Electron-phonon Interaction Time Research Articles

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.

Cylindrical and plane surface electromagnetic waves in laser technology of silicon surface texturing with a double femtosecond pulse

This paper presents experimental results of laser modification of the surface of monocrystalline silicon under the influence of a double femtosecond laser pulse with varying delays less than or close to the electron–phonon interaction time, as well as the results of numerical simulation of the photoexcitation of a semiconductor in the approximation of dielectric constant. It is shown that the excitation of cylindrical surface waves not only precedes the excitation of plane surface electromagnetic waves, but is also a determining factor in the cumulative change in the dielectric constant of the medium and the creation of conditions for the excitation of plane surface electromagnetic waves.

Comparison of Hot Spot Formation in NbN and MoN Thin Superconducting Films After Photon Absorption

In superconducting single-photon detectors (SSPD), the efficiency of local suppression of superconductivity and hot-spot formation is controlled by diffusivity and electron–phonon interaction time. Here, we selected a material, 3.6-nm-thick MoN ${}_{x}$ film, which features diffusivity close to those of NbN traditionally used for SSPD fabrication, but with electron–phonon interaction time an order of magnitude larger. In MoN ${}_{x}$ detectors, we study the dependence of detection efficiency on bias current, photon energy, and strip width, and compare it with NbN SSPD. We observe nonlinear current-energy dependence in MoN ${}_{x}$ SSPD and more pronounced plateaus in dependences of detection efficiency on bias current, which we attribute to longer electron–phonon interaction time.

Noise Thermal Impedance of a Diffusive Wire

The current noise density S2 of a conductor in equilibrium, the Johnson noise, is determined by its temperature T: S2 = 4k(B)TG, with G the conductance. The sample's noise temperature T(N) = S2/(4k(B)G) generalizes T for a system out of equilibrium. We introduce the "noise thermal impedance" of a sample as the ratio deltaT(N)omega/deltaP(J)omega of the amplitude deltaT(N)omega of the oscillation of T(N) when heated by an oscillating power deltaP(J)omega at frequency omega. For a macroscopic sample, it is the usual thermal impedance. We show for a diffusive wire how this (complex) frequency-dependent quantity gives access to the electron-phonon interaction time in a long wire and to the diffusion time in a shorter one, and how its real part may also give access to the electron-electron inelastic time. These times are not simply accessible from the frequency dependence of S2 itself.

Structural kinetics of laser-excited metal nanoparticles supported on a surface

Gold nanoparticles have been adsorbed as monolayers on silicon and glass substrates and the structure evolution following femtosecond laser excitation has been analyzed by means of time resolved X-ray scattering. The synchronization of the laser to the X-ray pulse structure emitted from a third generation synchrotron source allows to obtain a natural time resolution of 100 ps for the lattice kinetics. The prospects of using a picosecond X-ray streak camera are explored. The lattice kinetics are dominated by the fast heating of the particle lattice and nanosecond cooling times. However, the analysis of peak shapes reveals the presence of nonthermal motion within the lattice. Unexpectedly large relaxation times for the thermalization of vibrational modes are found and are attributed to the weak mechanical coupling to the substrate. Strong nonuniform strain develops within the domain of electron–phonon interaction time regime after the laser excitation as seen with the X-ray streak camera.

Geometric heat trapping in niobium superconductor–insulator– superconductor mixers due to niobium titanium nitride leads

We analyze the current–voltage characteristics of a Nb superconductor–insulator–superconductor mixer with NbTiN leads to identify the heating processes in this device. We argue that the electron–electron interaction is much faster than the electron–phonon interaction, and show that the heat flow to the bath temperature is limited by the electron–phonon interaction time in the junction. A solution is suggested to considerably reduce the heating.

Possibility of fabricating an inductive high-speed detector for electromagnetic radiation using thin YBa2Cu3O7−δ films

It is shown that an inductive high-speed nonequilibrium detector for electromagnetic radiation can be fabricated using thin YBaCuO films. An electronic detection regime has been obtained for the first time using a low-temperature inductive YBaCuO detector in the measuring frequency band Δf=1–50 MHz and it has been shown that no bolometric detection regime exists at operating temperatures far below the superconducting transition. The time constant of the low-temperature inductive YBaCuO detector in the electronic regime is determined only by the electron-phonon interaction time in the nodal regions τe-phd. The detector has the following limiting characteristics: when the operating temperature is reduced from 10 to 1K, the time constant τD varies between 10 and 100 ps and the sensitivity D* improves substantially from 109 to 4×1012 W−1 cm Hz1/2.

Fast nonequilibrium induction detectors based on thin superconducting films

A new type of fast detector is proposed. The operation of the detector is based on the change induced in the kinetic inductance of NbN and YBa2Cu3O7−δ superconducting films by nonequilibrium quasiparticles produced by electromagnetic radiation. The speed of a NbN detector is essentially temperature-independent and is less than 1 ps. A model based on the Omen-Scalapino scheme describes well the experimental dependence of the voltage-power sensitivity of a NbN detector on the modulation frequency of the radiation. A low equilibrium quasiparticle density and a high quantum yield give detecting power D*=1012W−1 · cm · Hz1/2 at temperature T=4.2 K and D*=1016 W−1 · cm · Hz1/2 at temperature T=1.6 K. The time constant of the low-temperature YBaCuO induction detector is determined only by the electron-phonon interaction time τ e-ph d in the nodal regions.

Hot carrier relaxation processes and nonequilibrium phonon effect in multiple quantum well structures

We have used a picosecond nonlinear optical correlation technique to investigate the relaxation processes of hot nonequilibrium carriers in multiple quantum well structures. The average energy loss time constant π avg of the hot electrons was measured. By solving the average energy loss rate equation of carriers and the hot phonon rate equation, taking both the phonon emission and reabsorption into account, we show that π avg is the sum of the electron-phonon interaction time constant and the nonequilibrium phonon decay time, assuming only a single mode of phonons are involved. It is found that the temperature will be higher and/or the energy loss rate of the electron system will be slower as the quantum well width decreases, or the excitation density increases, or the degree of the lattice mismatch between the well layer and the barrier layer increases in the case of strained quantum wells.

NONEQUILIBRIUM PHONON EFFECTS IN HOT CARRIER RELAXATION PROCESSES OF MULTIPLE QUANTUM WELL STRUCTURES

Considering the phonon absorption and the phonon emission, we solve the equation for average energy loss rate of carriers and the hot phonon Boltzmann equation. We find that τavg is a sum of the electron phonon interaction time constant and the nonequilibrium phonon. decay time.

Picosecond CO 2 laser pulses and their applications

The mechanisms for the generation of variable picosecond duration CO 2 laser pulses using the optical free induction decay method are reviewed. A few recent applications of the variable duration feature are described. They include the measurement of the T 2 relaxation time in a CO 2 laser medium, the measurement of multiphoton absorption in sf 6 and the determination of the electron-phonon interaction time in a semiconductor.

Gap relaxation time in superconducting niobium films

Gap relaxation time τ Δ is determined for thin Nb films in temperature interval T T c ⋍(0,87−0,97) with the help of the new experimental technique. The dependence of τ Δ and inelastic electron-phonon interaction time τ e on T c in Nb films is discussed.