Femtosecond dynamics of coherent THz phonons in transition metals

Abstract

We investigate dynamics of coherent optical phonons in Zn as a function of lattice temperature. We found that the coherent phonon decays via anharmonic phonon-phonon coupling and that the amplitude of the coherent phonon follows that of the photoexcited electron density under resonant condition. In metals, electron-phonon (e-ph) thermalization occurs on a sub-picosecond time scale and has been traditionally described by the use of the two temperature model (TTM). Since e-ph thermalization in metals proceeds on the sub picosecond timescale (THz frequency range), coherent optical phonons which are impulsively excited and decay within a few picoseconds, may play an important role in relaxation of hot electrons. Recently, Melnikov et al. have observed the surface coherent optical phonon (2.9 THz) in Gd using second harmonic generation (SHG) technique. Furthermore, Bovensiepen et al. studied both surface and bulk coherent optical phonons in Gd using the SHG and the transient reflectivity techniques, respectively. However, investigations of bulk coherent optical phonons in metals using conventional pump-probe techniques are still few, mainly due to the very short optical penetration depth and correspondingly weak changes in the refractive index associated with the coherent phonon oscillations. Here, we report on the observation of bulk coherent optical phonon in Zn using a pump-probe reflectivity technique with high sensitivity of ∆R/R =10. The sample used was a single crystal of Zn with cut and polished (0001) surface. The pump-probe measurements were carried out in a temperature range from 7 to 295 K. The light source used was a mode-locked Ti:sapphire laser (20 fs, 87 MHz) with the pump and probe powers fixed to 60 mW and 5 mW, respectively (spot size was 100 microns). We used a lock-in detection with the pump-beam chopped at 2 kHz. The penetration depth of the laser light with a wavelength of 800 nm was estimated to be 13 nm based on the absorption coefficient of 7.6x10cm. Therefore the contribution to the signal from the surface oxide layer can be considered negligible. Figure 1 shows ∆R/R signal observed at various temperatures as a function of the time delay after excitation pulse. Initial transient non-oscillatory response is due to the excitation and relaxation of nonequilibrium electrons. Since the interband electronic transition near the L-point occurs at around 800 nm, this transition dominates the generation of nonequilibrium electron distribution in Zn. The coherent E2g phonon, whose frequency ω0 is 2.3 THz at 295K, is clearly observed. The frequency and the line width of the Fourier transformed spectra of the E2g mode depend on the temperature as shown in the inset to Fig.1. In order to estimate the damping of the coherent optical phonon precisely, time-domain oscillatory signals after electron thermalization (t> 600 fs) are fitted using a damped harmonic oscillator formula. The damping rate of the coherent optical phonon determined this way is shown in Fig. 2 as a function of temperature, together with the mode frequency. The damping rate increases upon increasing the temperature. This behavior is well accounted for the anharmonic decay model, in which the optical phonon decays into the two acoustic phonons with half the frequency of the optical mode and with opposite wavevectors,