Electron Phonon Relaxation

Relaxation of CT samples also depends strongly on Sn content. For the most Sn-abundant sample x-ray analysis showed the presence of almost completely oriented Sn crystallites of 100-500 nm size, and electronic microscopy revealed that metal particles were separated by distances less than their size. It means that in this case we have practically metal film. The time response of such a sample appeared to be the same as that for pure Sn film. We attribute pulsewidth-limited rise of negative transmission and reflection (curves 10,11) to excitation of electrons in metal. Subsequent 5 ps rise reflects electron-phonon relaxation and further long decay is due to lattice cooling. Contribution of this dynamics is observed for the... 

Elsayed-Ali H. E., Norris T. B., Pessot M. A. and Mourou G. A. (1987), Time-resolved observation of electron-phonon relaxation in copper , Phys. Rev. Lett. 58, 1212-1215. 

BenistyH. (1995), Reduced electron-phonon relaxation rates in quantum-box systems theoretical analysis , Phys. Rev. B 51, 13281-13293. 

The energy of a laser pulse is absorbed by the electrons of the material. The absorption mechanism is different for opaque and transparent samples. Linear absorption is the main contribution in opaque materials (e.g., metals) whereas absorption has to be realized by nonlinear processes in transparent materials (e.g., dielectrics). These nonlinear processes are avalanche and multiphoton ionization [20-25]. After electronic excitation, the energy is transferred to the lattice and heats it up to boiling and/or to vaporization temperature. Then, electrons and lattice are in thermal equilibrium. The transfer time of the electronic energy to the lattice (electron-phonon-relax-ation time) is of the order of picoseconds [26]. 

Pump Power Dependence of the Electron-Phonon Relaxation... 

It is well established that the electronic heat capacity Q is a function of the electron temperature T. . Therefore, the effective rate constant g/CfT ) for the thermal relaxation of the electron gas 80 /87 decreases with increasing laser pump power. This is the reason why the electron-phonon relaxation time increases with increasing electron temperature/laser intensity. 

Figure 16.10 Power dependence of the electron-phonon relaxation time. Part (a) shows the results for a 15 nm spherical gold nanoparticle after 400 nm excitation. The bleach recovery is monitored at the bleach maximum of 520 nm. The decay curves were fitted with lifetimes of 1.5, 2.0, 3.3, and 3.6 ps for excitation powers of 50, 80, 100, and 160nj, respectively. Part (b) shows a plot of the electron-phonon relaxation times against the relative pump power. Extrapolation to zero laser power yields a decay time of 690 + 100 fs for 400 nm excitation. The result for 630 nm excitation is included as well, which features a hmiting decay time of 830 + 100 fs. This result corresponds to an electron-phonon couphng constant of around 2.5 + 0.5 X 10 Wm At high laser powers the bleach also shows a long time component as seen by the offset of the decay curves in (a). This corresponds to the phonon-phonon relaxation, which occurs on a time scale of 100 ps. The amphtude ratio of the phonon-phonon to the electron-phonon relaxation time increases with increasing power, as shown in (c). (Reproduced with permission from S. Link and M. El-Sayed, 1999. J. Phys. Chem. B 103 8410-8426. Copyright 1999 American Chemical Society.)...

The optoacoustic properties of plasmon-resonant gold nanoparticles originate from photoinduced cavitation effects. This process can be summarized as follows (i) thermalization of conduction electrons on the subpicosecond timescale/ (ii) electron-phonon relaxation on the picosecond timescale and thermalization of the phonon lattice, with a subsequent rise in temperature by hundreds to thousands of degrees (iii) transient microbubble expansion upon reaching the kinetic spinodal of the superheated medium, initiated on the nanosecond timescale (iv) microbubble collapse, resulting in shockwaves and other forms of acoustic emission. The expansion and collapse of a cavitation bubble takes place on a microsecond timescale, and are easily detected by ultrasonic transducers. 

Size and shape dependency is very important property in SPR [33, 34] of metal NPs observed in the range between 10 and 100 nm. The optical response of the SP absorption in these metal NPs can be demonstrated by the electron dynamics (electron-electron and electron-phonon scattering). It is found that the electron-phonon relaxation processes in NPs, which are smaller than the electron mean free path (MFP), are independent of their size or shape (Fig. 13.8). 

Second, the initial thermal nonequihbrium between electrons and phonons during ultrafast laser heating of nanoparticles requires special attention. A two-temperature model [75] describing the particle can be used to account for the electron-phonon relaxation time of the particle. Here, the electrons and the lattice of the particle are treated separately and the coupling-that is, the heat transfer from electrons to the lattice-is realized through the electron-lattice couphng factor, g. 

Owing to the fundamental and practical importance of electron-phonon coupling in photovoltaic nanomaterials, it is crucial to understand how the electron-phonon relaxation depends on a variety of factors including material, temperature, nanoparticle size and shape, surface terminations, surfactants, and so on. Recently, a non-adiabatic molecular dynamics approach has been developed to simulate the electron relaxation process in QDs s 60 process has been used to investigate several different sys-... 

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