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Proposed mechanism of interfacial energy transfer and experimental schematic a, Hot electron injection: the process typically assumed to occur at metal/semiconductor interfaces after photoexcitation of the metallic contact. In this case, hot electrons are first generated in the Au (1). At sufficiently high electron temperatures, the electrons traverse the interface and add charge to the conduction band (CB) of the semiconductor (2). b, Ballistic thermal injection (BTI): our proposed process for metal/semiconductor interfaces after an ultrafast excitation of the metal contact. This mechanism relies on hot-electron generation in the metal (1); prior to the electron–phonon coupling (less than a couple of picoseconds), energy propagates ballistically towards the metal/semiconductor interface. The electron energy front reaches the interface, whereby the electrons transfer their energy (2), rather than charge, to the pre-existing free electrons in the semiconductor’s conduction band. The pre-existing semiconductor’s electrons are now at an elevated temperature (see electron temperature profile in c, depicted by the purple and blue curves), and are promoted to elevated states in the conduction band (3) (for example, intraband excitations). Note that the scattering processes after either charge injection or the proposed BTI energy transfer mechanism, such as hot electron–electron scattering, are excluded for clarity. c, Schematic of our ultrafast ENZ experiment to spatially resolve the electron energy distribution after the potential injection processes. The 520 nm pump beam excites the Au surface at the Au/air interface, and a subpicosecond probe pulse monitors the ENZ mode of a thin Y:CdO film. EF, Fermi energy; MIR, mid-infrared; Te, electron temperature; VB, valence band.
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Light–matter interactions that induce charge and energy transfer across interfaces form the foundation for photocatalysis1,2, energy harvesting³ and photodetection⁴, among other technologies. One of the most common mechanisms associated with these processes relies on carrier injection. However, the exact role of the energy transport associated with...
Citations
... The concomitant electronic relaxation and scattering processes during these non-equilibrium conditions for metals can have significant influence on several of their physical properties [12,[17][18][19][20][21][22]. In particular, during such highly nonequilibrium conditions, thermal transport processes in noble metals can be drastically different as compared to equilibrium or near-to-equilibrium conditions [17,19,[23][24][25], which can critically underpin the design considerations and engineering of the aforementioned applications. For instance, in photothermal therapy, the knowledge of thermal transport coefficients is highly desirable to quantitatively describe the heat transfer processes [26][27][28], critical in quantitatively determining the amount of laser energy absorbed by gold nanoparticles. ...
... Using equation (23) in the integral with dash in equation (22), we get, ...
We report on the thermal transport properties of noble metals (gold, silver and copper) under conditions of extremely high electron temperatures (that are on the order of the Fermi energy). We perform parameter-free density functional theory calculations of the electron temperature-dependent electron–phonon coupling, electronic heat capacities, and thermal conductivities to elucidate the strong role played by the excitation of the low lying d-bands on the transport properties of the noble metals. Our calculations show that, although the three metals have similar electronic band structures, the changes in their electron–phonon coupling at elevated electron temperatures are drastically different; while electron–phonon coupling decreases in gold, it increases in copper and, it remains relatively unperturbed for silver with increasing electron temperatures of up to ∼60 000 K (or 5 eV). We attribute this to the varying contributions from acoustic and longitudinal phonon modes to the electron–phonon coupling in the three metals. Although their electron–phonon coupling changes with electron temperature, the thermal conductivity trends with electron temperature are similar for all three metals. For instance, the thermal conductivities for all three metals reach their maximum values (on par with the room-temperature values of some of the most thermally conductive semiconductors) at electron temperatures of ∼6000 K, and thereafter monotonically decrease due to the enhanced effect of electron–electron scattering for electronic states that are further away from the Fermi energy. As such, only accounting for electron–phonon coupling and neglecting electron–electron scattering can lead to large over-predictions of the thermal conductivities at extremely high electron temperatures. Our results shed light on the microscopic understanding of the electronic scattering mechanisms and thermal transport in noble metals under conditions of extremely high electron temperatures and, as such, are significant for a plethora of applications such as in plasmonic devices that routinely leverage hot electron transport.
... Several applications relevant for next generation electronics are based on metal semiconductor interfaces. These rely on ultrafast thermionic carrier injection from the metal film to the semiconductor layer under conditions when the metal electron temperatures are highly elevated with respect to its lattice temperature [11,12]. On a more fundamental level, the thermal management and ultrafast cooling dynamics of localized hot spots on optically excited metal surface [13] or metal nano structures [14] on dielectrics is important for investigations on heat transport. ...
Ultrafast laser induced thermionic emission from metal surfaces has several applications. Here, we investigate the role of laser polarization and angle of incidence on the ultrafast thermionic emission process from laser driven gold coated glass surface. The spatio-temporal evolution of electron and lattice temperatures are obtained using an improved three-dimensional (3D) two-temperature model (TTM) which takes into account the 3D laser pulse profile focused obliquely onto the surface. The associated thermionic emission features are described through modified Richardson-Dushman equation, including dynamic space charge effects and are included self-consistently in our numerical approach. We show that temperature dependent reflectivity influences laser energy absorption. The resulting peak electron temperature on the metal surface monotonically increases with angle of incidence for P polarization, while for S polarization it shows opposite trend. We observe that thermionic emission duration shows strong dependence on angle of incidence and contrasting polarization dependent behaviour. The duration of thermionic current shows strong correlation to the intrinsic electron-lattice thermalization time, in a fluence regime well below the damage threshold of gold. The observations and insights have important consequences in designing ultrafast thermionic emitters based on a metal based architecture.
... Unlike at metal/metal interfaces, at metal/non-metal interfaces, the negligible number density of free electrons available in the non-metal leads to the electron−electron thermal boundary conductance becoming a nonexistent heat transfer pathway, consistent with the diffuse mismatch model for electrons described above. In the case that the non-metal is doped to increase the free electron number density, this electron−electron thermal pathway can begin to contribute to thermal conductance, 138 which we describe in more detail later in this section. Given this, the three assumed mechanisms for electron−phonon coupling heat transfer at metal/non-metal interfaces are (i) electrons in the metal coupling to phonons in the non-metal across the metal/non-metal interface, (ii) electrons in the metal coupling to phonons in the metal on the metal side of the metal/non-metal interface, and (iii) subsequent phonon−phonon conductance across the interface. ...
... A CLE process between hot electrons in a metal and electrons and phonons in a doped non-metal was recently observed by Tomko et al. 138 at Au/doped CdO interfaces, a process deemed "ballistic thermal injection" (BTI). Ballistic thermal injection (BTI) is a recently discovered energy transduction mechanism that arises from the CLE dynamics at metal/non-metal interfaces. ...
... 154,155 However, this process has only begun to be studied, and thus, additional investigations into different material interfaces and the role of CLE on enhancing this BTI process are warranted and would result in hotelectron-based control over various material functionalities. For example, in the work from Tomko et al., 138 they used this BTI process to control the infrared plasmonic response of CdO via ultrafast thermal modulation of CdO's epsilon near zero mode. ...
The coupled interactions among the fundamental carriers of charge, heat, and electromagnetic fields at interfaces and boundaries give rise to energetic processes that enable a wide array of technologies. The energy transduction among these coupled carriers results in thermal dissipation at these surfaces, often quantified by the thermal boundary resistance, thus driving the functionalities of the modern nanotechnologies that are continuing to provide transformational benefits in computing, communication, health care, clean energy, power recycling, sensing, and manufacturing, to name a few. It is the purpose of this Review to summarize recent works that have been reported on ultrafast and nanoscale energy transduction and heat transfer mechanisms across interfaces when different thermal carriers couple near or across interfaces. We review coupled heat transfer mechanisms at interfaces of solids, liquids, gasses, and plasmas that drive the resulting interfacial heat transfer and temperature gradients due to energy and momentum coupling among various combinations of electrons, vibrons, photons, polaritons (plasmon polaritons and phonon polaritons), and molecules. These interfacial thermal transport processes with coupled energy carriers involve relatively recent research, and thus, several opportunities exist to further develop these nascent fields, which we comment on throughout the course of this Review.
... While Coulomb attraction between the transferred electrons and remaining holes limits the charge transfer, the energy transfer does not have such limitation and large amounts of energy can flow rapidly by the charge-charge scattering mechanism. Reproduced, with permission, from [79]. ...
... Focusing on an Au/CdO interface, Tomko et al. [79] reported experimentally that in cases of strong nonequilibrium, photoexcited metals can undergo an electron-mediated ballistic energy transfer to a non-metal substrate, without charge injection. They termed this effect ballistic thermal injection (BTI). ...
... TD-DFT also represents a unique tool to investigate the photoinduced charge transfer mechanisms taking place at the plasmonic NP surface [144,145]. First-principles techniques have for instance been employed to corroborate experimental observations of a ballistic thermal injection at a metal/semiconductor interface [146], to rationalize ultrafast electron transfer dynamics in heterostructures [147] and 2D materials [148], or to gain insight into the effects of plasmons in photocatalysis [149]. Unfortunately, these calculations suffer from an applicability upper limit, set by numerical complexity. ...
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.
... T he fundamental electronic transport processes in metals have been of foundational interest to a wide array of physics, chemistry, and engineering disciplines since Drude's work describing the scattering processes of a free electron gas. 1 A key process in Drude's theory, and subsequent more robust theories of electronic transport in metals, 2,3 is the electron− phonon scattering rate, which is the critical process that underpins a wide array of optical, electrical, magnetic, and thermal properties in materials processing, 4−6 plasmonics, 7−9 nanophotonics, 10 photovoltaics, 8,11,12 photocatalysis, and nonlinear optics. 13−16 For example, in plasmonic devices, the nonradiative plasmon decay involving the generation of hot carriers in a metallic layer, which can ballistically transfer their energies to underlying semiconductor layers without interacting with the relatively "colder" lattice, 9,17 can be used for applications in solar energy harvesting. Likewise, the fundamental understanding of electron−phonon scattering processes is also of significance to various phenomena in condensed matter physics such as spin caloritronics, 18,19 superconductivity, 20 and energy transport. ...
We experimentally show that the ballistic length of hot electrons in laser-heated gold films can exceed ∼150 nm, which is ∼50% greater than the previously reported value of 100 nm inferred from pump-probe experiments. We also find that the mean free path of electrons at the peak temperature following interband excitation can reach upward of ∼45 nm, which is higher than the average value of 30 nm predicted from our parameter-free density functional perturbation theory. Our first-principles calculations of electron-phonon coupling reveal that the increase in the mean free path due to interband excitation is a consequence of drastically reduced electron-phonon coupling from lattice stiffening, thus providing the microscopic understanding of our experimental findings.
... The main advantage of this system lies in facilitating charge separation across the interfacial Schottky barrier [109,148]. The carrier lifetimes can be extended by spatially separation of hot electrons from holes over metal-semiconductor interface [149]. Besides, the depletion region built within semiconductor can sweep hot-electrons away from the metal-semiconductor interface, further reducing the charge recombination [150]. ...
Plasmonics offer unprecedented control over light and stimulate fundamental research and engineering applications in solar energy. The surface plasmon resonance is responsible for both enhanced light scattering and absorption. The plasmon excitations in nanostructures can be tuned to control the hot-carrier emission. The damping dissipation of the kinetic energy of surface plasmons releases heat at nanoscale, which can be used to create high-performance nano-heaters and/or radiators. Spectrally and/or thermally engineered plasmonic nanomaterials attract considerable attention for solar energy application due to their distinct thermoplasmonic properties. In light of these advances, this paper provides a critical review of current research in thermoplasmonics with focus on its physic mechanisms, structure tuning strategies and solar energy applications. Basic mechanism of thermoplasmonics is described from the photothermal conversion and heat transfer physics to thermal-induced processes. Structure tuning strategies including self-tunable plasmons, plasmon coupling strategies and active plasmons with tunable gap distances are then fully discussed in terms of their principles and structures. Based on the flourishing development of novel thermoplasmonic structures, potential applications ranging from solar collector, solar radiator, thermo-photovoltaic, solar desalination and sterilization, solar degradation and catalysis, to solar fuels, and solar fertilizers are additionally highlighted. The advantages of using plasmonics over the conventional technologies are identified, and the areas where important basics involved when the thermoplasmonics bringing into application are stressed throughout the text. Finally, we provide our views on future challenges in solar thermoplasmonics, together with a few suggestions for further developments of this technology. This work would bring new insights and inspire innovative works on designing thermoplasmonics for solar energy application.
... While this work analyzed the carrier dynamics in SiNWs as a demonstration, the pump-probe s-SNOM can serve as a versatile and general optical diagnostic platform to study nanomaterials, including two-dimensional heterostructures 33 and quantum dots 34 . It also provides a promising tool to study other nonequilibrium thermodynamic phenomena, including phase transitions 35,36 , energy and charge transfer 37 , and phonon propagation 38 . ...
Semiconductor technology is continuing its vital role in the current industrial revolution. Investigating the carrier distribution and dynamics in semiconductor materials is essential to unraveling fundamental principles and developing advanced functionalities. The miniaturization of electronic and photonic devices further calls for new tools to probe carrier behaviors at the nanoscale. Here, we develop pump-probe scattering-type scanning near-field optical microscopy (s-SNOM) to characterize the carrier dynamics in semiconductor nanowires. By coupling experiments with the point-dipole model, we resolve the size-dependent photoexcited carrier lifetime in individual silicon nanowires. We further demonstrate local carrier decay time mapping in silicon nanostructures with a sub-50 nm spatial resolution. Our pump-probe s-SNOM enables the nanoimaging of ultrafast carrier kinetics, which is important for the future design of broad electronic, photonic, and optoelectronic devices.
... In other words, the electrons and phonons of the Au are thermalized and can be described by their respective thermal distribution functions. This critical aspect ensures that the measured surface temperature is indicative of only the plasma-Au energy transfer and subsequent thermal diffusion rather than ballistic mechanisms that traverse deep into the substrate 18,19 . ...
Plasmas are an indispensable materials engineering tool due to their unique ability to deliver a flux of species and energy to a surface. This energy flux serves to heat the surface out of thermal equilibrium with bulk material, thus enabling local physicochemical processes that can be harnessed for material manipulation. However, to-date, there have been no reports on the direct measurement of the localized, transient thermal response of a material surface exposed to a plasma. Here, we use time-resolved optical thermometry in-situ to show that the energy flux from a pulsed plasma serves to both heat and transiently cool the material surface. To identify potential mechanisms for this ‘plasma cooling,’ we employ time-resolved plasma diagnostics to correlate the photon and charged particle flux with the thermal response of the material. The results indicate photon-stimulated desorption of adsorbates from the surface is the most likely mechanism responsible for this plasma cooling. When a plasma interacts with a surface, different thermal effects may arise. Here, the authors explore plasma interactions with a surface that produce a surface cooling effect.
... While significant fraction of this research is concerned with the fundamental understanding of demagnetization processes in metallic multilayer systems [29,30], the photo-induced catalytic processes and energy conversion usually involve the transfer of energy and charge across interfaces [26,31]. Here, real-time TDDFT (RT-TDDFT) approaches contribute towards a fundamental understanding of the carrier dynamics related to the plasmon-mediated injection of hot electrons from metallic nanostructures into semiconductors or insulators [32][33][34][35]. The efficient transfer of excitations or dissipation of energy across interfaces has also evolved as an important topic of ultra-fast pump-probe experiments [3,[36][37][38]. ...
The interaction of a femtosecond optical pulse with a Fe$_{1}$/(MgO)$_{3}$(001) metal/oxide heterostructure is investigated using time-dependent density functional theory (TDDFT) calculations in the real-time domain. We systematically study electronic excitations as a function of laser frequency, peak power density and polarization direction. While spin-orbit coupling is found to result in only a small time-dependent reduction of magnetization (less than 10%), we find a marked anisotropy in the response to in-plane and out-of-plane polarized light, which changes its character qualitatively depending on the excitation energy: the Fe-layer is efficiently addressed at low frequencies by in-plane polarized light, whereas for frequencies higher than the MgO band gap, we find a particularly strong response of the central MgO-layer for cross-plane polarized light. For laser excitations between the charge transfer gap and the MgO band gap, the interface plays the most important role, as it mediates concerted transitions from the valence band of MgO into the $3d$ states of Fe closely above the Fermi level and from the Fe-states below the Fermi level into the conduction band of MgO. As these transitions can occur simultaneously altering charge balance of the layers, they could potentially lead to an efficient transfer of excited carriers into the MgO bulk, where the corresponding electron and hole states can be separated by an energy which is significantly larger than the photon energy.
