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关键词： Destructive interference   Light matter interactions   Light wavelength   Optical devices   Photonic crystals   Spontaneous emission  

摘要： Photonic bandgap design is one of the most basic ways to effectively control the interaction between light and matter. However, the traditional photonic bandgap is always dispersive (blueshift with the increase of the incident angle), which is disadvantageous to the construction of wide-angle optical devices. Hypercrystal, the photonic crystal with layered hyperbolic metamaterials (HMMs), can strongly modify the bandgap properties based on the anomalous wavevector dispersion of the HMM. Here, based on phase variation competition between HMM and isotropic dielectric layers, we propose for the first time to design nonreciprocal and flexible photonic bandgaps in one-dimensional photonic crystals containing magneto-optical HMMs. Especially the zero-shift cavity mode and the blueshift cavity mode are designed for the forward and backward propagations, respectively. Our results show maximum absorption about 0.99 (0.25) in an angle range of 20-75 degrees for the forward (backward) incident light at the wavelength of 367 nm. The nonreciprocal omnidirectional cavity mode not only facilitates the design of perfect unidirectional optical absorbers working in a wide-angle range, but also possesses significant applications for all-angle reflectors and filters. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

关键词： Beam diagnostics   Single- and few-photon ionization & excitation   Third order nonlinear optical processes   Ultrafast phenomena   X-ray beams & optics  

摘要： An intensity autocorrelation measurement is demonstrated to characterize a pulse duration of 9-keV x-ray free-electron laser (XFEL) pulses from a split-delay optical (SDO) system with four-bounce silicon 220 reflections in each branch. XFEL pulse replicas with variable time delays are generated by the SDO system itself. High intensity of >2×1016W/cm2 achieved in a self-seeding operation and careful data analysis allow the measurement with direct two-photon absorption. The autocorrelation trace gave a duration of 7.6±0.8fs in full width at half maximum for a Gaussian assumption. Furthermore, the trace shows good agreement with a simulation of the XFEL pulse shape propagating through the SDO system, irrespective of spectral chirps in the original XFEL pulses. Our results open the door toward direct temporal characterization of narrowband XFELs at the hard x-ray regime, such as self-seeded and future cavity-based XFELs, and indicate a solid way for temporal tailoring of ultrafast x-ray pulses with perfect crystals.

关键词： X ray absorption  

摘要： Chlorine K-edge X-ray absorption near edge structure (XANES) in actinide(IV) hexachlorides, [AnCl(6)](2-) (An = Th-Pu), is calculated with relativistic multiconfiguration wavefunction theory (WFT). Of particular focus is a 3-peak feature emerging from U toward Pu, and its assignment in terms of donation bonding to the An 5f vs. 6d shells. With or without spin-orbit coupling, the calculated and previously measured XANES spectra are in excellent agreement with respect to relative peak positions, relative peak intensities, and peak assignments. Metal-ligand bonding analyses from WFT and Kohn-Sham theory (KST) predict comparable An 5f and 6d covalency from U to Np and Pu. Although some frontier molecular orbitals in the KST calculations display increasing An 5f-Cl 3p mixing from Th to Pu, because of energetic stabilization of 5f relative to the Cl 3p combinations of the matching symmetry, increasing hybridization is neither seen in the WFT natural orbitals, nor is it reflected in the calculated bond orders. The appearance of the pre-edge peaks from U to Pu and their relative intensities are rationalized simply by the energetic separation of transitions to 6d t(2g)versus transitions to weakly-bonded and strongly stabilized a(2u), t(2u) and t(1u) orbitals with 5f character. The study highlights potential pitfalls when interpreting XANES spectra based on ground state Kohn-Sham molecular orbitals.

关键词： Acetone  

摘要： The unimolecular photodissociation dynamics of acetone spanning the entire S-1 <- S-0 absorption spectrum have been reinvestigated, with a focus on mechanisms that produce CO. At excitation wavelengths of lambda > 305.8 nm, all photoproducts are formed on the S-0 state after internal conversion. A roaming mechanism forming C2H6 + CO is active in the window lambda = 311.2-305.8 nm. From lambda = 305.8 to 262 nm, little or no CO is produced with the photochemistry dominated by the Norrish-type I C-C bond cleavage on the lowest excited triplet state, T-1. At higher energy (lambda < 262 nm), an increasing fraction of CH3CO radicals from the primary reaction have sufficient internal energy to spontaneously decompose to CH3 + CO. A new model is presented to account for the kinetic energy distribution of the secondary CH3 radical, allowing us to determine the height of the energetic barrier to CH3CO decomposition as 68 +/- 4 kJ mol(-1), which lies midway between previous measurements. The fraction of CO from triple fragmentation rises smoothly from 260 to 248 nm. We see no evidence of the return of roaming, or any other S-0 reaction, in this higher energy region of the first electronic absorption band. Published under an exclusive license by AIP Publishing.

关键词： X-ray absorption near edge spectroscopy   Extended X-ray absorption fine structure   Electrical energy   Amorphous materials   Materials  

摘要： X-ray absorption spectroscopy (XAS) plays a critical role in the characterization of energy materials, including thin-film electrocatalysts and battery materials. XAS is well-suited for this purpose because it is element-specific and can target distinct chemical environments within a material, even in a mixed or complicated matrix. Even so, some key energy materials are far from "ideal" XAS samples. This means that both sample preparation and experimental conditions need to be considered when collecting and interpreting data to ensure that conclusions are correct. This review outlines some of the key questions that an XAS experiment is well-suited to answering, including speciation of amorphous materials, understanding how multi-metal systems interact, and the different ways that we may observe single atoms. In addition, we show how XAS can be highly complementary to other analytical techniques in developing a full picture of a material over different scale bars. Importantly, we also examine instances where the sample matrix can distort XAS data, show an example where bond-length disorder can be confused with a change in the coordination number, and discuss some of the advantages and challenges of in situ electrocatalysis. Finally, we examine the future role that XAS will play in innovations in energy materials.

关键词： 光学薄膜   双谱段减反射   膜系设计   热蒸发离子束辅助沉积  

摘要： 在氟化钡光学元件上设计并制备多谱段减反射薄膜是提升光电系统探测性能的关键。在氟化钡基底上设计并制备了1 064 nm激光/长波红外双谱段减反射薄膜。基于周期对称结构膜系导纳计算方法,以及拟合膜层周期数与参考波长的优化算法,开展了复合谱段减反射薄膜初始膜系的设计方法研究。使用热蒸发离子束辅助沉积方法制备了多层减反射薄膜。测试结果表明,该薄膜在1 064 nm处透射率为94.0%,在8~12μm长波红外谱段平均透射率为96.3%,在8.2μm处的透射率高达99.4%。该激光/长波红外双谱段减反射薄膜具有良好的光学性能,可以应用于多模复合精确探测光电装备之中,对于提升探测系统的工作性能具有重大意义。