Thierry Barisien

Macroscopic coherence of a single exciton state in an organic quantum wire

Macroscopic quantum coherence has been observed in some many-body systems including superconductors, quantum liquids1 and cold atom condensates2, but never for a single quasi-particle state. In an ideal semiconductor, excitons (electron–hole pairs bound by the Coulomb interaction) can, in principle, exist as delocalized plane waves extending over the entire volume. However, any kind of disorder prevents long-range spatial coherence from emerging. There has been evidence for the formation of macroscopic coherent states only in condensate phases such as in the case of microcavity polaritons condensation3,4 or in a dense quasi-two-dimensional exciton gas5. It is unclear however, whether in this latter case the observations are really related to macroscopic coherence6. Here, we show that a single exciton state in an individual ordered conjugated polymer chain7,8, shows macroscopic quantum spatial coherence reaching tens of micrometres, limited by the chain length. The spatial coherence of the k=0 exciton state is demonstrated by selecting two spatially separated emitting regions of the chain and observing their interference.

Polymorphs and colors of polydiacetylenes: a first principles study.

Polydiacetylenes (PDAs) are exceptional polymeric materials with pi-conjugated backbones. Several of them can undergo chromogenic transitions under a wide range of external stimuli. Herein we investigate the electronic structure and the resulting properties of model and experimental PDAs, by means of first principles condensed matter calculations. It is shown that torsional isomers with a twist of the lateral groups can be formed at small energetic costs. We also show the relationship that exists between these twists and the observed changes in the electronic and physical properties. In particular, the calculated changes in the absorption, Raman and NMR spectra agree with the color and property changes as observed experimentally. Therefore, these isomers are excellent models for the structures involved in the chromogenic transitions.

Reversible quenching of a chromophore luminescence by color transition of a polydiacetylene.

A new reactive diacetylene molecule has been synthesized, incorporating a strongly luminescent chromophore, tetrazine (Tz). It readily polymerizes into the blue polydiacetylene (PDA) form, quenching the Tz luminescence already at concentrations ≤1 %. The blue to red PDA transition is thermally induced in the solid state and the original strong Tz emission is restored. This might lead to a new type of detection for sensors using the PDA color transition.

Coherent control of the optical emission in a single organic quantum wire.

We report on the first coherent control experiments on a purely electronic exciton state in an extended quasi-perfect organic quantum wire, a polydiacetylene chain isolated in the crystalline matrix of its own monomer. The time-integrated luminescence of a single wire is measured as the relative phase between two exciting sub-picosecond laser pulses is varied. From visibility functions the exciton dephasing time is extracted and its temperature dependence studied. Our work points the predominant role of thermalization upon the phase relaxation dynamics. By means of microscopic imaging spectroscopy we also show that despite local excitation coherent control is achieved on states delocalized over the chain at the micrometric scale.

Exciton-phonon coupling in a CsPbBr3 single nanocrystal

We have performed micro-photoluminescence measurements on a single CsPbBr3 nanocrystal (NC) with a size comparable to the Bohr diameter (7 nm). When the NC has an orthorhombic crystal symmetry, we observe an exciton fine structure composed of three peaks linearly polarized. We took advantage of the polarization properties of micro-photoluminescence to monitor in situ both the energy and linewidth of individual peaks when increasing temperature. We reveal that two regimes exist, at low and high temperature, which are dominated by acoustic or longitudinal optical phonon (Frohlich term) couplings, respectively. The acoustic contribution does not change when the energy of the excitonic transition varies in the range of 2.46–2.62 eV, i.e., with NC sizes corresponding to this range. We find that line broadening is mainly ruled by the Frohlich term, which is consistent with the polar nature of CsPbBr3.

Strategy to overcome recombination limited photocurrent generation in CsPbX3 nanocrystal arrays

We discuss the transport properties of CsPbBrxI3−x perovskite nanocrystal arrays as a model ensemble system of caesium lead halide-based perovskite nanocrystal arrays. While this material is very promising for the design of light emitting diodes, laser, and solar cells, very little work has been devoted to the basic understanding of their (photo)conductive properties in an ensemble system. By combining DC and time-resolved photocurrent measurements, we demonstrate fast photodetection with time response below 2 ns. The photocurrent generation in perovskite nanocrystal-based arrays is limited by fast bimolecular recombination of the material, which limits the lifetime of the photogenerated electron-hole pairs. We propose to use nanotrench electrodes as a strategy to ensure that the device size fits within the obtained diffusion length of the material in order to boost the transport efficiency and thus observe an enhancement of the photoresponse by a factor of 1000.

Coherent Exciton State In An Organic Quantum Wire

For the first time, we evidence the delocalization of a single quasiparticule state, an exciton, over the entire length of the organic quantum wire on which it is photocreated. This quantum wire, a polydiacetylene chain, has the properties of a perfect quasi‐1D semiconducting system where, moreover, a strong exciton‐photon coupling regime can be reached without the need of a cavity for the photons. The macroscopic coherence of the exciton state on the chain is demonstrated using an interference experiment.