In recent decades, terahertz wave has become an important research topic in the physical field because of its wide application prospect. Terahertz radiation is an electromagnetic wave with a frequency ranging from 0.1 to 10 THz. Such wave band, with abundant information, high spatial-temporal coherence, low photon energy, etc., is located between the microwave and the infrared wave, and is of great application values in astronomy, biology, computer science, communications and other applications. At present, the main application researches include terahertz time-domain spectroscopy, terahertz imaging, security inspection, terahertz radar, astronomy and communications.
At present, the generation of terahertz waves by laser filament in air is a relatively common, simple and reliable method. The principle of the method is that a laser with a wavelength of 800 nm is partially converted into a laser with a wavelength of 400 nm when being converged to pass through a BBO frequency doubling crystal. According to the relevant nonlinear properties of lasers, when the pulses of the two laser beams are overlapped, a strong terahertz wave can be radiated from the ionized gas medium.
Since two laser beams with different wavelengths have different refractive indexes and propagation speeds when propagate in a medium, and the pulse widths of the two laser beams are extremely small, the time for the two laser beams overlapping to generate strong terahertz waves is very short. When the pulses of the two laser beams are gradually separated from each other along with their propagation, the generated energy of terahertz wave signals will decrease quickly. The main advantages of the laser-filament method are that the obtained terahertz waves have a broad bandwidth, the device is convenient to build with a small occupying space, and the overall cost is relatively low. However, there is also a problem that the coherence length is short (usually on the order of millimeter), leading to ineffectively utilization of the laser energy during a long propagation distance.
In addition, the air contains water vapor, which has a stronger absorption for the terahertz waves. Therefore, the temperature, humidity and other environmental conditions have a significant impact on the generation, detection, and collection of the terahertz waves.
All of these problems greatly reduce the efficiency and cost performance of generating the terahertz waves.