Electron-phonon Relaxation Processes Research Articles

Wavelength dependence of laser-induced excitation dynamics in silicon

Effect of laser wavelength on the carrier-phonon dynamics and damage threshold of silicon is studied numerically. Laser excitation dynamics in silicon is studied using Three-Temperature Model (3TM). We consider the evolution of electron, hole, and lattice temperatures separately and including band-gap re-normalization effect on optical properties of silicon. Finite Difference Time Domain method is used to model the laser field. Damage threshold calculated using the 3TM is in reasonable agreement with the experiments. Our results indicate that the competition of inter-band excitation, plasma heating, and electron–phonon relaxation process defines the damage threshold for various wavelengths and pulse durations.

Anisotropic plasmonic Pd-tipped Au nanorods for near-infrared light-activated photoacoustic imaging guided photothermal-photodynamic cancer therapy.

The integration of photothermal therapy (PTT) and photodynamic therapy (PDT) has become a promising cancer treatment method. Herein, anisotropic metal hetero-nanostructure Pd-tipped Au nanorods (PTA NRs) were fabricated, which exhibit plasmon-enhanced photothermal performance under near-infrared laser irradiation. Due to their anisotropic nanostructure, PTA NRs promote the generation of energetic hot electrons and prolong the separation time of electrons and holes. The hot electrons could generate heat energy through the electron-phonon relaxation process and produce reactive oxygen species through energy and electron transformation processes. Because of their high NIR absorption cross-section, as well as good photostability, PTA NRs can be used for NIR-activated photoacoustic imaging-guided PTT-PDT combination cancer therapy. Experiments in tumor-bearing mice proved that PTA NRs exhibit excellent anti-tumor effects, with little side effects on normal organs, making them promising for NIR cancer phototherapy.

Copper, gold, and platinum under femtosecond irradiation: Results of first-principles calculations

The paper investigates the interaction of femtosecond laser pulses with the thin films of copper, gold, and platinum. It considers electron-phonon relaxation processes and melting in the metal system nonequilibrium heated by laser radiation. Instead of the approximated formula by Wang et al. [Phys. Rev. B 50, 8016 (1994)], which is widely used to determine the temperature dependence of the electron-phonon coupling factor, we propose an improved approach for its more accurate calculation from first principles. Comparison with experiments and other calculations shows our approach to provide good calculation accuracy. Melting time versus absorbed energy density was estimated for the films and shown to be markedly sensitive to latent heat at low absorbed energy densities $(l1$ MJ/kg). Our calculations taken to study the temporal evolution of the (220) diffraction peak intensity after femtosecond irradiation show good agreement between experimental and theoretical data, which was attained due to higher accuracy in our determination of the temperature dependence electron-phonon coupling factor.

Phonon-induced long-lasting nonequilibrium in the electron system of a laser-excited solid

Electron-electron thermalization and electron-phonon relaxation processes in laser-excited solids are often assumed to occur on different timescales. This is true for the majority of the conduction band electrons in a metal. However, electron-phonon interactions can influence the thermalization process of the excited electrons. We study the interplay of the underlying scattering mechanisms for the case of a noble metal with help of a set of complete Boltzmann collision integrals. We trace the transient electron distribution in copper and its deviations from a Fermi-Dirac distribution due to the excitation with an ultrashort laser pulse. We investigate the different stages of electronic nonequilibrium after an excitation with an ultrashort laser-pulse of 800 nm wavelength and 10 fs pulse duration. Our calculations show a strong nonequilibrium during and directly after the end of the laser pulse. Subsequently, we find a fast thermalization of most electrons. Surprisingly, we observe a long-lasting nonequilibrium, which can be attributed to the electron-phonon scattering. This nonequilibrium establishes at energies around peaks in the density of states of the electrons and persists on the timescale of electron-phonon energy relaxation. It influences in turn the electron phonon coupling strength.

Terasonic Excitations in 2D Gold Nanoparticle Arrays in a Water Matrix as Revealed by Atomistic Simulations

In this work we report on terahertz phononic excitations in two-dimensional (2D) gold nanoparticle arrays in a water matrix through a series of large-scale molecular dynamics simulations. For the first time, we observe acoustic Dirac-like crossings in H (H2O) atomic (molecular) networks that emerge due to an intraband phononic scattering. These crossings are the phononic fingerprints of ice-like arrangements of H (H2O) atomic (molecular) networks at nanometer scale. We reveal how phononic excitations in metallic nanoparticles and the water matrix reciprocally impact on one another providing the mechanism for the THz phononics manipulation via structural engineering. Furthermore, we show that by tuning the arrangement of 2D gold nanoparticle assemblies, the Au phononic polarizations experience subterahertz hybridization (Kohn anomaly) due to surface electron–phonon relaxation processes. This opens the way for the sound control and manipulation in soft matter metamaterials at nanoscale.

Ultrafast dynamics of high-contrast nano-grating formation on gold film induced by temporally shaped femtosecond laser

We theoretically investigated the ultrafast dynamics of high-contrast nano-grating formation on gold film surface with respect to non-equilibrium thermal excitation processes. It is proposed that the high-contrast nano-grating can be desirably achieved by optimizing the non-equilibrium thermal dynamics processes. The grating contrast can be largely promoted by 64% with adjusting thermal dynamics via tailoring temporally shaped femtosecond laser. The results are attributed to the competitive energy transfer routes between electrons thermal diffusion and electron–phonon relaxation processes during grating formation, which can be well manipulated by temporally shaped femtosecond laser. The study provides for strategy for production of high-quality periodic surface structures with sharp grating profiles.

Shape- and Symmetry-Dependent Mechanical Properties of Metallic Gold and Silver on the Nanoscale

The mechanical properties of anisotropic nanoparticles like gold nanorods (AuNRs) and silver nanorods (AgNRs) are different from those of isotropic shapes such as nanospheres. We probed the coherent lattice oscillations of nanoparticles by following the modulation of the plasmonic band frequency using ultrafast laser spectroscopy. We found that while the frequency of the longitudinal vibration mode of AgNRs is higher than that of AuNRs of similar dimensions, similarly sized gold and silver nanospheres have similar lattice vibration frequencies. Lattice vibrations calculated by finite element modeling showed good agreement with the experimental results for both AgNRs and AuNRs. The accuracy of the calculations was improved by using actual pentagonal shapes rather than cylinders that did not agree well with the experimental results. As the plasmon energy is transferred into lattice vibrations, the temperature of the nanoparticle necessarily increases as a result of this electron-phonon relaxation process. This results in a decrease in the Young's modulus that was accounted for in the calculations. Calculations showed that the tips of the nanorods are "softer" than the rest of the nanorod. Because the tips comprise a larger portion of the overall rod in the smaller rods, the smaller rods were more affected by the tip effects.

Observation of low-wavenumber out-of-plane optical phonon in few-layer graphene

Few-layer graphene grown by chemical vapor deposition has been studied by Raman and ultrafast laser spectroscopy. A low-wavenumber Raman peak of ~120 cm � 1 and a phonon-induced oscillation in the kinetic curve of electron–phonon relaxation process have been observed, respectively. The Raman peak is assigned to the low-wavenumber out-of-plane optical mode in the few-layer graphene. The phonon band shows an asymmetric shape, a consequence of so-called Breit-Wigner-Fano resonance, resulting from the coupling between the low-wavenumber phonon and electron transitions. The obtained oscillation wavenumber from the kinetic curve is consistent with the detected low-wavenumber phonon by Raman scattering. The origin of this oscillation is attributed to the generation of coherent phonons and their interactions with photoinduced electrons. Copyright © 2012 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article.

Nonequilibrium electrons in tunnel structures under high-voltage injection

We investigate electronic distributions in nonequilibrium tunnel junctions subject to a high voltage bias $V$ under competing electron-electron and electron-phonon relaxation processes. We derive conditions for reaching quasi-equilibrium and show that, though the distribution can still be thermal for low energies where the rate of the electron-electron relaxation exceeds significantly the electron-phonon relaxation rate, it develops a power-law tail at energies of order of $eV$. In a general case of comparable electron-electron and electron-phonon relaxation rates, this tail leads to emission of high-energy phonons which carry away most of the energy pumped in by the injected current.

Time-resolved optical characterization of proximized nano-bilayers for ultrafast photodetector applications

Time-resolved transient pump-probe spectroscopy measurements on proximized ferromagnet/superconductor (F/S) hybrid structures are presented. We focused our attention on both low and high critical temperature superconductors such as Nb and YBCO, while for F the weak-ferromagnetic alloy Ni0.48Cu0.52 has been used. Dynamics of the electron-phonon relaxation process has been investigated as a function of both the temperature and the F-film thickness. In the case of NiCu/Nb bilayers the presence of a thin F overlayer reduces the slow bolometric component of the photoresponse, while in YBCO-based structures faster relaxation times were measured. F/S nanobilayers are very attractive toward the development of novel hybrid superconducting photodetectors.

Comparative analysis of electron-phonon relaxation in a semiconducting carbon nanotube and a PbSe quantum dot

Abstract The key features of the phonon-induced relaxation of electronic excitations in the (7,0) zig-zag carbon nanotube (CNT) and the Pb16Se16 quantum dot (QD) are contrasted using a time-domain ab initio density functional theory (DFT) simulation. Upon excitation from the valence to the conduction band (CB), the electrons and holes nonradiatively decay to the band-edge in both materials. The paper compares the electronic structure, optical spectra, important phonon modes, and decay channels in the CNT and QD. The relaxation is faster in the CNT than in the QD. In the PbSe QD, the electronic energy decays by coupling to low-frequency acoustic modes. The decay is nonexponential, in agreement with non-Lorentzian line-shapes observed in optical experiments. In contrast to the QD, the excitation decay in the CNT occurs primarily via high-frequency optical modes. Even though the holes have a higher density of states (DOS), they relax more slowly than the electrons, due to better coupling to low-frequency vibrations. Further, the expected phonon bottleneck is not observed in the QD, as rationalized by a high density of optically dark states. The same argument applies to the CNT. The computed results agree well with experimentally measured ultrafast relaxation time-scales and provide a unique atomistic picture of the electron-phonon relaxation processes.

Recombination of an electron-hole plasma in silicon under the action of femtosecond laser pulses

Experimental data are obtained on the dynamics of conduction-electron relaxation at the stage preceding the melting of a silicon surface layer. The energy of a quantum of probe radiation is smaller than the band gap, making it possible to obtain information about the electron-phonon relaxation processes for an electron concentration of ∼ 1021 cm−3 in the conduction band.

Femtosecond transient-absorption dynamics of colloidal gold nanorods: Shape independence of the electron-phonon relaxation time

We studied the femtosecond dynamics of colloidal gold nanorods encapsulated in micelles after excitation with 400 nm pulses of 100 fs duration. It is found that the laser heating of the electron gas of gold nanorods with an average aspect ratio of 3.8 leads to the bleaching of both the transverse and longitudinal mode of the surface plasmon oscillation at 520 and 750 nm. The bleach recovers with the same time constant for both the transverse and longitudinal oscillation, for gold nanodots prepared by photothermal reshaping of the rods as well as for nanodots synthesized chemically by citrate reduction (and known to have twin boundaries and surface defects). Since the bleach recovery on the 3 ps time scale is assigned to electron-phonon relaxation processes, these results suggest that phonon dependent relaxation processes in gold nanoparticles are independent of the shape, size, type of the surfaces, or the mode of the surface plasmon, oscillation excited. The fact that the mean free path of the electron in metallic gold is in the nanometer length scale (\ensuremath{\sim}50 nm) raised the question of the importance of surface scattering to the electron-phonon relaxation process in gold nanoparticles. Our previous studies showed little dependence of the relaxation rate of the size of gold nanodots (from 9 to 48 nm). In the present study, the electron-phonon relaxation is measured in gold nanorods, which have different facets from those of gold nanodots.

Electron-phonon relaxation dynamics of niobium metal as a function of temperature.

The electron-phonon relaxation process in niobium has been measured with pump-probe technique and compared with theory for ambient (initial) temperatures ranging from 292 to 7 K. The time it takes for the electrons to thermalize with phonons was found to decrease from 370 fs at 292 K to 250 fs at 7 K, while the electron-phonon coupling parameter g decreases with decreasing temperature.

Electron-phonon relaxation in pure metals and superconductors at very low temperatures.

The electron-phonon relaxation processes in pure metals and superconductors are considered at low temperatures when electrons interact with both longitudinal and transverse phonons. The screening of the electron-ion interaction is treated in a gauge-invariant form, so that both longitudinal and transverse electromagnetic fields are taken into consideration. The results obtained are applied to the analysis of the heating effect of the electron system. The interaction of electrons with transverse phonons increases the electron-phonon relaxation rate by a factor of {congruent}15 in comparison with the case when only longitudinal phonons are taken into consideration. The peculiarity of the screening effects in a superconductor leads to the conclusion that at low temperatures {ital T}{much lt}{Delta} transverse phonons do not participate in the electron-phonon relaxation processes.