Can you imagine sound travels in the same way as light does? A research team
at City University of Hong Kong (CityU) discovered a new type of sound wave:
the airborne sound wave vibrates transversely and carries both spin and
orbital angular momentum like light does. The findings shattered scientists'
previous beliefs about the sound wave, opening an avenue to the development
of novel applications in acoustic communications, acoustic sensing and
imaging.
The research was initiated and co-led by Dr Wang Shubo, Assistant Professor
in the Department of Physics at CityU, and conducted in collaboration with
scientists from Hong Kong Baptist University (HKBU) and the Hong Kong
University of Science and Technology (HKUST). It was published in Nature
Communications, titled "Spin-orbit interactions of transverse sound".
Beyond the conventional understanding of sound wave
The physics textbooks tell us there are two kinds of waves. In transverse
waves like light, the vibrations are perpendicular to the direction of wave
propagation. In longitudinal waves like sound, the vibrations are parallel
to the direction of wave propagation. But the latest discovery by scientists
from CityU changes this understanding of sound waves.
"If you speak to a physicist about airborne transverse sound, s/he would
think you are a layman without training in university physics because
textbooks say that airborne sound (i.e., sound propagating in the air) is a
longitudinal wave," said Dr Wang. "While the airborne sound is a
longitudinal wave in usual cases, we demonstrated for the first time that it
can be a transverse wave under certain conditions. And we investigated its
spin-orbit interactions (an important property only exists in transverse
waves), i.e. the coupling between two types of angular momentum. The finding
provides new degrees of freedom for sound manipulations."
The absence of shear force in the air, or fluids, is the reason why sound is
a longitudinal wave, Dr Wang explained. He had been exploring if it is
possible to realise transverse sound, which requires shear force. Then he
conceived the idea that synthetic shear force may arise if the air is
discretised into "meta-atoms", i.e. volumetric air confined in small
resonators with size much smaller than the wavelength. The collective motion
of these air "meta-atoms" can give rise to a transverse sound on the
macroscopic scale.
Conception and realisation of "micropolar metamaterial"
He ingeniously designed a type of artificial material called "micropolar
metamaterial" to implement this idea, which appears like a complex network
of resonators. Air is confined inside these mutually connected resonators,
forming the "meta-atoms". The metamaterial is hard enough so that only the
air inside can vibrate and support sound propagation. The theoretical
calculations showed that the collective motion of these air "meta-atoms"
indeed produces the shear force, which gives rise to the transverse sound
with spin-orbit interactions inside this metamaterial. This theory was
verified by experiments conducted by Dr Ma Guancong's group in HKBU.
Moreover, the research team discovered that air behaves like an elastic
material inside the micropolar metamaterial and thus supports transverse
sound with both spin and orbital angular momentum. Using this metamaterial,
they demonstrated two types of spin-orbit interactions of sound for the
first time. One is the momentum-space spin-orbit interaction which gives
rise to negative refraction of the transverse sound, meaning that sound
bends in the opposite directions when passing through an interface. Another
one is the real-space spin-orbit interaction which generates sound vortices
under the excitation of the transverse sound.
The findings demonstrated that airborne sound, or sound in fluids, can be a
transverse wave and carry full vector properties such as spin angular
momentum the same as light does. It provides new perspectives and
functionalities for sound manipulations beyond the conventional scalar
degree of freedom.
"This is just a precursor. We anticipate more explorations of the intriguing
properties of the transverse sound," Dr Wang said. "In future, by
manipulating these extra vector properties, scientists may be able to encode
more data into the transverse sound to break the bottleneck of traditional
acoustic communication by normal sound waves."
The interaction of spin with orbital angular momentum enables unprecedented
sound manipulations via its angular momentum. "The discovery may open an
avenue to the development of novel applications in acoustic communications,
acoustic sensing and imaging," he added.
Reference:
Shubo Wang, Guanqing Zhang, Xulong Wang, Qing Tong, Jensen Li, Guancong Ma.
Spin-orbit interactions of transverse sound. Nature Communications, 2021; 12
(1)
DOI: 10.1038/s41467-021-26375-9
Tags:
Physics