Cellulose nanocrystal holograms

Cellulose nanocrystals (CNCs) are anisotropic and chiral colloidal nanorods able to self- assemble spontaneously into a cholesteric phase above a critical concentration. When the suspension is allowed to dry, it forms a solid film that retain the cholesteric order, enabling the reflection of specific wavelengths, typically in the visible range. ^[1] Due to their negative diamagnetic anisotropy, CNCs also tend to align perpendicular to an external magnetic field. [2] Experimentally, the cholesteric phase can be aligned under moderate magnetic fields (μ₀ H ≈0.5 T), enabling an angular control of the helical direction of the domains. ^[3,4] In this talk, we present a simple and robust method to spatially control the angular response of CNC films by casting them in close vicinity to commercially available templated magnets (Polymagnets ® ). We show how the spatially modulated magnetic field applied upon casting imprinted the orientation of the cholesteric structure in the bulk of the film thickness and produced its corresponding holographic pattern with a resulting illusion of depth. Angular-resolved optical spectroscopy and K-space optical microscopy analysis both converged toward a quantified illusion of depth in qualitative agreement with the profile expected from the magnetic field mapping and cross-section observations in SEM.

[1] R. M. Parker, B. Frka-Petesic et al., Adv. Mater. 2018, 30, 1704477.
[2] B. Frka-Petesic et al., Macromolecules 2015, 48, 8844.
[3] B. Frka-Petesic et al., Adv. Mater. 2017, 29, 1701469.
[4] B. Frka-Petesic et al., Phys. Rev. Materials 2019, 3, 045601.