3D Chiral Plasmonic Metamaterials Fabricated by Direct Laser Writing: The Twisted Omega Particle

Abstract

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][7][8][9][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] Results and Discussion Fabrication and Optical Characterization Fabrication ApproachTwisted omega architectures have already been theoretically studied and discussed as prototype for plasmonic structures with strong chiro-optical far-field response. [16][17][18][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 spectroscopicThe 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.

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