Self-starting mode locking of a Nd:glass fiber laser by use of the third-order nonlinearity of low-temperature-grown GaAs

We present a diode-pumped Nd:glass fiber laser, emitting at 1060 nm, that is passively mode locked by fast nonlinear loss in low-temperature-grown GaAs (LT-GaAs). This new mode-locking mechanism is based on intensity-dependent defocusing in LT-GaAs that occurs after nonresonant generation of free carriers by two-photon absorption. Mode locking is self-starting and produces pulses as short as 4.1 ps.     

Untersuchungen zur Kommutatorgruppe in zentralen und kompakten topologischen Gruppen II

Ohne ZusammenfassungHerrn Professor Dr. N. Hofreiter zum 70. Geburtstag     

Buchbesprechungen

Ohne Zusammenfassung     

The conjugacy problem for centre-by-metabelian groups

Archiv der Mathematik -     

A virtually free pro-p need not be the fundamental group of a profinite graph of finite groups

A subgroup of a pro-p product with amalgamation of two p-groups is given which cannot be presented as the pro-p fundamental group of a profinite graph of p-groups.     

Finite extensions of free pro-P groups of rank at most two

For a pro-p groupG, containing a free pro-p open normal subgroup of rank at most 2, a characterization as the fundamental group of a connected graph of cyclic groups of order at mostp, and an explicit list of all such groups with trivial center are given. It is shown that any automorphism of a free pro-p group of rank 2 of coprime finite order is induced by an automorphism of the Frattini factor groupF/F ^* . Finally, a complete list of automorphisms of finite order, up to conjugacy in Aut(F), is given. Supported by an NSERC grant. Supported by the Austrian Science Foundation.     

Finite-time deviations from exponential decay in the Weisskopf-Wigner model of spontaneous emission

For the first time to the authors' knowledge, a mathematically rigorous method is used for treating finite-time deviations from the exponential decay in the case of spontaneous Lyman- transition in a two-level hydrogenic atom. In the so-called Weisskopf-Wigner model (where the rotating-wave approximation is implied), finite-time deviations with a rigorous validity range, based on accurate error estimations, are derived.     

Reentrant mound formation in GaAs(001) homoepitaxy observed by ex situ atomic force microscopy

A study of the surface morphology of homoepitaxial GaAs(001) by means of ex situ atomic force microscopy in air reveals the reentrance of mounding behavior at low growth temperatures. A transition from statistical roughening to organized mound formation is observed as the growth temperature is reduced. We show by means of growth simulations that the observed morphology is compatible with anisotropic adatom diffusion in the presence of an Ehrlich-Schwoebel barrier. The mechanism leading to this kind of adatom kinetics at low temperatures is interpreted in terms of surfactant acting arsenic condensing on the surface.     

Cotorsion and wild homology

We solve the problem of describing the solutions of E-operators of order μ ≥ 1 admitting at z = 0 a basis over C of local solutions which are all holomorphic at z = 0. We prove that the components of such a basis can be taken of the form \(\sum {_{j = 1}^\ell } {P_j}\left( z \right){e^{{\beta _{{j^z}}}}}\), where ? ≤ μ, β ₁,...,β _? ∈ \(\overline {\mathbb{Q}} \) ^x, and P ₁(z),..., P _?(z) ∈ \(\overline {\mathbb{Q}} \)[z].