The growing field of quantum plasmonics lies at the intersection between nanophotonics and quantum optics. QUantum plasmonics investigate the quantum properties of single surface plasmons, trying to reproduce fundamental and landmark quantum optics experiment that would benefit from the light-confinement properties of nanophotonic systems, thus paving the way towards the design of basic components dedicated to quantum experiments with sizes inferior to the diffraction limit. Several groups have recently reproduced fundamental quantum optics experiments with single surface plasmons polaritons (SPPs). We have investigated two situations of quantum interference of single SPPs on lossy beamsplitters : a plasmonic version of the Hong-Ou-Mandel experiment, and the observation of plasmonic N00N states interferences. We numerically designed and fabricated several beamsplitters that reveal new quantum interference scenarios, such as the coalescence and the anti-coalescence of SPPs, or quantum non-linear absorption. Our work show that losses can be seen as a new degree of freedom in the design of plasmonic devices.
Thin layers of semiconductors where the permittivity crosses zero, support a particular polariton mode called epsilon-near-zero (ENZ) mode. This zero crossing can be obtained near optical phonon resonances in dielectrics or the plasma frequency in doped semiconductors. The coupling of metamaterial resonators to these ENZ modes leads to particularly large Rabi splittings. ENZ layers can be added to metamaterial-based strongly coupled systems to increase this coupling even further. I will discuss several examples of these coupled systems that include metasurfaces, phonons, intersubband transitions and ENZ modes.
Mid to far infrared is an important wavelength band for detection of substances. Incandescent sources are often used in
infrared spectroscopy because they are simple and cost effective. They are however broadband and quasi isotropic. As a
result, the total efficiency in a detection system is very poor. Yet it has been shown recently that thermal emission can be
designed to be directional and/or monochromatic. To do so amounts to shape the emissivity. Any real thermal source is
characterized by its emissivity, which gives the specific intensity of the source compared to the blackbody at the same
temperature. The emissivity depends on the wavelength and the direction of emission and is related to the whole
structure of the source (materials, geometry below the wavelength-scale...). Emissivity appears as a directional and
chromatic filter for the blackbody radiation. Playing with materials and structure resonances, the emissivity can be
designed to optimize the properties of an incandescent source. We will see how it is possible to optimize a plasmonic
metasurface acting as an incandescent source, to make it directional and quasi monochromatic at a chosen wavelength.
We will target a CO2 detection application to illustrate this topic.
We propose an experimental demonstration of a THz modulator with a visible optical command. The device is a n-doped
GaAs grating with subwavelength dimensions. The principle of this modulator is the control of the THz resonant
absorption by surface waves supported by the grating. This absorption is modulated with low power visible light, leading
to a modulation of the reflected THz beam. From experimental polarized THz reflectivity measurement of the grating,
we show that a depletion layer at the surface of the doped GaAs has to be taken into account to correctly describe the
observed resonant absorption. From experimental observation and modeling we are able to ascribe this absorption to the
coupling of incident THz light with surface plasmon-phonon polariton mode propagating along each wall of the grating.
Thus, each wall acts as a nano-antenna that resonantly absorbs light. The grating can be viewed as a metamaterial
composed of individual resonators. The theoretical model indicates that the reflectivity dip linked to the surface wave is
sensible to the electronic density in the walls of the grating. We performed an experiment to measure the THz
reflectivity while illuminating the grating with visible photons having energy higher than the bandgap of GaAs. The
created photoelectrons change the effective mode index, leading to a shift of the resonant absorption frequency. This
demonstrates the modulation of THz radiation around 8.5 THz with a visible optical command at room temperature.
We demonstrate the association of two-photon nonlinear microscopy with balanced homodyne detection for investigating second harmonic radiation properties at nanoscale dimensions. Variation of the relative phase between second-harmonic and fundamental beam is retrieved, as a function of the absolute orientation of the nonlinear emitters. Sensitivity of ≈ 1.6 photon per second, in the spatio-temporal mode of the local oscillator, is obtained. This value is high enough to efficiently detect the second-harmonic emission from a single KTiOPO<sub>4</sub> crystal of sub-wavelength size, embedded in a thin polymer film.