Perfect transmission through barrier using sound: research team led by Professor Xiang Zhang proves for the first time a century-old quantum theory
December 23, 2020 -- The perfect transmission of
sound through a barrier is difficult to achieve, if not impossible based on our
existing knowledge. This is also true with other energy forms such as light and
heat.
A research team led by Professor Xiang Zhang,
President of the University of Hong Kong (HKU) when he was a professor at the University of California, Berkeley,
(UC Berkeley) has for the first time experimentally proved a century old
quantum theory that relativistic particles can pass through a barrier with 100%
transmission. The research findings have been published in the top academic
journal Science.
Just as it
would be difficult for us to jump over a thick high wall without enough energy
accumulated. In contrast, it is predicted that a microscopic particle in the
quantum world can pass through a barrier well beyond its energy regardless of
the height or width of the barrier, as if it is “transparent”.
As early as
1929, theoretical physicist Oscar Klein proposed that a relativistic particle
can penetrate a potential barrier with 100% transmission upon normal incidence
on the barrier. Scientists called this exotic and counterintuitive phenomenon
the "Klein tunneling" theory. In the following 100 odd years,
scientists tried various approaches to experimentally test Klein tunneling, but
the attempts were unsuccessful and direct experimental evidence is still
lacking.
Professor
Zhang’s team conducted the experiment in artificially designed phononic
crystals with triangular lattice. The lattice’s linear dispersion properties
make it possible to mimic the relativistic Dirac quasiparticle by sound
excitation, which led to the successful experimental observation of Klein
tunneling.
"This is
an exciting discovery. Quantum physicists have always tried to observe Klein
tunneling in elementary particle experiments, but it is a very difficult task.
We designed a phononic crystal similar to graphene that can excite the
relativistic quasiparticles, but unlike natural material of graphene, the
geometry of the man-made phononic crystal can be adjusted freely to precisely
achieve the ideal conditions that made it possible to the first direct
observation of Klein tunneling,” said Professor Zhang.
The
achievement not only represents a breakthrough in fundamental physics, but also
presents a new platform for exploring emerging macroscale systems to be used in
applications such as on-chip logic devices for sound manipulation, acoustic
signal processing, and sound energy harvesting.
"In
current acoustic communications, the transmission loss of acoustic energy on
the interface is unavoidable. If the transmittance on the interface can be
increased to nearly 100%, the efficiency of acoustic communications can be
greatly improved, thus opening up cutting-edge applications. This is especially
important when the surface or the interface play a role in hindering the
accuracy acoustic detection such as underwater exploration. The experimental
measurement is also conducive to the future development of studying
quasiparticles with topological property in phononic crystals which might be
difficult to perform in other systems,” said Dr. Xue Jiang, a former member of
Zhang’s team and currently an Associate Researcher at the Department of
Electronic Engineering at Fudan University.
Dr. Jiang
pointed out that the research findings might also benefit the biomedical
devices. It may help to improve the accuracy of ultrasound penetration through
obstacles and reach designated targets such as tissues or organs, which could
improve the ultrasound precision for better diagnosis and treatment.
On the basis
of the current experiments, researchers can control the mass and dispersion of
the quasiparticle by exciting the phononic crystals with different frequencies,
thus achieving flexible experimental configuration and on/off control of Klein
tunneling. This approach can be extended to other artificial structure for the
study of optics and thermotics. It allows the unprecedent control of
quasiparticle or wavefront, and contributes to the exploration on other complex
quantum physical phenomena.
The article
published in Science: https://science.sciencemag.org/content/370/6523/1447.
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