Testing X-ray Gratings for Polarization Sensitivity

Transcription

1 1 Testing X-ray Gratings for Polarization Sensitivity Hannah Marlowe, 1 In collaboration with: Randall McEntaffer, 1 Casey DeRoo, 1 Drew M. Miles, 1 James H. Tutt, 1 Christian Laubis, 2 Victor Soltwisch, 2 Leonid Goray 3 1) University of Iowa, 2) Physikalisch-Technische Bundesanstalt (PTB), 3) I.I.G., Inc.

2 2 OUTLINE Off plane grating overview Confusion in literature regarding polarization response Recent theoretical results Empirical Measurements Model adjustment and comparison of theory and experiment Blazed Gratings

3 James Tutt 3 Casey DeRoo Randy McEntaffer Ryan Allured - SAO Tom Peterson Ben Donovan Drew Miles Jake McCoy

4 4 EXTERNAL COLLABORATORS PTB Beamline at BESSY II Frank Scholze Christian Laubis Victor Soltwisch I.I.G. inc Leonid Goray

5 5 MCENTAFFER TALK WEDNESDAY Randy McEntaffer Looking Forward to Future Soft X-ray Spectroscopy Missions Session on Missions I Wednesday 14:15-14:50

6 6 OFF-PLANE DIFFRACTION GEOMETRY Off-plane (conical) diffraction: light incident nearly parallel with grooves and diffracted into a cone Compared to in-plane where light is incident perpendicular to the groove axis Preferable packing geometry than inplane and high throughput and resolution in soft X-ray regime

7 POLARIZATION STUDY MOTIVATION Significant confusion in the literature regarding whether off-plane gratings exhibit strong polarization sensitivity Goray (G05): theoretical predictions for strong polarization sensitivity in off-plane Seely et al (S06): measured differences in efficiency for TM vs TE for blazed off-plane gratings (though not in agreement with model) Goray & Schmit (G10): more complex calculations no longer support strong polarization response We measured the polarization response of laminar (non-blazed) gratings and compare with model predictions

8 LAMINAR GRATINGS Au-coated, 6033 gr/mm developed at UIowa Details presented in McEntaffer et al Figure 3 (top) and 5b (bottom) in McEntaffer et al Grating efficiencies have previously been measured only in TM orientation We re-measured efficiencies at both TM and TE orientations at BESSY II synchrotron

9 MODELING WITH PCGRATE PCGrate software calculates efficiencies for diffraction gratings of various groove profiles and orientations in- and off-plane Input wavelength, groove profile shape, spacing, rms roughness, orientation Same SW used in previous studies by S06 and G05 Example groove profile input

10 PTB BEAMLINE Physikalisch-Technische Bundesanstalt (PTB) soft X-ray beamline at the BESSY II synchrotron in Berlin Highly linearly polarized in the plane of the storage ring Samples mounted in vacuum at arbitrary position relative to polarization angle using Ellipsometer Unique facility able to measure full off-plane diffraction

11 BEAMLINE TESTS Graze (º) Yaw (º) Energy (ev) Step (ev) Orders TE Measurements between ev ( nm) steps of 50 ev Incident light parallel to the groove direction at grazing incidence of 1.5º Oriented with E field to plane of incidence (TM) and rotated about beam axis 90º (TE) TM

12 MEASURED EFFICIENCIES No significant difference observed between polarizations Marlowe et al. 2016

13 PREDICTIONS VS. MEASUREMENT Measurements show no significant difference between TM or TE orientation despite predictions Able to replicate TE response well, but not TM Similar issue in S06 result Following discussions with the developers of PCGrate, these discrepancies appear to be due to model limitations

14 MODEL LIMITATIONS For the conical mount case, PCGrate v.6.1 relies on the Invarince theorem to solve efficiencies Representation of conical diffraction in the simpler in-plane geometry Only strictly valid in the case of perfectly conducting gratings G10 integral method calculations including finite conductivity for blazed gratings in the off-plane mount Calculation results are similar for TE compared to the perfect conductor case, but very different for TM Polarization dependence is no longer predicted with this treatment PCGrate calculations for the off-plane case assuming perfect conductivity describe TE efficiency well, but not TM

15 15 MODEL CORRECTIONS Newer version of PCGrate (v.6.6) is able to carry out finite conductivity calculations, but with some drawbacks.. Calculation time is ~4 times longer Not able to apply rms surface roughness approximations in a straightforward way (need to run Monte Carlo simulations) We don t really want to replace our software version if we don t have to G10 showed that finite conductivity results could be approximated via reflectivity coefficients with good agreement For grazing incidence, TM efficiency can be calculated from the reliable TE results using reflectivity coefficients for a given material/incidence angle

16 REFLECTIVITY COEFFICIENTS Coefficients of reflectivity derived from Fresnel equations Describe reflected fraction of incident light with complex index of refraction, n(ω) = 1- δ+iβ Rs/Rp 1 at shallow graze angles for our energies

17 Marlowe et al CORRECTED MODEL Perfect Conductivity Model Corrected Via Reflection Coeffs (Rtm/Rte very close to unity at 1.5º) Consistent with measurements

18 COMPARISON WITH FINITE CONDUCTIVITY MODELS Rigorous results over-plotted with reflectivity coefficient correction Marlowe et al Care of I.I.G.

19 BLAZED GRATINGS Verify results with blazed gratings Already have TM measurements of blazed gratings at BESSY in 2011 Pt-coated, holographically ruled gratings by Jobin Yvon Not current state of the art AFM measurements carried out at Upenn Input groove profiles extracted from AFM measurements into PCGrate 169 nm groove period 16 degree blaze Tutt et al. 2015

20 20 BLAZED MEASUREMENTS Carried out at BESSY synchrotron Similar measurements to laminar grating, but only taken at the TM orientation Measured -1 st, 1 st, 0 th order efficiency Oriented at Littrow, then Anti- Littrow configurations Compare efficiency to predicted PCGrate efficiency with new correction using multiple real groove profiles extracted from AFM

21 BLAZED GRATINGS Models match observed TM response at both orientations Tutt et al. 2015

22 PERFECT CONDUCTIVITY TM Tutt et al. 2015

23 CONCLUSIONS No measured polarization sensitivity for 6033 gr/mm laminar gratings in ev regime Supports more recent modeling results including finite conductivity PCGrate v.6.1 calculates only TE efficiencies well in the extreme off-plane mount at shallow graze angles Corrections may be made using reflectivity coefficients for the surface material and graze angle to obtain TM response Agrees well with finite conductivity model (and ~4 times shorter calculation time) Correctly predicts current measurements of laminar gratings and blazed gratings in TM orientation

24 24 THANK YOU! Acknowledgements: We would like to acknowledge the support of the University of Iowa Office of the Vice President for Research as well and the College of Liberal Arts and Sciences. This research as additionally supported by a NASA Earth and Space Science Fellowship (NNX13AM14H ), the NASA Roman Technology Fellowship (NNX12AI16), and an Astrophysics Research Advancement Grant (NNX13AD03G). The commercial software PCGrate-S(X) v.6.1 (I.I.G. Inc.). References 1) Leonid I. Goray, Proc. SPIE 5168, Optics for EUV, X-Ray, and Gamma-Ray Astronomy, 260 (2005) 2) J. F. Seely, L. I. Goray, Benjawan Kjornrattanawanich, J. M. Laming, G. E. Holland, K. A. Flanagan, R. K. Heilmann, C.-H. Chang, M. L. Schattenburg, and A. P. Rasmussen, Appl. Opt. 45, (2006) 3) Leonid I. Goray and Gunther Schmidt, J. Opt. Soc. Am. A 27, (2010) 4) McEntaffer, R. L., DeRoo, C., Schultz, T., Gantner, B., Tutt, J., Holland, A., O Dell, S., Gaskin, J., Kolodziejczak, J., Zhang, W. W., Chan, K.-W., Biskach, M., McClelland, R., Iazikov, D., Wang, X., & Koecher, L., Experimental Astronomy, 36, 389, 17 pages, 2013.

25 BACKUP SLIDES

26 4µm 26 Future of grating fabrication at Uiowa: e-beam lithography Graduate students Casey DeRoo and Jake McCoy are currently using e-beam lithography to fabricate gratings Have demonstrated laminar gratings on 5 mm X 5 mm field Next step will be to complete procedures to etch blazed grating profile NEW GENERATION GRATING MANUFACTURE E-beam (not SEM) image of 16 µm 2 FoV 4µm

27 PREVIOUS MEASUREMENTS S06 presented PCGrate modeling and measurements for 14º blazed, Au-coated gratings with 5000 grv/mm Good agreement with TE predictions, but not TM Still report significant polarization dependence Mismatch suggested to be due to imperfect knowledge of grating groove profile or profile variations over illuminated area (Figure 9 in Seely et al. 2006) Goddard Space Flight Center October 7, 2015

28 PREDICTED RESPONSE Goddard Space Flight Center October 7, 2015

29 31 FEM MODEL Calculations using the FEM code JCMwave Goddard Space Flight Center October 7, 2015

30 S06 RESULT IS NOT PREDICTED BY THIS METHOD The expected TM efficiencies for S06 using reflectivity coefficients would be very close the TE results (Adapted from Seely et al. 2006) TM result is not predicted well by PCGrate or by reflectivity corrections from the TE efficiencies Follow up with measurement of a blazed grating for more direct comparison SPIE Optics + Photonics: Synergy between Laboratory and Space - August, 2015