Advanced x-ray multilayer waveguide optics

Abstract

The aim of this thesis is to demonstrate that x-ray waveguide optics can be gen- eralized from a single guiding film to an array of planar waveguides, enabling more complex and controllable phenomena of field propagation both in partic- ular in the near-field in the vicinity of objects to be probed by coherent imaging. Two advanced x-ray multilayer waveguides (MWGs) structures, i.e. the waveg- uide array (WGA) and the multi-guide resonant beam couplers (RBCs) are de- signed and discussed. Starting from basic theoretical analysis, the structural model of MWGs is built up. Then the MWGs are studied in detail by numerical simulations based on finite-difference (FD) simulations, fabricated with preci- sion methods for controlled layer thickness, and finally characterized experi- mentally by phase retrieval methods. Chap. 1 introduces the basic theory of x-ray waveguides (see section 1.1), and3 presents geometries and mechanisms of different coupling devices (see section 1.2). FD simulations and phase retrieval methods are discussed to characterize the field propagation in the near-field and far-field (see section 1.3). In the final section 1.4, the fabrication processes of multilayers and MWGs are presented. Thereafter, the x-ray beams exiting from MWGs are characterized by x-ray re- flectivity and far field measurements with focused synchrotron radiation. Chap. 2 presents the concept of WGA, using the WGA structure with Mo/C multilayer at 19.9 keV photon energy. The controlled variation in guiding layer thickness is introduced to achieve the desired phase shifts between the guided output beams. The FD simulations show that multi-beam interference with the desired phase shifts can lead to a quasi-focal spot sized sub-50 nm in free space behind the waveguide. Chap. 3 uses two iterative phase retrieval algorithms to reconstruct the near- field distribution behind tailored WGA and - for comparison - simpler periodic waveguide multilayers (WGM) with N i /C multilayer structure for 13.8 keV pho- ton energy. These are shown to yield distinctly different near-field patterns. Im- portantly, the WGA also exhibits the desired secondary quasi-focal spot outside the structure. Chap. 4 presents the coupling of finite (sub-μm) x-ray beams into RBCs with three guiding layers in the [N i /C ] 3 /N i structure. Using especially resonant mode excitation, more than one reflected beams are generated with different beam offsets along the RBC surface constituting an exceptionally large Goos- Hänchen effect. Possible applications of such devices are beam splitters for co- herent imaging and interferometry. Chap. 5 summarizes the outcomes of this thesis, and discusses future applica- tions and investigations of the WGA and multi-guide RBCs structures.