Organic Anisotropic Excitonic Optical Nanoantennas

Abstract

Optical nanoantennas provide control of light at the nanoscale, which makes them important for diverse areas ranging from photocatalysis and flat metaoptics to sensors and biomolecular tweezing. They have traditionally been limited to metallic and dielectric nanostructures that sustain plasmonic and Mie resonances, respectively. More recently, nanostructures of organic J-aggregate excitonic materials have been proposed capable of also supporting nanooptical resonances, although their advance has been hampered from difficulty in nanostructuring. Here, the authors present the realization of organic J-aggregate excitonic nanostructures, using nanocylinder arrays as model system. Extinction spectra show that they can sustain both plasmon-like resonances and dielectric resonances, owing to the material providing negative and large positive permittivity regions at the different sides of its exciton resonance. Furthermore, it is found that the material is highly anisotropic, leading to hyperbolic and elliptic permittivity regions. Nearfield analysis using optical simulation reveals that the nanostructures therefore support hyperbolic localized surface exciton resonances and elliptic Mie resonances, neither of which has been previously demonstrated for this type of material. The anisotropic nanostructures form a new type of optical nanoantennas, which combined with the presented fabrication process opens up for applications such as fully organic excitonic metasurfaces.

Keywords: J-aggregates; Mie resonances; hyperbolic polaritons; localized surface exciton resonances; nanoantennas.

Figure 1

a) Schematic illustration of two basic types of optical nanoantennas: metallic nanoantennas that support localized surface plasmon resonances (left) and high‐index dielectric nanoantennas that support Mie resonances (right). The yellow areas indicate exemplified nearfield enhancements for plasmon and Mie resonances, with electric fields primarily being confined outside and inside the nanostructure, respectively. b) Example of complex permittivity of an excitonic material described by a single Lorentzian, showing an optically metallic negative permittivity region at wavelength shorter than the exciton resonance (red shaded area) and a large positive permittivity region at the other side of the exciton resonance (blue shaded area).

Figure 2

a) Fabrication scheme of TDBC nanocylinder arrays using SANE assisted with RIE, with the molecular structure of TDBC shown above the leftmost structure. The rightmost structure indicates the three characteristic dimensions of the nanocylinder arrays: periodicity p, diameter d, and height h. b) AFM images of resulting TDBC nanocylinder arrays with varying diameter and fixed periodicity at 800 nm. The scale bars are all 1 µm.

Figure 3

a) Experimentally measured extinction spectra for TDBC nanocylinder arrays with varying diameters. b) Anisotropic permittivity of TDBC obtained by ellipsometry measurements. Calculated extinction spectra using FDTD simulations assuming c) anisotropic and d) isotropic permittivity. Cylinder height of 150 nm and periodicity of 800 nm were used for the simulations. The vertical dashed lines indicate the exciton resonance position.

Figure 4

a) 2D GIWAXS image for a 200 nm thick TDBC film spin‐coated on a glass substrate. The incident angle was 0.16°, and the broad peak at q ≈ 1.8 Å^–1 is from the amorphous glass substrate. b) 3D molecular orientations reconstructed from (a). 1D scattering profiles along c) in‐plane and d) out‐of‐plane directions.

Figure 5

Calculated a) extinction and b) absorption cross‐section for single anisotropic TDBC nanocylinders. Solid curves indicate the extinction/absorption cross‐section for single nanocylinders and the dashed curves indicates the extinction/absorption spectra for array structures with periodicity of 800 nm. c) Representative electric (upper row panels) and magnetic (lower row panels) nearfield plots for xz‐plane. Axis directions and scale bars are in the leftmost panels. d–f) Results equivalent to (a–c) but for isotropic TDBC. The resonance positions are designated in (b) and (e) with the same color code.