Ultrafast Control of the Polarity of BiCoO3 by Orbital Excitation as Investigated by Femtosecond Spectroscopy

Abstract

BiCoO 3 is a perovskite-type cobalt oxide with a polar structure. We investigated dynamics of its polar state by using photoexcitation with femtosecond visible and THz pulses. The intensity of the second harmonic light, caused by a polar structure without inversion symmetry, was enhanced by more than 50% by irradiating the THz pulse showing ultrafast response following the fs THz pulse. In contrast, it was suppressed by more than 60% by exciting visible light with 100fs pulse width at room temperature. These results suggest not only an important role of the orbital excitation in the Co 3+ ion but also a key to improving the nonlinear optical response, e.g., a figure of merit in nonlinear crystals on the time scale of femtosecond.

Picosecond dynamics in layered cobalt perovskites studied by time-resolved Raman spectroscopy

“…Some SC materials, especially Fe 2+ complexes are known to show the perpetual photoinduced spin state transition between the HS and LS state in several iron complexes [1], and furthermore, the detailed dynamics of SC has also recently been investigated using a femtosecond (fs) laser and/or X-ray pulses [14,15]. From this respect, we have conducted ultrafast spectroscopy on several cobalt perovskites and so far demonstrated some works concerning the photoinduced electronic change involved in the SC phenomena [16][17][18]. One example is the LSCO system [19], on which we conducted time-resolved fs reflection spectroscopy and revealed that photoirradiation with 400 nm laser pulses could realize an exotic photoexcited state that is different from the thermally induced HS state.…”

Ultrafast polarization control by terahertz fields via π-electron wavefunction changes in hydrogen-bonded molecular ferroelectrics

“…First, we discuss the results of terahertz-pulse-pump SHG-probe measurements (Fig. 1b ), which detect the dynamics of polarization changes by an electric-field pulse 16 – 20 . Before the measurements, a static electric field F s of ±30 kV/cm (| F s | > F c ) is applied along the c axis to align the polarization and then it is released.…”

Ferroelectric Polarization Reversal vs Pump Spot Shape

“…The original proposal to switch the polarization by resonant excitation of the ferroelectric mode [11][12][13] has not yet been fully realized experimentally [14][15][16][17]. It was also shown that optical pumping with excitation of the electronic subsystem [18][19][20][21] leads only to a temporary decrease in polarization [22][23][24][25]. Following the rapid development and achievements of nonlinear phononics [26][27][28][29], it was proposed to pump the IR-active optical phonon mode, which is nonlinearly coupled with the ferroelectric soft mode [30].…”

“…In experiments with optical pumping [23][24][25], the penetration depth of the pump pulse varies from about 3 nm [25] upto 100 nm [24], depending on the crystal used. In this case, the depolarization factor should already be very small, and the depolarizing field is strongly reduced.…”