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Optical Properties of Energy-Dependent Effective Mass GaAs/GaxIn1?xAs and GaAs/AlxGa1?xAs Quantum Well Systems: A Shooting Method Study
摘要： In this paper, we study the effect of energy-dependent effective mass on optical properties of GaAs/GaxIn1?xAs and GaAs/AlxGa1?xAs quantum well systems through the compact density matrix approach. We solved the resulting non-linear Schro¨dinger equation by a simple shooting method and present the algorithm. We show that the energy-dependent effective mass effect is more important for systems with narrower quantum well systems. By an energy-dependent effective mass assumption, absorption coef?cient peak height increases with increasing the total system length L while in the constant effective mass limit, absorption coef?cient peak heights have not been in?uenced by changing L. In the GaAs/AlxGa1?xAs system, by increasing the number of wells, the linear absorption coef?cient amplitude at ?rst increases and then decreases in the ?xed effective mass approximation and monotonically decreases in the energy-dependent effective mass case. By increasing the number of wells, the linear absorption coef?cient peak position at ?rst shows a blue shift and then shows a redshift. In the GaAs/GaxIn1?xAs system, the situation is more complicated and it is described in more detail in the text. However, GaAs/GaxIn1?xAs quantum well systems have larger values of absorption coef?cient peak heights than GaAs/AlxGa1?xAs ones.
关键词： shooting method,GaAs/GaxIn1?xAs and GaAs/AlxGa1?xAs Quantum wells,refractive index changes,energy-dependent effective mass,absorption coef?cient
更新于2025-09-23 15:21:21
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Ultrafast X-ray Transient Absorption Spectroscopy of Gas-Phase Photochemical Reactions: A New Universal Probe of Photoinduced Molecular Dynamics
摘要： Time-resolved spectroscopic investigations of light-induced chemical reactions with universal detection capitalize recently on single-photon molecular probing using laser pulses in the extreme ultraviolet or X-ray regimes. Direct and simultaneous mappings of the time-evolving populations of ground-state reactants, Franck?Condon (FC) and transition state regions, excited-state intermediates and conical intersections (CI), and photoproducts in photochemical reactions utilize probe pulses that are broadband and energy-tunable. The limits on temporal resolution are set by the transit- or dwell-time of the photoexcited molecules at specific locations on the potential energy surface, typically ranging from a few femtoseconds to several hundred picoseconds. Femtosecond high-harmonic generation (HHG) meets the stringent demands for a universal spectroscopic probe of large regions of the intramolecular phase-space in unimolecular photochemical reactions. Extreme-ultraviolet and soft X-ray pulses generated in this manner with few-femtosecond or sub-femtosecond durations have enormous bandwidths, allowing the probing of many elements simultaneously through excitation or ionization of core?electrons, creating molecular movies that shed light on entire photochemical pathways. At free electron lasers (FELs), powerful investigations are also possible, recognizing their higher flux and tunability but more limited bandwidths. Femtosecond time-resolved X-ray transient absorption spectroscopy, in particular, is a valuable universal probe of reaction pathways that maps changes via the fingerprint core-to-valence resonances. The particular power of this method over valence-ionization probes lies in its unmatched element and chemical-site specificities. The elements carbon, nitrogen, and oxygen constitute the fundamental building blocks of life; photochemical reactions involving these elements are ubiquitous, diverse, and manifold. However, table-top HHG sources in the “water-window” region (280?550 eV), which encompasses the 1s-absorption edges of carbon (284 eV), nitrogen (410 eV), and oxygen (543 eV), are far from abundant or trivial. Recent breakthroughs in the laboratory have embraced this region by using long driving-wavelength optical parametric amplifiers coupled with differentially pumped high-pressure gas source cells. This has opened avenues to study a host of photochemical reactions in organic molecules using femtosecond time-resolved transient absorption at the carbon K-edge. In this Account, we summarize recent efforts to deploy a table-top carbon K-edge source to obtain crucial chemical insights into ultrafast, ultraviolet-induced chemical reactions involving ring-opening, nonadiabatic excited-state relaxation, bond dissociation and radical formation. The X-ray probe provides a direct spectroscopic viewport into the electronic characters and configurations of the valence electronic states through spectroscopic core-level transitions into the frontier molecular orbitals of the photoexcited molecules, laying fertile ground for the real-time mapping of the evolving valence electronic structure. The profound detail and mechanistic insights emerging from the pioneering experiments at the carbon K-edge are outlined here. Comparisons of the experimental methodology with other techniques employed to study similar reactions are drawn, where applicable and relevant. We show that femtosecond time-resolved X-ray transient absorption spectroscopy blazes a new trail in the study of nonadiabatic molecular dynamics. Despite table-top implementations being largely in their infancy, future chemical applications of the technique will set the stage for widely applicable, universal probes of photoinduced molecular dynamics with unprecedented temporal resolution.
关键词： time-resolved spectroscopy,photochemical reactions,high-harmonic generation,X-ray transient absorption spectroscopy,nonadiabatic molecular dynamics
更新于2025-09-23 15:21:21
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Setup to Study the in Situ Evolution of Both Photoluminescence and Absorption during the Processing of Organic or Hybrid Semiconductors
摘要： In situ measurement techniques, applied during the solution processing of novel semiconductors such as organic semiconductors or hybrid perovskites, have become more and more important to understand their film formation. In that context, it is crucial to determine how the optical properties, namely photoluminescence (PL) and absorption, evolve during processing. However, until now PL and absorption have mostly been investigated independently, significantly reducing the potential insights into film formation dynamics. To tackle this issue we present the development of a detection system that allows simultaneous measurement of full absorption and PL spectra during solution processing of the investigated film. We also present a spin-coater system attachable to the detection system, where the temperature of the substrate on which the film is processed can be changed. We performed test measurements by spin coating the well-known conjugated polymer P3HT demonstrating the potential of this technique. By considering absorption and corresponding PL, we extract the PL quantum yield (PLQY) during processing, which decreases with substrate temperature. Furthermore, we identify a significant red shift of the PL just prior to the onset of the aggregation process, indicating the importance of chain planarization prior to solid film formation.
关键词： photoluminescence,in situ measurement,organic semiconductors,hybrid perovskites,spin-coating,film formation,absorption,P3HT
更新于2025-09-23 15:21:21
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Ultrathin Flexible Carbon Fiber Reinforced Hierarchical Metastructure for Broadband Microwave Absorption with Nano Lossy Composite and Multi-scale Optimization
摘要： The implementation of thin structure for broadband microwave absorption is challenging due to requirement of impedance match across several frequency bands and poor mechanical properties. Herein, we demonstrate a carbon fiber (CF) reinforced flexible thin hierarchical metastructure (HM) composed of lossy materials including carbonyl iron (CI), multiwall carbon nanotube (MWCNT) and silicone rubber (SR) with thickness of 5mm and optimal concentration selected from twelve formulae. Optimization for the periodical unit size is applied and impacts of structural sizes on absorption performance is also investigated. An effective process combining the vacuum bag method and the hand lay-up technique is used to fabricate the HM. Experimental reflectivity of the absorber achieves broadband absorption below -10dB in 2-4GHz and 8-40GHz. Full band in 2-40GHz is covered below -8dB. Yielding stress of the HM is increased to 24MPa with attachment of CF while the fracture strain of the composite reaches 550%. The soft HM is suitable to adhere to curved surface of objects needed to be protected from microwave radiation detection and electromagnetic interference. Enhanced mechanical properties make it possible for further practical applications under harsh service environment such as ocean and machines with constant vibration.
关键词： broadband microwave absorption,hierarchical metastructure,dielectric-magnetic lossy material,flexible,mechanical properties
更新于2025-09-23 15:21:21
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Salt Effect in Ion-Pair Dynamics after Bimolecular Photoinduced Electron Transfer in a Room-Temperature Ionic Liquid
摘要： Bimolecular photoinduced electron transfer between perylene and two quenchers was investigated in an imidazolium room-temperature ionic liquid (RTIL) and in a dipolar solvent mixture of the same viscosity using transient absorption on the subpicosecond to submicrosecond timescales. Whereas charge separation dynamics were similar in both solvents, significant differences were observed in the temporal evolution of the ensuing radical ions: although small, the free ion yield is significantly larger in the RTIL, and recombination of the ion pair to the triplet state of perylene is more efficient in the dipolar solvent. The temporal evolution of reactant, ion and triplet state populations could be well reproduced using Unified Encounter Theory. This analysis reveals that the observed differences can be explained by the strong screening of the Coulomb potential in the ion pair by the ionic solvent. In essence, RTILs favor free ions compared to highly dipolar solvents of the same viscosity.
关键词： unified encounter theory,free ions,spin conversion,charge recombination,transient absorption,charge separation
更新于2025-09-23 15:21:21
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Nonradiative decay and absorption rates of quantum emitters embedded in metallic systems: Microscopic description and their determination from electronic transport
摘要： We investigate nonradiative decay and absorption rates of two-level quantum emitters embedded in a metal at low temperatures. We obtain the expressions for both nonradiative transition rates and identify a unique, experimentally accessible way to obtain both nonradiative transition rates via electronic transport in the host metallic system. Our findings not only provide a microscopic description of the nonradiative channels in metals, but they also allow one to identify, determine, and differentiate them from other decay channels, which is crucial to the understanding and controlling of the light-matter interactions at the nanoscale.
关键词： nonradiative decay,metallic systems,quantum emitters,electronic transport,absorption rates
更新于2025-09-23 15:21:21
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Effects of inter-subband couplings on bound and extended states of terahertz cascade structures
摘要： We theoretically analyze the effect of inter-subband couplings on the eigenstates of a terahertz heterostructure in the presence of coulombic donors. We find that the inter-subband electron-donor interaction causes a drastic reorganization of the eigenenergies and a strong admixture between the subbands that decisively influences the optical absorption spectrum of the structure. These results illustrate the importance of the inter-subband couplings in the formulation of the eigenstates of imperfect terahertz heterostructures.
关键词： inter-subband couplings,eigenstates,terahertz cascade structures,optical absorption spectrum
更新于2025-09-23 15:21:21
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Soft X-ray Absorption Spectroscopy Study of Spin Crossover Fe-Compounds: Persistent High Spin Configurations under Soft X-ray Irradiation
摘要： Metal-organic complex exhibiting spin crossover (SCO) behavior has drawn attention for its functionality as a nanoscale spin switch. The spin states in the metal ions can be tuned by external stimuli such as temperature or light. This article demonstrates a soft X-ray–induced excited spin state trapping (SOXEISST) effect in Hofmann-like SCO coordination polymers of FeII(4-methylpyrimidine)2[Au(CN)2]2 and FeII(pyridine)2[Ni(CN)4]. A soft X-ray absorption spectroscopy (XAS) study on these polymers showed that the high spin configuration (HS; S = 2) was prevalent in Fe2+ ions during the measurement even at temperatures much lower than the critical temperatures (>170 K), manifesting HS trapping due to the X-ray irradiation. This is in strong contrast to the normal SCO behavior observed in FeII(1,10-phenanthroline)2(NCS)2, implying that the structure of the ligand chains in the polymers with relatively loose Fe-N coordination might allow a structural adaptation to stabilize the metastable HS state under the soft X-ray irradiation.
关键词： soft X-ray induced excited spin state trapping,X-ray absorption spectroscopy,high spin,spin crossover
更新于2025-09-23 15:21:21
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Uncertainty budgets of major ozone absorption cross sections used in UV remote sensing applications
摘要： Detailed uncertainty budgets of three major ultraviolet (UV) ozone absorption cross-section datasets that are used in remote sensing application are provided and discussed. The datasets are Bass–Paur (BP), Brion–Daumont–Malicet (BDM), and the more recent Serdyuchenko–Gorshelev (SG). For most remote sensing application the temperature dependence of the Huggins ozone band is described by a quadratic polynomial in temperature (Bass–Paur parameterization) by applying a regression to the cross-section data measured at selected atmospherically relevant temperatures. For traceability of atmospheric ozone measurements, uncertainties from the laboratory measurements as well as from the temperature parameterization of the ozone cross-section data are needed as input for detailed uncertainty calculation of atmospheric ozone measurements. In this paper the uncertainty budgets of the three major ozone cross-section datasets are summarized from the original literature. The quadratic temperature dependence of the cross-section datasets is investigated. Combined uncertainty budgets is provided for all datasets based upon Monte Carlo simulation that includes uncertainties from the laboratory measurements as well as uncertainties from the temperature parameterization. Between 300 and 330 nm both BDM and SG have an overall uncertainty of 1.5 %, while BP has a somewhat larger uncertainty of 2.1 %. At temperatures below about 215 K, uncertainties in the BDM data increase more strongly than the others due to the lack of very low temperature laboratory measurements (lowest temperature of BDM available is 218 K).
关键词： uncertainty budgets,Monte Carlo simulation,temperature dependence,UV remote sensing,ozone absorption cross sections
更新于2025-09-23 15:21:21
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Revisited Dual Luminescence of 2,2′-Dipyridylamine Hydrochloride in Solution and Physical Processes behind It
摘要： Reaction of 2,2’-dipyridylamine (dpa) with hydrochloric acid results in formation of dpa hydrochloride (dpa?HCl), leading to substantial changes in its luminescent properties. Luminescence of dpa?HCl in solution has been previously interpreted as aggregation-induced phenomenon. In this work, the emission properties of dpa?HCl in solution were revisited, focusing on the impact of methanol and chloroform solvents. Luminescence, absorption, and 1H NMR spectroscopy were used to study physical state of the solute molecules and photophysical properties associated with them. The results clearly indicate that the concentration and solvent dependencies of luminescence are due to equilibrium between protonated dpaH + and neutral dpa forms, rather than aggregation in solution. In chloroform, strong emission with λem ? 430 nm is associated with the cationic dpaH + form. Polar and hydrogen bonding methanol not only facilitates dissociation of dpaH + cation, but also seems to quench its luminescence through pyridinium hydrogen bond, and the residual weak emission at λem ? 360 nm is associated with the neutral dpa form. The obtained results can be applied in design of new dpa based materials with desirable luminescent properties in both solution and solid state.
关键词： 2,2’-dipyridylamine,protonation,hydrogen bond,absorption,luminescence
更新于2025-09-23 15:21:21