Near-threshold reflectivity fluctuations in the independent-convective-hot-spot-model limit of a spatially smoothed laser beam

In the framework of the independent-hot-spot model, it is shown that the reflectivity resulting from scattering instabilities when a spatially smoothed laser beam interacts with a plasma exhibits large statistical fluctuations near threshold. The importance of the fluctuations is discussed in terms of a confidence interval for the reflectivity, which is more relevant to experimental measurements than the average reflectivity. An analytical model for the fluctuating reflectivity is developed and shown to be in good agreement with numerical simulations. The influence of the transverse size of the interaction region is studied.     

Water in contact with extended hydrophobic surfaces: direct evidence of weak dewetting

X-ray reflectivity measurements reveal a significant dewetting of a large hydrophobic paraffin surface floating on water. The dewetting phenomenon extends less than 15 A into the bulk water phase and results in an integrated density deficit of about one water molecule per 25-30 A(2) of water in contact with the paraffin surface. The results are supported by molecular dynamics simulations and related to the hydrophobic effect.     

Influence of the electron-hole density profile on the reflectivity of laser irradiated silicon

We study the influence of the spatial extension of the electron-hole plasma created by a pump pulse on the reflectivity of a probe pulse. We show that the density deduced from reflectivity measurements is the surface density value with a very good accuracy, except very close to the plasma resonance. We also show that the resonance broadening due to the spatial inhomogeneity can be larger than the one due to free carriers absorption and has to be included in the usual experimental determination of the plasma relaxation time.     

Periodic variations of plasma optical emission during repetitive pulsed-laser irradiation of aluminum in ambient air

Optical emission from plasma produced in ambient air by focusing a Nd:YAG laser beam on an aluminum surface was spectrally analysed. A periodic behaviour was observed in spectral line intensities associated with series of laser pulses. Based on simultaneous measurements of diffuse surface reflectivity and complementary measurements in other gases (He, O₂, N₂), this behaviour is ascribed to a competition between thermally-assisted surface oxidation and nitridation, and laser ablation. PACS 52.38.Mf; 52.50.Jm; 81.65; 78.68.+m; 52.70.Kz     

Reflection of high-intensity nanosecond Nd:YAG laser pulses by metals

Total reflectivity of silver and molybdenum samples irradiated by high-intensity nanosecond Nd:YAG laser pulses in air of atmospheric pressure is experimentally studied as a function of laser fluence in the range of 0.1–100 J/cm². The study shows that at laser fluences below the plasma formation threshold the total reflectivity of both silver and molybdenum remains virtually equal to the table room-temperature reflectivity values. The total reflectivity of these metals begins to decrease at a laser fluence of the plasma formation threshold. As the laser fluence increases above the plasma formation threshold, the reflectivity sharply drops to a low value and then remains unchanged with further increasing laser fluence. Calculation of the surface temperature at the plasma formation threshold fluence shows that the surface temperature value is substantially below the melting point that indicates an important role of the surface nanostructural defects in the plasma formation on a real sample due to their enhanced heating caused by both plasmonic absorption and plasmonic nanofocusing.     

Magnetoplasma reflectivity studies on Cd3As2

Magnetoplasma reflectivity measurements were performed on Cd₃As₂ in order to obtain more experimental details about the structure of the plasma reflectivity edge. The experimental results can be explained by assuming the existence of a surface layer with a carrier concentration-gradient which is responsible for optical interference effects.on leave from Dept. of Physics, University of Technology, 5600 MB, Eindhoven, the Netherlandson leave from University of Würzburg, D8700, Würzburg, FRG     

Structure of water adsorbed on a mica surface

Monte Carlo simulations of hydration water on the mica (001) surface under ambient conditions revealed water molecules bound closely to the ditrigonal cavities in the surface, with a lateral distribution of approximately one per cavity, and water molecules (interposed between K+ counter ions) in a layer situated about 2.5 A from a surface O along a direction normal to the (001) plane. The calculated water O density profile was in quantitative agreement with recent x-ray reflectivity measurements indicating strong lateral ordering of the hydration water but liquidlike disorder otherwise.     

X-ray reflectivity study on gold films during sputter deposition

We performed in-situ X-ray reflectivity measurements of gold films during sputter deposition on polished silicon substrates. The measurements were performed at several substrate temperatures and under two argon pressures. The gold surfaces were also examined by scanning tunneling microscopy after deposition to obtain their real-space topographic images. These images were used to complement the X-ray reflectivity measurements in determining the effect of argon pressure on the gold surface and its height-height difference functions. An approximation for height-height difference functions was employed to analyze the X-ray reflectivity data. The measured interface width during growth followed a simple power law, consistent with recent theoretical results of dynamic scaling behavior. The scaling exponents, however, do not agree well with predictions based on some models in 2 + 1 dimensions.     

Determination of layer-thickness fluctuations in Mo/Si multilayers by cross-sectional HR-TEM and X-ray diffraction

We present a method of deriving single layer thickness fluctuations of Mo/Si EUV multilayers from cross-sectional high-resolution transmission electron microscopy micrographs. The obtained thickness values for each layer are used in a layer model to calculate the grazing-incidence X-ray reflectivity (GIXRR) and the corresponding at-wavelength-reflectivity curves. Comparison with XRR measurements shows the strong effect of thickness fluctuations on the intensity of the secondary Kiessig fringes and the main Bragg maxima. This model results in substantially better reflectivity simulations than the standard periodic four-layer model or the assumption of statistically distributed (random) thickness errors. Results for reflectivity curves at 13-nm wavelength are discussed in terms of peak reflectivity, peak shift and further changes in the shape of the reflectivity curve. PACS 68.65.Ac; 68.37.Lp; 41.50.+h     

Weak-localization effect for light backscattered by surfaces with a complex structure

Results of laboratory measurements of the weak-localization effect for light scattered by surfaces with different characteristics are presented. The measurements were made in a range of phase angles of 0.2°–3.5°. The aim of the measurements was to study the influence of the choice of surface material (metal, dielectric), the size of particles of the scattering surface, their packing density, the reflectivity of a sample, and the wavelength of incident radiation on the effect. For the measurements, both nonpolarized and linearly polarized light was used. Powders of dielectric materials were found to enhance the weak-localization effect with increasing particle size, but the effect weakened starting with a certain size of the order of wavelength. The pressing of powders of dielectric transparent materials enhanced the opposition effect. This was most pronounced for samples with small-sized particles. The phase dependence of brightness became wider and more linear with decreasing reflectivity of the surface. A qualitative difference in the behavior of the phase dependence for the ratio of cross-and copolarized components was observed. For metals and metal-like materials, the ratio increased with decreasing phase angle. Dielectrics were found to have an opposite dependence.     

Effect of damaged surface layers on the parameters from plasma reflectivity spectra

This paper is an extension of the previous work, on surface characterization by the analysis of i.r. plasma reflectivity spectra. It is now shown that the apparent variations in the effective mass value, due to the presence of a damaged surface layer, can be approximately correlated with the nature of the surface. It has also been shown that the observation of a smaller optical relaxation time by the plasma reflectivity method compared to the corresponding d.c. relaxation time has nothing to do with the surface effects. Instead, the presence of a damaged surface layer always results in an increased estimate of relaxation time.     

Structure and morphology of Cu/Ni film grown by electrodeposition method: A study of neutron reflectivity and AFM

We present detailed study of structure and interface morphology of an electrodeposited Cu/Ni film using X-ray diffraction, X-ray reflectivity, neutron reflectivity and atomic force microscopy (AFM) techniques. The crystalline structure of the film has been determined by X-ray diffraction, which suggest polycrystalline growth of the film. The depth profile of density in the sample has been obtained from specular X-ray and neutron reflectivity measurements. AFM image of the air-film interface shows that the surface is covered by globular islands of different sizes. The AFM height distribution of the surface clearly shows two peaks and the relief structure (islands) on the surface in the film, which can be treated as a quasi-two-level random rough surface structure. We have demonstrated that the detailed morphology of air-film interfaces, the quasi-two-level surface structure as well as morphology of the buried interfaces can be obtained from off-specular neutron reflectivity data. AFM and off-specular neutron reflectivity measurements also show that the morphologies of electrodeposited surface is distinctively different as compared to that of sputter-deposited surfaces in the sample.     

Modification of optical properties of recrystallized tungsten due to the change in surface morphology induced by deuterium ion bombardment

Using ellipsometry and reflectometry, an anomalous change in the optical properties of recrystallized tungsten (W) as a result of bombardment with deuterium (D) ions was discovered at a sample temperature of ∼535 K. There is a qualitative difference between the value of reflectivity obtained from reflectometry measurements and that calculated from the ellipsometric data. A physical model of the discovered effect is proposed. It is shown that two processes take place at the surface of W exposed to plasma at 535 K: the appearance of blisters and a modification of the electronic structure of the surface layer.     

大面积表面波等离子体源微波功率吸收的数值模拟研究

建立了大面积矩形表面波等离子体(SWP)源全尺寸的三维模型,用数值模拟的方法研究了SWP源基于碰撞的功率吸收问题,给出了随等离子体参数变化的微波反射率曲线,分析了不同天线对微波功率沉积的影响,并讨论了微波功率吸收和表面波的定性关系. 结果发现,均匀放电的SWP源功率沉积本质是由表面波等离子体的性质决定的,等离子体密度太大或太小都不利于功率吸收. 在正常工作气压下,SWP源通过碰撞机理即可以实现微波功率的有效沉积,微波吸收率可达80%以上,与已有实验相符. 本研究同时发现,天线阵列激发的表面波模式越紧凑,强度越大,越有利于微波的吸收. 关键词： 时域有限差分法 等离子体表面波 功率吸收 狭缝天线     

Nuclear diffraction experiments with grazing incidence antireflection films

The energy dependent reflectivity of grazing incidence antireflection (GIAR) films containing the Mößbauer isotope⁵⁷Fe has been measured with recoilless 14.4 keV radiation from a radioactive source. The reflectivity is influenced by oxide layers on the surface. Assisted by Mößbauer spectroscopy in transmission geometry and X-ray reflectivity measurements the hyperfine interaction parameters in the layers could be determined. In a nuclear diffraction experiment with synchrotron radiation a GIAR-film has been used as a filter. The diffracted beam was analysed by the (002) reflection of Yttrium Iron Garnet (YIG). The evaluation of the observed quantum beat pattern yielded the energy differential reflectivity of the GIAR-film in agreement with the results from the Mößbauer measurements.     

Surface plasmon dispersion for surfaces covered by thin layers supporting longitudinal plasma waves

We analyse the effect of a superficial plasma layer on the substrate surface plasmon dispersion taking into account non local effects related to plasma oscillations in the surface layer. Surface plasmon dispersion and ATR reflectivity curves for an Aluminium surface covered by a potassium film 20 Å thick are computed. We show that new branches exist when longitudinal polarization waves are considered in the surface layer.     

表面等离子共振现象的新应用——微弱磁场的测量

提出了一种新的表面等离子体共振传感器, 它包含三层结构: 棱镜、金属薄膜及二能级介质. 通过理论分析发现, 与通常表面等离子体共振系统不同, 这一物理系统中同时存在两种共振效应 (表面等离子体共振和能级间量子跃迁的共振效应), 它们共同作用的结果导致一系列新的物理现象, 其中一个令人感兴趣的现象是入射光的反射率对外场导致的微小能级移动十分敏感 (这一现象是通常的表面等离子体共振系统所不具有的). 由于能级移动依赖于外场, 所以最终入射光的反射率对外场具有灵敏的响应. 本文以外磁场导致能级移动的情况进行了理论计算, 结果表明, 这种表面等离子体共振系统的入射光的反射率对外加磁场极其敏感. 这一特性可以用来测量物质表面附近的微弱磁场, 有可能发展成为一种新型检测技术.     

In situ reflectivity investigations of solid/liquid interface during laser backside etching

In situ reflectivity measurements of the solid/liquid interface with a pump-probe setup were performed during laser-induced backside wet etching (LIBWE) of fused silica with KrF excimer laser using toluene as absorbing liquid. The intensity, the temporal shape, and the duration of the reflected light measured in dependence on the laser fluence are discussed referring to the surface modification and the bubble formation.The vaporisation of the superheated liquid at the solid interface causes a considerable increase of the reflectivity and gives information about the bubble lifetime. The alterations of the reflectivity after bubbles collapse can be explained with the changed optical properties due to surface modifications of the solid surface. Comparative studies of the reflectivity at different times and the etch rate behaviour in dependence on the laser fluence show that the in situ measured surface modification begins just at the etch threshold fluence and correlates further with etch rate behaviour and the etched surface appearance. The already observed surface modification at LIBWE due to a carbon deposition and structural changes of the near surface region are approved by the changes of the interface reflectivity and emphasizes the importance of the modified surface region in the laser-induced backside wet etching process.     

A simultaneous multiwavelength dispersive X-ray reflectometer for time-resolved reflectometry

Aiming for the realization of time-resolved specular X-ray reflectivity measurements on the sub-second to millisecond timescales, a conceptually new method of measuring specular X-ray reflectivity curves is developed. Using this method the entire profile of the reflectivity curve of interest is measured in place. A horizontally convergent X-ray beam which has a one-to-one correlation between its direction and energy is realized using a curved crystal or laterally graded multilayers on an elliptic substrate. The X-ray beam is then incident on the surface of the specimen placed at the focus such that the glancing angle in the vertical direction is the same for all X-ray components, which are reflected in the vertical direction by the surface and diverge in the horizontal plane. The perpendicular momentum transfer continuously changes as a function of the horizontal ray direction even with fixed glancing angle since the wavelength (energy) changes. The X-ray intensity distribution across the beam direction measured downstream of the specimen using a one- or two-dimensional detector represents the X-ray reflectivity curve. Specular X-ray reflectivity curves are measured with exposure times ranging from 2?ms to 1?s for a gold film of thickness 14.3?nm on a silicon substrate. The potential of this method for time-resolved measurements is demonstrated by recording reflectivity curves with a time resolution of 20?ms from a rotating specimen.     

Optische Plasmaresonanz in CdO

The plasma resonance in semiconducting CdO with different electron concentrations has been investigated by means of reflectivity measurements on thin films with metallic substrate. The results — especially the dependence of the plasma frequency on carrier density — are discussed within the framework of the classical dispersion theory. At some films peculiarities of the reflectivity have been observed which suggest inhomogeneities of the electron concentration along the layer normal.