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| Illustration of the “meta-atom” and transverse sound. Credit: S. Wang et al |
Is it possible for sound to travel in the same manner that light does? City University of Hong Kong (CityU) researchers have found a new sort of sound wave: Like light, the sound wave in the air vibrates transversely and carries both spin and orbital angular momentum. The findings demolished scientists' preconceived notions about sound waves, paving the way for revolutionary applications in acoustic communications, sensing, and imaging.
Dr. Shubo
Wang, Assistant Professor in the Department of Physics at CityU, initiated and
co-led the study, which was carried out in partnership with experts from Hong
Kong Baptist University (HKBU) and Hong Kong University of Science and
Technology (HKUST). The paper was named "Spin-orbit interactions of
transverse sound" and was published in Nature Communications.
Beyond
the conventional understanding of sound wave
There are
two types of waves, according to physics textbooks. The vibrations of
transverse waves, such as light, are perpendicular to the wave propagation
direction. The vibrations of longitudinal waves, such as sound, are parallel to
the wave propagation direction. However, a recent finding by CityU scientists
has altered our perception of sound waves.
"If you ask a physicist about airborne transverse sound, he or she will assume you're a layperson who hasn't studied university physics," Dr. Wang added. "While airborne sound is usually a longitudinal wave, we showed for the first time that it can also be a transverse wave under specific circumstances. We also looked at its spin-orbit interactions, which is the coupling of two types of angular momentum (a property that only exists in transverse waves). The discovery gives sound manipulations new levels of flexibility."
Sound is a
longitudinal wave since there is no shear force in the air or fluids, according
to Dr. Wang. He was testing whether transverse sound, which requires shear
force, could be realized. Then he had the concept that if the air is
discretized into "meta-atoms," or volumetric air trapped in small
resonators with a size considerably smaller than the wavelength, synthetic
shear force may emerge. On a macroscopic scale, the aggregate motion of these
air "meta-atoms" can produce a transverse sound.
 |
Negative
refraction induced by the spin-orbit interaction in momentum space. Credit: S.
Wang et al |
Conception
and realization of 'micropolar metamaterial'
To put this notion into practice, he devised a sort of artificial material known as "micropolar metamaterial," which resembles a complicated network of resonators. The "meta-atoms" are formed when air is contained inside these mutually connected resonators. Only the air inside the metamaterial can vibrate and facilitate sound propagation since it is hard enough. Theoretical calculations revealed that the shear force is produced by the collective motion of these air "meta-atoms," resulting in transverse sound with spin-orbit interactions inside this metamaterial. Experiments undertaken by Dr. Ma Guancong's group at HKBU backed up this idea.
Furthermore, the researchers discovered that inside the micropolar metamaterial, air behaves like an elastic material, supporting transverse sound with both spin and orbital angular momentum. For the first time, they showed two types of sound spin-orbit interactions using this metamaterial. One is the momentum-space spin-orbit interaction, which causes negative transverse sound refraction, which means sound bends in opposite directions when it passes over an interface. Another is the real-space spin-orbit interaction, which produces sound vortices when transverse sound is excited.
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