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Liquid water producing terahertz waves confirmed
Liquid water has the property of absorbing terahertz light waves, and thus it has been considered impossible to act as a light source for terahertz waves. But recently, Zhang Xicheng, a special professor at Capital Normal University, led the team to use the femtosecond laser pulse to prove for the first time that liquid water can also produce terahertz waves. This important research, published in the latest issue of Applied Physics Letters, will provide a new possibility for terahertz waves to be widely used in wireless data transmission, industrial quality control and high-definition imaging.
Terahertz waves, also called far infrared waves, are electromagnetic waves with a frequency in the range of 0.1 to 10 terahertz. Due to the high frequency and short pulse, the terahertz wave time and space resolution are very high, and the terahertz energy is very small, which will not damage the material, so compared with X-ray, terahertz imaging technology and spectrum The technology has more advantages and has broad prospects in broadband communication, medical imaging, non-destructive testing, safety inspection, and selection of grain species.
There are four states of matter: solid, gaseous, liquid, and plasma. Previous studies have shown that solid, gaseous, and plasma materials can be used to generate terahertz waves, but terahertz waves from liquid materials have not been proven.
In the new study, the team of Zhang Xicheng creatively utilized a free-flowing layer of ultra-thin water film (less than 200 microns thick) to successfully produce terahertz waves from liquid water, thereby enclosing the liquid material into the terahertz source. They focus the femtosecond laser pulse into the water film, ionizing the water molecules, generating free electrons, and eventually emitting terahertz waves.
The researchers also found that terahertz waves from liquid water exhibit completely different characteristics than other terahertz sources such as air plasma previously discovered. For example, for air plasma, the shorter the duration of the laser pulse, the more terahertz waves are generated, and the liquid water is exactly the opposite. The longer the laser pulse lasts, the more terahertz waves are produced by liquid water. In addition, the intensity of the terahertz wave generated by the liquid water is related to the polarization of the laser beam, and the polarization of the laser beam hardly affects the intensity of the terahertz wave generated by the air plasma.
Zhang Xicheng said that these observations cannot be explained by the existing terahertz wave mechanism. Further study of the interaction between laser and liquid water can help reveal the physical principles behind these new phenomena and extend the application range of terahertz waves.
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