Mueller matrix spectroscopic ellipsometry study of nanocrystalline cellulose free-standing chiral films
2015 (English)Conference paper, Presentation (Other academic)
The astonishing colors exhibited by many birds, insects and other creatures have inspired the development of materials and structures for optical biomimetics. Particularly, aqueous suspensions of cellulose nanocrystals self-assembly in a chiral nematic liquid crystalline phase producing nanocrystalline cellulose (NCC) chiral films after slow evaporation  that mimic the left-handed helicoidal arrangement of chitin-protein fibrils found in some beetle cuticles. Owing to the helical structure, left-handed polarized light is selectively reflected from beetle cuticles and NCC chiral films at normal incidence in a spectral band centered at wavelength l0=nL where n is the in-plane average refractive index and L the helix pitch.
In this work we report the normalized Mueller matrix (M) of NCC free-standing chiral films measured with a dual rotating compensator ellipsometer (J. A. Woollam Co., Inc.) in the wavelength (l) range 250-1000 nm. Measurements performed on NCC films in reflection at angles of incidence (q) between 20 and 75° are shown in the contour map in Fig. 1 and display the same structure as those found in M of beetle cuticles . At q=20° the band of selective reflection of left-handed polarized light (m41=m14<0) is centered at 520 nm. However, NCC chiral films are characterized by a mosaic-like texture as can be observed in the optical microscopy image inserted on the right panel of Fig. 1. The multidomain texture indicates both random helix direction and pitch distribution. Therefore, measurements in different places show selective reflection bands with different spectral characteristics. On the other hand, the transmission of right-handed polarized light (m41=m14>0) is confirmed from measurements at normal incidence, as observed in the right panel of Fig. 1. Other properties of the transmitted light like degree of polarization, ellipticity, and azimuth are determined for incident unpolarized as well as for different polarizations of incident light. Also, circular dichroism and optical rotation of NCC chiral films are evaluated.
 J. A. Kelly et al, Acc. Chem. Res. 47 (2014) 1088−1096.
 E. Muñoz-Pineda et al, Thin Solid Films (2014) http://dx.doi.org/10.1016/j.tsf.2013.11.144
Place, publisher, year, edition, pages
Mueller-matrix ellipsometry, Cellulose, chiral films
Other Physics Topics
IdentifiersURN: urn:nbn:se:liu:diva-123602OAI: oai:DiVA.org:liu-123602DiVA: diva2:890085
9th Workshop Ellipsometry, Twente, 22-25 February 2015
FunderCarl Tryggers foundation Swedish Research CouncilKnut and Alice Wallenberg Foundation