Recent years have seen spectacular developments in the domain of nano-optics.
Alongside the well-known techniques of super-resolution microscopy progress
in nanofabrication has enabled important improvements in the fields
of optical imaging and spectroscopy.
Nowadays enhancements of specific near-field interactions
by nano-antennae/cavities allows the optical readout
of single molecule properties in a number of different systems.
The observation of normal modes of vibration for example carries
essential information on the orientation, link to the interface and environment
in which the molecule is embedded.

This thesis develops firstly a novel theoretical frame for these vibration-cavity
interactions by following the optomechanical formalism, which has proven
to be an accurate treatment of a wide variety of mechanical
systems interacting with a laser field. By leveraging this framework, we investigate
on the one hand several quantum limits. On the other hand, for the parameter space
experimentally available and for well-designed cavities, novel regimes of interaction between the
collective mode of vibration of an ensemble of molecules and an optical tone
seem within reach.

In parallel, this thesis experimentally explores one specific realization of
such a system. In collaboration with the group of Christophe Galland we constructed
the optical setups necessary to interrogate our nanostructures.
The class of structures constructed, known as self-assembled nanojunctions or
nanoparticles on mirror, reaches confinement of electric fields via
plasmonic modes close to its fundamental limit and enables the study of
many different mechanical systems whose thickness consist in a single molecule.
Along this research we have developed techniques to characterize mechanical,
electronic and thermal observables of nanojunctions interacting
with an optical field.

Our experimental study has enabled the discovery of a novel
effect related to the electronic confinement of these
quasi-0-dimensional
structures. The characteristic photoluminescence of metallic nanostructures
is found to consistently blink in nanojunctions akin to other quantum emitters.
The mechanisms explaining in detail this blinking remain debatable
but the numerous experimental data collected highlight the key role
of the interface in the specificities of the blinking observed.
This finding opens novel ways to characterize the important and to date
ill-described question of the interfaces in nanojunctions.
The observed modifications of the nanojunctions
optical properties might constitute an interesting step
towards the development of a novel class of functional materials ('gap-materials')
with enhanced and tailored electro-optical properties.

The last part of this thesis considers one technological application for
this class of material by conceptually developing a new kind of detector
based on the upconversion of an infrared photon to the visible; exploiting thus
in a unique way the optical properties of the molecular system.
The study of the quantum noises of these detectors highlights two exciting
and technologically relevant directions for such devices :
at room temperature
these systems operate with levels of noise
below state of the art devices,
at lower temperatures these systems open an unexplored path towards single photon detection
in a spectral range where this technology remains unavailable to date.