Femtosecond

About: Femtosecond is a research topic. Over the lifetime, 35106 publications have been published within this topic receiving 691405 citations. The topic is also known as: 1 E-15 s & fs.

Ti:sapphire-pumped high repetition rate femtosecond optical parametric oscillator

Abstract: A broadly tunable femtosecond optical parametric oscillator (OPO) based on KTiOPO4 is externally pumped by a self-mode-locked Ti:sapphire laser. The laser is capable of continuous tuning from 1.2 micrometers to 1.37 micrometers in the signal branch and 1.8 to 2.15 micrometers in the idler branch, when using one set of OPO optics. Other optics expand the tuning range of the OPO from 1.0 micrometers to 2.75 micrometers, for example, by using three sets of mirrors and two different crystals. Without prisms in the OPO cavity, 215 mW of chirped pulses is generated in the signal branch, while 235 mW is generated in the idler branch. The total conversion efficiency, as measured by pump depletion, is 50%. With prisms in the cavity, nearly transform-limited pulses of 135 femtoseconds are generated, which can be shortened to 75 fs by increasing the output coupling.

Femtosecond Dynamics of Double Proton Transfer in a Model DNA Base Pair: 7-Azaindole Dimers in the Condensed Phase

Abstract: The dynamics of double proton transfer in 7-azaindole (7-AI) dimers, a model DNA base pair, are investigated in real time using femtosecond transient absorption and fluorescence upconversion techniques. In nonpolar solvents we examine the isotope effect, the excitation energy dependence, and the structure analogue of the tautomer (7-MeAI). A detailed molecular picture of the nuclear dynamics in the condensed phase emerges with the relationship to the dynamics observed in molecular beams: Following the femtosecond excitation there are three distinct time scales for structural relaxation in the initial pair, proton (hydrogen) transfers, and vibrational relaxation or cooling of the tautomer. The molecular basis of tunneling and concertedness are elucidated by careful examination of the isotope effect and the time resolution. Comparison with the results in the isolated pair indicates the critical role of the N−H and N···N nuclear motions in determining the effective potential, and the thermal excitation in solution. Because the barrier is small, ∼1.3 kcal/mol, both are important factors and experiments at much higher energies will be unable to test either tunneling or concertedness. Finally, we compare the experimental results and the dynamical picture with detailed ab initio and molecular dynamics simulations.

Femtosecond laser irradiation of metallic surfaces: effects of laser parameters on superhydrophobicity.

Abstract: This work studies in detail the effect of femtosecond laser irradiation process parameters (fluence and scanning speed) on the hydrophobicity of the resulting micro/nano-patterned morphologies on stainless steel. Depending on the laser parameters, four distinctly different nano-patterns were produced, namely nano-rippled, parabolic-pillared, elongated sinusoidal-pillared and triple roughness nano-structures. All of the produced structures were classified according to a newly defined parameter, the laser intensity factor (LIF); by increasing the LIF, the ablation rate and periodicity of the asperities increase. In order to decrease the surface energy, all of the surfaces were coated with a fluoroalkylsilane agent. Analysis of the wettability revealed enhanced superhydrophobicity for most of these structures, particularly those possessing the triple roughness pattern that also exhibited low contact angle hysteresis. The high permanent superhydrophobicity of this pattern is due to the special micro/nano-structure of the surface that facilitates the Cassie-Baxter state.

Origins of waveguiding in femtosecond laser-structured LiNbO3

Abstract: Femtosecond laser-induced structural changes in LiNbO3 are studied. Depending on the laser processing parameters two different types of modification are identified and their origin is discussed. Both types of modification can be described within the framework of induced lattice defects. For strong material damage a refractive index increase can be obtained due to the induced stress field. By appropriate tailoring of this stress field thermally stable and highly symmetric waveguides can be obtained well suited for nonlinear integrated-optical applications.