# Simple and Complex Metafluids and Metastructures with Sharp Spectral Features in a Broad Extinction Spectrum: Particle-Particle Interactions and Testing the Limits of the Beer-Lambert Law

## TL;DR

This paper explores designing optical metafluids and metastructures with sharp spectral features across a broad spectrum, analyzing particle interactions and testing the limits of the Beer-Lambert law through experiments and theory.

## Contribution

It introduces a fundamental design approach for creating broad-spectrum optical filters with narrow transparency windows using controlled nanocrystal geometries and examines the impact of particle interactions on spectral properties.

## Key findings

- Metafluids and metastructures can achieve broad extinction spectra with narrow transparency bands.
- Particle-particle interactions can shift and modify plasmonic resonances, affecting filter performance.
- The Beer-Lambert law generally holds, except in cases of aggregation or nearly-touching nanocrystals.

## Abstract

Metallic nanocrystals (NCs) are useful instruments for light manipulation around the visible spectrum. As their plasmonic resonances depend heavily on the NC geometry, modern fabrication techniques afford a great degree of control over their optical responses. We take advantage of this fact to create optical filters in the visible-near IR. Our systems show an extinction spectrum that covers a wide range of wavelengths (UV to mid-IR), while featuring a narrow transparency band around a wavelength of choice. We achieve this by carefully selecting the geometries of a collection of NCs with narrow resonances that cover densely the spectrum from UV to mid-IR except for the frequencies targeted for transmission. This fundamental design can be executed in different kinds of systems, including a solution of colloidal metal NCs (metafluids), a structured planar metasurface or a combination of both. Along with the theory, we report experimental results, showing metasurface realizations of the system, and we discuss the strengths and weaknesses of these different approaches, paying particular attention to particle-particle interaction and to what extent it hinders the intended objective by shifting and modifying the profile of the planned resonances through the hybridization of their plasmonic modes. We have found that the Beer-Lambert law is very robust overall and is violated only upon aggregation or in configurations with nearly-touching NCs. This striking property favors the creation of metafluids with a narrow transparency window, which are investigated here.

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Source: https://tomesphere.com/paper/1702.03482