Transient Second Harmonic Generation Induced by Single Cycle THz pulses in Ba0.8Sr0.2TiO3/MgO

Abstract

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.

Resonant Excitation of the Ferroelectric Soft Mode by a Narrow-Band THz Pulse

“…It should be noted that for a correct comparison, the obtained frequency dependences were successively normalized to the probe power, film thickness with regard to the coherence length, and electric field of THz radiation. When performing FFT, we limited the consideration of temporal dependences by 17.5 ps (as in Figure 3 ) to avoid the distortion of the spectra by the reflection of THz pulses from the backside of the substrate as well as other effects of the pulse propagation [ 32 ]. The coherence length was estimated using the expression l coh = λ ω /4( n ω ± n 2ω ), where n ω and n 2ω are the refractive indices for the fundamental and SHG waves, and + (−) refers to the reflection (transmission) geometry, respectively.…”

“…The dynamics of a nonlinear optical response in BST films when they are excited by narrow-band THz pulses were investigated using a modified experimental setup described in [ 32 ].…”

Transient Polarization Reversal using an Intense THz Pulse in Silicon‐Doped Lead Germanate

“…Recently, it became known [ 20–22 ] that the oscillations observed in pump–probe experiments can be associated with propagation effect for mutually propagating THz pump and optical probe pulses. Then, based on the difference in the group velocities in material, where consideration of n 2ω is essential for the SHG detection, the frequency of these oscillations can be estimated.…”

Ultrafast Polarization Switching in BaTiO3 Nanomaterials: Combined Density Functional Theory and Coupled Oscillator Study

“…The closest successful result to date, which involved reversible polarization change, was achieved by Mankowsky et al in their work on lithium niobate . Other studies − have also explored the selective excitation of lattice vibrations under ultrashort optical or THz pulses, which are essential for achieving practical polarization switching. The absence of a predictive model poses a significant obstacle to experimentally observing ultrafast switching of electric polarization.…”