The finite-difference time-domain method can provide broadband results if the excitation source is a pulse. This demands that the parameters of modeled materials have to be applicable over broad frequency bands. We optimize the modified Debye model parameters for gold, silver, copper, platinum, and aluminum using a large-scale nonlinear optimization algorithm. The complex relative permittivities calculated using the optimized parameters agree well with experimental values over broad frequency bands. The associated root-mean-square deviations are 0.49%, 3.52%, 4.13%, 1.64%, and 0.66%, respectively. We also provide an example of broadband calculations. The obtained broadband results are verified by a series of steady-state calculations.

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A very-small-aperture laser (VSAL) with a circular aperture has a trade-off between the spot size and the output power. A nanometric annular aperture is fabricated to overcome this difficulty. The advantages of the annular aperture are demonstrated by measuring and comparing its near-field intensity distribution with that of a circular aperture. These apertures are fabricated on the same VSAL to ensure that they are under the same illumination conditions. The experimental results indicate that an annular aperture produces a smaller spot size and a higher peak intensity than a circular aperture. The confinement effect and the enhancement effect are attributed to the convergence of the power flow that passes through the annular aperture. The observed enhancement effect decreases when the distance from the VSAL facet is increased, but it does not vanish even when the distance is as large as 3.5 microm.

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