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Final Report Summary - NANOCHIRALITY (Polarimetric characterization of natural and artificial chiral anisotropic media)

The advent of nanoscale engineering has brought about the ability to create new types of chiral structures. Chiral metallic and semiconducting nanostructures have been shown to exhibit particularly strong optical activities in a wide region of the electromagnetic spectrum. The origin of high chiroptical activity of nanoscale assemblies is linked to high polarizability of nanoscale inorganic materials, leading to plasmonic and excitonic effects on the polarization rotation of incident photons. Large research efforts have been devoted to design and optimize the geometry of nanostructures that permit maximizing their near and far-field chiral behavior. The advantage of dealing with artificial materials is that their geometry can designed according to the interests and applications that the researcher wants to reach. For example the geometric chirality of resonant plasmonic nanostructures can be manipulated by modifying the geometrical parameters of the nanostrusctures enhancing the light confinement.

The NANOCHIRALY project was focused in the polarimetric study of the interaction of electromagnetic fields with natural and artificial chiral materials. Several polarization dependent effects such as circular dichroism, circular birefringence, linear dichroism, linear birefringence, depolarization, asymmetric light transmission, etc were studied using spectroscopic and/or imaging Mueller matrix polarimetry. In the firsts steps of the projects we focused in the design of novel Mueller matrix polarimeters that offered unparalleled sensitivity in the detection of optical activity and with micrometric resolution. In parallel we also investigated how the physical properties of a material can be recovered once its scattering (i.e. Mueller) matrix (e. g. Opt. Lett. 38, 1134-1136, 2013 & Opt. Lett. 40, 954-957, 2015). Our expertise in polarimetry allowed us to investigate the induction of chiroptical effects in nanotubular porphyrin aggregates

Some of our more remarkable results arised after investigating how apparently planar gammadion nanostructures fabricated by electron beam lithography provide a net chiral effect due to fabrication defects (Fig. 1), how chiroptical generated can be generated in stretchable films coated with plasmonic nanocolloids (Fig. 2) or how simple achiral 2D nanohole patterns can exhibit extraordinary optical transmission and that at certain orientations is asymmetric for right- and left- circularly polarized light (Fig. 3). The goal of our experimental and theoretical (through simulations) work in the field of nanostructures and metamaterials is understanding how the design of these artificial materials tailors its electromagnetic response. In particular, my recent publications in this field are focused in the following topics: the asymmetric transmission of circular polarized light in hole arrays (Opt. Exp., 22, 13719-13732 2015), the appearance of chiroptical effect in 2D nanostructures due to fabrication defects (to be published Opt. Exp., 2016), the modulation of chiroptical effects in nanocomposites (published as early view in Nature materials, 2016).

In this project we also pioneered the sensing of optical activity in reflection. Despite optical activity is typically associated to the light propagation through a medium, the spectroscopic ellipsometry analysis of the light reflected on gyrotropic media allows the determination of optical activity when transmission methods are not applicable. In particular, by doing Mueller matrix ellipsometry in a semiconductor crystal of AgGaS2 above the bandgap, we were able to report the first clear successful determination of reflection optical activity in an anisotropic medium and the largest value of optical activity ever measured in a natural material. This measurement was reported in Opt. Lett. 40, 4277-4280, 2015.

We think that the results of NANOCHIRALITY will significantly impact the way the chiroptical effects of anisotropic media are studied. We are proposing techniques and methods that allow more reliable and accurate measurements. We have shown that it is possible to use a general approach based on Mueller matrix polarimetry to study different optical samples that are of interest for the organic chemistry, molecular biology, crystallography and photonics communities.

Researcher contact details:

Oriol Arteaga
Dep. Física Aplicada i Òptica
C/ Martí i Franquès 1, 08028-Barcelona, Spain
Universitat de Barcelona
Tel: +34 934039221

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