This paper considers a range of plasmonic-black-metal polarizers suitable for ultra-short pulses. The polarizers consist of
a metal surface being nanostructured with a periodic array of ultra-sharp grooves with periods of 250-350 nanometers,
and groove depths around 500 nanometers. The surfaces can be designed such that practically all incident light with
electric field perpendicular to the groove direction is absorbed. The efficient absorption is due to incident light being
coupled into gap-plasmon polaritons that propagate downwards in the gaps between groove walls towards the groove
bottom, where it is then subsequently absorbed during propagation. Reflection is largely avoided due to an adiabatic
groove taper design. The other polarization, however, is very efficiently reflected, and the main point of this paper is that
the reflection is with negligible dispersive stretching even for ultra-short pulses of 5-10 femtoseconds temporal width in
the visible and near-infrared. Temporal pulse shapes after reflection are calculated by decomposing the incident laser
pulse into its Fourier components, multiplying with the reflection coefficient in the frequency domain, and then Fouriertransforming
the product back to the time-domain. Reflection of pulses is compared for polarizers based on different
metals (gold, nickel, chromium). Polarizers are studied for two groove-array designs and two directions of light
incidence, center wavelengths 650 nm and 800 nm, and pulse widths 5 fs and 10 fs for the incident pulse.
We produce and characterize randomly distributed, highly enhancing, large-area gold nanostructures formed on
templates made after anodization of Al with either oxalic acid or phosphoric acid, producing nanoporous alumina films.
The interpore distance of the fabricated templates can be tuned continuously, and by a subsequent selective dissolution of
the upper film making up the porous Al2O3 layers, the remaining embossed barrier layer can be used as a template for
sputter deposition of gold. The density (and structure) of gold nanoparticles covering the template is adjusted by varying
the sputtering conditions. We directly correlate the strong and broad surface plasmon (SP) resonances investigated by
reflection spectroscopy, as well as the field intensity enhancement (FE) factor investigated by far-field two-photon
luminescence (TPL) scanning optical microscopy measurements, to the density of the randomly distributed gold
nanoparticles on the templates. The position of high enhancements in the TPL-images and the magnitude of the average
FE are dictated by the laser excitation wavelength. We relate this large-area massive enhancement to constructive
interference of SP polaritons scattered from the densely packed gold particles on the fabricated templates.
Optical resonances in a single triangular-shaped metal groove and a periodic array of grooves are studied theoretically
with the Green's function surface integral equation method. In the case of a single groove we study the geometric
resonances for different groove heights, and show that the groove resonances can be explained by standing waves in the
gap being reflected at both the closed groove bottom and the open groove top. We also present the reflection that will be
obtained for different cases of picking up the reflected light within a small or large angular range. Large resonant fields
at the groove bottom are explained as being due to nanofocusing by the groove which can also be thought of as a closed
tapered gap. In the case of a periodic array of grooves we find that resonances of individual grooves are still present in
near-field enhancement spectra and reflection spectra but there are also e.g. very sharp resonances (Rayleigh anomalies)
at wavelengths near the cutoff wavelength of higher grating-reflection orders. Typical resonant enhancements can easily
be two times higher compared with the case of a single groove. The resonances can be realized in the wavelength range
from the visible to the infrared by varying groove height, angle, and periodicity.
We demonstrate the possibility of mapping the distribution of different biomolecules in living human embryonic stem
cells grown on glass substrates, without the need for fluorescent markers. In our work we improve the quality of
measurements by finding a buffer that gives low fluorescence, growing cells on glass substrates (whose Raman signals
are relatively weak compared to that of the cells) and having the backside covered with gold to improve the image
contrast under direct white light illumination. The experimental setup used for Raman microscopy is the commercially
available confocal scanning Raman microscope (Alpha300R) from Witec and sub-μm spatially resolved Raman images
were obtained using a 532 nm excitation wavelength.
Scattering resonances of metal nano-strip resonators are described as a consequence of formation of standing waves due
to counter-propagating short-range (and slow) surface plasmon polaritons and gap plasmon polaritons, which are
electromagnetic waves bound to and propagating along a nanometer-thin metal film, and a nanometer-sized gap between
metal surfaces, respectively. Scattering spectra and resonant fields are presented for single-metal-nano-strip resonators
and gap plasmon resonators with two closely spaced metal nano-strips. It is shown how strip resonators can be designed
for any resonance wavelength in the range from 600nm to 1600nm.
Using a scanning far-field second harmonic (SH) microscope we have excited surface plasmon polaritons (SPPs) on a 70 nm thick gold film surface covered with randomly distributed 80 nm wide gold particles. Multiple scattering of the SPPs by these gold particles leads to localization of the electromagnetic fields causing strongly enhanced and spatially localized SH generation. We investigate wavelength and polarization dependencies of both position and intensity of these SH bright spots for two different densities of random scatterers. Comparing SH and fundamental harmonic (FH) images, we conclude that the localized SH enhancement occurs due to overlap of FH and SH eigenmodes. Furthermore we confirm that for incident laser powers in the range 3-40mW, the bright spots exhibit quadratic intensity dependence. For the higher incident laser powers however, the strong heating of the surface seems to change the material properties causing some bright spots to disappear and others to emerge.