Photoluminescence from metal nanostructures
following intense ultrashort
illumination is a fundamental aspect of light–matter interactions.
Surprisingly, many of its basic characteristics are under ongoing
debate. Here, we resolve many of these debates by providing a comprehensive
theoretical framework that describes this phenomenon and support it
by an experimental confirmation. Specifically, we identify aspects
of the emission that are characteristic to either nonthermal or thermal
emission, in particular, differences in the spectral and electric
field dependence of these two contributions to the emission. Overall,
nonthermal emission is characteristic of the early stages of light
emission, while the later stages show thermal characteristics. The
former dominate only for moderately high illumination intensities
for which the electron temperature reached after thermalization remains
close to room temperature.
Micro and nano structures of titanium dioxide (TiO2) are well-known for their photocatalytic application. High surface area and high light scattering efficiency in such structures enhance their photocatalytic activity. The present work explores the possibility of enhancing photocatalytic activity through mesoporous TiO2 spheres by exploiting the coexistence of high porosity and morphology dependent resonance (MDR) modes. A controlled synthesis of TiO2 spheres with nano-crystalline grains of anatase phase and high surface area of about 96 m2/g has been successfully accomplished leading to mesoporous particles with uniformly distributed pores of small diameters much less than the wavelength of incident light. Despite the high porosity, MDR modes are observed in the photoluminescence spectrum of a single sphere. As inclusion of pores may produce significant changes in the refractive index (RI) of the resonator, and as the quality and density of the modes depend on the RI of the resonator, it is important to have a procedure to determine the RI of the resonator as well as to characterize the MDR modes. An iterative procedure that is quite general is presented for mode identification and for the determination of the porosity-induced reduction in the RI and for ascertaining the presence of chromatic dispersion. The presence of high surface area as well as of MDR modes of reasonably high Q-factor makes these particles promising for photo electrochemical applications.
Optical polymers are attractive for lightweight and cost-effective
refractive optical components, yet they reflect part of the incident
light. Traditional vacuum-deposited antireflective films purely adhere
to polymers and suffer from mechanical stresses due to the difference
in the thermal expansion coefficients. Alternatively, reflection can
be reduced by moth-eye structures; yet, their efficiency strongly
depends on their index-matching with the optical substrate, which
has not been demonstrated so far. Here, we introduce a new approach
to engineering highly effective antireflective structures on the surface
of the optical polymer, with an unprecedented ability to reduce the
surface reflection from 5 to 0.1%. The structures were produced by
high-throughput nanoimprint lithography, and their superior optical
performance was achieved due to the precise matching of their index
to that of the underlying substrate. We further applied these structures
on different polymers and showed that their antireflective effect
correlates with index-matching. We demonstrated that these structures
could be applied on flat surfaces and curved lenses and produce high
surface hydrophobicity. Overall, our work paves the way to an efficient
and scalable antireflective solution for polymer optics.
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