Can you imagine that sound travels in the same way as light travels? A research team from the City University of Hong Kong, China discovered a new type of sound wave: sound waves in the air vibrate laterally and carry spin and orbital angular momentum like light. This discovery broke the scientists’ previous view of sound waves and opened a way for the development of new applications in acoustic communication, acoustic sensing and imaging.
This research was initiated and co-led by Dr. Wang Shubo (transliteration), assistant professor of the Department of Physics, City University of Hong Kong, and was carried out in collaboration with scientists from Hong Kong Baptist University and Hong Kong University of Science and Technology. The research was published in Nature Communications.
Go beyond the traditional understanding of sound waves
Physics textbooks tell us that there are two kinds of waves. In transverse waves like light, the vibration is perpendicular to the direction of wave propagation. In a longitudinal wave like sound, the vibration is parallel to the direction of wave propagation. But the latest discovery of scientists from City University of Hong Kong has changed people’s understanding of sound waves.
“If you talk to a physicist about lateral sound in the air, he/she will think that you are a layman without college physics training, because the textbook says that the sound in the air (that is, the sound that propagates in the air) is one Plant longitudinal waves,” said Dr. Wang Shubo. “Although air-borne sound is usually a longitudinal wave, we have shown for the first time that it can be a transverse wave under certain conditions. And we have studied its spin-orbit interaction (an important property that only exists in transverse waves). , The coupling between the two angular momentums. This discovery provides a new degree of freedom for sound manipulation.”
Dr. Shubo Wang explained that there is no shear force in the air or fluid, which is why the sound is a longitudinal wave. He has been exploring whether it is possible to achieve lateral sound, which requires shear force. Then he thought that if the air is separated into “element atoms”, that is, the volume of air confined in a small resonator, whose size is much smaller than the wavelength, a synthetic shear force may be generated. The collective motion of these air “metatoms” can produce lateral sound on a macro scale.
The Conception and Realization of “Micropolar Metamaterials”
He cleverly designed an artificial material called “micropolar metamaterial” to realize this idea, which looks like a complex network of resonators. Air is confined in these interconnected resonators, forming “element atoms”. The metamaterial is hard enough so that only the air inside can vibrate and support the propagation of sound. Theoretical calculations show that the collective motion of these air “element atoms” does produce a shear force, which generates a lateral sound with spin-orbit interaction inside the metamaterial. The research team of Dr. Ma Guancong of Baptist University verified this theory through experiments.
In addition, the research team found that air behaves like an elastic material within the micropolar metamaterial, thus supporting lateral sound with spin and orbital angular momentum. Using this metamaterial, they demonstrated for the first time two types of sound spin or orbital interactions. One is the momentum space spin-orbit interaction, which causes negative refraction of lateral sound, which means that the sound bends in the opposite direction when passing through the interface. The other is the real-space spin-orbit interaction, which generates sound vortices under the excitation of lateral sound.
The research results show that the sound in the air, or the sound in the fluid, can be a kind of transverse wave and carry the complete vector properties like light, such as spin angular momentum. It provides new perspectives and functions for sound operations beyond traditional scalar degrees of freedom.
“This is just a prelude.” Dr. Wang said: “We expect to explore more intriguing characteristics of lateral sound. In the future, by manipulating these additional vector characteristics, scientists may be able to encode more data into lateral sound. In order to break the bottleneck of traditional acoustic communication.”
“The interaction of spin and orbital angular momentum allows unprecedented sound manipulation through its angular momentum.” He added: “This discovery may open up a way for the development of new applications in acoustic communication, acoustic sensing and imaging.”