Differential Reflectivity Measurements Research Articles

Analysis of Two Convective Storms Using Polarimetric X-Band Radar and Satellite Data

We analyzed two convective storms that passed over or near the Milešovka meteorological observatory. The observatory is located at the top of a hill and has been recently equipped with a Doppler polarimetric X-band radar FURUNO WR2120 for cloud investigations. Our analysis was based mainly on Doppler polarimetric radar data measured in vertical cross-sections (RHI-Range-Height Indicator). Radar data was also used for classifying hydrometeors by a newly developed XCLASS (X-band radar CLASSification) algorithm. We also used rapid scan data measured by the geostationary satellite Meteosat Second Generation to validate radar measurements at the upper parts of storms. Although an attenuation correction was applied to the reflectivity and differential reflectivity measurements, the attenuation typical of X-band radars was noticeable. It was mainly manifested in the differential reflectivity, co-polar correlation coefficient and specific differential phase. Nevertheless, radar measurements can be used to analyze the internal cloud structure of severe convective storms. The XCLASS classification was developed by major innovation of a previously published algorithm. The XCLASS algorithm identifies seven types of hydrometeors: light rain, rain, wet snow, dry snow, ice, graupel, and hail. It uses measured horizontal and vertical radar reflectivity, specific differential phase, co-polar correlation coefficient, and temperature, and applies fuzzy logic to determine the type of hydrometeor. The new algorithm practically eliminates unrealistic results around and below the melting layer provided by the original algorithm. It identifies wet snow in more cases, and areas with individual hydrometeors have more realistic shapes compared to the original algorithm. The XCLASS algorithm shows reasonable results for the classification of hydrometeors and can be used to study the structure of convective storms.

Fast Exciton Diffusion in Monolayer PtSe2

AbstractRecently, 2D noble metal dichalcogenides have drawn considerable attention due to their thickness‐tunable electronic and optical properties. However, the dynamical properties of photocarriers in these materials are less studied. Here, photocarrier dynamics in monolayer and bilayer PtSe samples prepared by chemical vapor deposition are studied by transient absorption microscopy. Spatially and temporally resolved differential reflectance measurements yield room‐temperature exciton lifetimes of 25 and 50 ps for monolayer and bilayer samples, respectively. The exciton diffusion coefficient in monolayer PtSe is found to be as large as 48 cm s. This value is higher than exciton diffusion coefficients of most known monolayer semiconductors. The deduced exciton mobility is close to the theoretical limit of charge carrier mobility of monolayer PtSe. The superior exciton transport property is unique to monolayers, as the exciton diffusion coefficient drops to 6.7 cm s in bilayers PtSe. The novel exciton transport properties, along with its high air stability, make monolayer PtSe an attractive material for ultrathin excitonic devices. These results provide insights on the exciton dynamic properties of 2D PtSe and help develop fundamental understanding on the performance of various optoelectronic devices based on 2D PtSe.

Tuning the Optical Properties of a MoSe2 Monolayer Using Nanoscale Plasmonic Antennas.

Nanoplasmonic systems combined with optically active two-dimensional materials provide intriguing opportunities to explore and control light-matter interactions at extreme subwavelength length scales approaching the exciton Bohr radius. Here, we present room- and cryogenic-temperature investigations of a MoSe2 monolayer on individual gold dipole nanoantennas. By controlling nanoantenna size, the dipolar resonance is tuned relative to the exciton achieving a total tuning of ∼130 meV. Differential reflectance measurements performed on >100 structures reveal an apparent avoided crossing between exciton and dipolar mode and an exciton-plasmon coupling constant of g = 55 meV, representing g/(ℏωX) ≥ 3% of the transition energy. This places our hybrid system in the intermediate-coupling regime where spectra exhibit a characteristic Fano-like shape. We demonstrate active control by varying the polarization of the excitation light to programmably suppress coupling to the dipole mode. We further study the emerging optical signatures of the monolayer localized at dipole nanoantennas at 10 K.

Spectroscopic Ellipsometry Investigation of Au‐Assisted Exfoliated Large‐Area Single‐Crystalline Monolayer MoS2

The recent Au‐assisted mechanical exfoliation technique has enabled scalable fabrication of millimeter‐sized, high‐quality monolayer 2D crystals. Herein, spectroscopic ellipsometry characterization of as‐exfoliated single‐crystalline monolayer MoS2 is reported. By developing layer‐by‐layer ellipsometry measurements and analysis, the complex dielectric function of monolayer MoS2 is extracted. Compared with bulk MoS2, the A and B exciton peaks in the dielectric function spectrum nearly disappear in the 1L MoS2–Au hybrid system. The charge transfer at the 1L MoS2–Au interface is responsible for this observation and the conclusion is also supported by Raman, photoluminescence, and differential reflectance measurements. These results reveal the impact of the Au substrate on the dielectric response of 2D monolayers with strong excitonic effect and will deepen our understanding of the 1L MoS2–Au interaction.

Light-Induced Charge Accumulation in PTCDI/Pentacene/Ag(111) Heterojunctions

The incorporation of singlet fission (SF) chromophores in solar cells is expected to bring significant increases in the power conversion efficiency thanks to multiexciton generation. However, efficient charge generation in the device is determined by the energy level alignment (ELA) between the active materials, which should favor exciton transport and separation under illumination. By combining ultraviolet photoemission spectroscopy and optical differential reflectance measurements, we determine the ELA in a prototypical SF heterojunction between pentacene (Pc) and perylene-tetracarboxylic-diimide (PTCDI) grown on Ag(111). Time-resolved X-ray photoelectron spectroscopy on such a system reveals light-induced modifications of the ELA; by measuring the transient shift of the core level photoemission lines we observe an accumulation of long-lived holes in the PTCDI within the first hundred picoseconds after the optical pump.

Improving Time-Efficiency of Variational Specific Differential Phase Estimation

This study presents a variational approach for optimized estimation of specific differential phase ( K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DP</sub> ) for polarimetric radars using a linear forward operator. A cubic B-spline interpolating filter is included to mitigate the impact of measurement error in the total differential phase and ensure the spatial continuity of K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DP</sub> . For rain, non-negative constraints are introduced to ensure that the K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DP</sub> estimates are within the physical bounds. The variational approach is flexible to incorporate the background information constructed from the measurements of horizontal reflectivity factor ( Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H</sub> ) and differential reflectivity ( Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DR</sub> ) based on the self-consistent relationship of polarimetric variables. The variational approach is evaluated using simulated experiments, as well as real observations from an S-band operational weather radar. Without including background information, the variational approach has slightly better performance compared to the approach based on linear programming (LP), and the background information helps to further improve the performance. In addition, the linear forward operator makes this variational approach computationally efficient. It needs less than 3% computational power required by the approach based on LP, making it more suitable for real-time operational applications.

Efficient interlayer electron transfer in a MoTe2/WS2/MoS2 trilayer heterostructure

Electron transfer and carrier dynamics in MoTe2/WS2/MoS2 trilayer heterostructures are investigated by transient absorption and photoluminescence measurements. Monolayer flakes of MoTe2, WS2, and MoS2 are obtained by mechanical exfoliation from their bulk crystals and are used to fabricate the heterostructures by a dry-transfer technique. Photoluminescence spectroscopic measurements indicate that the recombination of the MoS2 and WS2 intralayer excitons is significantly suppressed in the heterostructure, illustrating the efficient interlayer charge transfer processes. Layer-selective time-resolved differential reflectance measurements show that the electrons excited in MoTe2 can transfer to MoS2 within 0.3 ps. The transferred electrons show a long lifetime of several hundred picoseconds due to their slow recombination with the spatially separated holes that reside in MoTe2. Furthermore, the charge transfer and recombination processes are weakly dependent on the injected carrier density. These results demonstrate the feasibility of constructing van der Waals multilayer heterostructures involving the infrared-sensitive MoTe2 with emergent properties and provide important information to quantify the performance of MoTe2-based devices.

Optical properties of Pb0.52Zr0.48TiO3 nanorod arrays: second harmonic generation and multiphoton carrier dynamics

Nonlinear optical properties of poled and unpoled, lead zirconate-titanate (Pb0.52Zr0.48TiO3) nanorod arrays, grown on Pt-coated Si with ∼200 nm diameter and ∼600 nm height, were investigated. Clear signatures of second harmonic generations (SHG), from 490–525 nm (2.38–2.53 eV) at room temperature, were observed. Furthermore, time resolved differential reflectivity measurements were performed to study dynamical properties of photoexcited carriers in the range of 690–1000 nm where multiphoton processes were responsible for the photo-excitations. We compared this excitation scheme, which is sensitive mainly to the surface states, to when the photoexcited energy (∼3.1 eV) was close to the band gap of the nanorods. Our results offer promises for employing these nanostructures in nonlinear photonic applications.

Ultrafast carrier and spin dynamics in GaMnAs across the Curie temperature

The carrier and spin dynamics of the diluted ferromagnetic semiconductor GaMnAs are crucial to understanding its ferromagnetic nature, especially near the Curie temperature (TC). Herein, transient differential reflectivity measurements and the temporal evolution of Kerr rotation are shown using ultrafast optical spectroscopy at various temperatures. The energy and angular momentum transfer routes after femtosecond laser pumping are revealed. The hole dynamics consist of two processes: a fast process that arises from carrier recombination and a slow process caused by carrier–lattice interactions. The spin dynamics are determined from the spin–orbital coupling and spin–lattice interactions in the ferromagnetic and paramagnetic phases, respectively. Critical phenomena are observed from the competition between the hole-mediated ferromagnetic orders and thermal perturbations near TC. These experimental results provide deep insights into the ultrafast decay behavior in GaMnAs, which is regarded as one of the most promising spintronic materials.

Decoupling between metal–insulator transition and structural phase transition in an interface-engineered VO2

The coupling between the metal–insulator transition (MIT) and the structural phase transition (SPT) in VO2 has been at the center of discussion for several decades, while the underlying mechanisms of electron–lattice or electron–electron interactions remain an open question. Until recently, the equilibrium state VO2 is believed to be a non-standard Mott–Hubbard system, i.e., both of the two interactions cooperatively work on MIT, indicating the association between MIT and SPT. However, due to the pronounced contribution of strain in strongly correlated systems, it is desirable to explore the correspondence in an interface-engineered VO2. Herein, we investigate the carrier dynamics in the VO2 films with anomalous MIT on the basis of time-resolved transient differential reflectivity measurements. Unexpectedly, MIT is decoupled from SPT, in sharp contrast with the case of strain-free VO2 films: MIT is triggered by bandgap recombination below 75 °C during heating, while intense SPT-induced signal appears separately between 70 °C and 100 °C. The decoupling between MIT and SPT provides insights into the interfacial interactions in VO2 thin films.

Strain dependence of Auger recombination in 3 μm GaInAsSb/GaSb type-I active regions

We differentiate the effect of strain induced by lattice-mismatched growth from strain induced by mechanical deformation on cubic nonradiative Auger recombination in narrow-gap GaInAsSb/GaSb quantum well (QW) heterostructures. The typical reduction in the Auger coefficient observed with lattice-mismatched growth appears to be due to the concomitant compositional change rather than the addition of strain, with implications for mid-IR semiconductor laser design. We induced a range of internal compressive strain in five samples from −0.90% to −2.07% by varying the composition during the growth and mechanically induced a similar range of internal strain in analogous quantum well membrane samples. We performed time-resolved photoluminescence and differential reflectivity measurements to extract the carrier recombination dynamics, taken at 300 K with carrier densities from 2.7×1018 cm−3 to 1.4×1019 cm−3. We observed no change with strain in the cubic Auger coefficient of samples that were strained mechanically, but we did observe a trend with strain in samples that were strained by the QW alloy composition. Measured Auger coefficients ranged from 3.0×10−29 cm6 s−1 to 3.0×10−28 cm6 s−1.

Estimating Rain Microphysical Characteristics Using S-Band Dual-Polarization Radar in South Korea

AbstractRaindrop size distribution (DSD) observed using a disdrometer can be represented by a constrained-gamma (C-G) DSD model based on the empirical relationship between shape (µ) and slope (Λ). The C-G DSD model can be used to retrieve DSDs and rain microphysical parameters from dual-polarization radar measurements of reflectivity (ZH) and differential reflectivity (ZDR). This study presents a newµ–Λ relationship to characterize rain microphysics in South Korea using a two-dimensional video disdrometer (2DVD) and Yong-in S-band dual-polarization radar. To minimize sampling errors from the 2DVD and radar measurements, measured size distributions are truncated by particle size and velocity-based filtering and compared with rain gauge measurement. The calibration biases of radarZHandZDRwere calculated using the self-consistency constraint and vertical pointing measurements. The derivedµ–Λ relationship was verified using the mass-weighted mean diameter (Dm) and standard deviation of the size distribution (σm), calculated from the 2DVD, for comparison with existingµ–Λ relationships for Florida and Oklahoma. TheDm–σmrelationship derived from the 2DVD corresponded well with theµ–Λ relationship. Theµ–Λ relationship derived for the Korean Peninsula was similar to Florida, and both generally had largerµvalues than Oklahoma for the same Λ. The derivedµ–Λ relationship was applied to retrieve DSD parameters from polarimetric radar data, and the retrieved DSDs and derived physical parameters were evaluated and compared with the 2DVD measurements. The polarization radar-based C-G DSD model characterized rain microphysics more accurately than the exponential DSD model. The C-G DSD model based on the newly derivedµ–Λ relationship performed the best at retrieving rain microphysical parameters.

Optical Properties and Photocarrier Dynamics of Bi2O2Se Monolayer and Nanoplates

AbstractA comprehensive experimental study on optical properties and photocarrier dynamics in Bi2O2Se monolayers and nanoplates is presented. Large and uniform Bi2O2Se nanoplates with various thicknesses down to the monolayer limit are fabricated. In nanoplates, a direct optical transition near 720 nm is identified by optical transmission, photoluminescence, and transient absorption spectroscopic measurements and is attributed to the transition between the valence and conduction bands in the Γ valley. Time‐resolved differential reflection measurements reveal ultrafast carrier thermalization and energy relaxation processes and a photocarrier recombination lifetime of about 200 ps in nanoplates. Furthermore, by spatially resolving the differential reflection signal, a photocarrier diffusion coefficient of about 4.8 cm2 s−1 is obtained, corresponding to a mobility of about 180 cm2 V−1 s−1. A similar direct transition is also observed in monolayer Bi2O2Se, suggesting that the states in the Γ valley do not change significantly with the thickness. The temporal dynamics of the excitons in the monolayer is quite different from the nanoplates, with a strong saturation effect and fast exciton–exciton annihilation at high densities. Spatially and temporally resolved measurements yield an exciton diffusion coefficient of about 20 cm2 s−1.

Field dependent ultrafast carrier dynamics in InGaN/GaN p-i(MQW)-n structure

Abstract Field dependent ultrafast carrier dynamics in InGaN/GaN multiple quantum wells (MQW) embedded within a p-i-n structure (InGaN/GaNp-i (MQW)-n) was systematically investigated by photoluminescence (PL), time-resolved PL and transient differential reflectivity measurements. We observed two PL peaks (centered at 450 nm and 500 nm) originated from MQW and localized defect states. Excitation density dependent PL spectra shows that the amplified spontaneous emission participates in the quantum well emission. Surprisingly, time-resolved PL clearly exhibits that quantum well emission has an extremely long decay time, which is much slower than the lower energy peak from defect states. We infer that the built-in PN field affects dominantly the carrier recombination more than the piezoelectric polarization field. The distinct excitation-dependent decay kinetic allows us to identify the dynamic interplay of screening and descreening effect by photo-generated carriers. Additionally transient reflectivity measurements also exhibit that carrier thermalization process becomes faster with increasing excitation density, which could be due to the amplified spontaneous emission.

Anomalous temperature dependence of optical and acoustic phonons in Bi2Se3 arising from stacking faults

The class of layered 3D-topological insulators have shown intriguingly anomalous temperature dependence in their thermal expansion coefficients. It was proposed that stacking faults are the origin of the observed anomalous thermal expansion. Here, using femtosecond pump-probe differential reflectivity measurements we probe the carrier and coherently generated totally symmetric A1g optical phonons in Bi2Se3. Transient signals also show a low frequency (∼GHz) oscillations due to coherent longitudinal acoustic phonons. We extract temperature dependence of optical constants, sound velocity and Young’s modulus of Bi2Se3 using the strain pulse propagation model. A remarkable anomalous behavior around ∼180 K is observed in the temperature dependence of optical and acoustic phonons as well as the optical constants. First-principles density functional theory reveals that thermally activated formation of stacking faults is responsible for the anomalies observed in Bi2Se3, similar to case of Sb2Te3. We also show that inclusion of spin–orbit coupling plays an important role in reducing the total energy difference between the pristine and the faulted structures.

Ultrafast electron and spin dynamics of strongly correlated NdNiO3

Electron and spin dynamics of strongly correlated NdNiO3 are investigated across the metal–insulator transition with ultrafast optical spectroscopy at different temperatures, pump laser fluences, and laser polarizations. Transient differential reflectivity measurements on the insulating phase NdNiO3 show two characteristic electronic decay processes. The slow decay process, strongly dependent on coherent phonon excitation, is attributed to the electron–phonon interaction, while the fast decay process, to the electron-spin interaction. The temporal evolution of Kerr rotation reveals that net spin polarization can be induced by circularly polarized light in insulating NdNiO3 via electron excitation from the Ni t to e orbital, and two net spin polarizations with opposite directions are observed given the antiferromagnetic ordering in insulating NdNiO3. At the insulator-to-metal transition, the transient differential reflectivity reverses its sign; the metallic phase NdNiO3 also shows two characteristic electronic decay processes, a typical metallic decay processes and a slow decay process; the slow decay process is attributed to the interaction between electrons and the low frequency coherent phonons (2.45 THz). Our experiments demonstrate that the phase transition involves several processes upon continuous heating: the electron-spin interaction disappears at 9 K below the transition temperature; at the transition temperature, NdNiO3 transitions into the metallic phase and the antiferromagnetic ordering transitions into the paramagnetic ordering; at the transition temperature, the structure of NdNiO3 changes from a monoclinic structure () to a high temperature orthorhombic () structure. Such a structure change results in the change in phonon vibrations, and subsequently, leads to the disappearance of the electron–phonon interactions due to the insulating phase NdNiO3 and the emergence of other types of electron–phonon interactions due to the metallic phase.

Characterization of S-Band Dual-Polarized Radar Data for the Convective Rain Melting Layer Detection in A Tropical Region

In the tropical region, convective rain is a dominant rain event. However, very little information is known about the convective rain melting layer. In this paper, S-band dual-polarized radar data is studied in order to identify both the stratiform and convective rain melting layers in the tropical region, with a focus on the convective events. By studying and analyzing the above-mentioned two types of rain events, amongst three radar measurements of reflectivity ( Z ), differential reflectivity ( Z DR ), and cross correlation coefficient ( ρ HV ), the latter one is the best indicator for convective rain melting layer detection. From two years (2014 and 2015) of radar and radiosonde observations, 13 convective rain melting layers are identified with available 0 °C isothermal heights which are derived from radiosonde vertical profiles. By comparing the melting layer top heights with the corresponding 0 °C isothermal heights, it is found that for convective rain events, the threshold to detect melting layer should be modified to ρ HV = 0.95 for the tropical region. The melting layer top and bottom heights are then estimated using the proposed threshold, and it is observed from this study that the thickness of convective rain melting layer is around 2 times that of stratiform rain melting layer which is detected by using the conventional ρ HV = 0.97 .

Raindrop Size Distribution Parameters Retrieved from Guangzhou S-band Polarimetric Radar Observations

According to the statistical shape–slope (μ–Λ) relationship observed for the first time by several 2D-Video-Distrometers (2DVD) in southern China, a constrained gamma (C-G) model was proposed for the retrieval of rain drop size distributions (DSDs) from Guangzhou S-band polarimetric radar observations. Two typical precipitation processes were selected to verify the accuracy of the retrieval scheme. The μ–Λ relationship: Λ = 0.0241μ2 + 0.867μ + 2.453 was obtained based on the 2DVD observation results from at Huizhou Longmen station, which is a very representative location in the area. Relying on the Guangzhou polarimetric radar measurements of radar reflectivity (ZHH) and differential reflectivity (ZDR), the gamma (Γ) size distribution parameters (N0, μ, and Λ) can be retrieved by the C-G model retrieval scheme. The results show that the Guangzhou polarimetric radar retrievals of DSDs were close to the 2DVD observations at Guangzhou Maofengshan station. The rain rate, mass mean diameter, and normalized intercept parameter of radar retrievals were in good agreement with the 2DVD observations, and the relative errors were less than 10%. The overall accuracy of the retrieval scheme was high. The retrieval scheme has established the relationship between the polarimetric radar measurements and gamma size distribution parameters. It will be helpful to in-depth research and application of the dual-polarization radar data in microphysical precipitation processes analysis, as well as convection-resolved numerical model data assimilation and prediction effect evaluation.

A Simple and Effective Method for Separating Meteorological from Nonmeteorological Targets Using Dual-Polarization Data

AbstractTo satisfy the needs of the meteorological and aeroecological communities wanting a simple but effective way of flagging each other’s unwanted echo for a variety of different operational radar systems, we evaluated the ability of an estimate of depolarization ratio (DR) based on differential reflectivity (ZDR) and copolar correlation coefficient (ρHV) measurements to separate both types of echoes. The method was tested with data collected by S- and C-band radars used in the United States and Canada. The DR-based method that does not require training achieved 96% separation between weather and biological echoes. Since the misclassifications are typically caused by isolated pixels in the melting layer or at the edge of echo patterns, the addition of a despeckling algorithm considerably reduces further these false alarms, resulting in an increase in correct identification approaching 99% on test cases.

Electrocatalytic properties of adsorbed halides: The reduction of hexaaquairon(III) on polycrystalline gold in aqueous electrolytes

Enhancements in the rates of reduction of hexaaquairon(III), [Fe(H2O)6]3+, on polycrystalline Au, Au(poly), induced by the adsorption of bromide, Br−, have been examined by chronoamperometric techniques in 0.1M HClO4 aqueous solutions containing Br− at concentrations in the μM (<1ppm) range using a Au(poly)|Au(poly) rotating ring|disk electrode, RRDE. The extent of Br− adsorption on the Au(poly) disk was determined quantitatively by monitoring the diffusion-limited current for the oxidation of Br− on the Au(poly) ring in solutions devoid of [Fe(H2O)6]3+. Evidence that Br− adsorption is not affected by the reduction of [Fe(H2O)6]3+ was obtained from in situ differential reflectance measurements performed in the absence and in the presence of the metal complex in solution. Information derived from these measurements was then used to establish quantitative correlations between the disk current, idisk, associated with the reduction of [Fe(H2O)6]3+ and E, at constant θBrN, and between idisk and the normalized Br− coverage, θBrN, at constant potential, E. Rather surprisingly, significant increases in electrocatalytic activity over the otherwise bare Au(poly) surface could only be discerned for θBrN>0.15. Possible explanations for this unexpected behavior are discussed.