Jude Deschamps

The advent of free electron lasers has opened the opportunity to explore interactions between extreme ultraviolet (EUV) photons and collective excitations in solids. While EUV transient grating spectroscopy, a noncollinear four-wave mixing technique, has already been applied to probe coherent phonons, the potential of EUV radiation for studying nanoscale spin waves has not been harnessed. Here we report EUV transient grating experiments with coherent magnons in Fe/Gd ferrimagnetic multilayers. Magnons with tens of nanometers wavelengths are excited by a pair of femtosecond EUV pulses and detected via diffraction of a probe pulse tuned to an absorption edge of Gd. The results unlock the potential of nonlinear EUV spectroscopy for studying magnons and provide a tool for exploring spin waves in a wave vector range not accessible by established inelastic scattering techniques.

Equilibrium Bose-Einstein condensation of exciton-polaritons, demonstrated with a long-lifetime microcavity [Phys. Rev. Lett. 118, 016602 (2017)], has proven that driven-dissipative systems can undergo thermodynamic phase transitions in the limit where the quasiparticle lifetime exceeds the thermalization time. Here, we identify the role of dimensionality and polariton interactions in determining the degree of thermalization in optically generated traps. To distinguish the effect of trapping from interactions and lifetimes, we measured the polariton distribution under four nonresonant Gaussian pumps in a square geometry and compared it with polariton distributions measured with each pump individually. We found that significant redistribution of polaritons arises by trapping and modification of the density of states. Surprisingly efficient polariton-polariton scattering below the condensation threshold is evidenced by the depletion of the inflection-point polaritons. Our work provides a deeper understanding of polariton distributions and their interactions under various geometries of optically generated potentials.

Roaming dynamics in real time: Roaming is distinct from conventional reaction channels because of the unusual geometries that chemical systems use to bypass the minimum energy pathway. It is a relatively new phenomenon that is usually determined in experiments through spectroscopic characterization of the roaming products. Using a combination of time-resolved Coulomb explosion imaging and quasiclassical trajectory analysis, Endo et al. report real-time observation of individual fragments of the prototypical reaction of deuterated formaldehyde (D ₂ CO) dissociation as they roam on ultrafast time scales. They show that roaming not only occurs several orders of magnitude earlier than previously expected but also that it can terminate in a radical (D + DCO) rather than the well-known molecular (D ₂ + CO) product channel. Science , this issue p. 1072

The C₃-symmetric cyclotriveratrylene (CTV) was covalently bonded via click chemistry to 1, 2, 3, and 6 zinc(II) porphyrin units to various host for C₆₀. The binding constants, K_a, were measured from the quenching of the porphyrin fluorescence by C₆₀. These constants vary between 400 and 4000 M^–1 and are considered weak. Computer modeling demonstrated that the zinc(II) porphyrin units, [Zn], exhibit a strong tendency to occupy the CTV cavity, hence blocking the access for C₆₀ to land on this site. Instead, the pincer of the type [Zn]----[Zn] and in one case [Zn]----CTV, were found to be the most probable geometry to promote host–guest associations in these systems.

Two series of cofacial bis(dipyrrins) were prepared and their photophysical properties as well as their bimolecular fluorescence quenching with C ₆₀ were investigated.