Cyriaque Genet

Light in tiny holes

The presence of tiny holes in an opaque metal film, with sizes smaller than the wavelength of incident light, leads to a wide variety of unexpected optical properties such as strongly enhanced transmission of light through the holes and wavelength filtering. These intriguing effects are now known to be due to the interaction of the light with electronic resonances in the surface of the metal film, and they can be controlled by adjusting the size and geometry of the holes. This knowledge is opening up exciting new opportunities in applications ranging from subwavelength optics and optoelectronics to chemical sensing and biophysics.

Surface-plasmon circuitry

Electromagnetic waves at the surface of a metal have the enormous bandwidth of a light pulse and can be channeled into circuit components smaller than the diffraction limit.

Compact antenna for efficient and unidirectional launching and decoupling of surface plasmons.

Controlling the launching efficiencies and the directionality of surface plasmon polaritons (SPPs) and their decoupling to freely propagating light is a major goal for the development of plasmonic devices and systems. Here, we report on the design and experimental observation of a highly efficient unidirectional surface plasmon launcher composed of eleven subwavelength grooves, each with a distinct depth and width. Our observations show that, under normal illumination by a focused Gaussian beam, unidirectional SPP launching with an efficiency of at least 52% is achieved experimentally with a compact device of total length smaller than 8 μm. Reciprocally, we report that the same device can efficiently convert SPPs into a highly directive light beam emanating perpendicularly to the sample.

Modifying Chemical Landscapes by Coupling to Vacuum Fields

is typically achieved by placing the material in an optical cavity, such as that formed by two parallel mirrors, which is tuned to be resonant with a transition to an excited state. Theory, discussed below, shows that even in the absence of light, a residual splitting always exists due to coupling to vacuum (electromagnetic) fields in the cavity. While cavity strong coupling and the associated hybrid states have been extensively studied due to the potential they offer in physics such as room temperature Bose–Einstein condensates and thresholdless lasers, the implication for chemistry remains totally unexplored. This is despite the fact that strong coupling with organic molecules lead to exceptionally large vacuum Rabi splittings (hundreds of meV) due to their large transition dipole moments. The molecules plus the cavity must thus be thought of as a single entity with new energy levels and therefore should have its own distinct chemistry. We demonstrate here that one can indeed influence a chemical reaction by strongly coupling the energy landscape governing the reaction pathway to vacuum fields. In the absence of dissipation, the Rabi splitting energy h WR (Figure 1) between the two new hybrid light–matter states is given, for a two-level system at resonance with a cavity mode, by the product of the electric field amplitude E in the cavity and the transition dipole moment d :

Conductivity in organic semiconductors hybridized with the vacuum field.

Much effort over the past decades has been focused on improving carrier mobility in organic thin-film transistors by optimizing the organization of the material or the device architecture. Here we take a different path to solving this problem, by injecting carriers into states that are hybridized to the vacuum electromagnetic field. To test this idea, organic semiconductors were strongly coupled to plasmonic modes to form coherent states that can extend over as many as 10(5) molecules and should thereby favour conductivity. Experiments show that indeed the current does increase by an order of magnitude at resonance in the coupled state, reflecting mostly a change in field-effect mobility. A theoretical quantum model confirms the delocalization of the wavefunctions of the hybridized states and its effect on the conductivity. Our findings illustrate the potential of engineering the vacuum electromagnetic environment to modify and to improve properties of materials.

Efficiency and finite size effects in enhanced transmission through subwavelength apertures

We investigate transmission efficiency and finite size effects for the subwavelength hole arrays. Experiments and simulations show how the finite size effects depend strongly on the hole diameter. The transmission efficiency reaches an asymptotic upper value when the array is larger than the surface plasmon propagation length on the corrugated surface. By comparing the transmission of arrays with that of the corresponding single holes, the relative enhancement is found to increase as the hole diameter decreases. In the conditions of the experiments the enhancement is one to two orders of magnitude but there is no fundamental upper limit to this value.

Coherent coupling of molecular resonators with a microcavity mode

The optical hybridization of the electronic states in strongly coupled molecule–cavity systems have revealed unique properties, such as lasing, room temperature polariton condensation and the modification of excited electronic landscapes involved in molecular isomerization. Here we show that molecular vibrational modes of the electronic ground state can also be coherently coupled with a microcavity mode at room temperature, given the low vibrational thermal occupation factors associated with molecular vibrations, and the collective coupling of a large ensemble of molecules immersed within the cavity-mode volume. This enables the enhancement of the collective Rabi-exchange rate with respect to the single-oscillator coupling strength. The possibility of inducing large shifts in the vibrational frequency of selected molecular bonds should have immediate consequences for chemistry.

Generating Far-Field Orbital Angular Momenta from Near-Field Optical Chirality

We demonstrate orbital angular momentum (OAM) transfer by chiral plasmonic nanostructures designed on both sides of a thin suspended metallic membrane. We show how far-field vortex beams with tunable OAM indices can be tailored through nanostructure designs. We reveal the crucial role played by the central aperture that connects the two sides of the membrane from which OAM selection rules are derived in perfect agreement with experimental data.

Enhanced transmission through Penrose subwavelength hole arrays

Transmission through Penrose subwavelength hole arrays is studied in the optical regime. The loss of strict periodicity does not prevent surface plasmon modes from being excited. The authors study the spectral evolutions of these modes by increasing both the spacings between the holes in a commensurate way and the size of the array. When compared to the situation of a periodic array, their measurements show a reduction of the propagation length of the surface modes, most likely related to the influence of multiple scattering, which lowers the contribution of long-range interactions in the transmission spectra.

Optimization of bull's eye structures for transmission enhancement.

We present an exhaustive exploration of the parameter space defining the optical properties of a bulls eye structure, both experimentally and theoretically. By studying the resonance intensity variations associated with the different geometrical features, several parameters are seen to be interlinked and scale laws emerge. From the results it is possible to give a simple recipe to design a bulls eye structure with optimal transmission properties.