Development of Sub 10 fs Visible-NIR, UV, and DUV Pulses and Their Applications to Ultrafast Spectroscopy

Generation of sub-100 fs ultraviolet pulses from a Kerr-lens mode-locked Ce:LiCAF laser

We report the direct generation of mode-locked pulses as short as 91 fs from the broad-bandwidth gain medium of LiCaAlF₆ (Ce:LiCAF) by combining Kerr-lens mode locking with synchronous pumping. The latter of these schemes, and the broad bandwidth of Ce:LiCAF, resulted in dispersion tuning of wavelength via cavity length in the spectral region of 290 nm; this mechanism facilitated a practical means of estimating intra-cavity dispersion, which was compensated for using a Brewster's-cut prism pair. The pulse duration was measured via split-beam asynchronous cross-correlation using a Ti:sapphire reference laser and a known time reference. From the Ce:LiCAF laser cavity, output powers of 110 mW and a 9% slope efficiency were achieved.

Ultrashort visible energetic pulses generated by nonlinear propagation of necklace beams in capillaries

The generation of ultrashort visible energetic pulses is investigated numerically by the nonlinear propagation of infrared necklace beams in capillaries. We have developed a (3+1)D model that solves the nonlinear propagation equation, including the complete spatio-temporal dynamics and the azimuthal dependence of these structured beams. Due to their singular nonlinear propagation, the spectrum broadening inside the capillary extends to the visible region in a controlled way, despite the high nonlinearity, avoiding self-focusing. The results indicate that the features of these necklace beams enable the formation of visible pulses with pulse duration below 10 fs and energies of 50 μJ by soliton self-compression dynamics for different gas pressures inside the capillary.

Photoinduced Ultrafast Charge Transfer and Charge Migration in Small Gold Clusters Passivated by a Chromophoric Ligand

Because the development of attopulses, charge migration induced by short optical pulses has been extensively investigated. We report a computational purely electronic dynamical study of ultrafast few femtoseconds (fs) charge transfer and charge migration in realistic passivated stoichiometric Au₁₁ and Au₂₀ gold nanoclusters functionalized by a bipyridine ligand. We show that a net significant amount of electronic charge (0.1 to 0.4 |e| where |e| is the electron charge) is permanently transferred from the bipyridine chromophore to the gold cluster during the short 5-6 fs UV-vis strong pulse. This electron transfer to the metallic core is induced by the optical excitation of electronic states with a partial charge transfer character involving the chromophore before the onset of nuclei motion. In addition, the photoexcitation by the strong fs pulse builds a nonequilibrium electronic density that beats between the chromophore and the metallic core around the average of the transferred value. Modular systems made of a donor chromophore that can be photoexcited in the UV-vis range coupled to an efficient acceptor that could trap the charge are of interest for applications to nanodevices. Our study provides understanding on the very early, purely electronic dynamics built by the fs optical excitation and the initial charge separation step.

High-average-power femtosecond laser at 258 nm

We present an ultrafast fiber laser system delivering 4.6 W average power at 258 nm based on two-stage fourth-harmonic generation in beta barium borate (BBO). The beam quality is close to being diffraction limited with an M² value of 1.3×1.6. The pulse duration is 150 fs, which, potentially, is compressible down to 40 fs. A plain BBO and a sapphire-BBO compound are compared with respect to the achievable beam quality in the conversion process. This laser is applicable in scientific and industrial fields. Further scaling to higher average power is discussed.