Theory of Diffraction

A new approach is presented to predict radar cross section (RCS) of electrically large conducting geometries using iterative physical optics (IPO) in conjunction with physical theory of diffraction (PTD). IPO is based on iterative refinement of physical optics (PO) currents to include multiple reflections. However, IPO does not yield sufficient accuracy in diffraction dominant regions since it does not properly include effects of edge diffractions. Hence, the proposed approach includes diffraction effects using PTD, which introduces edge currents. Diffracted fields due to these edge currents are radiated on triangular facets to modify the PO currents before IPO iterations. Scattered fields are computed from converged IPO surface currents and PTD edge currents. A new set of PTD edge currents excited by the fields due to the IPO currents are also found and the fields radiated by these PTD edge currents are added to the scattered fields. Therefore, interactions between the surface and edge currents are included. Computations are accelerated by parallel processing using message passing interface (MPI). Monostatic and bistatic RCS are predicted using the proposed approach for tail-wing and trihedral corner reflector geometries. The IPO-PTD results are compared with method of moments (MoM) and multilevel fast multipole method results obtained using field calculations involving bodies of arbitrary shape (FEKO) commercial electromagnetic simulation software for validation.

Integrating rock physics and sequence stratigraphy for characterization of deep-offshore turbidite sand system

Definition and understanding deep water depositional environment is essential, especially in exploration and development phase. This study is aimed at improving characterization and environment of deposition assessment of deepwater turbidite sands systems as a way of evaluating reservoir geometry and quality in “NOJA” field, deep-offshore Niger Delta. The data used for the analysis comprise 3D full angle stack seismic, biofacies and logs from five wells. Seismic sequence analysis using reflection termination patterns were used for the mapping of depositional sequences. The sequences have internal reflection geometries ranging from parallel, subparallel and chaotic. Seismic structural maps gave insight into rock deformation and hydrocarbon potential of the field. The probable structure responsible for the trapping of oil and gas was a faulted anticlinal structure. Amplitude maps of the two stratigraphic surfaces gave insight into sand and shale distributions and possible environment of sediment deposition. Rock physics concepts were utilised to evaluate reservoir continuity, diagenetic and environment of deposition effects influencing reservoir properties. Two reservoirs (R1 and R2) of interest occurred at a depth interval of 2398–2461 m. Reservoir (R1) has average volume of shale of 22%, effective porosity 20%, hydrocarbon saturation 53%, and permeability 556 mD. Reservoir (R2) has average volume of shale of 10%, effective porosity 26%, hydrocarbon saturation 79%, and permeability 1044 mD. Environment of sediment deposition of the reservoirs was weakly confined with distributary channel complexes. Reservoir (R1) had low degree of connectivity (loosely amalgamated turbidite sand system) and reservoir (R2) possessed high degree of connectivity (highly amalgamated turbidite sand system). Rock physics templates of constant, contact and friable showed that the reservoir sands were poorly cemented to unconsolidated. These models indicated that the reservoir sands were both influenced by depositional and depth related diagenetic effects.

Average power scaling of THz spintronic emitters efficiently cooled in reflection geometry.

Metallic spintronic terahertz (THz) emitters have become well-established for offering ultra-broadband, gapless THz emission in a variety of excitation regimes, in combination with reliable fabrication and excellent scalability. However, so far, their potential for high-average-power excitation to reach strong THz fields at high repetition rates has not been thoroughly investigated. In this article, we explore the power scaling behavior of tri-layer spintronic emitters using an Yb-fiber excitation source, delivering an average power of 18.5 W (7 W incident on the emitter after chopping) at 400 kHz repetition rate, temporally compressed to a pulse duration of 27fs. We confirm that a reflection geometry with back-side cooling is ideally suited for these emitters in the high-average-power excitation regime. In order to understand limiting mechanisms, we disentangle the effects on THz power generation by average power and pulse energy by varying the repetition rate of the laser. Our results show that the conversion efficiency is predominantly determined by the incident fluence in this high-average-power, high-repetition-rate excitation regime if the emitters are efficiently cooled. Using these findings, we optimize the conversion efficiency and reach highest excitation powers in the back-cooled reflection geometry. Our findings provide guidelines for scaling the power of THz radiation emitted by spintronic emitters to the milliwatt-level by using state-of-the-art femtosecond sources with multi-hundred-Watt average power to reach ultra-broadband, strong-field THz sources with high repetition rate.

Probing electron and hole colocalization by resonant four-wave mixing spectroscopy in the extreme ultraviolet.

Extending nonlinear spectroscopic techniques into the x-ray domain promises unique insight into photoexcited charge dynamics, which are of fundamental and applied interest. We report on the observation of a third-order nonlinear process in lithium fluoride (LiF) at a free-electron laser. Exploring the yield of four-wave mixing (FWM) in resonance with transitions to strongly localized core exciton states versus delocalized Bloch states, we find resonant FWM to be a sensitive probe for the degree of charge localization: Substantial sum- and difference-frequency generation is observed exclusively when in a one- or three-photon resonance with a LiF core exciton, with a dipole forbidden transition affecting details of the nonlinear response. Our reflective geometry–based approach to detect FWM signals enables the study of a wide variety of condensed matter sample systems, provides atomic selectivity via resonant transitions, and can be easily scaled to shorter wavelengths at free-electron x-ray lasers.

Terahertz transverse electric modes in graphene with DC current in hydrodynamic regime

The dispersion, excitation, and amplification of electromagnetic transverse electric (TE) modes at terahertz (THz) frequencies in graphene in the hydrodynamic (HD) regime, with a direct electric current flowing perpendicular to the TE mode wavevector, were theoretically investigated. The expression for the nonlocal HD conductivity of graphene with a direct electric current flowing perpendicular to the TE mode wavevector was derived. The direct electric current in graphene leads to the capacitive nature of the graphene HD conductivity at THz frequencies, which makes TE modes exist in this frequency range. The excitation of TE modes in graphene by an incident THz wave was modeled for the attenuated total reflection geometry. A new physical mechanism of TE mode amplification in graphene effective for a low value of carrier drift velocity was predicted. THz lasing regimes with TE modes in graphene structure with direct electric current were found. The results of this work can be used to create miniature technologically feasible sources and amplifiers of THz radiation.