Highly efficient THz four-wave mixing in doped silicon.

Third-order non-linearities are important because they allow control over light pulses in ubiquitous high-quality centro-symmetric materials like silicon and silica. Degenerate four-wave mixing provides a direct measure of the third-order non-linear sheet susceptibility χ(3)L (where L represents the material thickness) as well as technological possibilities such as optically gated detection and emission of photons. Using picosecond pulses from a free electron laser, we show that silicon doped with P or Bi has a value of χ(3)L in the THz domain that is higher than that reported for any other material in any wavelength band. The immediate implication of our results is the efficient generation of intense coherent THz light via upconversion (also a χ(3) process), and they open the door to exploitation of non-degenerate mixing and optical nonlinearities beyond the perturbative regime.

Intensity-resolved IR multiple photon ionization and fragmentation of C60.

The sequential absorption of multiple infrared (IR) photons by isolated gas-phase species can lead to their dissociation and/or ionization. Using the newly constructed "Free-Electron Laser for IntraCavity Experiments" (FELICE) beam line at the FELIX facility, neutral C(60) molecules have been exposed to an extremely high number (approximately 10(23)) of photons/cm(2) for a total time duration of up to 5 micros. At wavelengths around 20 microm, resonant with allowed IR transitions of C(60), ionization and extensive fragmentation of the fullerenes are observed. The resulting photofragment distributions are attributed to absorption in fragmentation products formed once C(60) is excited to internal energies at which fragmentation or ionization takes place within the duration of the laser pulse. The high IR intensities available combined with the large interaction volume permit spatially resolved detection of the ions inside the laser beam, thereby disentangling the contributions from different IR intensities. The use of spatial imaging reveals intensity dependent mass distributions that are substantially narrower than what has been observed previously, indicating rather narrow energy distributions. A simple rate-equation modeling of the excitation process supports the experimental observations.

Structures of neutral Au7, Au19, and Au20 clusters in the gas phase.

The catalytic properties of gold nanoparticles are determined by their electronic and geometric structures. We revealed the geometries of several small neutral gold clusters in the gas phase by using vibrational spectroscopy between 47 and 220 wavenumbers. A two-dimensional structure for neutral Au7 and a pyramidal structure for neutral Au20 can be unambiguously assigned. The reduction of the symmetry when a corner atom is cut from the tetrahedral Au20 cluster is directly reflected in the vibrational spectrum of Au19.

Excited-state lifetimes of far-infrared collective modes in proteins.

Vibrational excitations of low frequency collective modes are essential for functionally important conformational transitions in proteins. Here we report the first direct measurement on the lifetime of vibrational excitations of the collective modes at 87 microm (115 cm(-1)) in bacteriorhodopsin, a transmembrane protein. The data show that these modes have extremely long lifetime of vibrational excitations, over 500 ps, accommodating 1500 vibrations. We suggest that there is a connection between this relatively slow anharmonic relaxation rate of approximately 10(9) sec(-1) and the similar observed rate of conformational transitions in proteins, which require multilevel vibrational excitations.