3D Chiral Plasmonic Metamaterials Fabricated ... - Wiley Online Library

FULL PAPER Chiral Metamaterials

www.advopticalmat.de

3D Chiral Plasmonic Metamaterials Fabricated by Direct Laser Writing: The Twisted Omega Particle Ioanna Sakellari,* Xinghui Yin, Maxim L. Nesterov, Konstantina Terzaki, Angelos Xomalis, and Maria Farsari* and by exploiting their acute response to their immediate environment, a variety of novel applications have been realized such as negative indices of refraction,[3,4] broadband circular polarization devices,[5] and strongly twisted local electromagnetic fields for sensitive detection of chiral molecules.[6–10] In chiral media, optical activity is a result of the cross-coupling between electric and magnetic fields. Two classical approaches have been proposed to model the mechanisms of optical activity:[11,12] (a) the coupled oscillator model system, where optical activity arises from the coupling of two separate, noncollinear oscillators, and (b) the one-electron model system, where an electron is bound on a helix, giving electric and magnetic character to the optical transitions. Recently, the plasmonic analogue of the coupled oscillator model system, which consists of a system of two cornerstacked gold nanorods, was experimentally demonstrated.[13] This system resembles two coupled, vertically displaced electrons that carry out orthogonal harmonic oscillation driven by an external light field. In this work, we experimentally and theoretically study the plasmonic version of the one-electron model system, which comprises a loop-wire structure, namely the so-called “twisted omega particle.” In this case, the 3D plasmonic meta-atom combines a small electric dipole antenna (the metallic wire) and a split-ring resonator (the loop), which exhibits a magnetic dipole resonance leading to a different electromagnetic response to RCP light and the left-handed one (LCP).[14,15]

The plasmonic version of a 3D chiral meta-atom which consists of a loopwire structure, namely the so-called twisted omega particle, is experimentally realized. The structure is fabricated by direct laser writing and subsequent electroless silver plating, a novel technique capable of producing truly 3D photonic nanostructures. In this case, the metallic wire of finite length supports an electric dipole resonance, whereas the loop acts as a split-ring resonator which exhibits a magnetic dipole resonance, leading to the separation of right-handed circularly polarized light and the left-handed one. The arising optical activity is discussed in terms of a single oscillator model system used classically to describe the generation of natural optical activity in chiral media, and it is shown that the twisted omega particle acts as its exact plasmonic analogue.

1. Introduction An object or a structure is called chiral if it displays no mirror symmetry plane.[1] A typical property characterizing the optical response of a chiral medium is optical activity, which relates to the polarization rotation of a linearly polarized light as it passes through such a medium. It manifests as circular dichroism (CD), i.e., different absorption between left- and right-handed circularly polarized (RCP) light, and as optical rotatory dispersion, i.e., wavelength dependence of optical rotation, and has been exploited in several applications in many disciplines of science including chemistry, physics, and biology. Recent developments in nanofabrication technologies have enabled the realization of artificial complex chiral materials composed of subwavelength scale building blocks.[2] These man-made materials, called metamaterials, display electromagnetic properties beyond those that can be found in nature mainly due to the geometric structure of their building blocks. By properly designing the structural properties of chiral metamaterials

2. Results and Discussion 2.1. Fabrication and Optical Characterization

Dr. I. Sakellari, Dr. X. Yin, Dr. M. L. Nesterov 4th Physics Institute and Research Center SCoPE University of Stuttgart Building 57, Stuttgart 70569, Germany E-mail: [email protected] Dr. K. Terzaki, A. Xomalis, Dr. M. Farsari IESL-FORTH N. Plastira 100, Heraklion, Crete 70013, Greece E-mail: [email protected]

DOI: 10.1002/adom.201700200

Adv. Optical Mater. 2017, 5, 1700200

2.1.1. Fabrication Approach Twisted omega architectures have already been theoretically studied and discussed as prototype for plasmonic structures with strong chiro-optical far-field response.[16–19] To this end, Helgert et al. utilized a layer-by-layer approach to fabricate an architecture reminiscent of the twisted omega by placing two L-shaped gold nanoparticles on top of each other and connected in the vertical direction.[20] Based on a combined spectroscopic 1700200 (1 of 6)

© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

www.advancedsciencenews.com


Figure 1. Dielectric template of twisted omega particles: a,b) overview of a 1.5 µm × 1.5 µm array of loop-wire meta-atoms with loop radius = 325 nm = wire length, c) top view, and d) single twisted omega particle.

and interferometric characterization, a rotation of polarization azimuth of linearly polarized light exceeding 50° at wavelengths around 1.08 µm was experimentally found, while circular dichroism and circular birefringence were also extracted using the classical Jones calculus. However, a direct realization of such a structure has not yet been realized, as the fabrication of such a highly curved architecture is seemingly difficult at optical frequencies. In this work, we combine direct laser writing (DLW) and subsequent electroless silver plating to realize truly 3D twisted omega meta-atoms. Our approach is based on DLW into a negative tone photoresist, where the dielectric template is coated with a conformal metal film.[21–24] In our case, the silver plating is achieved by incorporating metal binding monomers in the negative tone resin employed.[22–24] DLW into a positive tone photoresist followed by electroplating with gold is another approach already used to fabricate 3D plasmonic metaatoms.[5,25] Gold helix and tapered-helix mid infrared metamaterials have already been realized utilizing the latter approach. Initially, hollow cavities of the desired architectures are generated within the volume of a positive tone resin by DLW on a conductive substrate and, subsequently, filled with gold using an electrochemical deposition procedure. However, utilizing this latter approach in order to generate twisted omega architectures, restrictions on the template fabrication were encountered due to the structure design and the geometric parameters required. In particular, the development of such highly curved and narrow channels proved to be a trivial task, while scaling the twisted omegas down to a small loop-radius (80 s), silver adhesion on the substrate increases further leading to the formation of large clusters as silver nanoparticles merge together (see inset of Figure S2 in the Supporting Information). This results in a decrease in the overall transmitted intensity, while the reflection intensity increases and, the sample, in this case, acts like a silver mirror (Figure S2, Supporting Information). Furthermore, numerical calculations performed for silver plated twisted omegas with a uniform silver coating with a thickness of 30 nm covering the dielectric templates as well as the glass substrate (Figure S3, Supporting Information) show a total suppression of the overall transmission intensity, while a simultaneous increase in reflection intensity is observed in accordance with the experimental findings.

1700200 (3 of 6)




2.1.2. Optical Characterization When circularly polarized light impinges on a twisted omega particle, the wire part of the structure increases the coupling to the external light field resulting in an oscillating electric dipole along its axis, while a magnetic dipole moment is also induced in the same direction due to the coiled wire. An electric and a magnetic moment parallel to each other result in strong optical activity effects following heuristic considerations related to the Rosenfeld formula expressing the rotational strength (R) for chiral molecules[31] R = Im {µab ⋅ m ab }

(1)

where μab is the electric and mab is the magnetic dipole transition moment for electronic transitions between states a and b. Transmission measurements for left- and right-handed circularly polarized light passing through the silver plated structures are plotted in Figure 4a. The measurements were carried out at normal incidence in the spectral region from 2.5 to 6.5 µm with the incident light traveling in the +z direction. As one can see, when LCP light impinges on the fabricated left-handed twisted omega structures, a resonance is featured at 5.25 µm, whereas the resonance is suppressed for RCP incident light, demonstrating strong differential circular polarization response in accordance with theoretical predictions (Figure 4b and Figure S5 (Supporting Information)). Note that the overall transmitted intensity at resonance in the optical measurements is about 30% lower in relation to the one theoretically predicted, which can be probably attributed to losses due to the granular silver coating on top of the twisted omega dielectric templates as well as due to scattering from silver nanoparticles adhered on the

substrate. Furthermore, in order to ensure that the fundamental resonance is excited, the surface charge distribution at resonance is plotted in Figure 5, where the characteristic dipolar mode assures that the fundamental mode is the one observed. Also, note that the structure performance is not ideal as the transmission of LCP light does not drop down to 0 in the theoretical calculations shown in Figure 4b, while RCP transmittance does not reach a 100% value exhibiting a plasmon resonance dip at the same position as LCP light. The physical reason for this behavior lies in the fact that the geometry and, in particular, the pitch of the twisted omega is not perfectly matched to the pitch (i.e., the wavelength) of the incident light, which heuristically is the same reason why naturally occurring chiral molecules exhibit extremely low circular dichroism responses. Typically, CD spectra are measured from spatially isotropic samples,[32] while in our case the twisted omegas are all aligned in the same direction. In order to estimate the contribution of cross-converted light due to elliptical birefringence, simulations of the cross-polarized transmissions were carried out (Figure S4, Supporting Information). It can be seen that the contribution of cross-polarized light is negligible, which confirms that the observed differences in RCP and LCP transmission are actually a signature of optical activity.

3. The Twisted Omega Particle: One-Electron Model System The interest of the twisted omega structure lies in the fact that one can isolate the fundamental optical mode contrary to what is the case for many-turn helices. Since in the case of the

Figure 4. a) Measured and b) simulated transmission spectra and differential circular polarization response of the fabricated silver plated twisted omega meta-atoms for LCP and RCP incident light.


1700200 (4 of 6)




Condon expression is considered to be able to describe qualitatively the CD response.

4. Conclusion

Figure 5. Surface charge distribution along the twisted omega particle at the fundamental resonance at 5.25 µm.

twisted omega particles a single connected structure is considered, one can think of the omega particle as the prototypical example of a one-electron chiral system. Condon in 1937 established a working single-electron model based on resonance models by Lorentz, Drude, and others.[11,12] Instead of Drude’s helical path, the electron is situated in a general chiral potential. This model describes an oscillator in a chiral environment that enforces a helical oscillation.[33] We start from the constitutive relations for the bulk (homogeneous chiral) material, where a Condon model for the frequency dependence of the material parameter displaying chirality is assumed[34]

D = ε 0 ε E + ( χ − iκ )

H c

(2)

E c

(3)

B = µ0 µH + ( χ + iκ )

where E is the electric and H is the magnetic field strength, D is the electric, and B is the magnetic flux density, and χ is the Tellegen parameter. The material parameter of primary interest is the Pasteur or chirality parameter κ in the constitutive relations, which describes the degree of chirality and measures the effect of cross-coupling between electric and magnetic fields. According to Condon’s single-electron model, the frequency behavior for the imaginary part of the chirality parameter κ(ω) is given, as treated in Sihvola,[34,35] by

κ ′ (ω ) =

τω 0 x(1 − x 2 ) 1 − (2 − d 2 )x 2 + x 4

(4)

where x = ω /ω 0 is the relative frequency of the molecular transition, ω0 is the resonant frequency, d = Γ/ω 0 is a measure of the damping associated with the transition, and τ is a characteristic time constant describing the magnitude of chirality. Note that the analytic Condon expression for the singleelectron model system (Equation (4)), that can be used to describe the CD response for the case of the twisted omega particle, stands for individual isolated particles. When the particles are placed in an array, a shift in operational frequency is to be expected due to the interaction between the neighboring structures. Nevertheless, the interaction is weak and, thus, the


We demonstrate that direct laser writing in combination with subsequent electroless silver plating is a quite flexible fabrication method for the realization of truly 3D chiral plasmonic metamaterials. The plasmonic version of a 3D chiral metaatom which consists of a loop-wire medium, or the so called “twisted omega” particle, is experimentally realized following the above fabrication approach. Furthermore, Fourier transformed infrared (FTIR) measurements with circularly polarized light reveal a significant transmittance difference between RCP and LCP incident light, which is explained in terms of the geometry of the structure. Finally, the contribution of a single fundamental mode to the chiro-optical spectrum of the twisted omega particle demonstrates the plasmonic analogue to the one-electron theory of optical activity.

5. Experimental Section Direct Laser Writing: The 3D twisted omega architectures are fabricated by Direct Laser Writing via two photon absorption using a commercially available femtosecond laser lithography system (Photonic Professional GT, Nanoscribe GmbH, Germany). The laser beam was focused using a high numerical aperture microscope objective lens 100×, N.A. 1.3, Zeiss EC Plan Neofluar. The writing speed employed was 2.5 µm s−1 and the average power used was 2% of the optimal power measured before the objective lens. First, the dielectric templates of the twisted omega architectures were generated into the volume of a photosensitive organic–inorganic hybrid material containing metal binding moieties. After the completion of the component building process by DLW, the samples were developed for 20 min in a 50:50 solution of 1-propanol/2-propanol and were further rinsed with 2-propanol. Then, they were subsequently metallized using an electroless silver plating procedure. Material Synthesis: The synthesis of the photosensitive organic– inorganic hybrid composite used for the fabrication of the 3D twisted omega architectures has been described previously.[30] It consists of methacryloxy-propyl trimethoxysilane (MAPTMS), zirconium propoxide (ZPO, 70% in propanol), methacrylic acid (MAA), and 2-DMAEMA. MAPTMS and DMAEMA were used as the organic photopolymerizable monomers, while ZPO and the alkoxysilane groups of MAPTMS were used to form the inorganic network. DMAEMA served as a metalbinding moiety. In our case, the organic–inorganic hybrid material was synthesized by mixing the above chemicals in the following molar ratios: MAPTMS:ZPO = 8:2, ZPO:MAA = 1:1, and (MAPTMS+ZPO):DMAEMA = 7:3. Michler’s ketone (4,4-bis(diethylamino) benzophenone) was used as a photoinitiator and added at 1% w/w concentration to the final solution MAPTMS and DMAEMA composite. All chemicals were obtained from Sigma-Aldrich and used without further purification. The samples were prepared by drop casting onto 100 µm thick silanized glass substrates and the resultant films were dried in air for several days before photopolymerization. Electroless Silver Plating: The chemical metallization procedure followed is similar to the one described in ref. [24]. It mainly consists of three stages: (i) the seeding, (ii) the reduction, and (iii) the plating. Initially, the fabricated twisted omega dielectric templates are immersed into an aqueous solution of silver nitrate (AgNO3), where silver ions bind to amine moieties on surface and act like seeds for silver nanoparticles. In sequence, the sample is immersed into an aqueous solution of sodium borohydride (NaBH4), where the silver ions are reduced to form

1700200 (5 of 6)




silver nanoparticles. Finally, a silver bath/plating procedure takes place where the sample is immersed into an aqueous solution composed of silver nitrate (AgNO3), ammonia (NH3), and glucose (C6H12O6) serving as a reducing agent in order to obtain the metal coated structures. Typical plating times are 45–75 s. After each stage, the sample is rinsed twice in deionized water and dried in air at room temperature. Simulations: A commercial 3D full-wave finite element method solver (Comsol Multiphysics) is used for modeling of the silver plated twisted omega particles, where the refractive index for bulk silver (Ag) was employed according to Rakic.[36] A single unit cell is considered with periodic boundary conditions along the x- and y-directions, while an incident plane wave propagating in the +z-direction was used to excite the structure. Note that the source point of view was used for the reasoning; hence, for RCP light, the electric field vector propagated clockwise away from the source. The parameters employed for simulating the silver plated twisted omega particles were the following (Figure 3): wire length R1 = 380 nm, pitch R2 = 190 nm, loop radius R3 = 300 nm, silver shell thickness s = 30 nm. The wire was simulated to have an elliptical cross-section with a big diameter of the ellipsoid d1 = 300 nm and a small diameter of the ellipsoid d2 = 100 nm. The parameters employed for simulations were slightly smaller (≈25 nm) than those used for actual writing of the twisted omega particles in order to account for shrinkage effects. Measurements: FTIR transmittance/reflectance spectra were measured with a Vertex 80 spectrometer coupled to a Hyperion 2000 IR microscope (Bruker Optics) which was equipped with Cassegrain objectives (15×, 36×) and a liquid nitrogen cooled mercury cadmium telluride detector. Transmission measurements for circularly polarized light are carried out by inserting a custom ordered waveplate with a retardation of λ/4 at wavelengths from 2.5 to 7 µm (B. Halle Nf. Berlin) into the beam path between a linear polarizer and the sample. All transmittance spectra were normalized with respect to the bare glass substrate, which became opaque at wavelengths above 6.5 µm. All reflectance spectra were normalized with respect to a gold mirror.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This project had received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie SkłodowskaCurie grant agreement No 655249. X.Y. thanks the Carl Zeiss Foundation, and M.L.N. thanks the Alexander von Humboldt Foundation. The work was also supported by BW Stiftung and by the ERC Advanced Grant COMPLEXPLAS. The authors would also like to thank Dr. Anna Tasolamprou for fruitful discussions about the work presented in this paper.

Conflict of Interest The authors declare no conflict of interest.

Keywords chiral metamaterials, direct laser writing, electroless silver plating, plasmonics, twisted omega Received: March 2, 2017 Revised: May 15, 2017 Published online: June 16, 2017


[1] W. Kelvin, Baltimor Lectures on Molecular Dynamics and the Wave Theory of Light, C. J. Clay And Sons, London, 1904. [2] Z. Wang, F. Cheng, T. Winsor, Y. Liu, Nanotechnology 2016, 27, 412001. [3] J. B. Pendry, Science 2004, 306, 1353. [4] Z. Li, M. Mutlu, E. Ozbay, J. Opt. 2013, 15, 23001. [5] J. K. Gansel, M. Thiel, M. S. Rill, M. Decker, K. Bade, V. Saile, G. von Freymann, S. Linden, M. Wegener, Science 2009, 325, 1513. [6] E. Hendry, T. Carpy, J. Johnston, M. Popland, R. V. Mikhaylovskiy, A. J. Lapthorn, S. M. Kelly, L. D. Barron, N. Gadegaard, M. Kadodwala, Nat. Nanotechnol. 2010, 5, 783. [7] Y. Tang, A. E. Cohen, Science 2011, 332, 333. [8] M. Schäferling, N. Engheta, H. Giessen, T. Weiss, ACS Photonics 2016, 3, 1076. [9] M. Schäferling, X. Yin, N. Engheta, H. Giessen, ACS Photonics 2014, 1, 530. [10] M. Schäferling, D. Dregely, M. Hentschel, H. Giessen, Phys. Rev. X 2012, 2, 1. [11] E. U. Condon, Rev. Mod. Phys. 1937, 9, 432. [12] E. U. Condon, W. Altar, H. Eyring, J. Chem. Phys. 1937, 5, 753. [13] X. Yin, M. Schäferling, B. Metzger, H. Giessen, Nano Lett. 2013, 13, 6238. [14] D. L. Jaggard, A. R. Mickelson, C. H. Papas, Appl. Phys. 1979, 18, 211. [15] Y. Ra’di, S. A. Tretyakov, New J. Phys. 2013, 15, 53008. [16] S. A. Tretyakov, F. Mariotte, S. Member, C. R. Simovski, T. G. Kharina, J. Heliot, IEEE Trans. Antennas Propag. 1996, 44, 1006. [17] C. Rockstuhl, C. Menzel, T. Paul, F. Lederer, Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79, 35321. [18] M. S. Mirmoosa, Y. Ra’di, V. S. Asadchy, C. R. Simovski, S. A. Tretyakov, Phys. Rev. Appl. 2014, 1, 34005. [19] V. S. Asadchy, I. A. Faniayeu, Y. Ra’Di, S. A. Tretyakov, Photonics Nanostruct.: Fundam. Appl. 2014, 12, 298. [20] C. Helgert, E. Pshenay-Severin, M. Falkner, C. Menzel, C. Rockstuhl, E. B. Kley, A. Tünnermann, F. Lederer, T. Pertsch, Nano Lett. 2011, 11, 4400. [21] A. Radke, T. Gissibl, T. Klotzbücher, P. V. Braun, H. Giessen, Adv. Mater. 2011, 23, 3018. [22] N. Vasilantonakis, K. Terzaki, I. Sakellari, V. Purlys, D. Gray, C. M. Soukoulis, M. Vamvakaki, M. Kafesaki, M. Farsari, Adv. Mater. 2012, 24, 1101. [23] K. Terzaki, N. Vasilantonakis, A. Gaidukeviciute, C. Reinhardt, C. Fotakis, M. Vamvakaki, M. Farsari, Opt. Mater. Express 2011, 1, 586. [24] G. Kenanakis, A. Xomalis, A. Selimis, M. Vamvakaki, M. Farsari, M. Kafesaki, C. M. Soukoulis, E. N. Economou, ACS Photonics 2015, 2, 287. [25] J. K. Gansel, M. Latzel, A. Frölich, J. Kaschke, M. Thiel, M. Wegener, Appl. Phys. Lett. 2012, 100, 101109. [26] J. Kaschke, M. Wegener, Opt. Lett. 2015, 40, 3986. [27] J. Fischer, M. Wegener, Laser Photonics Rev. 2013, 7, 22. [28] G. Bickauskaite, M. Manousidaki, K. Terzaki, E. Kambouraki, I. Sakellari, N. Vasilantonakis, D. Gray, C. M. Soukoulis, C. Fotakis, M. Vamvakaki, M. Kafesaki, M. Farsari, A. Pikulin, N. Bityurin, Adv. Optoelectron. 2012, 2012, 1. [29] A. Pikulin, N. Bityurin, V. I. Sokolov, AIP Adv. 2015, 5, 127215. [30] I. Sakellari, E. Kabouraki, D. Gray, V. Purlys, C. Fotakis, A. Pikulin, N. Bityurin, M. Vamvakaki, M. Farsari, ACS Nano 2012, 6, 2302. [31] L. Rosenfeld, Z. Phys. 1929, 52, 161. [32] M. Decker, R. Zhao, C. M. Soukoulis, S. Linden, M. Wegener, Opt. Lett. 2010, 35, 1593. [33] M. Schäferling, Chiral Nanophotonics, Springer International Publishing, Cham, Switzerland, 2017. [34] A. Sihvola, Prog. Electromagn. Res. 1994, 9, 45. [35] A. H. Sihvola, J. Electromagn. Waves Appl. 1992, 6, 1177. [36] A. B. Djuris, J. M. Elazar, M. L. Majewski, A. D. Rakic, A. B. Djurisic, Appl. Opt. 1998, 37, 5271.

1700200 (6 of 6)
