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Calendar: ALS Calendar
Title: ALS-CXRO Special Seminar: Ralf K. Heilmann
When: 09/ 5/2014 10:00 AM - 11:00 AM
Description: “High-Efficiency broadband transmission grating for soft x-rays
and EUV photons: The critical angle transmission grating”

Presented By: Ralf K. Heilmann, Space Nanotechnology Laboratory, MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology

Date: Friday September 5 @ 10:00am
Location: 2-400F Conf Room

Transmission gratings have a number of advantages over reflection gratings in certain applications, especially in the soft x-ray and EUV bands: Certain alignment and figure tolerances can be relaxed by orders of magnitude, it is relatively easy to cover large apertures, they have low mass (easier thermal control, space applications), they are polarization insensitive, and they provide a straight-through zero-order beam (imaging). However, diffraction efficiency (due to absorption) and the ability to efficiently blaze in higher orders (for higher spectral resolving power) have been lacking. Recently developed critical-angle transmission (CAT) gratings combine the advantages of transmission and reflection gratings into an efficient (~ 50% broadband diffraction efficiency) blazed transmission grating. The CAT grating principle is based on grazing incidence reflection off the smooth sidewalls of freestanding ultra-high aspect-ratio silicon grating bars. I will review the design considerations for CAT gratings in the context of space-based large-aperture x-ray telescopes and objective grating spectrometers. Previously fabricated prototypes made by wet etching in KOH showed diffraction efficiencies in the range of 50-100% of theoretical predictions, but suffered from low throughput due to support structure blockage. We have recently fabricated ~10 cm2 high-throughput CAT gratings via a combination of anisotropic dry and wet etching techniques that are now ready for x-ray testing.
Location: 2-400F