Color Changing with Polar Oxide Materials

Second harmonic generation allows for conversion of incident red light into blue light. Irradiation with a terahertz laser pulse can significantly enhance the SHG effect in BiCoO3. (Source: Titech)

One popular method to change the color of laser light is the second harmonic gene­ration (SHG) effect, which doubles the frequency of light. However, observing this nonlinear effect requires a polar crystal in which inversion symmetry is broken. For this reason, iden­tifying crystals that can elicit strong SHG has been an important research topic, so that their pro­perties can be exploited for applications in materials science.

Observation of nonlinear optical phenomena like SHG requires a finite second-order electric suscep­tibility, which occurs within any polar structure without inversion symmetry, and strong laser light or pulses. In the perov­skite-type cobalt oxide BiCoO3 used, an apical oxygen shift along the c-axis and a Co-O5 pyramid are present in the unit cell, resulting in symmetry breaking and a large spontaneous pola­rization at room tempera­ture. For the laser pulse, the strong electro­magnetic wave with an electric field up to ~1 MV/cm in the THz energy region was developed by Hideki Hirori and his team at iCeMS and was used to achieve ultra­fast control of BiCoO3‘s nonlinear behavior.

Yoichi Okimoto at Tokyo Institute of Tech­nology and colleagues were speci­fically interested in under­standing how the intensity of SHG from the BiCoO3 crystal changes when irra­diated with a THz laser pulse at room temperature. Notably, an unpre­cedented enhance­ment of SHG by more than 50 % was observed, indi­cating that employing THz laser light in this fashion can signi­ficantly improve the figure of merit of nonlinear crystals. In addition, this effect occurs on the 100 femto­second time scale, sugges­ting possible appli­cation to ultra­fast opto­electronic devices.

Mechanis­tically, the ultrafast enhance­ment of the second harmonic signal can be under­stood in terms of d-d transitions from occupied to unoccupied states that exist around the wide energy band of the applied THz pulse. The photo­excited electrons elongate the apical oxygen atoms of the Co-O5 pyramids in the crystal structure via electron-phonon coupling, thereby aug­menting its polar structure and hence second-order electric suscep­tibility. Future inves­tigations of the photo­excited state of BiCoO3 and other polar oxide materials will consider higher-order non­linear optical responses as well as ultra­fast structural measure­ments using the THz pulse to eluci­date addi­tional mecha­nistic details of these fas­cinating materials. (Source: Titech)

Reference: Y. Okimoto et al.: Ultrafast Control of the Polarity of BiCoO3 by Orbital Excitation as Investigated by Femtosecond Spectroscopy, Phys. Rev. Applied 7, 064016 (2017); DOI: 10.1103/PhysRevApplied.7.064016

Link: Dept. of Chemistry, Tokyo Institute of Technology, Tokyo, Japan

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