Near- and far-field measurements of phase-ramped frequency selective surfaces at infrared wavelengths

Near- and far-field measurements of phase-ramped loop and patch structures are presented and compared to simulations. The far-field deflection measurements show that the phase-ramped structures can deflect a beam away from specular reflection, consistent with simulations. Scattering scanning near-field optical microscopy of the elements comprising the phase ramped structures reveals part of the underlying near-field phase contribution that dictates the far-field deflection, which correlates with the far-field phase behavior that was expected. These measurements provide insight into the resonances, coupling, and spatial phase variation among phase-ramped frequency selective surface (FSS) elements, which are important for the performance of FSS reflectarrays

Vibrational exciton nanoimaging of phases and domains in porphyrin nanocrystals.

Much of the electronic transport, photophysical, or biological functions of molecular materials emerge from intermolecular interactions and associated nanoscale structure and morphology. However, competing phases, defects, and disorder give rise to confinement and many-body localization of the associated wavefunction, disturbing the performance of the material. Here, we employ vibrational excitons as a sensitive local probe of intermolecular coupling in hyperspectral infrared scattering scanning near-field optical microscopy (IR s-SNOM) with complementary small-angle X-ray scattering to map multiscale structure from molecular coupling to long-range order. In the model organic electronic material octaethyl porphyrin ruthenium(II) carbonyl (RuOEP), we observe the evolution of competing ordered and disordered phases, in nucleation, growth, and ripening of porphyrin nanocrystals. From measurement of vibrational exciton delocalization, we identify coexistence of ordered and disordered phases in RuOEP that extend down to the molecular scale. Even when reaching a high degree of macroscopic crystallinity, identify significant local disorder with correlation lengths of only a few nanometers. This minimally invasive approach of vibrational exciton nanospectroscopy and -imaging is generally applicable to provide the molecular-level insight into photoresponse and energy transport in organic photovoltaics, electronics, or proteins

Exponential Decay Lifetimes of Excitons in Individual Single-Walled Carbon Nanotubes

The dynamics of excitons in individual semiconducting single-walled carbon nanotubes was studied using time-resolved photoluminescence (PL) spectroscopy. The PL decay from tubes of the same (n,m) type was found to be monoexponential, however, with lifetimes varying between less than 20 and 200 ps from tube to tube. Competition of nonradiative decay of excitons is facilitated by a thermally activated process, most likely a transition to a low-lying optically inactive trap state that is promoted by a low-frequency phonon mode