In situ scanning tunneling microscopy of corrosion of silver-gold alloys.

An in situ scanning tunneling microscope (STM) was used to observe the morphological changes accompanying the selective dissolution of Ag from low-Ag content Ag-Au alloys in dilute perchloric acid. This study was undertaken to explore the role of surface diffusion in alloy corrosion processes. These results are interpreted within the framework of the kink-ledge-terrace model of a crystal surface and a recent model of alloy corrosion based on a variant of percolation theory. The corrosion process leads to roughening of the surface by dissolution of Ag atoms from terrace sites. Annealing or smoothening of the surface occurs by vacancy migration through clusters and the subsequent annihilation of clusters at terrace ledges.

Visible continuum generation using a femtosecond erbium-doped fiber laser and a silica nonlinear fiber.

Supercontinuum extending to visible wavelengths is generated in a hybrid silica nonlinear fiber pumped at 1560 nm by a femtosecond, erbium-doped fiber laser. The hybrid nonlinear fiber consists of a short length of highly nonlinear, germano-silicate fiber (HNLF) spliced to a length of photonic crystal fiber (PCF). A 2 cm length of HNLF provides an initial stage of continuum generation due to higher-order soliton compression and dispersive wave generation before launching into the PCF. The visible radiation is generated in the fundamental mode of the PCF.

Coherence of supercontinua generated by ultrashort pulses compressed in optical fibers.

Femtosecond fiber lasers together with nonlinear fibers are compact, reliable, all-fiber supercontinuum sources. Maintaining an all-fiber configuration, however, necessitates pulse compression in an optical fiber, which can lead to nonlinearities for subhundred femtosecond, nanojoule pulses. In this work we show that using large-mode-area fibers for pulse compression mitigates the nonlinearity, resulting in compressed pulses with significantly reduced satellite pulses. Consequently, supercontinua generated with these pulses are shown to have as much as a 10 dB increase in coherence fringe contrast. By using a hybrid highly nonlinear fiber-photonic crystal fiber, the continuum can be extended to visible wavelengths while still maintaining high coherence.