Electron dynamics in chromium probed with 20-fs optical pulses

Abstract

Electron excitation and relaxation in chromium are probed with 20-fs time resolution using an ultrafast optical technique. We obtain good fits to the data for the transient reflectivity and transmittivity changes in a thin film using a simple model of electron relaxation, suggesting the existence of an efficient electron-electron thermalization process on ultrashort-time scales. Quantitative analysis allows the extraction of thermo-optic coefficients and dielectric constant variations related to both the electron and the lattice temperatures. DOI: 10.1103/PhysRevB.68.113102 PACS number͑s͒: 78.47.ϩp, 78.20.Nv, 78.20.Ci Nonequilibrium electron distributions can be excited in metals with an ultrashort light pulse.1 The subsequent energy exchange between the electrons and the lattice is governed predominantly by the electron-phonon ͑e-p͒ scattering time, but is also affected by the electron-electron ͑e-e͒ scattering time. The noble metals, possessing simple band structures, have provided a fertile testing ground for theories of nonequilibrium electron relaxation and diffusion. [2][3][4][5][6][7][8] It is now possible to investigate the electron dynamics on time scales of the order of the e-e scattering time, typically 10-50 fs for excess electron energies ϳ1 eV. Although in the noble metals and in the alloy CoPt 3 , where the e-p interaction is relatively weak, the evolution of such transient athermal electron distributions was investigated with ϳ20-fs time resolution, 7-9 in other transition metals no studies have been made on these time scales, to the best of our knowledge.The group-VIB transition metals ͑Cr, Mo, W͒ are interesting because they have large values of the e-p coupling constant, resulting in short electron energy relaxation times ϳ200 fs compared to the noble metals ͑ϳ1 ps͒.10 Their band structure is complicated, there being a significant density of states due to d electrons in the region around the Fermi level.11 Understanding the short-time electron dynamics in such metals with strong e-p coupling should become essential for applications in future ultrafast devices with ultrahigh switching speeds. The electron relaxation in thin films of chromium and tungsten under spatially homogeneous conditions was measured by Brorson et al. with optical pulses of duration 60 fs.10 But only the reflectivity change was probed, thus preventing access to the transient dielectric constant. In this report we monitor both the reflectivity and transmittivity changes in a thin polycrystalline film of chromium with 20-fs optical pulses to elucidate the ultrafast dynamics of the electrons therein.The film of Cr on a crown glass substrate is excited and probed from the front side with near Fourier-transformlimited optical pulses from a Ti:sapphire laser ͑Kapteyn-Murmane Labs͒ of central wavelength 790 nm, repetition rate 87 MHz, pulse duration L ϭ20 fs ͓full width at half maximum ͑FWHM͒ intensity͔, and spectral width ϳ50 nm ͑FWHM͒. A schematic diagram of the apparatus is shown in Fig. 1. Interband transitions of electro...

Laser-Induced Charge-Density-Wave Transient Depinning in Chromium

“…Contrary to many other CDW systems [19][20][21][22][23][24][25][26], the ultrafast dynamics of chromium has never been studied in bulks. Femtosecond reflectivity experiments have been reported in films, validating that the ultrafast electronic response is well accounted for by the two-temperature model [27,28], and confirmed by a recent study of a commensurate CDW in a Cr film as well [29].In this work, the CDW modulation (and the superimposed periodic lattice distortion) and the average lattice have been studied as a function of time after femtosecond laser excitation by picosecond time-resolved x-ray diffraction in a bulk. The strength of this technique is its wavevector selectivity, which allows to track the timedependent behaviour of the CDW and average lattice independently.…”

Ultrafast dynamics of coherent optical phonons and nonequilibrium electrons in transition metals

“…[2,3,4,5,6] The e − ph thermalization occurs on a subpicosecond time scale in metals and has been traditionally described by the use of the two temperature model (TTM). [6,7,8,9] Besides the observation of the dynamics of nonequilibrium electrons, coherent acoustic phonon pulses have been investigated by using the pumpprobe angular deflection technique. [10,11] The acoustic phonon pulse is generated through transient heating of * Electronic address: hase.muneaki@nims.go.jp the sample surface induced by excitation with intense laser pulses.…”

“…[8,24] the TR signal (∆R/R) was measured to extract the relaxation dynamics of nonequilibrium carriers and the bulk coherent optical phonon as a function of the time delay after excitation pulse. [25] III.…”

Two-Temperature Model of Nonequilibrium Electron Relaxation: A Review

“…They considered the weakly coupled electron and phonon subsystems and ignored the band structure of the metal, this work was further extended to the case of a metal exposed to ultrashort laser pulses 3 , and today known as TTM. It is a pioneering work, that provides the basic model for ultra-fast pump-probe laser studies in metals 4,5,6,7,8,12,13 . The theory of thermal relaxation of electrons in metals was further extended by P. B. Allen 14 .…”