光声角谱法研究二维栅格光共振吸收

基于严格的电磁场理论,利用有限差分法来计算任意形状的二维栅格的光声共振吸收。为了模拟实验栅格,引入一种二维准正弦栅格剖面模型。并根据光声角谱实验结果进行理论计算,数值计算表明通过选取合适的栅格参数和形状因子,能很好地拟合实验曲线。     

Resonance Absorption of Metallic Plate with Subwavelength Hole Array

The optical reflectance by a metallic plate arranged with array consisting of subwavelength periodic square hole is investigated by using the three-dimensional finite-difference time-domain method （3D-FDTD）. There are dips in the reflectivity spectra, which indicate the absorption peaks. The absorption peaks behave differently according to the ratio of hole width and the period of the hole array. Combined with the near fields of the absorption peaks, it is found that the surface plasmon （SP） resonance on the surface of plate and localized SP in the hole play a major role for the two absorptions.     

Transition between corrugated metal films and?split-ring-resonator arrays

We fabricate split-ring-resonator arrays via direct laser writing of polymers, followed by atomic-layer deposition of titania, chemical vapor deposition of silver, and focused-ion-beam milling. While structures like that have been fabricated previously by other means, our approach here allows for a direct comparison with the optical properties of corrugated metal surfaces, which are fabricated along the same lines. This comparison reveals substantial differences regarding the magnetic metamaterial properties. In particular, we find that the optical response of the corrugated metal surfaces is due to a higher-order magnetic resonance, whereas that of the split-ring resonators stems from their fundamental magnetic resonance.     

The photoacoustic spectroscopic investigations of the surface preparation of ZnSe crystals with the use of the optimization methods

This paper shows results of the photoacoustic (PA) spectral studies, with the microphone detection, of a series of ZnSe crystals with differently prepared surfaces. All samples exhibited the surface absorption connected with defects states located on their surfaces. The quality of the surface preparation is expressed by the surface absorption coefficient spectra of the samples times the thickness of a damaged layer. In this paper both theoretical and experimental photoacoustic amplitude and phase spectra as also the corresponding computed surface and volume optical absorption coefficient spectra of the samples with differently prepared surfaces are presented and discussed. The procedure of computations of the volume and surface absorption spectra with the use of the optimization method is presented in the paper too.     

Electrochemically intercalated MxC60 thin films in a solid state cell (M = Li,K): Optical and photoelectrochemical characterization

Solid-state electrochemical cells have been prepared with C₆₀ vacuum-evaporated thin films, a Li- or K-source counter electrode and a polymer PEO-LiClO₄ (PEO-KClO₄) electrolyte. The electrochemical intercalation in C₆₀ of Li⁺ (or K⁺) ions has been performed under constant current conditions up to a formal stoichiometry of the fulleride film equal to Li₁₂C₆₀ (K₅C₆₀). A complete charge-transfer pocess from the intercalated alkali to the alkali-metal compound has been assumed. Several quasi-equilibrium potential plateaux were observed during intercalation, that we associate with the coexistence of phases with different intercalant concentration. The electrochemical intercalation process is irreversible to a large extent. Optical and photoelectrochemical spectroscopy of the fulleride films was done in-situ at different moments of the intercalation reaction by illuminating the film electrodes through the transparent and conducting glass substrates. The photo electrochemical spectral response agrees well with the optical absorption spectra, both indicating a bandgap of 2.2 eV. The photoelectrochemical response shows a minimum forx = 3 (in the K_xC₆₀ compound) and a maximum atx = 4 (in both K_xC₆₀ andLi_xC₆₀ compounds), in agreement with previous conductivity results. NIR diffuse reflectance spectra of the solid-state cell show absorption bands in the fulleride films at a wavelength of 1100 nm. A band-energy diagram has been proposed for the M_xC₆₀/PEO-MClO₄/M electrochemical cell.On leave from Instituto de Fisica, Facultad de Ingenleria, C.C. 30, 11000 Montevideo, Uruguay     

Shape and size dependence of the surface plasmon resonance of gold nanoparticles studied by Photoacoustic technique

We report on the optical absorption properties of as prepared gold naoparticles of different shapes and sizes measured by photoacoustic (PA) method. The gold nanoparticles of two different shapes (dots, rods) have been prepared using the seed mediated growth method. The shape and the size of these different nanoparticles were determined by STM measurements. PA spectra show the splitting of the surface plasmon resonance (SPR) into two modes (transverse and longitudinal) in case of gold nanorods. The increase in the aspect ratio of the nanorods leads to clear redshifts of the longitudinal SPR. These shifts were used to determine the dielectric constant of the surrounding medium and its variation with the aspect ratios.     

In‐situ synthesis of transparent and conductive carbon nanotube networks

A method for the production of transparent carbon nanotube networks (CNTNs) over transparent substrates was developed. In this method, CNTNs were grown directly in the target surface by applying the catalyst in specific zones of the substrate through lithographic techniques. The networks can be also transferred from the original substrate to other surfaces. The newly grown carbon nanotubes have a very high aspect ratio (>50000). Thus far, networks with an optical transmittance of 94% at 550 nm and a surface resistivity of 3.6 kΩ/sq have been produced. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The effects of heating on color centers in germanium-doped quartz

Optical absorption, electron paramagnetic resonance absorption, and thermoluminescence have been measured in germanium-doped quartz crystals which have been X-irradiated at room temperature and subsequently heated. Strong bleaching of the 280 nm optical absorption and the A and C center magnetic resonance absorptions coincided with maxima of the total thermoluminescent glow curve. The temperature at which these several phenomena occurred varied for different quartz samples.     

Scanning plasmon near-field microscopy on dye clusters

A scanning plasmon near-field microscope has been used to image dye clusters on a silver surface. This demonstrates the potential of optical near-field microscopy for the investigation of surfaces and non-conducting adsorbate layers. Images recorded in parallel with the conventional scanning tunneling microscope show the underlying conductive substrate.     

Optimization of light transmission efficiency for nano-grating assisted MSM-PDs by varying physical parameters

Metal–semiconductor–metal photodetectors (MSM-PDs) are utilized for suitable nano-structured fabrications that can play an important role for the development of future high-speed devices to achieve very high responsivity–bandwidth characteristics. Finite-difference time-domain (FDTD) method is used to calculate the light absorption enhancement factor of nano-grating assisted MSM-PDs near the design wavelength by changing the detector's geometrical parameters in comparison with a conventional device (i.e., without the nano-gratings). The substrates containing the aperture and the nano-gratings are optimized in thickness to trigger an extraordinary optical transmission. Besides, this paper demonstrates how the coupling condition satisfies the oscillating charge densities evolution to the surface plasmon polaritons under illumination of a polarized light and the existence of a band gap in the dispersion curve of surface plasmons propagating on periodically corrugated surfaces. The simulated results confirm that the light absorption enhancement factor of nano-grating assisted MSM-PDs is ∼12-times better than the conventional type of MSM-PDs.     

Incubation/ablation patterning of polymer surfaces with sub-μm edge definition for optical storage devices

By exposure to low fluence UV laser radiation, the optical absorption coefficient of subsurface polymer material can be increased (incubation) with spatial control, using a suitable contact mask, proper imaging of the mask, or laser direct writing. Spatially selective ablation of polymethylmethacrylate (PMMA) is then achieved with large area XeCl excimer laser pulses at 308 nm. In this way, the transfer of spatial information to the material can be decoupled from the high laser fluence removal (ablation) step. The advantages are: The mask is exposed to only low fluence laser radiation — damage is avoided. Since the mask can be removed before the ablation step, mask contamination by the ablated plume cannot occur. Using this incubation/ablation method, PMMA surfaces can be patterned (248 nm/308 nm) with submicrometer spatial control and edge contrasts better than 0.2 m. This has impact on optical storage technology and laser surface processing techniques in general. The smallest single structure obtained was somewhat smaller than 0.5 m in diameter up to now, given by the mask.Presented at Laserion '91, June 12–14, 1991     

介质填充浅槽周期结构表面上的太赫兹表面等离子体激元

通过在金属表面刻成浅的垂直凹槽,并在槽内填充不同的介质,对金属表面浅槽周期结构上传播的表面等离子体激元的色散特性与填充介质的关系进行了研究.研究表明通过在周期凹槽内填充介质可以有效降低人工表面等离子体激元的渐近频率,并增强金属表面对电磁场的约束.分析了太赫兹波段金属的吸收损耗对人工表面等离子体激元特性的影响,结果显示基于填充介质的浅槽周期表面结构可以获得长距离传输以及场的亚波长约束.通过对波传输的数值仿真,验证了该表面结构在太赫兹波段良好的导波能力.这种表面结构对太赫兹波段新型集成导波器件的设计具有参考价值.     

介质填充浅槽周期结构表面上的太赫兹表面等离子体激元

通过在金属表面刻成浅的垂直凹槽,并在槽内填充不同的介质,对金属表面浅槽周期结构上传播的表面等离子体激元的色散特性与填充介质的关系进行了研究.研究表明通过在周期凹槽内填充介质可以有效降低人工表面等离子体激元的渐近频率,并增强金属表面对电磁场的约束.分析了太赫兹波段金属的吸收损耗对人工表面等离子体激元特性的影响,结果显示基于填充介质的浅槽周期表面结构可以获得长距离传输以及场的亚波长约束.通过对波传输的数值仿真,验证了该表面结构在太赫兹波段良好的导波能力.这种表面结构对太赫兹波段新型集成导波器件的设计具有参考价值.     

基于SPPs-CDEW混合模式的亚波长单缝多凹槽结构全光二极管

全光二极管是集成光子回路上最基本的光子器件,如何有效增强全光二极管的单向透射性,提高消光比一直是学者们研究的重点.当前,利用表面等离极化激元(surface plasmon polaritons,SPPs)和复合衍射衰逝波(composite diffracted evanescent wave,CDEW)的亚波长金属微纳结构构建全光二极管器件还鲜有研究.因此,开发出一种可调制的全光二极管,对未来制备复杂的光子回路具有重要意义.本文提出了一种基于SPPs-CDEW混合模式设计全光二极管的方法和结构,该结构结合纳米缝中的类法布里-珀罗共振效应,利用结构参数对SPPs进行调控,实现了光束单向透过的功能.首先,利用理论推导和有限元算法分析了单缝-对称双凹槽纳米结构的透射增强现象,提出了透射增强和削弱的物理机理.其次,计算出规约化透射率随单狭缝和凹槽对之间距离变化的远场透射谱,给出的理论和数值计算结果符合得很好.最后,通过此透射谱精确确定凹槽的位置和数量,得出上表面对称分布五对增强透射凹槽、下表面六对抑制透射凹槽的最优全光二极管结构,有效增强了全光二极管的单向透射性,提高了消光比,最大消光比可以达到38.3 dB,即正向透射率是反向透射率的6761倍,比已有文献提高了14.6 dB,并在850 nm左右有70 nm宽的工作波长带宽(20 dB).本文提出的光二极管结构简单,宽带宽工作,易于集成,耦合效率高,研究结果对光学信号传输、集成光回路、超分辨率光刻等相关领域具有潜在的应用价值.     

Numerical analysis of the photoacoustic spectra of silicon samples with differently treated surfaces

This paper presents results of the photoacoustic (PA) spectral studies of a series of silicon samples with differently prepared surfaces. The PA amplitude and phase spectra of the samples indicated existence of the damaged surface layers. In the paper the two layer mathematical model of a sample with a damaged surface layer, that was used for numerical interpretation of the amplitude and phase PA spectra of the investigated samples, is presented and discussed. This model comprises the Urbach edge contribution to the optical absorption coefficient spectrum and it enabled observation of its influence on the PA amplitude spectra of silicon samples.     

Vision-based system for automatic stabilization of mode,beam steer,and output power of a high-power laser

As advancements in laser technology result in higher-power devices, increased thermal loads experienced by resonator optical components generate instabilities in the output beam. Consequently, methods for maintaining and optimizing output-beam parameters are necessary for efficient long-term operation. This study examines a vision-based feedback control system which monitors a sampled cross-sectional image of the laser's output, and regulates both cavity mirrors, as well as the excitation level, to stabilize beam uniformity, beam steer, and output-power level.The system optimizes the output-power distribution achievable from a given laser, while significantly reducing beam steering effects. Direction of beam propagation is restricted to within 60 rad of the desired setpoint, while the output power is stabilized to within 4%. Results clearly document a considerable improvement in laser performance through maximization of beam uniformity, minimization of beam steering, and stabilization of total output power. As such, an important degree of consistency and repeatability in output-beam parameters, essential to precision laser applications, is easily achieved. Valuable diagnostic capabilities pertaining to output beam intensity and energy profiles are also incorporated into the control system. These data may be monitored on-line or stored for subsequent analysis.     

Optical absorptions of an exciton in a quantum ring: Effect of the repulsive core

We study the optical absorptions of an exciton in a quantum ring. The quantum ring is described as a circular quantum dot with a repulsive core. The advantage of our methodology is that one can investigate the influence of the repulsive core by varying two parameters in the confinement potential. The linear, third-order nonlinear and total optical absorption coefficients have been examined with the change of the confinement potential. The results show that the optical absorptions are strongly affected by the repulsive core. Moreover, the repulsive core can influence the oscillation in the resonant peak of the absorption coefficients.     

Transmission and absorption based photoacoustic methods of determination of the optical absorption spectra of Si samples — Comparison

This paper is a comparison of two photoacoustic methods of determination of the optical absorption coefficient spectra of semiconductors illustrated with results obtained from Si samples. It presents experimental transmission and absorption photoacoustic spectra of Si samples as also the appropriate models leading to the determination of optical absorption coefficient spectra. The idea and the experimental set-up of the analyzed methods are presented in the paper too. From the fitting procedure of theoretical characteristics to experimental transmission and absorption photoacoustic spectra and after computations of the optical absorption coefficient spectra three components of the optical absorption coefficient spectra of Si samples were identified i.e. band to band transitions, Urbach tail and free carrier absorption. Their parameters are given and discussed in the paper. At the end the advantages and disadvantages of both methods are discussed. To the best of our knowledge, no such proof of the correctness of the PA method of determination of optical absorption spectra has been done.     

Absorption Spectra and Refractive Index Changes of an Exciton in a Core/Shell Quantum Dot

The absorption spectra and the refractive index changes are calculated theoretically for an exciton in a core/shell quantum dot. The advantage of our methodology is that one can investigate the influence of the repulsive core by varying two parameters in the confinement potential. The dimensionality effect of exciton quantum dots on the optical absorptions has been studied. It has been found that in the same regime, the optical absorption intensities of excitons are much smaller for the core/shell quantum dots than for the two-dimensional quantum rings. The linear and the nonlinear optical absorption coefficients and refractive index changes have been examined with the change of the confinement potential. The results show that the optical absorptions and the refractive index changes are strongly affected by the repulsive core of core/shell quantum dots. Moreover, the calculated results also reveal that as the inner radius increases, the peak values of the absorption coefficients and the refractive index changes of an exciton will show the optical Aharonov–Bohm oscillation in core/shell quantum dots.     

Linear and Nonlinear Intersubband Optical Absorptions and Refractive Index Changes in InGaN Strained Single Quantum Wells： Strong Built-in Electric Field Effects

Considering the strong built-in electric field （BEF） effects due to the spontaneous and piezoelectric polarizations, the intersubband optical absorptions and refractive index changes for an InxGa1-xN/AlyGa1-yN strained single quantum well are studied theoretically within the framework of the density matrix method and effective-mass approximation. The linear, third-order nonlinear and total absorption coefficients and refractive index changes are calculated as a function of the incident optical intensity and photon energy. Our results show that both the incident optical intensity and the strong BEF have great influence on the total absorptions and refractive index cllanges. The results are significant for designing some important photodetectors and the photonic crystal devices with adjustable photonic band structures.