New opportunities in optical testing given by photochromic materials Giorgio Pariani, Martino Quintavalla, Rossella Castagna, Letizia Colella, Chiara Bertarelli, Frederic Zamkotsian, Andrea Bianco
Photochromic materials F Change of: F 3 - UV-Vis absorption spectra 2 C H 3 A R 2 - Luminescence H 3 C S S X 1 R 1 X A - Polarizability/Refractive indices 300 400 500 600 700 800 l - Conductivity/charge mobility COLORLESS FORM - Dipole moment - Redox potential. Vis UV F F 3 And: 2 A - No thermal switching X S X S R 1 R 2 1 - Good fatigue resistance B - Fast responce 300 400 500 600 700 800 l COLORED FORM Bertarelli, C., Bianco, A., Castagna, R., Pariani, G., J. Photochem. Photobiol. C: Photochemistry Reviews , 12 (2), 106 – 125 (2011)
Photochromic Focal Plane Masks 100 1 - Photochromic polymer (6%) in PMMA 70 m m thick film 80 0.8 - Contrast 60 0.6 A T % F F F F F F F F 40 0.4 F F F F vis UV 20 0.2 S S S S B m m A B 0 0 400 500 600 700 800 l (nm) - large contrast in a specific spectral 100 range of interest (Ha, Na,...) - filters are needed to cover the entire 80 spectral range Filter1 60 Filter2 - easy to produce (with direct laser T% Mask opaque writing) and easy to use 40 20 0 400 500 600 700 800 Bianco, A., et al., Astron. Nachr., 326(5), 370-374 (2005) l (nm)
Photochromic FPM @ Asiago Telescope Bianco, A., Bertarelli C., Gallazzi, M.C., Zerbi G., Giro, E., Molinari, E., Astron. Nachr., 326(5), 370-374 (2005)
Process POST PROCESS Chemical development DESIGN PRINTING TESTING CGH pattern Irradiation: Vis light Interferometry ERASING Irradiation: UV light make the element re-writable: it is a reconfigurable platform
Interferometry • Amplitude Computer Generated Holograms for optical testing • Photochromic Point Diffraction Interferometer
Computer Generated Holograms J. C. Wyant, 1971-72 Null Test INTERFEROGRAM spherical reference surface CGH aspherical mirror spherical aspherical wavefront wavefront CGH : binary representation of the interferogram between the spherical and aspherical wavefront under test. Each line adds m l of OPD and changes the wavefront slope by sin( q ) = m l / L , L is the local line spacing. L = down to 1 micron REQs: Line period e = 50 nm PV Accuracy Size D = 150 mm
Writing strategies Mask projection Direct Laser Writing raster mode scanning mode Objective Mask Photochromic Photochromic substrate substrate Moving stages • • High pattern density/slow process Fast process/stitching for large areas • • High accuracy Distorsions from projection • • Compex system/very versatile Very versatile (according to the mask)
Amplitude Fresnel CGH – scanning mode Laser Plotter f 20 He-Ne laser shutter beam splitter mirror 1 cm focusing screen focusing lens Spot diameter: 4 m m film Centering precision: 7 m m stages Translation precision: 15 m m 2 Light density: 50 W/mm
Test results th 0 order Fresnel st 1 order Plane reference CGH surface spherical spherical mirror wavefront PV: 1680 nm RMS: 238 nm 3l PV, l /2 RMS CGH Wavefront analysis Interference fringes Fringes are well visible: the produced Fresnel CGH satisfies the basic requirements of optical quality and contrast Other aberrations: some errors are introduced by the film surface and by the accuracy of the written pattern Pariani, G., Bertarelli, C., Dassa, G., Bianco, A., Zerbi, G., Optics Express , 19 (5), 4536 – 4541 (2011)
Production with standard DLW – scanning mode Production of photochromic CGHs at the Institut für Technische Optik - Universität Stuttgart CLWS300M (Production System) Smallest feature sizes: < 1µm (binary) 30 micron Max. substrate size: Ø 300 mm Max. substrate thickn.: 25 mm Write speed (typ.): on-axis: 9 mm/h Autofocus beam in the visible off-axis: 4 mm/h Wavelength: 457-488-514 nm Positioning increment: radial: 0.6 nm azimuthal: 1'' @ 600 rpm Results: - High material resolution, up to the writing beam size f 1 - High beam power may affect the surface - Contrast low for the autofocus beam and wrong writing beam (at the limit of the material sensitivity) A custom DLW machine is required! Pariani, G., Bertarelli, C., Bianco, A., Schaal, F., Pruss, C., Proc. of SPIE , 8450 (1), (2012).
Photochromic Point Diffraction Interferometer Linnik 1933, Smartt 1972 pinhole A B outer region Diffraction from a pinhole in a semi-transparent film • Simple use: PDI is positioned in the focal plane of the optics under test • Common path: very low sensitivity to vibrations and turbulence Quintavalla et al., Opt. Laser Eng. 2014
Why a photochromic PDI? Optical writing of the substrate: • single step process, no post process required • tunable transparency with irradiation time to maximize the fringe visibility 0.9 0.85 0.8 0.75 Visibilita 0.7 0.65 0.6 0.55 0.5 0 50 100 150 200 250 300 350 Tempo di schiarimento (s)
Why a photochromic PDI? Optical writing of the substrate: • single step process, no post process required • tunable transparency with irradiation time to maximize the fringe visibility • wide range of pinhole size to match the optics under test o Pinhole size tunable from 1 to 50 microns o The size depends on the photons dose o Auto-confinement of the beam inside the photochromic substrate
Why a photochromic PDI? Optical writing of the substrate: • single step process, no post process required • tunable transparency with irradiation time to maximize the fringe visibility • wide range of pinhole size to match the optics under test • the pinhole may be written by the test optic! no fine alignments required continous monitoring easily possible works properly for low aberrated optics
Results self-referencing test mirror at 0° 150mm dia., f # 2 spherical mirror 2.5 m m pinhole dia. orientation PtV (nm) RMS (nm) (deg) 0 128±15 27±3 180 126±16 22±3 mirror at 180°
Results comparison with a standard Fizeau inteferometer Fizeau PDI 150mm dia., f # 2 spherical mirror INT PtV (nm) RMS (nm) Fizeau 144±9 29±2 PDI 132±15 26±3 absolute accuracy Certified 150mm dia., f # 8, λ/8 PtV and λ/40 RMS spherical mirror PtV (nm) RMS (nm) SPEC 40 8 PDI 56±17 8.5±2
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