Ultrafast Control of the Polarity of ${\mathrm{BiCoO}}_3$ by Orbital Excitation as Investigated by Femtosecond Spectroscopy

${\mathrm{BiCoO}}_3$ is a perovskite-type cobalt oxide with a polar structure. We investigate the dynamics of its polar state by using photoexcitation with femtosecond visible and terahertz pulses. The intensity of the second-harmonic light, caused by a polar structure without inversion symmetry, is enhanced by more than 50% by irradiating the terahertz pulse showing an ultrafast response following the femtosecond terahertz pulse. By contrast, it is suppressed by more than 60% by exciting visible light with a 100-fs pulse width at room temperature. These results suggest not only an important role of the orbital excitation in the ${\mathrm{Co}}^{3+}$ 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 a femtosecond.

Figure 1

(a) The crystal structure and (b) the spin configuration of ${\mathrm{BiCoO}}_3$ after Ref. [10]. (c) Schematics of the terahertz pump and SHG probe measurement system, in which $M$, $L$, and HPS are the mirror, the lens, and the high-pass filter, respectively. The terahertz pump pulse is focused onto the sample using the parabolic mirror, and the 800-nm probe pulse with the lens through the parabolic mirror. The generated SH pulse is introduced to the monochromator and is detected by the photomultiplier.

Figure 2

(a) Delay time ($t_d$) dependence of relative change of the reflectivity ($\mathrm{\Delta }R/R$) at 0.25 and 0.8 eV and (b) second-harmonic intensity ($\mathrm{\Delta }{I}_{\text{SH}}/I_{\text{SH}}$) after photoirradiation of an approximately 2.6-eV pulse. (Inset) The transient reflectance spectrum just after the photoirradiation ($t_d\approx 0\mathrm{ps}$). The fluence of the pump pulse is about $8.2\mathrm{mJ}/{\mathrm{cm}}^2$.

Figure 3

(a): Time profiles of the relative change of the second-harmonic intensity ($\mathrm{\Delta }{I}_{\text{SH}}/I_{\text{SH}}$) with several different terahertz pulses. The color of each profile corresponds to that of the terahertz pulse shown in (b). (b) Waveforms of the terahertz field used for the photoexcitation in the time domain. In the orange, green, and blue profiles, intensities of the field are $3/4$, $1/2$, and $1/4$ that of the red profile, respectively.

Figure 4

Normalized maximum value of the second-harmonic change at about 0 ps ($\mathrm{\Delta }{I}_{\text{SH}}^{\text{peak}}$) as a function of the peak value in the terahertz electric field ($E^{\text{peak}}$). (Inset) Time profiles of $\mathrm{\Delta }{I}_{\text{SH}}/I_{\text{SH}}$ and the square of the applied terahertz field ($|E_{\text{THz}}{|}^2$).