Eloïse Devaux

Launching and decoupling surface plasmons via micro-gratings

Controlling separately the launching of surface plasmons and their recovery as freely propagating light is essential for the development of surface plasmon photonic circuits. With this target in mind, we have studied in the near-field the launching of surface plasmons in a well-defined direction by micro-arrays of subwavelength holes milled in a thick metal film. We show that surface plasmons can then be converted back to freely propagating light by means of another appropriately designed array. These results not only provide insight into the efficient decoupling of surface plasmons but also into their role in the enhanced transmission mechanism.

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 :

Extraordinary Optical Transmission Enhanced by Nanofocusing

We demonstrate that the phenomenon of extraordinary optical transmission (EOT) through perforated metal films can be further boosted up by utilizing nanofocusing of radiation in tapered slits. For one-dimensional arrays of tapered slits in optically thick suspended gold films, we show that the maximum transmission at resonance is achieved for taper angles in the range of 7-10 degrees increasing significantly in comparison with the transmission by straight slits. Transmission spectroscopy of fabricated 500 and 700 nm period tapered slits in a 180 nm thick gold film on a glass substrate demonstrates the enhanced EOT with the resonance transmission being as high as approximately 0.18 for the filling ratio of approximately 0.13 and showing good correspondence with theoretical results. It is also shown that the enhanced transmission can be achieved with either weak (2.5%) or strong (43%) reflection depending on the direction of light (normal) incidence.

Surface plasmon interferometry : measuring group velocity of surface plasmons

Optical transmission spectroscopy on metal films with slit-groove pairs is conducted. Spectra of the light transmitted through the slit exhibit Fabry-Perot-type interference fringes due to surface plasmons propagating between the slit and the groove. The spectral dependence of the period of interference fringes is used to determine the group velocity of surface plasmons on flat gold and silver surfaces.

Compact gradual bends for channel plasmon polaritons

We report the design, fabrication and characterization of compact gradual bends for channel plasmon polaritons (CPPs) being excited at telecom wavelengths. We obtain high-quality near-field optical images of CPP modes propagating along a bent V-groove in gold, which indicate good CPP mode confinement in the groove and efficient guiding around the compact S-bend connecting two 5-mum-offset grooves over a distance of 5 mum. Using averaged cross sections of the CPP intensity distributions before and after the S-bend, the total bend loss is evaluated and found to be close to 2.3 dB for the wavelengths in the range of 1430-1640 nm.

Bend loss for channel plasmon polaritons

Using near-field optical microscopy, the authors investigate propagation of channel plasmon polaritons excited in the wavelength range of 1425–1640nm along smoothly bent and split V-shaped grooves milled in a gold film. We find that for 0.6-μm-wide and 1.1-μm-deep grooves bent with the smallest curvature radius of ≅0.83μm, the double bend (for S bends) and splitting (for Y splitters) losses decrease for longer wavelengths approaching (in the wavelength range of 1600–1640nm) the levels of ∼0 and 0.5dB, respectively.

Thermodynamics of Molecules Strongly Coupled to the Vacuum Field

The thermodynamics of strong coupling between molecules and the vacuum field is analyzed and the Gibbs free energy, the enthalpy, and entropy of the coupling process are determined for the first time. The thermodynamic parameters are a function of the Rabi splitting and the microscopic solvation. The results provide a new framework for understanding light-molecule strong coupling.

Ground-State Chemical Reactivity under Vibrational Coupling to the Vacuum Electromagnetic Field.

Abstract The ground‐state deprotection of a simple alkynylsilane is studied under vibrational strong coupling to the zero‐point fluctuations, or vacuum electromagnetic field, of a resonant IR microfluidic cavity. The reaction rate decreased by a factor of up to 5.5 when the Si−C vibrational stretching modes of the reactant were strongly coupled. The relative change in the reaction rate under strong coupling depends on the Rabi splitting energy. Product analysis by GC‐MS confirmed the kinetic results. Temperature dependence shows that the activation enthalpy and entropy change significantly, suggesting that the transition state is modified from an associative to a dissociative type. These findings show that vibrational strong coupling provides a powerful approach for modifying and controlling chemical landscapes and for understanding reaction mechanisms.

Field enhancement and extraordinary optical transmission by tapered periodic slits in gold films

We investigate field enhancements by one-dimensional periodic arrays of tapered slits fabricated to a high quality (nm precision) using focused ion beam milling in a 180 nm-thick gold film. Tapering of periodic slits in metal was recently shown to boost the extraordinary optical transmission (EOT) exhibited by similar, but non-tapered, plasmonic structures. Here, both simulated and experimental reflection spectra, along with high-resolution two-photon luminescence (TPL) scanning optical images and simulated electric field plots of the metal slits, are compared, revealing good correspondence between spectral dependences and field intensity enhancements (FEs) estimated via the local TPL. Experimentally investigated structures had a fixed taper angle α=20.5° for two different widths, w=80 and 130 nm, having gaps g=25 and 65 nm, respectively, both fabricated at two different periods, Λ=500  and 700 nm. We attributed the obtained FE reaching ~110 to nanofocusing and resonant interference of counter-propagating plasmons by the periodic tapered gaps. As both simulated and experimentally achieved FEs depend on taper angle, gold film thickness, period and gap of the slit arrays, the resonances can actually be tuned in the wavelength range from visible to infrared, making this configuration promising for a wide range of practical applications, e.g. within surface-enhanced spectroscopies.