Angle-dependent Reflectivity Research Articles

Design and fabrication of composite structures in ZnSe providing broadband mid-infrared anti-reflection

A comparative study of anti-reflection microstructures (ARMs) and ARMs with Al2O3 coatings (Al2O3/ARMs) on zinc selenide (ZnSe) has been carried out. The transmittance and electric field intensities were analyzed and presented using finite-difference time-domain (FDTD) simulations. ARMs were successfully fabricated on ZnSe surfaces by reactive ion etching and an Al2O3 layer was deposited onto the ARMs by an ion beam sputtering method. The theoretical analysis and experimental results showed that the transmittance of Al2O3/ARMs-treated ZnSe had a close relationship with the thickness of Al2O3 layer and was higher than that of ARMs-treated ZnSe in the 2–5 μm wavelength range, which was due to the fact that the Al2O3/ARMs had a better graded refractive index profile. The angle-dependent reflectance of ZnSe with Al2O3/ARMs was lower than that with ARMs. A difference of the spatial distribution of the electric field intensity between ARMs and Al2O3/ARMs was observed due to the surface Al2O3 coating, which predicted qualitative laser damage thresholds for both structures. The results of this study provide potential applications in high-efficiency IR optoelectronic devices and high power mid-infrared laser systems.

The accuracy of AVA approximations in isotropic media assessed via synthetic numerical experiments: Implications for the determination of porosity

Analyses of seismic amplitude vs. angle are widely used to estimate hydrocarbon reservoir properties. This paper investigates the accuracy of existing approximations based on the Zoeppritz equation, using synthetic numerical experiments that correlate P-wave reflectivity in isotropic media with reservoir porosity. An effective medium non-interacting approach (NIA) in rock physics modelling was used to compute the properties of fluid-saturated (water + gas) reservoir, which were then used in seismic modelling. In parallel, a Bayesian approach was used to estimate reservoir porosities from angle-dependent reflection coefficients and seismic amplitudes. A Maximum a posteriori solution of the Bayesian approach was also utilised to obtain an inverted porosity distribution in the reservoir model. The results of our forward models are important as they suggest that most of the approximations deviate from the exact Zoeppritz solutions with increasing angles of incidence of seismic waves. The results from the Bayesian inversion show that the Rüger and Bortfeld approximations agree with the exact Zoeppritz solutions to accurately estimate reservoir porosity. All the other approximations, except for Smith's, underestimate reservoir porosity and should be used in pre-stack inversion with caution. Smith's and Fatti's approximations failed to estimate reservoir porosity because of associated uncertainties.

Spectral characterization of silicon photonic crystal slab using out-of-plane light coupling arrangement

Fano resonance is analyzed by measuring the transmission and reflection from a photonic crystal slab containing a hexagonal symmetry of holes in silicon fabricated by electron beam lithography. The spectral features in the measured transmission and angle dependent reflection are in good agreement with those calculated using rigorous coupled wave analysis method (using DiffractMOD module of RSoft $$^{\mathrm {TM}}$$ ). This provides a convincing proof to this versatile and convenient out-of-plane measurement technique. In our photonic crystal, the quality factors measured at 1314, 1328, 1377, 1599, and 1630 nm are high and can be utilized for designing low-threshold micro-lasers and narrowband optical filters in the silicon slab platform.

Including and using internal multiples in closed-loop imaging — Field data examples

Nowadays, it is more widely accepted that multiple reflections should not be considered as noise, but as signal that can provide additional illumination of the subsurface. However, one of the challenges in seismic imaging is including all multiples in the migration process for field data in a reliable manner. Although including surface multiples in imaging has been demonstrated already on field data in recent years, the proper imaging of internal multiples is less established. We have determined successful field data applications on imaging that takes all internal multiples into account. This is done via so-called full-wavefield migration (FWM), an inversion-based method in which, given the migration velocity model, the angle-dependent reflectivity is iteratively estimated by minimizing the misfit between the modeled and the measured data. Its forward model is based on a multidimensional version of the so-called Bremmer series, which allows modeling of transmission effects and any type of multiple scattering in the subsurface and, thereby, is able to minimize the data misfit correctly. An application of FWM on deepwater field data from the Norwegian North Sea validates its capabilities to explain and image internal multiples. Furthermore, it is demonstrated on the same field data that the FWM framework can also be used for data interpolation and primary/multiple separation.

Study of soft x-ray optical properties of niobium carbide (NbC) thin film in 6–15 nm wavelength region

In the present study, the optical constants of niobium carbide (NbC) in soft x-ray wavelength region of 6–15 nm have been determined using angle dependent x-ray reflectivity measurements. Experimentally measured optical constants (δ and β) are compared with tabulated values of Henke et al. The δ values are found 5–30% lower than the bulk value whereas deviation in beta values is around 5–38%. To the best of our knowledge, the present study gives the first reported experimental values of optical constants for niobium carbide in 6–15 nm wavelength range. The knowledge of experimentally measured optical constants of NbC will help in practical calculation of optical devices.

Robust exciton–polariton Rabi splitting in graphene nano ribbons: the means of two-coupled semiconductor microcavities

Recent work on the exciton-photon coupling is presented. The proposed structure is a two-coupled semiconductor microcavity, composed of distributed Bragg reflectors, each consists of Si3N4 / SiO2, AlAs / Al0.1Ga0.9As, and GaAs/AlAs. Assuming that armchair graphene nanoribbon is located in the maximum of electric field amplitude inside the first semiconductor microcavity, the transfer matrix method is used to obtain the upper and lower polariton (UP and LP) branches and angle-dependent reflectance spectrum. A clear anticrossing between the neutral excitons and the cavity modes is observed for different polarization states. The obtained magnitude of splitting from the results is 10 to 12 meV, which indicates the possibility of enhancing the vacuum Rabi splitting for the proposed structure. This can pave the ways toward implementation of graphene in polaritonic devices.

Clarification of surface modes of a periodic nanopatch metasurface.

We study the angle-dependent optical reflectance spectrum of a metasurface consisting of a periodic array of film-coupled plasmonic nanopatch particles. The nanopatch metasurface exhibits a strong, angle-independent absorption resonance at a wavelength defined by the nanopatch geometry and relative density. When the nanopatches are arranged in a regular lattice, a second, sharp absorption dip is present that varies strongly as a function of the incidence angle. This second resonance is a collective effect involving the excitation of surface plasmon modes and relates to a Wood's anomaly. Using an analytical model, we compute the surface modes of the structure and confirm details about the various mechanisms that contribute to the reflection spectra. The measured reflectance spectra are in excellent agreement with both analytical calculations and full-wave numerical simulations.

Out-of-Plane Designed Soft Metasurface for Tunable Surface Plasmon Polariton.

Reliable and repeatable tunability gives functional diversity for reconfigurable plasmonics devices, while reversible and large mechanical deformation enabled by soft materials provides a new way for the global or partial regulation of metadevices. Here, we demonstrate a soft metasurface with an out-of-plane design for tuning the energy of surface plasmon polaritons (SPPs) bloch wave using theory, simulation, and experiments. Our metasurface is composed of two-layered gold nanoribbon arrays (2GNRs) on a soft substrate. The out-of-plane coupling mechanism is systematically analyzed in terms of separation height effect. Moreover, by harnessing mechanical deformation, continuously tunable plasmonic resonance has been achieved in the visible and near-infrared ranges. We further studied the angle-dependent reflection spectra of our metastructure. Compared with its planar counterpart, our soft and two-layered metastructure exhibits diverse tunability and significant field enhancement by out-of-plane interactions. Our approach in designing soft metasurface with out-of-plane structures can be extended to more-complex photonic devices and finds prominent applications such as biosensing, high-density plasmonic circuits, surface-enhanced luminescence, and surface-enhanced Raman scattering.

Charged Polaron Polaritons in an Organic Semiconductor Microcavity.

We report strong coupling between light and polaron optical excitations in a doped organic semiconductor microcavity at room temperature. Codepositing MoO_{3} and the hole transport material 4, 4^{'}-cyclohexylidenebis[N, N-bis(4-methylphenyl)benzenamine] introduces a large hole density with a narrow linewidth optical transition centered at 1.8eV and an absorption coefficient exceeding 10^{4} cm^{-1}. Coupling this transition to a Fabry-Pérot cavity mode yields upper and lower polaron polariton branches that are clearly resolved in angle-dependent reflectivity with a vacuum Rabi splitting ℏΩ_{R}>0.3 eV. This result establishes a path to electrically control polaritons in organic semiconductors and may lead to increased polariton-polariton Coulombic interactions that lower the threshold for nonlinear phenomena such as polariton condensation and lasing.

Experimental verification of spatially varying fracture-compliance estimates obtained from amplitude variation with offset inversion coupled with linear slip theory

The elastic compliance of a fracture can be spatially varying, reflecting the variation of microscale properties of the fracture, e.g., aperture, contact asperities, and fracture infill. Characterizing the spatial heterogeneity of a fracture is crucial in explaining the apparent frequency dependence of fracture compliance and in addressing the spatially varying mechanical and hydraulic properties of the fractured medium. Apparent frequency dependence of the estimated fracture compliance is caused when the used seismic wavelength is very large compared to the scale of heterogeneity. We perform ultrasonic laboratory experiments, and characterize the spatially varying compliance along a fluid-filled fracture. We simulate a horizontal fracture, and introduce heterogeneous fluid distribution along the fracture. We perform amplitude variation with offset (AVO) inversion of the P-P reflections, in which we obtain the theoretical angle-dependent reflection responses by considering the linear-slip model. The estimated compliance distribution clearly separates the dry region from the wet region of the fracture. The effective bulk modulus of the fluid is estimated using the derived values of the compliance. We find that the obtained bulk modulus is well-explained by the presence of minute quantity of air bubbles in the water. We also find new evidence of the existence of scattered waves generated at the boundary representing a sharp change in fracture compliance. The estimated boundary between the dry and the wet regions of the fracture, which is detected by AVO inversion, is slightly shifted compared with the actual location. This is possibly due to the interference of the scattered waves that are generated at the boundary. The linear-slip model can represent thin structures in rocks in a wide range of scale. Therefore, our methodology, results, and discussion will be useful in developing new applications for assessing laterally varying mechanical and hydraulic properties of thin nonwelded discontinuities, e.g., fractures, joints, and faults.

Formation of a conducting LaAlO3/SrTiO3 interface studied by low-energy electron reflection during growth

The two-dimensional electron gas occurring between the band insulators SrTiO$_3$ and LaAlO$_3$ continues to attract considerable interest, due to the possibility of dynamic control over the carrier density, and the ensuing phenomena such as magnetism and superconductivity. The formation of this conducting interface is sensitive to the growth conditions, but despite numerous investigations, there are still questions about the details of the physics involved. In particular, not much is known about the electronic structure of the growing LaAlO$_3$ layer at the growth temperature (around 800 $^o$C) in oxygen (pressure around $5\times 10^{-5}$ mbar), since analysis techniques at these conditions are not readily available. We developed a pulsed laser deposition system inside a low-energy electron microscope in order to study this issue. The setup allows for layer-by-layer growth control and in-situ measurements of the angle-dependent electron reflection intensity, which can be used as a fingerprint of the electronic structure of the surface layers during growth. By using different substrate terminations and growth conditions we observe two families of reflectivity maps, which we can connect either to samples with an AlO$_2$-rich surface and a conducting interface; or to samples with a LaO-rich surface and an insulating interface. Our observations emphasize that substrate termination and stoichiometry determine the electronic structure of the growing layer, and thereby the conductance of the interface.

Enhanced omnidirectional and weatherability of Cu2ZnSnSe4 solar cells with ZnO functional nanorod arrays

This paper presents the use of nanorods of different sizes, deposited from a chemical solution, as an antireflection layer in copper–zinc–tin selenide (CZTSe) solar cells. With the aid of the nanorods, the surface reflection of the CZTSe solar cells was reduced from 7.76% to 2.97%, and a cell efficiency of 14% was obtained as a result. Omni-directional anti-reflection was verified by the angle-dependent reflection measurements. The nanorod arrays also provided the CZTSe solar cells with a hydrophobic surface, allowing it to exhibit high resistance against humidity during weatherability tests. This shows that the surface passivation brought by the nanorod layer at the surface could effectively extend the lifetime of the CZTSe solar cells. The rate of efficiency decay of the CZTSe solar cells was reduced by 46.85% from that of the device without a nanorod array at the surface, indicating that this surface layer not only provided effective resistance against reflection at the device surface, but also served as a passivation layer and humidity-resistant surface-protection layer.

Design of ITO/SiO2/TiO2 distributed Bragg reflectors as a p-type electrode in GaN-based flip-chip light emitting diodes

Two different SiO2/TiO2 distributed Bragg reflectors (DBR) with ITO ohmic contacts were investigated as p-type reflective electrodes in InGaN/GaN flip-chip light-emitting diodes (FC-LEDs). The DBR structures were designed to have reflectance over 95% at wavelengths of 400–520nm (DBR 1) and 400–720nm (DBR 2). These ranges are wider than those of the electroluminescence spectrum of blue FC-LEDs. The FC-LEDs with DBR 1 showed an output power of 114mW at 200mA, while those with DBR 2 presented higher output power of 135mW. A ray tracing simulation showed that the anomalously low output power of the FC-LEDs with DBR 1 can be attributed to the low incidence angle to the DBR due to the diffraction of the light at the patterned sapphire substrate. A finite-difference time-domain simulation and angle-dependent reflectance measurement of the DBRs were also carried out. The results showed that DBR 2 yielded high reflectance even at low incidence angles, while DBR 1 presented low reflectance at incidence angles lower than 30°, followed by a reduction of the output power.

High efficient and wide-angle solar absorption with a multilayered metal-dielectric film structure

A typical 6-layered metal-dielectric film structure of SiO2 (57.3 nm)/Cr (2.9 nm)/SiO2 (72.9 nm)/Cr (6.6 nm)/SiO2 (57.6 nm)/Cu (>100.0 nm) was designed and fabricated by magnetron sputtering. It showed a high solar absorption of about 95.8% in the wavelength range of 250–2000 nm, a low thermal emittance of about 0.104 at 600 K and good thermal stability at 673 K after annealing for 12 h in vacuum condition. The angle-dependent reflectance spectra indicated that the proposed 6-layered film structure has a great angular tolerance even when the incident angle increases to 50°. All of the excellent spectral properties and good thermal stability demonstrate the 6- layered planar film structure will be suitable for practical applications of solar-thermal conversion.

Effects of sand-shale anisotropy on amplitude variation with angle (AVA) modelling: The Sawan gas field (Pakistan) as a key case-study for South Asia's sedimentary basins

Amplitude variation with angle (AVA) is a technique widely used in the characterisation of hydrocarbon reservoirs and assumes the Earth’s crust to be an isotropic medium. Yet, anisotropy is ubiquitous in stratigraphic sequences and has first-order effects on seismic AVA responses when investigating subsurface prospects. This work analyses the effects of anisotropic strata on AVA responses using the Lower Goru Formation, middle Indus basin (Pakistan) as a case study. In the study area, shale intervals are interbedded with reservoir sands of the Sawan gas field. Shales in this field form laminae or are dispersed within reservoir sands, making the Lower Goru Formation an example of a vertically transversely isotropic (VTI) medium. In this work, we calculate the effective (saturated) mechanical properties of the Lower Goru Formation based on rock physics templates; the Backus (1962) average typically designed for layered media, combined with the empirical relations of Brown and Korringa (1975) and Wood (1955). The input data used in our rock physics modelling is based on detailed petrophysical analyses of well data. Using the saturated effective mechanical properties of the Lower Goru Formation, we generate angle-dependent reflection coefficient curves (and seismic AVA responses) based on exact and approximate solutions, for both isotropic and anisotropic reservoir scenarios. Our results suggest that the effects of lithological anisotropy are more pronounced in places with thick shale beds within reservoir sands. Conversely, angle-dependent reflection curves, and seismic AVA responses based on isotropic or anisotropic cases, give similar solutions in the presence of thin shale beds. As a corollary of this work, we present a Bayesian inversion method for the estimation of porosity in VTI media.

Excitation of surface plasmon polaritons on silicon with an intense femtosecond laser pulse

We report the experimental observation of anomalies appearing in the reflection of intense $p$-polarized 100-femtosecond (fs) laser pulses at a nonmetallic material surface with a grating structure. The reflectivity was measured in air as a function of the angle of incidence at a Si grating. The results have exhibited an abrupt decrease to create a sharp dip at a specific incident angle of $\ensuremath{\sim}{24}^{\ensuremath{\circ}}$, where the grating surface was deeply ablated along the edge of the grooves. Similar to the so-called Wood's anomalies, the observed angle-dependent reflectivity provides direct evidence that surface plasmon polaritons (SPPs) can resonantly be excited at the interface between air and the nonmetallic material surface, as the intense fs laser pulse produces a high density of free electrons to form a metal-like layer on the Si grating surface. Calculation for a model target reproduces well the experimental results to confirm the excitation of SPPs on the Si grating, demonstrating the generation of enhanced near fields for the periodic ablation of a target surface.

Uncertainty analysis for the determination of B4C optical constants by angle-dependent reflectance measurement for 40 nm to 80 nm wavelength.

The index of refraction and the extinction coefficient for thin films of boron carbide were determined by angle-dependent reflectance measurements in the vacuum-ultraviolet spectral range. The numerical approximation was done using transfer-matrix formalism in combination with particle swarm optimization for the fitting algorithm. By this, not only for the reflectance measurement but also for the numerical approximation, a profound uncertainty budget was developed. This includes possible effects due to contamination and intermediate layers. Thus it was possible to establish a method for determination of n and k with reliable and highly traceable uncertainties, and to significantly improve the consistency of existing data required for current developments in optical technology.

Ultraviolet GaN Light-Emitting Diodes with Porous-AlGaN Reflectors

A GaN/AlGaN ultraviolet light emitting diode (UV-LED) structure with a porous AlGaN reflector structure has been demonstrated. Inside the UV-LED, the n+-AlGaN/undoped-AlGaN stack structure was transformed into a porous-AlGaN/undoped-AlGaN stack structure through a doping-selective electrochemical etching process. The reflectivity of the porous AlGaN reflector was 93% at 374 nm with a stop-bandwidth of 35 nm. In an angle-dependent reflectance measurement, the central wavelength of the porous AlGaN reflector had blueshift phenomenon by increasing light-incident angle from 10° to 50°. A cut-off wavelength was observed at 349 nm due to the material absorption of the porous-AlGaN/u-AlGaN stack structure. In the treated UV-LED structure, the photoluminescence emission wavelength was measured at 362 nm with a 106° divergent angle covered by the porous-AlGaN reflector. The light output power of the treated UV-LED structure was higher than that of the non-treated UV-LED structure due to the high light reflectance on the embedded porous AlGaN reflector.

Imaging optical scattering of butterfly wing scales with a microscope.

A new optical method is proposed to investigate the reflectance of structurally coloured objects, such as Morpho butterfly wing scales and cholesteric liquid crystals. Using a reflected-light microscope and a digital single-lens reflex (DSLR) camera, we have successfully measured the two-dimensional reflection pattern of individual wing scales of Morpho butterflies. We demonstrate that this method enables us to measure the bidirectional reflectance distribution function (BRDF). The scattering image observed in the back focal plane of the objective is projected onto the camera sensor by inserting a Bertrand lens in the optical path of the microscope. With monochromatic light illumination, we quantify the angle-dependent reflectance spectra from the wing scales of Morpho rhetenor by retrieving the raw signal from the digital camera sensor. We also demonstrate that the polarization-dependent reflection of individual wing scales is readily observed using this method, using the individual wing scales of Morpho cypris. In an effort to show the generality of the method, we used a chiral nematic fluid to illustrate the angle-dependent reflectance as seen by this method.

Frequency‐ and angle‐dependent poroelastic seismic analysis for highly attenuating reservoirs

ABSTRACTWe extend the frequency‐ and angle‐dependent poroelastic reflectivity to systematically analyse the characteristic of seismic waveforms for highly attenuating reservoir rocks. It is found that the mesoscopic fluid pressure diffusion can significantly affect the root‐mean‐square amplitude, frequency content, and phase signatures of seismic waveforms. We loosely group the seismic amplitude‐versus‐angle and ‐frequency characteristics into three classes under different geological circumstances: (i) for Class‐I amplitude‐versus‐angle and ‐frequency, which corresponds to well‐compacted reservoirs having Class‐I amplitude‐versus‐offset characteristic, the root‐mean‐square amplitude at near offset is boosted at high frequency, whereas seismic energy at far offset is concentrated at low frequency; (ii) for Class‐II amplitude‐versus‐angle and ‐frequency, which corresponds to moderately compacted reservoirs having Class‐II amplitude‐versus‐offset characteristic, the weak seismic amplitude might exhibit a phase‐reversal trend, hence distorting both the seismic waveform and energy distribution; (iii) for Class‐III amplitude‐versus‐angle and ‐frequency, which corresponds to unconsolidated reservoir having Class‐III amplitude‐versus‐offset characteristic, the mesoscopic fluid flow does not exercise an appreciable effect on the seismic waveforms, but there exists a non‐negligible amplitude decay compared with the elastic seismic responses based on the Zoeppritz equation.