The ability to switch ferroics (magnets, ferroelectrics, multiferroics) between two stable bit states is the main principle of modern data storage technology. Due to many new ideas, originating from fundamental research during the last 50 years, this technology has developed in a breath-taking fashion. Ever increasing demands for faster and more energy efficient data storage strongly motivate fundamental studies of dynamics in ferroics triggered by ultrashort stimuli. It has been recently realized that nearly single cycle intense THz pulses and the phenomenon of the second harmonic generation are appealing tools for excitation and detection of poorly understood ultrafast dynamics of electric polarization in ferroelectrics at the picosecond timescale. Here we investigate picosecond dynamics of second harmonic from near-infrared pulse in ferroelectric heterostructure Ba0.8Sr0.2TiO3/MgO triggered by the electric field of a nearly single cycle intense THz pulse. The dynamics of the nonlinear optical signal is characterized by a step and oscillations at the frequency of about 1.67 THz. Although the observations can be mistakenly interpreted as oscillations of the electric polarization at the frequency of the soft mode and switching of the order parameter to another metastable state, here we show that the THz modulation of second harmonic generation in Ba0.8Sr0.2TiO3/MgO has a purely optical origin. The observation can be explained assuming that the THz pulse is a relativistically propagating inhomogeneity which induces center of symmetry breaking and linear birefringence. Our work reveals the role of propagation effects in interpretation of time-resolved non-linear optical experiments and thus it has important implications for experimental studies of ultrafast dynamics in ferroics.
We report here an experimental study of ultrafast response of the dielectric polarization in (Ba0.8Sr0.2)TiO3 thin films to a strong electric field of a nearly single-cycle THz pulse. The phenomenon of Second Harmonic Generation (SHG) is used as a probe of the polarization in the terahertz pump-optical probe experiment. SHG loops for THz pulses of different amplitudes were obtained. The SHG response is modelled assuming that the ferroelectric material is split into 180-degree domains. It is shown that intuitive model based on forced harmonic oscillator does not fully describe to the observed ultrafast ferroelectric response
Objectives. Advances in laser physics over the last decade have led to the creation of sources of single-period electromagnetic pulses having a duration of about 1 ps, corresponding to the terahertz (THz) frequency range and a field amplitude of several tens of MV/cm. This allows the electrode-free application of an electric field to a ferroelectric for observing not only the excitation of coherent phonons, but also ultrafast (at the sub-picosecond timescale) dynamic polarization switching. To detect polarization switching, a pump-probe technique is used in which a THz pulse is used with an optical probe. Since its intensity is proportional to the square of the polarization, the signal of the optical second harmonic is used to measure polarization switching under the action of a THz pulse. To evaluate switching efficiency, both linear (refractive index and absorption coefficient) and non-linear optical characteristics (quadratic and cubic susceptibilities) are required. For any application of ferroelectric crystals in the THz range, knowledge of the relevant linear optical characteristics is also necessary.Methods. The technique of THz spectroscopy in the time domain was used; here, a picosecond THz pulse transmitted through the crystal is recorded by strobing the detector with a femtosecond optical pulse. The THz-induced dynamics of the order parameter in a ferroelectric was studied by detecting the intensity of a nonlinear optical signal at the frequency of the second optical harmonic.Results. The transmission of a THz wave and the intensity of second harmonic generation on a lead germanate crystal doped with silicon in the time and spectral domains were measured. On this basis, the absorption coefficient dispersion and cubic nonlinear susceptibility were calculated in the range of 0.5-2.0 THz. The presence of a region of fundamental absorption near the phonon modes was confirmed along with a resonant enhancement of the cubic nonlinear susceptibility for two phonon modes Ω1 = 1.3 THz and Ω2 = 2.0 THz.Conclusions. The proposed technique is effective for analyzing the dispersion of the optical characteristics of ferroelectric crystals. The significantly improved spectral resolution (0.1 THz) increases the accuracy of determining nonlinear susceptibility due to the detailed analysis of the linear and nonlinear contributions to the second harmonic intensity.
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