Photons produced in the process of spontaneous parametric down-conversion (SPDC) are typically entangled in multiple
degrees of freedom. Although polarization is often the parameter of interest, the presence of spectral or spatial
entanglement can complicate experiments, either by reducing the visibility of certain interference effects or by reducing
collection efficiency. Several recent works have shown that the collection efficiency can be improved by focusing the
pump beam. This approach has the added benefit of increasing the bandwidths of the emitted photons. We show that, in
the case of type II SPDC, a focused pump can result in different spatial profiles for the signal and idler. If the crystal is
configured for emission of polarization-entangled photon pairs, this effect will reduce the fidelity of that entanglement.
Moreover, this spatial asymmetry leads to different spectral profiles for the two photons, even when the pump is
monochromatic. The spectral and spatial asymmetries can be attributed to the difference in the angular dispersion (walkoff)
of the two polarizations, along with a strong correlation between wavelength and emission direction. We also
examine the link between spatial entanglement and single-mode coupling efficiency. We find that efficiency is maximized
when spatial entanglement can be eliminated. For the case in which walk-off does not play a role, this can be
accomplished by properly focusing the pump.
We present theoretical studies and experimental results on the optical properties of gold, octahedra-shaped nanoparticles.
We show that the optical spectrum varies quite dramatically as two nanoparticles are brought into close proximity. AFM
images and optical spectra have been obtained for nanooctahedra dimers in uncoupled and strongly coupled
configurations. The former displays a single peak in the optical spectrum, while the latter shows an additional peak at
longer wavelengths. Calculations indicate that the additional spectral feature originates from a strongly coupled plasmon
state that oscillates along the extended axis of the dimer. We investigate theoretically the distances over which the
dimers couple and find these results to be particularly orientation dependent. The anisotropic particle shape and sharp
apices contribute significantly to the orientational dependence of the interparticle couplings.