ICMAB THESIS

PhD researcher José Mendoza from the Nanopto group at ICMAB-CSIC.

Supervisors: Agustín Mihi, ICMAB-CSIC, Spain and María Isabel Alonso, ICMAB-CSIC, Spain

Abstract: Advancements in nanotechnology fabrication have introduced new methods for studying material properties, often using either directly or indirectly light as a key tool. The polarization state of light, in particular, reveals structural and compositional details, making precise polarization control essential. In situ characterization requires miniaturized light sources with controllable polarization. For circularly polarized light, chiral nanoantenna arrays effectively control both polarization and radiation patterns. Unlike bulky optics, chiral metasurfaces offer a compact solution for electromagnetic control. However, while they can be produced with high-resolution techniques like electron beam or focused ion beam lithography, these methods are costly and slow, limiting large-scale, cost-effective production for industrial applications.

Alternatively, Nanoimprint lithography (NIL) is a great candidate for the production of large-area metasurfaces. NIL, compatible with industrial roll2roll fabrication methods, has proved excellent results in metasurface nanofabrication implemented in optoelectronic devices. However, the implementation of NIL into the production of large-area chiral metasurfaces remains yet elusive. In this thesis, we propose the pioneering combination of unconventional Nanoimprint lithography for the nanofabrication of large-area chiral metasurfaces. These metasurfaces endow chiral properties to light-emitting nanomaterials deposited on top, coupling the emission processes to the chiral collective modes sustained by the 2D-chiral arrays.

First, we initiate the experimental phase of this thesis by nanoimprinting achiral emitters into chiral gammadion patterns. This process demonstrates that achiral emitters produce circularly polarized light when arranged directly in chiral architectures. This effect is tested for two different perovskite inks, achieving a dissymmetric circular emission glum of 0.15, a two-order-of-magnitude improvement compared to chemical methods. Additionally, we experimentally demonstrate the polarization inversion characteristic of 2D-chiral metasurfaces, with preferential emission switching across emitted half-spaces. Moreover, we show an enhanced out-coupling efficiency through the addition of a TiO2 layer, which doubled the circular emission dissymmetry (0.3) via collective resonances.

Second, we decouple the emitters from the metasurface and examine the role of chiral lattice resonances in producing circularly polarized emissions. We analyze the dependence of glum when overlapping the emission band and the metasurface's chiroptical response. When these overlap, experimental glum values reached up to 0.56 for green perovskite nanocrystals. We test various functional coatings (TiO2, Si, and Au) on gammadion metasurfaces to spectrally adjust the chiroptical response across the visible spectrum. We further examine the origins of chiral dissymmetry based on absorption and scattering, experimentally and computationally for dielectric (TiO2) and plasmonic (Au) coatings. Finally, we propose a hybrid architecture with a double-imprinted metasurface to enable experimental simultaneous chiral emission at two different wavelengths.

Third, we introduce the hexagonal triskelion array, a new chiral structure capable of sustaining lattice resonances across the visible spectrum. Building on previous work, we test various achiral emitting materials on these metasurfaces to impart chiral emissive properties. By utilizing materials of different compositions and characteristics, we demonstrate the versatility of this approach, achieving glum >0.5 across a broadband range when coupled with diffractive modes. We then propose a "white mixture" solution combining blue (CdSe/CdS nanoplatelets), green (CsPbBr3 perovskites), and red (CdSe/CdS quantum dots) emissive materials for potential white chiral light applications.

In the fourth and last part of this thesis, we conduct an in-depth investigation of this new chiral metasurface. For this structure, we experimentally and computationally characterize the resonances' angular dispersion, examining the origins of the chiral response in both linear and circular polarization space. Benefitting from extrinsic chirality, we achieve tunable circular polarization emission in different spatial directions, obtaining adaptable emissions from a static metasurface. We conclude this chapter by examining the metasurface response under higher excitation power densities, demonstrating high-purity circularly polarized lasing emission with a glum > 1.9, which was tunable via the TiO2 coating.