Investigation polarization characteristics of vertical-cavity surface-emitting lasers

We have improved transmission line laser model to study the polarization characteristics by incorporating into spin-flip model (SFM). Theoretical studies on the influence of the Bragg reflectivity on the polarization switching of vertical-cavity surface-emitting laser (VCSEL) have been discussed in this paper. We have demonstrated that reflectivity of Bragg reflectors can be optimized to provide suitable hysteresis loops width which indicates the tolerance of the input power.     

高双折射光子晶体光纤中均匀布拉格光栅的特性

研究了具有高双折射的光子晶体光纤(HB PCF)中均匀布拉格光栅(FBG)的光谱特性。利用紧凑的超格子模型,对光子晶体光纤的传输特性进行分析,研究正向传输和反向传输的模式之间的耦合规律,从而研究写入光子晶体光纤中的均匀布拉格光栅的特性。首先给出具有C6v对称性的零双折射光子晶体光纤中光纤布拉格光栅的布拉格波长λB随光纤结构参量的变化规律;然后分析一种高双折射光子晶体光纤中的光纤布拉格光栅的光谱特性,高双折射使两个不同偏振态的反射峰分开较大;最后分析了一种常用的双模双折射光子晶体光纤中光纤布拉格光栅的光谱特性,LP01模和LPe11模的两个偏振态对应的反射谱都由于高双折射而分开。     

X-ray resonant magnetic scattering by Fe/Cr superlattices

We have studied the structural and magnetic properties of an antiferromagnetically (AF) coupled Fe/Cr superlattice by means of soft x-ray resonant magnetic scattering. Strong and purely magnetic Bragg peaks are observed at the half-order positions in reciprocal space parallel to the [001] growth direction and in between the structural Bragg reflections from the superlattice periodicity. The magnetic hysteresis loops measured at the first-order and at the half-order Bragg peaks clearly demonstrate the strong AF coupling of the Fe/Cr multilayer. Transverse scans and off-specular reflectivity measurements confirm an AF domain structure of the superlattice in remanence with large perpendicular correlation. In addition, the transverse scan of the half-order Bragg peak exhibits a Lorentzian line shape at zero field, which diminishes in higher fields, indicative of a remanent multidomain state approaching a single-domain state towards saturation.     

Geometrical confinement effects on the magnetization and polarization response in resonant magneto-optic rotator waveguides

Control of magnetization is central to the performance of magneto-optical switches and isolators. Photonic crystal technology on these devices can yield significant improvements in polarization rotation efficiency and an overall reduction in device dimensions. The optical response and field reversal characteristics of resonant magneto-optic polarization rotators fabricated on chip are presented herein and analyzed by micromagnetic simulation. By introducing domain-strip structures into the resonant cavity of Bragg gratings formed on magnetic garnet films, a bi-stable magnetic state is demonstrated and the enhancement of characteristic saturation field is studied. Domain closure loops between the strips affect the hysteresis response in the resonant cavity. Large magneto-optic rotations exceeding 45° are produced near resonance between 1500 and 1580 nm in the stop-bands, although the presence of linear birefringence in these gyrotropic waveguides strongly suppresses the Faraday rotation outside the stop-bands and degrades the linearity of the output polarization.     

X-ray circular dichroism under reflection from antiferromagnetically coupled multilayers

The fluorescence yield under the Bragg reflection of right and left circular polarized radiation with a wavelength in the vicinity of the FeL _{2, 3} absorption edges from the periodic multilayer [Fe(1.5 nm)/V(1.5 nm)]₁₀ that is characterized by the antiferromagnetic interlayer exchange coupling has been theoretically analyzed in L-MOKE geometry. It has been shown that the largest polarization asymmetry of reflectivity takes place at the ??magnetic?? Bragg reflection, which appears due to the doubling of the magnetic period. The expected occurrence of the dichronic signal in the fluorescence yield by its selective excitation in the layers with the same magnetization direction was confirmed, but the effect is suppressed by the specific behaviour of the standing waves in the strongly absorbing multilayers where the standing wave antinodes are situated in layers with low absorption.     

Measuring magnetic profiles at manganite surfaces with monolayer resolution

The performance of manganite-based magnetic tunnel junctions (MTJs) has suffered from reduced magnetization present at the junction interfaces that is ultimately responsible for the spin polarization of injected currents; this behavior has been attributed to a magnetic “dead layer” that typically extends a few unit cells into the manganite. X-ray magnetic scattering in resonant conditions (XRMS) is one of the most innovative and effective techniques to extract surface or interfacial magnetization profiles with subnanometer resolution, and has only recently been applied to oxide heterostructures. Here we present our approach to characterizing the surface and interfacial magnetization of such heterostructures using the XRMS technique, conducted at the BEAR beamline (Elettra synchrotron, Trieste). Measurements were carried out in specular reflectivity geometry, switching the left/right elliptical polarization of light as well as the magnetization direction in the scattering plane. Spectra were collected across the Mn L_2,3 edge for at least four different grazing angles to better analyse the interference phenomena. The resulting reflectivity spectra have been carefully fit to obtain the magnetization profiles, minimizing the number of free parameters as much as possible. Optical constants of the samples (real and imaginary part of the refractive index) in the interested frequency range are obtained through absorption measurements in two magnetization states and subsequent Kramers–Krönig transformation, allowing quantitative fits of the magnetization profile at different temperatures. We apply this method to the study of air-exposed surfaces of epitaxial La_2/3Sr_1/3MnO₃ (0 0 1) films grown on SrTiO₃ (0 0 1) substrates.     

Demonstration of polarization-independent resonant subwavelength grating filter arrays

We demonstrate a two-dimensional (2D) polarization-independent resonant subwavelength grating (RSG) in a filter array. RSGs, also called guided mode resonant filters, are traditionally one-dimensional gratings; however, this leads to TE and TM resonances at different wavelengths and with different spectral shape. A 2D grating can remove the polarization dependence at normal incidence, while maintaining the desirable RSG properties of high reflectivity, narrow passband, and low sidebands without ripple. We designed and fabricated 2D gratings with near-identical responses for both polarizations at normal incidence in the telecommunication band. Ninety percent reflectivity is achieved at the resonant wavelengths.     

Layer magnetization canting in 57Fe/FeSi multilayer observed by synchrotron Mössbauer reflectometry

Synchrotron Mössbauer reflectometry and CEMS results on a [⁵⁷Fe(2.55 nm)/FeSi\break(1.57 nm)]₁₀ multilayer (ML) on a Zerodur substrate are reported. CEMS spectra are satisfactorily fitted by α‐Fe and an interface layer of random α‐(Fe, Si) alloy of 20% of the ⁵⁷Fe layer thickness on both sides of the individual Fe layers. Kerr loops show a fully compensated AF magnetic layer structure. Prompt X‐ray reflectivity curves show the structural ML Bragg peak and Kiessig oscillations corresponding to a bilayer period and total film thickness of 4.12 and 41.2 nm, respectively. Grazing incidence nuclear resonant Θ–2Θ scans and time spectra (E = 14.413 keV, λ = 0.0860 nm) were recorded in different external magnetic fields (0 < B^ext < 0.95 T) perpendicular to the scattering plane. The time integral delayed nuclear Θ–2Θ scans reveal the magnetic ML period doubling. With increasing transversal external magnetic field, the antiferromagnetic ML Bragg peak disappears due to Fe layer magnetization canting, the extent of which is calculated from the fit of the time spectra and the Θ–2Θ scans using an optical approach. In a weak external field the Fe layer magnetization directions are neither parallel with nor perpendicular to the external field. We suggest that the interlayer coupling in [Fe/FeSi]₁₀ varies with the distance from the substrate and the ML consists of two magnetically distinct regions, being of ferromagnetic character near substrate and antiferromagnetic closer to the surface.     

Combined neutron and synchrotron studies of magnetic films

We discuss specular reflectivity and off-specular scattering of neutrons and X-rays from magnetic films. Both these techniques are capable of providing information about the morphology of the chemical and magnetic roughness and the magnetic domain structure. The use of neutrons with polarization analysis enables the spatial distribution of different vector components of the magnetization to be determined, and the use of resonant magnetic X-ray scattering enables magnetization in a compound system to be determined element-selectively. Thus both these methods provide powerful and complementary new probes for studying magnetism at the nanoscopic level in a variety of systems such as those exhibiting exchange bias, giant magnetoresistance, spin injection, etc. We shall illustrate with an example of both techniques applied to an exchange bias system consisting of a single crystal of antiferromagnetic FeF₂ capped with a ferromagnetic Co film, and discuss what has been learned about how exchange bias works in such a system.     

Real-time observation of Bragg grating formation in As2S3 chalcogenide ridge waveguides

Real-time study of the formation of Bragg gratings in monolayer As₂S₃ ridge waveguides is presented. A transverse holographic method is used to record such gratings. The periodic photodarkening of the layer leads to the formation of a Bragg grating with resonant wavelength 1.55 μm. The experiment allowed to control in situ the grating parameters such as the reflectivity, the resonant wavelength (between 1500 and 1630 nm) and the bandpass at −3 dB.     

Design and performance of two-channel EUV multilayer mirrors with enhanced spectral selectivity

In this paper, we present a study on two-channel multilayer mirrors which can operate at two wavelengths in Extreme Ultraviolet (EUV) spectral range. We propose a new method to design two-channel EUV multilayer mirrors with enhanced spectral selectivity. The mirror structure is a stack of two periodic multilayers separated by a buffer layer. We have defined the main parameters which allow adjustment of the distance between different order Bragg’s peak and of wavelength positions of reflectivity minima. Two mirrors have been designed and deposited for solar EUV telescope applications by using this method. The first mirror reflects Fe IX–X line (17.1 nm) and Fe XVI (33.5 nm) lines with attenuation of the He II line (30.4 nm). The second mirror reflects Fe IX–X and He II lines with attenuation of Fe XV (28.4 nm) and Fe XVI lines. Measurements with synchrotron radiation source confirm that, in both cases, for these mirrors, we are able to adjust reflectivity maxima (Bragg peak position) and minima. Such multilayers offer new possibilities for compact design of multi-wavelength EUV telescopes and/or for high spectral selectivity.     

Ce 5d and Fe 3d magnetic profiles in CeH2/Fe multilayers probed by XRMS

The element and electronic shell specificities of X-ray resonant magnetic scattering have been used to investigate the magnetization of Ce 5d and Fe 3d states in [CeH₂(19.6 ?)/Fe(25.4 ?)]*38 a multilayer. We show that the measurement of the magnetic contribution to the intensities reflected at low angles at the Ce L₂ and Fe L_2,3 edges allows us to investigate the profile of the Ce 5d and Fe 3d magnetic polarization. The Fe 3d polarization is found to be uniform across the Fe layer and the Ce 5d polarization appears to be restricted close to the interface with Fe. Received: 22 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Influence of dynamic effects in nuclear resonant Bragg scattering on the accuracy of determination of self-diffusion coefficient in 56Fe/57Fe periodic multilayers

The angular dependences of nuclear resonant scattering are considerably different from respective X-ray reflectometry curves because of some special features of their detection and nuclear decay acceleration (speedup effect) in the conditions of coherent scattering. Model calculations show that the kinematic formula relating the intensity of the nuclear Bragg reflection maximum at different stages of annealing to the diffusion coefficient is inapplicable when the Bragg peak falls near the critical angle.     

X-ray reflectivity data analysis using Bayesian inference: The study of induced Pt magnetization in Pt/Co/Pt

The induced Pt magnetization in a Pt/Co/Pt thin film structure is studied. The normally nonmagnetic Pt acquires a magnetic moment due to the magnetic proximity effect at the Co–Pt interfaces. Element specific Pt structural and magnetic properties are characterized by synchrotron-based resonant x-ray reflectivity and x-ray resonant magnetic reflectivity measurements. An advanced analysis method based on Bayesian inference is used for model fitting of the x-ray data. Using this method, we retrieve the best fit values of material parameters (e.g., thickness, interfacial roughness) from the data. Analysis of x-ray reflectivity data of this specific system shows that the Pt magnetization and Co–Pt interfacial roughness is significantly different between the top and bottom Pt layers, with both values being larger in the top Pt. The successful application of this Bayesian method to study the magnetic and structural properties of a thin film system demonstrates its effectiveness for x-ray reflectivity data analysis.     

Resonant soft x-ray scattering from stepped surfaces of SrTiO3

We studied the resonant diffraction signal from stepped surfaces of SrTiO(3) at the Ti 2p → 3d (L(2,3)) resonance in comparison with x-ray absorption (XAS) and specular reflectivity data. The steps on the surface form an artificial superstructure suitable as a model system for resonant soft x-ray diffraction. A small step density on the surface is sufficient to produce a well defined diffraction peak. We determined the optical parameters of the sample across the resonance and found that the differences between the energy dependence of the x-ray absorption signal, the specular reflectivity and the step-related peak reflect the different quantities probed in these signals. When recorded at low incidence or detection angles, XAS and specular reflectivity spectra are strongly distorted by the changes of the angle of total reflection with energy. The resonant diffraction spectrum is less affected and can be used as a spectroscopic probe even in less favorable geometries.     

低温退火的X射线W/Si多层膜应力和结构性能

W/Si多层膜反射镜在硬X射线天文望远镜中有重要应用. 为减小其应力对反射镜面形和望远镜分辨率的影响, 同时保证较高的反射率, 采用150, 175和200 ℃ 的低温退火工艺对采用磁控溅射镀制的W/Si周期多层膜进行后处理. 利用掠入射X射线反射测试和样品表面面形测试对退火前后W/Si多层膜的应力和结构进行表征. 结果表明, 在150 ℃ 退火3 h 后, 多层膜1级峰反射率和膜层结构几乎没有发生变化, 应力减少约27%; 在175 ℃ 退火3 h后, 多层膜膜层结构开始发生变化, 应力减少约50%; 在200 ℃退火3 h 后, 多层膜应力减小超过60%, 但1级布拉格峰反射率相对下降17%, 且膜层结构发生了较大变化. W, Si界面层的增大和相互扩散加剧是应力和反射率下降的主要原因.     

Pump geometry for resonant and quasi-resonant optical excitation of microcavity lasers

A simple new pump geometry for optical excitation of microcavities and vertical-cavity surface-emitting lasers is presented. The technique circumvents the high reflectivity of the cavity stop band by excitation through the substrate at a large angle of incidence. Under these conditions, the reflectivity of the bottom Bragg reflector is small, and optical pumping at any desired photon energy becomes possible. Experimental results for optical excitation with this new geometry are compared with resonant optical pumping through the cavity mode.     

微腔有机电致发光器件的谐振腔反射镜性能

根据微腔原理运用传输矩阵法对构成微腔有机电致发光器件(MOLED)谐振腔的两个反射镜进行模拟计算并比较,可观察到:随金属反射镜的反射率增大,微腔器件的电致发光(PL)谱的半峰全宽(FWHM)逐渐窄化;峰值逐渐蓝移至设计的谐振峰值520nm处;峰值强度和光谱积分强度逐渐增强。结果表明:金属反射镜反射率越大越好。随DBR反射镜的周期数从1增加到9,EL的峰值均为520nm,半峰全宽逐渐窄化,积分强度逐渐减弱;峰值强度由弱增强再减弱,4个周期时峰值强度最大,所以设计微腔器件时,DBR的周期是一项很重要的参数。DBR反射率太大不利于出光,太小微腔效应小。需要根据制作目的和需要进行合理选择。     

刻蚀光纤布拉格反射器特性的理论分析

从理论上研究了波导耦合型刻蚀光纤布拉格反射器的特性。采用正交模式耦合理论描述其中的光栅辅助正反向模式耦合，用非正交模式耦合理论描述输入输出接口的模式激发，给出了分析耦合波导布格瓣反射器的角析表达式，分析表明刻光张布拉格反应器中存在三种布拉格反应，在频谱中表现为六个分立的反射峰，与实验结果相符，其反射率不仅决定事系数，还与其结构参数，尤其是Ａｌ２Ｏ３层的厚度，Ａｌ２Ｏ３层与光纤芯的距离以及匹配层折射     

Magnetic ordering in a [Fe/Co]<Subscript>35</Subscript> BCC film studied by nuclear resonant reflectometry

An analysis of the angular dependences of nuclear resonant reflectivity time spectra for different models of magnetic ordering in films reveals an ambiguity in the magnetization direction determined from spectra measured at one orientation of the sample. This analysis explains features in the spectra of the nuclear resonant reflectivity from a MgO/[Fe(6 ML)/Co(3 ML)]₃₅/V (1 nm) film measured before and after sample rotation by 90° about the normal to the surface. It is shown that the spectrum measured only at one orientation of the sample determines only the effective azimuth angle of magnetization γ^eff. This does not exclude the occurrence of a domain structure, while the angle γ^eff does not correspond to the true direction of the preferred orientation of magnetization. The results of measurements at two orientations of the sample can be satisfactorily matched using a model that considers a coherent mixture of states with magnetization directed along the 〈110〉 axis (77%) and with a chaotic orientation of the magnetic hyperfine field B _hf in the film plane for the other nuclei.