Transcript Investigation of polarizing mirrors at 121.6 nm

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 Lyman a Lyot Coronograph Imager project.
Parameters
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.
Polarizing mirror in the LYOT optical design
Justification
Spécifications
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
Experimental techniques
Grazing X-ray reflectometry
Experimental setup for evaporation
nm
Point
Layer
thickness
Substrate0
MgF1
*10
roughness
°°0
33.8
2
indice
0.3
1.4
Reflectivity measurement at PTB (Bessy II, Berlin)
M.Richter, A. Gottwald, U. Kroth
6
absorption
6.49
8.804
0.299
0.156
0.7
MgF2/Al
Jobin-Yvon sample
LCFIO sample
log(exR)
log(fR)
5
log (reflectance) a.u
Reflectvity ( measured)
6
4
3
2
0.6
0.5
0.4
0.3
0.2
1
0.1
aa1451
0
80
0
1000
2000
3000
4000
90
100
110
120
130
140
Wavelength (nm)
5000
grazing angle (sec)
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
Reflectivity (calculated)
Al (for reflectivity),
Fluoride (for transparency)
Al + Mg2F2 (25 nm)
Al
0.6
The 80-120 nm spectral range is characterized by
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.
0.8
Based on optical properties:
Knowledge of optical constants
Al + Al2O3 (2.5 nm)
0.4
both low transparency and low reflectivity of materials
which makes optical measurements specifically difficult :
In this case, a capper layer (as MgF2) is
necessary.
0.2
Optical constants are often unavailable or found strongly different
from an author to the other.
0.0
40
60
80
100
120
140
160
Wavelength (nm)
Optical constants of the deposited materials have to be
determined before modelization of polarizing multilayers.
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
Principle [4]
Application to MgF2
MgF2
Iso-reflectivity diagrams
Intersection of iso-reflectiviy diagrams selected from measurements gives the result: (n, k).
0.8
0.9
0.7
Reflectivity (measurement)
0.5
0.4
k
0.3
n,k
0.2
0.1
n,k,e
o
o
38.3 /float-glass
39.9 /Al
33.6 /Al
24.6 /Al
12.5 /Al
0.4
k
0.4
0.2
0.0
80
100
120
140
60
Wavelength (nm)
0.5
0.4
0.3
0.2
More than two measurements are necessary.
0.1
Calculations with:
Indices of "Palik"
Indices of "Palik"+ 0.8 nm Al2O3
Indices given by this method
+ 0.8 nm Al2O3
0.6
0.2
0.6
k
Mesurement MgF2 (24.6 nm) /Al
0.8
k
k
MgF2thickness (nm)
60
n
0.9
0.8
0.7
2
ns,ks
k
0.6
0.0
R(e )= f (n,k,ns,ks)
X
e=
‡ R=0.3 e=5 nm R=0.2
n
Reflectivity of a thin layer
of known thickness
on Al substrate
Al
1.0
0.6
2
1
Iso-reflectivity diagram
at l = 122 nm
l  122 nm
R=f (n,k)
1
Reflectivity
Bulk material reflectivity
80
100
120
140
Wavelength (nm)
n
Reflectivity measurements at PTB
under normal incidence with various
layer thicknesses
Verification: good agreement between
the experimental and calculated R(l) plots
when using the thus determined n (l) and k (l)
n = 1.73, k= 0.04
(k = 0 in Palik tables)
n
Fabrication and test of polarizing mirrors
In collaboration with PTB where Polarized reflectivity measurements were performed [5,6]
Present development
1
1.0
With optimized thicknesses (calculation)
Measurements
Rs
, Rp
Fit of measurements
Rs , Rp
Rs,Rp
0.6
0.4
PTB is working to install the setup on a new dedicated VUV beam
line: the MLS
(Metrology Light Source).
Mes
i = 67°
Rs= 0.45
Rp=0.023
Rp
0.2
0.0
0
20
40
60
The precision of measurements
should be increased.
(First measurements are expected
in the middle of 2008).
80
Incidence angle (deg)
Multilayer
1.0
0.8
Mesurements
0.8
Simulation
0.6
Al2O3 (<1 nm)
Rs
MgF2
Al
MgF2
Experimental values
i= 69°
Rs = 0.71
Rp = 0.058
0.4
0.2
0.6
i = 62°
Rs = 0.75
-3
Rp = 1.6 10
0.4
0.2
Al
Rp
0.0
0.0
20
40
60
80
0
20
40
60
80
Incidence angle (deg)
Incidence angle (deg)
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
Réflectivity Rs, Rp
Initial calculation
Rs, Rp
0.8
Al
Ongoing development
Present development
2
Rs, Rp
MgF2/float glass
MgF2
Ongoing development
Rs
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
CONCLUSION
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 a Lyot
Coronograph Imager project
References
[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.