No abstract

No abstract

The effect of a thin metal coating in near-field fluorescence imaging is studied by using a model based on the plane wave decomposition, that takes into account the dynamic nature of the molecule's electronic structure, namely the modification of the fluorescence lifetime due to the near-field environment (the metal layer and the probe's presence). The electromagnetic wave crossing the metal layer generates a plasmon excitation, the effect of which is the more important when the molecules, considered as dipoles, are polarized perpendicularly to the surface and when they are in a saturated excitation state. The consequence of the plasmon excitation is a decrease in fluorescence lifetime and, for the case of molecules polarized perpendicularly to the surface, a decrease in lateral resolution. When the molecules are polarized parallel to the surface, plasmon excitation is weaker and its effect on the resolution seems to be beneficial. The interaction with the probe is also studied; its effect on imaging of molecules is weak in the presence of the metal coating.

The envelope-function method with generalized boundary conditions is applied to the description of localized and resonant interface states. A complete set of phenomenological conditions which restrict the form of connection rules for envelope functions is derived using the Hermiticity and symmetry requirements. Empirical coefficients in the connection rules play role of material parameters which characterize an internal structure of every particular heterointerface. As an illustration we present the derivation of the most general connection rules for the one-band effective mass and 4-band Kane models. The conditions for the existence of Tamm-like localized interface states are established. It is shown that a nontrivial form of the connection rules can also result in the formation of resonant states. The most transparent manifestation of such states is the resonant tunneling through a single-barrier heterostructure.

We study transport properties of an arbitrary two terminal Hermitian system within a tightbinding approximation and derive the expression for the transparency in the form, which enables one to determine exact energies of perfect (unity) transmittance, zero transmittance (Fano resonance) and bound state in the continuum (BIC). These energies correspond to the real roots of two energy-dependent functions that are obtained from two non-Hermitian Hamiltonians: the Feshbach's effective Hamiltonian and the auxiliary Hamiltonian, which can be easily deduced from the effective one. BICs and scattering states are deeply connected to each other. We show that transformation of a scattering state into a BIC can be formally described as a "phase transition" with divergent generalized response function. Design rules for quantum conductors and waveguides are presented, which determine structures exhibiting coalescence of both resonances and antiresonances resulting in the formation of almost rectangular transparency and reflection windows. The results can find applications in construction of molecular conductors, broad band filters, quantum heat engines and waveguides with controllable BIC formation.