The second harmonic effect enables the conversion of incident red light into blue light. The terahertz laser pulse irradiation can significantly increase the frequency doubling effect of ceramics. How to change the color of laser pointer? A popular method is to use the so-called second harmonic (SHG) effect, which doubles the frequency of light, thereby changing its color. However, if such a nonlinear effect is to be achieved, a polar crystal is required, and the inverse symmetry of the structure therein is destroyed. Therefore, the discovery of crystals that can cause strong frequency doubling is an important research topic in materials science.
Observation of nonlinear optical phenomena such as second harmonic effects requires a finite second-order polarization rate in the structure, which occurs in any polar structure without the need for symmetric inversion and strong laser pulses. In this work, a perovskite cobalt oxide is used, in which an apex angle oxygen atom moves along the c axis, forming a conical structure in the molecular structure, resulting in an asymmetric structure, and spontaneous polarization at room temperature .
For the laser pulse, it realizes a strong electromagnetic wave in the terahertz energy region with an electric field strength of about 1 MV per centimeter. It was developed by his team using international motors and systems, and achieved nonlinear ultrafast control of the crystal.
Colleagues at Tokyo Institute of Technology are particularly interested in the change in the intensity of the second harmonic of the crystal under terahertz irradiation at room temperature. It is worth noting that the second harmonic effect has been unprecedentedly enhanced by more than 50%, which shows that using this method of terahertz green laser pointer can significantly improve the advantages of nonlinear crystals. In addition, this effect occurs on a time scale of 100 femtoseconds (10-13 seconds), which may be applicable to some ultra-high-speed optoelectronic devices.
From the physical mechanism, the ultra-fast enhancement of the second harmonic signal can be regarded as the excessive transition from the occupation of the electron hole to the non-occupancy of the d-d transition process, and the wide energy band of the terahertz pulse exists. The photo-excited electrons elongate the oxygen atoms at the top of the conical structure in the crystal due to the photoelectric effect, thereby increasing the polarity of its structure (thus forming a second-order polarizability). In future research, the research focus will be on the high-order nonlinear optical response of other polar oxide materials in the light-excited state, and the use of terahertz pulses to achieve rapid measurement of the mechanical details of these special materials.