Thomas P. Dougherty

Femtosecond time-resolved spectroscopy of polarization dynamics in KNbO3

Femtosecond time-resolved impulsive stimulated scattering experiments were conducted on KNbO3 at temperatures spanning the orthorhombic phase. A heavily damped soft optic phonon mode associated with the displacive character of the transition and relaxational modes associated with order-disorder character were examined. Previous frequency-domain light scattering experiments had yielded conflicting results for the dynamics of the soft mode and the symmetry of the order-disorder motions. We find the relaxational motion to be of A1 symmetry and the soft mode to be the only low-frequency exictation of B2 symmetry. The soft mode is well described by a damped polariton model without any coupling to a relaxational mode. These results are consistent with a previously proposed eight-site model for the phase transitions.

Examination of order-disorder and soft modes in perovskite ferroelectrics by impulsive stimulated Raman scattering

The dynamics of low frequency soft modes and order-disorder modes in the orthorhombic phase of KNbO3 and the tetragonal phase of BaTiO3 have been examined using a time-domain technique, impulsive stimulated Raman scattering (ISRS). There was no evidence in either crystal for an order-disorder mode with the same symmetry as the soft mode. Order-disorder modes were found to have different symmetry from the soft modes. Excellent agreement was found in both crystals between capacitance measurements and the LST value of the component of the low frequency dielectric constant which depends on the soft mode frequency. These results are consistent with the predictions of the eight-site model for the crystals and their transitions.

Femtosecond impulsive stimulated Raman scattering studies of LiTaO3

Abstract Impulsive stimulated Raman scattering (ISRS) experiments on LiTaO3 were conducted over a wide range of scattering wavevectors. The results permit explanation of the anomalously high and wavevector-dependent damping rates of the low-frequency phonon-polariton mode in terms of coupling of the polariton to a weakly Raman-active low-frequency relaxational mode and to two heavily damped vibrational modes. ISRS experiments at low temperatures were made possible through the use of multiple-pulse ISRS experiments where photorefractive damage precluded experiments with single excitation pulses.

Anomalous polariton dynamics in LiTaO3 polariton dynamics in LiTaO3

The techniques of single and multiple-pulse impulsive stimulated Raman scattering (ISRS) are used to explore the dynamics of the lowest frequency A1 polariton mode in the ferroelectric crystal lithium tantalate. This time-domain technique which allows unambiguous characterization of the frequency and damping dispersion of the polariton reveals anomalously high damping rates for the A1 mode. The damping behavior is explained by a model that includes coupling of the polariton to a weakly Raman-active relaxational mode and to two E-symmetry modes.

Impulsive stimulated scattering study of the coupled acoustic and soft modes in KD2PO4

Abstract The temperature and wave vector dependence of the soft shear acoustic mode and the soft polarization mode of KD2PO4 are examined in the high-temperature phase through impulsive stimulated Brillouin and Raman scattering. With different scattering wave vectors the regimes ωτ 1 are probed where ω is the acoustic frequency and τ is the polarization relaxation time. The results illustrate how the coupled acoustic and polarization response may be observed in the time domain, and indicate advantages for the time-domain approach in the ωτ τ 1 regime. In this regime the softening of the acoustic mode is followed by hardening (increasing of the frequency) as the transition temperature is approached from above. On femtosecond time scales, it is possible to test critically for any inertial component to the polarization response, which would indicate an overdamped oscillator rather than relaxational description of the polar optic phonon mode.

Polarization dynamics in ferroelectric perovskite crystals

The polar low frequency vibrational and relaxational modes crucial to understanding the structure and phase transitions of ferroelectric perovskite crystals are examined using impulsive stimulated Raman scattering (ISRS). At wavevectors near the Brillouin zone center, these modes are strongly coupled to light, and the pure optic mode behavior is deduced from the wavevector dependence of the mixed phonon-polaritons.

Femtosecond Time-Resolved Observation of Soft Mode Dynamics in Ferroelectric Crystals

Despite extensive study of the barium titanate (perovskite) family of ferroelectrics, the dynamics and microscopic mechanism for the phase transitions between the paraelectric cubic phase and the tetragonal, orthorhombic, and rhombohedral ferroelectric phases remain in doubt. From a dynamical point of view, the degree to which collective motion of the order parameter, i.e. the polarization, is vibrational (phonon-like) in character versus relaxational (hopping dynamics) is in dispute due to the conflicting results of recent low-frequency Raman spectroscopy studies [1,2]. In terms of microsopic mechanism, relaxational dynamics are indicatative of an order-disorder transition involving collective hopping of ions between different crystalline arrangements. Vibrational dynamics indicate a displacive transition involving motions of ions within a single free energy region whose minimum position shifts with temperature. The vibrational component of the order parameter is often very heavily damped (sometimes overdamped) and so Raman spectral responses are central peaks which merge with relaxational responses. It is often difficult to discern and analyze quantitatively the vibrational and relaxational dynamics.