Observation of Nonequilibrium Electron Heating in Copper

Abstract

Picosecond laser pulses are used to sequentially heat and probe the thermally induced reflectivity transients in copper, and the nonequilibrium heating of electrons to a temperature above that of the lattice is observed. The ability to temporally resolve electronic and lattice heating contributions is of considerable significance to thermomodulation spectroscopy of solids. PACS numbers: 72.15.Eb, 65.90.+ i, 78.20.Nv Thermomodulation spectroscopy is an important method for studying the critical points associated with the interband optical absorption in solids/ The early work of Scouler^ demonstrated the significance of this technique to bandstructure studies in metals, and the use of this technique continues through the present day.^The traditional approach to a thermomodulation measurement is to pass a low-frequency (^50 Hz) alternating current through an optically thin (-40 nm) sample film, and synchronously detect the corresponding heating-induced reflectivity change as a function of optical wavelength. Temperature changes of -10 K result in reflectivity changes on the order of AJR -lO""*. A variety of effects contribute to the reflectivity change, including thermal expansion, electron-photon interactions, and shifting of the Fermi level.^ Scouler^ and Rosei and Lynch'* were the first to observe a sign reversal of A/? when the photon energy was tuned through the <i-band to Fermilevel interband transition energy of Au and Cu. This unique feature was interpreted as the result of Fermi distribution smearing, i.e., a heating-induced increase in unoccupied states below the Fermi level and a decrease in unoccupied states above the Fermi level/ However, other mechanisms influence the thermomodulation spectrum in this energy range, and the detailed contributions are not readily separated by use of the traditional measurement method.In this work, I demonstrate the use of picosecond lasers to sequentially heat and probe the thermally induced reflectivity transients of Cu in the vicinity of the d-band edge. I observe an anomalous rapid heating and cooling component in the reflectivity change, which I believe can be attributed to the initial heating of electrons above the lattice temperature. The phenomenon of inequality between the electron temperature and the lattice temperature is referred to as nonequilibrium heating,^ and to my knowledge, it has not previously been observed in metals. Observation of this phenomenon results from the ability to temporally resolve the electronic and lattice contributions to the reflectivity change and to show their different spectral behavior. By electronic contributions I mean changes resulting from hot electrons alone, whereas by lattice contributions I mean changes resulting from a hot lattice in equilibrium with the electrons.The apparatus used for transient thermodulation spectroscopy (TTMS) consists of a modelocked argon-ion laser which synchronously pumps two ring dye lasers at a pulse repetition rate of 246 MHz. The heating dye laser is fixed in wavelength at 645 nm (1.92 eV),...

Investigation of thermomechanical responses in ultrafast laser heating of metal nanofilms

“…(2). Both experimentally measured [3] and theoretically predicted [13] heat propagation speeds in hot electron gas of Au bulk are around 8×10 5 m/s, while the speed in the crystal Au nanofilms is reduced to 5 × 10 5 m/s since the size effect greatly decreases the thermal conductivity. Therefore, the propagation distance of electron heat waves at time 260 fs is about 140 nm as shown in Fig.…”

“…The nonequilibrium process occurs as the pulse width of the ultrashort pulse laser is comparable to the characteristic time for electrons to change their states. In the studies on nonequilibrium process, the energy exchange between electrons and the lattice was analyzed theoretically [1,2] and observed experimentally [3,4]. Two-temperature model [2] was used to describe the evolution of electron and lattice temperatures in the process of nonequilibrium heating of metal nanofilms.…”

“…Interaction between ultrashort pulse laser and metallic nanostructures involves the nonequilibrium energy exchange between electrons and lattice [1][2][3][4][5] and the size effects [6][7][8][9][10][11][12][13][14][15][16][17] occurring in microscale energy transport processes. The nonequilibrium process occurs as the pulse width of the ultrashort pulse laser is comparable to the characteristic time for electrons to change their states.…”

Picosecond Transient Thermoreflectance: Time-Resolved Studies of Thin Film Thermal Transport

“…Only recently have we been able to observe this phenomenon using picosecond pulsed lasers [13,14]. This nonequilibrium heating can be obse!ygd in both semiconductors and metals.…”

Collective Electron Dynamics in Metallic and Semiconductor Nanostructures

“…Experimentally, thanks to the recent development of ultrafast spectroscopy techniques, it is now possible to monitor the femtosecond dynamics of an electron gas confined in metallic nanostructures such as thin films [1,2,3,4,5,6,7,8], nanotubes [9], metal clusters [10,11] and nanoparticles [6,7,12,13]. Therefore, meaningful comparisons between experimental measurements and numerical simulations based on microscopic theories are becoming possible.…”