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Ultrafast electron-lattice thermalization in copper and other noble metal nanoparticles
摘要： Using time-resolved ultrafast pump-probe spectroscopy we investigated the electron-lattice energy transfer in small copper nanospheres with diameters ranging from 3.2 to 23 nm, either embedded in a glass or dispersed in a solvent. Electron-lattice scattering rate is shown to increase with size reduction, in agreement with our previous results obtained on gold and silver nanoparticles in the low excitation regime. We attribute this effect to the reduction of the screening efficiency of electron–phonon interactions close to the nanoparticle surface. To understand the discrepancy between the results on the electron-lattice scattering in different metals reported in the literature (reduction, no dependence or increase with nanoparticle size), we discuss the experimental conditions required for the accurate determination of electron-lattice energy transfer time from time-resolved investigations in the weak and strong excitation regimes and present power-dependent experiments on gold nanospheres in solution. Our findings are derived from a theoretical analysis based on the two-temperature model predictions and on a complete modeling of the nanoparticle transient extinction cross-section through the resolution of Boltzmann equation in the presence of hot electrons.
关键词： pump-probe spectroscopy,copper nanoparticles,electron-lattice interactions,two-temperature model,ultrafast electron dynamics
更新于2025-09-23 15:23:52
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Non-Fourier Estimate of Electron Temperature in Case of Femtosecond Laser Pulses Interaction with Metals
摘要： This work is devoted to the electron temperature variation in metals through interaction with femtosecond laser pulses. Our study was inspired by the last mathematical breakthroughs regarding the exact analytical solutions of the heat equation in the case of flash laser-matter interaction. To this purpose, the classical Anisimov’s two temperature model was extended via the 3D telegraph Zhukovsky equation. Based upon this new approach, the computational plots of electron thermal fields during the first laser pulse interaction with a gold surface were inferred. It is shown that relaxation times and coupling factors over electron thermal conductivities (g/K) govern the interaction between the laser pulse and metal sample during the first picoseconds. The lower the factor g/K, the higher the electron temperature becomes. In contrast, the lower the relaxation time, the lower the electron temperature.
关键词： two temperature model,3D telegraph equation,femtosecond lasers-metal interaction,metals
更新于2025-09-23 15:21:01
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Transient optics of gold during laser irradiation: From first principles to experiment
摘要： Intense femtosecond laser pulses can induce dramatic changes on different properties of materials. Laser induced changes of the optical properties are particularly relevant, since they can lead to a modi?cation of the amount of energy that the material absorbs from the laser pulse. In noble metals, changes of re?ectivity upon femtosecond laser illumination are expected to be strong due to the excitation of d electrons. In this work we perform measurements of the re?ectivity of laser excited gold in the infrared and in the ultraviolet range, respectively. We ?nd a remarkable dependence of the re?ectivity on laser ?uence, which is in turn different in both ranges of photon energy. In order to understand the behavior of the re?ectivity in laser excited solids and to explain our measured re?ectivity curves we develop a theoretical scheme in the framework of the two-temperature model with the electronic temperature as the key parameter. Our approach is based on all-electron calculations of the interband contribution to the re?ectivity and a careful determination of the intraband, Drude-like terms and a realistic model for the space and time resolved energy transfer from a Gaussian laser pulse into the electronic system. We obtain very good agreement between experiment and theory and identify the main mechanisms for re?ectivity changes as a function of laser ?uence.
关键词： intraband,Drude-like terms,interband contribution,electronic temperature,Gaussian laser pulse,re?ectivity,two-temperature model,femtosecond laser pulses,optical properties,gold
更新于2025-09-23 15:19:57
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Laser-Induced Breakdown in Dielectrics: Strong Electron Superheating
摘要： Laser-induced breakdown hinders the operation of modern lasers, photonic elements and devices, and can be also employed as an important operational regime for high-intensity laser technology of new materials. In this report we generalize the two-temperature model of laser-matter interaction to simulate the regimes of laser-induced breakdown in dielectrics. This generalization allow us to associate the onset of breakdown with a sequence of the step-wise increase and decrease of the mean electron energy, (cid:2013)(cid:3032), and related evolution of the free electron density, (cid:1866)(cid:3032). The model set of the rate equations includes photo-ionization and recombination kinetics, radiation absorption, energy release and exchange effects defining the time evolution of (cid:2013)(cid:3032) to the band gap energy (cid:1831)(cid:3008), and critical energy of ionization, (cid:1831)(cid:3030)(cid:3045)(cid:124)1.5(cid:1831)(cid:3008). Additionally, our model includes an effect of a strong electron superheating, Δ(cid:2013)(cid:3032), above (cid:1831)(cid:3030)(cid:3045). Namely, in treating the onset of impact ionization by generated free electrons the related impact ionization rate, (cid:1875)(cid:3036)(cid:3040)(cid:3043) = (cid:2028)(cid:3035)(cid:2879)(cid:2869), is associated with the time of the electron heating to the critical energy, (cid:2028)(cid:3035) = (cid:1831)(cid:3030)(cid:3045)((cid:1856)(cid:2013)(cid:3032) (cid:1856)(cid:1872)? )(cid:2879)(cid:2869) ≈ 1 ((cid:2009)(cid:1835))(cid:2879)(cid:2869), where (cid:2009) = (cid:2026)(cid:3090) (cid:1831)(cid:3030)(cid:3045)? is the avalanch coefficient, (cid:2026)(cid:3090) is the related absorption cross-section of free electrons and I is the radiation intensity). However, when the neutral atoms start to deplet the impact ionization can be controlled by an additional collision time, (cid:2028)(cid:3030) = (cid:1856)(cid:3028)((cid:1865)(cid:3032) (cid:1831)(cid:3030)(cid:3045)? )(cid:3117)/(cid:3118)((cid:2869)(cid:2879)(cid:3041)(cid:3280) (cid:3041)(cid:3276)? )(cid:2870)(cid:3117)/(cid:3118) (where (cid:1856)(cid:3028) is the interatomic distance, (cid:1865)(cid:3032) is the electron mass and (cid:1866)(cid:3028) is the atomic density). That is, if (cid:1866)(cid:3032) ? (cid:1866)(cid:3028) and (cid:2028)(cid:3030) ? (cid:2028)(cid:3035) the impact ionization is controlled by (cid:2028)(cid:3035) and the ionizing impacts can occur at (cid:2013)(cid:3032) > (cid:1831)(cid:3030)(cid:3045)within the time (cid:2028)(cid:3030)(cid:124) 0.1 fs. However, when (cid:1866)(cid:3032) tends to (cid:1866)(cid:3028) by reaching the level of (cid:2013)(cid:3032) > (cid:1831)(cid:3030)(cid:3045) and seeking new neutral atoms the free electrons continue to experience the impacts with the ionized atoms and recombining electron-ion pairs. By making these interactions the free electrons can acquire an additional energy of Δ(cid:2013)(cid:3032) ≈ (cid:2028)(cid:3030) before producing the final ionizing impacts and loosing the energy of (cid:1831)(cid:3008).
关键词： electron superheating,impact ionization,two-temperature model,Laser-induced breakdown,dielectrics
更新于2025-09-16 10:30:52
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Hydrodynamic modeling and time-resolved imaging reflectometry of the ultrafast laser-induced ablation of a thin gold film
摘要： The ablation of thin metal films using ultrafast laser radiation represents a standard processing procedure in laser technology. However, looking closer to the resulting structures, the arising topology deviates strongly from the expected one. Ablation of thin gold films using ultrafast laser radiation with a Gaussian-shaped spatial intensity distribution results in flat topologies. By synergistically combining space and time-resolved reflectometry with expanded two-temperature hydrodynamic modeling, we explain the formation of bulging and rupture of the thin film-surface by the expansion of strongly stretched liquid material due to the induced rarefaction wave propagating towards the substrate. Also, the formation of a flat ablation structure is described by the interplay of two rarefaction waves, one propagating towards the substrate and the other one propagating towards the vacuum boundary, within the completely molten film. The simulated topology agrees excellent with the experimentally observed ablation structure. Besides, all simulated stages of excitation of the gold film, namely electron-phonon non-equilibrium, hydrodynamic expansion, and rupture, are confirmed by space and time-resolved pump-probe reflectometry.
关键词： Imaging reflectometry,Thin film ablation,Femtosecond laser radiation,Two temperature model hydrodynamics (TTMHD),Gold thin film,Ultrafast metrology
更新于2025-09-16 10:30:52
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Ultrafast Plasmon Dynamics and Hole–Phonon Coupling in NIR Active Nonstoichiometric Semiconductor Plasmonic Cu <sub/> 2– <i>x</i> </sub> S Nanocrystals
摘要： Nonstoichiometric Cu2?xS nanocrystals (NCs) have been synthesized by hot-injection method after changing the copper to sulfur ratio. Optical studies confirmed well-defined localized surface plasmon resonance (LSPR) absorbance band from near-infrared to mid-infrared arising due to p-type hole vacancy in the doped semiconductor. Carrier concentrations of the Cu2?xS NCs are calculated to be on the order of ～1021 cm?3 after following the Drude model, which is one order of magnitude lower as compared to the metallic (Au/Ag) system. To understand ultrafast plasmon dynamics of Cu2?xS NCs, femtosecond broadband (visible?near IR) pump?probe spectroscopy has been employed. Ultrafast plasmon dynamics of the Cu2?xS NCs have been monitored after changing composition (copper to sulfur ratio), pump wavelength, and laser intensity. At moderate pump power hole?phonon relaxation time constant has been observed to be in the range of 240?440 fs for Cu2?xS NCs depending upon pump wavelengths (400, 800 nm). From the ultrafast transient data, hole?phonon coupling constant (G) has been determined in the range (1.6?2.7) × 1010 J K?1 s?1 cm?3 for Cu2?xS NCs at different excitation wavelengths, which is also one order lower as compared to the metallic system. We have proposed a new mechanistic scheme for hot carrier relaxation dynamics, in accordance with the two-temperature model (TTL) as reported in literature for plasmon dynamics.
关键词： Nonstoichiometric Cu2?xS nanocrystals,femtosecond broadband pump?probe spectroscopy,localized surface plasmon resonance,hole?phonon coupling constant,two-temperature model,Drude model
更新于2025-09-12 10:27:22
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Gate switching of ultrafast photoluminescence in graphene
摘要： The control of optical properties by electric means is the key to optoelectronic applications. For atomically thin two-dimensional (2D) materials, the natural advantage lies in that the carrier doping could be readily controlled through the electric gating effect, possibly affecting the optical properties. Exploiting this advantage, here we report the gate switching of the ultrafast upconverted photoluminescence from monolayer graphene. The luminescence can be completely switched off by the Pauli-blocking of one-photon interband transition in graphene, with an on/off ratio exceeding 100 times, which is remarkable compared to other 2D semiconductors and 3D bulk counterparts. The chemical potential and pump fluence dependences of the luminescence are nicely described by a two-temperature model, including both the hot carrier dynamics and carrier-optical phonon interaction. This gate switchable and background-free photoluminescence can open up new opportunities for graphene-based ultrafast optoelectronic applications.
关键词： ultrafast photoluminescence,gate switching,hot carriers,graphene,optoelectronics,two-temperature model
更新于2025-09-09 09:28:46