Final Report Summary - METAMECH (Template assisted assembly of METAmaterials usingMECHanical instabilities)
The index of refraction of homogenous transparent materials for visible light is typically in the range of 1 to 4. The basic concept of optical metamaterials is to go beyond this limitation and create materials, which have even negative index of refraction. This would open completely new horizons for controlling the flow of light. Metamaterials are feasible by creating structures consisting of metals and insulators of sizes smaller than the wavelength of light. Consequently, light interacts with these metamaterials as if they were homogenous, but the index of refraction can now be varied by design of the nanoscopic features. First proofs of this concept have been done for metamaterials made by complex lithographic techniques and consequently, these materials are only available in microscopic sizes of a fraction of the diameter of a human hair and at very high cost. Our approach is radically different, since we aim to create such materials by the synthesis of nanoparticles and their self-assembly into complex patterns, rather than by lithography. Thus, these materials will become cheaper and available in large size.
The perfection of the material is limited by the uniformity of the nanoparticles they are made from. We have thus pushed shape control of metal nanoparticles beyond the state of the art. We have found that soft coatings of these particles can serve as “bumper layer” facilitating well-defined separations. In order to create complex patterns, we use wrinkled surfaces. Thus, the particles can be deposited from solutions into ordered arrangements and later even printed to other surfaces of interest. Controlled wrinkling is a novel scalable approach towards surface structuring: We use a rubber material, which we coat with a thin hard layer while stretched. When the material is subsequently relaxed, wrinkles form. Their size can be as narrow as 1/1000 of the diameter of a human hair. Still, we can create those structures uniformly on large areas at very low cost. We showed that the optical properties of nanoparticle-surface assemblies can be tuned by the pattern of their assembly. Thus we can use the same particulate building block and drastically change the optical properties just by changing the assembly pattern. In the following, we achieved the template-assisted self-assembly of macroscopic magnetic metasurface with magnetic resonances in the optical frequency range. We created an ordered array of rod-like gold nanoparticles supported on a thin gold film acting as a plasmonic mirror. For visible light, this metastructure expresses the electric current flow necessary for magnetic resonances, i.e. negative magnetic permeability. Because all nanorods exhibit close to perfect ordering, the magnetic moment accumulates over the area covered (109 particles / cm2) resulting in a macroscopic effect detectable by conventional UV/Vis spectrometry. The obtained optical metamaterial was fabricated guided by rational design and its optical and structural properties have been closely correlated. Our modular approach is highly transferable to other particle systems for optical functional materials with tailored light-matter interactions.
Our work has already found applications in optical detection of molecules, because certain detection techniques (Raman spectroscopy) can be strongly enhanced by our structures and thus molecules can be detected in hand held devices rather than large lab-based setups. Further possible applications are envisaged in photovoltaics, where similar nanostructures can be used to capture light more efficiently and thus enhance the performance of photovoltaic systems.
The perfection of the material is limited by the uniformity of the nanoparticles they are made from. We have thus pushed shape control of metal nanoparticles beyond the state of the art. We have found that soft coatings of these particles can serve as “bumper layer” facilitating well-defined separations. In order to create complex patterns, we use wrinkled surfaces. Thus, the particles can be deposited from solutions into ordered arrangements and later even printed to other surfaces of interest. Controlled wrinkling is a novel scalable approach towards surface structuring: We use a rubber material, which we coat with a thin hard layer while stretched. When the material is subsequently relaxed, wrinkles form. Their size can be as narrow as 1/1000 of the diameter of a human hair. Still, we can create those structures uniformly on large areas at very low cost. We showed that the optical properties of nanoparticle-surface assemblies can be tuned by the pattern of their assembly. Thus we can use the same particulate building block and drastically change the optical properties just by changing the assembly pattern. In the following, we achieved the template-assisted self-assembly of macroscopic magnetic metasurface with magnetic resonances in the optical frequency range. We created an ordered array of rod-like gold nanoparticles supported on a thin gold film acting as a plasmonic mirror. For visible light, this metastructure expresses the electric current flow necessary for magnetic resonances, i.e. negative magnetic permeability. Because all nanorods exhibit close to perfect ordering, the magnetic moment accumulates over the area covered (109 particles / cm2) resulting in a macroscopic effect detectable by conventional UV/Vis spectrometry. The obtained optical metamaterial was fabricated guided by rational design and its optical and structural properties have been closely correlated. Our modular approach is highly transferable to other particle systems for optical functional materials with tailored light-matter interactions.
Our work has already found applications in optical detection of molecules, because certain detection techniques (Raman spectroscopy) can be strongly enhanced by our structures and thus molecules can be detected in hand held devices rather than large lab-based setups. Further possible applications are envisaged in photovoltaics, where similar nanostructures can be used to capture light more efficiently and thus enhance the performance of photovoltaic systems.