New research femtosecond time scale changes the color of the laser: the laser industry or a new version

How to change the color of the Laser? One popular method is to use the so-called second harmonic (SHG) effect, which doubles the frequency of light and thereby changes its color. However, if this non-linear effect is achieved, a polar crystal is required in which the inversion symmetry of the structure is destroyed. Therefore, the discovery of crystals that cause strong octaves is an important research topic in materials science.

Observation of nonlinear optical phenomena, such as second harmonic effects, requires a finite second-order electrical polarization in the structure that occurs in any polar structure without the need for symmetric inversion and intense laser pulses. In this work, BiCoO3, a perovskite cobalt oxide, is used in which one of the top-corner oxygen atoms moves along the c-axis to form a Co-O5 conical structure in the molecular structure, resulting in an asymmetric structure and at room temperature Spontaneous polarization.

For laser pulses, which achieve strong electromagnetic waves in the terahertz region with electric field intensities of up to about 1 MV per centimeter, Hideki Hirori and his team developed using international motors and systems and achieved nonlinear supercooled BiCoO3 crystals Quick control.

Yoichi Okimoto, a colleague at Tokyo Institute of Technology, is particularly interested in the changes in the second harmonic intensity of a BiCoO3 crystal at terahertz radiation at room temperature. It is noteworthy that the second harmonic effect has been enhanced by more than 50%, which shows that the use of terahertz laser in this way 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 suitable for some very high speed optoelectronic devices.

Speaking from the physical mechanism, the ultrafast enhancement of the second harmonic signal can be reasonably seen in the dd transition, ie from the occupation of the electron hole to the unoccupied excess, within a wide band of terahertz pulses. Photoexcited electrons elongate the top oxygen atoms of the Co-O5 cone structure in the crystal due to the photo-electric effect, thereby increasing the polarity of the structure (thus forming a second-order electrode polarization).

Future research will focus on the higher-order nonlinear optical responses of photo-excited BiCoO3 and other polar oxide materials and the use of terahertz pulses for rapid measurement of mechanical details of these special materials.