May 17, 2019 feature
Extraordinarily transparent compact metallic metamaterials
In materials science, achromatic optical components can be designed with high transparency and low dispersion. Materials scientists have shown that although metals are highly opaque, densely packed arrays of metallic nanoparticles with more than 75 percent metal by volume can become more transparent to infrared radiation than dielectrics such as germanium. Such arrays can form effective dielectrics that are virtually dispersion-free across ultra-broadband ranges of wavelengths to engineer a variety of next-generation metamaterial-based optical devices.
Scientists can tune the local refractive indices of such materials by altering the size, shape and spacing of nanoparticles to design gradient-index lenses that guide and focus light on the microscale. The electric field can be strongly concentrated in the gaps between metallic nanoparticles for the simultaneous focusing and 'squeezing' of the dielectric field to produce strong, doubly enhanced hotspots. Scientists can use these hotspots to boost measurements made using infrared spectroscopy and other non-linear processes across a broad frequency range.
In a recent study now published in Nature Communications, Samuel J. Palmer and an interdisciplinary research team in the departments of Physics, Mathematics and Nanotechnology in the U.K., Spain and Germany, showed that artificial dielectrics can remain highly transparent to infrared radiation and observed this outcome even when the particles were nanoscopic. They demonstrated the electric field penetrates the particles (rendering them imperfect for conduction) for strong interactions to occur between them in a tightly packed arrangement. The results will allow materials scientists to design optical components that are achromatic for applications in the mid-to-infrared wavelength region.
Palmer and colleagues were able to tune the local refractive index of these components by altering the size, shape and spacing of nanoparticles with sensitivity to the local refractive index of the surrounding environment. The scientists enhanced the electric field in the gaps between the metallic nanoparticles in the array and simultaneously exploited their transparency, tunability and high metallic filling fraction to design a gradient-index lens. The work focused light on the microscale and squeezed the electric field in the nanoscale to produce the doubly enhanced electric field hotspot throughout the infrared (IR) region. The scientists envision that the new work will boost measurements made using IR spectroscopy and other nonlinear processes across a broad-range of frequencies.
Materials scientists are presently able to develop new and advanced materials; however, no new material is truly homogenous in its constitution. Nevertheless, most materials can be characterized using homogenous microscopic properties such as refractive indices wherein the atomistic inhomogeneities are smaller than the average wavelengths of optical light incident on the material. Artificially constructed materials known as metamaterials are described by an effective index when the material contains a sufficiently subwavelength structure. Early metamaterials included artificial dielectrics composed of centimeter-scale arrays of metallic particles capable of guiding and focusing radio waves like a dielectric. The metallic particles of early artificial dielectric materials were so large they behaved as perfect conductors with high transparency to radio waves. Recent research in materials science aims to build effective dielectrics for the visible and infrared spectrum using nanoscale metallic particle arrays. Advances in the assembly of metallic nanoparticles can then allow for sophisticated engineering of unprecedented light-matter interactions in the optical domain.
In the present work, Palmer et al. contrasted the transparency of nanocylinder arrays and nanospheres (although nanoparticles can have other shapes) to germanium in order to demonstrate that the arrays could guide and focus light. The arrays of nanocylinders behaved as effective dielectrics with transverse electric polarized light; where a transverse force on the electrons led to oscillating surface charges that mimicked the oscillating dipoles of an atom in a real dielectric.
In contrast, the response of the cylinders to transverse magnetic polarized light was similar to the bulk metal, since electrons were free to move under the action of the longitudinal electric field without encountering the cylinder surfaces. The arrays of nanospheres in the study behaved as effective dielectrics, regardless of the incident polarization—focusing the electrons in any direction to result in surface charges that imitated the oscillating dipoles of a dielectric. Such arrays showed high transparency compared to real dielectrics such as germanium—even when the system had higher than 75 percent metal.
To test the accuracy of the proposed theory, Palmer et al. produced a highly ordered colloidal supercrystal using gold nanoparticles of 60 nm diameter. They deposited the supercrystal on a germanium substrate and characterized the material (tested physical properties) using a UV-vis-NIR spectrophotometer. The scientists observed outstanding transparency of the materials, demonstrating the feasibility of experimentally producing metamaterials. Using magnetic near-fields, they showed that the effective dielectrics were transparent enough to act as micrometer-scale lenses to infrared radiation. Despite containing 82 percent metal by volume, the scientists observed that breaking the solid gold into an array of gold nanocylinders produced a transparent lens capable of focusing light, closely resembling the behavior of a homogenous dielectric lens.
The scientists then compared different types of metals (aluminum, silver, gold and titanium) to show that materials with longer skin depths produced the most transparent and least dispersive nanoparticle arrays. Palmer et al. showed that at a fixed wavelength, the ratio of the particle diameter to the skin depth of the metal determined if the particle would behave as quasiparticle dipoles or as perfect conductors.
In addition to high transparency, the scientists could tune the system by controlling the size, shape and space of the particles. For instance, Palmer et al. controlled the aspect ratio of arrays of elliptical cylinders to show that the anisotropic response of the material could be tuned. The numerical results showed that the effective index could be easily tuned to vary by more than 50 percent when the system was rotated. Thereby the scientists were able to tune the effective index by fixing the particle positions and tuning their sizes.
To highlight this potential to tune the local effective index, Palmer et al. then constructed a gradient-index (GRIN) lens using triangular lattices of gold cylinders and varied the diameters of the cylinders with position. Using the GRIN lens, the scientists were able to simultaneously focus light on the microscale and then 'squeeze' light on the nanoscale to produce the intense, 'doubly enhanced' electric field hotspots. Unlike plasmonic enhancements, the effect did not rely on lossy resonances, demonstrating broadband and low-loss properties.
They showed that the focal point of the GRIN lens had to coincide with the region of closest packing to maximize squeezing of the electric field. Unlike magnetic fields that were continuous across the air-metal interfaces in the study, the electric field strongly localized in the gaps. As a result, squeezing a 2 µm wavelength into 2 nm gaps produced strong hotspots of high intensity in the study.
In this way, Palmer et al. constructed low-loss, effective dielectrics from arrays of metallic nanoparticles. The scientists obtained highly transparent arrays that exceeded the transparency of real dielectrics such as germanium; renowned for their transparency to low energy radiation. They were also able to locally tune and control the size, shape and space of the particles forming the new metamaterials. The scientists showed the effective index to be essentially constant for all wavelengths greater than 2 µm. This work will allow materials scientists to design and engineer sophisticated optical devices with metamaterials that guide or enhance light across a broad range of frequencies, essentially without an upper bound on wavelength.
J. B. Pendry et al. Extremely Low Frequency Plasmons in Metallic Mesostructures, Physical Review Letters (2002). DOI: 10.1103/PhysRevLett.76.4773
D. R. Smith. Metamaterials and Negative Refractive Index, Science (2004). DOI: 10.1126/science.1096796
Seungwoo Lee. Colloidal superlattices for unnaturally high-index metamaterials at broadband optical frequencies, Optics Express (2015). DOI: 10.1364/OE.23.028170
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