Relaxation Dynamics Of Hot Electrons Research Articles

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.

Electron–Phonon Relaxation Dynamics of Hot Electrons in Gold Nanoparticles Are Independent of Excitation Pathway

Electron–Phonon Relaxation Dynamics of Hot Electrons in Gold Nanoparticles Are Independent of Excitation Pathway

Surface hopping dynamics in periodic solid-state materials with a linear vibronic coupling model.

We report a surface hopping approach in which the implemented linear vibronic coupling Hamiltonian is constructed and the electronic wavefunction is propagated in the reciprocal space. The parameters of the linear vibronic coupling model, including onsite energies, phonon frequencies, and electron-phonon couplings, are calculated with density-functional theory and density-functional perturbation theory and interpolated in fine sampling points of the Brillouin zone with maximally localized Wannier functions. Using this approach, we studied the relaxation dynamics of the photo-excited hot carrier in a one-dimensional periodic carbon chain. The results show that the completeness of the number of Hilbert space k points and the number of phonon q points plays an important role in the hot carrier relaxation processes. By calculating the relaxation times of hot carriers under different reciprocal space sampling and extrapolating with the stretched-compressed exponential function, the relaxation times of hot electrons and holes in the quasi-continuous energy band are obtained. By considering the feedback effect in the hopping processes and analyzing the time-dependent phonon energy in different normal modes, we found that the long-wave longitudinal optical phonons play a major role in the relaxation dynamics of hot electrons and holes. We, therefore, provided herein an efficient and accurate approach for modeling the photophysical processes in periodic solid-state material systems.

Relaxation dynamics of hot electrons in the transition metals Au, Ag, Cu, Pt, Pd, and Ni studied by ultrafast luminescence spectroscopy

The ultrafast relaxation dynamics of photoexcited electrons in six transition metals, Au, Ag, Cu, Pt, Pd, and Ni, were investigated using femtosecond luminescence spectroscopy in the infrared region between 0.4 and 1.05 eV. The behaviors of the time-resolved spectra are significantly different between group 11 noble metals (Au, Ag, and Cu) and group 10 transition metals (Pt, Pd, and Ni), which are neighbors in the periodic table of elements. In the latter group, the instantaneous luminescence intensities are one order of magnitude lower and the lifetimes (around 200 fs) are far shorter than those of the group 11 metals (typically 700 fs). The time-resolved spectra, decay profiles, and excitation power dependence were analyzed using a phenomenological model that considers both the nonthermal and thermal electrons. It was found that the nonthermal component is remarkably small in the group 10 transition metals. These systematic differences between group 10 and 11 metals are ascribed to the differences in their electron band structures and/or electron–phonon coupling strengths.

Ultrafast hot-hole injection modifies hot-electron dynamics in Au/p-GaN heterostructures.

A fundamental understanding of hot-carrier dynamics in photo-excited metal nanostructures is needed to unlock their potential for photodetection and photocatalysis. Despite numerous studies on the ultrafast dynamics of hot electrons, so far, the temporal evolution of hot holes in metal-semiconductor heterostructures remains unknown. Here, we report ultrafast (t < 200 fs) hot-hole injection from Au nanoparticles into the valence band of p-type GaN. The removal of hot holes from below the Au Fermi level is observed to substantially alter the thermalization dynamics of hot electrons, reducing the peak electronic temperature and the electron-phonon coupling time of the Au nanoparticles. First-principles calculations reveal that hot-hole injection modifies the relaxation dynamics of hot electrons in Au nanoparticles by modulating the electronic structure of the metal on timescales commensurate with electron-electron scattering. These results advance our understanding of hot-hole dynamics in metal-semiconductor heterostructures and offer additional strategies for manipulating the dynamics of hot carriers on ultrafast timescales.

Doublon bottleneck in the ultrafast relaxation dynamics of hot electrons in 1T−TaS2

Employing time-resolved photoelectron spectroscopy we analyze the relaxation dynamics of hot electrons in the charge density wave / Mott material 1T-TaS_2. At 1.2 eV above the Fermi level we observe a hot electron lifetime of 12 +- 5 fs in the metallic state and of 60 +- 10 fs in the broken symmetry ground state - a direct consequence of the reduced phase space for electron-electron scattering determined by the Mott gap. Boltzmann equation calculations which account for the interaction of hot electrons in a Bloch band with a doublon-holon excitation in the Mott state provide insight into the unoccupied electronic structure in the correlated state.

Adsorbate-mediated relaxation dynamics of hot electrons at metal/organic interfaces

Hot-electron dynamics at metal surfaces are crucial for charge transport properties and chemical reactions at metal/organic interfaces. In order to study the influence of electron-donating adsorbates on hot-electron lifetimes we performed femtosecond time-resolved two-photon photoemission measurements of tetrathiafulvalene (TTF)-covered Au(111) surfaces. The electron-donating nature of TTF provides density of states near the Fermi energy (${E}_{F}$) increasing the possibility for electron-electron scattering events. This leads to an additional fast electron-electron scattering assisted hot-electron relaxation channel for low intermediate state energies (${E}_{i}\ensuremath{-}{E}_{F}$) at the TTF/Au(111) interface with hot-electron lifetimes up to $340\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$. However, suppressing the electron donation via intercalation of electron-accepting tetracyanoquinodimethane molecules causes quenching of this fast electron relaxation channel. This results in an asymptotic increase of the hot-electron lifetimes for infinitesimal small intermediate state energies up to about $1000\phantom{\rule{0.28em}{0ex}}\mathrm{fs}$ as predicted by the pure Landau theory of Fermi liquids.

Linear and Nonlinear Absorption Properties of Diamond-Like Carbon Doped With Cu Nanoparticles

Ultrafast relaxation processes in diamond-like carbon (DLC) thin films with embedded Cu nanoparticles (DLC:Cu nanocomposites) were investigated by means of transient absorption spectroscopy focusing on localized surface plasmon resonance (LSPR) of photoexcited Cu nanoparticles. Absorption spectra of the composite films correspond to the sum of absorption spectra of DLC matrix and Cu nanoparticles; however, Cu nanoparticles strongly dominate in the transient differential absorption. Excitations of DLC matrix and of Cu nanoparticles relax independently revealing no strong interaction. High sensitivity measurements enabled to obtain the hot electron relaxation dynamics in Cu nanoparticles in the low excitation intensity conditions. The relaxation time was found to be independent of the excitation intensity up to tens of microjoule per square centimeter per pulse and to increase at higher intensities. The relaxation time obtained at low excitation intensity was also found to increase by about 30 % in the samples with high Cu concentration, where larger nanoparticles were formed.

Study of Surface Plasmon Induced Hot Electron Relaxation Process and Third-Order Optical Nonlinearity in Gold Nanostructures

The relaxation dynamics of hot electrons and the third-order optical nonlinearity in gold nanorods (GNRs) with different aspect ratios were investigated with the help of femtosecond optical Kerr (OKE) technique. Relaxation process on the time scale of picosecond (ps) was obtained at the longitudinal surface plasmon resonant (SPR) wavelength. As the aspect ratio (AR) of the GNRs varied from 4.2 ± 0.3 to 3.3 ± 0.3, the SPR wavelength changed accordingly from 808 to 750 nm, but the corresponding relaxation time and the third-order optical susceptibility did not change much. However, as the pump power increased from 150 mW to 400 mW, the relaxation time of the nanorod with AR = 4.2 ± 0.3 increased by 50% from 1.20 ± 0.01 ps to 1.84 ± 0.04 ps, indicating that the pump power is an important factor that affects the response time of the nanostructures. Comparative studies between Au nanorods, Au nanobipyramids, and Au triangular prisms measured under the same setup demonstrated that excitation conditions (excitation wavelength or pump power) and structure shapes were key factors to modulate the relaxation time of hot electrons. These results are important for the control and modulation of the response time in metallic nanostructures, which is essential for their applications in photovoltaic, photocatalytic, and ultrafast optic devices.

Wannier interpolation of the electron-phonon matrix elements in polar semiconductors: Polar-optical coupling in GaAs

We generalize the Wannier interpolation of the electron-phonon matrix elements to the case of polar-optical coupling in polar semiconductors. We verify our methodological developments against experiments, by calculating the widths of the electronic bands due to electron-phonon scattering in GaAs, the prototype polar semiconductor. The calculated widths are then used to estimate the broadenings of excitons at critical points in GaAs and the electron-phonon relaxation times of hot electrons. Our findings are in good agreement with available experimental data. Finally, we demonstrate that while the Fr\"ohlich interaction is the dominant scattering process for electrons/holes close to the valley minima, in agreement with low-field transport results, at higher energies, the intervalley scattering dominates the relaxation dynamics of hot electrons or holes. The capability of interpolating the polar-optical coupling opens new perspectives in the calculation of optical absorption and transport properties in semiconductors and thermoelectrics.

Shape and Temperature Dependence of Hot Carrier Relaxation Dynamics in Spherical and Elongated CdSe Quantum Dots

Time-domain nonadiabatic ab initio simulations are performed to study the phonon-assisted hot electron relaxation dynamics in a CdSe spherical quantum dot (QD) and an elongated quantum dot (EQD) with the same diameter. The band gap is smaller, and the electron and hole states are denser in the EQD than in the QD. Also, the band gap shows a stronger negative temperature dependence in the EQD than in the QD. Higher frequency phonons are excited and scattered with electrons at higher temperatures for both QD and EQD. The electron―phonon coupling is generally stronger in the EQD than in the QD. The hot electron decay rates calculated from nonadiabatic molecular dynamics show a weaker temperature dependence than the T ―1 trend in both QD and EQD, which is attributed to the thermal expansion effect. Furthermore, the relaxation of hot electrons proceeds faster and shows stronger temperature dependence in the EQD than in the QD. Our work demonstrates that the shape of quantum dots has a strong impact on the electron decay dynamics.

Theory of ultrafast intraband relaxation in carbon nanotubes

AbstractUltrafast intraband carrier relaxation in carbon nanotubes can be described by Carbon Nanotube Bloch Equations (CNBE) treating the Coulomb interaction beyond the Hartree‐Fock level to include carrier‐carrier scattering. We obtain equations of motion for carrier densities and two‐particle carrier correlations that are solved numerically. The complex non‐Markovian relaxation dynamics of hot electrons is calculated for various carbon nanotubes of different diameter and chirality. All calculated nanotubes show an ultrafast relaxation to equilibrium within approximately 100 femtoseconds. The observed relaxation is only weakly dependent on chirality, but depends on the diameter of the nanotube. With increasing diameter of the CNT the relaxation is slowed down, because of the weaker Coulomb interaction in larger diameter CNT. Additionally, we find oscillations of the electron plasma that are due to memory effects in the electron equilibration. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Hot Electron Relaxation Dynamics of Gold Nanoparticles Embedded in MgSO4 Powder Compared To Solution: The Effect of the Surrounding Medium

To test the influence of the surrounding medium on the relaxation dynamics of the plasmon band bleach recovery of gold nanoparticles after excitation with femtosecond laser pulses, we embedded 14.5 and 12.1 nm colloidal gold nanoparticles (synthesized electrochemically) in MgSO4 powder and investigated these samples by femtosecond diffuse reflectance spectroscopy. By measuring the relaxation dynamics over a wide range of excitation energies, we found that the fast decay component is slower by about a factor of 2 for the particles in the MgSO4 powder compared to those in solution while no significant change in the slow decay component is observed. In agreement with this observation, we found that adding solvent to the particles embedded in the powder caused a decrease in the relaxation time from about 10 ps to 5 ps for the fast decay component. This leads to the conclusion that the electron−phonon relaxation in these gold nanoparticles depends on the chemical nature and/or physical phase (solid vs solution...

Ultrafast dynamics of hot electrons and holes in copper: Excitation, energy relaxation, and transport effects

Time-resolved two-photon photoemission (2PPE) at various photon energies is used to investigate the relaxation dynamics of hot electrons in Cu(111), applying auto- and cross-correlation techniques. The relaxation times vary from 250 fs at 0.1 eV above the Fermi level to 20 fs at 2 eV and show a strong wavelength dependence in the vicinity of the $d$-band feature in the 2PPE spectra. Electrons not directly excited from the $d$ band exhibit a much longer relaxation time than $d$-band electrons excited to the same intermediate-state energy. We attribute these apparently longer lifetimes to a delayed electron generation via Auger decay of $d$-band holes. Based on a band structure calculation, a simulation of the ballistic transport effect and its implication on the observed electron relaxation dynamics is presented for the three low-index copper surfaces. These observations suggest that $d$-band holes have a substantially longer lifetime than excited $\mathrm{sp}$-band electrons of the corresponding excitation energy.

Hot electron relaxation dynamics in ZnSe

The ultrafast relaxation dynamics of hot electrons, initially photoexcited with an excess energy of 300 meV, are monitored in a high-quality ZnSe epilayer grown on a GaAs substrate by exploiting the intrinsic interferrometric asymmetric Fabry–Perot sample structure. The results are consistent with the expected characteristic electronic LO-phonon emission time of 40–50 fs and provide evidence for the influence of the “hot phonon effect”(or “LO-phonon bottleneck”) on the electron cooling dynamics at carrier densities above ∼3×1017 cm−3.

Two-photon photoemission spectroscopy at clean and oxidized Cu(110) and Cu(100) surfaces

Two-photon photoemission spectra of Cu(110) and Cu(100) are measured using ∼ 3.2 eV excitation laser with ∼ 20 fs pulse width. The spectra are measured as a function of laser polarization, excitation energy, bias voltage, and oxygen coverage. They are discussed in terms of the bulk and surface electronic structure of Cu and hot electron relaxation dynamics, and compared to previous nanosecond laser studies. In addition, three-photon photoemission through the n = 1 image potential state on Cu(100) is observed.

Femtosecond dynamics of hot-electron relaxation in Cu(110) and Cu(100)

Hot-electron relaxation dynamics due to electron-electron scattering at Cu(110) and Cu(100) surfaces are measured with < 10 fs time resolution by two-photon time-resolved photoemission spectroscopy. Comparison of experimental population decay rates for hot electrons with 1.3-3.2 eV energy above the Fermi level with those calculated by the Fermi liquid theory shows significant disagreement. The experimental rates are on average ∼ 5 times slower, have a different energy dependence than predicted by theory, and depend on the crystal face of Cu. The time scales for hot-electron thermalization measured here are important for understanding and controlling hot-electron-induced chemistry at metal surfaces.

Time-resolved IR-absorption spectroscopy of hot-electron dynamics in satellite and upper conduction bands in GaP.

The relaxation dynamics of hot electrons in the ${\mathit{X}}_{6}$ and ${\mathit{X}}_{7}$ satellite and upper conduction bands in GaP was directly measured by femtosecond UV-pump--IR-probe absorption spectroscopy. From a fit to the induced IR-absorption spectra the dominant scattering mechanism giving rise to the absorption at early delay times was determined to be intervalley scattering of electrons out of the ${\mathit{X}}_{7}$ upper conduction-band valley. For long delay times the dominant scattering mechanism is electron-hole scattering. Electron transport dynamics of the upper conduction band of GaP has been time resolved.