CMB S-4: Broadband Optics

July 27, 2016

Version 1.0

CMB-S4 Collaboration

Contents1 Solicitation List 1

2 Goal of S4 Technical Papers 3

3 Introduction 3

4 Requirements 3

5 Lens Material 4

6 Half-Wave Plates 46.1 Achromatic Half-Wave Plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56.2 Sapphire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76.3 Metamaterial Silicon Broadband Half-Wave Plates . . . . . . . . . . . . . . . . . 8

7 Anti-Reflection Coating 117.1 Thermal Spray Anti-Reflection Coating . . . . . . . . . . . . . . . . . . . . . . . 127.2 Epoxy Anti-Reflection Coating . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137.3 Metamaterial Silicon AR coatings . . . . . . . . . . . . . . . . . . . . . . . . . . 14

8 Filters 178.1 Metal Mesh Filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178.2 Silicon Substrate Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

9 Windows 26

10 Research and Development for CMB S-4 26

11 Conclusion 26

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1 Solicitation List

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Topics POC Author StatusIntroduction Jeff M Jeff McMahon Draft In - Waiting for CommentGoal/ Approach Shaul H Shaul Hanany DoneRefractive Lens MaterialSilicon Jeff M Jeff McMahon Waiting for DraftAlumina Toki S Oliver Jeong First DraftPlastic Zeesh A Keith Thompson 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 Kevin Coughlin Draft In - Waiting for CommentWiregrid Pol Mod Jeff M David Thuss Waiting for DraftAnti-Reflection CoatingAR Intro Jeff M Jeff McMahon Decide authorEpoxy 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 Shaul H Minnesota Waiting for DraftHoles in Plastic Jeff M Tobias Marriage Waiting for DraftFilterFilter Intro Zeesh A Zeesh Ahmed Waiting for DraftMetal Mesh Filter Zeesh A Lorenzo Moncelsi Draft In - Need .tex and .bibSilicon Substrate Filter Jeff M C. D. Munson Draft In - Waiting for CommentStanford Laser Cut Al Mylar Zeesh A Zeesh Ahmed Waiting for DraftPlastic Filter Zeesh A Keith Thompson Waiting for DraftAlumina Filter Toki S Oliver Jeong Draft InRT-MLI Toki S Osamu Tajima Waiting for DraftWindowZotefoam Zeesh A Kovac Group Waiting for DraftHDPE Jeff M Mike Niemack Waiting for DraftCold Window Jeff M Mike Niemack Waiting for Draft

Table 1: Lists of solicited topics

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2 Goal of S4 Technical PapersSummarize the current state of the technology and identify R&D efforts necessary to advanceit for possible use in CMB-S4. CMB-S4 will likely require a scale-up in number of elements,frequency coverage, and bandwidth relative to current instruments. Because it is searching forlower magnitude signals, it will also require stronger control of systematic uncertainties.

3 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.

4 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)

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• Application over curved surface

• No delamination

5 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.

6 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).

6.1 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

. (6.1)

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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Figure 2: 512mm-diameter sapphire window cut from a 200kg ingot of HEM sapphire grown atTuizhou Haotian Optoelectronics Technology in China

6.2 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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6.3 Metamaterial Silicon Broadband Half-Wave PlatesDescription of technology Birefringent metamaterial silicon is fabricated by making asymmet-ric features in the surface of silicon plates. There are several advantages of this technology. First,HWP the difference in the index of refraction between the two principle axes can be made large(∆n > 1). Second, the loss of this material can be extremely low as silicon has a low loss tangentand with the high ∆n each HWP layer can be made very thin.

To make a broadband half-wave plate (HWP), we designed a 3-layer Pancharatnam geometrywith three layer antireflection (AR) coatings on both sides. The HWP layers consist of a set ofevenly spaced grooves cut into the silicon. The AR layers consist of two orthogonal sets of grooves,leaving rectangular cross-sectioned stepped pyrimids. The coating is designed to be birefringentto minimize polarization dependancies in reflections. To get the 3-layer HWP, we first start withtwo silicon wafers, and cut the central HWP layer into the thicker of the two wafers. The twowafers are then glued together, and the outer layers are cut. The HWP layers just touch, leaving nointerstitial silicon, so small holes permiate the entire plate. Dispite this, the plates have proven tobe mechanically robust.

Demonstrated performance and metrics At writing one broadband HWP has been deployedon the Atacama Cosmology Telescope as part of the Advanced ACT upgrade. The HWP had adiameter of 34 cm, and was optimized for 75-165 GHz range. In lab, it was demonstrated to havehigh (greater than 90%) modulation efficiency, low reflections (averaging less than 3%). On thetelescope, it was mesured to have low emission ( 2K at 90 GHz, 4K at 150 GHz). Preliminarydata analyzed from polarized point sources indicate that this HWP behaves as expected. Soon, anadditional HWP for 125-270 GHz will be finished and deployed, with similar performance to beexpected.

Challenges in scaling to S4 The major challenge to overcome scaling this to S4 level produc-tion is the fabrication time. Our current system can create one HWP in approximatly three weeks.In order to scale this to the needs of S4, we would need to create a more automated system. Iwould estimate that a fully automated system could get the fabrication time down to three days.The primary component needed to reduce the time would be a saw with several blades, capible ofmaking several layers of cuts simultaneously. Additionally, a rotation stage to automatically repo-sition the HWP for the second set of AR coating cuts would reduce machine down time further. Anadditional challenge is developing broader band HWPs. Design work and prototypes are needed tomeet this challenge. Finally, it would be advantageous to have a cold HWP rotator. Work should beinvested in developing such a system as it would reduce the requirements on the HWP simplifyingdesign and fabricaiton.

R&D path forward The next steps of R&D is to continue to refine our fabrication technique toincrease our yeild. More development of our gluing procedure can mitigate the risk of delaminationof the two silicon plates durring the fabrication process. After working on the gluing process, thenext step would be to begin working on further automation of our system by constructing a new

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Figure 3: Birefringent silicon is cut only in one direction, with evenly spaced cuts. To form the 3-layer HWP, onesilicon wafer is cut all the way through, leaving only strips which remain in place due to the glue layer between thetwo wafers. By tuning the width and depth of the cuts, the index of refraction can be tuned within a range.

saw, including the upgrades listed above. We should also invest in developing broader band HWPsand a cold rotator.

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Figure 4: Picured is the fully fabricated HWP currently deployed on ACT. It is placed in the encoder ring to measurethe angle of the HWP as it rotates in front of the telescope.

Figure 5: The modulation efficiency of the HWP was measured using a vector network analyzer at Goddard SpaceFlight Center. This was done in two bands, roughly corresponding to the bands of the telescope. The modulationefficiency was found to be approximatly 90% in the high band and 95% in the low band. Due to incomplete cleaningat the time of measurement, it is possible that the true modulation efficiency is higher than these values.

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7 Anti-Reflection Coating

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Figure 6: 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].

7.1 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 6, 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 7: 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]

7.2 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 7, 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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7.3 Metamaterial Silicon AR coatingsDescription of technology Metamatieial AR coatings are fabricated by cutting sub-waevlengthfeatures into surfaces of refractive optical elements. At Michigan we have developed a capabilityto fabricate these coatings on silicon optics using a custom three-axis silicon dicing saw. Thissystem allows us to produce micron accurate arrays of square-based stepped pyramids. By tuningthe fill factor we can create layers that behave a simple dielectric sheets with an index of refractionbetween that of silicon and vacuum. Multi-layer coatings can be realized by cutting progressivelythinner grooves at greater depth which are centered in the wider first layers. We have created anumber of full scale optics with near zero defect rate, and the quality of our process is continuallybeing refined.

Demonstrated performance and metrics Thus far we have deployed 12 full scale lenses on theACTPol and the AdvACT experiments and we have delivered a set of lenses to the PIPER experi-ment. The largest lens yet produced is 33 cm in diameter, but there is no restriction in fabricatinglenses up to the maximum diameter available for single crystal silicon. We have fabricated thesecoatings on both concave and convex surfaces. The demonstrated bandwidth of these coatings isin excess of an octave for three layer coatings. In addition, we have the ability to achieve widerbandwidth with the proven three layer coatings by trading reflection performance for bandwidth.In the current coatings we achieve ∼ 0.1% average reflections in two CMB bands. If we designedfor 1% refleciton we could achieve 3 : 1 ratio bandwidth with three layers. We have designed andprototyped the fabrication of a five layer coating that would cover 75-300 GHz. This proof of prin-ciple shows that 4:1 bandwidth is possible with these coatings. We will complete the second sideof this five layer prototype in the coming months to confirm its performance. Wider bandwidth(5:1) is possible if we optimized with out the restriction of the blades we had in stock.

Challenges in scaling to S4 The primary challenge for applying this technology to S4 is reduc-ing the time it takes to AR coat a single lens. Currently, fabricating the coatings takes roughly twoweeks per lens. Based on our experience with the existing first generation AR coating machine wehave a preliminary design for a new machine which would automate a number of the time intensiveset up tasks. We forecast that with this machine we could reduce the time to fabricate a coating ona lens to 1-2 days. The key features of this new machine would be: (1) rotation stage to changethe orientation of the the lens, (2) automated metrology to acquire lens positioning after mountingand rotations, (3) multiple independent spindles set up with different dicing blades to minimizetime intensive blade changes. This system would make it practical to fabricate the large number oflenses required for S4.

The next potential challenge is in fabricating lenses larger than 46 cm. The issue here has todo with the availability of large silicon rather than limits of the machine. Therefore we discuss thischallenge in the note about silicon properties.

R&D path forward The most important R&D issue is to build a new machine and demonstratea speed up of the fabrication of these optics. An additional issue is fabricating larger diameter

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silicon lenses. This requires bonding multiple pieces of silicon together as discussed in the singlecrystal silicon white paper.

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Figure 8: (Left) The performance of metamaterial silicon AR coatings on fabricated on lensesthat have been or will soon be fielded including: the ‘MF’ (90/150 band) lenses for ACTPol, the‘HF’ (150/220 band) lenses for AdvACT, and the PIPER 200 GHz lenses. Simulations are shownas dashed lines and measurements are shown as solid lines. The MF and HF lenses use threelayer AR coatings while the PIPER lenses use a single layer coating. A preliminary design for afive layer coating is shown in gray dashed line. A prototype of this five layer coating has beenfabricated on one side of a 25 cm diameter test wafer. (Right) Shown is a close-up picture of amechanical prototype of our 3-layer coating for our 75-165 GHz antireflection coating. For thiscoating, the top cut is 250 um wide, and 500 um deep. The middle layer is 110 um wide, 310 deep.The last layer is 25 um wide, 257 um deep. The pitch between the sets of cuts is 450 um.

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8 Filters

8.1 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

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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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4

Reflective Metal Mesh

Absorptive & Scattering Epoxy/Powder Mix

Antireflection Coated Silicon

Composite IR-Blocking Filter Construction

Antireflection Coated Silicon

Figure 9: Our silicon-substrate composite filter is composed of several components. In the pass-band the metamaterial antireflection coated silicon couples light into and out of the filter stack fromfree space. In the stop-band, a set of lithographically patterned reflective metal features reflect asignificant portion of the incident light, and an absorptive and scattering layer of optical epoxyloaded with powdered Reststrahlen materials blocks much of the remaining light. Inset imagesshow the specific components.

8.2 Silicon Substrate FiltersAt Michigan we are develping a hybrid filtering approach based on reflective frequency selectivestructures patterned on silicon substrates, scattering/absorptive layers comprised of crystal pow-ders embedded in an epoxy binder, and metamaterial antireflection coatings to control the in-bandreflections from the vacuum-silicon interfaces. We have prototyped and IR blocking filter designedto pass the 70-170 GHz band. This approach can be adapted to all ground based CMB bands andcould realize metal mesh band pass filters. These filters could be integrated with silicon lenses.

Composite Filter Construction Figure 9 shows the anatomy of our composite absorptive/reflectiveIR-blocking filter. This filter consists of lithographically defined frequency selective surfaces pat-terned on two silicon wafers, a 25µm layer of an absorptive mixture of epoxy and reststrahlenpowders placed between the two patterned surfaces, and a metamaterial antireflection coating onboth vacuum silicon interfaces. At IR wavelengths, light is reflected off the front silicon waferand frequency selective surface. The front metamaterial surface scatters light both specularly anddiffusely for frequencies above the single-moded limit of the structure. Infrared light not reflectedby this frequency selective metal mesh is subject to both scattering and absorption by the powder-epoxy composite. An additional metal mesh layer reflects most of the remaining light back into theepoxy-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 IR power, and uses anabsorbing layer to further attenuate IR power passing the first reflective layer at arbitrary angles of

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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 10: The performance of composite filter parts. Left: The measured (black dots), and sim-ulated (blue dashed) low frequency performance for a single 75um layer of Reststrahlen powdermix, in epoxy, on a silicon wafer. Additionally shown are the best fit simulated performance (trans-mission and absorption) for a stack consisting of AR coated silicon on either side of a 75um powdermix layer. The target transmission band for the AR coating is 70-170 GHz and is marked with theblue band on the plot. Middle: The IR blocking performance of a full composite filter is shown,with a 300K blackbody overlaid. Right: A drawing of the integrated test cryostat, wherein the filter(shown in white, and held at 20K) is used to block power from a 300K blackbody falling on a 5Kdisk bolometer (black). A total blocking efficiency of >98% was demonstrated using this setup, inkeeping with the prediction from the FTS measurements. This was determined by measuring theheating of the bolometer when exposed to an aperture open to 300K, and determining the incidentpower relative to the same environment without a blocking filter.

incidence.At millimeter and sub millimeter wavelengths the frequency selective surfaces have a high

transmission, and the absorbing layer is inconsequentially thin leading to low absorption. Thusin the bands of interest, this structure behaves nearly as if it were a slab of solid low loss silicontreated with a high quality antireflection coating [36].

Composite Filter Performance The performance of our composite filters was evaluated usingFourier Transform Spectrometer (FTS) measurements, as well as integrated measurements madewith a disk bolometer in a cryostat open to a 300K blackbody.

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IR Blocking Performance: The infrared blocking performance of these filters was measuredon an FTS up to 5000 icm (2 um wavelength, 150 THz), giving a full characterization of thetransmission across the spectrum of a 300K blackbody. In these measurements, the composite filterspecularly reflected >40 % of the light incident from a 300K blackbody (indicating reflection offthe front silicon surface and metal mesh features), and diffusely reflected another 10%, indicativeof backscattering off the powder layer. It transmitted <1% of a 300K blackbody in FTS tests, and<2% in an integrated cryostat test, confirming excellent IR blocking performance.

Low Frequency Performance: The low frequency performance of a 75um layer of the powderfilter component was measured down to 10 icm using an FTS. These data were then fit with asimple transmission line model. This model was then used to extrapolate to the signal band andsimulate the effect of adding an three-layer antireflective coating. This model shows that the filterintroduces minimal loss (dominated by the epoxy carrier) in a signal band from 70-170 GHz,and that an instrument-band transmission of >99% should be achievable for a filter using thistechnology (with the total transmission limited by the antireflection coating performance).

Cold Performance: The Cold Performance was characterized to ensure the proper functioningof these filters at cryogenic temperatures. It is a known phenomenon that some Reststrahlen mate-rials have absorption bands that open up when the material is cooled down. In particular, alumina(Al2O3) is known to have a section of its absorption band (between 30 and 300 microns) open upat temperatures of tens of Kelvin [37], [38]. A powder filter consisting of a mixture of calciumcarbonate (CaCO3) and magnesium oxide (MgO) was measured in an FTS at a range of temper-atures between 4K and 300K to confirm no substantive performance change was introduced bycooling.

Thermal Performance and Cryostat testing: An integrated test of the composite filter perfor-mance was carried out in a cryostat, to measure the total blocking efficiency of a 15cm diameterprototype. For a 15cm diameter sample composite filter, there was no measurable heating of thecenter of the filter, when the filter was cooled to 20K and used to block the power from a 7cm diam-eter window open to 300K. In this configuration, the power deposited on a carbon disk bolometerat 5K was measured, and this measurement established a lower limit on the blocking of >98% ofa 300K blackbody. This limit is in agreement with the FTS measurements of the full compositefilter.

Potential Applications on S4 In addition to forming effective free-space IR blocking filters, thisfiltering approach offers several novel possibilities for silicon-substrate optical elements. Lowerfrequency-selective metal elements can be incorporated into these filters to aid in defining theinstrument signal band. These filters can also be easily and inexpensively integrated into otheroptical components, such as silicon lenses.

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) demonstrate band defining

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filters. All of these prototypes should be tested cryogenically, and optically. The first generationprototype filter has smaller than expected reflection of IR power (40% vs 90% predicted). Weattribute this to a near-field coupling of IR power into the absorbing layer. A second prototypethat spaces out the first reflector from the absorbing layer is an important next step. The opticalmeasurements must test for diffraction and scattering to wide angles at a wide range of frequencies.

25

9 Windows

10 Research and Development for CMB S-4Measurement of material property at 4K. Optics / Scattering Simulation?

11 Conclusion

26

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