Electron-lattice Thermalization Research Articles

Ultrafast Surface Plasmon Probing of Interband and Intraband Hot Electron Excitations.

Upon the interaction of light with metals, nonthermal electrons are generated with intriguing transient behavior. Here, we present femtosecond hot electron probing in a noveloptical pump/plasmon probe scheme. With this, we probed ultrafast interband and intraband dynamics with 15 nm interface selectivity, observing a two-component-decay of hot electron populations. Results are in good agreement with a three-temperature model of the metal; thus, we could attribute the fast (∼100 fs) decay to the thermalization of hot electrons and the slow (picosecond) decay to electron-lattice thermalization. Moreover, we could modulate the transmission of our plasmonic channel with ∼40% depth, hinting at the possibility of ultrafast information processing applications with plasmonic signals.

Tunable ultrafast thermionic emission from femtosecond-laser hot spot on a metal surface by control of laser polarization and angle of incidence: A numerical investigation

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 the 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 the angle of incidence for the P polarization, while for the S polarization it shows the opposite trend. We observe that thermionic emission duration shows a strong dependence on the angle of incidence and contrasting polarization dependent behavior. The duration of the 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 using a metal based architecture.

Intrapulse Correlated Dynamics of Self-Phase Modulation and Spontaneous Raman Scattering in Synthetic Diamond Excited and Probed by Positively Chirped Ultrashort Laser Pulses

In synthetic diamond plates, the intrapulse-correlated dynamics of self-phase modulation and spontaneous nonresonant Raman scattering by center-zone optical phonons were for the first time directly investigated for tightly focused (focusing numerical aperture NA = 0.25) positively chirped visible-range high-intensity laser pulses with variable durations (0.3–9.5 ps) and energies transmitted through the sample. The observed self-phase modulation broadening and modulation of the transmitted light and Stokes Raman spectra for the (sub)picosecond pulse durations indicate the considerable Raman–Kerr contribution to the nonlinear polarization. The latter appears through plasma emission of the optical phonons, which emerges on the (sub)picosecond timescale and dominates at ≈1 ps. Later, this phonon contribution is eventually suppressed in the material due to picosecond-scale electron-lattice thermalization and the related thermally enhanced symmetrical decay of optical phonons into lower-frequency acoustic ones.

Nonthermal Dynamics of Dielectric Functions in a Resonantly Bonded Photoexcited Material

AbstractA fundamental question of whether it is thermal or nonthermal has often been argued, when the system undergoes an ultrafast phase change by femtosecond laser photoexcitation. One of the difficulties therein is how to consider the thermalization of a photoexcited system, in which the “lattice” and “electron” systems are decoupled from an equilibrium state just after photoexcitation. This work shows a methodology to identify a thermalization‐time domain of a photoexcited semiconductor, where it can be detected by monitoring the time‐resolved photoabsorption broadband spectrum and comparing that to thermally induced spectrum changes, by which the electron–lattice thermalization time is evaluated to be 12 ps for PbTe. Further analyzing the dynamic dielectric functions substantiates that thermalization time is related to the recovery time of the resonant bonding. Such a crucial determination of thermalization time would eliminate an uncertainty of “thermal or nonthermal” in ultrafast phase change of the resonantly bonded materials.

Ultrafast electron-lattice thermalization in copper and other noble metal nanoparticles

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.

Revealing Electron-Phonon Interactions and Lattice Dynamics in Nanocrystal Films by Combining in Situ Thermal Heating and Femtosecond Laser Excitations in 4D Transmission Electron Microscopy.

We present a comparative investigation on static equilibrium and transient structural dynamics of nanocrystalline gold films on silicon nitride supports performed at various in situ temperatures and by ultrafast laser excitations in a four-dimensional ultrafast transmission electron microscope (4D-UTEM). The change of relative diffraction intensity and lattice spacing with rising temperatures was systematically measured for {220} Debye-Scherrer rings via the in situ heating technique, which leads to a precise determination of the actual Debye temperature and a finding of significant depression of lattice expansions in the films. The diffraction intensity/lattice spacing-temperature relationship calibrated by the static, thermally equilibrium observations was then employed for investigating ultrafast transient dynamics on the same specimen region. The electron-phonon coupling constant g was determined to be 7.2 × 1015 W/m3 K in combination with simple two-temperature model analysis. We found a marked variation of temperature rise maximum (at quasi-equilibrium states) in between the temporal evolutions of lattice spacing and diffraction intensity, a phenomenon which may only be explained by the effect of nonthermal equilibrium relaxation dynamics following femtosecond laser excitations. The method demonstrated here can thus be applied to quantitative evaluations of nonthermal equilibrium contributions during the electron-lattice thermalization.

Picosecond relaxation of X-ray excited GaAs

In this paper we present the current status of our theoretical studies on ultrafast relaxation of X-ray/XUV excited gallium arsenide. First, we discuss our previous approach, the unified model based on rate equations, two-temperature model and the extended Drude approach. By fitting the model to the available experimental data, we obtained realistic estimates on transient electronic temperature and electron-lattice thermalization timescale. Next, we make a step towards a rigorous description of the relaxation process with our hybrid code, XTANT. We extend the XTANT to include the band-specific effect of the suppression of collisional processes in GaAs, and perform dedicated simulations. We find that the extended model correctly describes the predicted transient non-isothermality of conduction and valence bands, however, currently, it cannot reproduce the experimentally observed reflectivity overshooting at 5−10 ps. The reason for this discrepancy is that the electron-phonon coupling rate implemented in XTANT, although successfully applied for diamond and silicon, clearly underestimates the strength of this coupling in GaAs. The outline for a respective model improvement is discussed.

Pulse-width-dependent surface ablation of copper and silver by ultrashort laser pulses

The single-shot spallation thresholds for copper and silver surfaces demonstrate a considerable IR-laser (1030 nm) pulse-width dependent increase over a range of 0.2–12 ps for the former material and a very weak increase for the latter one, while the corresponding thresholds for visible (515 nm) laser pulses remain almost constant. The IR-laser increase of the ablation thresholds is related to two-photon interband (d–s) absorption in copper, contrasting with the linear absorption of visible laser pulses in this material. In silver, common weakly sublinear dependences on the laser pulsewidth were observed, ruling out possible multi-photon—either three(four)-photon in IR, or two-photon in the visible range—interband transitions in this material. Moreover, electron-lattice thermalization times of 1–2 ps were evaluated for these materials in the spallative ablation regime, contrasting strongly with the previously theoretically predicted multi-picosecond thermalization times.

Time-resolved observation of band-gap shrinking and electron-lattice thermalization within X-ray excited gallium arsenide.

Femtosecond X-ray irradiation of solids excites energetic photoelectrons that thermalize on a timescale of a few hundred femtoseconds. The thermalized electrons exchange energy with the lattice and heat it up. Experiments with X-ray free-electron lasers have unveiled so far the details of the electronic thermalization. In this work we show that the data on transient optical reflectivity measured in GaAs irradiated with femtosecond X-ray pulses can be used to follow electron-lattice relaxation up to a few tens of picoseconds. With a dedicated theoretical framework, we explain the so far unexplained reflectivity overshooting as a result of band-gap shrinking. We also obtain predictions for a timescale of electron-lattice thermalization, initiated by conduction band electrons in the temperature regime of a few eVs. The conduction and valence band carriers were then strongly non-isothermal. The presented scheme is of general applicability and can stimulate further studies of relaxation within X-ray excited narrow band-gap semiconductors.

Ultrafast surface strain dynamics in MnAs thin films observed with second harmonic generation

Optical second harmonic generation (SHG) is used to probe surface strain in 150 and 190-nm thin films of MnAs grown epitaxially on GaAs(001). The $p$-polarized SHG signal produced by $p$-polarized 775-nm, 200-fs pulses is theoretically and experimentally shown to be sensitive to the normal component of surface strain from $\ensuremath{-}20$ to 70 ${}^{\ensuremath{\circ}}$C, which includes the ferromagnetic/paramagnetic striped coexistence phase region that exists from $\ensuremath{\sim}$10 to 40 ${}^{\ensuremath{\circ}}$C. We use this dependence to time-resolve the surface strain dynamics in MnAs following pumping with 200-fs pulses of 1.0 or 2.0 mJ cm${}^{\ensuremath{-}2}$ that raise the surface temperature by tens of degrees. For a film at $\ensuremath{-}20{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C the strain reaches a minimum value in $\ensuremath{\sim}$10 ps, indicative of electron-lattice thermalization, before recovering on a 500-ps time scale consistent with a one-dimensional heat diffusion model. For a film at 20 ${}^{\ensuremath{\circ}}$C the minimum strain is reached only after $\ensuremath{\sim}$200 ps and attains a value higher than predicted by the heat diffusion model; recovery, however, still occurs in $\ensuremath{\sim}$500 ps. The long strain fall time possibly reflects the influence of latent heat and stripe dynamics in the coexistence phase. The larger calculated drop in surface strain may be due to deficiencies in the one-dimensional heat diffusion model. The nonequilibrium surface strain also may not be determined by the local temperature alone but by the constraints throughout the film/substrate system, which are certainly known to govern the strain and stripe characteristics under equilibrium conditions.

Femtosecond electron-lattice thermalization dynamics in a gold film probed by pulsed surface plasmon resonance

The dynamics of electronic excitations and their relaxation in a gold film is studied on the femtosecond time scale with a pump-probe technique. For the pump beam we use pulses with wavelengths centered at 800 nm, 400 nm or both. The surface plasmon resonance (SPR) in Kretschmann's configuration is used as a sensitive and fast-response probe of the dynamics of the dielectric properties of the gold film. The quantity that is monitored is the intensity of the reflected light at an incidence angle close to the SPR. With changes of the dielectric properties induced by the pump beam and during subsequent relaxation, the amount of the reflected light of the probe beam, sent with a variable delay, also changes, thus providing information on the temporal characteristics of the thermalization process. Special features of SPR probing with short pulses are also accounted for in this work. The thermalization of the electronic subsystem and energy transfer to the lattice are discussed in connection with the two-temperature relaxation model that takes into account temperature dependences of the electronic heat capacity and the electron-phonon coupling.

Volume fraction dependence of transient absorption signal and nonlinearities in metal nanocolloids

Electron–lattice thermalization dynamics in metal nanoparticles or in bulk metal is usually estimated by measuring the decay time of the change in transmission following an optical excitation. Such measurements can be performed in transient absorption geometry using a femtosecond laser. We find that for silver nanoplatelet/water colloids, the decay time of the transient absorption depends on the volume fraction of silver in water. By estimating the volume fraction dependence of nonlinearities in the same samples, we show that the variation in the measured decay time is due to pump-depletion effects present in the sample. The correct correction factor for taking into account pump-depletion effects in fifth- and higher-order nonlinearities is also presented.

Properties of plasmas produced by short double pulse laser ablation of metals

We investigate the composition of plasmas produced by laser ablation of metals with two time-delayed short laser pulses using fast imaging and time- and space-resolved optical emission spectroscopy. The ablated material is deposited on mica substrates and analyzed by atomic force microscopy. The laser-produced craters are inspected by optical microscopy to evaluate the ablated material quantity. It is shown that the fraction of nanoparticles in the ablation plume is strongly altered when a second laser pulse of sufficiently large delay is applied. Comparing the results obtained for different metals, we observe a significant nanoparticle reduction for interpulse delays of the order of the characteristic time of electron-lattice thermalization. More detailed analyses show that the plume changes occur on two different characteristic times, indicating two different mechanisms at its origin. Here, we discuss the involved processes and we propose a simple and efficient technique for the measurement of electron-lattice thermalization times based on plume observations during double pulse laser ablation.

Reducing nanoparticles in metal ablation plumes produced by two delayed short laser pulses

Using fast imaging and atomic force microscopy, we demonstrate that the fraction of nanoparticles in ablation plumes produced by short pulse laser irradiation of metals is strongly altered when a second laser pulse of sufficiently large delay is applied. Comparing the results obtained for gold and copper, it is shown that a significant nanoparticle reduction is only observed if the delay between both laser pulses exceeds the characteristic time of electron-lattice thermalization. We propose the reduced electronic heat transport at large lattice temperature as the dominant mechanisms for the observed nanoparticle reduction.

Nonequilibrium electron dynamics in noble metals

Electron-electron and electron-lattice interactions in noble metals are discussed in the light of two-color femtosecond pump-probe measurements in silver films. The internal thermalization of a nonequilibrium electron distribution created by intraband absorption of a pump pulse is followed by probing the induced optical property changes in the vicinity of the frequency threshold for the d band to Fermi surface transitions. This is shown to take place with a characteristic time constant of 350 fs, significantly shorter than previously reported in gold. This difference is ascribed to a weaker screening of the electron-electron interaction by the d-band electrons in silver than in gold. These results are in quantitative agreement with numerical simulations of the electron relaxation dynamics using a reduced static screening of the electron-electron Coulomb interaction, and including bound electron screening. Electron-lattice thermalization has been studied using a probe frequency out of resonance with the interband transitions. In both materials, the transient nonthermal nature of the electron distribution leads to the observation of a short-time delay reduction of the energy-loss rate of the electron gas to the lattice, in very good agreement with our theoretical model.

Temperature-dependent electron-lattice thermalization in GaAs

Thermalization of hot electron with the lattice is precisely investigated in bulk GaAs in the temperature range 50--300 K using a high-sensitivity two-color femtosecond absorption saturation technique. The results show that for lattice temperatures larger than 100 K, the photoexcited electron gas eventually thermalizes with the lattice with a characteristic time close to the LO phonon lifetime as a consequence of the nonequilibrium hot-phonon effect. For lower temperatures, the electron-lattice thermalization time is found to be longer than the LO phonon lifetime in agreement with numerical simulations of the coupled carrier-LO phonon dynamics.

Optical Phonon Coherence and Population Decays in III–V Semiconductors

Investigations of optical phonon coherence and population decays in bulk III–V semiconductors are discussed. The results are compared to our lifetime determinations using a non phonon-mode selective technique based on precise measurement of electron-lattice thermalization dynamics. The measured LO phonon lifetimes were found to be consistent with the dephasing times measured in intrinsic samples. In the presence of carriers (for the density range 1016–1017 cm -3), due to LO phonon-plasmon hybridization, the dephasing time is strongly reduced and becomes much smaller than the lifetime which is almost not altered. The roles of pure dephasing and population decay mechanisms in optical phonon relaxation are discussed for various carrier environment.

Femtosecond investigation of the hot-phonon effect in GaAs at room temperature

Slowing down of the hot electron-lattice thermalization by generation of nonequilibrium phonons (hot-phonon effect) is investigated in GaAs at room temperature using high-sensitivity femtosecond single- and double-wavelength absorption saturation techniques. Measurements were performed for different amplitudes of the hot-phonon effect by changing either the individual excess energy of the photoexcited carriers or their density in the range 7\ifmmode\times\else\texttimes\fi{}${10}^{15}$--4\ifmmode\times\else\texttimes\fi{}${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. After an initial regime dominated by nonequilibrium LO-phonon k-space redistribution, the long-time delay (\ensuremath{\ge}3 ps) electron thermalization is found to occur with a characteristic time of \ensuremath{\sim}1.9 ps, independent of the total energy initially injected into the carriers and essentially reflecting the LO-phonon energy relaxation. This is in agreement with numerical simulations of the coupled carrier-phonon relaxation dynamics, indicating that energy transfers to holes are responsible for this slight reduction of the thermalization time compared to the LO-phonon lifetime (identified with the dephasing time ${\mathit{T}}_{2}$/2\ensuremath{\sim}2.1 ps). \textcopyright{} 1996 The American Physical Society.