Circular dichroism of distorted double gyroid thin film metamaterials

Strong circular dichroism (CD) has been reported in triply periodic, co-continuous gyroid thin films for certain orientations and surface terminations. However, processing of gyroid thin films introduces distortions experimentally, creating a mismatch between the structures created practically and those explored computationally. This work explores the impact of compression normal to the substrate ($z$-compression) with conserved volume in (110)-oriented plasmonic silver double gyroid thin films on CD using finite-difference time-domain (FDTD) simulations. As compression reaches 15% and above, new features emerge including termination-dependent opposite-handed CD responses and, at larger compressions, shorter wavelength responses that span many surface terminations. The longest wavelength responses of the system redshift with increasing compression. The top surface structure contributes strongly to the emerging opposite-handed features and redshifting of wavelengths. However, the less surface termination-dependent features arise from a mixture of contributions from the top surface and interior of the films. Interplay of these leads to CD-switching phenomena as a function of compression for certain terminations and wavelengths. When alternative methods are utilized to compress the system, such as compression with a Poisson's ratio of 0.33 (comparable to polystyrene) or the generation of compressed equilibrium structures with nonaffine strut changes via self-consistent field theory, similar optical responses persist. Overall, this study highlights the significant impact experimentally relevant distortions (especially compression and some nonaffine structural shifts) can have on the CD response of block copolymer templated plasmonic double gyroid thin films, and it provides mechanistic insight into the film interior versus surface contributions to the CD response during compression.