20 research outputs found
Near-unity broadband omnidirectional emissivity via femtosecond laser surface processing
It is very challenging to achieve near perfect absorption/emission that is
both broadband and omnidirectional while utilizing a scalable fabrication
process. Femtosecond laser surface processing is an emerging low-cost and
large-scale manufacturing technique used to directly and permanently modify the
surface properties of a material. The versatility of this technique to produce
tailored surface properties has resulted in a rapidly growing number of
applications. Here, we demonstrate near perfect, broadband, omnidirectional
emissivity from aluminum surfaces by tuning the laser surface processing
parameters including fluence, pulse count, and the ambient gas. Full-wave
simulations and experimental results prove that the obtained increase in
emissivity is mainly a result of two distinct features produced by femtosecond
laser surface processing: the introduction of microscale surface features and
the thick oxide layer. This technique leads to functionalized metallic surfaces
that are ideal for emerging applications, such as passive radiative cooling and
thermal management of spacecraft
Unraveling the temperature dynamics and hot electron generation in tunable gap-plasmon metasurface absorbers
Localized plasmons formed in ultrathin metallic nanogaps can lead to robust
absorption of incident light. Plasmonic metasurfaces based on this effect can
efficiently generate energetic charge carriers, also known as hot electrons,
owing to their ability to squeeze and enhance electromagnetic fields in
confined subwavelength spaces. However, it is very challenging to accurately
identify and quantify the dynamics of hot carriers, mainly due to their
ultrafast time decay. Their non-equilibrium temperature response is one of the
key factors missing to understand the short time decay and overall transient
tunable absorption performance of gap-plasmon metasurfaces. Here, we
systematically study the temperature dynamics of hot electrons and their
transition into thermal carriers at various timescales from femto to
nanoseconds by using the two-temperature model. Additionally, the hot electron
temperature and generation rate threshold values are investigated by using a
hydrodynamic nonlocal model approach that is more accurate when ultrathin gaps
are considered. The derived temperature dependent material properties are used
to study the ultrafast transient nonlinear modification in the absorption
spectrum before plasmon-induced lattice heating is established leading to
efficient tunable nanophotonic absorber designs. We also examine the damage
threshold of these plasmonic absorbers under various pulsed laser
illuminations, an important quantity to derive the ultimate input intensity
limits that can be used in various emerging nonlinear optics and other tunable
nanophotonic applications. The presented results elucidate the role of hot
electrons in the response of gap-plasmon metasurface absorbers which can be
used to design more efficient photocatalysis, photovoltaics, and photodetection
devices
Photothermal Circular Dichroism Induced by Plasmon Resonances in Chiral Metamaterial Absorbers and Bolometers
Chiral photochemistry remains a challenge
because of the very small asymmetry in the chiro-optical absorption of
molecular species. However, we think that the rapidly developing fields of
plasmonic chirality and plasmon-induced circular dichroism demonstrate very
strong chiro-optical effects and have the potential to facilitate the
development of chiral photochemistry and other related applications such as
chiral separation and sensing. In this study, we propose a new type of chiral
spectroscopy – photothermal circular dichroism. It is already known that the
planar plasmonic superabsorbers can be designed to exhibit giant circular
dichroism signals in the reflection. Therefore, upon illumination with chiral
light, such planar metastructures should be able to generate a strong asymmetry
in their local temperatures. Indeed, we demonstrate this chiral photothermal effect
using a chiral plasmonic absorber. Calculated temperature maps show very strong
photothermal circular dichroism. One of the structures computed in this paper could
serve as a chiral bolometer sensitive to circularly polarized light. Overall, this
chiro-optical effect in plasmonic metamaterials is much greater than the
equivalent effect in any chiral molecular system or plasmonic bio-assembly. Potential
applications of this effect are in polarization-sensitive surface photochemistry and chiral bolometers
Development of a Theoretical Model That Predicts Optothermal Energy Conversion of Gold Metallic Nanoparticles
Gold nanoparticles (AuNPs) can be found in different shapes and sizes, which determine their chemical and physical characteristics. Physical and chemical properties of
metallic NPs can be tuned by changing their shape, size, and
surface chemistry; therefore, this has led to their use in a wide variety of applications in many industrial and academic sectors. One of the features of metallic NPs is their ability to act as optothermal energy converters, where they absorb light at a specific wavelength and heat up their local nanosurfaces. This feature has been used in many applications where metallic NPs get coupled with thermally responsive systems to trigger an optical response. In this study, we synthesized AuNPs that are spherical in shape with an average diameter of 20.07 nm. This work assessed simultaneously theoretical and experimental techniques to evaluate the different factors that affect heat generation at the surface of AuNPs when exposed to a specific light wavelength. The results indicated that laser power, concentration of AuNPs, time × laser power interaction, and time illumination, were the most important factors that contributed to the temperature change exhibited in the AuNPs solution. We report a regression model that allows predicting heat generation and temperature changes with residual standard errors of less than 4%. These results are highly relevant in the future design and development of applications where metallic NPs are incorporated into systems to induce a temperature change triggered by light exposure
Localization of Excess Temperature Using Plasmonic Hot Spots in Metal Nanostructures: Combining Nano-Optical Antennas with the Fano Effect
It
is challenging to strongly localize temperature in small volumes
because heat transfer is a diffusive process. Here we show how to
overcome this limitation using electrodynamic hot spots and interference
effects in the regime of continuous-wave (CW) excitation. We introduce
a set of figures of merit for the localization of excess temperature
and for the efficiency of the plasmonic photothermal effect. Our calculations
show that the local temperature distribution in a trimer nanoparticle
assembly is a complex function of the geometry and sizes. Large nanoparticles
in the trimer play the role of the nano-optical antenna, whereas the
small nanoparticle in the plasmonic hot spot acts as a nanoheater.
Under the specific conditions, the temperature increase inside a nanoparticle
trimer can be localized in a hot spot region at the small heater nanoparticle
and, in this way, a thermal hot spot can be realized. However, the
overall power efficiency of local heating in this trimer is much smaller
than that of a single nanoparticle. We can overcome the latter disadvantage
by using a trimer with a nanorod. In the trimer assembly composed
of a nanorod and two spherical nanoparticles, we observe a strong
plasmonic Fano effect that leads to the concentration of optical energy
in the small heater nanorod. Therefore, the power efficiency of generation
of local excess temperature in the nanorod-based assembly greatly
increases due to the strong plasmonic Fano effect. The Fano heater
incorporating a small nanorod in the hot spot has obviously the best
performance compared to both single nanocrystals and a nanoparticle
trimer. The principles of heat localization described here can be
potentially used for thermal photocatalysis, energy conversion and
biorelated applications
DNA-Assembled Nanoparticle Rings Exhibit Electric and Magnetic Resonances at Visible Frequencies
Metallic nanostructures can be used
to manipulate light on the subwavelength scale to create tailored
optical material properties. Next to electric responses, artificial
optical magnetism is of particular interest but difficult to achieve
at visible wavelengths. DNA-self-assembly has proved to serve as a
viable method to template plasmonic materials with nanometer precision
and to produce large quantities of metallic objects with high yields.
We present here the fabrication of self-assembled ring-shaped plasmonic
metamolecules that are composed of four to eight single metal nanoparticles
with full stoichiometric and geometric control. Scattering spectra
of single rings as well as absorption spectra of solutions containing
the metamolecules are used to examine the unique plasmonic features,
which are compared to computational simulations. We demonstrate that
the electric and magnetic plasmon resonance modes strongly correlate
with the exact shape of the structures. In particular, our computations
reveal the magnetic plasmons only for particle rings of broken symmetries,
which is consistent with our experimental data. We stress the feasibility
of DNA self-assembly as a method to create bulk plasmonic materials
and metamolecules that may be applied as building blocks in plasmonic
devices