13 research outputs found
Parametric study of temperature distribution in plasmon-assisted photocatalysis
Recently, there has been a growing interest in the usage of mm-scale
composites of plasmonic nanoparticles for enhancing the rates of chemical
reactions; the effect was shown recently to be predominantly associated with
the elevated temperature caused by illumination. Here, we study the parametric
dependence of the temperature distribution in these samples, and provide
analytic expressions for simple cases. We show that since these systems are
usually designed to absorb all the incoming light, the temperature distribution
in them is weakly-dependent on the illumination spectrum, pulse duration,
particle shape, size and density. Thus, changes in these parameters yield at
most modest quantitative changes. We also show that the temperature
distribution is linearly dependent on the beam radius and the thermal
conductivity of the host. Finally, we study the sensitivity of the reaction
rate to these parameters as a function of the activation energy and interpret
various previous experimental reports. These results would simplify the
optimization of photocatalysis experiments, as well as for other energy-related
applications based on light harvesting for heat generation
The photothermal nonlinearity in plasmon-assisted photocatalysis
We show that the temperature rise in large ensembles of metal nanoparticles
under intense illumination is dominated by the temperature dependence of the
thermal conductivity of the host, rather than by the optical properties of the
metal or the host. This dependence typically causes the temperature rise to
become sublinear, with this photothermal nonlinear effect becoming unusually
strong, reaching even several tens of percent. We then show that this can
explain experimental observations in several recent plasmon-assisted
photocatalysis experiments. This shows that any claim for dominance of
non-thermal electrons in plasmon-assisted photocatalysis must account for this
photothermal nonlinear mechanism
An electronic-based model of the optical nonlinearity of low-electron-density-Drude materials
Low electron density Drude (LEDD) materials such as indium tin oxide (ITO)
are receiving considerable attention because of their combination of CMOS
compatibility, unique epsilon-near-zero (ENZ) behavior, and giant ultrafast
nonlinear thermo-optic response. However, the understanding of the electronic
and optical response of LEDD materials is so far based on simplistic extensions
of known models of noble metals, frequently without the inclusion of the
interplay among the lower electron density, relatively high Debye energy, and
the non-parabolic band structure. To bridge this knowledge gap, this work
provides a complete understanding of the nonlinear electronic-thermal-optical
response of LEDD materials. In particular, we rely on state-of-the-art electron
dynamics modeling, as well as the newly derived time-dependent permittivity
model for LEDD materials under optical pumping within the adiabatic
approximation. We find that unlike noble metals, the electron temperatures can
reach the Fermi temperature, in which case the effective chemical potential
dramatically decreases and even becomes negative, thus, transiently converting
the Drude metal into a semiconductor. We further show that the nonlinear
optical response of LEDD materials originating from the changes to the real
part of the permittivity is due to the generation of non-thermal electrons.
This resolves the argument about the rise time of the permittivity and shows
that it is instantaneous. In this vein, we show that referring to the LEDD
permittivity as having a ``saturable'' nonlinearity is unsuitable since its
permittivity dynamics does not originate from population inversion. Finally, we
analyze the probe pulse dynamics and unlike previous work, we obtain a
quantitative agreement with the results of recent experiments
The electronic and thermal response of low electron density Drude materials to ultrafast optical illumination
Many low electron density Drude (LEDD) materials such as transparent
conductive oxide or nitrides have recently attracted interest as alternative
plasmonic materials and future nonlinear optical materials. However, the
rapidly growing number of experimental studies has so far not been supported by
a systematic theory of the electronic, thermal and optical response of these
materials. Here, we use the techniques previously derived in the context of
noble metals to go beyond a simple electromagnetic modelling of low electron
density Drude materials and provide an electron dynamics model for their
electronic and thermal response. We find that the low electron density makes
momentum conservation in electron-phonon interactions more important, more
complex and more sensitive to the temperatures compared with noble metals;
moreover, we find that electron-electron interactions are becoming more
effective due to the weaker screening. Most importantly, we show that the low
electron density makes the electron heat capacity much smaller than in noble
metals, such that the electrons in LEDD materials tend to heat up much more and
cool down faster compared to noble metals. While here we focus on indium tin
oxide (ITO), our analytic results can be easily applied to any LEDD materials.Comment: Comments are welcome
Theory of Non-equilibrium "Hot" Carriers in Direct Band-gap Semiconductors Under Continuous Illumination
The interplay between the illuminated excitation of carriers and subsequent
thermalization and recombination leads to the formation of non-equilibrium
distributions for the "hot" carriers and to heating of both electrons, holes
and phonons. In spite of the fundamental and practical importance of these
processes, there is no theoretical framework which encompasses all of them and
provides a clear prediction for the non-equilibrium carrier distributions.
Here, a self-consistent theory accounting for the interplay between excitation,
thermalization, and recombination in continuously-illuminated semiconductors is
presented, enabling the calculation of non-equilibrium carrier distributions.
We show that counter-intuitively, distributions deviate more from equilibrium
under weak illumination than at high intensities. We mimic two experimental
procedures to extract the carrier temperatures and show that they yield
different dependence on illumination. Finally, we provide an accurate way to
evaluate photoluminescence efficiency, which, unlike conventional models,
predicts correctly the experimental results. These results provide a starting
point towards examining how non-equilibrium features will affect properties
hot-carrier based application.Comment: Version accepted in New Journal of Physic
The role of heat generation and fluid flow in plasmon-enhanced reduction-oxidation reactions
Recently, we have shown that thermal effects play a crucial role in speeding
up the rate of bond-dissociation reactions. This was done by applying a simple
temperature-shifted Arrhenius Law to the experimental data, corroborated with
detailed account of the heat diffusion occurring within the relevant samples
and identification of errors in the temperature measurements. Here, we provide
three important extensions of our previous studies. First, we analyze thermal
effects in reduction-oxidation (redox) reactions, where charge transfer is an
integral part of the reaction. Second, we analyze not only the spatial
distribution of the temperature, but also its temporal dynamics. Third, we also
model the fluid convection and stirring. An analysis of two exemplary
experimental studies allows us to show that thermal effects can explain the
experimental data in one of experiments (Baumberg and coworkers), but not in
the other (Jain and coworkers), showing that redox reactions are not
necessarily driven by non-thermal charge carriers