###### Abstract  
Poly(fluorene-alt-benzothiadiazole) (PFBT) is a promising chiral polymer for use in metamaterials and other photonic applications, due to its large chiral optical activity at visible wavelengths. However, its usages are very limited, since it is not readily patternable into microstructures and challenging to apply to applications. In this paper, we demonstrate photo patterning of chiral PFBT/Achiral SU-8 photoresist to produce high quality structures while retaining the extraordinary chiral optical activity of our previously-reported PFBT/SU8 nanocomposite films. The ability to produce cleanly patterned microfeatures with high chirality may enable wider use of PFBT in chiral metamaterials and other photonic applications. polymerization; fabrication; chirality; nanocomposites Article  
In Situ Polymerization of Chiral Poly(fluorene-alt-benzothiadiazole) Nanocomposites with Enhanced Chirality  
[PERSON], [PERSON], [PERSON], J. An alternative approach to achieving a negative refractive index employs chiral materials with high optical activity [21]. In a chiral material, the effective refractive index is given by  
\[n_{eff}=\sqrt{\varepsilon\mu}\pm\kappa \tag{1}\]  
where \(\kappa\) is the chirality parameter, and the effective refractive index is increased for light of one circular polarization and decreased for light of the other circular polarization [22]. As shown by Equation (1), for a sufficiently large value of \(\kappa\) and a sufficiently small value of \(\varepsilon\), the effective refractive index for one circular polarization could become negative even if \(\mu\) remained near one. Although most biological materials are chiral, the chirality parameter in naturally-occurring materials is typically below 0.001 [23], far too small to make substantive contributions to the effective refractive index. Moreover, most naturally-occurring materials have a maximum chiro-optical activity at ultraviolet wavelengths. In contrast, co-polymers with a donor-acceptor structure such as chiral poly(fluorene-alt-benzothiadiazole) (PFBT), can have optical transitions in the visible spectral regime and exhibit maximum values of \(\kappa\) at visible wavelengths [24]. Along with their potential for use in this chiral route to negative-index metamaterials, such highly optically active materials also show promise for other interesting chiral photonic devices such as polarization-sensitive phase-shifters. We have previously demonstrated the plasmonic enhancement of chirality in PFBT films using gold nanoparticles [25], and more recently, we reported on thin film fabrication of nanocomposites of a PFBT in a photopolymer matrix [26]. This optically clear nanocomposite shows exceptionally large circular dichroism produced by the enhanced helical conformation of PFBT within the fully cured SU-8 matrix. These chiral PFBT/SU-8 nanocomposites exhibit exceptionally high circular dichroism (CD) (dissymmetry ratio of gabs = \(-\)0.32), two orders of magnitude larger than that of pure PFBT films of the same optical density (gabs = \(-\)0.003). SU-8 is well known for its processibility and its outstanding chemical and mechanical properties (chemical resistance, mechanical strength, high temperature strength, thermal stability, and biocompatibility), which have made it the material of choice for MEMS and related applications. As described above, the dramatic enhancement of CD was achieved in PFBT/SU-8 nanocomposites after ultraviolet (UV) light exposure, development, and annealing. However, using a typical SU-8 processing and development process, the PFBT/SU-8 nanocomposite film was not fully removed from the nominally unexposed area. A thin film of residue remained in the regions between patterned structures. Aggregates of PFBT were clearly visible within the SU-8 matrix in this residual film. Although topographic patterning was achieved in that work, with a thick film in exposed areas and much thinner film in unexposed areas, both the exposed and unexposed areas contributed to the overall chiro-optical response.