|
Epsilon-near-zero (ENZ) material possesses the dielectric permittivity near-zero in a range of optical spectrum. Real part of Drude-type dielectric permittivity goes through zero at the plasma frequency of noble metals, doped semiconductor, and conducting oxide. Another example is hyperbolic metamaterial, where transverse negative and transverse positive hyperbolic dispersions are distinguished at ENZ spectral position. In organic molecular aggregates a van der Waals coupling between ground and excited states of neighboring molecular excitons leads to resonance Davydov splitting, resulting in a coherent super dipole moment. A collection of Lorentzian oscillators in a narrow spectral range is associated with a negative real part dielectric permittivity from Kramers-Kronig dispersion relation, permitting the existence of ENZ spectral range. We study a series of optical films of donor-acceptor-donor organic pi-conjugated molecules to relate the strength of donor group with ENZ property. In order to characterize ENZ spectral response, spectroscopic ellipsometry (SE), attenuated total internal reflection (ATR) spectra, GIWAXS and NEXAFS measurements as well as linear optical spectra of reflection and absorption are employed. Also to relate linear optical spectra with ATR spectra, FDTD simulation is carried out by use of dielectric permittivity spectra obtained from SE measurement. Preliminary data show that both strength of donor group of individual organic molecules and the orientation of molecular planes as well as degree of aggregates of molecular aggregates in film are associated with ENZ response. Advantages of organic ENZ materials are found to be the simple sample preparation by spin-coating or thermal evaporation and the tunability of ENZ spectral range covering visible and near IR spectrum.
|
