Femtosecond-tunable measurement of electron thermalization in gold

The dynamics of plasmon-induced hot carrier creation in colloidal gold

The generation and dynamics of plasmon-induced hot carriers in gold nanoparticles offer crucial insights into nonequilibrium states for energy applications, yet the underlying mechanisms remain experimentally elusive. Here, we leverage ultrafast X-ray absorption spectroscopy (XAS) to directly capture hot carrier dynamics with sub-50 fs temporal resolution, providing clear evidence of plasmon decay mechanisms. We observe the sequential processes of Landau damping (~25 fs) and hot carrier thermalization (~1.5 ps), identifying hot carrier formation as a significant decay pathway. Energy distribution measurements reveal carriers in non-Fermi-Dirac states persisting beyond 500 fs and observe electron populations exceeding single-photon excitation energy, indicating the role of an Auger heating mechanism alongside traditional impact excitation. These findings deepen the understanding of hot carrier behavior under localized surface plasmon resonance, offering valuable implications for applications in photocatalysis, photovoltaics, and phototherapy. This work establishes a methodological framework for studying hot carrier dynamics, opening avenues for optimizing energy transfer processes in nanoscale plasmonic systems.

Plasmonic Ultrafast All-Optical Switching with a Superior On-Off Ratio

Plasmonic ultrafast all-optical switching holds great promise for advancing next-generation optical communication and optical computing technologies. However, achieving subpicosecond all-optical switching with a high on-off ratio remains challenging due to the slow dynamics of electron-phonon scattering in plasmonic materials. Here, we report an innovative method that utilizes the negative signal induced by plasmonic excited hot electrons in the transient spectrum and the positive signal caused by hot electrons excited by off-resonant pumping, both designed at the same wavelength to effectively offset slow dynamics. This approach enables plasmonic ultrafast all-optical switching with a 500 fs response time and a superior on-off ratio exceeding 20 within 1 ps. The strategy offers a promising path for high-performance all-optical modulation and can be widely applied across various plasmonic materials.

Mechanism of Amplified Photoacoustic Effect for Silica-Coated Spherical Gold Nanoparticles

Plasmonic nanomaterials are effective photoacoustic (PA) contrast agents with diverse biomedical applications. While silica coatings on gold nanoparticles (AuNPs) have been demonstrated to increase PA efficiency, the underlying mechanism remains elusive. Here, we systematically investigated the impact of silica coatings on PA generation under picosecond and nanosecond laser pulses. Experimentally, we demonstrated a record high PA amplification of up to 400% under noncavitation conditions with a thin silica coating and only under picosecond laser pulses. We provide a clear mechanism for the observed PA amplification that identifies two competing effects, including transient absorption, which reduces photon energy absorption, and electron-phonon energy transfer at the gold-silica interface, which partly reverses the transient absorption effect. This study provides the first evidence and mechanistic insight on the impact of nonlinear optical effects on the nanomaterial-property relationship in PA contrast agents and offers insights for designing highly efficient contrast agents for biomedical applications.

Hot Electron Cooling in n-Doped Colloidal Nanoplatelets Following Near-Infrared Intersubband Excitation

Intersubband transition was recently discovered in colloidal nanoplatelets, but the associated intersubband carrier relaxation dynamics remains poorly understood. In particular, it is crucial to selectively excite the intersubband transition and to follow the hot electron dynamics in the absence of valence-band holes. This is achieved herein by exciting the predoped electrons in CdSe/ZnS nanoplatelets using near-infrared femtosecond pulses and monitoring nonequilibrium electron dynamics using broad-band visible pulses. We find that the n = 2 electrons relax to the n = 1 subband and establish a Fermi-Dirac distribution within 200 fs, and finally reach an equilibrium with the lattice within a few ps. The cooling dynamics depend mainly on the excitation fluence but weakly on the doping density and the lattice temperature. These characteristics are well captured by our numerical simulation that explicitly accounts for the state occupation effect and optical phonon scattering.

Harnessing collisional nonlinearity for enhanced harmonic generation by ultraviolet plasmonic nanoparticles

We investigate the contribution of inelastic electron collisions to nonlinear (NL) dynamics in ultraviolet plasmonic nanoparticles, exploring their potential for harmonic generation. Employing the Landau weak coupling formalism to model radiation-driven electron dynamics in sodium and aluminum, we account for both electron-electron and electron-phonon scattering processes by a set of hydrodynamic equations, which we solve perturbatively to obtain third-order NL susceptibilities. Furthermore, we model high harmonic generation enhanced by localized surface plasmons in nanospheres composed of such poor metals, demonstrating their efficient operation for extreme ultraviolet generation. Our investigation reveals that plasmonic nanospheres composed of sodium and aluminum produce a large field intensity enhancement of ≃103-105, boosting the harmonic generation process. Our findings indicate that poor metals hold great promise for advanced extreme ultraviolet nano-sources with potential applications in nano-spectroscopy.

Quantifying Ultrafast Energy Transfer from Plasmonic Hot Carriers for Pulsed Photocatalysis on Nanostructures

Photocatalysis with plasmonic nanostructures has lately emerged as a transformative paradigm to drive and alter chemical reactions using light. At the surface of metallic nanoparticles, photoexcitation results in strong near fields, short-lived high-energy "hot" carriers, and light-induced heating, thus creating a local environment where reactions can occur with enhanced efficiencies. In this context, it is critical to understand how to manipulate the nonequilibrium processes triggered by light, as their ultrafast (femto- to picoseconds) relaxation dynamics compete with the process of energy transfer toward the reactants. Accurate predictions of the plasmon photocatalytic activity can lead to optimized nanophotonic architectures with enhanced selectivity and rates, operating beyond the intrinsic limitations of the steady state. Here, we report on an original modeling approach to quantify, with space, time, and energy resolution, the ultrafast energy exchange from plasmonic hot carriers (HCs) to molecular systems adsorbed on the metal nanoparticle surface while consistently accounting for photothermal bond activation. Our analysis, illustrated for a few typical cases, reveals that the most energetic nonequilibrium carriers (i.e., with energies well far from the Fermi level) may introduce a wavelength-dependence of the reaction rates, and it elucidates on the role of the carriers closer to the Fermi energy and the photothermally heated lattice, suggesting ways to enhance and optimize each contribution. We show that the overall reaction rates can benefit strongly from using pulsed illumination with the optimal pulse width determined by the properties of the system. Taken together, these results contribute to the rational design of nanoreactors for pulsed catalysis, which calls for predictive modeling of the ultrafast HC-hot adsorbate energy transfer.

Decoupling Plasmonic Hot Carrier from Thermal Catalysis via Electrode Engineering

Increased attention has been directed toward generating nonequilibrium hot carriers resulting from the decay of collective electronic oscillations on metal known as surface plasmons. Despite numerous experimental endeavors, demonstrating hot carrier-mediated photocatalysis without a heating contribution has proven challenging, particularly for single electron transfer reactions where the thermal contribution is generally detrimental. An innovative engineering solution is proposed to enable single electron transfer reactions with plasmonics. It consists of a photoelectrode designed as an energy filter and photocatalysis performed with light function modulation instead of continuously. The photoelectrode, consisting of FTO/TiO2 amorphous (10 nm)/Au nanoparticles, with TiO2 acting as a step-shape energy filter to enhance hot electron extraction and charge-separated state lifetime. The extracted hot electrons were directed toward the counter electrode, while the hot holes performed a single electron transfer oxidation reaction. Light modulation prevented local heat accumulation, effectively decoupling hot carrier catalysis from the thermal contribution.

The role of the plasmon in interfacial charge transfer

The lack of a detailed mechanistic understanding for plasmon-mediated charge transfer at metal-semiconductor interfaces severely limits the design of efficient photovoltaic and photocatalytic devices. A major remaining question is the relative contribution from indirect transfer of hot electrons generated by plasmon decay in the metal to the semiconductor compared to direct metal-to-semiconductor interfacial charge transfer. Here, we demonstrate an overall electron transfer efficiency of 44 ± 3% from gold nanorods to titanium oxide shells when excited on resonance. We prove that half of it originates from direct interfacial charge transfer mediated specifically by exciting the plasmon. We are able to distinguish between direct and indirect pathways through multimodal frequency-resolved approach measuring the homogeneous plasmon linewidth by single-particle scattering spectroscopy and time-resolved transient absorption spectroscopy with variable pump wavelengths. Our results signify that the direct plasmon-induced charge transfer pathway is a promising way to improve hot carrier extraction efficiency by circumventing metal intrinsic decay that results mainly in nonspecific heating.

Ultrafast Surface Plasmon Probing of Interband and Intraband Hot Electron Excitations

Upon the interaction of light with metals, nonthermal electrons are generated with intriguing transient behavior. Here, we present femtosecond hot electron probing in a noveloptical pump/plasmon probe scheme. With this, we probed ultrafast interband and intraband dynamics with 15 nm interface selectivity, observing a two-component-decay of hot electron populations. Results are in good agreement with a three-temperature model of the metal; thus, we could attribute the fast (∼100 fs) decay to the thermalization of hot electrons and the slow (picosecond) decay to electron-lattice thermalization. Moreover, we could modulate the transmission of our plasmonic channel with ∼40% depth, hinting at the possibility of ultrafast information processing applications with plasmonic signals.

Evolution of Excited-State Behaviors of Gold Complexes, Nanoclusters and Nanoparticles

Metal nanomaterials have been extensively investigated owing to their unique properties in contrast to bulk counterparts. Gold nanoparticles (e. g., 3-100 nm) show quasi-continuous energy bands, while gold nanoclusters (<3 nm) and complexes exhibit discrete energy levels and display entirely different photophysical properties than regular nanoparticles. This review summarizes the electronic dynamics of these three types of gold materials studied by ultrafast spectroscopy. Briefly, for gold nanoparticles, their electronic relaxation is dominated by heat dissipation between the electrons and the lattice. In contrast, gold nanoclusters exhibit single-electron transitions and relatively long excited-state lifetimes being analogous to molecules. In gold complexes, the excited-state dynamics is dominated by intersystem crossing and phosphorescence. A detailed understanding of the photophysical properties of gold nanocluster materials is still missing and thus calls for future efforts. The fundamental insights into the discrete electronic structure and the size-induced evolution in quantum-sized nanoclusters will promote the exploration of their applications in various fields.

"Nano-egg" superstructures of hydrophobic nanocrystals dispersed in water

In this feature article, we use hydrophobic ferrite (Fe3O4) nanocrystal shells filled with Au nanocrystals self-assembled into 3D superlattices and dispersed in water. These superstructures act as nano-heaters. The stability of such superstructures is very high, even for several years, when stored at room temperature. When subjected to an electron beam, the inverted structure of Fe3O4 structures is gradually dissolved due to the formation of hydrated electrons and hydroxyl radicals.

Impacts of hot electron diffusion, electron-phonon coupling, and surface atoms on metal surface dynamics revealed by reflection ultrafast electron diffraction

Metals exhibit nonequilibrium electron and lattice subsystems at transient times following femtosecond laser excitation. In the past four decades, various optical spectroscopy and time-resolved diffraction methods have been used to study electron-phonon coupling and the effects of underlying dynamical processes. Here, we take advantage of the surface specificity of reflection ultrafast electron diffraction (UED) to examine the structural dynamics of photoexcited metal surfaces, which are apparently slower in recovery than predicted by thermal diffusion from the profile of absorbed energy. Fast diffusion of hot electrons is found to critically reduce surface excitation and affect the temporal dependence of the increased atomic motions on not only the ultrashort but also sub-nanosecond times. Whereas the two-temperature model with the accepted physical constants of platinum can reproduce the observed surface lattice dynamics, gold is found to exhibit appreciably larger-than-expected dynamic vibrational amplitudes of surface atoms while keeping the commonly used electron-phonon coupling constant. Such surface behavioral difference at transient times can be understood in the context of the different strengths of binding to surface atoms for the two metals. In addition, with the quantitative agreements between diffraction and theoretical results, we provide convincing evidence that surface structural dynamics can be reliably obtained by reflection UED even in the presence of laser-induced transient electric fields.

Non-thermal emission in gap-mode plasmon photoluminescence

Photoluminescence from spatially inhomogeneous plasmonic nanostructures exhibits fascinating wavelength-dependent nonlinear behaviors due to the intraband recombination of hot electrons excited into the conduction band of the metal. The properties of the excited carrier distribution and the role of localized plasmonic modes are subjects of debate. In this work, we use plasmonic gap-mode resonators with precise nanometer-scale confinement to show that the nonlinear photoluminescence behavior can become dominated by non-thermal contributions produced by the excited carrier population that strongly deviates from the Fermi-Dirac distribution due to the confinement-induced large-momentum free carrier absorption beyond the dipole approximation. These findings open new pathways for controllable light conversion using nonequilibrium electron states at the nanoscale.

Ab initio-simulated optical response of hot electrons in gold and ruthenium

Optical femtosecond pump-probe experiments allow to measure the dynamics of ultrafast heating of metals with high accuracy. However, the theoretical analysis of such experiments is often complicated because of the indirect connection of the measured signal and the desired temperature transients. Establishing such a connection requires an accurate model of the optical constants of a metal, depending on both the electron temperature Te and the lattice temperature Tl. In this paper, we present first-principles simulations of the two-temperature scenario with Te ≫ Tl, showing the optical response of hot electrons to laser irradiation in gold and ruthenium. Comparing our simulations with the Kubo-Greenwood approach, we discuss the influence of electron-phonon and electron-electron scattering on the intraband contribution to optical constants. Applying the simulated optical constants to the analysis of ultrafast heating of ruthenium thin films we highlight the importance of the latter scattering channel to understand the measured heating dynamics.

Molecular scale nanophotonics: hot carriers, strong coupling, and electrically driven plasmonic processes

Plasmonic modes confined to metallic nanostructures at the atomic and molecular scale push the boundaries of light-matter interactions. Within these extreme plasmonic structures of ultrathin nanogaps, coupled nanoparticles, and tunnelling junctions, new physical phenomena arise when plasmon resonances couple to electronic, exitonic, or vibrational excitations, as well as the efficient generation of non-radiative hot carriers. This review surveys the latest experimental and theoretical advances in the regime of extreme nano-plasmonics, with an emphasis on plasmon-induced hot carriers, strong coupling effects, and electrically driven processes at the molecular scale. We will also highlight related nanophotonic and optoelectronic applications including plasmon-enhanced molecular light sources, photocatalysis, photodetection, and strong coupling with low dimensional materials.

Resonant third-harmonic generation driven by out-of-equilibrium electron dynamics in sodium-based near-zero index thin films

We investigate resonant third-harmonic generation in near-zero index thin films driven out-of-equilibrium by intense optical excitation. Adopting the Landau weak coupling formalism to incorporate electron-electron and electron-phonon scattering processes, we derive a novel set of hydrodynamic equations accounting for collision-driven nonlinear dynamics in sodium. By perturbatively solving hydrodynamic equations, we model third-harmonic generation by a thin sodium film, finding that such a nonlinear process is resonant at the near-zero index resonance of the third-harmonic signal. Thanks to the reduced absorption of sodium, we observe that third-harmonic resonance can be tuned by the impinging pump radiation angle, efficiently modulating the third-harmonic generation process. Furthermore, owing to the metallic sodium response at the pump optical wavelength, we find that the third-harmonic conversion efficiency is maximised at a peculiar thin film thickness where evanescent back-reflection provides increased field intensity within the thin film. Our results are relevant for the development of future ultraviolet light sources, with potential impact for innovative integrated spectroscopy schemes.

Phototransformation of achiral metasurfaces into handedness-selectable transient chiral media

Chirality is a geometric property describing the lack of mirror symmetry. This unique feature enables photonic spin-selectivity in light-matter interaction, which is of great significance in stereochemistry, drug development, quantum optics, and optical polarization control. The versatile control of optical geometry renders optical metamaterials as an effective platform for engineered chiral properties at prescribed spectral regimes. Unfortunately, geometry-imposed restrictions only allow one circular polarization state of photons to effectively interact with chiral meta-structures. This limitation motivates the idea of discovering alternative techniques for dynamically reconfiguring the chiroptical responses of metamaterials in a fast and facile manner. Here, we demonstrate an approach that enables optical, sub-picosecond conversion of achiral meta-structures to transient chiral media in the visible regime with desired handedness upon the inhomogeneous generation of plasmonic hot electrons. As a proof of concept, we utilize linearly polarized laser pulse to demonstrate near-complete conversion of spin sensitivity in an achiral meta-platform-a functionality yet achieved in a non-mechanical fashion. Owing to the generation, diffusion, and relaxation dynamics of hot electrons, the demonstrated technique for all-optical creation of chirality is inherently fast, opening new avenues for ultrafast spectro-temporal construction of chiral platforms with on-demand spin-selectivity.

Ultrafast optical properties of stoichiometric and non-stoichiometric refractory metal nitrides TiNx, ZrNx, and HfNx

Refractory metal nitrides have recently gained attention in various fields of modern photonics due to their cheap and robust production technology, silicon-technology compatibility, high thermal and mechanical resistance, and competitive optical characteristics in comparison to typical plasmonic materials like gold and silver. In this work, we demonstrate that by varying the stoichiometry of sputtered nitride films, both static and ultrafast optical responses of refractory metal nitrides can efficiently be controlled. We further prove that the spectral changes in ultrafast transient response are directly related to the position of the epsilon-near-zero region. At the same time, the analysis of the temporal dynamics allows us to identify three time components: the "fast" femtosecond one, the "moderate" picosecond one, and the "slow" at the nanosecond time scale. We also find out that the non-stoichiometry does not significantly decrease the recovery time of the reflectance value. Our results show the strong electron-phonon coupling and reveal the importance of both the electron and lattice temperature-induced changes in the permittivity near the ENZ region and the thermal origin of the long tail in the transient optical response of refractory nitrides.

Ultrafast hot-carrier dynamics in ultrathin monocrystalline gold

Applications in photodetection, photochemistry, and active metamaterials and metasurfaces require fundamental understanding of ultrafast nonthermal and thermal electron processes in metallic nanosystems. Significant progress has been recently achieved in synthesis and investigation of low-loss monocrystalline gold, opening up opportunities for its use in ultrathin nanophotonic architectures. Here, we reveal fundamental differences in hot-electron thermalisation dynamics between monocrystalline and polycrystalline ultrathin (down to 10 nm thickness) gold films. Comparison of weak and strong excitation regimes showcases a counterintuitive unique interplay between thermalised and non-thermalised electron dynamics in mesoscopic gold with the important influence of the X-point interband transitions on the intraband electron relaxation. We also experimentally demonstrate the effect of hot-electron transfer into a substrate and the substrate thermal properties on electron-electron and electron-phonon scattering in ultrathin films. The hot-electron injection efficiency from monocrystalline gold into TiO2, approaching 9% is measured, close to the theoretical limit. These experimental and modelling results reveal the important role of crystallinity and interfaces on the microscopic electronic processes important in numerous applications.

Femtosecond Laser Direct Writing of Flexible Electronic Devices: A Mini Review

By virtue of its narrow pulse width and high peak power, the femtosecond pulsed laser can achieve high-precision material modification, material additive or subtractive, and other forms of processing. With additional good material adaptability and process compatibility, femtosecond laser-induced application has achieved significant progress in flexible electronics in recent years. These advancements in the femtosecond laser fabrication of flexible electronic devices are comprehensively summarized here. This review first briefly introduces the physical mechanism and characteristics of the femtosecond laser fabrication of various electronic microdevices. It then focuses on effective methods of improving processing efficiency, resolution, and size. It further highlights the typical progress of applications, including flexible energy storage devices, nanogenerators, flexible sensors, and detectors, etc. Finally, it discusses the development tendency of ultrashort pulse laser processing. This review should facilitate the precision manufacturing of flexible electronics using a femtosecond laser.

Coherent and Incoherent Ultrafast Dynamics in Colloidal Gold Nanorods

The study of the mechanisms that control the ultrafast dynamics in gold nanoparticles is gaining more attention, as these nanomaterials can be used to create nanoarchitectures with outstanding optical properties. Here pump-probe and two-dimensional electronic spectroscopy have been synergistically employed to investigate the early ultrafast femtosecond processes following photoexcitation in colloidal gold nanorods with low aspect ratio. Complementary insights into the coherent plasmonic dynamics at the femtosecond time scale and incoherent hot electron dynamics over picosecond time scales have been obtained, including important information on the different sensitivity to the pump fluence of the longitudinal and transverse plasmons and their different contributions to the photoinduced broadening and shift.

Ultrafast Laser-Induced Dynamics of Non-Equilibrium Electron Spill-Out in Nanoplasmonic Bilayers

Contemporary quantum plasmonics capture subtle corrections to the properties of plasmonic nano-objects in equilibrium. Here, we demonstrate non-equilibrium spill-out redistribution of the electronic density at the ultrafast time scale. As revealed by time-resolved 2D spectroscopy of nanoplasmonic Fe/Au bilayers, an injection of the laser-excited non-thermal electrons induces transient electron spill-out thus changing the plasma frequency. The response of the local electronic density switches the electronic density behavior from spill-in to strong (an order of magnitude larger) spill-out at the femtosecond time scale. The superdiffusive transport of hot electrons and the lack of a direct laser heating indicate significantly non-thermal origin of the underlying physics. Our results demonstrate an ultrafast and non-thermal way to control surface plasmon dispersion through transient variations of the spatial electron distribution at the nanoscale. These findings expand quantum plasmonics into previously unexplored directions by introducing ultrashort time scales in the non-equilibrium electronic systems.

Leveraging plasmonic hot electrons to quench defect emission in metal-semiconductor nanostructured hybrids

Modeling light-matter interactions in hybrid plasmonic materials is vital to their widening relevance from optoelectronics to photocatalysis. Here, we explore photoluminescence (PL) from ZnO nanorods (ZNRs) embedded with gold nanoparticles (Au NPs). A progressive increase in Au NP concentration introduces significant structural disorder and defects in ZNRs, which paradoxically quenches defect related visible PL while intensifying the near band edge (NBE) emission. Under UV excitation, the simulated semi-classical model realizes PL from ZnO with sub-bandgap defect states, eliciting visible emissions that are absorbed by Au NPs to generate a non-equilibrium hot carrier distribution. The photo-stimulated hot carriers, transferred to ZnO, substantially modify its steady-state luminescence, reducing NBE emission lifetime and altering the abundance of ionized defect states, finally reducing visible emission. The simulations show that the change in the interfacial band bending at the Au-ZnO interface under optical illumination facilitates charge transfer between the components. This work provides a general foundation to observe and model the hot carrier dynamics and strong light-matter interactions in hybrid plasmonic systems.

Superstructures of water-dispersive hydrophobic nanocrystals: specific properties

Here, we describe water-soluble superstructures of hydrophobic nanocrystals that have been developed in recent years. We will also report on some of their properties which are still in their infancy. One of these structures, called "cluster structures", consists of hydrophobic 3D superlattices of Co or Au nanocrystals, covered with organic molecules acting like parachutes. The magnetic properties of Co "cluster structures" a retained when the superstructures is dispersed in aqueous solution. With Au "cluster structures", the longer wavelength optical scattered spectra are very broad and red-shifted, while at shorter wavelengths the localized surface plasmonic resonance of the scattered nanocrystals is retained. Moreover, the maximum of the long-wavelength signal spectra is linearly dependent on the increase in assembly size. The second superstructure was based on liquid-liquid instabilities favoring the formation of Fe3O4 nanocrystal shells (colloidosomes) filled or unfilled with Au 3D superlattices and also spherical solid crystal structures are called supraballs. Colloidosomes and supraballs in contact with cancer cells increase the density of nanocrystals in lysosomes and near the lysosomal membrane. Importantly, the structure of their organization is maintained in lysosomes for up to 8 days after internalization, while the initially dispersed hydrophilic nanocrystals are randomly aggregated. These two structures act as nanoheaters. Indeed, due to the dilution of the metallic phase, the penetration depth of visible light is much greater than that of homogeneous metallic nanoparticles of similar size. This allows for a high average heat load overall. Thus, the organic matrix acts as an internal reservoir for efficient energy accumulation within a few hundred picoseconds. A similar behavior was observed with colloidosomes, supraballs and "egg" structures, making these superstructures universal nanoheaters, and the same behavior is not observed when they are not dispersed in water (dried and deposited on a substrate). Note that colloidosomes and supraballs trigger local photothermal damage inaccessible to isolated nanocrystals and not predicted by global temperature measurements.

Two-dimensional filtering in the Fourier domain of transient grating coherent artifacts in time-resolved spectroscopy

Removal of coherent artifacts is important in the analysis of time and wavelength resolved spectroscopy data. By taking advantage of the strong correlation between spectra acquired sequentially in time, artifact removal can be formulated as a 2D problem for improved effectiveness. This paper proposes a 2D method to remove transient grating coherent artifacts from femtosecond time-resolved spectroscopy data based on filtering in the Fourier domain, leading to better estimation of the material parameters from the measured data. The method is simple, intuitive, and light on computation resources. The effectiveness of the method is demonstrated with experimental data acquired from a bare gold film with and without coherent artifacts using mutually parallel and perpendicular pump/probe polarizations, as well as with more complex samples (nanostructured gold film on a glass substrate and rhodamine fluorophores in solution). The proposed method is expected to be applicable to coherent artifact removal in other types of time and wavelength-resolved spectroscopy data.

Determining hot-carrier transport dynamics from terahertz emission

Understanding the ultrafast excitation and transport dynamics of plasmon-driven hot carriers is critical to the development of optoelectronics, photochemistry, and solar-energy harvesting. However, the ultrashort time and length scales associated with the behavior of these highly out-of-equilibrium carriers have impaired experimental verification of ab initio quantum theories. Here, we present an approach to studying plasmonic hot-carrier dynamics that analyzes the temporal waveform of coherent terahertz bursts radiated by photo-ejected hot carriers from designer nano-antennas with a broken symmetry. For ballistic carriers ejected from gold antennas, we find an ~11-femtosecond timescale composed of the plasmon lifetime and ballistic transport time. Polarization- and phase-sensitive detection of terahertz fields further grant direct access to their ballistic transport trajectory. Our approach opens explorations of ultrafast carrier dynamics in optically excited nanostructures.

Effect of Photoinitiators Doped in PDMS for Femtosecond-Laser Writing: Characterization and Outcomes

Herein, we have characterized in depth the effect of femtosecond (fs)-laser writing on various polydimethylsiloxane (PDMS)-based composites. The study combines systematic and nanoscale characterizations for the PDMS blends that include various photoinitiators (organic and inorganic agents) before and after fs-laser writing. The results exhibit that the photoinitiators can dictate the mechanical properties of the PDMS, in which Young's modulus of PDMS composites has higher elasticity. The study illustrates a major improvement in refractive index change by 15 times higher in the case of PDMS/BP-Ge [benzophenone (BP) allytriethylgermane] and Irgacure 184. Additional enhancement was achieved in the optical performance levels of the PDMS composites (the PDMS composites of Irgacure 184/500, BP-Ge, and Ge-ATEG have a relative difference of less than 5% in comparison with pristine PDMS), which are on par with glasses. This insightful study can guide future investigators in choosing photoinitiators for particular applications in photonics and polymer chemistry.

The effect of enhanced heat transfer across metal-nonmetal interfaces subject to femtosecond laser irradiation

The heat transfer across metal-nonmetal interfaces inevitably affects the femtosecond laser processing of thin metal films coated on nonmetal substrates. In the present work, a two-temperature model with a metal-nonmetal interface is employed to numerically investigate the heat transfer across a metal-nonmetal interface. A parallel-series thermal circuit is considered under the drastic electron-phonon nonequilibrium induced by femtosecond laser irradiation. The interfacial thermal resistance affects temporal evolutions of surface electron temperature and phonon temperature, as well as the optical response simulated by the Drude-Lorentz model. By inserting an interlayer and reducing the interfacial thermal resistance, the enhanced heat transfer across Au-Al₂O₃ and Au-Si interfaces is confirmed. More heat transfers from a metal to a nonmetal due to lower total interfacial thermal resistance, which reshapes the temperature distributions of metal-electrons, metal-phonons, and nonmetal-phonons. Consequently, the higher damage threshold of thin Au films and the lower sensitivity of damage threshold versus film thickness are determined. It implies that the heat transfer across metal-nonmetal interfaces is found to affect the transient thermal reflectivity detection and the repeatable femtosecond laser processing of thin metal films.

Optimizing Hot Electron Harvesting at Planar Metal-Semiconductor Interfaces with Titanium Oxynitride Thin Films

Understanding metal-semiconductor interfaces is critical to the advancement of photocatalysis and sub-bandgap solar energy harvesting where electrons in the metal can be excited by sub-bandgap photons and extracted into the semiconductor. In this work, we compare the electron extraction efficiency across Au/TiO₂ and titanium oxynitride (TiON)/TiO_2-x interfaces, where in the latter case the spontaneously forming oxide layer (TiO_2-x) creates a metal-semiconductor contact. Time-resolved pump-probe spectroscopy is used to study the electron recombination rates in both cases. Unlike the nanosecond recombination lifetimes in Au/TiO₂, we find a bottleneck in the electron relaxation in the TiON system, which we explain using a trap-mediated recombination model. Using this model, we investigate the tunability of the relaxation dynamics with oxygen content in the parent film. The optimized film (TiO_0.5N_0.5) exhibits the highest carrier extraction efficiency (N_FC ≈ 2.8 × 10¹⁹ m^-3), slowest trapping, and an appreciable hot electron population reaching the surface oxide (N_HE ≈ 1.6 × 10¹⁸ m^-3). Our results demonstrate the productive role oxygen can play in enhancing electron harvesting and prolonging electron lifetimes, providing an optimized metal-semiconductor interface using only the native oxide of titanium oxynitride.

Principles and advances in ultrafast photoacoustics; applications to imaging cell mechanics and to probing cell nanostructure

In this article we first present the foundations of ultrafast photoacoustics, a technique where the acoustic wavelength in play can be considerably shorter than the optical wavelength. The physics primarily involved in the conversion of short light pulses into high frequency sound is described. The mechanical disturbances following the relaxation of hot electrons in metals and other processes leading to the breaking of the mechanical balance are presented, and the generation of bulk shear-waves, of surface and interface waves and of guided waves is discussed. Then, efforts to overcome the limitations imposed by optical diffraction are described. Next, the principles behind the detection of the so generated coherent acoustic phonons with short light pulses are introduced for both opaque and transparent materials. The striking instrumental advances, in the detection of acoustic displacements, ultrafast acquisition, frequency and space resolution are discussed. Then secondly, we introduce picosecond opto-acoustics as a remote and label-free novel modality with an excellent capacity for quantitative evaluation and imaging of the cell's mechanical properties, currently with micron in-plane and sub-optical in depth resolution. We present the methods for time domain Brillouin spectroscopy in cells and for cell ultrasonography. The current applications of this unconventional means of addressing biological questions are presented. This microscopy of the nanoscale intra-cell mechanics, based on the optical monitoring of coherent phonons, is currently emerging as a breakthrough method offering new insights into the supra-molecular structural changes that accompany cell response to a myriad of biological events.

Dual-Modal Nanoplasmonic Light Upconversion through Anti-Stokes Photoluminescence and Second-Harmonic Generation from Broadband Multiresonant Metal Nanocavities

Metal nanocavities can generate plasmon-enhanced light upconversion signals under ultrashort pulse excitations through anti-Stokes photoluminescence (ASPL) or nonlinear harmonic generation processes, offering various applications in bioimaging, sensing, interfacial science, nanothermometry, and integrated photonics. However, achieving broadband multiresonant enhancement of both ASPL and harmonic generation processes within the same metal nanocavities remains challenging, impeding applications based on dual-modal or wavelength-multiplexed operations. Here, we report a combined experimental and theoretical study on dual-modal plasmon-enhanced light upconversion through both ASPL and second-harmonic generation (SHG) from broadband multiresonant metal nanocavities in two-tier Ag/SiO₂/Ag nanolaminate plasmonic crystals (NLPCs) that can support multiple hybridized plasmons with high spatial mode overlaps. Our measurements reveal the distinctions and correlations between the plasmon-enhanced ASPL and SHG processes under different modal and ultrashort pulsed laser excitation conditions, including incident fluence, wavelength, and polarization. To analyze the observed effects of the excitation and modal conditions on the ASPL and SHG emissions, we developed a time-domain modeling framework that simultaneously captures the mode coupling-enhancement characteristics, quantum excitation-emission transitions, and hot carrier population statistical mechanics. Notably, ASPL and SHG from the same metal nanocavities exhibit distinct plasmon-enhanced emission behaviors due to the intrinsic differences between the incoherent hot carrier-mediated ASPL sources with temporally evolving energy and spatial distributions and instantaneous SHG emitters. Mechanistic understanding of ASPL and SHG emissions from broadband multiresonant plasmonic nanocavities marks a milestone toward creating multimodal or wavelength-multiplexed upconversion nanoplasmonic devices for bioimaging, sensing, interfacial monitoring, and integrated photonics applications.

d-Band Hole Dynamics in Gold Nanoparticles Measured with Time-Resolved Emission Upconversion Microscopy

The performance of photocatalysts and photovoltaic devices can be enhanced by energetic charge carriers produced from plasmon decay, and the lifetime of these energetic carriers greatly affects overall efficiencies. Although hot electron lifetimes in plasmonic gold nanoparticles have been investigated, hot hole lifetimes have not been as thoroughly studied in plasmonic systems. Here, we demonstrate time-resolved emission upconversion microscopy and use it to resolve the lifetime and energy-dependent cooling of d-band holes formed in gold nanoparticles by plasmon excitation and by following plasmon decay into interband and then intraband electron-hole pairs.

Observation of Charge Separation Enhancement in Plasmonic Photocatalysts under Coupling Conditions

Surface plasmon resonance-induced charge separation plays key roles in plasmon-related applications, especially in photocatalysis and photovoltaics. Plasmon coupling nanostructures exhibit extraordinary behaviors in hybrid states, phonon scattering, and ultrafast plasmon dephasing, but plasmon-induced charge separation in these materials remains unknown. Here, we design Schottky-free Au nanoparticle (NP)/NiO/Au nanoparticles-on-a-mirror plasmonic photocatalysts to support plasmon-induced interfacial hole transfer, evidenced by surface photovoltage microscopy at the single-particle level. In particular, we observe a nonlinear increase in charge density and photocatalytic performance with an increase in excitation intensity in plasmonic photocatalysts containing hot spots as a result of varying the geometry. Such charge separation increased the internal quantum efficiency by 14 times at 600 nm in catalytic reactions as compared to that of the Au NP/NiO without a coupling effect. These observations provide an improved understanding of charge transfer management and utilization by geometric engineering and interface electronic structure for plasmonic photocatalysis.

Recent Trends in Plasmon-Assisted Photocatalytic CO2 Reduction

Direct photocatalytic reduction of CO₂ has become an highly active field of research. It is thus of utmost importance to maintain an overview of the various materials used to sustain this process, find common trends, and, in this way, eventually improve the current conversions and selectivities. In particular, CO₂ photoreduction using plasmonic photocatalysts under solar light has gained tremendous attention, and a wide variety of materials has been developed to reduce CO₂ towards more practical gases or liquid fuels (CH₄ , CO, CH₃ OH/CH₃ CH₂ OH) in this manner. This Review therefore aims at providing insights in current developments of photocatalysts consisting of only plasmonic nanoparticles and semiconductor materials. By classifying recent studies based on product selectivity, this Review aims to unravel common trends that can provide effective information on ways to improve the photoreduction yield or possible means to shift the selectivity towards desired products, thus generating new ideas for the way forward.

Ultrafast hot electron dynamics in plasmonic nanostructures: experiments, modelling, design

Metallic nanostructures exhibit localized surface plasmons (LSPs), which offer unprecedented opportunities for advanced photonic materials and devices. Following resonant photoexcitation, LSPs quickly dephase, giving rise to a distribution of energetic 'hot' electrons in the metal. These out-of-equilibrium carriers undergo ultrafast internal relaxation processes, nowadays pivotal in a variety of applications, from photodetection and sensing to the driving of photochemical reactions and ultrafast all-optical modulation of light. Despite the intense research activity, exploitation of hot carriers for real-world nanophotonic devices remains extremely challenging. This is due to the complexity inherent to hot carrier relaxation phenomena at the nanoscale, involving short-lived out-of-equilibrium electronic states over a very broad range of energies, in interaction with thermal electronic and phononic baths. These issues call for a comprehensive understanding of ultrafast hot electron dynamics in plasmonic nanostructures. This paper aims to review our contribution to the field: starting from the fundamental physics of plasmonic nanostructures, we first describe the experimental techniques used to probe hot electrons; we then introduce a numerical model of ultrafast nanoscale relaxation processes, and present examples in which experiments and modelling are combined, with the aim of designing novel optical functionalities enabled by ultrafast hot-electron dynamics.

Simultaneous capturing phonon and electron dynamics in MXenes

Plasmonic MXenes are of particular interest, because of their unique electron and phonon structures and multiple surface plasmon effects, which are different from traditional plasmonic materials. However, to date, how electronic energy damp to lattice vibrations (phonons) in MXenes has not been unraveled. Here, we employed ultrafast broadband impulsive vibrational spectroscopy to identify the energy damping channels in MXenes (Ti3C2Tx and Mo2CTx). Distinctive from the well-known damping pathways, our results demonstrate a different energy damping channel, in which the Ti3C2Tx plasmonic electron energy transfers to coherent phonons by nonthermal electron mediation after Landau damping, without involving electron-electron scattering. Moreover, electrons are observed to strongly couple with A1g mode (~60 fs, 85-100%) and weakly couple with Eg mode (1-2 ps, 0-15%). Our results provide new insight into the electron-phonon interaction in MXenes, which allows the design of materials enabling efficient manipulation of electron transport and energy conversion.

Multi-temperature modeling of femtosecond laser pulse on metallic nanoparticles accounting for the temperature dependences of the parameters

This review considers the fundamental dynamical processes of metal nanoparticles during and after the impact of a femtosecond laser pulse on a nanoparticle, including the absorption of photons. Understanding the sequence of events after photon absorption and their timescales is important for many applications of nanoparticles. Various processes are discussed, starting with optical absorption by electrons, proceeding through the relaxation of the electrons due to electron–electron scattering and electron–phonon coupling, and ending with the dissipation of the nanoparticle energy into the environment. The goal is to consider the timescales, values, and temperature dependences of the electron heat capacity and the electron–phonon coupling parameter that describe these processes and how these dependences affect the electron energy relaxation. Two- and four-temperature models for describing electron–phonon relaxation are discussed. Significant emphasis is paid to the proposed analytical approach to modeling processes during the action of a femtosecond laser pulse on a metal nanoparticle. These consider the temperature dependences of the electron heat capacity and the electron–phonon coupling factor of the metal. The entire process is divided into four stages: (1) the heating of the electron system by a pulse, (2) electron thermalization, (3) electron–phonon energy exchange and the equalization of the temperature of the electrons with the lattice, and (4) cooling of the nanoparticle. There is an appropriate analytical description of each stage. The four-temperature model can estimate the parameters of the laser and nanoparticles needed for applications of femtosecond laser pulses and nanoparticles.

Tuning Light Emission Crossovers in Atomic-Scale Aluminum Plasmonic Tunnel Junctions

Atomic-sized plasmonic tunnel junctions are of fundamental interest, with great promise as the smallest on-chip light sources in various optoelectronic applications. Several mechanisms of light emission in electrically driven plasmonic tunnel junctions have been proposed, from single-electron or higher-order multielectron inelastic tunneling to recombination from a steady-state population of hot carriers. By progressively altering the tunneling conductance of an aluminum junction, we tune the dominant light emission mechanism through these possibilities for the first time, finding quantitative agreement with theory in each regime. Improved plasmonic resonances in the energy range of interest increase photon yields by 2 orders of magnitude. These results demonstrate that the dominant emission mechanism is set by a combination of tunneling rate, hot carrier relaxation time scales, and junction plasmonic properties.

Second harmonic generation under doubly resonant lattice plasmon excitation

Second harmonic generation is enhanced at the surface lattice resonance in plasmonic nanoparticle arrays. We carried out a parametric investigation on two-dimensional lattices composed of gold nanobars where the centrosymmetry is broken at oblique incidence. We study the influence of the periodicity, the incidence angle and the direction of the linear input polarization on the second harmonic generation. Excitation of the surface lattice resonance either at the fundamental or second harmonic wavelength, achieved by varying the incidence angle, enhance the conversion efficiency. As a special case, we demonstrate that both the wavelengths can be simultaneously in resonance for a specific period of the lattice. In this double resonant case, maximum second harmonic power is achieved.

Second-Order Photoinduced Reflectivity for Retrieval of the Dynamics in Plasmonic Nanostructures

Measuring the change in reflectivity (ΔR) using the traditional pump-probe approach can monitor photoinduced ultrafast dynamics in matter, yet relating these dynamic to physical processes for complex systems is not unique. By applying a simple modification to the classical pump-probe technique, we simultaneously measure both the first and second order of ΔR. These additional data impose new constraints on the interpretation of the underlying ultrafast dynamics. In the first application of the approach, we probe the dynamics induced by a pump laser on the local-surface plasmon resonance (LSPR) in gold nanoantennas. Measurements of ΔR over several picoseconds and a wide range of probe wavelengths around the LSPR peak are followed by data fitting using the two-temperature model. The constraints, imposed by the second-order data, lead us to modify the model and force us to include the contribution of nonthermalized electrons in the early stages of the dynamics.

Recent Development of Ultrafast Optical Characterizations for Quantum Materials

The advent of intense ultrashort optical pulses spanning a frequency range from terahertz to the visible has opened a new era in the experimental investigation and manipulation of quantum materials. The generation of strong optical field in an ultrashort time scale enables the steering of quantum materials nonadiabatically, inducing novel phenomenon or creating new phases which may not have an equilibrium counterpart. Ultrafast time-resolved optical techniques have provided rich information and played an important role in characterization of the nonequilibrium and nonlinear properties of solid systems. Here, some of the recent progress of ultrafast optical techniques and their applications to the detection and manipulation of physical properties in selected quantum materials are reviewed. Specifically, the new development in the detection of the Higgs mode and photoinduced nonequilibrium response in the study of superconductors by time-resolved terahertz spectroscopy are discussed.

Active Tuning of Plasmon Damping via Light Induced Magnetism

Circularly polarized optical excitation of plasmonic nanostructures causes coherent circulating motion of their electrons, which in turn gives rise to strong optically induced magnetization, a phenomenon known as the inverse Faraday effect (IFE). In this study we report how the IFE also significantly decreases plasmon damping. By modulating the optical polarization state incident on achiral plasmonic nanostructures from linear to circular, we observe reversible increases of reflectance by up to 8% and simultaneous increases of optical field concentration by 35.7% under 10⁹ W/m² continuous wave (CW) optical excitation. These signatures of decreased plasmon damping were also monitored in the presence of an external magnetic field (0.2 T). We rationalize the observed decreases in plasmon damping in terms of the Lorentz forces acting on the circulating electron trajectories. Our results outline strategies for actively modulating intrinsic losses in the metal via optomagnetic effects encoded in the polarization state of incident light.

Unbiased Plasmonic-Assisted Integrated Graphene Photodetectors

Photonic integrated circuits (PICs) for next-generation optical communication interconnects and all-optical signal processing require efficient (∼A/W) and fast (≥25 Gbs-1) light detection at low ( Collapse

Key Words

• graphene
• photodetectors
• integrated photonics
• plasmonics
• photothermoelectric effect

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MESH Headings Collapse

Grants

• Horizon 2020 Framework Programme
• Quantum Flagship
• Engineering and Physical Sciences Research Council
• H2020 European Research Council

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Affiliation(s)

Ioannis Vangelidis Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece
Dimitris V. Bellas Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece Department of Informatics, Center for Interdisciplinary Research and Innovation, Aristotle University of Thessaloniki, Thessaloniki 57001, Greece
Stephan Suckow AMO GmbH, Advanced Microelectronic Center Aachen (AMICA), Otto-Blumenthal-Strasse 25, Aachen 52074, Germany
George Dabos Department of Informatics, Center for Interdisciplinary Research and Innovation, Aristotle University of Thessaloniki, Thessaloniki 57001, Greece
Sebastián Castilla ICFO - Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, Castelldefels, Barcelona 08860, Spain
Frank H. L. Koppens ICFO - Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology, Castelldefels, Barcelona 08860, Spain ICREA - Institució Catalana de Recerca i Estudis Avançats, Barcelona 08010, Spain
Andrea C. Ferrari Cambridge Graphene Centre, University of Cambridge, Cambridge CB3 0FA, U.K.
Nikos Pleros Department of Informatics, Center for Interdisciplinary Research and Innovation, Aristotle University of Thessaloniki, Thessaloniki 57001, Greece
Elefterios Lidorikis Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece University Research Center of Ioannina (URCI), Institute of Materials Science and Computing, Ioannina 45110, Greece

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Electromagnetically induced modification of gold optical properties

The reflection of light from a metal film, i.e., a mirror, is among the most fundamental and well-understood effects in optics. If the film thickness is greater than the wavelength, reflection is strong and is explained in simple terms by the Fresnel equations. For film thickness much less than the wavelength, reflection is far weaker and more exotic effects become possible. This is especially so if the light illuminating the film is pulsed at the femtosecond time scale. In this work, a phenomenon is proposed where few-femtosecond laser pulses temporarily modify a thin metal film's optical properties via processes that appear linear and classical in nature. By casting a pulsed standing-wave pattern across the metal surface, we consider the possibility that conduction electrons are redistributed to create temporary regions of partly enhanced or reduced density without the excitation of inter-band transitions. The process would constitute a temporary change to the conductivity of the metal, and thus, may be observable as changes to the metal's transmittance and reflectance. In regions where the density is enhanced (reduced), the transmittance is decreased (increased). The concept is termed Electromagnetically Induced Modification (EIM) and is premised on the fact that the pulse length is shorter than the relaxation time of the conduction electrons. An experiment is conducted to test the concept by measuring the change in reflectance and transmittance of gold films with thickness ranging from 20-300 Angstrom. The results show that the film's transmittance decreases only when the standing-wave pattern is present. As the pulse length is increased, or as the film thickness is increased, the changes disappear. The changes show little dependence on the pulse intensity as it is varied by a factor of two. To gain further insight, the Drude theory is used to develop a simplified model for EIM, which qualitatively agrees with the observations. However, neither the experiment nor the model can prove the validity of the EIM concept. As such, an assessment is made for the potential of alternative well-known processes to explain the observations.

Influence of Electronic Non-Equilibrium on Energy Distribution and Dissipation in Aluminum Studied with an Extended Two-Temperature Model

When an ultrashort laser pulse excites a metal surface, only a few of all the free electrons absorb a photon. The resulting non-equilibrium electron energy distribution thermalizes quickly to a hot Fermi distribution. The further energy dissipation is usually described in the framework of a two-temperature model, considering the phonons of the crystal lattice as a second subsystem. Here, we present an extension of the two-temperature model including the non-equilibrium electrons as a third subsystem. The model was proposed initially by E. Carpene and later improved by G.D. Tsibidis. We introduce further refinements, in particular, a temperature-dependent electron-electron thermalization time and an extended energy interval for the excitation function. We show results comparing the transient energy densities as well as the energy-transfer rates of the original equilibrium two-temperature description and the improved extended two-temperature model, respectively. Looking at the energy distribution of all electrons, we find good agreement in the non-equilibrium distribution of the extended two-temperature model with results from a kinetic description solving full Boltzmann collision integrals. The model provides a convenient tool to trace non-equilibrium electrons at small computational effort. As an example, we determine the dynamics of high-energy electrons observable in photo-electron spectroscopy. The comparison of the calculated spectral densities with experimental results demonstrates the necessity of considering electronic non-equilibrium distributions and electron-electron thermalization processes in time- and energy-resolved analyses.

Chemically-Controlled Ultrafast Photothermal Response in Plasmonic Nanostructured Assemblies

Plasmonic nanoparticles are renowned as efficient heaters due to their capability to resonantly absorb and concentrate electromagnetic radiation, trigger excitation of highly energetic (hot) carriers, and locally convert their excess energy into heat via ultrafast nonradiative relaxation processes. Furthermore, in assembly configurations (i.e., suprastructures), collective effects can even enhance the heating performance. Here, we report on the dynamics of photothermal conversion and the related nonlinear optical response from water-soluble nanoeggs consisting of a Au nanocrystal assembly trapped in a water-soluble shell of ferrite nanocrystals (also called colloidosome) of ∼250-300 nm in size. This nanoegg configuration of the plasmonic assembly enables control of the size of the gold suprastructure core by changing the Au concentration in the chemical synthesis. Different metal concentrations are analyzed by means of ultrafast pump-probe spectroscopy and semiclassical modeling of photothermal dynamics from the onset of hot-carrier photogeneration (few picosecond time scale) to the heating of the matrix ligands in the suprastructure core (hundreds of nanoseconds). Results show the possibility to design and tailor the photothermal properties of the nanoeggs by acting on the core size and indicate superior performances (both in terms of peak temperatures and thermalization speed) compared to conventional (unstructured) nanoheaters of comparable size and chemical composition.

Ultrafast Plasmonics Beyond the Perturbative Regime: Breaking the Electronic-Optical Dynamics Correspondence

The transient optical response of plasmonic nanostructures has recently been the focus of extensive research. Accurate prediction of the ultrafast dynamics following excitation of hot electrons by ultrashort laser pulses is of major relevance in a variety of contexts from the study of light harvesting and photocatalytic processes to nonlinear nanophotonics and the all-optical modulation of light. So far, all studies have assumed the correspondence between the temporal evolution of the dynamic optical signal, retrieved by transient absorption spectroscopy, and that of the photoexcited hot electrons, described in terms of their temperature. Here, we show both theoretically and experimentally that this correspondence does not hold under a nonperturbative excitation regime. Our results indicate that the main mechanism responsible for the breaking of the correspondence between electronic and optical dynamics is universal in plasmonics, being dominated by the nonlinear smearing of the Fermi-Dirac occupation probability at high hot-electron temperatures.

Disentangling Light- and Temperature-Induced Thermal Effects in Colloidal Au Nanoparticles

We present temperature-dependent (from room temperature to 80 °C) absorption spectra of Au/SiO₂ core-shell nanoparticles (NPs) (core diameter: ∼25 nm) in water in the range from 1.5 to 4.5 eV, which spans the localized surface plasmon resonance (LSPR) and the interband transitions. A decrease in absorption with temperature over the entire spectral range is observed, which is more prominent at the LSPR. These changes are well reproduced by theoretical calculations of the absorption spectra, based on the experimentally measured temperature-dependent real (ε₁) and imaginary (ε₂) parts of the dielectric constant of Au NPs and of the surrounding medium. In addition, we model the photoinduced response of the NPs over the entire spectral range. The experimental and theoretical results of the thermal heating and the simulations of the photoinduced heating are compared with the ultrafast photoinduced transient absorption (TA) spectra upon excitation of the LSPR. These show that while the latter is a reliable monitor of heating of the NP and its environment, the interband region mildly responds to heating but predominantly to the population evolution of charge carriers.

In Situ Investigation of Ultrafast Dynamics of Hot Electron-Driven Photocatalysis in Plasmon-Resonant Grating Structures

Understanding the relaxation and injection dynamics of hot electrons is crucial to utilizing them in photocatalytic applications. While most studies have focused on hot carrier dynamics at metal/semiconductor interfaces, we study the in situ dynamics of direct hot electron injection from metal to adsorbates. Here, we report a hot electron-driven hydrogen evolution reaction (HER) by exciting the localized surface plasmon resonance (LSPR) in Au grating photoelectrodes. In situ ultrafast transient absorption (TA) measurements show a depletion peak resulting from hot electrons. When the sample is immersed in solution under -1 V applied potential, the extracted electron-phonon interaction time decreases from 0.94 to 0.67 ps because of additional energy dissipation channels. The LSPR TA signal is redshifted with delay time because of charge transfer and subsequent change in the dielectric constant of nearby solution. Plateau-like photocurrent peaks appear when exciting a 266 nm linewidth grating with p-polarized (on resonance) light, accompanied by a similar profile in the measured absorptance. Double peaks in the photocurrent measurement are observed when irradiating a 300 nm linewidth grating. The enhancement factor (i.e., reaction rate) is 15.6× between p-polarized and s-polarized light for the 300 nm linewidth grating and 4.4× for the 266 nm linewidth grating. Finite-difference time domain (FDTD) simulations show two resonant modes for both grating structures, corresponding to dipolar LSPR modes at the metal/fused silica and metal/water interfaces. To our knowledge, this is the first work in which LSPR-induced hot electron-driven photochemistry and in situ photoexcited carrier dynamics are studied on the same plasmon resonance structure with and without adsorbates.

A self-supported ultrathin plasmonic film for ultrafast optical switching

Self-supporting gold nanowire (AuNW) gratings with a thickness of about 200 nm are produced by solution-processing and flexible-transfer techniques. Such an ultrathin structure is applied as an ultrafast optical switch that enables low-threshold optical modulation with a high signal contrast and a high signal-to-noise ratio. Transient energy-band modification in gold under excitation by femtosecond laser pulses is the main responsible mechanism. For a pump fluence of about 170 μJ cm^-2, a modulation depth of about 10% is achieved for the optical switching signal. Self-supporting metallic plasmonic photonic thin films with a large area and flexible structures are important for applications in a large variety of circumstances and on different interfaces for optical signal processing, optical logic circuits, and optical communication systems.