吸收波长和吸收效率可控的超材料吸收器

为了使超材料完美吸收器(metamaterial perfect absorber,MPA)能够同时实现吸收效率和吸收波长的控制,本文提出利用二氧化钒(VO2)和石墨烯作为MPA的材料,通过对MPA的结构设计,在红外波段实现了高吸收,吸收效率最高可达99％.研究发现通过改变VO2的温度和石墨烯的化学势,可同时实现MPA...     

Relationship between Helmholtz-resonance absorption and panel-type absorption in finite flexible microperforated-panel absorbers

Microperforated panels (MPPs) can provide wide-band absorption without fibrous and porous materials and are recognized as next-generation absorption materials. Although the fundamental absorbing mechanism of an MPP absorber is Helmholtz-resonance absorption, sound-induced vibration of an MPP itself can affects the absorption characteristics. There have been some studies considering the effects of the sound-induced vibration and there even is a proposal to widen the absorption bandwidth by positively utilizing the vibration of an MPP itself. On the other hand, in a previous study, the relationship between MPP absorbers and panel-type absorbers was investigated with infinite theory. However, the relationship between Helmholtz-resonance absorption and panel-type absorption in finite flexible MPP absorbers has not been clarified. Herein, from the viewpoint of an absorption-characteristics transition with the perforation ratio, the relationship between Helmholtz-resonance absorption and panel-type absorption including the effects of eigen-mode vibrations of the panel is theoretically and experimentally investigated. The analytical model considers a finite flexible MPP supported in a circular duct, and the predicted data for the absorption coefficient under normal incidence is validated by an experiment using an acoustic tube. From this investigation, it is found that panel-type absorption due to eigen-mode vibrations of the panel occurs independently from Helmholtz-resonance absorption, while panel-type absorption due to a mass-spring resonance of a panel and a back cavity has a trade-off relationship with Helmholtz-resonance absorption with respect to the perforation ratio.     

Two-photon absorption in the vicinity of the exciton absorption in CdS

The two-photon absorption spectrum of CdS in the wavelength region of the free and bound exciton lines has been measured using a pulsed dye laser and a differential transmittance technique. An intense absorption band is observed which is possibly due to the interference of the contributions of different intermediate states to the interband two-photon absorption. No absorption is observed which can be attributed to the formation of free excitonic molecules.     

Three-photon absorption and subsequent excited-state absorption in CdS

The nonlinear absorption of single picosecond light pulses (λ = 1.06 μm) in CdS is investigated at very high light intensities. Three-photon absorption and subsequent excited-state absorption of the generated electrons and holes explain the rapid decrease of transmission with increasing intensity. A three-photon absorption cross-section of σ⁽³⁾ = (2±0.5) × 10^-80 cm⁶ s² and an average excited state absorption cross-section of σ_ex = (7 ± 3) × 10^-18 cm² was determined.     

Resonance absorption band model

A model is derived for the resonance absorption of molecular bands composed of Lorentzian lines that is suitable for calculating fluorescence from irradiated air. The model is based on an explicit functional form derived for the resonance absorption of a single Lorentzian line which properly accounts for the possibility of unequal emission and absorption line widths. A procedure for rapid numerical computation of this function is given. This function yields a model for the resonance absorption of molecular bands composed of either overlapping or separated lines. A significant savings in computation time is obtained by using the model in place of exact expressions for resonance absorption. Examples are given for the resonance absorption of the N₂⁺ First Negative band system.     

Expressions for the local spectral absorption coefficient for correlation absorption spectroscopy

The apparent spectral absorption coefficient for correlation spectroscopy is independent of the spectral source-steradiancy for all optical depths if proper average values are chosen for the contributions by line centers and line wings to the absorption of optically thin source lines. It is shown that the absorption coefficient can be expressed by explicit relations if (a) the source line-widths are small compared with the absorber line-widths or (b) the absorbing medium is optically thin. In the latter case, the effective absorption coefficient depends on the source line-widths, as in the general case when the absorption coefficient has to be evaluated numerically; then the contributions by coincident lines and by line wings can be separated only if the latter are slowly varying functions of the wave-number over the full widths of the transmitting lines. It is shown that for many practical applications the approximation of negligible source line-widths is a valid one.     

Holographic investigation of induced absorption

A holographic technique for studying induced absorption in transmitting media using a scheme with an additional phase measuring grating is described. A matrix analysis is conducted, and expressions for the induced absorption are derived from the asymmetry of the transmitted beam intensity variation. The dynamics of the induced absorption in a photochromic glass is studied by the standard single-beam diffraction method and a new dual-beam interference method. The influence of a residual absorption grating resulting from the measuring phase grating on the induced absorption dynamics is analyzed. Measuring data obtained by the two methods are shown to be in good agreement     

The average absorption coefficient for enclosed spaces with non uniformly distributed absorption

This study concerns the determination of an equivalent acoustic absorption model of the flat heterogeneous walls present in industrial rooms. Numerous measurements of the reverberation time in reverberant room were carried out for several facings with different distributed spatial absorption. Experimental results were compared to classical reverberation time models. The measurements showed that the change in average acoustic absorption depends on the relative distance between the sound source and the absorbent panels, as it is this which creates heterogeneity. Therefore, taking into consideration, in the theoretical models of average acoustic absorption studied, the solid angles representing the equivalent area of the panels as viewed by the source, improved the accuracy of the calculated reverberation time compared to the measurements. This equivalent acoustic absorption model, based on Sabine's absorption coefficient and employing the solid angle ratio, was used to calculate the reverberation time of several industrial rooms. The results obtained are better than those obtained with the standard formula.     

水下弹性微穿孔吸声结构吸声系数研究

利用模态叠加法建立了水介质微穿孔板的数学模型,基于声电类比法得到其等效电路模型.研究了弹性微穿孔板和弹性背腔对垂直入射吸声系数的影响.与空气介质中的微穿孔板不同,水下微穿孔板因结构阻抗不足,难以取得满意的吸声效果,为此提出了增强型微穿孔吸声结构,并在水介质阻抗管内对理论结果予以验证.结果表明,随着增强型弹性微穿孔板弯曲...     

相长干涉：电磁诱导吸收

揭示修饰态原子相干对吸收的相长干涉产生电磁诱导吸收，这与修饰态原子相干对吸收的相消干涉导致电磁诱导透明的情形形成鲜明的对照. 以三个电偶极跃迁构成N型链的四能级系统为例，中间跃迁作为探测跃迁，在修饰态表象中分裂成四个相互耦合的跃迁. 这些耦合源于介质的自发辐射和介质激发态相干到基态相干的自发转移. 前者对所有四个探测跃迁产生相同符号的耦合，表现为对吸收的抑制. 而后者对其中两个跃迁产生正号的耦合，对另两个跃迁产生负号的耦合. 正是这些正负参半的耦合显著改变修饰态相干对吸收的贡献，使之由抑制吸收改变为增强吸 关键词： 电磁诱导吸收 相消干涉 相长干涉 原子相干的自发转移     

Pion absorption in the resonance region

Pion absorption in the resonance region is studied microscopically in terms of the elementary coupling of mesons to nucleons and isobars. Particular emphasis is put on calculating the contribution from genuine two-body absorption and three-body absorption. At low energies the two-body absorption is the most important channel, but around resonance and beyond the three-body absorption becomes important and can account for about 50% of the total absorption cross section. Genuine multiparticle absorption by more than three particles is estimated to be a small fraction of the total. Comparison with the experimental data is made and good agreement with experiment is found for different nuclei and different energies.     

Laser intracavity absorption spectroscopy

Emission spectra of multimode lasers are very sensitive to spectrally selective extinction in their cavity. This phenomenon allows the quantitative measurement of absorption. The sensitivity of measurements of intracavity absorption grows with the laser pulse duration. The ultimate sensitivity obtained with a cw laser is set by various perturbations of the light coherence, such as quantum noise, Rayleigh scattering, four-wave mixing by population pulsations, and stimulated Brillouin scattering. It depends on the particular laser type used, and on its operative parameters, for example pump power, cavity loss, cavity length, and length of the gain medium. Nonlinear mode-coupling dominates the dynamics of lasers that feature a thin gain medium, such as dye lasers, whereas Rayleigh scattering is more important in lasers with a long gain medium, such as doped fibre lasers, or the Ti:sapphire laser. The highest sensitivity so far has been obtained with a cw dye laser. It corresponds to 70000 km effective length of the absorption path. The ultimate spectral resolution is determined by the spectral width of mode emission, which is 0.7 Hz in this dye laser. High sensitivity and high temporal and spectral resolution allow various practical applications of laser intracavity spectroscopy, such as measurements and simulations of atmospheric absorption, molecular and atomic spectroscopy, process control, isotope separation, study of free radicals and chemical reactions, combustion diagnostics, spectroscopy of excited states and nonlinear processes, measurements of gain and of spectrally narrow light emission. Intracavity absorption in single-mode lasers shows enhanced sensitivity as well, although not as high as in multimode lasers. Received: 10 May 1999 / Published online: 29 July 1999     

双光子吸收的Franz-Keldysh效应

从实验上证实Hg_0.695Cd_0.305Te 光电二极管空间电荷区中存在双光子吸收的Franz-Keldysh效应.利用一个皮秒Nd:YAG激光器抽运的光学参量产生器和差频产生器作为激发光源,测量了入射波长为λ₀=7.92μm的脉冲激光所激发的光响应随入射光强的变化关系.脉冲光响应峰值强度随入射光强的增大呈现二次幂函数增强趋势.采用等效RC电路模型将脉冲光伏信号峰值与入射光强相关联,得到空间电荷区中强电场下单光束 关键词： Franz-Keldysh效应 碲镉汞 双光子吸收 脉冲光伏信号     

Reverse-saturated absorption and narrow line in the absorption spectrum of a V-type three-level medium

We study the absorption spectrum of a V-type three-level atom when the laser beam interacts only with one of its two transitions. With the presence of vacuum-induced quantum interference, the absorption spectrum shows some new interesting features, which include ultranarrow absorption line, transparency, and a reverse-saturated absorption that is characterized by a larger absorption with increasing laser intensity.     

Time-resolved UV absorption of polyimide

The 355-nm transient absorption of polyimide thin films has been measured following excitation withsubablative, 24-ps long, 355-nm laser pulses. The 355-nm absorption increases by 25% following 355-nm, 20 mJ/cm² excitation and recovers with a fast time constant 34 ps, and a slow time constant which is much longer than 6 ns. The data are fitted by a three-level rate equation model incorporating the temperature dependence of the ground state absorption coefficient. The fast component is attributed to the decay ofS ₁ and the slow component results from increased ground state absorption caused by a laser-induced temperature rise. The nonlinear intensity dependence is attributed to excited state (S ₁) absorption. These results indicate the importance of considering the dynamic absorption in modelling ablation.     

弱吸收基底上弱吸收薄膜的光学常数计算方法

采用一种相对简单而又精确的光度法来计算弱吸收基底上弱吸收薄膜的光学常数,为低损耗紫外薄膜的设计与实现提供了理论基础。采用JGS1型熔融石英基底,制备了MgF2与LaF3材料的单层膜,获得了JGS1型熔融石英基底及MgF2与LaF3薄膜的光学常数色散曲线。结果显示:在200 nm左右处,JGS1型熔融石英基底的吸收已经比较明显,消光系数在10-8量级,因此,应考虑基底的弱吸收,以提高薄膜光学常数的计算精度。     

Calculating vibrational mode contributions to sound absorption in excitable gas mixtures by decomposing multi-relaxation absorption spectroscopy

Sound relaxational absorption spectroscopy of excitable gas mixtures is potentially applied for gas composition detection. The relaxation of vibrational modes of gas molecules determines the sound relaxational absorption. However, to our knowledge, the contribution of each vibrational mode available in gas mixtures to sound multi-relaxation absorption has not been calculated in existing literature. In this paper, based on the decoupled expression of the effective isochoric molar heat for a gas mixture, a sound multi-relaxation absorption spectrum is decomposed into the sum of single-relaxation spectra. From this decomposable characteristic, the contribution of each vibrational mode available in the gaseous medium to the multi-relaxation absorption is obtained at room temperature. For various gas compositions including carbon dioxide, methane, nitrogen etc., the calculated contributions of vibrational modes are verified by the comparison with experiment data. We prove the following views with quantifiable outcomes that the primary molecular relaxation process associated with the lowest mode plays the major role in acoustic relaxational absorption of gas mixtures; the mode with lower vibrational frequency provides higher contribution to the primary relaxation process. This work could provide a deeper insight into the relationship between the sound relaxational absorption spectroscopy and gas molecules.     

红外吸收峰的强度与反常吸收现象

红外光谱测量有时需要在强的背景信号下得到弱的样品吸收峰,如在大量溶剂中测量溶质的红外光谱.理论上,只要某一组分在背景样品和待测样品的含量(分子个数)完全一致,则该组分的光谱信号就不会出现在最终的红外光谱图上.而实际测量时结果并非总如此.本文在测量聚苯乙烯及硬脂酸的“基线”时发现,在样品吸收较强的峰位(吸光度A＞1.0)...     

Volume absorption laser energy meter for high energy laser by water absorption

Since the energy density of high energy laser can be extremely high and destructive, it is difficult to directly measure the laser energy with custom methods. A volume absorption method directly using water as absorption substance is introduced. In the energy meter, water is pumped into an absorption cavity sealed by a quartz window, and energy increment of water by laser is calculated. The new energy meter has excellent power and energy linearity and is almost not affected by power, energy and flow, its relative deviation to norm middle-power energy meter is 1.9 %, which infers that the new energy meter can measure higher laser energy and maintain higher precision as well.     

Optical absorption in silicon doping superlattices

The optical absorption beyond the fundamental edge in moderate to large period silicon doping superlattices of various designs is studied theoretically. At 1.3 μm, the largest absorption coefficient found is 0.2 cm⁻¹. Only moderate external biases are required to reach maximum absorption.