Long-lived optical phonons in ZnO studied with impulsive stimulated Raman scattering

The anharmonic properties of the low-frequency $E_2$ phonon in ZnO were measured using impulsive stimulated Raman scattering. At 5 K, the frequency and lifetime are $(2.9789\pm 0.0002)\mathrm{THz}$ and $(211\pm 7)\mathrm{ps}$. The unusually long lifetime and the high accuracy in the determination of the frequency hold promise for applications in metrology, quantum computation and materials characterization. The temperature dependence of the lifetime is determined by two-phonon upconversion decay contributions, which vanish at zero temperature. Results suggest that the lifetime is limited by isotopic disorder and that values in the nanosecond range may be achievable in isotopically pure samples.

Figure 1

(Color online). Differential change in transmission at 20 K and 292 K showing coherent $E_2$ (LF) oscillations in ZnO. The black rectangle indicates the range of the scan shown in the inset.

Figure 2

Raman line shapes obtained from the time-domain coherent phonon oscillations (circles). The solid lines represent Lorentzian fits to the data. An arbitrary vertical offset was applied to the data for clarity.

Figure 3

Temperature dependence of the Raman line shape FWHM obtained from Lorentzian fits to the frequency-domain data (circles) and time-domain fits to the coherent-oscillation amplitude using a simple decaying exponential (squares); see the text and also Fig. 4. The diameter of the circles and the side of the squares give the vertical errors. Full and dotted lines are fits neglecting, respectively, down- and upconversion processes. Inset: frequency versus temperature. The dotted curve is a guide to the eye.

Figure 4

Average oscillating amplitude over 4 ps segments as a function of the average delay time for the segment. The solid line represents the best fit with a single decaying exponential.