Coherent Longitudinal Acoustic Phonons in Three-Dimensional Supracrystals of Cobalt Nanocrystals

We use broadband picosecond acoustics to detect longitudinal acoustic phonons with few-gigahertz frequency in three-dimensional supracrystals (with face-centered cubic lattice) of 7 nm cobalt nanocrystal spheres. In full analogy with atomic crystals, where longitudinal acoustic phonons propagate with the speed of sound through coherent movements of atoms of the lattice out of their equilibrium positions, in these supracrystals atoms are replaced by (uncompressible) nanocrystals and atomic bonds by coating agents (carbon chains) that act like mechanical springs holding together the nanocrystals. By repeating the measurements at different laser angles of incidence it was possible to accurately determine both the index of refraction of the supracrystal (<i>n</i> = 1.26 ± 0.03) and the room-temperature longitudinal speed of sound (<i>v</i>_s= 1235 ± 12 m/s), which is quite low due to the heavy weight of the spheres (with respect to atoms in a crystal) and the soft carbon chains (with respect to atomic bonds). Interestingly, the speed of sound inside the supracrystal was found to dramatically increase by decreasing the sample temperature due to a change in the stiffness of the dodecanoic acid chains which coat the Co nanocrystals

Coherent two-dimensional micro-spectroscopy: Application on plasmon propagation and TMDC materials

Poster from Plasmonica 2018 conference Nano-antennas have the unique ability to channel far-field radiation to sub-wavelength dimensions. The resulting strongly confined and enhanced electromagnetic fields boost nonlinear optical effects at the nanoscale. For this purpose, we introduce coherent two-dimensional (2D) micro-spectroscopy which probes the nonlinear optical response of the nano-antennas with sub-micron spatial resolution. An LCD-based pulse shaper in 4f geometry is used to create collinear trains of 12-fs visible/NIR laser pulses in the focus of a numerical aperture of a 1.4 immersion-oil microscope objective. We motivate this new method for getting nonlinear third-order information of the ultrafast dynamics of plasmon propagation via phase cycling, e.g., for the local spatial investigation of the strong coupling between a transition metal dichalcogen-ide (TMD) monolayers and a nano-antenna on top of it.</p

Unveiling the Role of Electron-Phonon Scattering in Dephasing High-Order Harmonics in Solids

High-order harmonic generation (HHG) in solids is profoundly influenced by the dephasing of the coherent electron-hole motion driven by an external laser field. The exact physical mechanisms underlying this dephasing, crucial for accurately understanding and modelling HHG spectra, have remained elusive and controversial, often regarded more as an empirical observation than a firmly established principle. In this work, we present comprehensive experimental findings on the wavelength-dependency of HHG in both single-atomic-layer and bulk semiconductors. These findings are further corroborated by rigorous numerical simulations, employing ab initio real-time, real-space time-dependent density functional theory and semiconductor Bloch equations. Our experimental observations necessitate the introduction of a novel concept: a momentum-dependent dephasing time in HHG. Through detailed analysis, we pinpoint momentum-dependent electron-phonon scattering as the predominant mechanism driving dephasing. This insight significantly advances the understanding of dephasing phenomena in solids, addressing a long-standing debate in the field. Furthermore, our findings pave the way for a novel, all-optical measurement technique to determine electron-phonon scattering rates and establish fundamental limits to the efficiency of HHG in condensed matter

Charge Photogeneration in Donor–Acceptor Conjugated Materials: Influence of Excess Excitation Energy and Chain Length

We investigate the role of excess excitation energy on the nature of photoexcitations in donor–acceptor π-conjugated materials. We compare the polymer poly­(2,6-(4,4-bis­(2-ethylhexyl)-4H-cyclopenta­[1,2-<i>b</i>;3,4-<i>b</i>′]­dithiophene)-4,7-benzo­[2,1,3]­thiadiazole) (PCPDTBT) and a short oligomer with identical constituents at different excitation wavelengths, from the near-infrared up to the ultraviolet spectral region. Ultrafast spectroscopic measurements clearly show an increased polaron pair yield for higher excess energies directly after photoexcitation when compared to the exciton population. This effect, already observable in the polymer, is even more pronounced for the shorter oligomer. Supported by quantum chemical simulations, we show that excitation in high-energy states generates electron and hole wave functions with reduced overlap, which likely act as precursors for the polaron pairs. Interestingly, in the oligomer we observe a lifetime of polaron pairs which is one order of magnitude longer. We suggest that this behavior results from the intermolecular nature of polaron pairs in oligomers. The study excludes the presence of carrier multiplication in these materials and highlights new aspects in the photophysics of donor–acceptor small molecules when compared to polymers. The former are identified as promising materials for efficient organic photovoltaics