Strong Circular Dichroism in Single Gyroid Optical Metamaterials
Cédric Kilchoer, Narjes Abdollahi, James A. Dolan, Doha Abdelrahman, Matthias Saba, Ulrich Wiesner, Ullrich Steiner, Ilja Gunkel, Bodo D. Wilts
SStrong Circular Dichroism in Single Gyroid OpticalMetamaterials
Cédric Kilchoer , Narjes Abdollahi , James A. Dolan , Doha Abdelrahman ,Matthias Saba , Ulrich Wiesner , Ullrich Steiner , Ilja Gunkel and Bodo D. Wilts ∗ Over the past two decades, metamaterials have led to an increasing number of biosensingand nanophotonic applications due to the possibility of a careful control of light propagat-ing through subwavelength features. Chiral nanostructures (characterized by the absenceof any mirror symmetry), in particular, give rise to unique chiro-optical properties such ascircular dichroism and optical activity. Here, we present a gyroid optical metamaterial witha periodicity of
65 nm exhibiting a strong circular dichroism at visible wavelengths. Ourbottom-up approach, based on metallic replication of the gyroid morphology in triblockterpolymer films, generates a large area of periodic optical metamaterials. We observe astrong circular dichroism in gold and silver gyroid metamaterials at visible wavelengths. Weshow that the circular dichroism is inherently linked to the handedness of the gyroid nanos-tructure, and demonstrate its tuneability. The optical effects are discussed and comparedto other existing systems, showing the potential of bottom-up approaches for large-scalecircular filters and chiral sensing.
Three-dimensional (3D) chirality, i.e. the absence of mirror or inversion symmetry, can befound in a large number of natural morphologies including proteins and various crystalstructures [1]. Light propagation in a chiral medium depends on its polarization state,leading to chiro-optical effects such as circular dichroism (CD) and optical activity [1].Traditionally, CD is the difference in absorption between circularly polarized light of op-posite handedness, while optical activity refers to the rotation of linearly polarized lightpropagating through a chiral medium due to a difference in phase velocity, also known ascircular birefringence [2]. These two effects are typically weak in natural materials, butthey can be strongly enhanced in specifically engineered nanostructured dielectric [3] andplasmonic composites [4]. The latter are better known as chiral metamaterials and possessa subwavelength structure, which furnishes them with optical properties that are often notfound in nature. Optical metamaterials – nanostructured materials engineered to yield adesired response in the visible spectrum – have received special attention due to excitingpotential nanophotonic applications including plasmonic biosensing [5], and subdiffractionimaging [6]. In addition, chiral nanostructures can strongly enhance both optical activityand CD [4], leading to attractive effects at visible frequencies with chiral metamaterialsserving as highly efficient broadband circular polarizers [7] and chiral sensors [8, 9]. Ulti-mately, a strong chirality may gives access to negative refractive indices as predicted byTretyakov [10] and Pendry [11]. 3D negative refraction may lead to a number of tech-nological applications, most notably the ‘perfect lens’ [12], and the invisibility cloak [13].Chiral metamaterials with an optical response at visible and near-infrared frequencies areparticularly challenging to fabricate because of the required 3D chiral structures on the C. Kilchoer, N. Abdollahi, Dr. J. A. Dolan, D. Abdelrahman, Dr. M. Saba, Prof. U. Steiner, Dr. I. Gunkel, Dr.B. D. WiltsAdolphe Merkle Institute, University of Fribourg, Ch. des Verdiers 4, 1700 Fribourg, Switzerland ∗ E-mail: [email protected] Prof. U. WiesnerCornell University, 214 Bard Hall, Ithaca, NY 14853-1501, USA et al . demonstrated periodic gold helices exhibiting a strong CD at mid-infraredwavelengths in transmission by employing direct laser writing followed by gold electrodepo-sition [7]. These helical structures and their CD can be further optimized by increasing thenumber of intertwined helices within the unit cell [27–29]. Employing focused ion beam-induced deposition techniques, helices with a smaller radius were fabricated, showing astrong CD between 500 and 1000 nm [30]. By carefully controlling the deposition angle,helices can be grown from gold nanoparticle seeds yielding sub-100 nm gold helices with amaterial-based tuneable CD at visible frequencies [31, 32]. Existing top-down techniquesare often cumbersome, costly, and limited in the achievable feature sizes and coverage ar-eas. On the other hand, bottom-up manufacture of chiral metamaterials has virtually nolimitations in the feature size or morphology. While metamaterials based on self-assemblytechniques such as DNA origami [33–35], peptides [36, 37] and cellulose nanocrystals tem-plates [38] show CD at visible wavelengths, its strength is several orders of magnitudeslower compared to helical metamaterials [4, 9].Here, we present an optical metamaterial fabricated by replication of a self-assembledgyroid in block copolymer (BCP) films [39, 40] exhibiting strong CD at visible frequen-cies. Triblock terpolymer films with an alternating gyroid morphology exhibit an intricatenanostructure with a cubic unit cell of approximately 65 nm [41]. Large single gyroidgold domains, inclinated along their cubic h i direction, display a linear polarization-dependent optical response [39, 41, 42], which has only very recently been explained [41].While the same sample displayed no discernible CD when illuminated at normal incidence,a weak gyrotropic effect was observed when the sample was tilted to align its h i axiswith the illumination direction [39].In this article, using the same gyroid morphology, we demonstrate a strong CD in silverand gold gyroid metamaterials with a peak at ∼
510 nm and ∼
550 nm, respectively. Wefurther show that the sign of the CD is linked to the handedness of the gyroid structureand can be tuned using different metals (Au, Ag) and dielectric surrounding materials.The CD strength in these self-assembled gyroid metamaterials is comparable to the bestresults achieved with top-down fabrication techniques [4], demonstrating the viability ofself-assembly in fabricating metamaterials with strong chiro-optical responses. We thusshow that our bottom-up approach is a promising, fast, and cheap alternative to establishedtop-down techniques.
Continuous nanostructured gold and silver networks exhibiting the alternating gyroid mor-phology (body-centered cubic space group I
32, 214 in [43]) were fabricated by a pro-cedure that is discussed in detail in a separate publication [44]. In brief, the fabricationstarts with the self-assembly of a polyisoprene- b -polystyrene- b -poly(ethylene oxide) BCPfilm into the alternating gyroid morphology. The polyisoprene (PI) gyroid phase is de-graded, followed by electrochemical backfilling of the voided single gyroid network withgold or silver ( Figure 1 a) [40]. The (in–plane) single gyroid morphology of the polymertemplates and the metal replica was previously confirmed by small–angle X–ray scatter-2 ) Polymer film Selective removal of PI Silvereletrodeposition PolymerremovalSilver gyroid in polymer Silver gyroid in air b) c) d)
400 500 600 700 8000123456 t r an s m i tt an c e ( % ) wavelength (nm) airpolymer Figure 1.
Fabrication and optics of silver single gyroids. a) Sketch of the fabrication process of singlegyroid optical metamaterials. b) SEM top-view image of a silver gyroid metamaterial. The gyroidstructure exhibits a h i out-of-plane orientation. Inset: Schematic gyroid structure viewed from the h i direction. c) Multi-domain gyroid optical metamaterial in transmission under unpolarized light atnormal incidence. Large domains appear green, while the borders between the domains are clearly visibleby their darker coloration. d) Unpolarized transmission spectrum of a single silver gyroid domain withand without polymer. Scale bars: (b)
500 nm , (c)
200 µm . ing [40]. We note that grazing-incidence small-angle X-ray scattering of similar gyroidterpolymer films revealed a slight distortion in the out–of–plane direction, which is relatedto the film processing [45]. Importantly, the subsequent processing of the gold and silvergyroid manufacture differ. In the case of gold gyroids, the remaining polymer scaffold isetched away in an oxygen plasma, yielding a free-standing single gyroid gold film [40]. Be-cause of the lower oxidation resistance of silver compared to gold, this approach does notyield metallic silver gyroids. Instead, polymer degradation is carried out in an argon plasmafor the manufacture of free-standing silver gyroids. As explained in detail in [44], this re-sults in stable metallic single silver gyroids that are covered by a very thin carbonaceouscoating.We first focus on silver gyroid metamaterials that have a stronger optical activity com-pared to gold. Top-view SEM images show that the fabricated free-standing silver singlegyroid has a periodic unit cell of circa 65 nm and a h i out-of-plane orientation (Figure1b), in agreement with previous work using the same triblock terpolymer [40, 41]. Theperiodicity the gyroid morphology was confirmed throughout the entire film thickness byfocused ion beam SEM (Figure S3). Transmission optical micrographs of silver gyroid filmson FTO-coated glass substrates reveal large green-coloured domains (Figure 1c). Thesecorrespond to single domains with uniform gyroid orientation (Figure 1b, see also ref. [40]).The green color stems from a transmission band in the 450 to 600 nm wavelength range,with a peak transmittance of ∼ SEM images of the nanostructure (
Figure 2 a) show the presence of single gyroid domainsof different handedness. Since our fabrication method does not induce a unique handed-ness, a racemic coexistence of both handednesses across the sample is expected. Uponillumination with circularly-polarized (CP) light, multidomain silver gyroid films show amosaic of bimodally colored domains (Figure 2b,c). The color and intensity of individualdomains swaps when the handedness of the light is altered. The difference in transmittance3
00 500 600 700 800-0.4 - Domain 2 c i r c u l a r d i c h r o i s m wavelength (nm) Domain 1 ↻↺ RCPLCP
400 500 600 700 8000123456 t r an s m i tt an c e ( % ) wavelength (nm) RCPLCP b)a) c) d)e) Figure 2.
Circular dichroism in a silver gyroid optical metamaterial. (a) SEM image of a single gyroidnanostructure, encompassing a grain boundary, indicated by the white line. To identify the handednessof single gyroid, the [110]-oriented sample was tilted by 35° around the [ ] axis, providing a view ofthe [111] orientation [46]. The right- and left-handed domains at the top and bottom of the image areillustrated above and below the SEM image, respectively. Illumination of the [110]-oriented silver gyroidfilm with (b) right-handed and (c) left-handed circularly polarized light shows a mosaic of bimodallycolored domains, where the contrast is inverted with handedness. (d) The CP-dependent transmittancespectra of an individual domain (domain 1) shows a strong difference. (e) CD measurements of 7 differentdomains show either positive or negative spectra of identical shape. Domains 1 and 2 are indicated in(b). Scale bars: (a)
500 nm , (b,c)
200 µm . of individual domains when illuminated under right-handed (RCP) and left-handed (LCP)circularly polarized light (defined from the receiver’s point of view) indicates the presence ofa strong CD, which was spectrally quantified using a custom-built microspectrophotometerwith a measurement spot size diameter of ∼
50 µm. This measurement allows the charac-terization of individual domains that typically span a few hundred µm. Figure 2d shows apronounced difference in the transmitted intensity between 450 and 550 nm. As commonfor photonic crystals [47] and metamaterials [23], we define the CD asCD = T RCP − T LCP T RCP + T LCP , (1)where T RCP and T LCP denote the transmitted intensity for RCP and LCP illumination,respectively. Note that the CD in this definition is generally a combination of two effects:a difference in absorption of RCP and LCP light as in the traditional definition [1], anda different distribution between reflection and transmission for the two polarizations as inphotonic crystals [3].The CD of different domains is plotted in Figure 2e. While the overall strength of CDis uniform across different domains (maximal absolute value of ∼ ∼
510 nm for allmeasured domains), it is either positive or negative indicating a bimodal behavior. Thisimplies that the CD arises from the inherent chirality of right- and left-handed single gyroidnetwork domains.
To determine the effect of the plasmonic metal, from which the gyroid is formed, on theCD, we compare the optical responses of single gyroid networks made from gold and silver.
Figure 3 a shows the CP-dependent transmittance of gold and silver single gyroid networks.The gold spectra (solid lines) show two transmittance peaks at ∼
490 nm and 580 nm and adip at ∼
520 nm, as previously observed [48, 49]. While the short-wavelength peak is nearly4 transmittance (%) w a v e l e n g t h ( n m ) R C P A u L C P A u R C P A g L C P A g a ) b ) circular dichroism w a v e l e n g t h ( n m ) A u A g
Figure 3.
Circular polarization resolved transmittance. (a) CP-dependent spectra of gold and silver singlegyroid metamaterials. (b) The CD of gold sample is red-shifted and has a lower intensity compared tothe silver metamaterial. s i l v e r transmittance (%) w a v e l e n g t h ( n m ) n = n = n = g o l d c )b ) transmittance (%) w a v e l e n g t h ( n m ) n = n = n = a ) circular dichroism w a v e l e n g t h ( n m )
A g n = n = n = n = n = n =
Figure 4.
Effect of the dielectric medium on the CD. Measured CD for (a) gold and (b) silver singlegyroid metamaterials before removing the polymer scaffold ( n ≈ . ) and after infiltration of the gyroidswith several dielectric media (air n ≈ . , heptane n ≈ . , Cargille refractive index liquid n ≈ . ).(c) Wavelength-dependent CD intensity as a function of the refractive index of the dielectric medium. insensitive to the CP handedness, the intensity and wavelength of the second peak and thedip differ. The maximum CD of ∼ ∼
550 nm (Figure 3b). In silver gyroidmetamaterials, the transmittance arising from the resonant nanostructure is spectrally moreconfined, at wavelengths between 450 and 600 nm. A dip at ∼
510 nm is present for one ofthe two handednesses. T RCP and T LCP show a pronounced difference that leads to a muchstronger CD with a peak value above 0.25 at 510 nm, 5-6 times higher compared to thegold metamaterial.The optical response of nano-structured plasmonic metals is known to be sensitive to theenvironment, i.e. the refractive index of the dielectric medium surrounding the plasmonicfeatures [48]. To study this effect, the silver gyroids were measured before the polymermatrix was etched away ( n ≈ .
60) and, furthermore, the voided gold and silver gyroidmetamaterials were infiltrated with several dielectric media.
Figure 4 a,b compares theCD of gold and silver single gyroids infiltrated by air ( n = 1 . n = 1 . n = 1 . ∼ ∼ transmittance (%) w a v e l e n g t h ( n m ) T R R T L L T R L T L R a ) p o l y m e r T R R T L L T R L T L R transmittance (%) w a v e l e n g t h ( n m ) b ) a i r
Figure 5.
Polarization conversion efficiency of silver gyroids in air and embedded in the polymer scaffold.Transmittance spectra of the four elements of the Jones matrix, where T RL and T LR are measures forthe conversion of CP light by silver single gyroid metamaterials surrounded by (a) polymer and (b) air.Gold gyroid metamaterials were exhibiting a extremely weak polarization conversion, therefore this resultin not shown. A more careful analysis of the spectra in Figure 4b reveal two opposing trends in thepresence of dielectric media. In the presence of the polymer scaffold, the silver is surroundedby the polystyrene block of the BCP ( n ≈ . n = 1 .
55 liquid suggests that the etching process, which removes the polymer,significantly changes the immediate environment of the silver gyroid struts.Similar to the Au data, the CD is strongly enhanced for the polymer-containing silvergyroid sample. While the liquid-infiltrated silver gyroid samples also exhibit a CD increasecompared to the voided air gyroid, this has to be seen in context of an overall transmissionreduction in both CP channels for these two samples.
To gain an understanding how different dielectric background media influence the CDstrength and spectral position, the optical response was analyzed by two sets of a lin-ear polarizer and a quarter wave plate, one each in the illumination and detection opticalpaths. This optical configuration allows the quantification of the conversion of the circularpolarization by the silver gyroid samples.
Figure 5 shows the presence of polarizationconversion between LCP and RCP light. The conversions from right-to-left ( T RL ) and fromleft-to-right ( T LR ) circularly polarized light have similar intensities, so that the CD evi-dently arises from a difference between T RR and T LL . This behaviour is expected as thegeometry exhibits a C rotational symmetry perpendicular to the optical axis for light atnormal incidence on a h i inclinated gyroid (cf. generalized chiral structures in [50]). Wenote that the above statement additionally requires reciprocal materials and that the slabgeometry is terminated such that the bulk C axis lies in its center. The former requirementis clearly satisfied, while the latter is only approximately valid in our self-assembled opticalmetamaterials.For the free-standing silver gyroid (Figure 5a), the majority of the circularly polarizedlight is transmitted without polarization conversion ( T RR and T LL ) below 570 nm. Part ofthe light is, however, converted into light of the other handedness (compare dashed lineswith the solid lines). For a gyroid with the polymer scaffold in place ( n = 1 .
60, Figure6b), the light transmitted without polarization conversion is increased by approximately30%, while the polarization conversion is comparable to that of the voided gyroid. Inboth materials, the polarization conversion has a minimum around 510 nm, where the CDis strongest. Over the whole spectral range, silver gyroid embedded in polymer showsa much lower ratio of polarization conversion compared to air. This difference in theratio of polarization conversion explains the weaker CD in silver gyroid surrounded by air.Interestingly, in both cases, the converted light becomes dominant at longer wavelengths,where up to 90% (at at 670 nm in air) of the transmitted light is converted into the otherhandedness.Chiral metamaterials can be extremely efficient in converting the polarization state ofincident light [51–53]. Due to the lack of a higher rotational symmetry ( C n with n >
2, [3])in h i oriented gyroids, light propagation in the metamaterial is not restricted to onepolarization channel. As a result, RCP light can be partially converted into LCP light andvice-versa. The effect of bulk and slab symmetries on the chiro-optical response will bediscussed below. Silver and gold single gyroid metamaterials show a strong and variable CD in the visiblewavelength range. Gyroids are intrinsically chiral, although their chiro-optical responsestrongly depends on the observation direction if realized as a photonic crystal [54] or amicrowave metamaterial [55]. The gyroid handedness is defined by the rotational senseof the 6-fold helix along the h i axis (Figure 2a). The origin of the CD in plasmonicsingle gyroids has been theoretically studied by Oh et al . for gyroids which are orientedalong the h i and h i directions [55, 56]. For a perfect electric conductor, a right-handed single gyroid network has a negative CD when oriented along the h i direction.For the h i direction, the CD is virtually absent [55]. Theory suggests that the CDvanishes in any loss-less metamaterial with an inherent C axis along the optical axis, i.e. the h i direction of a gyroid, in a converged simulation [3]. The fabricated singlegyroid metamaterial investigated here is however h i out-of-plane oriented, for which atheoretical description is yet to be performed.The single gyroid optical response is experimentally characterized by a strong CD (Fig-ure 2), with a finite circular polarization conversion (Figure 5). These findings can beassociated with the metamaterial symmetry properties. In terms of symmetry, the gyroidmorphology belongs to the non-symmorphic space group I
32 [57], with underlying isogo-nal octahedral O point group. For the gyroid slab geometry, oriented along the cubic h i direction, this is reduced to the dihedral D group, which includes a two-fold ( C ) rota-tional symmetry axis along the optical axis and two C axes perpendicular to the opticalaxis. Based on the classification introduced by Menzel et al . [50,58], the homogenized meta-material slab thus exhibits elliptical counter-rotating eigenstates along the h i direction.In contrast to what has been observed in “metallic gyroid photonic crystals” [59], noneof these eigenstates are of the Bloch form in lossy metamaterials, but evanescent Floquetmodes [60] with higher order surface modes involved in the scattering process [41]. Withoutadditional approximations, symmetry implies the presence of a finite CD with symmetricalcircular polarization conversion and a linear dichroism, as experimentally observed in [41].The polarization conversion is further responsible for the CD observed in reflection (FigureS1) [4]. All experimental characterizations performed here are thus consistent with thesymmetry properties of a h i oriented gyroid slab.A careful investigation of the spectral shapes reveal features that are not an immediatesignature of the gyroid lattice: The minimum in the transmission spectra in Figure 4a atca. 520 nm seems to be indicative of a strong resonance that is not predicted in any of theprevious theories or simulation results [41,55]. However, due to the fabrication process of thethe gyroid metamaterials, involving an electrodeposition process on rough FTO substrates(see Methods), surfaces on both sides of the gyroid slab show non-planar terminations.7he observed polarization-sensitive transmission dip may thus stem from the non-planarterminations of the fabricated gyroid samples that lead to chiral protrusions supportingassociated localized plasmonic modes. Since the plasma resonance of silver is blue-shiftedcompared to gold, combined with the lower plasmonic losses in silver, a blue-shift of thisresonance by approximately 100 nm and a substantial increase of its strength is expected insilver gyroids. In the absence of absorption at ∼
400 nm for any of the constituent materials,we deduce that the strong decay of the silver spectra below 500 nm has the same origin asthe 520 nm minimum in gold.A further interesting spectral feature in Figure 4b is the minimum at ca. 520 nm, whichappears only in the LCP channel. Preliminary full wave simulations (not shown) indicatethat this spectral feature arises from a special topography at one of the surfaces in theabsence of bulk structural elements which provide filtering.
The spectra in Figures 3 and 4 show that the CD is strongly influenced and tunable bythe material composition. The fabrication process allows the replication of the copolymertemplate into different (plasmonic) materials, where the focus here is on silver and gold dueto their favorable plasmonic properties at optical wavelengths [61]. Figure 3 shows thatthe CD of gold metamaterials is strongly reduced compared to silver metamaterials. Thereduced CD can be associated with the higher intrinsic losses of the gold material properties[62]. The smaller losses of silver facilitates the resonant behavior of the nanostructure,resulting in a strong chiro-optical activity. The peak CD intensity is observed at ∼
510 nmfor silver gyroids and, bathochromically shifted, at 550 nm for gold gyroids, consistent withthe difference in the plasma frequency between the two materials [62].A higher refractive index environment results in an increase in the transmission forboth polarizations in gold at higher wavelengths above the resonance dip (Figure 4a). Thisarises from a shift of the surface plasmon polariton fields into the dielectric domain when thepermittivity of the latter is increased, leading to decreased effective absorption. At the sametime, the higher refractive index also increases the scattering cross-section of the localisedresonance as expected from Mie theory, leading to decreased transmission at the resonancedip. The CD increases and red-shifts as seen in Figure 4c. The underlying mechanismseems to be a circular polarization sensitivity of the resonance at its long wavelength tail,indicating that it originates from chiral protrusions.In silver gyroids, the optical response of the material depends strongly on the samplehistory. For the sample, in which the polymer was not removed, a similar result to goldwas observed, with a marked increase in peak transmission in the RCP channel. Interest-ingly, the polymer filled gyroid leads to an increased LCP resonance dip at 520 nm, leavingthe overall transmission in this channel approximately unchanged, resulting in a stronglyenhanced CD (see Figure S2). For the samples that were voided in an Ar plasma andsubsequently filled with dielectric oils, an opposite trend is observed - the transmissiondecreases with increasing refractive index in both channels. Since the optical differencebetween the polymer and oil-filled gyroids cannot be an effect of the dielectric constantalone, this must arise from differences in the sample preparation, i.e. the plasma removalof the polymer. As indicated above, there is evidence for the formation of a very thin car-bonaceous layer at the silver surface during this process. From the spectra in Figure 4b, itis evident that this layer causes losses in the propagation of plasmon-polaritons across thegyroid network, thereby reducing the transmission in both channels. The overall dichroismof the oil-infiltrated samples is, however, increased compared to the air, which is indicativeof a stronger transmission reduction in the LCP channel compared to the RCP signal.
The strength of the CD is well above previously reported values for other materials cre-ated using self-assembly techniques [4] and equals CD values in metamaterials fabricatedvia top-down approaches: in a comparable wavelength range (450 - 550 nm), where only afew examples have been reported so far, the best CD results were achieved by Esposito8 t al . [30]. In their work, the transmitted light under CP illumination through intertwin-ing triple-helical nanowires oriented along the optical axis has a CD contrast of ∼ T RCP = 0 . T LCP = 0 . T RCP = 0 . T LCP = 0 . h i out-of-plane orientation. Comparable experiments on single gyroid metamaterialswith other orientations would facilitate the understanding of the role of the metamaterialorientation on the optical properties. h i -oriented single gyroids have a higher degree ofrotational symmetry ( C ) so that the associated eigenstates of the metamaterial seperateinto the RCP and LCP channels, respectively [3, 50]. Therefore, a gyroid metamaterialoriented h i out-of-plane would exhibit a CD in transmission but not in reflection, that isdue to different absorption in the two channels, and zero polarization conversion. The ab-sence of polarization conversion around an n -fold ( n >2) symmetry [3] has been previouslyemployed as a design principle to eliminate polarization conversion in top-down fabricatedmetamaterials [30] and in photonic crystals [3, 63]. The gyroid naturally exhibits 3-foldaxes along its crystallographic h i direction, so that a gyroid metamaterial with this out-of-plane orientation would be very interesting both from a fundamental and an applicationpoint-of-view. Gyroids oriented along their h i direction are, on the other hand, expectedto lead to a much stronger CD [55]. In conclusion, we have demonstrated the presence of a strong CD in metallic gyroid op-tical metamaterials. Through a block copolymer-based fabrication method, we fabricatedsilver and gold metamaterials exhibiting a single gyroid morphology with a cubic unitcell with ∼
65 nm lattice constant. Optical micrographs and spectroscopic measurementswith circularly polarized light reveal a strong CD between 470 and 570 nm. The materialproperties and the refractive index of the surrounding medium have a large influence onthe CD. Compared to silver single gyroids, the CD in gold single gyroid metamaterials issignificantly weaker and spectrally shifted. Both of these observations are expected fromthe larger losses and the frequency-shifted plasmonic response. Additionally, the CD isstrongly affected by the host medium, where a higher refractive index results in a higherCD. The measured CD is much enhanced in comparison to structures previously fabricatedby top-down lithography.Our results further demonstrate an extreme sensitivity of the optical response to minutedetails in the 3D network morphology. The unexplained resonances in both the gold andsilver spectra most likely arise from as yet unexplained surface resonances that have beenshown to depend extremely sensitively on structural details at the gyroid surfaces. Par-ticularly the spectral differences in the two CP channels in silver are puzzling and couldstem from the presence of polarization-sensitive local resonances in chiral metamaterialprotrusions. Further investigations are necessary to test this hypothesis. As expected forplasmonic nanostructures, the plasmonic excitations at the metal-dielectric interface withinthe metamaterial are highly sensitive to the particular interface composition. In the caseof the silver gyroids studied here, this gives rise to a delicate interplay. While the passi-vation of the silver surface is essential to avoid its oxidation, the presence of carbonaceouslayer at the silver surface is detrimental for the plasmonic performance. Creating a silvernetwork within a polymer template (excluding air), on the other hand, provides excellentmetamaterial properties without the need to additionally stabilize the silver.Our results pave the way toward the bottom-up manufacture of 3D self-assembled silveroptical metamaterials. In particular, the feasibility of electrochemical silver replication of9olymeric 10-nm network morphologies demonstrates their utility for the creation of opticalmetamaterials. Applications of this process include tunable CD filters and a promisinglarge-scale material for chiral sensing. Accessing other high symmetry orientations of thegyroid containing screw axes and associated helical elements is expected to generate evenstronger chiro-optical properties, such as strong CD along the cubic h i direction [55]and optical activity in the absence of ellipticity along the C symmetric h i direction [3]. Sample Fabrication
A 10% solution of polyisoprene- b -polystyrene- b -poly(ethylene oxide) (ISO) triblock terpoly-mer (80 kg/mol, f PI = 0 . f PS = 0 . f PEO = 0 .
17) in anhydrous anisole (Sigma-Aldrich)was further spincoated for 60 s at 1200 rpm onto a fluorine-doped tin oxide coated glasssubstrate (FTO glass, Sigma-Aldrich). The FTO glass was immersed in a Piranha bathand then silanized by immersion in a 0.2% solution of octyltrichlorosilane (Sigma-Aldrich)in anhydrous anisole for 15 s. As-spun samples were annealed in a controlled solvent vapouratmosphere using an experimental set-up detailed in [40].Annealed films were treated with UV light (Mineralight ® XX-15S, 254 nm) for 15 min-utes and washed with ethanol absolute for 30 minutes to remove PI from the matrix and getthe voided structure. The sample was backfilled by plasmonic material through electrode-position. MetSil 500CNF (Metalor) silver solution and ECF 60 (Metalor) gold solutionmodified with arsenic trioxide were used for electrodeposition. The electrodeposition wascarried out using a Metrohm AutoLab PGSTAT302N potentiostat. Ag/AgCl with KCl(Metrohm) and platinum electrode tip (Metrohm) were used as the reference and counterelectrodes. The first step of the electrodeposition, the nucleation of the metallic structureon the conductive substrate, was performed using a start and lower vertex potential of -0.4V and -1.15 V. The potential for the growing step was -0.756 V for gold and -0.656 V forsilver.To remove the polymer from the replica, plasma etching was used for 20 minutes (PlasmaEtch Inc., PE-100-RIE, 50 W and 40CC/min Ar).
Optical Characterization
A Zeiss Axio Scope.A1 (Zeiss AG, Oberkochen, Germany) polarized light microscope and axenon light source (Thorlabs SLS401; Thorlabs GmbH, Dachau, Germany) was used for alloptical experiments. For spectroscopic measurements, the transmitted and reflected lightwas collected by an optical fibre (QP230-2-XSR, 230 µm core) with a measurement spot sizeof ≈
20 µm. The spectra were recorded by a spectrometer (Ocean Optics Maya2000 Pro;Ocean Optics, Dunedin, FL, USA). Light microscopy images were acquired with a PointGrey GS3-U3-28S5C-C (FLIR Integrated Imaging Solutions Inc., Richmond, Canada) CCDcamera. The linear polarizers (Thorlabs WP25M-UB, Thorlabs) and quarter waveplates(B. Halle, Germany) were both superachromatic, allowing an effective measurement rangeof 400 - 800 nm.
Scanning electron microscopy (SEM)
Samples were imaged using a TESCAN (TESCAN, a.s., Brno, Czech Republic) MIRA3field emission scanning electron microscope.
Supporting InformationAcknowledgements
This research was supported by the Swiss National Science Foundation through grant num-ber 163220and 188647, the Ambizione program (168223 to BDW), and the National Centreof Competence in Research “Bio-Inspired Materials”.
Conflict of Interest
The authors declare no conflict of interest. 10 eywords circular dichroism, chiral media, optical metamaterials, 3D nanostructure
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Strong Circular Dichroism in Single Gyroid Optical Metamaterials
Cédric Kilchoer, Narjes Abdollahi, James A. Dolan, Doha Abdelrahman, Matthias Saba, Ulrich Wiesner, Ullrich Steiner, Ilja Gunkel and Bodo D. Wilts
Figure S1 : Circular dichroism in reflection under RCP and LCP illumination with polymer and air as surrounding medium.
Figure S2: Circular dichroism in silver gyroid optical metamaterial with polymer as surrounding medium. Samples are illuminated by right-handed (a) and left-handed (b) circularly polarized light. Scale bar is 500 µm. (c) Transmittance spectra of an individual domain collected after the RCP and LCP filtering. (d)
𝐶𝐶𝐶𝐶 measured on 10 different domains (d). The circular dichroism of domains 1 and 2, marked in (a), is highlighted.measured on 10 different domains (d). The circular dichroism of domains 1 and 2, marked in (a), is highlighted.