image: Fig. 1 Variation of the output THz pulse energy as a function of (a) pumping laser energy in different gases and (b) tilting angle of -BBO in argon. The top axis of (b) presents the time delay between the dual-color laser beams induced by the tilting angle of -BBO. view more
Credit: OEA
A new publication from Opto-Electronic Advances; DOI 10.29026/oea.2022.210065 considers an over 20 mJ THz laser pulse generated at 1 kHz in gas media.
Terahertz (THz) science and technology have received extensive attention from scientific researchers from all over the world in the past 20 years because of its prospective application potential in security imaging, medical diagnosis, military, wireless communications and astronomy. However, the development of high-power broadband THz radiation source has been challenging task in the above-mentioned fields.
Among various THz radiation sources, THz radiation source based on femtosecond laser filament has the advantages of broadband (~200 THz), high amplitude (100 MV/cm) and no limitation of damage threshold. In addition, the THz generation method based on the femtosecond laser filamentation confines the THz wave inside the filament, which can eliminate the diffraction and absorption during the propagation of the THz wave in the atmosphere and make the remote delivery of the THz wave become possible.
The THz generation scheme based on the dual-color femtosecond laser filamentation has higher energy conversion efficiency than that using single-color femtosecond laser. In this scheme, the intensity, bandwidth, polarization and other characteristics of the THz radiation can be affected by many laser parameters including the temporal delay, dispersion, polarization, wavelength, spatial departure of the dual-color fields. Even the ambient gas species plays crucial role too. In order to develop an efficient THz radiation source, all these parameters need to be carefully designed and manipulated.
The research group led by Prof. Weiwei Liu from Nankai university used femtosecond laser with single pulse energy of 6 mJ to generate the dual-color laser filamentation by frequency doubling the fundamental laser via a b-BBO crystal. The dual-color laser beams achieve the perfect spatial-temporal overlap through a tilted a-BBO crystal. Meanwhile, a dual-wavelength plate was used to make the dual-color laser beams have the identical polarization. The energy of the THz pulse generated from the laser filament in argon can be up to 21 μJ and the corresponding THz conversion efficiency reaches 0.35%.
In this work, the ambient gas species’ influence on the THz generation efficiency by the dual-color laser filamentation was investigated experimentally (Fig. 1a). The experimental results show that the highest conversion efficiency of THz radiation is achieved in argon gas. The relationship between the tilting angle of a-BBO and the generated THz power in argon was also investigated (Fig. 1b). a-BBO with optimal tilting angle and pre-designed thickness can simultaneously compensate the time delay and spatial walk-off of the two-color laser beams, playing the critical role in improving the generation efficiency of THz wave. This research work achieved a breakthrough in the energy conversion efficiency of THz wave generated by the two-color femtosecond laser filamentation, which is of great significance for the study of high-intensity THz sources and the exploration of the interaction between strong THz beam and materials.
Article reference: Yu ZQ, Zhang N, Wang JX, Dai ZJ, Gong C et al. 0.35% THz pulse conversion efficiency achieved by Ti:sapphire femtosecond laser filamentation in argon at 1 kHz repetition rate. Opto-Electron Adv 5, 210065 (2022). doi: 10.29026/oea.2022.210065
Keywords: femtosecond laser filaments / walk-off effect / conversion efficiency
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The research interests of the group led by Prof. Weiwei Liu from Nankai University focuses on ultrafast laser optics and application. Particularly, in a new sub-field of nonlinear optics, namely, femtosecond (fs) laser filamentation nonlinear optics. The group has carried out original and pioneering experiments discovering many unique and important phenomena. These results have found promising applications in intense THz wave generation, remote sensing of air pollutant and novel imaging technique.
Prof. Weiwei Liu has led more than 30 research projects financially supported by the National Key Research and Development Program, National manned space program, National Natural Science Foundation of China, etc. He has published more than 200 scientific papers in refereed journals. The total citation number is more than 9000 and H index reaches 49 (Google scholar). Prof. Liu has won the First Prize of Optical Optical Science and Technology Award of Chinese Optical Society, the First Prize of Science of Tianjin Natural Science and etc. The research works have been widely reported by China Central Television (CCTV), Tianjin Television and other media, and also documented by the Centennial Anniversary Achievement Exhibition of Nankai University. Prof. Weiwei Liu has recently been elected as the Optica (former OSA) fellow.
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