Ultrathin niobium nanofilms on fiber optical tapers--a new route towards low-loss hybrid plasmonic modes.

Torsten Wieduwilt,Uwe Hübner,Sebastian Goerke,Markus A Schmidt,Jan Dellith,Sven Linzen,Alessandro Tuniz

doi:10.1038/srep17060

Abstract

Due to the ongoing improvement in nanostructuring technology, ultrathin metallic nanofilms have recently gained substantial attention in plasmonics, e.g. as building blocks of metasurfaces. Typically, noble metals such as silver or gold are the materials of choice, due to their excellent optical properties, however they also possess some intrinsic disadvantages. Here, we introduce niobium nanofilms (~10 nm thickness) as an alternate plasmonic platform. We demonstrate functionality by depositing a niobium nanofilm on a plasmonic fiber taper, and observe a dielectric-loaded niobium surface-plasmon excitation for the first time, with a modal attenuation of only 3–4 dB/mm in aqueous environment and a refractive index sensitivity up to 15 μm/RIU if the analyte index exceeds 1.42. We show that the niobium nanofilm possesses bulk optical properties, is continuous, homogenous, and inert against any environmental influence, thus possessing several superior properties compared to noble metal nanofilms. These results demonstrate that ultrathin niobium nanofilms can serve as a new platform for biomedical diagnostics, superconducting photonics, ultrathin metasurfaces or new types of optoelectronic devices.

Highlights

Environment adjacent to the metal layer[10]
The adhesion of gold on glass substrates is rather poor, imposing the need for additional adhesion layers (e.g. Ti or Cr) between the dielectric and the metal, which can deteriorate the optical properties of the device, if the film thickness falls below ten nanometers, as adhesion layers typically impose more optical loss
Even though ultrathin niobium films reveal superior mechanical, chemical and nanostructuring properties, the immediate question from a plasmonics point of view is: Do ultrathin niobium films support SPPs and how do their properties compare to state-of-the-art materials such as noble metals?

Summary

Introduction

Environment adjacent to the metal layer[10]. SPPs are promising candidates for substantially downscaling current optoelectronic components[9,15], with the vision of integrating nanoelectronics and nanophotonics on an ultra-compact platform with an unprecedentedly small footprint. The adhesion of gold on glass substrates is rather poor, imposing the need for additional adhesion layers (e.g. Ti or Cr) between the dielectric and the metal, which can deteriorate the optical properties of the device, if the film thickness falls below ten nanometers, as adhesion layers typically impose more optical loss This additional deposition step often requires that the sample be transferred into a different deposition apparatus, resulting in oxide formation on the adhesion layer (especially in the case of Ti), significantly altering or even degrading the optical properties of the gold. Niobium films reveal substantially better adhesion to glass, in particular to silica, leading to solid metallic films which cannot be mechanically removed from glass surfaces without destroying the actual sample surface This is especially relevant in the framework of ultrathin layers, as no additional adhesion layer is required as in the case of gold. Oxide film formation can be controlled and even enforced on the nanometer level via electrolytic passivation, i.e. anodization[29]; since the adhesion of the oxide layer to the niobium film is extremely strong, this leads to an intrinsically robust multilayer with predefined thickness, making additional film deposition steps obsolete

Methods

Results

Conclusion