Spectroscopy Research Articles

Microwave-optical spectroscopy of Rydberg excitons in the ultrastrong driving regime

Abstract We study the ultrastrong driving of Rydberg excitons in Cu$_2$O by a microwave field. The effect of the field is studied using optical absorption spectroscopy, and through the observation of sidebands on the transmitted laser light. A model based on Floquet theory is constructed to study the system beyond the rotating wave approximation. We obtain near quantitative agreement between theory and experiment across a 16-fold range of microwave field strengths spanning from the perturbative to the deep strong driving regime. Compared to Rydberg atoms, the large non-radiative widths of Rydberg excitons leads to new behaviour such as the emergence of an absorption continuum without ionization.

Spectroscopic Evidence of Ultrafast Topological Phase Transition by Light-Driven Strain.

Enabling reversible control over the topological invariants, transitioning them from nontrivial to trivial states, has fundamental implications for quantum information processing and spintronics. It offers a promising avenue for establishing an efficient on/off switch mechanism for robust and dissipationless spin-currents. While mechanical strain has traditionally been advantageous for such manipulation of topological invariants, it often comes with the drawback of in-plane fractures, rendering it unsuitable for high-speed, time-dependent operations. This study employs ultrafast optical and THz spectroscopy to explore topological phase transitions induced by light-driven strain in Bi2Se3. Bi2Se3 requires substantial strain for Z2 switching. Our observations provide experimental evidence of ultrafast switching behavior, demonstrating a transition from a topological insulator with spin-momentum-locked surfaces to hybridized states and normal insulating phases under ambient conditions. Notably, applying light-induced strong out-of-plane strain effectively suppresses surface-bulk coupling, facilitating the differentiation of surface and bulk conductance even at room temperature─significantly surpassing the Debye temperature. We expect various time-dependent sequences of transient hybridization and manipulation of topological invariant through photoexcitation intensity adjustments. The sudden surface and bulk transport alterations near the transition point enable coherent conductance modulation at hypersound frequencies. Our findings on the potential of light-triggered ultrafast switching of topological invariants hold promise for high-speed topological switching and its related applications.

Classification of Vaginal Cleanliness Grades through Surface‐Enhanced Raman Spectral Analysis via The Deep‐Learning Variational Autoencoder–Long Short‐Term Memory Model

In this study, it is aimed to establish a novel method based on a deep‐learning‐guided surface‐enhanced Raman spectroscopy (SERS) technique to achieve rapid and accurate classification of vaginal cleanliness levels. We proposed a variational autoencoder (VAE) approach to enhance spectral quality, coupled with a deep learning algorithm long short‐term memory (LSTM) neural network to analyze SERS spectra produced by vaginal secretions. The performance of various machine learning (ML) algorithms is assessed using multiple evaluation metrics. Finally, the reliability of the optimal model is tested using blind test data (N = 10/group for each cleanliness level). The data quality of the SERS fingerprints of four types of vaginal secretions is significantly improved after VAE decoding and reconstruction. The signal‐to‐noise ratio of the generated spectra increased from the original 2.58–11.13. Among all algorithms, the VAE–LSTM algorithm demonstrates the best prediction ability and time efficiency. Additionally, blind test datasets yielded an overall accuracy of 85%. In this study, it is concluded that the deep‐learning‐guided SERS technique holds significant potential in rapidly distinguishing between different levels of vaginal cleanliness through human vaginal secretion samples. This contributes to the efficient diagnosis of vaginal cleanliness levels in clinical settings.

Effects of Biostimulants on the Eco-Physiological Traits and Fruit Quality of Black Chokeberry (Aronia melanocarpa L.).

Biostimulants contribute to the physiological growth of plants by enhancing the quality characteristics of fruit without harming the environment. In addition, biostimulants applied to plants strengthen nutritional efficiency, abiotic stress tolerance, and fruit biochemical traits. We investigated the effectiveness of specific organic biostimulants. Five treatments were tested: (1) control (H2O, no biostimulants); (2) Magnablue + Keyplex 350 (Mgl + Kpl350); (3) Cropobiolife + Keyplex 120 (Cpl + Kpl120); (4) Keyplex 120 (Kpl120); and (5) Magnablue + Cropobiolife + Keyplex 120 (Mgl + Cpl + Kpl120) on the mineral uptake and physiology in black chokeberry (Aronia) plants, as well as the quality of their berries. The different treatments were applied to three-year-old chokeberry plants, and the experimental process in the field lasted from May to September 2022 until the harvest of ripe fruits. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) revealed that the fifth treatment significantly increased concentrations of P, Ca, and K. Additionally, the fifth treatment enhanced photochemical efficiency (Fv/Fm), water-splitting efficiency (Fv/Fo) in PSII, and the performance index (PI) of both PSI and PSII in chokeberry leaves. Improvements in photosynthesis, such as CO2 assimilation (A), transpiration (E), and water-use efficiency (A/E), were also noted under biostimulant applications. Upon harvesting the ripe fruits, part of them was placed at room temperature at 25 °C, while the rest were stored at 4 °C, RH 90% for 7 days. The cultivation with biostimulants had beneficial effects on the maintenance of flesh consistency, ascorbic acid concentration, and weight of berries at 4 and 25 °C, especially in the 5th treatment. Moreover, the total antioxidant capacity, anthocyanin concentration, and total phenols of the berries were notably higher in the third and fifth treatments compared to the control. These data suggest that selecting appropriate biostimulants can enhance plant yield and fruit quality by potentially activating secondary metabolite pathways.

Green Pathway of Urea Synthesis Through Plasma–Ice Interaction: Optimization and Mechanistic Insights With N2 + CO2 and NH3 + CO2 Gas Mixtures

ABSTRACTThis study presents a green approach for urea synthesis using plasma–ice interaction with N₂ + CO₂ and NH₃ + CO₂ gas mixtures. Plasma characterization via electrical and optical emission spectroscopy revealed that urea formation involves complex reactions driven by high‐energy species, producing reactive nitrogen and carbon intermediates. Plasma‐treated ice exhibited increased pH, electrical conductivity (EC), and reduced oxidation–reduction potential (ORP). Optimization of parameters, especially treatment time, significantly influenced urea formation. The NH₃ + CO₂ plasma produced a higher urea concentration (7.7 mg L−1) compared to the N₂ + CO₂ plasma (0.55 mg L−1) due to ammonia's greater reactivity. This method offers a sustainable, energy‐efficient alternative to traditional urea synthesis processes.

Iodine‐DMSO Catalyzed β‐carboline Derivative Chemoselective Fluorescent Molecule for Detection of Fluoride and Cyanide Anion

AbstractIn this work, a simple and highly sensitive fluorescent receptor 1‐(p‐tolyl)‐4,9‐dihydro‐3H‐pyrido[3,4‐b]indole (receptor 1), has been synthesized for the detection of fluoride and cyanide ions. The receptor 1 was very selective towards fluoride and cyanide anions over other anions such as Cl−, Br−, I−, HSO4−, NO3−, HPO42−, and OAc−. Synthesized receptor 1 shows low detection limits 3.29 × 10−8 M for fluoride ion, which was determined by UV–vis absorption. While LOD for cyanide was observed to be 61.5 nM. Importantly, while, receptor 1 produced binding constant for fluoride and cyanide in DMSO‐H2O (1:9, v/v) was found to be 5 × 10−4M−1 and 3.28 × 10−5 M−1, respectively. The variation in the optical and fluorescence spectroscopy confirms the formation of N─H⋯F− and N─H⋯CN− hydrogen bond of the pyrrolic proton The sensing mechanism displayed by receptor 1 for fluoride and cyanide ions was confirmed by 1H‐NMR and FT‐IR spectroscopic along with density functional theory (DFT) calculations. Solution with the fluoride and cyanide ions in DMSO‐H2O (1:9, v/v) shows a color change from light yellow to blue under UV light and yellow to colorless in visible light.

Boosted photoelectrochemical activity of anodic titanium dioxide nanotubes by electrophoretically decorated nickel–aluminum layered double hydroxide

The splitting of water using photoelectrochemical (PEC) processes is a promising method for generating renewable hydrogen. However, the practical efficiency of converting solar energy to fuel in PEC systems remains limited by inadequate light absorption and the swift recombination of photogenerated charge carriers within the photoelectrode material. In our research, we present a photoanode that addresses these challenges. In this work, we report on the electrophoretic deposition of nickel–aluminum layered double hydroxide (NiAl-LDH) under different voltages onto anodic TiO2 nanotube arrays (TNTAs) as a convenient and economically viable fabrication route of versatile and durable photoanodes. PEC and optical spectroscopy examinations, including linear sweep voltammetry, electrochemical impedance spectroscopy, Mott-Schottky plots, and diffuse reflectance spectroscopy, reveal that NiAl-LDH/TNTAs composites exhibit a superior enhancement of visible light absorption and consequently water splitting photocurrent. We interpret the improvement of PEC water splitting performance of the NiAl-LDH/TNTAs with the band structure speculated by the binding energy and the flat-band potential and band-gap measurements which features this nanocomposite as a novel photocatalyst for future hydrogen energy applications.

Ionophore-Based SERS Sensing for Electrolyte Cations.

We present here a surface-enhanced Raman spectroscopy (SERS)-based platform for selectively detecting electrolyte cations. This platform facilitates the reorientation of the chromoionophore I (CHI) molecule positioned on the surface of silver nanoparticles, regulated by the targeted electrolyte cation within the ionophore-based ion-selective sensing framework. When exposed to the target ions, leading to deprotonation, the aromatic plane of the CHI molecule shifts from an endwise to an edgewise configuration on the SERS substrate surface. This reorientation improves the coupling of the induced dipole within the molecule and the induced electromagnetic field normal to the surface, leading to a heightened and more discernible SERS signal. Additionally, NMR data revealed a protonation-dependent conformational change in the CHI molecules. Upon protonation, the side chain of the CHI molecule extends away from its aromatic ring, whereas upon deprotonation, the carbon chain folds back and closely approaches the edge of the aromatic plane, further confirming this conclusion. Using Ca2+ and Na+ as examples, this method shows a detection limit of 0.1 μM for Ca2+ and 1 μM for Na+ with high selectivity. The successful application of the versatile and rapidly advancing SERS technique for electrolyte cation detection shows a promising pathway for enhancing current ion detection capabilities. As a preliminary demonstration, the total Ca2+ concentration in undiluted human serum was successfully determined, with common matrix interference effectively circumvented.

Convenient Au@Ag Double‐Layer Nanoarray Fabricated by Rapid Thermal Annealing and Chemical Replacement Method for Surface‐Enhanced Raman Spectroscopy Sensing

Surface‐enhanced Raman spectroscopy (SERS) has a wide range of applications in molecular recognition, environmental pollutant detection, and other fields. However, the intensity and number of “hot spots” in 1D and 2D nanostructures are limited due to the scale‐dependent localized plasmonic effect of nanostructures, making it difficult to increase the detection limit. Herein, a kind of 3D substrate called Au@Ag double‐layer nanoarray (Au@Ag DLA) is prepared using rapid thermal annealing and chemical replacement methods. The energy‐dispersive spectrometer spectra confirm the successful growth of AgNPs on the gold nanoarray (Au SLA) by showing no presence of the copper element, indicating complete replacement of the Cu film deposited on Au SLA by Ag atoms. The detection limit of malachite green in Au@Ag DLA is 10−8 mol L−1, four and three orders of magnitude higher than that of Au SLA and AgNPs, respectively. This stronger SERS effect of Au@Ag DLA arises from the larger number of intense hot spots generated not only on the horizontal surface but also in the vertical direction. This finding provides a method for developing efficient and stable 3D SERS substrate, which can be utilized for the trace detection of water pollutants and pesticide residue.

Direct One-Stage Plasma Chemical Synthesis of Nanostructured Thin Films of the System β-Ga<sub>2</sub>O<sub>3</sub>-GaN of Different Composition

For the first time, nanostructured thin films of the β-Ga2O3−GaN system were obtained by plasma chemical deposition from the gas phase (PECVD) on c-sapphire substrates. High-purity metallic gallium, as well as high-purity gaseous nitrogen and oxygen were used as sources of macro components. The low-temperature nonequilibrium plasma of an inductively coupled HF (40.68 MHz) discharge at a reduced pressure (0.01 Torr) was the initiator of chemical transformations between the starting substances. A mixture of oxygen and nitrogen was used as a plasma-forming gas. The plasma chemical process was studied using the optical emission spectroscopy (OES) method. The obtained thin films of the β-Ga2O3−GaN system with a GaN phase content of 2 to 7% were characterized by various analytical methods.

Corrosion resistance and biocompatibility of carbon ion implanted AZ31B magnesium alloy

The poor corrosion resistance of magnesium limits its clinical applications. Accordingly, in the present study, carbon ions were incorporated into a AZ31b magnesium alloy surface via carbon plasma immersion ion-implantation to improve its corrosion resistance and biocompatibility. The surface morphology and properties of the modified alloy were evaluated by field-emission scanning electron microscopy, water contact angle measurement, Raman scattering, X-ray photoelectron spectroscopy, and energy dispersive X-ray spectroscopy. Furthermore, compositional depth profiles were obtained by glow discharge optical emission spectroscopy, revealing a Gaussian-like distribution of carbon concentration. Electrochemical and hydrogen-evolution analysis demonstrated the successfully improved corrosion resistance of the AZ31b Mg alloy, while its biocompatibility was demonstrated by MTT and cell-adherence assays.

Hydroxy-Carboxylic Acids as Green and Abundant Ligands for Sustainable Recovery of Copper from a Multimetallic Powder: A Proof of Concept.

The recovery of copper and other valuable metals had become increasingly strategic for the future of the global economy, particularly in regions lacking abundant mineral resources, such as most European countries. In this study, we investigated the viability of utilizing environmentally friendly, cost-effective, abundant and bio-based ligands, specifically carboxylic acids and their derivatives, for copper leaching in a low-temperature hydrometallurgical process. Our investigation focused on elucidating the impact of substituents in the α position of hydroxy-carboxylic acids on copper solubilization efficacy. Notably, hydroxy-carboxylic acids, like malic acid and lactic acid, were evidenced as particularly promising ligands for leaching copper from a custom-made multimetallic powder. By thoroughly characterizing the obtained complexes (Raman, UV-Vis) and by supporting the experimental efforts by a Design of Experiment (DoE) approach, we optimized the leaching process. The influence of experimental parameters such as pH, temperature, leaching time, and Cu/ligand molar ratio on process yield (determined through Inductively Coupled Plasma - Optical Emission Spectroscopy, ICP-OES, analysis) was thoroughly investigated. Additionally, we developed a subsequent copper recovery step by precipitating copper (II) hydroxide in an alkaline environment, guided by speciation diagrams tailored for each copper-ligand system.

Diffuse Optical Spectroscopy: Technology and Applications: introduction to the feature issue

Welcome to the 2024 Feature Issue on Diffuse Optical Spectroscopy: Technology and Applications in Biomedical Optics Express! This feature issue provides an exemplary sample of established and emerging DOS technologies as well as their biomedical applications through 27 contributed research papers and 1 invited review article. DOS researchers are inherently multidisciplinary, advancing topics spanning the basic theory of light-tissue interactions, computational modeling, technique and system development and preclinical and clinical applications. You will find this full range of topics represented in this feature issue.

SERS Performance Factor: A Convenient Parameter for the Enhancement Evaluation of SERS Substrates.

Surface-enhanced Raman spectroscopy (SERS), with molecular fingerprint information and single-molecule sensitivity, has been widely used for qualitative and quantitative analysis in various fields. Plenty of nanostructured plasmonic materials have been fabricated to achieve high SERS activity. Currently, great difficulty lies in evaluating the SERS performance among substrates, making it difficult to standardize. Addressing this problem, this work proposed the SERS performance factor () as a practically operational parameter to evaluate the sensitivity of SERS substrates. Experimentally, SPF can be obtained by taking the ratio of the slopes (i.e., the sensitivity) for concentration-dependent SERS and normal Raman measurements in the linear range of the intensity response under identical experimental conditions. Theoretically, SPF quantitatively describes the overall contribution to the SERS performance, (i.e., the electromagnetic (EM) enhancement of the SERS substrate and the interfacial interaction between the probe and substrate). The use of SPF as the criterion for evaluating the SERS performance was validated on Au nanoparticles in colloidal and solid states, where the tendency of SPF is consistent with that of the sensitivity of the probe molecules. Derived from the typically used surface enhancement factor EF in which accurate parameters are hardly achievable and different from concentration-dependent analytical enhancement factor AEF, SPF distinguishes itself with a simpler calculation and thereby offers a convenient and reliable protocol for the evaluation of the performance of different SERS substrates, which is very important to the practical application of SERS.

Experimental characterization and 3D simulations of turbulent flames assisted by nanosecond plasma discharges

This paper presents quantitative experimental data generated for the validation of plasma-assisted combustion (PAC) simulations. These data are then used to validate the phenomenological model of Castela et al. They are also useful to test other PAC models. In the experiment presented here, Nanosecond Repetitively Pulsed (NRP) discharges are applied to a lean-premixed turbulent methane-air flame initially near the lean blow-off limit. The discharges significantly enhance the combustion and stabilize the flame after a few pulses. Electrical and optical diagnostics are employed to extensively quantify the transient and steady state of the plasma-stabilization process. The flame shape is characterized by OH* chemiluminescence imaging. In the discharge region, OH density profiles are obtained by 1D laser-induced fluorescence, and the gas temperature is measured by optical emission spectroscopy measurements. The local gas temperature increases by 1250 K, and the OH number density rises sevenfold when NRP discharges are applied. These results evidence the cumulative thermal and chemical effects of NRP discharges, which are especially challenging to replicate numerically. A Large Eddy Simulation (LES) of the experiment is performed. Combustion chemistry is modeled by an analytically reduced mechanism, while the plasma discharge is described by the low-CPU cost phenomenological model of Castela et al., which aims to capture the main thermal and chemical effects induced by the discharges. The model of Castela et al. is validated in the burnt gases by the remarkable agreement between the simulations and the experiments regarding the flame shape, the local gas temperature, and the OH number density. More generally, this work demonstrates the relevance of simplified plasma models in LES solvers to simulate complex plasma-assisted burners.

Single Source Precursor Path to 2D Materials: A Case Study of Solution‐Processed Molybdenum‐Rich MoSe2‐x Ultrathin Nanosheets

AbstractTwo‐dimensional Molybdenum diselenide is a versatile and promising material used for a wide range of applications and its synthesis in high quality and yield is currently the subject of intense research. Low temperature solution phase synthesis of nanomaterials using designed molecular precursors enjoys tremendous advantages over traditional high‐temperature solid‐state synthesis. However, molecular precursor chemistry of molybdenum selenide nanomaterials is almost non‐existent. We report here synthesis and structural characterization of new molybdenum molecular precursors with selenoether and selenourea ligands, which due to their easy synthesis, desired Mo/Se composition and moderate processing properties, are highly promising as single source precursors for the scale‐up production of 2D molybdenum diselenide materials. This is demonstrated by the solution‐phase decomposition of the Mo‐selenourea precursor which produced Mo‐rich MoSe2‐x ultrathin nanosheets (NSs) in good yield. These NSs were characterized by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP‐OES), Powder X‐ray diffraction (XRD), Raman spectroscopy, Fourier Transform Infrared Spectroscopy (FT‐IR), Thermogravimetric analysis (TGA), Transmission electron microscopy (TEM) and X‐ray photoelectron spectroscopy (XPS) studies.

Discharge characteristics of silicon-based DC Helium microplasmas: comparison between Through Silicon Via and closed cavity type micro-reactors

Abstract This paper explores the microfabrication process of plasma reactors, from the production of micro-reactors to the addition of Through Silicon Vias (TSV) using proprietary etching techniques and on site cleanroom facilities. The investigation focuses on both newly fabricated closed and open-cavity Micro-Hollow Cathode Discharge (MHCD) reactors, with particular emphasis on electrical and optical diagnostics to characterize their plasma properties. All experiments, including those on closed-cavity reactors, were specifically conducted for this study under identical conditions to ensure a rigorous comparison between reactor types. When electrical diagnostics were carried out in helium at various pressures, interesting phenomena such as the evolution of voltage breakdown were observed. These diagnostics also provided new insights into the impact of surface electrode and overall geometry properties and pressure on reactor performance. 
Additional investigation into instability mechanisms revealed self-pulsing oscillations, with different reactor types having different oscillation amplitudes for similar operating conditions. Optical emission spectroscopy (OES) emerged as a strong tool for spatial plasma analysis, revealing consistent gas temperature distributions across reactor diameters. The Inglis-Teller series provided an estimate of the upper limit of electron density, while the Stark broadening analysis of Hα offered valuable insights into electron density variations, particularly in the self-pulsing regime.

Theoretical framework for calibrating the depth-dependent optical scattering in layered human skin using spatially offset measurements.

Spatially offset spectroscopy offers an alternative non-invasive method for enabling deep probing of structures and chemical molecules, which is clinically significant for the characterization of chemical and physical alterations in human skin. However, a more precise depth-resolved quantification using the spatially offset measurements still remains a challenge due to the mixed inhomogeneous scattering. Herein, we report a Monte-Carlo-based quantification modeling platform combined with a novel, to the best of our knowledge, scattering spectrum decomposition method to explore the depth-dependent optical scattering contributions in human skin. In the simplified modeling, human skin was empirically set to be composed of multiple layers, and each layer possessed different photon weights for the spatially offset scattering intensity measurements. The modeling results of photon transportation in-and-out of the layered skin substantially discovered that the layer-dependent scattering contribution was compositely encoded into the spatially offset measurements and varied with the illumination incidence angle. For calibrating the layer-dependent scattering contribution, a modified nonlinear independent component processing algorithm was applied to the spatially offset measurements by decomposing the photon weights of each layer. The calibration results figured out the major scattering contribution of each layer along the offset axis under different incidence angles, which were consistent with previous experimental observations. The proposed theoretical framework establishes a feasible approach for spatially offset optical spectroscopies enabling non-invasive quantitative A-line characterization of the concentrations of skin components.

Charged Impurity Scattering and Electron-Electron Interactions in Large-Area Hydrogen Intercalated Bilayer Graphene.

Intercalation is a promising technique to modify the structural and electronic properties of 2D materials on the wafer scale for future electronic device applications. Yet, few reports to date demonstrate 2D intercalation as a viable technique on this scale. Spurred by recent demonstrations of mm-scale sensors, we use hydrogen intercalated quasi-freestanding bilayer graphene (hQBG) grown on 6H-SiC(0001), to understand the electronic properties of a large-area (16 mm2) device. To do this, we first analyze Shubnikov-de Haas (SdH) oscillations and weak localization, permitting determination of the Fermi level, cyclotron effective mass, and quantum scattering time. Our transport results indicate that at low temperature, scattering in hQBG is dominated by charged impurities and electron-electron interactions. Using low- temperature scanning tunneling microscopy and spectroscopy (STS), we investigate the source of the charged impurities on the nm-scale via observation of Friedel oscillations. Comparison to theory suggests that the Friedel oscillations we observe are caused by hydrogen vacancies underneath the hQBG. Furthermore, STS measurements demonstrate that hydrogen vacancies in the hQBG have an extremely localized effect on the local density of states, such that the Fermi level of the hQBG is only affected directly above the location of the defect. Hence, we find that the calculated Fermi level from SdH oscillations on the millimeter scale agrees with the value measured locally on the nanometer scale with STS measurements.

Operando Investigation of Al Plating Regimes on HOPG in [EMImCl]:AlCl3 by Electrochemical Reflection Anisotropy Spectroscopy

Rechargeable aluminium batteries show promise as next‐generation systems with a more abundant material base than lithium technology. However, the stable native oxide on top of aluminium metal electrodes leads to poor cell performance. Graphite, on the other hand, is a so far rarely investigated alternative that can be used as both the anode and cathode. Here, metallic aluminium is deposited at the anode, while AlCl4– is intercalated at the cathode. For both cases, understanding the electrode–electrolyte interface is crucial for improving the performance of the battery. In this work, we use reflection anisotropy spectroscopy to study the evolution of the interface under applied potentials. We find that the cathode exhibits an irreversible swelling of the top‐most graphite layer due to AlCl4– intercalation as well as the formation of an SEI during the first voltammetry cycle. On the anode, the electrodeposition of aluminium is initially well‐ordered. However, the evolution of the surface morphology depends on the applied potential, with island‐like growth at less cathodic potentials, and layer‐by‐layer growth at more anodic potentials. With the optical operando spectroscopy, we can follow these qualitatively different plating and stripping regimes in a time‐resolved manner.