Scalable Fabrication of Composite Ti/Ag Plasmonic Helices: Controlling Morphology and Optical Activity by Tailoring Material Properties

Abstract

245 wileyonlinelibrary.com COMMUNICATION www.MaterialsViews.com www.advopticalmat.deThe interest in the interaction of metal helices with electromagnetic radiation has a long history. Traditionally, these interactions have been primarily applied in antenna engineering in the design of end fi re and polarization-insensitive receivers. [ 1 ] More recently, metal helices have been investigated for applications as chiral metamaterials, and have demonstrated some unique effects such as broadband circular polarization and negative refraction. [2][3][4][5] In particular, chiral metamaterial applications of metal helices in the visible wavelength region rely on effects associated with the strong resonances of localized surface plasmons of noble metals, especially gold and silver. However, due to the three-dimensional shape and requisite small feature sizes, the fabrication of plasmonic helices that are active in the visible wavelength region remains challenging. Most fabrication methods to date have relied on the use of rather complicated, expensive, and multi-step techniques to generate small-sized passive templates or scaffolds that are metalized to achieve plasmonic activity. For example, direct laser writing of helical cavities in positive tone photoresist, followed by development and electrochemical deposition of gold, has been used to produce square-lattice arrays of plasmonic helices that show a broadband response in the infrared region. [ 4 ] Additionally, selfassembled helical superstructures of noble metal nanoparticles and biological macromolecules (e.g., DNA, peptides, and proteins) have shown plasmonic chiral optical properties. [ 6 ] While these methods are certainly novel, one is consistently forced to choose between the simplicity of the method and the strength of optical activity in the production of plasmonic helices. In order to fulfi ll the promise of metal helices as chiral metamaterials, a scalable fabrication method capable of producing plasmonic helices with signifi cant optical activity will need to be developed.Glancing angle deposition (GLAD), a simple and scalable physical vapor deposition method based on geometric shadowing effect, [ 7 ] is a potential method to achieve this goal, and has already been used to design dielectric helices for circular polarizers based on the Bragg phenomenon. [ 8 ] Clearly, the incorporation of Ag and Au within the GLAD technique is a very appealing method of fabricating plasmonic helices for chiral metamaterial applications, especially given the recent demonstrated potential for GLAD roll-to-roll processing. [ 9 ] However, the high surface mobility of Ag or Au adatoms results in kinetic growth processes that overwhelm the shadowing effect, and precludes the production of non-equilibrium helical structures using conventional GLAD. [ 10 ] Large diameter, squareshaped Ag helices have been fabricated using oblique angle deposition (OAD), [ 11 ] a variation of GLAD, but the assimilation of Ag or Au within the full spectrum of GLAD structures has previously eluded...

Chiroptical response of a single plasmonic nanohelix

“…The experimentally retrieved CD reaches a maximum value of approximately 2.5 %. It should be emphasized again that this value is recorded for a single nanohelix, in contrast to most of the studies reporting on CD values for large arrays of chiral nanostructures [13,40,43,[46][47][48]51]. In the case discussed here, a single nanohelix with an approximate diameter of only 120 nm is excited by a Gaussian beam focused weakly down to a spot with diameter of around 1.2 µm (full width at half maximum at λ = 1450 nm).…”

Hot-Volumes as Uniform and Reproducible SERS-Detection Enhancers in Weakly-Coupled Metallic Nanohelices

“…According to far-field studies, the optical response of a nanohelix is governed by localized surface plasmon resonances (LSPR)1314151617181920. Therefore, regular metallic nanohelices bear the potential of converting incoming light into strongly enhanced and localized optical near-fields18.…”

Optical and Infrared Helical Metamaterials

“…However, one must note that in this case, unwanted circular polarization conversions will again play a role and diminish the achievable extinction ratio. Similarly, the Zhao group has recently reported the fabrication of titanium-silver composite helices via GLAD [51].…”