Femtosecond laser-induced air plasma can generate terahertz waves. The sawtooth-like asymmetric photoelectric field can be made by the superposition of three-color laser with appropriate amplitude ratio, frequency ratio and phase difference. Based on the transient photocurrent model and numerical simulation, it is found that intense terahertz waves can be generated by the sawtooth-like asymmetric photoelectric field with different frequency ratios of three-color femtosecond laser, such as 1:2:4 and 1:2:6. By investigating time-varying parameters, such as electron density, electron velocity and photocurrent density, it was further demonstrated that the shape of the asymmetric photoelectric field plays an important role in the formation of net photocurrent and the generation of terahertz waves. The investigation will be helpful to the development of key techniques on intense terahertz waves generated by laser-induced air plasma.
In order to satisfy needs which are efficient detection and miniaturization of terahertz scanning imaging instrument, a rectangular wave-guide antenna with lengthwise slits on the broad face is designed. The antenna has the advantages which are high aperture efficiency, compact structure, small volume, light weight, low cost, low side-lobe performance, narrow beam scanning and so on. At first, these parameters include slit length, slit distance, slit width and slit offset along the central axial of the broad face are calculated with the aid of related theory, then the antenna is modeled and optimized in the software. The simulation results show that the antenna can directly transmit and receive the terahertz wave at 0.1THz, the gain of the main lobe is higher, the gain of the side lobe is lower and main lobe presents the pencil beam. The results can provide reference for the revised design of terahertz scanning imaging instrument.
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