CMB S-4: Broadband Optics
July 12, 2016
Version 1.0
Zeeshan Ahmed, Jeff McMahon, Aritoki Suzuki, Joaquin VieiraSLAC, University of Michigan, University of California Berkeley, University of Illinois Urbana
Champaign
Contents1 Solicitation List 1
2 Introduction 2
3 Requirements 2
4 Lens Material 3
5 Half-Wave Plates 35.1 Achromatic Half-Wave Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45.2 Sapphire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6 Anti-Reflection Coating 86.1 Thermal Spray Anti-Reflection Coating . . . . . . . . . . . . . . . . . . . . . . . 96.2 Epoxy Anti-Reflection Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106.3 Si Grooved AR Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
7 Filters 147.1 Metal Mesh Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147.2 Si Substrate Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
8 Research and Development for CMB S-4 25
9 Conclusion 25
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1 Solicitation List
Topics POC Author StatusIntroduction Jeff M Jeff McMahon Draft In - Waiting for CommentRefractive Lens MaterialSilicon Jeff M TBD Waiting for DraftAlumina Zeesh A Ki-Won Yoon Waiting for DraftPolarization ModulatorAHWP Intro Toki S Charlie Hill Draft In - Waiting for CommentSapphire HWP Toki S Charlie Hill Draft In - Waiting for CommentGrooved Si HWP Jeff M Michigan Waiting for DraftWiregrid Pol Mod Jeff M Goddard/Hopkins Waiting for DraftAnti-Reflection CoatingEpoxy Toki S Oliver Jeong Draft In - Waiting for CommentThermal Spray Toki S Oliver Jeong Draft In - Waiting for CommentDice Grooved Si Jeff M Kevin Coughlin Draft In - Waiting for CommentPlastic Sheet Joaquin V Andrew Nadolski Waiting for DraftLaser Grooved Anything Joaquin V Shaul Haneny? Waiting for DraftFilterMetal Mesh Filter Zeesh A Lorenzo Moncelsi Draft In - Waiting for CommentSilicon Substrate Filter Jeff M C. D. Munson Draft In - Waiting for CommentPlastic Filter Zeesh A Zeesh A Waiting for DraftAlumina Filter TBD TBD Waiting for DraftRT-MLI TBD TBD Waiting for Draft
Table 1: Lists of solicited topics
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2 IntroductionBroad-band antirefrection (AR) coated lenses are required for nearly all of the currently proposedCMB-S4 optical designs including: small aperture refracting telescopes and large angular scaletelescopes using reimaging lenses. Similar optical coatings can also be used to realize efficienthalf-wave plate polarization modulators which could dramatically improve the ability of S4 tomeasure polarization on the largest angular scales. In this note we review the requirements for thesecoatings, discuss the state of the art, and outline the next steps required ready these technologiesfor the CMB-S4 project.
3 RequirementsTelescope Bandwidth, Mapping speed, Material loss
Multiple design studies have found that high index of refraction lenses (n & 3) are required forre-imaging optics to realize large fields of view on > 3 m telescopes (what can we reference, askNiemack, Halverson, and others for ideas) and to maximize the number of detectors per telescopein refractors (eg BICEP 3). At the same time broad-band detector designs have evolved suchthat 2:1 ratio bandwidth detectors have been deployee, 3:1 ratio bandwidth detectors will soon bedeployed, and even broader bandwidths are envisioned.
Requirements on lens material
• Index (covered by this paragraph)
• Loss
• Size, diameter, available thickness
• Thermal conductivity
• Mechanical strength - should not crack
• Cost (?)
Requirements on AR coating
• Index - ability to hit/control index, stability of index over frequency band
• Low loss (absorption and scatter)
• Application over curved surface
• No delamination
2
4 Lens MaterialSilicon, Alumina, and Sapphire represent the naturally occurring materials which have high indexof refraction, low dielectric losses, and for which coatings are currently under development. Thesematerials have tradeoffs that drive their use for different applications. Sapphire has extremely lowdielectric loss (tanδ . 10−4), is available in single crystal pieces up to 510 mm in diameter, butis birefringent. This last property makes it suitable for wave plates, but not lenses. Silicon has ahigh index of refraction (n = 3.4), extremely low dielectric loss (tanδ < 7 × 10−5), but is onlyavailable in pieces up to 46 cm in diameter. Alumina also has a high index of refraction (n = 3.15),reasonably low dielectric loss (tanδ < 1 × 10−4), and has the advantage that it can be fabricatedas a single piece for parts up to 1 m in diameter. These differences in performance and availabilitydrive the applications of these materials in different optical systems. For example, ACTPol whichrequires lenses 33 cm diameter and below uses silicone to take advantage of its machinability andlow loss, while BICEP3, SPT3G and Polar bear use Alumina since they require larger diameterlenses. More details on the materials properties are presented in Appendix A.
5 Half-Wave Plates
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Figure 1: The modulation efficiency and phase for various AHWP stacks, referenced to the mod-ulator’s central frequency. Increasing the number of plates increases the polarization efficiencyand decreases the phase variation across an increasing bandwidth. Various percent bandwidths areshown for reference: 2:1 (dash), 3:1 (dot), and 5:1 (dash-dot).
5.1 Achromatic Half-Wave PlateEven though a half-wave plate (HWP) is naturally a narrow-band instrument whose thicknessis tuned to preserve linear polarization at a single frequency, a Pancharatnam achromatic HWP(AHWP) can preserve linear polarization over a broad range of frequencies, hence making AHWPspractical polarization modulators for multi-frequency CMB polarization experiments [1].
An AHWP consists of an odd number of identical “single HWPs” stacked in an optimized ori-entation [2]. Incoming light with linear polarization fraction Pin is rotated by twice its polarizationangle with respect to the principle axes of the AHWP plus a frequency-dependent phase φ(ν), andwith a frequency-dependent modulation efficiency ε(ν) [3]
∆θ = 2θin + φ(ν) ; ε(ν) =Pout(ν)
Pin
. (5.1)
The modulation efficiency and phase for various AHWP stacks is shown in Figure 1, referencedto the central frequency of the modulator, which is set by the thickness of the identical “singleHWPs” [4]. A greater number of plates gives increased polarization efficiency and decreasedphase variation across a larger bandwidth.
AHWP technology is relatively new to CMB polarization experiments. EBEX flew a 4K 5-stack sapphire AHWP during an 11-day balloon observation of 150, 250, and 410GHz in 2012/2013.Though the EBEX AHWP successfully completed ∼1 million revolutions during the flight, dataanalysis is still ongoing and the impact of the AHWP on data quality is still under investigation[5, 6]. Simons Array will deploy a 300K 3-stack sapphire AHWP on PB2a to observe at 90 and150GHz starting in 2017, marking the first demonstration of a CMB AHWP from the ground.Though in-lab characterization of the PB2a AHWP looks promising, demonstration in the field isyet to be achieved [7].
Primary considerations of AHWP implementation for CMB-S4 include large-diameter bire-fringent plates, broadband anti-reflection coatings, mitigation of increased absorption and thermal
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emission due to multi-plate stacks, and control of frequency dependent effects such as modulationefficiency and phase [7]. Though these requirements are challenging, each is already being activelyaddressed within the CMB community.
Various birefringent materials—including sapphire, meta-material silicon, and metal-mesh substrates—have been suggested for large-diameter AHWP design [8, 9, 10]; various anti-reflection techniques—such as laser-ablated sub-wavelength structures, thermal-sprayed ceramic, and epoxy—have beensuggested for large-bandwidth AHWP construction [11, 12, 13]; cryogenic rotation stages are cur-rently being developed to facilitate AHWP cooling and suppress thermal emission [7, 14]; andhardware and analysis techniques have been developed to control AHWP frequency-dependent ef-fects [15, 3]. Lessons from EBEX data analysis, from in situ characterization of the PB2a AHWP,and from HWP R&D associated with other broadband CMB experiments such as LiteBIRD willhelp define the role and construction of AHWP polarization modulators for CMB-S4 [16].
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Sapphire HWPs: Metamaterial Silicon HWPs:
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Figure 2: 512mm-diameter sapphire window cut from a 200kg ingot of HEM sapphire grown atTuizhou Haotian Optoelectronics Technology in China
5.2 SapphireHalf wave plate (HWP) polarization modulators are becoming increasingly common in CMBexperiments [17, 18, 4, 5, 19] due to their effectiveness at suppressing 1/f noise and mitigat-ing temperature-to-polarization leakage [4, 20]. In order to implement HWPs, CMB-S4 needsbirefringent windows (i.e. large diameter, small thickness) offered at reasonable prices and leadtimes. Sapphire is an appealing HWP candidate due to its low loss tangent (tanδ = 10−4 at 300K,tanδ < 10−6 at 50K) and large differential index (no ≈ 3.1, ne ≈ 3.4) at microwave frequencies[21]. Despite its alluring characteristics, sapphire is difficult to manufacture at large diameters andhigh purities [22]. Fortunately, industry techniques are evolving such that sapphire HWPs may bepractical for S4 experiments.
The Heat Exchanger Method (HEM) is the standard growth technique for large sapphire boules[23]. GHTOT (in China) is now reaching > 500mm diameters while achieving low levels ofimpurities and crystal defects via their Advanced HEM method [24]. Arc-Energy has developed aControlled Heat Extraction System (CHES) furnace which controls seed orientation during HEMgrowth to push beyond 500mm [25]. Despite its successes producing HWPs for stage 3 CMBexperiments [26], the HEM process is limited by its omnidirectional nature and inherent thermalgradients.
In reaction to demand for larger windows, industry is developing alternative sapphire growthtechniques. The edge-defined film-fed growth (EFG) method aims to create windows duringgrowth rather than via post-process machining by drawing the crystal through shaping aids [23].The Clear Large Aperture Sapphire Sheets (CLASS) line of EFG products at Saint-Gobain crystalsreach 300mm , where Kyocera (in Japan) can go up to 200mm but is pushing larger [27, 28].
In the event that single-crystal growth does not meet its diameter and purity requirements,CMB-S4 can turn to other sapphire solutions, including composite plates. For example, sapphirebonding is a common technique that can be pushed to large diameters for low-stress applications[29]. Combining the power of precision dicing and novel bonding techniques may further accom-modate large fields of view in S4 optical systems.
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6 Anti-Reflection CoatingSilicon grooving, thermal spray, epoxy, meta material
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Figure 3: Tunability of dielectric constant for plasma spray AR technologies. Dielectric constantof an alumina-based coating is controlled by mixing hollow microspheres (Red) and/or varyingplasma energy with different spray parameters (Blue), such as flow rate of plasma gas.[31].
6.1 Thermal Spray Anti-Reflection CoatingPlasma spray AR is a process by which a base material of alumina and silica are melted with aplasma jet and sprayed onto a lens surface, cooling immediately upon impact to form a stronglyadhered coating without the need for any glues or adhesion promoters. The ability to tune thedielectric constant by varying porosity within the coatings, as shown in Figure 3, and the low loss-tangent (tan δ < 10−3 at 140 K) of plasma sprayed coatings allow for an AR coating with therange of dielectric constants for broadband multi-layer application. Furthermore, it is technicallysimple, requiring no additional processes than spraying. Due to matching coefficient of thermalexpansion between the alumina-silica coatings and alumina optics, plasma sprayed AR coating isrobust against cryogenic delamination. Additionally, spraying with the robotic arm allows for afast and simple programmable spraying technique on a variety of surface profiles, whether theybe flat, large curved lenses (∼ 700 mm diameter), or a large array of small hemispherical lenslets(∼ 6.35 mm diameter). The SPT-3G receiver will be deploying with its 720 mm diameter infraredfilter and lenses, multi-layer AR coated using plasma spraying for high optical throughput andbandwidth to cover 90, 150, and 220 GHz bands. [30] For broadband AR coating, it is desirebleto have dielectric constant as low as 1.8. The lowest dielectric constant currently achieved by theplasma spray technique is 2.6. Stage-3 experiment combined plasma sprayed layer with porousteflon sheet (εr = 1.6) to create broadband AR coating. It would be desireble to have plasma sprayAR coating that covers necessary dielectric constant range.
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Figure 4: Tunability of dielectric constant for epoxy-based AR technologies. Dielectric constantof an epoxy-based dielectric coating is controlled by mixing different Stycasts and SrTiO3.[35]
6.2 Epoxy Anti-Reflection CoatingEpoxy-based dieletric AR is a technology which uses a negative mold to coat a lens surface anda CNC mill to cut the coating to the correct thickness. The tunable dielectric constant, as shownin Figure 4, and low loss-tangent of Stycast-based coatings allows for broadband multi-layer ARcoatings. Loss-tangent of epoxy and epoxy-filler mixture increases with frequency, that it can beused as low-pass infrared filter[32]. Stress relief grooves are required on epoxy AR coating dueto thermal constraction mismatch between epoxy and alumina lens. Epoxy can be laser machinedfor narrow groove width to prevent scattering. Epoxy-based dielectric AR coating is applied withsingle-layer on the 600 mm diameter infrared filter and lenses of the 95 GHz BICEP3 receiver [33]and will be deploying with multi-layer on the 500 mm diameter lenses and infrared filters of thePOLARBEAR-2a receiver covering 95 and 150 GHz bands [34].
Loss-tangent of epoxy and strontium titanate mixture is high for application of AR coating. Itis necessary to develop high dielectric constant filler with low loss. Currently, the epoxy techniquerequires a complicated and laborious process of coating and machining, limiting its applicabilityfor high volume fabrication. Furthermore, epoxy-based AR coating require a large CNC to ma-chine large diameter lenses. Highly capable machine center should be studied for scalability ofthis technique.
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6.3 Si Grooved AR Coating
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Metamaterial Silicon AR coatingsKevin Coughlin, Charles Munson, Rahul Datta, Jeff McMahon
Description of technology Metamatieial AR coatings are fabricated by cutting sub-waevlength features into surfaces of refractive optical elements. At Michigan we havedeveloped a capability to fabricate these coatings on silicon optics using a custom three-axis silicon dicing saw. This system allows us to produce micron accurate arrays of square-based stepped pyramids. By tuning the fill factor we can create layers that behave a simpledielectric sheets with an index of refraction between that of silicon and vacuum. Multi-layer coatings can be realized by cutting progressively thinner grooves at greater depthwhich are centered in the wider first layers. We have created a number of full scale opticswith near zero defect rate, and the quality of our process is continually being refined.
Demonstrated performance and metrics Thus far we have deployed 12 full scalelenses on the ACTPol and the AdvACT experiments and we have delivered a set of lensesto the PIPER experiment. The largest lens yet produced is 33 cm in diameter, but there isno restriction in fabricating lenses up to the maximum diameter available for single crystalsilicon. We have fabricated these coatings on both concave and convex surfaces. Thedemonstrated bandwidth of these coatings is in excess of an octave for three layer coatings.In addition, we have the ability to achieve wider bandwidth with the proven three layercoatings by trading reflection performance for bandwidth. In the current coatings weachieve ∼ 0.1% average reflections in two CMB bands. If we designed for 1% refleciton wecould achieve 3 : 1 ratio bandwidth with three layers. We have designed and prototypedthe fabrication of a five layer coating that would cover 75-300 GHz. This proof of principleshows that 4:1 bandwidth is possible with these coatings. We will complete the secondside of this five layer prototype in the coming months to confirm its performance. Widerbandwidth (5:1) is possible if we optimized with out the restriction of the blades we hadin stock.
Challenges in scaling to S4 The primary challenge for applying this technology to S4is reducing the time it takes to AR coat a single lens. Currently, fabricating the coatingstakes roughly two weeks per lens. Based on our experience with the existing first generationAR coating machine we have a preliminary design for a new machine which would automatea number of the time intensive set up tasks. We forecast that with this machine we couldreduce the time to fabricate a coating on a lens to 1-2 days. The key features of thisnew machine would be: (1) rotation stage to change the orientation of the the lens, (2)automated metrology to acquire lens positioning after mounting and rotations, (3) multipleindependent spindles set up with different dicing blades to minimize time intensive bladechanges. This system would make it practical to fabricate the large number of lensesrequired for S4.
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Figure 1: The performance of meta-material silicon AR coatings on fabri-cated on lenses that have been or willsoon be fielded including: the ‘MF’(90/150 band) lenses for ACTPol, the‘HF’ (150/220 band) lenses for Ad-vACT, and the PIPER 200 GHz lenses.Simulations are shown as dashed linesand measurements are shown as solidlines. The MF and HF lenses use threelayer AR coatings while the PIPERlenses use a single layer coating. A pre-liminary design for a five layer coatingis shown in gray dashed line. A proto-type of this five layer coating has beenfabricated on one side of a 25 cm di-ameter test wafer.
The next potential challenge is in fabricating lenses larger than 46 cm. The issue herehas to do with the availability of large silicon rather than limits of the machine. Thereforewe discuss this challenge in the note about silicon properties.
R&D path forward The most important R&D issue is to build a new machine anddemonstrate a speed up of the fabrication of these optics. An additional issue is fabricatinglarger diameter silicon lenses. This requires bonding multiple pieces of silicon together asdiscussed in the single crystal silicon white paper.
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7 FiltersAlumina IR filter, Nylon, plastic filter, MMF
7.1 Metal Mesh Filter
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Metal mesh
Peter Ade, Lorenzo Moncelsi, Giampaolo Pisano, Carole Tucker
June 23, 2016
Abstract
Metal-mesh technology has been employed for decades to build high-performance filters workingat millimeter and sub-millimeter wavelengths. They have found a wide range of applications, mainlytargeted to astronomical instrumentation. Mesh filters, dichroics, beam dividers and polarizers havebeen used in a multitude of ground-based, balloon-borne and satellite projects. The technology is wellproven and space qualified. The same technology can be used to develop more general quasi-opticaldevices able to manipulate and transform the electromagnetic field amplitude and phase across theirsurface. We summarize recent developments in this field discussing devices such as mesh half-waveplates, artificial dielectrics and flat mesh lenses.
1 Filters
For many years we have employed multiple-layered metal-meshes embedded in polymeric dielectrics todefine FIR photometric bandwidths, reject unwanted optical/NIR radiation and control the thermalenvironment in cryogenic instruments. By employing typically 8-layered devices at sequential temperaturestages, it is possible to reject optical/NIR radiation, whilst maintaining the transmission performance ofthe filter stack to > 80%. Crucially the randomly-oriented patterned metal-mesh layers essentially act asreflectors of the high frequency radiation, thus rejected thermal power is reflected rather than absorbed,and thus reduce thermal loading in the instrument. These devices can currently be manufactured withdiameters up to 300 mm, with excellent uniformity and reproducibility.
1.1 Low-pass, high-pass and band-pass mesh filters
By using multiple layers of well well-known inductive, capacitive or resonant metal mesh patterns andtheir combinations it is possible to achieve high-pass, low-pass and band-pass optical filtering respectively(Ade et al., 2006; Marcuvitz, 1951; Ulrich, 1967). Good in-band transmission and excellent out-of-bandrejection over an extended measurement range has been met by the use of typically four or five multilayermesh filters in series. The first filter of the stack, located at the detector array, could be a band-passor an edge-defining low-pass filter, followed by a series of low-pass edge filters at different temperaturestages which are necessary to reject the rising power from thermal blackbody sources. The use of severalfilter elements is a bonus because it enables us to distribute the placement of them on the various coldstage shield apertures within the photometric system and thus limit the thermal power reaching thecolder stages. This is important since the heat lift available at sub-Kelvin stages is very limited. Finally,high-pass filters can be installed just above the detectors at the focal plane to mitigate radio-frequencyinterference originating outside of the receiver.
1.2 Blocking filters (thermal filters)
Following on from above, crucial to all cryogenic instruments is thermal filtering of optical/NIR. We haveexploited new porous polypropylene materials in combination with fine capacitive structures to improvethe radiation environment in large aperture cryogenic systems. The porous material scatters most of theincident shortwave radiation for wavelengths equivalent to the pore size of the material but maintains
1
high transmission (> 95%) at FIR wavelengths. When operated with metal-mesh reflectors, designed toremove the longer NIR wavelengths, the combination becomes very effective and essentially rejects nearlyall of the optical/NIR, whilst providing excellent transmission for the FIR/mm wavelengths. Indeed atmillimeter wavelengths the loss is ≪ 1%.
2 Artificial dielectrics and anti-reflection coatings (ARCs)
2.1 Porous dielectric material ARCs
The material used as substrate for most mesh-devices is polypropylene, which has a refractive indexn ∼ 1.5. In order to minimize the reflection losses it is necessary to use anti-reflection coatings. Theeasiest way to match a material to the free-space impedance consists of using a quarter-wavelength slabwith an intermediate refractive index: n′ =
√(n) ≈ 1.22. Single layers of porous polypropylene or porous
Teflon can be efficiently used for this purpose to cover bandwidths up to 40%.
2.2 Artificial dielectrics and metamaterial ARCs
There are cases where the refractive indices required for the ARCs are not available. This is the casefor broadband multilayer coatings or high refractive indices materials to be AR-coated like sapphire,quartz or silicon. In these cases quarter-wavelength layers made of artificial dielectrics can be synthesizedand used for very broadband applications, more than 100% in bandwidth. Artificial dielectrics can berealized by loading dielectric materials with stacked metal mesh grids. In addition to the requirement ofhaving sub-wavelength structures as in mesh-filter type applications, the periodic grids need to be stackedwithin their near-field distances. The stacked grids will look like a uniform medium to the electromagneticradiation passing through them. The “equivalent” refractive index will depend on the number of gridsand their spacing. Refractive indices ranging from 1.2 to 4 can be easily achieved with negligible losses(Zhang et al., 2009).
3 Mesh retarders
The symmetry in the geometrical patterns of normal mesh filters guarantees their polarization inde-pendence when used on-axis. As we mentioned earlier, when the symmetry is broken the grids showanisotropic behavior. Parallel continuous lines and parallel dashed-lines are “extreme” examples ofanisotropic grids with strong inductive and capacitive reactance in one direction, while almost transparentin the orthogonal one. Stacking capacitive and inductive grids in orthogonal directions, the phase-shiftof the relative polarization vectors will change in opposite directions. The overall effect is similar tothat introduced by the ordinary and the extra-ordinary axes in birefringent crystals and so, by using theappropriate number of grids and geometries, it is possible to realize phase retarders. These, in turn, canbe used to manipulate the polarization state of the light. Other types of grid geometries allow capacitiveand inductive behavior to be on the same grid.
3.1 Quarter-Wave Plates
Stacks of three capacitive and three inductive grids is enough to achieve 90 degree differential phase-shift between two orthogonal axes. A mesh-QWP (or circular polarizer) can be used to convert linearpolarization into circular and vice-versa (Pisano et al., 2012). Mesh-QWPs used in combination withpolarizers have been used to rotate the polarization angle of the light.
3.2 Half-Wave Plates
Differential phase-shifts equal to 180 degrees can be achieved using capacitive and inductive stacks madeof 4 to 6 grids, depending on the bandwidth required. Rotating HWPs are used to rotate the polarization
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direction of the incoming light for further modulation. Most applications, mainly astronomical, requirelarge bandwidths. This means maintaining high in-band transmission while keeping the differential phase-shift close to 180 degrees. The first mesh-HWPs had bandwidths of the order of 30% (Pisano et al.,2008; Zhang et al., 2011; Pisano et al., 2012) while most recent broadband realizations have exceeded90% bandwidths.
3.3 Reflective Half-Wave Plates
Simple reflective HWPs can be built by locating a polarizer at a quarter-wavelength distance from aplane mirror. These devices work only within periodic narrow bands. However, it is possible to realizedielectrically embedded reflective HWPs with extraordinarily large bandwidths, more than 150%, byusing polarizers and artificial dielectrics (Pisano et al., 2014).
4 Mesh lenses
Mesh technology has been recently used to realize flat devices with focusing properties. This can beachieved either by manipulating the effective refractive index of the medium or manipulating the phaseacross the surface of the lens.
4.1 Graded index metamaterial lenses
Graded Index (GrIn) lenses require changing refractive indices across the lens surface following specificrules. Mesh-embedded artificial dielectrics with variable geometries have been used to develop this typeof thin and flat lenses (Savini et al., 2012).
4.2 Mesh lenses
A device able to modify a planar wavefront into a converging wavefront has the focusing effect of a lens.A mesh-lens can be imagined as a simple planar transmissive device that locally modifies the phase ofthe radiation across its surface to re-create the effect of a classical lens. The mesh-lens is discretizedinto pixels that are optimized to provide high transmission and a phase-shift, relative to a central point,with the required frequency dependence. Each pixel is a column of aligned capacitive unit-cells designedlike a normal mesh-filter. A 54 mm diameter mesh-lens, ∼ 2.3mm thick, working across the full W-band(75-110GHz) has been realized and tested (Pisano et al., 2013). The beam measurements showed verygood agreement down to the fourth sidelobe. This device did not require anti-reflection coatings and theoverall modelled transmission was above 97%.
4.3 Mesh lens arrays
There is an increasing need for pixel integration at focal plane level in millimeter-wave telescopes. Hun-dreds to thousands of detectors need to be coupled to the telescope optics necessarily avoiding massiveand expensive horn antennas. Different solutions can be adopted such as lens-let and phased-array an-tennas, although their manufacturing processes might not be straightforward. An alternative solutionconsists in realizing arrays of miniaturized mesh-lenses. A whole array consists of a single flat devicemanufactured using exactly the same processes required for a single mesh-lens.
References
Ade, P. A. R., Pisano, G., Tucker, C., & Weaver, S. 2006, in Proceedings of SPIE, Vol. 6275, Society ofPhoto-Optical Instrumentation Engineers (SPIE) Conference Series
Marcuvitz, N. 1951, Waveguide Handbook, Electromagnetics and Radar Series (P. Peregrinus)
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Pisano, G., Maffei, B., et al. 2014, in Proceedings of SPIE, Vol. 9153, Millimeter, Submillimeter, andFar-Infrared Detectors and Instrumentation for Astronomy VII, 915317
Pisano, G., Ng, M. W., Haynes, V., & Maffei, B. 2012, in PIERS Proceedings, Kuala Lumpur, Malaysia
Pisano, G., Ng, M. W., Haynes, V., & Maffei, B. 2012, Progress In Electromagnetics Research M, 25, 101
Pisano, G., Ng, M. W., Ozturk, F., Maffei, B., & Haynes, V. 2013, Appl. Opt., 52, 2218
Pisano, G., Savini, G., Ade, P. A. R., & Haynes, V. 2008, Appl. Opt., 47, 6251
Savini, G., Ade, P. A. R., & Zhang, J. 2012, Optics Express, 20, 25766
Ulrich, R. 1967, Infrared Physics, 7, 37
Zhang, J., Ade, P. A. R., Mauskopf, P., Moncelsi, L., Savini, G., & Whitehouse, N. 2009, Appl. Opt., 48,6635
Zhang, J., Ade, P. A. R., Mauskopf, P., Savini, G., Moncelsi, L., & Whitehouse, N. 2011, Appl. Opt., 50,3750
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7.2 Si Substrate Filters
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Silicon Substrate Filters
C. D. Munson, Jeff McMahon, Kevin Coughlin
July 6, 2016
At Michigan we are develping a hybrid filtering approach based on re-flective frequency selective structures patterned on silicon substrates, scatter-ing/absorptive layers comprised of crystal powders embedded in an epoxy binder,and metamaterial antireflection coatings to control the in-band reflections fromthe vacuum-silicon interfaces. We have prototyped and IR blocking filter de-signed to pass the 70-170 GHz band. This approach can be adapted to all groundbased CMB bands and could realize metal mesh band pass filters. These filterscould be integrated with silicon lenses.
1 Composite Filter Construction
Figure 1 shows the anatomy of our composite absorptive/reflective IR-blockingfilter. This filter consists of lithographically defined frequency selective sur-faces patterned on two silicon wafers, a 25µm layer of an absorptive mixtureof epoxy and reststrahlen powders placed between the two patterned surfaces,and a metamaterial antireflection coating on both vacuum silicon interfaces. AtIR wavelengths, light is reflected off the front silicon wafer and frequency selec-tive surface. The front metamaterial surface scatters light both specularly anddiffusely for frequencies above the single-moded limit of the structure. Infraredlight not reflected by this frequency selective metal mesh is subject to bothscattering and absorption by the powder-epoxy composite. An additional metalmesh layer reflects most of the remaining light back into the epoxy-powder layer,boosting absorption and (to a lesser extent) reflection. This approach reducesthe load on the cryogenic stage by reflecting a significant portion of the IRpower, and uses an absorbing layer to further attenuate IR power passing thefirst reflective layer at arbitrary angles of incidence.
At millimeter and sub millimeter wavelengths the frequency selective sur-faces have a high transmission, and the absorbing layer is inconsequentiallythin leading to low absorption. Thus in the bands of interest, this structurebehaves nearly as if it were a slab of solid low loss silicon treated with a highquality antireflection coating [1].
1
Reflective Metal Mesh
Absorptive & Scattering Epoxy/Powder Mix
Antireflection Coated Silicon
Composite IR-Blocking Filter Construction
Antireflection Coated Silicon
Figure 1: Our silicon-substrate composite filter is composed of several compo-nents. In the pass-band the metamaterial antireflection coated silicon coupleslight into and out of the filter stack from free space. In the stop-band, a set oflithographically patterned reflective metal features reflect a significant portion ofthe incident light, and an absorptive and scattering layer of optical epoxy loadedwith powdered Reststrahlen materials blocks much of the remaining light. Insetimages show the specific components.
2 Composite Filter Performance
The performance of our composite filters was evaluated using Fourier TransformSpectrometer (FTS) measurements, as well as integrated measurements madewith a disk bolometer in a cryostat open to a 300K blackbody.
IR Blocking Performance: The infrared blocking performance of these fil-ters was measured on an FTS up to 5000 icm (2 um wavelength, 150 THz),giving a full characterization of the transmission across the spectrum of a 300Kblackbody. In these measurements, the composite filter specularly reflected >40% of the light incident from a 300K blackbody (indicating reflection off the frontsilicon surface and metal mesh features), and diffusely reflected another 10%,indicative of backscattering off the powder layer. It transmitted <1% of a 300Kblackbody in FTS tests, and <2% in an integrated cryostat test, confirmingexcellent IR blocking performance.
Low Frequency Performance: The low frequency performance of a 75umlayer of the powder filter component was measured down to 10 icm using anFTS. These data were then fit with a simple transmission line model. Thismodel was then used to extrapolate to the signal band and simulate the effectof adding an three-layer antireflective coating. This model shows that the filterintroduces minimal loss (dominated by the epoxy carrier) in a signal band from70-170 GHz, and that an instrument-band transmission of >99% should be
2
Composite Filter Performance
0 1000 2000 3000 4000 500010-4
10-3
10-2
10-1
100
Pow
er (
norm
alized)
Reflectance, specular
Total Hemispherical Reflectance
Transmission
300K Blackbody (normalized)
FTS Transmission DataModel: noAR
Model:with AR Coating
Transm
ission
Abso
rptio
n
Transmission
Abs
orpt
ion70-170 GHz
Refe
ctio
n / T
rans
mission
In Band Performance1
10-1
10-2
10-3
Wavenumber [icm]0 5 10 15 20 25
Full Filter Performance
Wavenumber [icm]
Figure 2: The performance of composite filter parts. Left: The measured (blackdots), and simulated (blue dashed) low frequency performance for a single 75umlayer of Reststrahlen powder mix, in epoxy, on a silicon wafer. Additionallyshown are the best fit simulated performance (transmission and absorption) fora stack consisting of AR coated silicon on either side of a 75um powder mixlayer. The target transmission band for the AR coating is 70-170 GHz and ismarked with the blue band on the plot. Middle: The IR blocking performanceof a full composite filter is shown, with a 300K blackbody overlaid. Right: Adrawing of the integrated test cryostat, wherein the filter (shown in white, andheld at 20K) is used to block power from a 300K blackbody falling on a 5K diskbolometer (black). A total blocking efficiency of >98% was demonstrated usingthis setup, in keeping with the prediction from the FTS measurements. Thiswas determined by measuring the heating of the bolometer when exposed to anaperture open to 300K, and determining the incident power relative to the sameenvironment without a blocking filter.
3
achievable for a filter using this technology (with the total transmission limitedby the antireflection coating performance).
Cold Performance: The Cold Performance was characterized to ensure theproper functioning of these filters at cryogenic temperatures. It is a knownphenomenon that some Reststrahlen materials have absorption bands that openup when the material is cooled down. In particular, alumina (Al2O3) is knownto have a section of its absorption band (between 30 and 300 microns) open up attemperatures of tens of Kelvin [2], [3]. A powder filter consisting of a mixture ofcalcium carbonate (CaCO3) and magnesium oxide (MgO) was measured in anFTS at a range of temperatures between 4K and 300K to confirm no substantiveperformance change was introduced by cooling.
Thermal Performance and Cryostat testing: An integrated test of thecomposite filter performance was carried out in a cryostat, to measure the totalblocking efficiency of a 15cm diameter prototype. For a 15cm diameter samplecomposite filter, there was no measurable heating of the center of the filter,when the filter was cooled to 20K and used to block the power from a 7cmdiameter window open to 300K. In this configuration, the power deposited on acarbon disk bolometer at 5K was measured, and this measurement establisheda lower limit on the blocking of >98% of a 300K blackbody. This limit is inagreement with the FTS measurements of the full composite filter.
3 Potential Applications on S4
In addition to forming effective free-space IR blocking filters, this filtering ap-proach offers several novel possibilities for silicon-substrate optical elements.Lower frequency-selective metal elements can be incorporated into these filtersto aid in defining the instrument signal band. These filters can also be easily andinexpensively integrated into other optical components, such as silicon lenses.
4 R&D Path Forward
To prepare for CMB-S3 we should (1) demonstrate a full scale prototype, (2)demonstrate merging this approach with a silicon lens, and (3) demonstrateband defining filters. All of these prototypes should be tested cryogenically,and optically. The first generation prototype filter has smaller than expectedreflection of IR power (40% vs 90% predicted). We attribute this to a near-fieldcoupling of IR power into the absorbing layer. A second prototype that spacesout the first reflector from the absorbing layer is an important next step. Theoptical measurements must test for diffraction and scattering to wide angles ata wide range of frequencies.
4
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5
8 Research and Development for CMB S-4Measurement of material property at 4K. Optics / Scattering Simulation?
9 Conclusion
25
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