Abstract-Electrically Controllable Terahertz Second‐Harmonic Generation in GaAs

Joo Kang, Won Tae Kim, Hyeon‐Don Kim, Soojeong Baek, Kwang Jun Ahn, Bumki Min,Fabian Rotermund

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.202000359?af=R

Terahertz radiation and its nonlinear optical manipulation may possess potential for a variety of applications in next‐generation electronics and optics. Pioneering studies have shown that the nonlinearity of carrier drift in semiconductors and graphene can be utilized for nonlinear optical processes at terahertz frequencies. However, because of the symmetric response of carriers to the terahertz field direction, most experiments have confirmed only the presence of odd‐order nonlinear processes. In this study, electric‐field‐induced terahertz second‐harmonic generation (SHG) in photoexcited gallium arsenide is demonstrated, where an applied bias field breaks the directional symmetry of the drift transport of electrons. The amplitudes of odd‐ and even‐harmonic waves are found to be highly controllable using the bias field. The measured conversion efficiency of SHG reaches beyond 10−5, substantially higher than the value previously reported. This terahertz harmonic generation platform with electrical controllability may be useful for future nonlinear applications at terahertz frequencies.

Abstract-Polarization switching in ferroelectric thin film induced by a single-period terahertz pulse

Elena Mishina,  Kirill Grishunin, Vladislav Bilyk, Natalia Sherstyuk, 

https://www.cambridge.org/core/journals/mrs-advances/article/polarization-switching-in-ferroelectric-thin-film-induced-by-a-singleperiod-terahertz-pulse/91CA5CE74306D2C39DC594B0BE31C92E

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.

COPYRIGHT: © Materials Research Society 2018 

Changing the color of laser light on the femtosecond time scale

How BiCoO3 achieves second harmonic generation

https://www.sciencedaily.com/releases/2017/06/170614091737.htm

SUMMARY-Using femtosecond visible and terahertz (THz) pulses as external perturbations, scientists have investigated the second harmonic generation effect in photoexcited BiCoO3. Driven by the THz pulse, this research highlights the importance of orbital excitation in the Co3+ ion and provides clues for improving the performance of nonlinear optical phenomena in nonlinear crystals on the femtosecond time scale.

How can the color of laser light be changed? One popular method to achieve this is the so-called second harmonic generation (SHG) effect, which doubles the frequency of light and hence changes its color. 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 in this work, 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 (i.e., far-infrared) 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 (10-13 s) 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.

---

Story Source:

Materials provided by Tokyo Institute of Technology. Note: Content may be edited for style and length.

---

Journal Reference:

1. Y. Okimoto, S. Naruse, R. Fukaya, T. Ishikawa, S. Koshihara, K. Oka, M. Azuma, K. Tanaka, H. Hirori. Ultrafast Control of the Polarity of BiCoO3 by Orbital Excitation as Investigated by Femtosecond Spectroscopy. Physical Review Applied, 2017; 7 (6) DOI: 10.1103/PhysRevApplied.7.064016