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A Low-Temperature Annealing Method for Alloy Nanostructures and Metasurfaces: Unlocking a Novel Degree of Freedom

Jeonghyeon Kim, Olivier Martin, Debdatta Ray, Christian Santschi, Hsiang-Chu Wang

The material and exact shape of a nanostructure determine its optical response, which is especially strong for plasmonic metals. Unfortunately, only a few plasmonic metals are available, which limits the spectral range where these strong optical effects can be utilized. Alloying different plasmonic metals can overcome this limitation, at the expense of using a high-temperature alloying process, which adversely destroys the nanostructure shape. Here, a low-temperature alloying process is developed where the sample is heated at only 300 degrees C for 8 h followed by 30 min at 450 degrees C and Au-Ag nanostructures with a broad diversity of shapes, aspect ratios, and stoichiometries are fabricated. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses confirm the homogeneous alloying through the entire sample. Varying the alloy stoichiometry tunes the optical response and controls spectral features, such as Fano resonances. Binary metasurfaces that combine nanostructures with different stoichiometries are fabricated using multiple-step electron-beam lithography, and their optical function as a hologram or a Fresnel zone plate is demonstrated at the visible wavelength of lambda = 532 nm. This low-temperature annealing technique provides a versatile and cost-effective way of fabricating complex Au-Ag nanostructures with arbitrary stoichiometry.

WILEY-V C H VERLAG GMBH2022

Fabrication and applications of AuAg alloy metasurfaces and hybrid metal dielectric nanoantennas

Debdatta Ray

The most prevalent materials used in the field of plasmonics are Au and Ag. However, for the past few years, the plasmonic community has also been looking for alternative materials that have lower losses than Au and higher stability than Ag. This thesis is an attempt to provide some nanotechnological solutions in that direction. In this thesis, I have experimentally demonstrated plasmonic anostructures and metasurfaces with other alternative materials like AuAg alloys and hybrid metal-dielectric nanoantennas: Al disk stacked on top of Si cylinder. The development of new processes for the incorporation of novel materials for the fabrication of nanostructures requires rigorous optimization procedures which have been detailed in the greater part of the thesis. In the first part of the thesis, I describe the development of a novel low-temperature alloying process for fabricating AuAg alloyed nanostructures. With this process, nanostructures of varying stoichiometries were fabricated by annealing a bilayer of Au and Ag, the relative thickness of the bilayer determined the composition. The process made possible to perfectly retain the shape of the nanostructures after annealing, withoutthe use of insulating material like SiO2 as a protective cover. Further, it only required a single annealing step after the completion of fabrication by conventional lithography and enabled the fabrication on the same substrate of alloyed nanostructures with different stoichiometries, unlocking an additional degree of freedomfor nanostructure design. I extended this versatile and cost-effective technique successfully to the fabrication of complex 4-rod Fano resonant nanostructures and to binary plasmonic metahologram and metalenses. The metasurfaces were built from similar shape nanostructures with the different phase responses provided by a variation of composition (Au0.2Ag0.8 and Au0.8Ag0.2 ) rather than the conventionally used variation in geometrical parameters. This technique paves the way for fabricating plasmonic metasurfaces with meta-atoms varying in both geometry and composition. In the second part of the thesis I investigate the effect of introducing dielectric to metal nanostructures. In order to do this analysis, I use a hybrid metal-dielectric nanoantenna with stacked geometry comprising of a metal disk on a Si cylinder. I perform a systematic study showing the evolution of the multipoles along with the spectra for this hybridmetal-dielectric nanoantenna as its dimensions are varied one by one. I broaden the analysis to demonstrate the "magnetic light" at energies above 1 eV by varying the height of the Ag on the Si cylinder and below 1 eV by introducing insulating spacing between them. The appearance of the anapole state is also explored along with some exceptionally narrow spectral features by varying the radius of the Ag disk. This geometry has also been fabricated using Al as the metal which also acted as an etching mask, simplifying the fabrication process and Si as the dielectric, together with a SiO2 spacer. These nanostructures have been used for bulk refractive index sensing with glucose solution showing a sensitivity of 208 nm/RIU, which has been further improved to 245 nm/RIU by under etching the SiO2 spacer. I believe the nanotechnological processes developed in this thesis will facilitate the fabrication of metasurfaces with these alternative materials resulting in new and superior performances.

EPFL2021

Multipolar scattering analysis of hybrid metal-dielectric nanostructures

Andrei Kiselev, Olivier Martin, Debdatta Ray

We perform a systematic study showing the evolution of the multipoles along with the spectra for a hybrid metal-dielectric nanoantenna, a Si cylinder and an Ag disk stacked one on top of another, as its dimensions are varied one by one. We broaden our analysis to demonstrate the "magnetic light" at energies above 1 eV by varying the height of the Ag on the Si cylinder and below 1 eV by introducing insulating spacing between them. We also explore the appearance of the anapole state along with some exceptionally narrow spectral features by varying the radius of the Ag disk. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

OPTICAL SOC AMER2021