Engineering chiral and topological resonances in perovskite metasurfaces

Abstract

Light-matter interaction is enhanced by surface plasmons, which can confine the light onto the metal surface within the subwavelength scale. However, surface plasmons cannot avoid optical loss of metal in visible range. One potential solution to this limit is found in low loss dielectric metamaterials, which can obtain specular properties artificially designing sub-wavelength scale unit cells. Furthermore, to avoid complex structure, dielectric metasurfaces have been largely studied. This thesis investigates the potential of active dielectric (perovskite) metasurfaces for chiral and topological resonances. In our sample, the dielectric metasurface itself is defined by light-emitting perovskite materials. We construct metasurfaces that consist of emissive material, perovskite, spin- coated onto the nanostructured patterned substrate. The refractive index of the perovskite material is higher than that of surrounding materials (PMMA and quartz). Applying the principle of bound states in the continuum (BIC) and topological interface states, we theoretically and experimentally demonstrate how to adjust the chiral and topological properties by designing the metasurfaces. We select 2D PEPI as for the active material, which shows excellent excitonic properties at room temperature. We can divide the thesis into two parts. The first is the chiral BIC metasurface and their chiral emission properties. The second part deals with exciton-polaritons in topological junction metasurfaces. 1) Chiral Quasibound states in the continuum For the first part, chiral quasibound states in the continuum are realized in the visible range, and maximally chiral emission from a perovskite metasurface is demonstrated. Grayscale lithography is employed to control the etching depths in the substrate and induce out-of-plane symmetry breaking. A perovskite film is spin-coated on a patterned glass substrate. An extremely high level of chiral emission is experimentally achieved in the normal direction at room temperature. Chiral emission is maximally enhanced for one helicity via critical coupling, while strongly suppressed for the other helicity. The physical mechanism is explained using the reciprocity principle. For another study, we deal with circularly polarized emission measurements from chiral metasurfaces and characterize the polarization distortion caused by the beam splitter. We detail the procedures for the Stokes parameters and Mueller matrix measurements. Then, we directly measure the Mueller matrix of the beam splitter and retrieve the original polarization state of circularly polarized emission from our metasurface sample. Using the measured Mueller matrix of the beam splitter, we specifically identify what contributes to polarization distortion in circularly polarized emission. 2) Topological exciton polaritons in compact perovskite junction metasurfaces In the last chapter, topological exciton polaritons are experimentally demonstrated in organic– inorganic hybrid perovskite thin films, taking synergic advantages of room-temperature perovskite excitons and topological photonic structures. Topological junction structures based on perovskite gratings are realized using a momentum-space analog of the 1D Dirac system. Desired enhancement phenomena are observed including narrow-beam polariton emission from a tightly localized junction region, polaritonic nonlinearity boost, and enhanced luminescence. These remarkable features are obtained from highly compact devices with their footprint widths on the order of a few micrometers and efficiently tailorable with simple unit-cell geometry control.