Achromatic x-ray focusing using diffractive and refractive elements

Abstract

ABSTRACT We develop a new type of X-ray lens system which is achromatic in a limited energy range. For such achromatswe combine dierent types of refractive and diractive e lements. For example, Fresnel zone plates and planarparabolic concave SU-8 lenses are comb ined with lenses with a biconvex parabolic shape and with Fresnel lenses,respectively. We present numerical re sults from a theoretical study of such optical systems. We determine thefocal spot size for an energy range of about E ± E with E/E 17%. Amongst other results we “nd that,compared with conventional lens systems, the spot siz e can be reduced by several tens of percent by using suchachromatic lens systems.Keywords: achromats, compound refractive lenses, CRL, X-ray, zone plates, numerical simulation 1. INTRODUCTION The development of third generation s ynchrotron sources and the improvem ent in micro-system technologies,have simultaneously provoked great advances in X-ray micro-focusing. For many applications of X-rays such asX-ray spectroscopy, dir action, small angle scattering, and imaging, focusing is an issue of primary importancein order to study small objects or to spatially resolve and distinguish all kinds of structures. Thus, in thelast decades dierent chromatic opti cal devices for monochromatic X-rays have been developed using dierentmaterials and various micro-fabrication techniques. Such optical systems are by now successfully working, evenin the hard X-ray range, but they are chromatic. If the same optical device is used for a dieren t energy, theresulting focal length diers, i.e., chromatic aberration occurs. Consequ ently for focusing devices, the focalspot size increases up to several tens of microns for a broad energy band, whereas it can be smaller than onemicron for a discrete energy or a suciently narrow energy band. For the improvement of micro-focus lensesdierent physical phenomena are being used, depending on the “eld of application. For instance, mirrors, whichare especially used in astronomy, as they need a relatively wide working distance, and capillaries, which areprimarily used for illumination, are both based on total re”ection. Fresnel zone plates, which are based ondir action, are widespread in astronomy and synchrotron applications. Compound refractive lenses (CRL),which utilize refraction, are applicable in a notably large energy range (5keV-500keV). In particular they areused for microscopy and spectroscopy purposes. CRL which are used in the hard X-ray range and Fresnel zoneplates require a highly monochromatic beam. Since sever al applications require a variety of X-ray energies oreven tunability, a search for achromatic optical devices has started. Such applications include, e.g. , absorptionspectroscopy, which requires an energy scan within a de“ned energy range, or pink beam applications, such asselective ”uorescence spectrosc opy or anomalous diraction.In the following two alternatives for achromatic X-ray lenses are proposed, which are based on dir action andrefraction. The results of numerical simulations are also presented, which have been performed for an in-depthstudy of the potential performance.