1-Research

The group research activities focus on two themes: nanophysics and biophotonics. The common denominator of our work is the detection and analysis of properties of individual nano-objects. Our goal is to deeply understand light with matter interactions at the nanoscale in order to explore groundbreaking applications at the interface between different domains in physics or different research disciplines.

Some achievements of the group:

We introduce original microscopy and spectroscopy approaches to study the properties of nanoobjects (molecules, metal and semiconductor nanoparticles, carbon nanotubes, ...) under various conditions of: light illumination, temperature (ambient or cryogenic), static fields (magnetic or electric) and environments (liquid, solid or on surfaces). We develop applications in quantum physics, condensed matter physics and bio-imaging.

Spectroscopy of charged complexes in semiconductor nanocrystals

The first research theme of the group concerns the photo-physics of individual nano-objects at cryogenic temperatures. Among available systems chemically synthesized II-VI semiconductor nanocrystals and most recently perovskite nanostructures are particularly attractive due to their bright luminescence, tunable across the visible spectrum, and their suitable properties for applications ranging from optoelectronics to biological labeling. Using magneto-optical spectroscopy we have been able to fully elucidate the luminescence properties and their band-edge exciton fine structure.

We also performed the first study of the spin properties of the trion state in colloidal CdSe nanocrystals. We engineer a suitable nanocrystal structure to enable efficient photo-charging, while judicious choice of crystal structure enables different charge carrier properties to be accessed. Cryogenic magneto-optical spectroscopy allowed us to identify the trion, assign its charge state and directly measure the g-factors of charge carriers spins. Most significantly, we show that spin relaxation is inhibited in nanocrystals because of an acoustic phonon bottleneck effect.

Single molecules at low temperatures

Important achievements of the group in single molecule spectroscopy include the first realization of single photon sources based on the control of the excitation of a single emitter. Recently we demonstrated an efficient source of indistinguishable near infrared photons based on a solid-immersion-lens confocal microscope. We have also proposed a simple super-resolution microscopy method operating at cryogenic temperature and showed that sub-10 nm spatial resolutions can be achieved. This super-resolution nanoscopy of single molecules paves the way to the study of coherent interactions between single quantum emitters and to the manipulation of their degree of entanglement in the view of applications in quantum networks. We have also shown that Stark shifts of sharp single molecule lines can be used to probe the local electric fields e.g. generated by flexomagnetoelectric effect in multiferroic material. We could obtain the first direct evidence of the electric polarization induced by a magnetization inhomogeneity in an iron garnet film. This inhomogeneity was created by the non-uniform magnetic fields generated at domain boundaries of a Type-I superconductor in the intermediate state.

Single walled carbon nanotubes

We have extended our single molecule techniques to study the optical properties of individual single-walled carbon nanotubes through absorption and photoluminescence spectroscopy, and we demonstrated the efficient, all-optical generation of trions and provided a clear signature of biexciton generation. We have also shown that single nanotube microscopy is a valuable technique for the understanding of the thermal motion of the nanotubes in crowded environments. We found that even a small bending flexibility strongly enhances their motion. This last study definitively establishes the reptation dynamics of stiff filaments and provides a framework to tailor the mobility of nanotubes in confined environments.

Optical manipulation of single flux quanta

We have introduced a new optical method for fast and precise manipulation of individual Abrikosov vortices in Type-II superconductors. The method uses the local photothermal heating of the superconductor with a focused laser beam. The later creates a temperature gradient in the superconductor and generates a thermal force, which drive vortices toward higher temperature regions. The obtained results unveil new aspects of the interaction between laser radiation and the vortex matter in superconductors and led to new projects on the interplay between photonics magnetism and superconductivity.

Development of photothermal microscopy

We have developed ultrasensitive optical techniques for the detection and study of non-fluorescing nano-objects. These new methods are based on the photothermal effect, and have unprecedented sensitivity which allowed for optical detection and absorption spectroscopy of tiny metal nanoparticles, non-luminescent quantum dots, and single walled carbon nanotubes. We have also developed an innovative method for single nanoparticle tracking using triangulation, a “Nano-GPS”, that can record arbitrarily long particle trajectories. We have designed and characterized a full functional nano-sized probe for long-term single-molecule imaging. We synthesized 5 nm gold nanoparticles functionalized with nanobodies; a small fragment of camelid antibody which recognizes widely used GFPs with a very high affinity. We showed that these small, functional gold nanoparticles could be detected and tracked for unlimited periods of time in living cells using photothermal imaging. We demonstrated the versatility and targeting efficiency of our probe by labeling several types of GFP-tagged proteins, both in in vitro and cellular systems. We also show that these probes can be used to study proteins not only in confined structures but also in intracellular compartments by tracking adhesive proteins and microtubule associated proteins in living cells.

Development of new super- resolution methods based on STED nanoscopy

Super-resolution microscopies have broken the long standing barrier of diffraction and achieved nanometer scale resolutions. One of the great challenges for these techniques is to achieve fast wide-field imaging while maintaining the nanoscale super-resolution. STED-Nanoscopy stands out by instantly providing signal from predetermined nanosized regions in the sample and can in principal provide better imaging speeds. STED nanoscopy remained a point-scanning technique, which needed parallelization. We have achieved massive parallelization of STED nanoscopy using wide-field excitation together with well-designed optical lattices for depletion and a fast camera for detection. Acquisition of large field of view super-resolved images requires scanning over a single unit cell of the optical lattice, which can be as small as 300nm x 300 nm. Optical lattices achieve efficient fluorescence depletion with moderate laser powers (Optics Express 2014). Our approach opens many perspectives in fast wide-field super-resolution imaging (e.g. endogenous proteins in living cells).

Exploiting nanosocopy to study the spatiotemporal orchestration of proteins in cell adhesion sites

In a study performed in a close synergistic association with the group of G. Giannone (IINs), we have unraveled the key spatiotemporal molecular events leading to integrins activation by their main activator talin in cell adhesion sites (AS). We performed single protein super-resolution microscopy and tracking to study integrin and talin displacements and distributions outside versus inside mature AS. We demonstrated that AS are specialized platforms priming integrins immobilization. Integrins are recruited to AS through a membrane diffusion/trapping mechanism, while talin is recruited to AS directly from the cytosol. Integrins reside in AS through free-diffusion and immobilization cycles, during which activation promotes immobilization (Nature Cell Biology 2012). This constituted one of the pioneering studies using single protein super-resolution tracking to tackle in-depth a biological function and an important step towards the understanding of the assembly/disassembly of AS and actin networks, which control critical cellular functions such as adhesion, mechano-transduction, migration.