Polarized Reflectivity Measurements Research Articles

Controllable Spin-Orbit Torque Induced by Interfacial Ion Absorption in Ta/CoFeB/MgO Multilayers with Canted Magnetizations.

Electrically generated spin-orbit torque (SOT) has emerged as a powerful pathway to control magnetization for spintronic applications including memory, logic, and neurocomputing. However, the requirement of external magnetic fields, together with the ultrahigh current density, is the main obstacle for practical SOT devices. In this paper, we report that the field-free SOT-driven magnetization switching can be successfully realized by interfacial ion absorption in perpendicular Ta/CoFeB/MgO multilayers. Besides, the tunable SOT efficiency exhibits a strong dependence on interfacial Ti insertion thicknesses. Polarized neutron reflection measurements demonstrate the existence of canted magnetization with Ti inserted, which leads to deterministic magnetization switching. In addition, interfacial characterization and first-principles calculations reveal that B absorption by the Ti layer is the main cause behind the enhanced interfacial transparency, which determines the tunable SOT efficiency. Our findings highlight an attractive scheme to a purely electric control spin configuration, enabling innovative designs for SOT-based spintronics via interfacial engineering.

Ion-beam-milled graphite nanoribbons as mesoscopic carbon-based polarizers

We demonstrate optical reflectivity and Raman responses of graphite microstructures as a function of light polarization when the incident light is applied perpendicular to the material's stacking direction (c-axis). For this, we employed novel graphite nanoribbons with edges polished through ion-beam etching. In this unique configuration, a strong polarization dependence of the D, G, and 2D Raman modes is observed. At the same time, polarized reflectivity measurements demonstrate the potential of such a device as a carbon-based, on-chip polarizer. We discuss the advantages of the proposed fabrication method as opposed to the mechanical polishing of bulk crystals.

Identifying, Resolving, and Quantifying Anisotropy in ReS2 Nanomechanical Resonators.

As an emerging two-dimensional semiconductor, rhenium disulfide (ReS2 ) is renowned for its strong in-plane anisotropy in electrical, optical, and thermal properties. In contrast to the electrical, optical, optoelectrical, and thermal anisotropies that are extensively studied in ReS2 , experimental characterization of mechanical properties has largely remained elusive. Here, it is demonstrated that the dynamic response in ReS2 nanomechanical resonators can be leveraged to unambiguously resolve such disputes. Using anisotropic modal analysis, the parameter space for ReS2 resonators in which mechanical anisotropy is best manifested in resonant responses is determined. By measuring their dynamic response in both spectral and spatial domains using resonant nanomechanical spectromicroscopy, it is clearly shown that ReS2 crystal is mechanically anisotropic. Through fitting numerical models to experimental results, it is quantitatively determined that the in-plane Young's moduli are 127 and 201GPa along the two orthogonal mechanical axes. In combination with polarized reflectance measurements, it is shown that the mechanical soft axis aligns with the Re-Re chain in the ReS2 crystal. These results demonstrate that dynamic responses in nanomechanical devices can offer important insights into intrinsic properties in 2D crystals and provide design guidelines for future nanodevices with anisotropic resonant responses.

Anisotropic optical properties of single Si2Te3 nanoplates

We report a combined experimental and computational study of the optical properties of individual silicon telluride (Si2Te3) nanoplates. The p-type semiconductor Si2Te3 has a unique layered crystal structure with hexagonal closed-packed Te sublattices and Si–Si dimers occupying octahedral intercalation sites. The orientation of the silicon dimers leads to unique optical and electronic properties. Two-dimensional Si2Te3 nanoplates with thicknesses of hundreds of nanometers and lateral sizes of tens of micrometers are synthesized by a chemical vapor deposition technique. At temperatures below 150 K, the Si2Te3 nanoplates exhibit a direct band structure with a band gap energy of 2.394 eV at 7 K and an estimated free exciton binding energy of 150 meV. Polarized reflection measurements at different temperatures show anisotropy in the absorption coefficient due to an anisotropic orientation of the silicon dimers, which is in excellent agreement with theoretical calculations of the dielectric functions. Polarized Raman measurements of single Si2Te3 nanoplates at different temperatures reveal various vibrational modes, which agree with density functional perturbation theory calculations. The unique structural and optical properties of nanostructured Si2Te3 hold great potential applications in optoelectronics and chemical sensing.

Study of reactively sputtered nickel nitride thin films

Nickel nitride (Ni–N) thin film samples were deposited using reactive magnetron sputtering process utilizing the different partial flow of N2 (RN2). They were characterized using x-ray reflectivity (XRR), x-ray diffraction (XRD) and x-ray absorption near-edge spectroscopy (XANES) taken at N K-edge and Ni L-edges. From XRR measurements, we find that the deposition rate and the density of Ni–N films decrease due to successively progression in RN2, signifying that Ni–N alloys and compounds are forming both at Ni target surface and also within the thin film samples. The crystal structure obtained from XRD measurements suggest an evolution of different Ni–N compounds given by: Ni, Ni(N), Ni4N, Ni3N, and Ni2N with a gradual rise in RN2. XANES measurements further confirm these phases, in agreement with XRD results. Polarized neutron reflectivity measurements were performed to probe the magnetization, and it was found Ni–N thin films become non-magnetic even when N incorporation increases beyond few at.%. Overall growth behavior of Ni–N samples has been compared with that of rather well-known Fe–N and Co–N systems, yielding similarities and differences among them.

Two-scale Monte Carlo ray tracing for canopy-leaf vector radiative transfer coupling

Canopy reflectance simulation was widely developed using Monte Carlo (MC) ray tracing. Nevertheless, the proposed models assume leaf as Bi-Lambertian medium. Based on the recently developed leaf MC ray tracing code, this study proposes to couple MC ray tracing simulations on leaf and canopy levels. Firstly, at the first level, the bidirectional scattering distribution function (BSDF) is computed for unpolarized incoming rays as well as some common states of polarization producing together a polarization decomposition basis, allowing to derive the Stokes phase function. Moreover, discretizing both incident and scattering angles over the sphere allows to produce a database of all possible Stokes phase functions. Secondly, at the canopy level, the reflectance is simulated using ray tracing technique, when ray is intercepted, the scattering is done considering the appropriate Stokes phase function in the leaf database. Simulation of multiple wavelengths is accelerated based on a new MC weighted sampling technique permitting to consider the same tracing for all the wavelengths together. Simulation results show the relevance of such modeling compared to traditional models. Indeed, from one side the canopy bidirectional reflectance depends on the leaf BSDF thus if leaves are assumed Bi-Lambertian surfaces leads to inaccurate results, and from the other side, the reflectance is sensitive to polarization and neglecting it affects the results mainly when the incident light is polarized. Comparison with actual polarized reflectance measurements presented in literature shows good agreement with same trends and variation ranges. Quantitative validation of our canopy level model is done using the ROMC web-tool. Reflectance RMSE between our simulations and the ROMC reference is lower than 0.02.

Enhanced anisotropy and study of magnetization reversal in Co/C60 bilayer thin film

The interface between the organic semiconductor (OSC)/ferromagnetic (FM) material can exhibit ferromagnetism due to their orbital hybridization. Charge/spin transfer may occur from the FM to OSC layer leading to the formation of “spinterface,” i.e., the interface exhibiting a finite magnetic moment. In this work, the magnetic properties of the Co/C60 bilayer thin film have been studied to probe the interface between the Co and C60 layer. Polarized neutron reflectivity (PNR) measurement indicates that the thickness and moment of the spinterface are ∼2 ± 0.18 nm and 0.8 ± 0.2 μB/cage, respectively. The comparison of the magnetization reversal between the Co/C60 bilayer and the parent single layer Co thin film reveals that spinterface modifies the domain microstructure. Further, the anisotropy of the bilayer system shows a significant enhancement (∼two times) in comparison to its single layer counterpart which is probably due to an additional interfacial anisotropy arising from the orbital hybridization at the Co/C60 interface.

Dynamics of reactive sputtering affecting phase formation of Co–N thin films

Target poisoning is one of the major drawbacks of reactive sputtering (RS) affecting the stoichiometry of a compound film depositing onto a substrate. The dynamics of RS has been understood in terms of the well-known ‘Berg’s Model’ (Berg et al. in Thin Solid Films 565:186, 2014; Berg and Nyberg in Thin Solid Films 476(2):215, 2005) which demonstrates that by reducing the size of a sputter target, the drawbacks of RS can be minimized. In the present work, we compare the evolution of cobalt–nitride (Co–N) phases in an RS process from two different sized magnetron sources having diameter of 1 and 3 in. Samples were studied for their long-range ordering and local structure using X-ray diffraction and X-ray absorption spectroscopy. Quantification of N at.% was carried out using secondary ion mass spectroscopy. Polarized neutron reflectivity and bulk magnetization measurements were carried out to study the magnetic properties of Co–N thin film samples. We found that the N incorporation in Co–N films can be accelerated utilizing the smaller source. In addition, by reducing the target size from 3 to 1 in., the range of metallic state can be extended. The $${\mathrm {Co}}_{4}{\mathrm {N}}$$ phase emerging from the metallic state of the smaller target has a lattice parameter more closer to its theoretical value. An insight has been presented on the RS process and it can be readily seen from this work that the poisoning of the target can be minimized for the growth of the $${\mathrm {Co}}_{4}{\mathrm {N}}$$ film from the smaller target and can be understood in accordance with the available theoretical models.

Polarimetric retrievals of cloud droplet number concentrations

Cloud droplet number concentration (Nd) is an important parameter of liquid clouds and is crucial to understanding aerosol-cloud interactions. It couples boundary layer aerosol composition, size and concentration with cloud reflectivity. It affects cloud evolution, precipitation, radiative forcing, global climate and, through observation, can be used to partially monitor the first indirect effect.With its unique combination of multi-wavelength, multi-angle, total and polarized reflectance measurements, the Research Scanning Polarimeter (RSP) retrieves Nd with relatively few assumptions. The approach involves measuring cloud optical thickness, mean droplet extinction cross-section and cloud physical thickness. Polarimetric observations are capable of measuring the effective variance, or width, of the droplet size distribution. Estimating cloud geometrical thickness is also an important component of the polarimetric Nd retrieval, which is accomplished using polarimetric measurements in a water vapor absorption band to retrieve the amount of in-cloud water vapor and relating this to physical thickness. We highlight the unique abilities and quantify uncertainties of the polarimetric approach.We validate the approach using observational data from the North Atlantic and Marine Ecosystems Study (NAAMES). NAAMES targets specific phases in the seasonal phytoplankton lifecycle and ocean-atmosphere linkages. This study provides an excellent opportunity for the RSP to evaluate its approach of sensing Nd over a range of concentrations and cloud types with in situ measurements from a Cloud Droplet Probe (CDP). The RSP and CDP, along with an array of other instruments, are flown on the NASA C-130 aircraft, which flies in situ and remote sensing legs in sequence.Cloud base heights retrieved by the RSP compare well with those derived in situ (R = 0.83) and by a ceilometer aboard the R.V. Atlantis (R = 0.79). Comparing geometric mean values from 12 science flights throughout the NAAMES-1 and NAAMES-2 campaigns, we find a strong correlation between Nd retrieved by the RSP and CDP (R = 0.96). A linear least squares fit has a slope of 0.92 and an intercept of 0.3 cm−3. Uncertainty in this comparison can be attributed to cloud 3D effects, nonlinear liquid water profiles, multilayered clouds, measurement uncertainty, variation in spatial and temporal sampling, and assumptions used within the method. Radiometric uncertainties of the RSP measurements lead to biases on derived optical thickness and cloud physical thickness, but these biases largely cancel out when deriving Nd for most conditions and geometries. We find that a polarimetric approach to sensing Nd is viable and the RSP is capable of accurately retrieving Nd for a variety of cloud types and meteorological conditions.

Atomic-scale engineering of ferroelectric-ferromagnetic interfaces of epitaxial perovskite films for functional properties

Besides epitaxial mismatch that can be accommodated by lattice distortions and/or octahedral rotations, ferroelectric-ferromagnetic interfaces are affected by symmetry mismatch and subsequent magnetic ordering. Here, we have investigated La0.67 Sr0.33 MnO3 (LSMO) samples with varying underlying unit cells (uc) of BaTiO3 (BTO) layer on (001) and (110) oriented substrates in order to elucidate the role of symmetry mismatch. Lattice mismatch for 3 uc of BTO and symmetry mismatch for 10 uc of BTO, both associated with local MnO6 octahedral distortions of the (001) LSMO within the first few uc, are revealed by scanning transmission electron microscopy. Interestingly, we find exchange bias along the in-plane [110]/[100] directions only for the (001) oriented samples. Polarized neutron reflectivity measurements confirm the existence of a layer with zero net moment only within (001) oriented samples. First principle density functional calculations show that even though the bulk ground state of LSMO is ferromagnetic, a large lattice constant together with an excess of La can stabilize an antiferromagnetic LaMnO3-type phase at the interface region and explain the experimentally observed exchange bias. Atomic scale tuning of MnO6 octahedra can thus be made possible via symmetry mismatch at heteroepitaxial interfaces. This aspect can act as a vital parameter for structure-driven control of physical properties.

Optical properties of lithium gallium oxide

The optical dielectric function tensor of orthorhombic single-crystal LiGaO2 was determined for polarizations along a, b, and c crystal-axis in the photon energy range from 0.04eV to 6.5eV by the generalized spectroscopic ellipsometry. In the far-infrared spectral range from 12.4meV to 40meV, the dielectric function was determined from conventional polarized transmittance and polarized reflectance measurements. Lineshape analysis of the dielectric function tensor major components allowed for a determination of the long-wavelength optical phonon characteristics, refractive indices dispersion, and parameters of interband and excitonic optical transitions.

Polarization-selective branching of stop gaps in three-dimensional photonic crystals

We study the direction- and wavelength-dependent polarization anisotropy in light scattering at the air-photonic crystal interface as a function of angle of incidence for TE and TM polarized light. This is done using optical reflectivity measurements at high-symmetry points in the Brillouin zone of a three-dimensional photonic crystal with fcc symmetry. Polarized reflectivity measurements indicate the presence of stop gap branching for TE polarization, which is absent for TM polarization till the Brewster angle at K point. In contrast, stop gap branching is present for both TE and TM polarizations at W point due to the intricate mixing of crystal planes. This characteristic behavior signifies the inevitable role of energy exchange in the stop gap branching. The measured polarization anisotropy shows a prominent shift in the Brewster angle for on-resonance wavelength as compared to the off-resonance along both K and W points, and that, in accordance with theory. Our results have implications in polarization-induced light scattering in sub-wavelength photonic structures like plasmonic crystals, and meta-materials.

Highly efficient selective metamaterial absorber for high-temperature solar thermal energy harvesting

In this work, a selective solar absorber made of nanostructured titanium gratings deposited on an ultrathin MgF2 spacer and a tungsten ground film is proposed and experimentally demonstrated. Normal absorptance of the fabricated solar absorber is characterized to be higher than 0.9 in the UV, visible and, near infrared (IR) regime, while the mid-IR emittance is around 0.2. The high broadband absorption in the solar spectrum is realized by the excitation of surface plasmon and magnetic polariton resonances, while the low mid-IR emittance is due to the highly reflective nature of the metallic components. Further directional and polarized reflectance measurements show wide-angle and polarization-insensitive high absorption within solar spectrum. Temperature-dependent spectroscopic characterization indicates that the optical properties barely change at elevated temperatures up to 350°C. The solar-to-heat conversion efficiency with the fabricated metamaterial solar absorber is predicted to be 78% at 100°C without optical concentration or 80% at 400°C with 25 suns. The performance could be further improved with better fabrication processes and geometric optimization during metamaterial design. The strong spectral selectivity, favorable diffuse-like behavior, and good thermal stability make the metamaterial selective absorber promising for significantly enhancing solar thermal energy harvesting in various systems at mid to high temperatures.

Ferromagnetism and spin-dependent transport inn-type Mn-doped bismuth telluride thin films

We describe a detailed study of the structural, magnetic, and magneto-transport properties of single-crystal, n-type, Mn-doped Bi2Te3 thin films grown by molecular beam epitaxy. With increasing Mn concentration, the crystal structure changes from the tetradymite structure of the Bi2Te3 parent crystal at low Mn concentrations towards a BiTe phase in the (Bi2Te3)m(Bi2)n homologous series. Magnetization measurements reveal the onset of ferromagnetism with a Curie temperature in the range 13.8 K - 17 K in films with 2 % - 10 % Mn concentration. Magnetization hysteresis loops reveal that the magnetic easy axis is along the c-axis of the crystal (perpendicular to the plane). Polarized neutron reflectivity measurements of a 68 nm-thick sample show that the magnetization is uniform through the film. The presence of ferromagnetism is also manifest in a strong anomalous Hall effect and a hysteretic magnetoresistance arising from domain wall scattering. Ordinary Hall effect measurements show that the carrier density is n-type, increases with Mn doping, and is high enough (> 2.8 x 10^{13} cm^{-2}) to place the chemical potential in the conduction band. Thus, the observed ferromagnetism is likely associated with both bulk and surface states. Surprisingly, the Curie temperature does not show any clear dependence on the carrier density but does increase with Mn concentration. Our results suggest that the ferromagnetism probed in these Mn-doped Bi2Te3 films is not mediated by carriers in the conduction band or in an impurity band.

Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 2: Application to the Research Scanning Polarimeter

Abstract. A new method to retrieve ice cloud asymmetry parameters from multi-directional polarized reflectance measurements is applied to measurements of the airborne Research Scanning Polarimeter (RSP) obtained during the CRYSTAL-FACE campaign in 2002. The method assumes individual hexagonal ice columns and plates serve as proxies for more complex shapes and aggregates. The closest fit is searched in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with a virtually continuous selection of aspect ratios and distortion. The asymmetry parameter, aspect ratio and distortion of the hexagonal particle that leads to the best fit with the measurements are considered the retrieved values. Two cases of thick convective clouds and two cases of thinner anvil cloud layers are analyzed. Median asymmetry parameters retrieved by the RSP range from 0.76 to 0.78, and are generally smaller than those currently assumed in most climate models and satellite retrievals. In all cases the measurements indicate roughened or distorted ice crystals, which is consistent with previous findings. Retrieved aspect ratios in three of the cases range from 0.9 to 1.6, indicating compact particles dominate the cloud-top shortwave radiation. Retrievals for the remaining case indicate plate-like ice crystals with aspect ratios around 0.3. The RSP retrievals are qualitatively consistent with the CPI images obtained in the same cloud layers. Retrieved asymmetry parameters are compared to those determined in situ by the Cloud Integrating Nephelometer (CIN). For two cases, the median values of asymmetry parameter retrieved by CIN and RSP agree within 0.01, while for the two other cases RSP asymmetry parameters are about 0.03–0.05 greater than those obtained by the CIN. Part of this bias might be explained by vertical variation of the asymmetry parameter or ice shattering on the CIN probe, or both.

Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 1: Methodology and evaluation with simulated measurements

Abstract. We present a new remote sensing technique to infer the average asymmetry parameter of ice crystals near cloud top from multi-directional polarization measurements. The method is based on previous findings that (a) complex aggregates of hexagonal crystals generally have scattering phase matrices resembling those of their components; and (b) scattering phase matrices systematically vary with aspect ratios of crystals and their degree of microscale surface roughness. Ice cloud asymmetry parameters are inferred from multi-directional polarized reflectance measurements by searching for the closest fit in a look-up table of simulated polarized reflectances computed for cloud layers that contain individual, randomly oriented hexagonal columns and plates with varying aspect ratios and roughness values. The asymmetry parameter of the hexagonal particle that leads to the best fit with the measurements is considered the retrieved value. For clouds with optical thickness less than 5, the cloud optical thickness must be retrieved simultaneously with the asymmetry parameter, while for optically thicker clouds the asymmetry parameter retrieval is independent of cloud optical thickness. Evaluation of the technique using simulated measurements based on the optical properties of a number of complex particles and their mixtures shows that the ice crystal asymmetry parameters are generally retrieved to within 5%, or about 0.04 in absolute terms. The retrieval scheme is largely independent of calibration errors, range and sampling density of scattering angles and random noise in the measurements. The approach can be applied to measurements of past, current and future airborne and satellite instruments that measure multi-directional polarized reflectances of ice-topped clouds.

Accuracy assessments of cloud droplet size retrievals from polarized reflectance measurements by the research scanning polarimeter

We present an algorithm for the retrieval of cloud droplet size distribution parameters (effective radius and variance) from the Research Scanning Polarimeter (RSP) measurements. The RSP is an airborne prototype for the Aerosol Polarimetery Sensor (APS), which was on-board of the NASA Glory satellite. This instrument measures both polarized and total reflectance in 9 spectral channels with central wavelengths ranging from 410 to 2260nm. The cloud droplet size retrievals use the polarized reflectance in the scattering angle range between 135° and 165°, where they exhibit the sharply defined structure known as the rain- or cloud-bow. The shape of the rainbow is determined mainly by the single scattering properties of cloud particles. This significantly simplifies both forward modeling and inversions, while also substantially reducing uncertainties caused by the aerosol loading and possible presence of undetected clouds nearby. In this study we present the accuracy evaluation of our algorithm based on the results of sensitivity tests performed using realistic simulated cloud radiation fields.

Influence of a depletion layer on localized surface waves in doped semiconductor nanostructures

We report THz polarized reflectivity measurements on a doped GaAs grating made of thin walls with subwavelength dimensions and periodicity. A clear dip appears in the TM-polarized reflectivity spectrum, due to the excitation of a localized surface plasmon-phonon mode along the grating walls. The theoretical model used to describe the electromagnetic response of the grating shows the importance of the depletion layer at the GaAs surface and indicates a high sensitivity of the reflectivity dip frequency to the surface potential.

Engineering anisotropic dielectric response through amorphous laminate structures

AbstractNanolaminates built from ultra‐thin, ultra‐smooth films of amorphous metals, and solution‐processed oxides represent a new platform for dispersion engineering. Materials exhibiting anisotropic elliptical dispersion and hyperbolic dispersion with positive refraction are realized through choice of amorphous metal and laminate design. Transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS), polarized reflectance measurements, and effective medium theory are combined to demonstrate the precision and predictability of the fabrication techniques and optical properties from ultra‐violet to infra‐red frequencies.magnified image Engineering anisotropic dielectric response through amorphous laminate structures.

Charge-density fluctuations probed by vibronic modes of K0.3MoO3

Polarized reflection measurements have been performed on the charge-density-wave compound K0.3MoO3 from room temperature down to T=50K. For the first time, the least conducting direction (perpendicular to the planes of MoO6-octahedrons) was investigated in the entire infrared range of frequency. The diatomic MoO stretching vibrations centered around 940cm−1 exhibit an unusual temperature dependence; below some characteristic temperature T⁎≈200K the line shape changes significantly. The observed features are attributed to change-density-wave fluctuations that are present well above the charge-density-wave transition TCDW, and start to develop short-range order at T⁎.