Passive Optical Components: Coating Technology

Transcription

1 Passive Optical Components: Coating Technology Angela Piegari ENEA Optical Coatings Group (Italy) International School on Space Optics, ESA/ESTEC 2-6 October

2 Foreword Passive Components in the optical domain operate to handle (transmit, reflect, filter, split, combine, block, ) the incoming radiation. They are used for all space segments (payload, satellites, launchers) and are found in most satellite subsystems dedicated to Astronomy, Earth Observation, Satellite Navigation, Telecommunication, Health monitoring, Microgravity, etc. The first optical device (photo-camera, NASA) flown is space dates back to the Forties, and today optical components are found everywhere. Space instruments may contain several optical components, belonging to different categories: lenses and windows (flat, spherical, aspheric, cylindrical, freeform surfaces, prisms), often required with coatings on the surface. components dedicated to optical networks (fibers, connectors, couplers, splitters, collimators, attenuators, wavelength division multiplexers). optical filters and mirrors. International School on Space Optics, ESA/ESTEC 2-6 October Corning

3 Optical Components in the School programme The field of passive optical components is extremely vast and only part of it will be covered in this lecture. However, many optical components (both passive and active) are included in the programme of the school. Most lectures will mention or describe optical components, e.g. gratings (Bernd Harnisch Spectrometers), fibers, waveguides (Nikos Karafolas Photonics in Satellite Telecom), lightweight and large mirrors (Roberto Ragazzoni Telescope & Adaptive Optics), etc. This lecture is dedicated to optical components based on thin-film materials and coating technology, which is the most versatile method for filtering the radiation: Optical Filters Metallic and Dielectric Mirrors Antireflection Coatings Beam splitters Polarizers Many types of optical components are available on the market, however not all of them are space qualified and, in several cases, they must be custom designed and fabricated for the specific instrument. International School on Space Optics, ESA/ESTEC 2-6 October

4 Which is the use in space instruments? Role of some passive Optical Components: - radiation limited to a narrow wavelength range should enter the optical instrument: bandpass filters are needed - background radiation, out of the wavelength region of interest, could disturb the observed signal: blocking filters are needed - radiation in a narrow band, or at a single wavelength (laser), should be rejected or attenuated: notch filters are needed - unwanted reflected light from optical surfaces can create ghost images: antireflection coatings are needed International School on Space Optics, ESA/ESTEC 2-6 October

5 Contents Optical materials (bulk and thin films) Design, fabrication, testing of coated optics (coatings are treatments with a primary role of modifying in a desired way the optical properties of surfaces) Examples of application of optical filters in instruments for space missions (ESA Earth Observation) Space environment effects International School on Space Optics, ESA/ESTEC 2-6 October

6 Wavelength range Currently the spectral region for which optical coatings and filters are constructed extends from about 5 nm to 500 µm. The ultraviolet - visible - near infrared spectrum is the most common operating range and many applications are concentrated at such wavelengths. X-rays on the shortwavelength side and microwaves on the long-wavelength side are out of the optical domain International School on Space Optics, ESA/ESTEC 2-6 October

7 Optical materials: properties Complex refractive index (n-ik) n refractive index, k extinction coefficient ( =4 k/ absorption coefficient) The refractive index changes with wavelength: dispersion f( ) and could change with operating conditions (temperature, humidity, ) Short wavelengths (ultraviolet): limited choice because of high absorption below the cut wavelength Long wavelengths (infrared): presence of absorption bands High refractive index materials: TiO2, Ta2O5, HfO2, LaF3, ZnSe, ZnS, Si,... (semiconductors with large dispersion: AlAs, GaAs, AlGaAs, InGaAs..) Intermediate refractive index materials: Y2O3, Al2O3, Low refractive index materials: SiO2, MgF2, CaF2... The material selection is important for both substrates and coatings International School on Space Optics, ESA/ESTEC 2-6 October

8 Optical Substrates Space qualified substrates Flatness wavefront distortion Roughness scattered light Inclusions laser damage Radiation hard change of performance Scratches and digs Parallelism. Some of these requirements are valid also for coated surfaces International School on Space Optics, ESA/ESTEC 2-6 October

9 Optical materials for coatings: dielectrics, metals H. Angus Macleod: Optical Coatings 2007 International School on Space Optics, ESA/ESTEC 2-6 October

10 Interference in layered media Optical Filters are often associated with a wide class of components and can be based on several physical phenomena: Absorption, reflection, holography, diffraction, scattering, interference in thin films, etc. Optical Coatings consist of thin uniform layers of material deposited over a surface to modify its properties. Filters based on absorption Their performance is mainly based on interference of the electromagnetic radiation, even though in some cases is determined exclusively by the properties of materials. Interference in thin films A method that allows almost any optical performance to be achieved, both in terms of spectral and spatial behavior. Optical Coatings Optical Filters International School on Space Optics, ESA/ESTEC 2-6 October

11 Basic definitions Optical interference effects in thin layers, together with the optical characteristics of materials, will ensure the performance of optical coatings Materials: characterized by their refractive index n Layers: characterized by the thickness d (comparable to the wavelength) and the phase thickness δ = 2 n d/λ Reflectance & Transmittance: Simple interface: R = ǀ r ǀ 2 = (n0-n1)/(n0+ n1) 2 (r is named Fresnel amplitude reflection coefficient) Double interface (single layer reflectance): R = (r1 + r2 e-2iδ)/(1 + r1 r2 e-2iδ) 2 n0 n1 n2 d with r1 = (n0-n1)/ (n0+n1), r2 = (n1-n2)/ (n1+ n2) T = 1 R if there is no absorption International School on Space Optics, ESA/ESTEC 2-6 October

12 Multilayer optical coatings Constructive and destructive interference thin films (from 1 to some hundred) substrate (e.g.: glass) For more than two layers is difficult to express R and T with analytical formulas. Their calculation is usually based on the matrix method. from Wikimedia Commons Calculation of coating spectral performance based on Maxwell equations The electric B and magnetic C fields are calculated through a transfer matrix Reflectance R= (η 0 -Y)/(η 0 +Y) 2 Y= C/B H. Angus Macleod: Thin-Film Optical Filters 4 th ed. (CRC Press, New York 2010) q = number of layers, ηj = complex refractive index of the layer j Transmittance International School on Space Optics, ESA/ESTEC 2-6 October

13 Coatings design the detailed theory is not necessary to understand the functioning of coatings. There are a few simple design principles to be accepted. Quarter-wave layers: nd = λ/4 Half-wave layers: nd = λ/2 n d = optical thickness d: geometrical layer thickness substrate International School on Space Optics, ESA/ESTEC 2-6 October

14 Single-layer coatings Single layer on a substrate (periodic spectral behavior) Quarter-wave layers: nfilmd = λ0 /4 nfilm nsub =2 n d/ = /2, if λ = λ0 R λo = [(1- nfilm2/nsub)/(1+nfilm2/nsub)]2 nfilm > or < nsub The same condition holds for odd submultiples of λ0 Half-wave layers: nfilmd = λ 0 /2 =2 n d/ = R λ'o = R substrate = [(1- nsub)/(1+nsub)]2 for any real nfilm Reflectance of a single layer of index n=2.25 or n=1.38, on a glass substrate of index 1.52 The low index layer is a potential antireflection coating The high index layer could be used as a beam splitter at a single wavelength International School on Space Optics, ESA/ESTEC 2-6 October

15 Components commonly used in space applications Antireflection coatings Single wavelength Wideband High reflection coatings Narrow wavelength range Large spectrum Filters Edge (short- or long-wave pass) Bandpass Narrow band Dichroic Notch Beam splitters Induced transmittance AR coating on glass Variable filters International School on Space Optics, ESA/ESTEC 2-6 October

16 Antireflection Coatings: single wavelength d Air n o = 1 Fluoride n film =1.38 Glass n sub = quarter-wave layer (nd = /4) Uncoated glass R=0 if nfilm = nsub MgF2 film 2 quarter-wave layers Uncoated glass n1 n2 nsub R=0 if n2/n1 = nsub (V-coat) International School on Space Optics, ESA/ESTEC 2-6 October

17 Antireflection Coatings: more complex solutions To improve the overall performance, non-quarter-wave layers are often needed SiO 2 n 1 =1.45 Ta 2 O 5 n 2 =2.15 glass n=1.52 Low index High index Structured coatings An ideal AR coating can be obtained with a layer with the refractive index decreasing from the substrate index to the air index. two non quarter-wave layers: multiple solutions (single wavelength) glass four non quarter-wave layers (wideband AR) Low High Low High blue: 1.67 high 0.71 low, green: 0.31 high 1.32 low (optical thickness in terms of quarter-waves) AR coating Uncoated glass 500 nm AR plasma-etched structures on PMMA (U.Schulz - in Optical thin films and coatings Eds. A. Piegari, F. Flory - Elsevier, ) International School on Space Optics, ESA/ESTEC 2-6 October

18 Mirrors: metal Metal layers as front surface mirrors from H.A. Macleod: Optical Coating Design 2014 Metal layer > 100 nm Various types of substrates depending on the application substrate R= ǀrǀ2 = 1 - nmetal 1 + nmetal 2 nmetal = n ik (low n, high k values) (1 - n)2 + k2 R= (1 + n)2 + k2 Aluminum has high reflectance in the ultraviolet spectrum. International School on Space Optics, ESA/ESTEC 2-6 October

19 Mirrors: overcoating Protection of front surface mirrors from H.A. Macleod: Optical Coating Design 2014 Aluminum λ 0 = 550 nm oxide metal substrate Protective layers (oxides, nitrides, etc ) are often used to avoid metal degradation in operating conditions. Attention should be paid to the possible interaction between the protective layer and the metal. International School on Space Optics, ESA/ESTEC 2-6 October

20 Mirrors: high reflectance Enhanced Aluminum mirror from H.A. Macleod: Optical Coating Design 2014 H L H L Metal Glass λ 0 = 550 nm Four layers of low and high index over the metal layer International School on Space Optics, ESA/ESTEC 2-6 October

21 All-dielectric Mirrors High reflectance coatings (q.w.layers) nh = 2.15 nl = 1.45 glass nsub=1.52 Quarter-wave high reflectance coatings: at 0 the coated substrate can be represented by a single refractive index Neff R = [ (1-Neff)/(1+Neff) ]2 Neff = Neff = nh nh 2 X nl nl nh. nh nl.. number of layers nsub 2 x 1 nsub even odd odd number of layers with external H Dielectric mirror (quarter-wave): 5 and 19 layers Glass/ HLHLHLHLHLHLHLHLHLH /Air H,L n (H,L) d (H,L) = 0 /4, 0 =600 nm The maximum reflectance Rλ0 increases with the number of layers and with the index contrast. The bandwidth is function of the two indices. International School on Space Optics, ESA/ESTEC 2-6 October

22 Broadband mirrors A single quarter-wave stack cannot cover a wide spectrum because the index contrast is insufficient from H.A. Macleod: Optical Coating Design 2014 Glass/ HLHLHLHLHLHL.H L H L H L H L H L H L..H /Air ref. wavelength: 0 0 An alternative way is the progressive increase or decrease of layer thicknesses International School on Space Optics, ESA/ESTEC 2-6 October

23 Interference Filters Types of optical filters: Edge (short- or long-wave pass) Bandpass Narrow band Notch Dichroic Beam splitters Induced transmittance. from H.A. Macleod: Optical Coating Design rd order 1 st order ( λ 0 ) International School on Space Optics, ESA/ESTEC 2-6 October

24 Edge filters Short-wave pass and long-wave pass filters Edge filters are used also as dichroic filters, to separate two regions of the wavelength spectrum (long-wave pass are preferred to short-wave pass) Quarter-wave stack with thickness of external layers divided by 2 (external: nd = 0 /8) Short-wave pass filter (19 layers; external L/2) T(%) R(%) L: MgF 2 H: TiO 2 0 = 600 nm Long-wave pass filter (21 layers; external H/2) T (%) R(%) Glass/L /2 HLHLHLHLHLHLHLHLHL /2/Air Glass/H /2 LHLHLHLHLHLHLHLHLHLH /2/Air Improvement of performance can be obtained by software with an optimization process International School on Space Optics, ESA/ESTEC 2-6 October

25 Short pass filters: optimization Pass band Stop band from H.A. Macleod: Optical Coating Design 2014 Higher order suppression In the case of the quarter-wave stack the interference condition that exists at λ 0 also exists at λ 0 /3, λ 0 /5, λ 0 /7, and so on, leading to the higher order reflectance zones that limit its usefulness for some applications. The higher orders can be suppressed using more than two materials with a structure based on the sequence LMHML, or even with only two materials using non quarter-wave layers. A further possibility is given by inhomogeneous layers with a refractive index changing along the thickness (rugate filters). 3rd order Filter with 85 non q.w.layers and 2 materials (nh = 2.15, nl =1.45) International School on Space Optics, ESA/ESTEC 2-6 October

26 Narrow-band transmission filters Fabry-Perot filters Narrow-band transmission (Fabry-Perot) filters single cavity: central half-wave layer Glass/HLHLHLHLH 2L HLHLHLHLH/Air Glass/LHLHLHLHL 2H LHLHLHLHL/Air 19 layers L:SiO 2, H:Ta 2 O 5 0 = 600 nm double cavity: two half-wave layers Glass/HLHL2HLHLH L HLHL2HLHLH/Air International School on Space Optics, ESA/ESTEC 2-6 October

27 Multiple cavity filters For obtaining a broad-band-pass filter, it is sufficient to combine a shortwave pass with a longwave pass filter This type of dielectric filters will be used in the example of application International School on Space Optics, ESA/ESTEC 2-6 October

28 Induced transmission filters The induced transmission filter is obtained by canceling the reflectance of a metal layer by matching its refractive index with the surrounding media, with the aid of dielectric stacks on both sides of the metal Glass/ (...HLHL)L M L (LHLH )/Air Optical constants of metals at λ0 =550 nm Metal n k k/n Ag (Schultz) Ag (Palik) Al Ni Cu from H.A. Macleod: Optical Coating Design 2014 The out-of-band rejection improves with a higher ratio k/n of the metal This type of filter will be used in the example of application International School on Space Optics, ESA/ESTEC 2-6 October

29 Oblique Incidence Snell s law : n 0 sin 0 = n 1 sin 1 The refractive index will be modified as function of the incidence angle (and polarization) n 1 s = n 1 cos 1, n 1 p = n 1 /cos 1 0 n 0 n 1 1 d glass The path differences reduce 1 = 2 n 1 d cos 1 / (phase thickness) from H.A. Macleod: Optical Coating Design 2014 Null p-reflection at the Brewster angle: B = arctg (n 1 /n 0 ) for a single interface International School on Space Optics, ESA/ESTEC 2-6 October

30 Oblique incidence effects Curve shift (towards shorter wavelengths) Curve modification (typically related to the polarization state s o p) T(%) Fabry-Perot filter Long-wave pass filter 0 = 30, 45 pol: s R(%) 4-layer wideband AR coating s p 0 0 = 45 pol: s, p International School on Space Optics, ESA/ESTEC 2-6 October

31 Oblique incidence: cone angle Collimated and convergent beam Transmittance of a narrow band filter at different incidence angles, with a collimated beam (orange curve) compared to the transmittance with an oblique convergent beam (blue curve). International School on Space Optics, ESA/ESTEC 2-6 October

32 Beam splitters The beam splitter divides a beam in two parts and is usually tilted so that the incident and reflected beam are separated (at a single wavelength or over a wide band). Polarization could be an issue. Metal or dielectric layers are used T R Wideband beam splitter (65/35) with 2 dielectric materials air/hll/glass from H.A. Macleod: Optical Coating Design 2014 International School on Space Optics, ESA/ESTEC 2-6 October

33 Separates or combines the polarizations Polarizers Polarizers separate or combine the polarizations based on: material properties (e.g. birefringence, metamaterials, etc.), component structure (e.g. chiral structure, grating, etc.), thin film interference (coatings) From Wikimedia Commons Wideband Cube Polarizer Coating Separates the s- from p-polarized light, effective inside a cube made of two prisms, commonly with Angle Of Incidence = 45 Degree of polarization in transmission & reflection: Pol = (Tp-Ts)/(Tp+Ts) Pol = (Rs-Rp)/(Rs+Rp) p λ 1 Thin-film plate polarizer (θ 0 =45 ) at a single wavelength: 33 layers (nl=1.45, nh=2.25) s the p-reflectance disappears completely at the Brewster angle in the two index materials of the multilayer coating, which is achieved if the incident medium has a high index (e.g.glass) International School on Space Optics, ESA/ESTEC 2-6 October

34 Absorption The absorption can create problems in some applications. A typical example is the laser damage that will occur in coatings with absorption greater than few ppm. It depends mainly on material properties but also on the fabrication process. R + T + A + S = 1 R = Reflectance T = Transmittance A = Absorption S = Scattering When there is absorption in the filter or in the substrate, the front reflectance will be different from the back reflectance, while the transmittance is the same from both sides. In some cases it could be useful to have, inside the optical instrument, surfaces that can absorb any light that falls on them to minimize the amount of stray light. Example: R (%) OG 570 is a Schott glass absorbing below 550 nm. H. Angus Macleod Black absorber coating made of metal and oxide materials, with a reflectance < 3.5% from 300 to 1700 nm ( ) International School on Space Optics, ESA/ESTEC 2-6 October

35 Undesired effects Stray light H.A. Macleod International School on Space Optics, ESA/ESTEC 2-6 October

36 Further developments Pixelated coatings directly deposited on each single pixel of the detector using microfabrication techniques Metamaterials Microscope image of a 5-pixel matrix of multispectral Fabry-Perot filters, with different pixel sizes, and spectral transmittance (F.Pradal OIC2016) Compensation of phase distortion caused by non uniform coating thickness H.A. Macleod International School on Space Optics, ESA/ESTEC 2-6 October

37 Fabrication techniques Thin film deposition techniques (PVD and CVD) - The most widely diffused at commercial level are: electron beam evaporation and sputtering (with or without ion assistance) Electron Beam Evaporation Ion Assisted Deposition Ion Plating Magnetron Sputtering Ion Beam Sputtering Atomic Layer Deposition each technique has pros and cons. Dual Ion Beam Sputtering (DIBS) Electron beam evaporation with ion assistance (e-iad) «Optical Coatings» Angela Piegari Poltu Quatu 18 May 2017 Pag. 37 International School on Space Optics, ESA/ESTEC 2-6 October

38 Issues in coating manufacturing During the design, the real material properties (not from the literature) should be used to avoid discrepancies with the experimental results. The effects of thickness (and index) variation should be simulated in advance to study the stability of the coatings against fabrication errors. H. Angus Macleod The fabrication process is often more complicated than the theoretical prediction; in addition to material properties other factors must be taken into account (as defects, uniformity, etc.). International School on Space Optics, ESA/ESTEC 2-6 October

39 Measurement of performance Accurately measuring the performance of an optical thin-film coating can be as challenging as designing and manufacturing it. Understanding measurement techniques and uncertainty when specifying and procuring optical coatings is important for optical system designers. H. A. Macleod International School on Space Optics, ESA/ESTEC 2-6 October

40 Narrow band filters for space instruments Two examples with different characteristics (developed in the frame of ESA projects) 1. Spatially variable filter (for an imaging spectrometer) - wavelength shift of the transmission peak over few mm critical issues: high variation of performance over small dimensions, wide operating spectrum (VIS-NIR with extension to SWIR) ESA: ULTRA-COMPACT MEDIUM-RESOLUTION SPECTROMETER FOR LAND APPLICATIONS 2. Very narrow band filter (for a lightning imager) - high uniformity over large dimensions (>100 mm) critical issues: very narrow bandwidth ( 1 nm) at a defined wavelength, operation at oblique incidence ESA: LIGHTNING IMAGER LARGE AREA NARROW BAND FILTER DEVELOPMENT Fabrication techniques and measurements methods will be shortly described together with each example International School on Space Optics, ESA/ESTEC 2-6 October

41 1. Imaging Spectrometer for Earth Observation Polar sun-synchronous orbit at an altitude of 700 km Compact image spectrometer with a variable narrow-band transmission filter coupled to an array detector Each line of a two-dimensional array detector, which is equipped with a variable narrow-band filter (transmission band displaced to different wavelengths along the surface), will detect radiation in a different pass band. Replacing classical optical components (prisms, gratings) with a spatially variable filter allows the construction of a spectrometer with reduced size and weight and with no moving parts. Telescope The compact spectrometer is not limited to Earth observation, but is also useful for planetary missions Detector Filter International School on Space Optics, ESA/ESTEC 2-6 October

42 Linearly variable filter for spectrometry The variable narrow-band transmission filter is combined with the array detector by either direct deposition on the CCD or on a separate glass substrate The spatial variation is required along only one direction, the other is uniform Data cube This optical sensor is the core element of a compact low-mass spectrometer for hyper-spectral imaging On a 2D detector the spatial and spectral information are recorded simultaneously during its movement t International School on Space Optics, ESA/ESTEC 2-6 October

43 Filter specifications and design The variable filter shows a narrow-band transmittance which peak wavelength is displaced over its surface The filter is coupled to a CCD detector Extended spectral range: nm, Spectral resolution: 10 nm CCD detector Variable filter VIS-NIR (2 mm length, not to scale) Induced Transmission filter: Ag - SiO 2 Ta 2 O 5 (21 layers) Back-side blocking filter: SiO 2 Ta 2 O 5 (38 layers) Spectral gradient: 250 nm/mm IT Filter Substrate Blocking filter The transmittance curve is displaced over the filter surface, by a variation of the coating thickness with a linear gradient (IT filter in the VIS-NIR: min thickness 1000 nm, max 2500 nm) International School on Space Optics, ESA/ESTEC 2-6 October

44 wavelength (nm) thickness (nm) Peak-wavelength and thickness gradient An alternative variable filter can be made with adjacent uniform stripes Layer thickness d = q (λ 0 /4n) (q is related to the required variation of λ 0 ) d max /d min =(1000/400) (n 400 /n 1000 ) d The thickness gradient over the surface can be easily calculated from the required peak wavelength gradient or combining several variable filters peak gradient distance (mm) thickness gradient distance (mm) 10 silica SiO 2 tantala Ta 2 O 5 A sketch of the OVIRS-NASA five-segment LVF assembly (K.Hendrix,OIC2016) A. Piegari, A. Krasilnikova Sytchkova, J. Bulir, Variable transmission filters for spectrometry from Space 2.Fabrication process,, Applied Optics 47, (2008) C151-C156 International School on Space Optics, ESA/ESTEC 2-6 October

45 Masking apparatus for the fabrication of graded coatings Masking blade moved during film deposition Coating profile controlled by mask speed substrate r.f.sputtering plant Fixed shield to cover part of the substrate, for any adjacent filter Movable mask Uniform area for optical monitoring Reflectance spectral curve (blue) as appears on the computer screen, during the monitoring process, compared with the theoretical reflectance curve (red) Variable filter International School on Space Optics, ESA/ESTEC 2-6 October

46 Localized Transmittance measurements Optical tests carried out by a dedicated set-up: Characterization range: nm 2-D translation micrometric system: min step 25 μm Spectral resolution: < 2 nm Spatial resolution: < 20 μm Transmittance (%) 70 Variable area of the IT filter Scan track mm LINEARITY/G13TS40 Lineare (LINEARITY/G13TS40) LVF spectral dispersion 59.5nm/mm y = x R = Blocking filter fit Transmittance (%) Band # band position Transmission measurements Linearity LVF basic structure sample G13TS Wavelength (nm) Linearity Wavelength (nm) Wavelength (nm) Wavelength (nm) International School on Space Optics, ESA/ESTEC 2-6 October

47 2. Lightning Imager The Lightning Imager is an instrument of METEOSAT (MTG), for the study of lightning phenomena in the atmosphere In order to operate during daylight hours or in the presence of stray light, a filtering technology must be integrated into the imager to isolate the signal generated by lightning Monitoring of lightning activities on Earth is an essential element in the Weather Prediction The strongest emission features in the cloud top optical spectra are produced by the neutral oxygen and neutral nitrogen lines. The Oxygen line triplet is located between and nm Filter requirements Operating wavelength range: nm Transmission bandwidth: 0.45 nm 160 mm In-band Transmission: 0.8 Out-band Transmission: 10-4 Dimensions: 160 mm diameter 30 mm A. Piegari, A. Sytchkova, I. Di Sarcina, M. L. Grilli, S. Scaglione, Optical transmission filters for observation of lightning phenomena in the Earth atmosphere, Applied Optics 50, (2011) C100-C105 International School on Space Optics, ESA/ESTEC 2-6 October

48 Filter optical requirements The central band (0.45 nm) must be transmitted in the operating conditions of the instrument Solar Rejection Filter Narrow Band Filter Objective: 6 lenses Detector Optical layout T Filters and AR coatings are needed The wavelengths of interest must pass through the narrow-band filter at all incidence angles in the range +/- 5.5 A very narrow passband (< 1 nm) would not allow the transmission at all angles double-cavity filter (51 layers, TiO2/SiO2, bandwidth 1 nm) with the incidence angle from 0 (black) to ±5.5 degrees (red) International School on Space Optics, ESA/ESTEC 2-6 October

49 transmittance (%) Narrow- and broad-band filters A wider filter bandwidth would be adequate Fabry-Perot double cavity 35 layers (SiO 2 - TiO 2 ) or 51 layers (SiO 2 - HfO 2 ) bandwidth FWHM = 3 nm The out-of-band spectrum close to the pass band is rejected by the narrow-band transmittance filter itself, while to reject the radiation over the whole operating range ( nm) an additional blocking filter is needed design: 70 layers wavelength (nm) Measured transmittance of the complete filter over a wide -spectrum International School on Space Optics, ESA/ESTEC 2-6 October

50 Manufacturing challenges Precise spectral positioning Bandwidth accuracy High uniformity (diameter mm) The experimental results are often worse than the calculated values owing to manufacturing errors. Effect of random errors of 0.1% and 1% on all layer thickness International School on Space Optics, ESA/ESTEC 2-6 October

51 Masking apparatus for large area coatings Ion beam sputtering deposition Profiled mask to improve uniformity, designed by software 160 mm Mask Substrate Fabricated mask Second ion source (Ion assistance) First ion source (Sputtering) International School on Space Optics, ESA/ESTEC 2-6 October

52 Optical testing: mapping Ad-hoc apparatus for transmittance measurements is required control of the performance over a large surface high spectral resolution Measured 3D profile (transmittance-peak wavelength over the surface) for a filter manufactured without mask Setup for mapping the transmittance over the whole surface of large optics International School on Space Optics, ESA/ESTEC 2-6 October

53 Typical tests Environmental durability Changes induced by the environmental conditions on the characteristics of materials used in components for space instrumentation, during its lifetime, can be responsible for the failure of the optical performance of the system. Mechanical resistance adhesion, abrasion, humidity... Thermal cycling (cryogenic temperature) Exposure to ionizing radiation: gamma rays, protons, etc. Solar irradiance Cross-sectional TEM image of a Mo/Si multilayer coating, (a) before and (b) after proton bombardment (M.G.Pelizzo et al. Opt Exp. 2011). Laser damage associated with contamination in vacuum Damaged morphology in defect sites of clean (a) and contaminated (b) samples (J.Shao, App Surf. Sci 2013) Silver mirrors flown on MISSE-7 showing particulate contamination and haze near its center Impact crater on a MISSE-6 silver mirror (C.Panetta, OIC2013) International School on Space Optics, ESA/ESTEC 2-6 October

54 Temperature variations Changes in temperature lead to variations in the final performance of optical components due to both coating and substrate modification Variation of the refractive index of different glass types as function of temperature (-100, +140 C), at 587 nm. Predicted characteristics of a band-pass infrared filter on different substrates, after a temperature increase of 100 C H.A. Macleod Refractive index of a TiO 2 film vs temperature (A.Sytchkova-OIC2016) International School on Space Optics, ESA/ESTEC 2-6 October

55 Thermal cycling Cryogenic temperature measurements Typical measurement conditions Thermal cycling test Eight thermal cycles: 333 K to 203 K Level of one hour, slope 2K/min Transmittance of a metal-dielectric filter and a silver mirror measured in vacuum, by an online spectrometer inside the cryostat during the thermal cycle, at several temperatures Cryostat T&R measurements Wavelength range : nm Temperature range: K (cryostat) K (heated vacuum chamber) Internal pressure: 10-4 Pa Stress failure due to vacuum thermal cycle of a space optic (D. Wernham - Elsevier) International School on Space Optics, ESA/ESTEC 2-6 October

56 Proton irradiation Low-energy protons are expected to be necessary to prove the effects on the coating, whereas the high-energy proton tests shall verify mainly the substrate susceptibility to induced damage. The behavior of thin-film multilayer structures, irradiated with protons, can be simulated by the Monte Carlo method (SRIM) The flux of low-energy protons is usually higher than the flux of high-energy protons. High-energy protons 30 MeV (with total fluence of 10 8 p + /cm 2 ) Low-energy protons 60 kev (with total fluence of p + /cm 2 ) Typical values ion range of 30 MeV protons, in the target High-energy protons in a glass substrate come at rest at 4.5 mm from the surface The coating thickness is typically 1-2 µm and the protons energy is lost mostly in the substrate. Low-energy protons come at rest in nm. International School on Space Optics, ESA/ESTEC 2-6 October

57 oxygen vacancies (at/cm 3 ) Low energy protons (60 kev) Monte Carlo simulations for a thin-film filter Energy is lost in the first layers of the coating structure In the bare substrate, 60 kev protons are stopped within 800 nm Target: UV narrow-band filter (29 layers: HfO 2 /SiO 2 ) on fused silica substrate During the collisions, the protons are able to displace the atoms of the target from their equilibrium position. Vacancies are created in some layers. 2.5x x D distribution of protons inside the coating (the left grey area corresponds to the coating surface) Distribution (trajectory) of protons in the first 14 layers (640 nm) of the 29-layer filter Distribution of Oxygen vacancies in the SiO 2 layers of the filter that could be related to material changes 1.5x x x depth (nm) International School on Space Optics, ESA/ESTEC 2-6 October

58 Transmittance (%) Transmittance (%) Transmittance (%) Experiments on bulk material and thin films Protons 60 kev, total fluence p + /cm 2 UV transmittance decrease, in the fused silica substrate Decrease and shift of transmittance peak, in the UV narrow-band filter Irradiated samples: Substrates Single layers UV filters No variations in single layers of HfO before irradiation after irradiation Wavelength (nm) Transmittance of the fused silica substrate before and after irradiation: changes in the UV Measured transmittance of a narrow band filter (29 layers HfO 2 /SiO 2 ) before and after low-energy proton irradiation Wavelength (nm) before irradiation after irradiance Wavelength (nm) The peak change may be due to modification of the SiO 2 material. No changes detected in VIS-NIR. I. Di Sarcina, M.L. Grilli, F. Menchini, A. Piegari, S. Scaglione, A. Sytchkova, D. Zola, "Behavior of optical thin-film materials and coatings under proton and gamma irradiation," Applied Optics 53, (2014) 314A-320A International School on Space Optics, ESA/ESTEC 2-6 October

59 Transmittance (%) Transmittance (%) Gamma irradiation Example of required conditions Radiation The coating shall withstand a total radiation dose of 20 Krad, (gamma) accumulated during a mission lifetime of 7.5 years. Rack with 60 Co sources Low inclination LEO: 1-10 Gy/year High inclination LEO: Gy/year Samples irradiated at different total doses up to 5 MGy HIGH DOSE: 1.8 MGy Sample: Sup Total Dose: 1.79MGy Wavelength (nm) before irr. after irr. 10 Gy = 1 krad Transmittance of a fused silica substrate before and after irradiation: a decrease can be noticed at short wavelengths 100 radioisotope source (ENEA-Calliope plant) Maximum dose rate (rack center): 5000 Gy/h LOW DOSE: 50 Gy (5 krad) Wavelength (nm) Filter 19 Filter 19 after irr Metaldielectric filter: no detected change International School on Space Optics, ESA/ESTEC 2-6 October

60 transmittance transmittance irradiance (watt m -2 nm -1 ) Solar irradiance testing Tests with the beamline of the synchrotron light source (BEAR), which delivers photons in the energy range 3 ev 1600 ev ( nm), and covers the DUV (deep ultraviolet) and UV region of the electromagnetic spectrum E E-4 sun BEAR substrate wavelength (nm) 11 hrs as The ultraviolet irradiation induces in the fused silica substrate the formation of a broad absorption band, in the wavelength range nm 1E wavelength (nm) Irradiated Samples at the synchrotron beamline Fused silica substrate HfO 2 thick layer (1 µm) UV narrow band filters (HfO 2 /SiO 2 ) HfO 2 film wavelength (nm) HfO 2 13 hrs No changes detected in the hafnia films. as International School on Space Optics, ESA/ESTEC 2-6 October

61 Absorbance (a.u.) absorbance (a.u.) Substrate modifications The solar irradiation and the irradiation with high-dose gamma rays (>1 MGy) induces a decrease in the transmittance of the fused silica substrate at short wavelengths (< 400 nm). Such decrease can be explained with the formation of so-called E -centers. In fact the absorbance of the irradiated substrate, calculated from the measured transmittance, can be well fitted in the spectral range nm, by combining two Gaussian curves centered at two wavelengths λ 1 and λ 2 typically associated with the formation of E -centers nm 213 nm nm nm wavelength (nm) Gamma rays: substrate absorbance wavelength (nm) UV solar photons: substrate absorbance International School on Space Optics, ESA/ESTEC 2-6 October

62 Space environment Main environmental components of space that can have an impact on optical coatings: D.Wernham in Optical thin films and coatings Eds. A.Piegari, F.Flory (Elsevier, 2013) More information in the lecture by Dominic Doyle: The Space Environment and its Effects on Optics MISSE on the ISS - photo NASA Many interesting experiments on material behavior are carried out directly on the International Space Station: MISSE (Materials International Space Station Experiment) and this is the best way to study synergic effects, even though more expensive than experiments on the ground. «Optical Coatings» Angela Piegari Poltu Quatu 18 May 2017 Pag. 62 International School on Space Optics, ESA/ESTEC 2-6 October

63 Further reading H. A. Macleod, (2010), Thin-Film Optical Filters (4th ed.). CRC Press, Boca Raton, London, New York. J. A. Dobrowolski, (2010), in M. Bass (Ed.) Handbook of Optics, Vol. 4 (3rd ed.) McGraw- Hill, New York, article 7. N. Kaiser, H. Pulker, (Eds.) (2003), Optical Interference Coatings, Springer Series in Optical Sciences, Vol. 88. Springer-Verlag, Berlin, Heidelberg. A. Piegari, F. Flory, (Eds.) ( ), Optical Thin Films and Coatings. Elsevier, Cambridge. The Optics Encyclopedia, John Wiley and Sons Acknowledgments Some slides are courtesy of Angus Macleod Other slides are from works made under ESA contracts International School on Space Optics, ESA/ESTEC 2-6 October

64 Angela Piegari Senior Scientist Angela Piegari has been working for 30 years in the field of optical materials and coated optics for several applications, at ENEA (Italian National Agency for New Technologies, Energy and the Sustainable Development) where she has been in charge of the Optical Coatings Laboratory. She collaborates with many European Institutes as well as with International Organizations, and is member of various technical committees. She acted as Topical Editor of the journal Applied Optics of the Optical Society of America and recently edited a book on Optical Thin Films and Coatings (Elsevier ). International School on Space Optics, ESA/ESTEC 2-6 October