32 research outputs found
Understanding Hot-Electron Generation and Plasmon Relaxation in Metal Nanocrystals: Quantum and Classical Mechanisms
Generation of energetic (hot) electrons is an intrinsic property of any
plasmonic nanostructure under illumination. Simultaneously, a striking
advantage of metal nanocrystals over semiconductors lies in their very large
absorption cross sections. Therefore, metal nanostructures with strong and
tailored plasmonic resonances are very attractive for photocatalytic
applications. However, the central questions regarding plasmonic hot electrons
are how to quantify and extract the optically-excited energetic electrons in a
nanocrystal. We develop a theory describing the generation rates and the
energy-distributions of hot electrons in nanocrystals with various geometries.
In our theory, hot electrons are generated owing to surfaces and hot spots. The
formalism predicts that large optically-excited nanocrystals show the
excitation of mostly low-energy Drude electrons, whereas plasmons in small
nanocrystals involve mostly hot electrons. The energy distributions of
electrons in an optically-excited nanocrystal show how the quantum many-body
state in small particles evolves towards the classical state described by the
Drude model when increasing nanocrystal size. We show that the rate of surface
decay of plasmons in nanocrystals is directly related to the rate of generation
of hot electrons. Based on a detailed many-body theory involving kinetic
coefficients, we formulate a simple scheme describing the plasmon's dephasing.
In most nanocrystals, the main decay mechanism of a plasmon is the Drude
friction-like process and the secondary path comes from generation of hot
electrons due to surfaces and electromagnetic hot spots. This latter path
strongly depends on the size, shape and material of the nanocrystal,
correspondingly affecting its efficiency of hot-electron production. The
results in the paper can be used to guide the design of plasmonic nanomaterials
for photochemistry and photodetectors.Comment: 90 pages, 21 figures, including Supplementary Informatio
What's so Hot about Electrons in Metal Nanoparticles?
Metal nanoparticles are excellent light absorbers. The absorption processes
create highly excited electron-hole pairs and recently there has been interest
in harnessing these hot charge carriers for photocatalysis and solar energy
conversion applications. The goal of this Perspectives article is to describe
the dynamics and energy distribution of the charge carriers produced by photon
absorption, and the implications for the photocatalysis mechanism. We will also
discuss how spectroscopy can be used to provide insight into the coupling
between plasmons and molecular resonances. In particular, the analysis shows
that the choice of material and shape of the nanocrystal can play a crucial
role in hot electron generation and coupling between plasmons and molecular
transitions. The detection and even calculation of many-body hot-electron
processes in the plasmonic systems with continuous spectra of electrons and
short lifetimes are challenging, but at the same time very interesting from the
point of view of both potential applications and fundamental physics. We
propose that developing an understanding of these processes will provide a
pathway for improving the efficiency of plasmon-induced photocatalysis.Comment: To be published in ACS Energy. 41 pages and 8 figures, including
Supplementary Informatio
Temporal plasmonics: Fano and Rabi regimes in the time domain in metal nanostructures
The Fano and Rabi models represent remarkably common effects in optics. Here
we study the coherent time dynamics of plasmonic systems exhibiting Fano and
Rabi resonances. We demonstrate that these systems show fundamentally different
dynamics. A system with a Fano resonance displays at most one temporal beat
under pulsed excitation, whereas a system in the Rabi regime may have any
number of beats. Remarkably, the Fano-like systems show time dynamics with very
characteristic coherent tails despite the strong decoherence that is intrinsic
for such systems. The coherent Fano and Rabi dynamics that we predicted can be
observed in plasmonic nanocrystal dimers in time-resolved experiments. Our
study demonstrates that such coherent temporal plasmonics includes nontrivial
and characteristic relaxation behaviors and presents an interesting direction
to develop with further research.Comment: 31 pages [main text (4 figures) + supplemental (12 figures)
Aluminum Nanoparticles with Hot Spots for Plasmon-Induced Circular Dichroism of Chiral Molecules in the UV Spectral Interval
Plasmonic nanocrystals with hot spots are able to localize optical energy in
small spaces. In such physical systems, near-field interactions between
molecules and plasmons can become especially strong. This paper considers the
case of a nanoparticle dimer and a chiral biomolecule. In our model, a chiral
molecule is placed in the gap between two plasmonic nanoparticles, where the
electromagnetic hot spot occurs. Since many important biomolecules have optical
transitions in the UV, we consider the case of Aluminum nanoparticles, as they
offer strong electromagnetic enhancements in the blue and UV spectral
intervals. Our calculations show that the complex composed of a chiral molecule
and an Al-dimer exhibits strong CD signals in the plasmonic spectral region. In
contrast to the standard Au- and Ag-nanocrystals, the Al system may have a much
better spectral overlap between the typical biomolecule's optical transitions
and the nanocrystals' plasmonic band. Overall, we found that Al nanocrystals
used as CD antennas exhibit unique properties as compared to other commonly
studied plasmonic and dielectric materials. The plasmonic systems investigated
in this study can be potentially used for sensing chirality of biomolecules,
which is of interest in applications such as drug development.Comment: 31 pages, 7 figure
Universal imprinting of chirality with chiral light by employing plasmonic metastructures
Chirality, either of light or matter, has proved to be very practical in
biosensing and nanophotonics. However, the fundamental understanding of its
temporal dynamics still needs to be discovered. A realistic setup for this are
the so-called metastructures, since they are optically active and are built
massively, hence rendering an immediate potential candidate. Here we propose
and study the electromagnetic-optical mechanism leading to chiral optical
imprinting on metastructures. Induced photothermal responses create anisotropic
permittivity modulations, different for left or right circularly polarized
light, leading to temporal-dependent chiral imprinting of hot-spots, namely
imprinting of chirality. The above effect has not been observed yet, but it is
within reach of modern experimental approaches. The proposed nonlinear
chiroptical effect is general and should appear in any anisotropic material;
however, we need to design a particular geometry for this effect to be strong.
These new chiral time-dependent metastructures may lead to a plethora of
applications.Comment: Main (29 pages, 6 figures) and supplemental (46 pages, 35 figures