Shining a new light on materials

Researchers from the Soft Matter Physics group at the Adolphe Merkle Institute are designing new materials that display intriguing optical properties, with a fabrication process that promises to be more efficient than previously employed methods.

Metamaterials are artificial materials that can be tuned to reflect and redirect light in a highly controlled manner. In a sense, they act like lenses, but they display certain unique characteristics that cannot be found in traditional optical elements. Over the past two decades, metamaterials have led to an increasing number of biosensing and nanophotonic applications, for example to identify proteins, thanks to the careful control of light propagating through subwavelength features.

Researchers from the AMI Soft Matter Physics group have investigated chiral nanostructures that are characterized by the absence of any mirror symmetry. These metamaterials most notably display unique properties such as circular dichroism (CD) and optical activity, and others not found in nature. CD is the difference in absorption of left and right circularly polarized light, and is applied in different types of spectroscopy to the structural study of proteins, or to determine geometric and electronic structures of molecules for example. It has even been predicted that these nanostructures could have negative refractive indices, leading potentially to the development of an invisibility cloak or other new optical devices.

The researchers focused on creating three-dimensional (3D) chiral structures that could display strong CD at visible and near-infrared wavelengths, while being relatively simple to assemble. Nanostructures with those features have been particularly challenging to create, and are limited by the very nature of their structures, or because of the time required  to produce a satisfactory result. Most studies have focused on helical structures arranged in 2D arrays, which produce strong CD, but are cumbersome and costly, with limited features.  The AMI scientists chose to develop self-assembled structures instead, an approach that offers no limitations to feature size and can be of a chosen 3D morphology. Their metamaterial was based on the replication of a gyroid, an inherently chiral 3D structure, in block copolymer films. One of the polymer elements is removed from the film, and replaced with gold or silver by electrochemical backfilling. The remaining polymer is then removed, leaving only the metallic gyroid structure.  

The method used by the AMI researchers is more efficient than previous attempts using self-assembled techniques such as DNA origami, peptide, and cellulose nanocrystal templates, with much stronger CD. It affords materials that display similar results as structures made with more costly techniques. The AMI nanostructures are also easier to tune thanks to their material composition, and respond to a wider range of wavelengths. With the silver gyroid structures investigated proving to have a stronger CD than the gold ones, the results pave the way to a simpler fabrication of 3D self-assembled silver optical materials. Applications could include tunable CD filters and large-scale materials for chiral sensing, while variations of the gyroid structure could further improve CD.  

Reference: Kilchoer, C.; Abdollahi, N.; Dolan, J. A.; Abdelrahman, D.; Saba, M.; Wiesner, U.; Steiner, U.; Gunkel, I.; Wilts, B. D. Strong Circular Dichroism in Single Gyroid Optical Metamaterials. Advanced Optical Materials 2020, 8 (13), 1902131