Investigation of polarizing mirrors at 121.6 nm

1. Parameters Spécifications Justification Incidence angle 30° à 70° Limited dimensions of the instrument Angle of acceptance 2.6° (-/+ 1.3°) Field of view of the instrument Rp/Rs 4. 10-3 Good rejection of the perpendicular polarisation Rs min 25% Photometry condition k k n n Polarizing mirror in the LYOT optical design Grazing X-ray reflectometry 1 2 n,k 1 2 Principle [4] n,k,e X ns,ks Al Iso-reflectivity diagrams Intersection of iso-reflectiviy diagrams selected from measurements gives the result: (n, k). Bulk material reflectivity R=f (n,k) k e=5 nm R=0.2 e=‡ R=0.3 Reflectivity of a thin layer of known thickness on Al substrate k o o More than two measurements are necessary. n R(e )= f (n,k,ns,ks) MgF2 Al Al2O3 (<1 nm) MgF2 Al MgF2 Al 1st SMESE Workshop, IAS Paris, 10-12 mars 2008 Investigation of polarizing mirrors at 121.6 nm F. Bridou, M. Cuniot-Ponsard, J-M. Desvignes Laboratoire Charles Fabry de l’Institut d’Optique Palaiseau, France Required specifications Summary From IAS team: A. Millard, F. Auchère, F. Rouesnel, J-C. Vial The goal of this study is to demonstrate the feasibility of polarizing mirrors at l = 121.6 nm designed within the framework of the Lymana Lyot Coronograph Imager project. Optical constants of the materials involved in the multilayers are determined experimentally. Reflectivity measurements in the VUV wavelength domain have been performed at PTB (Synchrotron at BESSY II, Berlin). Simulations, deposits, measurements and characterizations were made and first experimental results are in good agreement with calculated predictions. Experimental techniques Reflectivity measurement at PTB (Bessy II, Berlin) M.Richter, A. Gottwald, U. Kroth Experimental setup for evaporation Thin films are evaporated on glass substrates, 2 cm in diameter, in a UHV chamber equipped with an electron gun and four targets. The initial pressure in the chamber is close to 10-8 mbar. The successive deposition rates and thickness are controlled by a programmable quartz. The grazing X-ray reflectometry allows determining the thickness, interfacial roughness, and complex index (at the source wavelength) of each of the successive films deposited on a substrate [1]. Experimental reflectivity versus wavelength under near normal incidence Preliminary requirements Choice of materials Necessity to avoid oxidization Knowledge of optical constants The 80-120 nm spectral range is characterized by both low transparency and low reflectivity of materials which makes optical measurements specifically difficult : Optical constants are often unavailable or found strongly different from an author to the other. Optical constants of the deposited materials have to be determined before modelization of polarizing multilayers. As it can be seen on this graph, the spontaneous formation a a thin alumina layer upon air contact causes the reflectance to collapse to a value lower than 10% at 120 nm. In this case, a capper layer (as MgF2) is necessary. Based on optical properties: Al (for reflectivity), Fluoride (for transparency) From Palik tables indices[ 2,3 ], calculated reflectivity versus wavelength of pure Al, Al+Al2O3 (2.5 nm),Al+MgF2 (25 nm) Determination of the optical indices from reflectivity measurements Application to MgF2 MgF2 Iso-reflectivity diagram at l = 122 nm kk Reflectivity measurements at PTB under normal incidence with various layer thicknesses Verification: good agreement between the experimental and calculated R() plots when using the thus determined n () and k () n = 1.73, k= 0.04 (k = 0 in Palik tables) Fabrication and test of polarizing mirrors In collaboration with PTB where Polarized reflectivity measurements were performed [5,6] Ongoing development Present development Ongoing development 2 Present development 1 With optimized thicknesses (calculation) MgF2/float glass Multilayer PTB is working to install the set-up on a new dedicated VUV beam line: the MLS (Metrology Light Source). The precision of measurements should be increased. (First measurements are expected in the middle of 2008). Measurement: i = 67°, Rs = 0.45, Rp = 0.023, Rp/Rs = 0.05 Fit : i = 67°, Rs =0.45, Rp= 1. 10-4, Rp/Rs = 2.2 10-4 i = 69° Rs = 0.71 Rp = 0.06 Rp/Rs = 0.085 i = 62° Rs = 0.75 Rp = 1.6 10-3 Rp/Rs = 2.1 10-3 The precision of measurements at PTB is presently not sufficient to give the value of the actual minimum of Rp References CONCLUSION [1] F. Bridou, B. Pardo, J. Optics, 21(4) 183 (1990).  [2]  Handbook of Optical Constants, E.D. Palik, G. Ghosh, (CD Rom), (Academic Press, 1999). [3] http://ftp.esrf.fr/pub/scisoft/xop/DabaxFiles/OptConst_Palik.dat [4] F.Bridou, M. Cuniot-Ponsard, J-M. Desvignes, Opt. Comm, 271 (2007), pp 353-360. [5] A. Gottwald, U. Kroth, W. Paustian, M. Richter, H. Schoeppe, R. Thornagel, F. Bridou, M. Cuniot-Ponsard, J-M Desvignes, 15th International Conference on Vacuum Ultraviolet Radition Physics, Berlin, Germany, 23thJul-3Aug 2007. |6] A. Gottwald, F. Bridou, M. Cuniot-Ponsard,,J-M. Desvignes, S. Kroth, U. Kroth, W. Paustian, M. Richter, H. Shöppe,R. Thornagel, Appl. Opt., 46 (2007), pp 7797-7804. A preliminary determination of indices in the wavelength range 80-140 nm was performed in order to select the materials and design of polarizing mirrors. Two different designs of polarizing mirrors at l = 121.6 nm have been prepared and tested at PTB. The first results show that each of these two designs allows to meet the requirements of the Lyman  Lyot Coronograph Imager project