Quantifying Figures of Merit for Localized Surface Plasmon

Jan 18, 2019 - Using localized surface plasmon resonances (LSPR) to focus electromagnetic radiation to the nanoscale shows the promise of unprecedente...
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Quantifying figures of merit for localized surface plasmon resonance applications: a materials survey Brock Doiron, Monica Mota, Matthew P Wells, Ryan Bower, Andrei Mihai, Yi Li, Lesley F. Cohen, Neil McN. Alford, Peter K. Petrov, Rupert F Oulton, and Stefan A. Maier ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01369 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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ACS Photonics

Quantifying figures of merit for localized surface plasmon resonance applications: a materials survey Brock DoironA, Monica MotaA, Matthew P. WellsB, Ryan BowerB, Andrei MihaiB, Yi LiA, Lesley F. CohenA, Neil McN. AlfordB, Peter K. PetrovB, Rupert F. OultonA, Stefan A. MaierD,A Department of Physics, Imperial College London, London, UK Department of Materials, Imperial College London, London, UK D Nanoinstitut München, Chair in Hybrid Nanosystems, Faculty of Physics, Ludwig-Maximilians Universität München, München, Germany A

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ABSTRACT Using localized surface plasmon resonances (LSPR) to focus electromagnetic radiation to the nanoscale shows the promise of unprecedented capabilities in optoelectronic devices, medical treatments and nanoscale chemistry, due to a strong enhancement of light-matter interactions. As we continue to explore novel applications, we require a systematic quantitative method to compare suitability across different geometries and a growing library of materials. In this work, we propose application-specific figures of merit constructed from fundamental electronic and optical properties of each material. We compare seventeen materials from four material classes (noble metals, refractory metals, transition metal nitrides, and conductive oxides) considering eight topical LSPR applications. Our figures of merit go beyond purely electromagnetic effects and account for the materials’ thermal properties, interactions with adjacent materials, and realistic illumination conditions. For each application we compare, for simplicity, an optimized spherical antenna geometry and benchmark our proposed choice against the state-of-the-art from the literature. Our propositions suggest the most suitable plasmonic materials for key technology applications and can act as a starting point for those working directly on the design, fabrication and testing of such devices. Keywords: Plasmonics, Mie Theory, Material Characterization, Hot Electron Devices, Photothermal Applications

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The strong interaction between light and the free electrons of metals gives rise to extraordinary phenomena not possible with conventional dielectric photonic systems. The oscillating electric field of light can resonantly drive the oscillation of these free electrons in what is called a surface plasmon resonance (SPR) due to the generation of interfacial charges that coherently exchange energy with the electromagnetic field. From the coining of the term “surface plasmon” as a quantization of plasma oscillation in the late 1950s in thin metal foils,1 plasmonics has developed into an active area of research2. With recent developments in nanofabrication techniques, it is possible to fabricate metallic particles with sizes below the wavelength of light that still readily interact with incident radiation. This excitation is termed a localized surface plasmon resonance (LSPR) distinguished from a propagating surface plasmon polariton (SPP) in thin metal films. The ability to confine and control light on length scales below the diffraction limit was unprecedented and stimulated a surge of proposed novel applications3 including integrated photonic circuits3, imaging4,5 and sensing6,7. Although these and many other plasmonic applications have been demonstrated experimentally, they have yet to be integrated into widespread industrial settings as is seen with photonics and electronics with the exception of surface enhanced Raman scattering (SERS) substrates.8 This is primarily due to the incompatibility with industrial fabrication techniques due to the relatively low melting temperatures of conventional plasmonic metals resulting in diffusion into silicon9 and the difficulty of patterning inert metals using gas phase chemical etching10 . In addition, the large absorptive losses at visible wavelengths result in pronounced heating, which can speed up the degradation of surrounding components.11 Recently, there has been particular interest in alternative materials and the development of non-radiative applications12,13 to circumvent these issues without compromising performance. We divide the diverse range of materials currently being investigated into four classes: noble metals, refractory (high-temperature stable) metals, transition metal nitrides, and conductive oxides. The extensive range of physical properties (hardness, heat tolerance, thermal dissipation) and electronic characteristics (free carrier concentration, effective mass) are taken into account to examine materials which will facilitate plasmonic devices that can be efficiently implemented across the ultraviolet (UV), visible and infrared (IR) regimes. The summary of the operation ranges and advantageous properties is summarized in Table 1. The most widely used and wellstudied materials in plasmonics are silver (Ag), gold (Au), and copper (Cu). These noble metals were originally preferred due to their stability and high conductivity resulting in sharp resonances. However, these materials are also plagued by low melting temperatures (