In groundbreaking new research, an international team of researchers led by
the University of Minnesota Twin Cities has developed a unique process for
producing a quantum state that is part light and part matter.
The discovery provides fundamental new insights for more efficiently
developing the next generation of quantum-based optical and electronic
devices. The research could also have an impact on increasing efficiency of
nanoscale chemical reactions.
The research is published in Nature Photonics, a high-impact, peer-reviewed
scientific journal published by the Springer Nature Publishing Group.
Quantum science studies natural phenomena of light and matter at the
smallest scales. In this study, the researchers developed a unique process
in which they achieved “ultrastrong coupling” between infrared light
(photons) and matter (atomic vibrations) by trapping light in tiny, annular
holes in a thin layer of gold. These holes were as small as two nanometers,
or approximately 25,000 times smaller than the width of a human hair.
These nanocavities, similar to a highly scaled-down version of the coaxial
cables that are used to send electrical signals (like the cable that comes
into your TV), were filled with silicon dioxide, which is essentially the
same as window glass. Unique fabrication methods, based on techniques
developed in the computer-chip industry, make it possible to produce
millions of these cavities simultaneously, with all of them simultaneously
exhibiting this ultrastrong photon-vibration coupling.
“Others have studied strong coupling of light and matter, but with this new
process to engineer nanometer-sized version of coaxial cables, we are
pushing the frontiers of ultrastrong coupling, which means we are
discovering new quantum states where matter and light can have very
different properties and unusual things start to happen,” said Sang-Hyun Oh,
a University of Minnesota professor of electrical and computer engineering
and the senior author of the study. “This ultrastrong coupling of light and
atomic vibrations opens up all kinds of possibilities for developing new
quantum-based devices or modifying chemical reactions.”
The interaction between light and matter is central to life on earth—it
allows plants to convert sunlight into energy and it allows us to see
objects around us. Infrared light, with wavelengths much longer than what we
can see with our eyes, interacts with the vibrations of atoms in materials.
For example, when an object is heated, the atoms that make up the object
start vibrating faster, giving off more infrared radiation, enabling
thermal-imaging or night-vision cameras.
Conversely, the wavelengths of infrared radiation that are absorbed by
materials depend on what kinds of atoms make up the materials and how they
are arranged, so that chemists can use infrared absorption as a
“fingerprint” to identify different chemicals.
These and other applications can be improved by increasing how strongly
infrared light interacts with atomic vibrations in materials. This, in turn,
can be accomplished by trapping the light into a small volume that contains
the materials. Trapping light can be as simple as making it reflect back and
forth between a pair of mirrors, but much stronger interactions can be
realized if nanometer-scale metallic structures, or “nanocavities," are used
to confine the light on ultra-small length scales.
When this happens, the interactions can be strong enough that the
quantum-mechanical nature of the light and the vibrations comes into play.
Under such conditions, the absorbed energy is transferred back and forth
between the light (photons) in the nanocavities and the atomic vibrations
(phonons) in the material at a rate fast enough such that the light photon
and matter phonon can no longer be distinguished. Under such conditions,
these strongly coupled modes result in new quantum-mechanical objects that
are part light and part vibration at the same time, known as
“polaritons.”
The stronger the interaction becomes, the stranger the quantum-mechanical
effects that can occur. If the interaction becomes strong enough, it may be
possible to create photons out of the vacuum, or to make chemical reactions
proceed in ways that are otherwise impossible.
"It is fascinating that, in this coupling regime, vacuum is not empty.
Instead, it contains photons with wavelengths determined by the molecular
vibrations. Moreover, these photons are extremely confined and are shared by
a minute number of molecules,” said Professor Luis Martin-Moreno at the
Instituto de Nanociencia y Materiales de Aragón (INMA) in Spain, another
author of the paper.
“Normally, we think of vacuum as basically nothing, but it turns out this
vacuum fluctuation always exists,” Oh said. “This is an important step to
actually harness this so-called zero energy fluctuation to do something
useful.”
Reference:
Yoo, D., de León-Pérez, F., Pelton, M. et al. Ultrastrong plasmon–phonon
coupling via epsilon-near-zero nanocavities. Nat. Photonics, 2020 DOI:
10.1038/s41566-020-00731-5