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Optical Metamaterials by Polymer Self-assembly

Periodic Reporting for period 1 - cOMPoSe (Optical Metamaterials by Polymer Self-assembly)

Reporting period: 2016-04-01 to 2018-03-31

Metamaterials are artificially structured materials whose interaction with electromagnetic waves is determined by their structure rather than by their chemical composition. The resulting material properties are not found in nature. Metamaterials that operate at optical frequencies, known as optical metamaterials, have attracted special attention due to their potentially ground-breaking technical applications such as sub-diffraction imaging or invisibility cloaking. The creation of optical metamaterials remains technologically challenging, as it requires fabricating nanometer scale features over macroscopic areas. Top-down lithographic techniques were utilized to create infrared metamaterials, and negative refraction was found in parts of the visible spectrum. However, state-of-the-art lithography is limited by the accessible feature sizes and often results in only microscopic patterning areas. Furthermore, these optical metamaterials aren’t truly three-dimensional (3D) as they are limited to a narrow range of light propagation directions. This research project investigated an alternative bottom-up approach toward the fabrication of 3D optical metamaterials by replicating continuous network structures of self-assembled block copolymers. The ultimate goal has been to realize a material that exhibits a negative refractive index in the visible optical spectrum. Advanced in situ x-ray scattering techniques were used to investigate and fundamentally understand the self-assembly of 3D network structures by means of well-controlled annealing experiments. The goal of these experiments was to overcome the limitations of “self-assembled” optical metamaterials made by current empirical approaches.
The project employed gyroid-forming block copolymers as model systems of a three-dimensional network structure. The unique geometry of the gyroid - a bicontinuous and triply periodic cubic morphology with inherent chirality - holds promise for enabling applications like optical metamaterials, the main target of this project.

In the first part of the project we studied a fundamental aspect of polymer physics, the crystallization of polymers within the confinement of the continuous network structure of the gyroid. We demonstrated that crystallization proceeds along the least-convoluted pathways of the tortuous gyroid network of a self-assembled block copolymer. Importantly, this crystallization renders the polymer film birefringent. We could show that this birefringence can be used to identify individual gyroid grains by polarization microscopy despite the morphology being structurally isotropic.

The second part of the project focused on characterizing polymer self-assembly in films of two different gyroid-forming block copolymers during well-controlled annealing experiments by means of in situ x-ray scattering. These scattering experiments revealed the effects of several key annealing parameters on the morphology, degree of order, and orientation of the gyroid in block copolymer films. This allowed identifying protocols for the reproducible generation of gyroid polymer films with controlled orientation and well-ordered structures in three dimensions. These gyroid polymer films were used as templates for the fabrication of optical metamaterials by replication of the continuous network structure into gold in the third part of the project. The resulting gyroid metamaterials exhibited a depressed plasma frequency as well as circular and linear dichroism, which were found to be sensitive to the orientation of the gyroid and its surface terminations.
Confined crystallization of polymers as well as polymeric nanostructures with controlled orientation and long-range order both have been described for simple morphologies like spheres, cylinders, and lamellae. However, this has not yet been studied for 3D network structures such as the gyroid. The results of this project are therefore expected to be of broad interest to scientists in all areas of polymer science and engineering. Furthermore, the fundamental understanding of self-assembled 3D block copolymer networks based on in situ structural characterization will have a profound impact on the rational design and engineering strategies of future 3D optical metamaterials. The ability to generate 3D network structures with long-range order and controlled orientation will be key to creating 3D optical metamaterials with a negative refractive index in the visible that can be tested using geometrical optics. Similarly, the visualization of grains in semicrystalline gyroid morphologies by simple polarized light microscopy will be useful for the fabrication of optical metamaterials using polymer templates and other nanotechnology applications using polymer self-assembly, as it provides a rapid and simple means for quality control of self-assembled block copolymer templates.
Micrographs (SEM,POM) of a gold gyroid metamaterial fabricated using a well-ordered polymer template