Observation of coherent phonons in strontium titanate: Structural phase transition and ultrafast dynamics of the soft modes

Abstract

Coherent phonons were observed in SrTiO 3 by using ultrafast polarization spectroscopy. The ultrafast dynamics and softening of phonon modes, which contribute to the structural phase transition at 105 K, are studied. The temperature dependences of the phonon frequency and the relaxation rate are obtained from the observed damped oscillation of coherent phonons. The observed phonon relaxation is explained well by a population decay due to anharmonic phonon-phonon coupling for the soft mode phonons.

Temperature dependent photoexcited carrier dynamics in multiferroic BiFeO3 film: a hidden phase transition

“…Figure 3(a) shows an abnormal (slowing down) temperature dependence of DR (t ¼ 0 ps) at zero time delay, and it is seen that the anomalies occur with beginning at T 1 $ 137.5 K and ending at T 2 $ 195 K. A maximum photo-transition probability appears at around 160 K. The arising of the R peak at around 100 K could be attributed to the structural phase transition of the SrTiO 3 substrate. 30 Apart from this instantaneous amplitude, the onset of the two anomalies is further observed in the extracted time constants, as shown in Fig. 3(c).…”

“…The temperature range encompasses the low temperature phase transitions of BFO film but was set right beyond the structure phase transition temperature of SrTiO 3 (103 K). 30 Figure 2(a) shows the typical transient reflectivity DR(t)/R at temperatures of 294 K and 160 K, respectively. Electronic photo-excitations by the pump pulses result in a swift rise of DR(t)/R at zero time delay.…”

“…21 In general, as the temperature is lower than the one of a succession of phase transitions, besides the conventional electron-phonon relaxation, additional scattering channels will modify the charge transport and relaxation dynamics. 22,30 For better design flexibility, the larger ferroelectric polarization, more steerable magnetism, BFO films have caught more attention. 5,25,26 In our previous work, we have observed the electron-phonon interaction of BiFeO 3 thin film at room temperature.…”

Excitation and Time‐Evolution of Coherent Optical Phonons

“…At frequencies close to the resonance, the polarization have strong phonon character and the polariton dispersion is very flat, which implies that the polaritons will propagate very slowly. Temperature-driven phase transitions were monitored by observing coherent phonon-polaritons in these perovskite ferroelectrics using four-wave mixing [50] and ultrafast polarization spectroscopy [51]. The former technique enables the wave vector-selected generation of phonon-polaritons by varying the angle between the two pump beams.…”

Observation of coherent phonons in Ca-doped SrTiO3: Doping-induced ferroelectric phase transition and ultraviolet-illumination effect

“…The waist size of the beams at the sample is about 0.5 mm. Coherent phonons are created by femtosecond optical pulses through the process of impulsive stimulated Raman scattering, and are detected by monitoring the timedependent anisotropy of refractive index induced by the pump pulse [9]. The source of UV illumination is provided by the second harmonics (380 nm, 3.3 eV) of the output from another modelocked Ti:sapphire laser, whose energy is larger than the optical band gap of SrTiO 3 (3.2 eV),…”