Effects of ambient air pressure on surface structures produced by ultrashort laser pulse irradiation.

We report an experimental analysis addressing striking effects of residual air ambient pressure, from atmospheric conditions (103 mbar) to high vacuum (10-4 mbar), on the surface structures induced on a silicon target by direct femtosecond laser irradiation. We observe an interesting direct impact of the ambient pressure on the period and depth of the generated ripples as well as on the formation of microgrooves. Moreover, a significant correlation is observed between the ripples' period and depth. The change of pressure is accompanied by a variation of the degree of nanoparticle coverage, which is eventually recognized as an important factor for the development of the final surface structures. These results shed light on the intriguing mechanisms underlying the formation of the various surface textures, also evidencing that the ambient pressure can act as an effective parameter to tailor some characteristic features of the processed surface.

Ultrafast supercontinuum spectroscopy of carrier multiplication and biexcitonic effects in excited states of PbS quantum dots.

We examine the population dynamics of multiple excitons in PbS quantum dots using spectrally resolved ultrafast supercontinuum transient absorption (SC-TA) measurements. We simultaneously probe the first three excitonic transitions. The transient spectra show the presence of bleaching of absorption for the 1S(h)-1S(e) transition, as well as transients associated with the 1P(h)-1P(e) transition. We examine signatures of carrier multiplication (multiple excitons arising from a single absorbed photon) from analysis of the bleaching features in the limit of low absorbed photon numbers (left angle bracket N(abs) right angle bracket ∼ 10(-2)) for pump photon energies from two to four times that of the band gap. The efficiency of multiple-exciton generation is discussed both in terms of the ratio between early- to long-time transient absorption signals and of a broadband global fit to the data. Analysis of the population dynamics shows that bleaching associated with biexciton population is red shifted with respect to the single exciton feature, which is in accordance with a positive binding energy for the biexciton.

Enhanced light coupling in sub-wavelength single-mode silicon on insulator waveguides.

We report on NIR efficient end-coupling in single-mode silicon on insulator waveguides. Efficient coupling has been achieved using Polymer-Tipped Optical Fibers (PTOF) of adaptable radius of curvature (ROC). When compared with commercial micro lenses, systematic studies as a function of PTOF ROC, lead for subwavelength PTOF to a coupling factor enhancement as high as 2.5. This experimental behavior is clearly corroborated by radial FDTD simulations and an absolute coupling efficiency of about 50% is also estimated.

Field localization and enhanced Second-Harmonic Generation in silicon-based microcavities.

High-quality amorphous Silicon Nitride (a-Si(1-x)N(x):H) Fabry-Pérot microcavities can show resonant surface Second Harmonic Generation (SHG) effect. We consider two different layouts of planar microcavities with almost identical linear reflectance and show how the structure geometry can strongly affect SHG yield. In particular, a difference of more than one order of magnitude in the SHG intensity is observed when the fundamental beam is tuned at the cavity resonance frequency. We explain this finding on the basis of a theoretical model taking into account the spatial distribution of the electric fields of the pump and harmonic frequencies inside the structure. A satisfactory matching of experimental data with the theoretical model is obtained by considering the source of second-order nonlinearity as limited to surface contributions.