Light-matter interactions form the core of the research of the laboratory. This research spans everything from optical forces, chirality and plasmonics to weak and strong coupling regimes, as detailed below. It is interdisciplinary in nature at the interface of physics and chemistry, sometimes even venturing into biological systems.
It involves the assembly of optical setups, the spectroscopic characterization both through static and femto-second time resolved measurements, theoretical simulations and fabrication of plasmonic nanostructures using state-of-the-art tools such as FIB. The vast zoology of molecules and molecular materials (the molecular tool-box) is exploited for given targets. The members of the group come from diverse backgrounds, typically physics, engineering and physical chemistry, source of stimulating discussions.
Light-matter interactions: Coupling in the weak and strong coupling
Light-matter interactions can be enhanced by placing a material in a confined optical mode that is resonant with a transition of the material. The interaction is typically classified in the weak and strong coupling regimes. In weak coupling, the radiative properties are modified, a typical example being the Purcell effect. As the interaction increases, the strong coupling regime can be reached when a molecular transition and a resonant optical mode whereby they exchange energy faster than any relaxation process. In such a case new hybrid light-matter states are formed. These states, having both features of light and matter, are known as polaritonic states. They are separated by the Rabi splitting ħωVR, as illustrated in the figure. The strong coupling process involves the vacuum field of the cavity and the molecular transition; therefore it occurs even without light. The light-matter hybridization is expected to alter the material properties of the system. Indeed we have demonstrated that by strongly coupling molecular electronic transitions with a cavity or plasmonic mode, properties can be modified such as a chemical reaction rate, the conductivity and work function of organic materials. However, light-matter strong coupling is not limited to electronic transitions, which has been extensively studied during the last two decades.
Recently, we were able to show that vibrational transitions in the ground state can also be strongly coupled to an optical mode using micro-cavities in the infrared (IR) region. The resonant coupling is achieved by tuning the frequency ωc of micro-cavities to a given molecular vibrational frequency ωv of a specific bond in the molecule resulting in the formation of two new hybrid vibro-polariton states VP+ and VP-. The properties of these states are also being explored and the first results show that the Raman cross-section is enhanced by up to 3 orders of magnitude.
Surface plasmon optics, plasmonic spin Hall effects and spin-orbit interactions
Although no longer the main focus of the laboratory, we have been extensively working on plasmonics for the past fifteen years following the discovery of the extraordinary optical transmission through subwavelength apertures in metal films. This work has been focused on the optical properties of subwavelength holes, plasmonic devices and circuitry. Examples of such work include the diffraction of light emanating from a subwavelength aperture, the plasmonic sorting of photons, the plasmonic control of light polarization states, including the design of plasmonic wave plates, etc. Overall, these results have revealed how surface plasmon modes are efficient in offering new types of optical modes that can be tailored by controlling surface designs at the nanoscale.
Recently, our interest has shifted towards spin Hall effects that we studied in a plasmonic context, leading to measuring with high sensitivity the helicity of a light beam. Plasmonic spin Hall effects can thus open the way to various high-resolution sensing applications involving the chirality of light. This issue is in direct relation with the possibility to induce true optical chirality involving for instance surface plasmons propagating on twisted planar metallic nanostructures. The notion of chirality is particularly interesting ton investigate in the near field and this pushed us to further demonstrate orbital angular momentum transfers determined by and controlled on chiral plasmonic nanostructures. Exploiting plasmonic spin-orbit couplings on metallic spirals, we have been able to generate far-field optical vortices with tunable topological charges, revealing the important properties of singular plasmonic interactions in the near field.
Optical forces, brownian motion in designer force fields and chirality
As spinning fields, surface plasmons give rise to specific types of forces and torques that can be exerted on nano-objects immersed in the associated near field. We are currently developing a whole activity in this area, from both experimental and theoretical perspectives. Experimentally, we have shown that surface plasmon modes can drive optical tweezers efficiently. This led us to study near-field optical forces associated with negatively refracted surface plasmons capable of controlling nanoparticle trajectories. Such experiments based on Brownian statistical analysis of the kinetic motions open many new ways and applications for high-resolution all-optical control of particle flow. More recently, we started to develop new optical trapping configurations as test-systems for non-equilibrium statistical physics at the level of which various stochastic protocols can be implemented, involving external optical forces.
In parallel, we have started to explore the fascinating possibilities offered by chiral plasmonic modes for generating genuine chiral optical forces on chiral nano-objects. Combining chiral contents of light and matter, we have recently calculated new types of optical forces, genuinely chiral, that are exerted on chiral dipoles -or extended objects- by chiral light fields, both from far-field and near-field perspectives. We have revealed how these chiral optical forces and torques are dependent on the enantiomeric form of the chiral objects. This remarkable consequence suggests promising strategies for all-optical chiral discrimination schemes, in particular in the plasmonic near field.