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Abstract:
Photonic cavities are an essential part of many modern optical devices.
The characterization of the plasmonic modes of these cavities is complex
due to their tiny size, which makes access difficult by external
signals.
Researchers from the Autonomous University of Madrid, IMDEA Nanociencia
and IFIMAC have developed a new method for the fabrication and
characterization of atomic-sized photonic cavities by exploiting the
mechano-quantum tunnel effect. This discovery may be fundamental for the
design of nanometric-size opto-electronic devices.

Light in the tunnel

Madrid, Spain | Posted on March 26th, 2020

Photonic cavities are an essential part of many modern optical devices,
from a laser pointer to a microwave oven. Just as we can store water in
a tank and create standing waves on the surface of the water, we can
confine light in a photonic resonator whose walls are strongly
reflective. Just as water surface waves depend on the geometry of the
tank (shape, depth), specific optical modes can be created in a photonic
cavity whose properties (colour and spatial distribution of intensity)
can be tuned by changing the dimensions of the cavity. When the size of
the cavity is very small, much smaller than the wavelength of the light
confining it (nano-cavity in the case of visible light), an
intensification effect of the light is produced that is so strong that
it influences the electrons on the walls of the cavity. A mixture
between photons and electrons is then produced, giving rise to hybrid
modes between light and matter known as plasmons.
Plasmons in optical nano-cavities are extremely important for many
applications such as chemical sensors that allow the detection of
individual molecules, or the manufacture of nanolasers that could
operate with hardly any electrical current consumption. However, the
characterisation of these plasmonic modes is generally very complex,
because of the tiny size of the cavities that makes extremely difficult
to access them by external signals.
On the other hand, the tunnel effect is one of the most characteristic,
mysterious and best documented effects of Quantum Mechanics. In a tunnel
process, a particle (e.g. an electron) can pass through a narrow barrier
(the space that separates two metals at nanometric distances) despite
not having enough energy to overcome it. It is as if we could pass from
one side to the other of the Great Wall of China without having to jump
over it. Incredible as it may seem, particles from the quantum world can
do this under certain conditions. In most of these processes, the energy
of the particle before and after the process is the same. However, in a
small fraction of these events, the particle can give up some of its
energy, for example, by generating light, which is known as the
inelastic tunnel process. Although it is well known that the properties
of the light emitted in the inelastic tunnel process between two metals
depend on the plasmonic modes that exist in the cavity, it also depends
strongly on the energy distribution of the particles performing the
tunnel process. Until now, it had been impossible to distinguish
unequivocally between these two effects and therefore extract the
information on the plasmonic modes from the analysis of the light
emitted by the tunnel effect.
Researchers from Universidad Autónoma de Madrid, IMDEA Nanociencia and
IFIMAC have developed a method to overcome this problem by
simultaneously determining the energy distribution of the tunnelling
electrons and the light emitted in a scanning tunnel microscope. They
have exploited the tunnelling effect to create optical resonators of
atomic dimensions and study their optical properties, unravelling for
the first time the contributions due to the energy of the tunnelling
particles from the effects originated by the plasmonic modes in the
cavity.
This work proposes a novel methodology for the characterization of
light-matter interaction at atomic size, and may have important
technological implications for the development of chemical sensors of
single molecules, new sources of single or interlaced photons or
nanolasers that are active at extremely low pumping powers.
The research has been published in the prestigious journal Nature
Communications.