Yasushi Iwata

Plume dynamics during film and nanoparticles deposition by pulsed laser ablation

The gas dynamics of pulsed laser ablation of silicon target in the helium gas ambient is investigated via direct simulation Monte Carlo method with a real physical scale of target-substrate configuration. A shock driven process is clearly observed. It is shown that the interaction of the shock front with the target surface and the vapor front induce significant backward flux of ablated particles and oscillating behavior of vapor front. A confined layer mixed with high density Si and He atoms is formed around the contact front. Its behavior is important to the nanoparticle formation and deposition.

Cluster formation dynamics in a locally-confined gas layer mixed with the plume ablated by pulsed laser irradiation

Abstract The dynamical process of pulsed laser ablation is investigated by means of Monte Carlo simulations in order to develop a new cluster source. The interaction between the ablated Cu vapor plume and the He ambient gas induces strong shock compression of the gas by which the gas density is one order of magnitude increased and forms a stable gas layer mixed with the plume at the contact front. It is found that even with the target-wall distance as short as about 80 μm, a mixed layer more than 10 μm in thickness can be formed at about 15 μm away from the source wall. This layer can be locally-confined for a considerable long time of hundreds of ns. Frequent collisions occur among the atoms of vapor and gas within the confined layer which induce effective cluster aggregation. The thermodynamic state of this region is well defined, so that the mono-dispersed growth condition might form cluster beam with well-defined internal freedom.

EELS imaging analysis of surface plasmon polaritons confined in silicon cluster superlattice

Silicon cluster superlattices, which have been generated by direct deposition of a silicon cluster beam with a monodisperse size distribution, give a just solution to bring forth new functional nanomaterials consistent with a sustainable society. Silicon clusters correctively form a unclosed-packed body centered cubic (bcc) superlattice structure with a lattice constant of 2.134±0.002 nm, retaining a sp3 diamond structure of the same lattice constant as in crystalline silicon (c-Si). The sp3 structures are spread uniformly across the superlattice and the endogenous surfaces on the silicon clusters successively form a topologically new nanostructure. The low-loss EELS spectra with a 60 keV probing electron beam have demonstrated surface plasmon losses in the energy range 3.0 eV ≤ E, along with an intense peak at 16.5 eV of the bulk plasmon losses in c-Si. The results are consistent with the analytical results of the endogenous surface Tamm states with a metallic electronic structure. The characteristic surface plasmon loss peaks appearing at even intervals of nearly 1 eV reveal that each incident probing electron absorbs multi-quanta nħω (n = 1, 2, 3, .., 10) from surface plasmons localized in the silicon cluster superlattice. Silicon cluster superlattices possibly clear a path to Nanophotonic Silicon in future photonics.