Elemental nanomaterials, such carbon nanotubes (CNTs), graphene and more recently exfoliated black phosphorus (bP), are studied in my group for their low-dimensional semiconducting properties. Graphene is a semi-metal, whereas bP and CNTs are semiconductors having tunable band gaps in the near infrared, which depend on sample thickness for bP (Eg≈0.3-1.9 eV) and diameter for CNTs (Eg≈0.6-1.1 eV). Not present in bulk materials, the confinement effects also induce unexpected chemical, electrical, and optical properties, which are of interest for applications.
An overview of the general characteristics of quantum confinement will be presented first to highlight the impact on the optical and electrical properties of bP, graphene and NTs. This talk will more specifically present recent studies on exfoliated black phosphorus prepared in a glovebox as mono-, bi- and multilayered bP. Similar to the case of graphene, thin bP layers exhibit strong and unexpected optical resonances in Raman, the so-called D-modes, which can be understood in the context of coherent scattering involving defects and phonons. Because the process relaxes the optical selection rule forbidding access to q ≠ Γ phonons, the mechanism can produce strong and unexpected spectral features in different conditions, giving for instance anti-resonances in the mid-IR part of the spectrum and second-order Raman modes. Our work show that the defect-mediated resonances are ubiquitous in exfoliated bP, CNTs, and graphene and of interest for potential applications, such as THz wave modulators, nanoprobes and photodetectors. They are also useful to nanoscience because they help analyze interface properties, reactivity, and solve processing hurdles for making electronic nanodevices.

I will present a technique on how to inspect causal links between time-series based on dynamical systems theory, mainly, on the Takens theorem. As an application, I will show how to determine climatic drivers of malaria epidemics in a particular site (Northern Argentina). A discussion of new applications and extensions closes the talk.

Monolayers of semiconducting transition metal dichalcogenides are direct band gap semiconductors for which the direct band gap is located in the two inequivalent K and K' valleys. Because of the lack of inversion symetry and of the large spin orbit interaction, the spin and valley degrees of freedom are strongly coupled and a particular valley exciton population can be initialized using circularly polarized light excitation. As two dimensional materials, monolayers can be stacked on top of one another in the form of van der waals heterostructures to create artifical materials. Heterostructure of monolayer of TMD encapsulated in hBN offer a rich plateform to study the properties of 2D excitons.

In this talk, I will describe optical properties of bright and dark excitons in monolayers of TMD encapsulated in hexagonal boron nitride. Because of their 2D nature, excitons in monolayers of TMDs arze tighly bound and their propertie are strongly influenced by the dielectric environment. The relevent spectroscopic signature of such modifications is inscribed in their excitation spectrum and I will present a model to describe such influence. Dark excitons are activated by magnetic fields applied in different configurations. The exchange interaction mixes dark excitons in the two valleys and I will show how this mixing can be monitored by external magbetic fields.

Dense colloidal suspensions are paramount in many industrial applications. However, as they are governed by Brownian motion and particle interactions, the ability to quantify their dynamics in flowing systems remains challenging. Using nanocelluloses as model systems, we focus on the dynamics of a colloidal nanorod suspension forming a thread in a flow-focusing channel. I will first demonstrate an experimental method for characterization of the nanorod orientation dynamics using a flow-stop technique coupled with a thorough analysis of the orientation relaxation. The coupling of rotational diffusion coefficients to particle size distributions as well as flow-induced orientation mechanisms in the nanorod network will be discussed. Then, I will focus on the shape of the thread formed by the suspension. Using experiments performed with Optical Coherence Tomography and numerical simulations with a Volume of Fluid method, I will exhibit that the development of the thread in the channel is controlled by an effective interfacial tension between the colloidal suspension and water.

Optical resonators are basic elements for a huge variety of devices, from lasers to sensors. They are also at the basis of important quantum electrodynamic effects, such as tailoring the spontaneous emission via the Purcell effect. Optical resonators are therefore of the utmost relevance in many different fields and they have been extensively investigated during the last century, still some important revisions of their understanding are recently emerged in the framework on non-Hermitian theories.
The basis of the novel models is the open nature of any optical resonator, since photons cannot be confined in a finite volume for infinite time. This simple and well known fact implies complex eigenvalues and eigenvectors of the photonic master equations, leading to a deep revision of the famous expression for the Purcell factor, with counterintuitive effects, such as complex modal volume, Fano-like local density of states and even negative radiative rate contributions [1-3].
We experimentally demonstrate all these predictions in cavities on photonic crystal slabs with relatively low optical loss. Near-field hyperspectral imaging via quantum dot photoluminescence is proved to be a direct tool for detecting the counterintuitive effects of non-Hermitian theories. The experimental results fully agree with the predictions [4,5].
References
[1] C. Sauvan, PRL 110, 237401 (2013)
[2] E. A. Muljarov, W. Langbein, Phys. Rev. B 94, 235438 (2016)
[3] P. Lalanne et al., Laser Photonics Rev. 12, 1700113 (2018) 
[4] K-G. Cognée et al. Optica, 6, 269 (2019)
[5] D. Balestri et al, submitted to PRL.

Single chain nanoparticles (SCNPs) are an emergent class of synthetic nano-objects, of size 5-20 nm, obtained through intramolecular cross-linking of single polymer chains. They are the basis of the so-called single-chain technology, a rapidly growing research area of enormous possibilities and applications in fields as biosensors, catalysts, drug delivery or rheological agents. In this presentation we summarize the state-of-the art and our most recent contributions to the field through the combination of synthesis, simulations, scattering (neutron and X-ray) and relaxation techniques (dielectric spectroscopy and rheology), and discuss the main challenges for the next years.

SCNPs are topologically polydisperse. The standard synthesis protocols produce SCNPs with open sparse topologies consisting of tightly linked domains connected by flexible linkers, displaying analogies with intrinsically disordered proteins (IDPs) and suggesting SCNPs as a model system -free of specific interactions- to discriminate steric effects on IDPs in crowded cellular solutions As opposite to the gaussian conformations adopted by linear chains, sparse SCNPs in concentrated solutions and bulk exhibit an entropically driven collapse to fractal globular conformations. This observation is independent of the specific architecture of the macromolecular crowders (linear or complex) surrounding the SCNP [3,4] and on the lifetime of the SCNP bonds (irreversible or dynamic).

Simulations can be used to envisage a broad set of realistic synthesis routes to control the compactness of SCNPs, leading to globular cross-linked objects much smaller than microgels. Concentrated solutions of globular SCNPs exhibit, in contrast to hard spheres, structural and dynamic anomalies, as soft caging, reentrant behaviour (ordering and speeding up by increasing concentration) and absence of dynamic heterogeneities. This constitutes a realization in a real macromolecular system of the qualitative scenarios proposed for a long time by generic models of structureless ultrasoft particles.

Finally we explore the possibility of using SCNPs as nanofillers for linear matrixes in fully-polymeric nanocomposites and even for designing bulk materials based only in SCNPs. We characterize several dynamic scales going from the alpha-relaxation to the reorientation of the SCNPs. Whereas the glass transition is just slightly affected with respect to their linear counterparts, sparse SCNPs exhibit a much faster relaxation of the chain modes, reflecting a strong suppression of entanglements. When SCNPs are used in nanocomposites, their specific topology has a different effect on the dynamics of the linear chains. By characterizing the entanglement network in the nanocomposite we find that sparse and globular SCNPs lead respectively to mild dilation and strong narrowing of the tube for the reptational motion of the linear chains.

The increased flexibility in how commercial microscopes can be modified to add external features has opened the possibility of numerous new
types of experiments. In this talk I will present work on two such projects.



In a first study, the growth of condensed colloidal states from a fluid phase is explored in the presence of an external drive of the system by a controlled thermal
field and thermophoresis. A thermal gradient is formed in a fluid bath of colloids by adding and trapping with optical tweezers a high refractive index particle.
By absorbing a portion of the laser light from the tweezers, the particle acts as a point-like heat source and causes a temperature gradient in the mixture that is
highly localized in both space and time. This leads to the clustering and phase transition by thermophoresis of colloids from the bath around the trapped particle.
I will show that the steady-state size of the aggregates can be determined by the shape of the external field and the bulk volume fraction of the system and that condensed
structures with various order can be generated (crystalline, amorphous and icosahedral order). This novel technique to trigger phase transitions opens the way to probe the dynamics
of growth of phase transitions from highly out-of-equilibrium conditions and to observe the behaviour of interfaces in potentials that are inhomogeneous at the scale of the fluctuations
of the interface.



In the second project, I explore the strength of soft materials against puncture and penetration by sharp objects. These are properties of condensed matter relevant to various
situations spanning the design of strong gloves, to understanding animals bite mechanics or the invasion of filament-shaped yeasts breaking through human skin. Puncture and penetration
are described by an interplay between several mechanisms: the material elastic response, fracture, crack propagation, friction and adhesion. This variety of mechanisms raises the question
of which are mostly relevant to any particular situation, and my focus will be here on two types of micron-scale objects indenting soft polydimethyl siloxane (PDMS). First, nanoindentation
is used to measure the force acting on micropipettes as they are pushed into a PDMS block. I will show that, here, the critical force to puncture is highly dependent on the speed of the indenter
and that there is an optimal polymer crosslinker concentration to resist against puncture. Second, deformations of PDMS are measured as an invasive yeast, Candida albicans, grows in this
substrate in the shape of a filament with a tip continuously moving forward by cell wall creation. Using a dedicated "compression force microscopy" technique, the 3d the displacement of
fluorescent particles in the PDMS is monitored and highlights that it sustains deformations that are surprisingly smaller compared to the nanoindentation tests as well as other detailed
features of the growth.

We will encode the quantum entropy function into modular structures such as Jacobi Forms and Siegel modular forms to uncover interesting structures pointing to an underlying Calogero model as a putative CFT that is the holographically dual model to quantum gravity in ADS_2.

Observing and controlling macroscopic quantum systems has long been a driving force in research on quantum physics. In this endeavor, coupling individual quantum systems to mechanical oscillators is of great interest. While both read-out of mechanical motion using two-level spin systems and spin read-out using oscillators have been demonstrated, temperature control of the motion of a macroscopic object using electronic spins is still a daunting task. We will present our observations of spin-dependent torque and dynamical back-action from a micro-diamond levitating in a Paul Trap [2]. Using a combination of microwave and laser excitation enables the spin of nitrogen-vacancy centers to act on the diamond orientation and to observe cooling of the diamond libration. Further, driving the system in the non-linear regime, we demonstrate bistability and self-sustained lasing of the librational mode. The final temperature is currently limited by microwave excitation on the blue side of the spin-resonance since the trapping frequency is lower than the spin decoherence rate. We discuss ways to achieve spin-cooling to the ground state and present first steps towards a platform that uses levitating magnets and CVD grown diamond samples in order to reach the sideband resolved regime [3].

[1] Delord T. et al. Phys. Rev. Lett. 121 053602 (2018)
[2] Delord T., et al. arXiv :1905.11509 (2019)
[3] Huillery P. et al. ArXiv 1903.09699 (2019)