Translocation of a stiff polymer in a microchannel.

The voltage-driven dynamics of a stiff polymer through a nanopore are treated with a bend elastic model. In contrast to flexible polymers described by a stretch elasticity, bend elastic chains can be oriented in an external field, here the anchoring field created by the pore atoms. The trajectory of the chain is calculated using the Langevin equation of motion. The dynamical equation is solved by a normal mode analysis of the elastic curve with free ends. Interaction with the pore walls acts to align the chain, and with the electric field induced inside the pore controls the translocation time. Application of a force proportional to the distance of the exit from the end of the pore such as an optical trap slows down the motion, and reduces the chain response to the wall potential and the extension along the pore axis. DNA is a well-known semirigid polymer, and a comparison is made to the molecular dynamics simulation of translocation of DNA through a synthetic nanopore.

Melting-induced enhancement of the second-harmonic generation from metal nanoparticles.

We report on the generation of second-harmonic signals by irradiating monolayers of high purity Ga nanoparticles, embedded in a SiOx matrix, with femtosecond laser pulses at 800 nm. A remarkable melting-induced enhancement of the second-harmonic generation is observed in correspondence of the phase transition. In addition, the hysteresis cycle of nonlinear transmittance is shown to be amplified a factor of 80-100 with respect to the linear response and interpreted in the framework of a nonlinear effective-medium model.