基于多层膜的软X射线偏振测量

综述了过去10年中德国BESSY同步辐射装置在软X射线偏振测量方面所做的工作。在BESSY同步辐射装置中,有10条椭圆波动器光束线,这可使同步加速器辐射的偏振态从线偏振光(水平或者垂直)转变为左旋或右旋圆偏振光。由于很多偏振敏感实验(例如,MCD光谱测量)需要归一化量,因此对偏振度进行量化非常重要。对于偏振实验,即对光的偏振态测量来说,需要两个光学元件分别起相位片和检偏器作用。因此,专门研制了在软X射线区有透射和反射功能的多层膜,并对其做了优化。通过使多层膜参数(周期,厚度比)与构成材料的吸收边相匹配,即可获得共振加强的偏振灵敏度。由此可知,基于多层膜的偏振测量与这些偏振光学元件工作波长处性能测量密切相关,文中对仪器的设置和测试结果做了介绍,同时给出了磁性薄膜或光活化物质的磁光光谱测量和偏振测量的示例(法拉第和克尔效应)。

Multilayer-based soft X-ray polarimetry

An overview of the soft X-ray polarimetry udertaken at BESSY over the last 10 years is presented. At BESSY, ten elliptical undulator beamlines are operating in the VUV and soft X-ray range, which enables the polarisation state of the synchrotron radiation to be changed from linear (horizontal or vertical) to left- or right-handed circular. It is essential that the degree of polarisation is known quantitatively, since this is a normalization quantity for many polarisation-sensitive experiments (e.g. MCD-spectroscopy). For a polarimetry experiment i.e. the measurement of the polarisation state of light, two optical elements are required acting as a phase retarder and a linear polariser, respectively. In the soft X-ray range, specially tailored multilayers (ML) operating in transmission and in reflection have been developed and optimized for this purpose. By matching the ML-parameters (period, thickness ratio) to an absorption edge of one of the constituent materials, a resonantly enhanced polarisation sensitivity can be achieved. Thus, ML-polarimetry is strongly connected with At-Wavelength Metrology of these polarisation optical elements, for which the instrumentation and results are presented.Examples of magneto-optical spectroscopy and polarimetry to determine properties of magnetic thin films or optically active substances are also presented (Faraday and Kerr effect, L-MOKE).