Application of ellipsometry techniques to biological materials

Ellipsometry is well-suited for bioadsorption studies and numerous reports, mainly using null ellipsometry, are found on this subject whereas investigations addressing structural properties of thin biolayers are few. Here two examples based on the use of spectroscopic ellipsometry (SE) on the latter are briefly discussed. In the first example, time evolution of thickness, spectral refractive index and surface mass density of a fibrinogen matrix forming on a silicon substrate are investigated with SE and a structural model of the protein matrix is discussed. In the second example a model dielectric function concept for protein monolayers is presented. The model allows parameterization of the optical properties which facilitates monitoring of temperature induced degradation of a protein layer. More recently, photonic structures in beetles have been studied with SE. It is shown here that full Mueller-matrix SE can resolve very complex nanostructures in scarab beetles, more specifically chiral structures causing reflected light to become circularly polarized.     

Improvement for sensitivity of biosensor with total internal reflection imaging ellipsometry (TIRIE)

The biosensor based on the total internal reflection imaging ellipsometry (TIRIE) is realized as an automatic analysis method for protein interaction processes in real-time, with high throughput and label-free. An evanescent wave is used as the optical probe to monitor bio-molecular interactions on a chip surface with a high sensitivity due to its phase sensitive property. In this paper, the technique is optimized with a polarization setting, a spectroscopic light source and a low noise CCD detector to improve the performance of the biosensor in sensitivity and detection limit, as evidenced by a quantitative detection of Hepatitis B virus surface antigen (HbsAg) with concentrations of 8, 16, 32, 64, 125 and 250 ng/ml. The sensitivity is increased by one order of magnitude and the detection limit has been extended more than 50 times for HbsAg detection.     

Femtosecond laser spectroscopy of spins: Magnetization dynamics in thin magnetic films with spatio-temporal resolution

Based on the Magneto-Optical Kerr Effect (MOKE), we have developed an experimental set-up that allows us to fully characterize the magnetization dynamics in thin magnetic films by measuring all three real space components of the magnetization vector M. By means of the pump-probe technique it is possible to extract the time dependence of each individual projection with sub-picosecond resolution. This method has been exploited to investigate the temporal evolution of the magnetization (modulus and orientation) induced by an ultrashort laser pulse in thin epitaxial iron films.According to our results, we deduced that the initial, sub-picosecond demagnetization is established at the electronic level through electron-magnon excitations. The subsequent dynamics is characterized by a precessional motion on the 100 ps time scale, around an effective, time-dependent magnetic field. Following the full dynamics of M, the temporal evolution of the magneto-crystalline anisotropy constant can be unambiguously determined, providing the experimental evidence that the precession is triggered by the rapid, optically-induced misalignment between the magnetization vector and the effective magnetic field.These results suggest a possible pathway toward the ultrarapid switching of the magnetization.     

Total internal reflection ellipsometry of metal-organic compound structures modified with gold nanoparticles

Total internal reflection ellipsometry (TIRE) technique was used to investigate the optical response of different hybrid multilayer systems. It was shown that the optical response was significantly changed by gold nanoparticles, which have been introduced for modification of functional properties of hybrid system. Nevertheless, the dispersion of optical parameters for gold nanoparticles was quite close in various hybrid systems in the case of adequate models used for interpretation of TIRE data.     

Engineering magnetic nanostructures with inverse hysteresis loops

Top-down lithography techniques allow the fabrication of nanostructured elements with novel spin configurations,which provide a new route to engineer and manipulate the magnetic response of sensors and electronic devices and understand the role of fundamental interactions in materials science.In this study, shallow nanostructure-pattemed thin films were designed to present inverse magnetization curves,i.e.,an anomalous magnetic mechanism characterized by a negative coercivity and negative remanence.This procedure involved a method for manipulating the spin configuration that yielded a negative coercivity after the patterning of a single material layer.Patterned NiFe thin films with trench depths between 15％-25％ of the total film thickness exhibited inverse hysteresis loops for a wide angular range of the applied field and the trench axis.A model based on two exchange-coupled subsystems accounts for the experimental results and thus predicts the conditions for the appearance of this magnetic behavior.The findings of the study not only advance our understanding of patterning effects and confined magnetic systems but also enable the local design and control of the magnetic response of thin materials with potential use in sensor engineering.     

On circular Bragg regimes in ellipsometry spectra of ambichiral sculptured thin films

The generalized ellipsometry formalism was used for a right-handed ambichiral sculptured thin film. The amplitude ratios and phase differences were calculated using an experimental dielectric function for bulk titania. The results showed that the occurrence of the circular Bragg regimes can be adduced from ellipsometry spectra.     

Monitoring plasma etching of biomolecules by imaging ellipsometry

Low-pressure plasma discharges can be applied to remove various biomolecules from surfaces. However, the knowledge on the interaction between plasma and biomolecules and the kinetics of their removal is still rather poor, which is a major limiting factor for the optimization of this type of plasma treatment. This is, among other reasons, because of the restrictions of currently used techniques for the evaluation of the rates of biomolecule removal during plasma treatment. Therefore, an alternative method based on imaging ellipsometry is applied in this study. It is shown that this method allows reliable semi-quantitative comparison of the treatment efficiency of plasma discharges sustained in different gas mixtures.     

Pulsed laser deposition and optical characterizations of the magnetic samarium orthoferrite

Pulsed Laser Deposition of magnetically ordered polycrystalline SmFeO₃ films has been optimized onto SiO₂ glass substrates as function of substrate temperature, oxygen pressure and pulsed laser fluency. Using a KrF excimer laser, crystallization temperature is found to be about 1048 K for a weak fluency of only 1.7 J cm^− 2. We show that this growth temperature can be reduced using higher fluency and that it is possible to obtain a film texturation along the c axis by reducing the oxygen pressure at given temperature and fluency. In a second part, we focus on the SmFeO₃ optical constants determined by in situ ellipsometry using a stacking model and the Cauchy dispersion relation for SmFeO₃ layer. We show a good correlation between the transmission and reflection calculated from these data and measured by ex situ spectrophotometry in the visible range.     

Application of wide angle beam spectroscopic ellipsometry for quality control in solar cell production

Wide angle beam ellipsometry developed by our group uses non-collimated illumination with a special light source and arrangement giving multiple-angle-of-incidence and multiwavelength information. Our aim was to make our wide angle beam ellipsometer suitable for spectral measurement and to obtain the spectra of many points along a long line (presently 0.2 m but it could be increased up to 1 m if necessary) of an entire sample simultaneously. The prototype uses a xenon lamp as a light source with film polarizers and a concave optical grating to reach the desired 6 nm spectral resolution over the range of 360-630 nm. This new technique mixed with an appropriate ellipsometric model has the capability to make “in situ” control in solar cell fabrication. In order to demonstrate the ability of our instrument, wide angle beam spectroscopic ellipsometry measurements were carried out on Al-doped ZnO samples, which have different physical properties such as specific resistance and transparency.     

Designed synthesis and magnetic properties of Co hierarchical nanostructures

Size and shape controlled fabrication of magnetic Co microsphere, nanoribbon, nanochain and rose-like microarchitecture has been successfully realized via a simple hydrothermal route. X-ray diffraction analysis suggests that Co hierarchical nanostructures are identified as hexagonal phase. Magnetic hysteresis measurements demonstrate that the obtained different Co hierarchical structures show structure-dependent magnetic properties. Saturation magnetization (M_S) found for Co spherical flowers and spherical powders are larger than Co nanoribbons, smaller than sphere-rebuilt micro particles or chain-like structures. Chain-like and nanorribon structures have abnormally large coercivity (H_C). H_C values of Co nanoribbons and one dimensional chains become as large as 256 Oe and 316 Oe.     

功能碳薄膜化学键成分的椭偏光谱研究

用磁过滤真空离子溅射系统，改变加在单晶Si(100)村底上的衬底偏压．制备不同sp2C键与sp2C键比例的碳薄膜样品。测量了2．0～5．0eV光子能量范围内各个碳薄膜样品的椭偏光谱。发现该谱与碳薄膜中的sp键比例有明显的关系。我们提出一种较简便的分析解谱方法，分析所测得的各个椭偏光谱．半定量地确定各碳薄膜样品的sp3C键与sp2C键比例。所得结果还与同类样品的拉曼光谱与吸收光谱测量结果相比较．结果基本上是一致的。研究结果表明椭偏光谱方法可以发展成一种较简便的、对样品无损伤的测定功能碳薄膜中的sp3C键成分的新方法。     

Linear and nonlinear magnetooptics of planar Au/Co/Si nanostructures

Linear and nonlinear magnetooptics are applied to study the magnetic properties of Au/Co/Si planar nanostructures. The dependence of the nonlinear magneto-optical Kerr effect on the thickness of the cobalt layer, d_Co, reaches a maximum at d_Co ≈ 2 nm. This characteristic thickness is consistent with the formation of surface magnetization. An enhancement of the second harmonic generation (SHG) intensity and of magnetization-induced SHG is attained for an island-like structure of the cobalt layer, and is associated with the excitation of localized surface plasmons in cobalt nanoislands.     

基于光谱干涉椭偏法的薄膜厚度测量

薄膜厚度的测量在芯片制造和集成电路等领域中发挥着重要作用。椭偏法具备高测量精度的优点,利用宽谱测量方式可得到全光谱的椭偏参数,实现纳米级薄膜的厚度测量。为解决半导体领域常见的透明硅基底上薄膜厚度测量的问题并消除硅层的叠加信号,本文通过偏振分离式光谱干涉椭偏系统,搭建马赫曾德实验光路,实现了近红外波段硅基底上膜厚的测量,以100 nm厚度的二氧化硅薄膜为样品,实现了纳米级的测量精度。本文所提出的测量方法适用于透明或非透明基底的薄膜厚度测量,避免了检测过程的矫正步骤或光源更换,可应用于化学气相沉积、分子束外延等薄膜制备工艺和技术的成品的高精度检测。     

Detection and characterization of single nanoparticles by interferometric phase modulated ellipsometry

We introduce a new measurement system called Nanopolar interferometer devoted to monitor and characterize single nanoparticles which is based on the interferometric phase modulated ellipsometry technique. The system collects the backscattered light by the particles in the solid angle subtended by a microscope objective and then analyses its frequency components. The results for the detection of 2 μm and 50 nm particles are explained in terms of a cross polarization effect of the polarization vectors when the beam converts from divergent to parallel in the microscope objective. This explanation is supported with the results of the optical modelling using the exact Mie theory for the light scattered by the particles.     

Studies of optical anisotropy in opals by normal incidence ellipsometry

Anisotropy of thin opal films was studied by ellipsometric technique in a visible spectral range. At normal light incidence, the ellipsometric data were directly related to anisotropy parameters measured by polarization modulation technique. In the (111)-oriented thin films, the optical anisotropy was mainly caused by internal strain-induced birefringence with anisotropy axes oriented along [110] and [-112] directions. The deviation from 180°-symmetry, which has been observed for ellipsometric parameters in the in-plane sample rotation experiments at normal incidence, was enhanced at oblique incidence and assigned to particular properties of opal. Experimental data were discussed in the model of stacked anisotropic layers.     

Accuracy of color determination from spectroscopic ellipsometry measurements

In many coating and display applications, quantitative determination of the sample color is required. Standard procedures exist for converting an experimentally measured reflectivity spectrum into color coordinates such as CIE L*a*b*. In this paper we evaluate CIE L*a*b* color coordinates using a reflectivity spectrum which is calculated from an optical model determined by a spectroscopic ellipsometry (SE) analysis of the sample. The accuracy of the SE-determined color coordinates are compared with traditional color measurements, and the advantages of using SE for color determination are discussed.     

Insight into excimer laser crystallization exploiting ellipsometry: Effect of silicon film precursor

The optical diagnostic of spectroscopic ellipsometry is shown to be an effective tool to investigate the mechanism of excimer laser crystallization (ELC) of silicon thin films. A detailed spectroscopic ellipsometric investigation of the microstructures of polycrystalline Si films obtained on SiO₂/Si wafers by ELC of a-Si:H and nc-Si films deposited, respectively, by SiH₄ plasma enhanced chemical vapor deposition (PECVD) and SiF₄-PECVD is presented. It is shown that ellipsometric spectra of the pseudodielectric function of polysilicon thin films allows to discern the three different ELC regimes of partial melting, super lateral growth and complete melting. Exploiting ellipsometry and atomic force microscopy, it is shown that ELC of nc-Si has very low energy density threshold of 95 mJ/cm² for complete melting, and that re-crystallization to large grains of ∼ 2 μm can be achieved by multi-shot irradiation at an energy density as low as 260 mJ/cm² when using nc-Si when compared to 340 mJ/cm² for the ELC of a-Si films.     

Development of biosensor based on imaging ellipsometry and biomedical applications

So far, combined with a microfluidic reactor array system, an engineering system of biosensor based on imaging ellipsometry is installed for biomedical applications, such as antibody screen, hepatitis B markers detection, cancer markers spectrum and virus recognition, etc. Furthermore, the biosensor in total internal reflection (TIR) mode has be improved by a spectroscopic light, optimization settings of polarization and low noise CCD which brings an obvious improvement of 10 time increase in the sensitivity and SNR, and 50 times lower concentration in the detection limit with a throughput of 48 independent channels and the time resolution of 0.04 S.     

Virtual separation approach to study porous ultra-thin films by combined spectroscopic ellipsometry and quartz crystal microbalance methods

Spectroscopic ellipsometry (SE) and quartz crystal microbalance with dissipation (QCM-D) techniques have been extensively used as independent surface characterization tools to monitor in-situ thin film formation. They provide different information for ultra-thin films because QCM-D is sensitive to the solvent content while SE is not. For using these two techniques in tandem, we present a virtual separation approach to enable the determination of both ultra-thin film thickness and porosity. Assumptions for the intrinsic molecular polarizability (index of refraction n_o) and density (ρ_o) of the organic adsorbent must be made, and the consequences for these parameters' values are discussed.     

Enhanced magnetic field sensitivity of spin-dependent transport in cluster-assembled metallic nanostructures

The emerging field of spintronics explores the many possibilities offered by the prospect of using the spin of the electrons for fast, nanosized electronic devices. The effect of magnetization acting on a current is the essence of giant or tunnel magnetoresistance. Although such spintronics effects already find technological applications, much of the underlying physics remains to be explored. The aim of this article is to demonstrate the importance of spin mixing in metallic nanostructures. Here we show that magnetic clusters embedded in a metallic matrix exhibit a giant magnetic response of more than 500% at low temperature, using a recently developed thermoelectric measurement. This method eliminates the dominating resistivity component of the magnetic response and thus reveals an intrinsic spin-dependent process: the conduction-electron spin precession about the exchange field as the electron crosses the clusters, giving rise to a spin-mixing mechanism with strong field dependence. This effect appears sensibly only in the smallest clusters, that is, at the level of less than 100 atoms per cluster.