■ Energy transfer between fluorescent emitters : from weak to strong coupling regime

Fluorescent radiation emitted by a fluorophore strongly depend on the structuration of the environment at sub-wavelength scale [Surf. Sci. Rep. 70, 1 (2015)]. Enhancement and control of light-matter interaction at the nanometer scale is a key issue of modern photonics and has largely been explored. On the contrary, the role of a nanostructured environment in assisting the long-range (micrometer scale) interaction between two emitters is still an emerging topic in nanophotonics and experimental studies are scarce. This field has very interesting perspectives for both fundamental physics (e.g. cooperative emission, qubits entanglement, nano-optical logical gates) and applications (e.g. solar harvesting, sensing). At Institut Langevin we have demonstrated, for the first time, that energy transfer can occur between fluorescent emitters (donor and acceptor) located at distances of several micrometers, thanks to the weak coupling to surface plasmons (i.e. collective excitations of electrons at a metal-dielectric interface) propagating over distances of several microns on the top of a silver film [Phys. Rev. Lett. 116, 037401 (2016)], beating the records by almost three orders of magnitude. The PhD proposal concerns the extension of energy transfer to the strong coupling regime between plasmons and organic molecules. Strong coupling with surface plasmons results in the hybridization of the surface plasmon with the molecular excited states, that are coherently coupled on a scale up to tens of microns. This long range molecular interaction has opened new perspectives for tailoring the optical properties and energy landscapes of molecules [Nat. Comm. 6, 5981 (2015)], and could provide an alternate effective path for energy transfer. Based on the large spatial extension of the hybrid state, energy transfer on micron distances will be performed by hybridizing donor and acceptor molecules through the same surface plasmon. During the PhD thesis, we will perform experiments on planar metal films covered with J-aggregates (donors and acceptors), in collaboration with J. Bellessa (ILM, Lyon) who is a pioneer of plasmonic strong coupling [Phys. Rev. Lett. 93, 036404 (2004)]. Accurate studies of both the crossover from weak to strong coupling and the occurrence of strong-coupling energy transfer will be performed with a fluorescent near-field scanning optical microscope (f-SNOM), recently developed at Institut Langevin [Opt. Expr. 21, 11536 (2013)], that allows a nanometric control of the distance between the fluorescent probe and the substrate. A molecular patch will be deposited on the apex of the f-SNOM tip. The reduction of the distance between the probe and an organic film on metal allows to gradually include supplementary molecules to the coherent state, changing the hybrid state energy. This experimental method will bring a totally new insight into phenomena studied in the strong coupling regime.

Near-field microscopy, Fluorescence microscopy, Plasmonics, Nanophotonics, Strong coupling, Energy transfer, Fluorescent emitters

UMR7587
Langevin Institute "Waves and Images"

Description of the research Unit/subunit

The Institut Langevin is specialized in the study of physics of waves in many domains going from optics to acoustic to electromagnetism. It brings together high-level fundamental research, applied research and company start-ups with strong interdisciplinary components. It is divided in four research themes, i.e. “Waves in complex media”, “Waves Physics for Medicine and biology”, “Non-conventional imaging and sensing”, “Subwavelength physics”. The PhD supervisors’ group is part of the “Subwavelength physics” theme, which involves researchers who study phenomena which are specific to the physics of waves at subwavelength scales in various regions of the electromagnetic radiation spectrum. Topics of interest concern the interaction of electromagnetic or acoustic waves with subwavelength resonators, plasmonics and optical antennas, thermal radiation at subwavelength scales and near-field interactions of fluorescent quantum emitters with nanostructures, and the development of novel super-resolved imaging methods. These research topics are often related with other themes of the Institut Langevin.

Name of the supervisor
Valentina Krachmalnicoff

Strong coupling between molecular excitations and surface plasmons or cavity modes, which occurs when the plasmon/emitters interaction overcomes the damping in the system, has become a very active topic of research [Rep. Prog. Phys. 78, 013901 (2015)] : sessions on this subject are present in quasi all the Photonic conferences and full conferences are devoted to this subject. The project is related to KET on “nanotechnology”, “photonics”, “advanced materials” in that it will open new perspectives for the development of new nanostructured materials that will lay research groundwork for photovoltaic conversion applications. Strong coupling regime has been recently applied to the modification of intrinsic properties of materials like the chemical reactivity [Acc. Chem. Res. 49, 2403 (2016)] or the conductivity [Nature Mat. 14, 1123 (2015)]. The possibility of observing such properties in regions of larger spatial extension will be investigated in the project and will potentially lead to materials with original properties. The project will benefit from an ongoing collaboration between V. Krachmalnicoff and the Italian company Micro Photon Devices (http://www.micro-photon-devices.com/) for the development of single-photon avalanche detectors and detector arrays with outstanding specifications in terms of quantum efficiency and time resolution.

The candidate is required to possess a Master Degree in Physics, with an important component of quantum physics and/or nanophotonics. Well-developed experimental skills as well as good knowledge in simulations, data analysis, interfacing of scientific instruments will be important. Knowledge in nanofabrication techniques (optical/electric lithography and other clean-room techniques for the fabrication of nanostructured samples) as well as in organic molecules chemistry will be welcome even though not mandatory.