Amplitude and phase characterization of weak blue ultrashort pulses by downconversion

We present difference-frequency generation cross-correlation frequency-resolved optical gating, a new method of characterizing the amplitude and phase of weak ultrashort pulses in the blue spectral region. The method uses the spectrally resolved downconversion signal of the blue pulse and a fully characterized reference pulse with a lower center frequency. The amplitude and phase of the blue test pulse are retrieved from the corresponding spectrogram and electric field of the reference pulse with an interactive Fourier-transform algorithm.     

Characterization of biological thin films at the solid-liquid interface by x-ray reflectivity

We demonstrate that 18 keV x-rays can be used to study organic thin films at the solid-liquid interface by x-ray reflectivity. We establish that this is a powerful technique for investigating biological systems in a previously inaccessible manner. Our measurements enabled the density distribution of single phospholipid bilayer membranes in bulk water to be measured with unprecedented precision. Previously, characterization of biomimetic structures normal to a "buried" interface was a domain of neutron reflectivity.     

Tailoring the ultrafast dephasing of quasiparticles in metallic photonic crystals

The ultrafast dephasing of waveguide-plasmon polaritons in metallic photonic crystal slabs is investigated in the femtosecond regime by second-order nonlinear autocorrelation. We find a drastic modification of the dephasing rates due to interaction between localized particle plasmons and optical waveguide modes and subsequent modification of the photonic density of states. In the strong coupling regime our measurements give clear evidence for the appearance of ultrafast polaritonic beat phenomena. All experimental results agree well with theoretical simulations based on a coupled damped harmonic oscillator model.     

Efficient intracavity generation of visible pulses in a femtosecond near-infrared optical parametric oscillator

We have achieved efficient simultaneous intracavity doubling of the signal frequency and sum-frequency generation between the signal and the pump in a periodically poled LiNbO(3) femtosecond optical parametric oscillator pumped by a Ti:sapphire laser. The responsible mechanisms are second-, third-, and fourth-order quasi-phase matching (QPM). An ~56% poling duty cycle permits efficient even-order QPM. Femtosecond pulses tunable in the visible were generated with a total efficiency of more than 8% for only 480-mW pump power at 80 MHz. Optimization of the poling duty cycle can improve the efficiency further.     

A three-field,bi-domain based approach to the strongly coupled electromechanics of the heart

We propose a novel, unconditionally stable and fully coupled finite element method for the bidomain based approach to cardiac electromechanics. To this end, the transmembrane potential, the extracellular potential, and the displacement field are treated as independent variables such that the already coupled electrophysiology problem in the bidomain setting is further extended to the electromechanical coupling. In this multifield problem, the intrinsic coupling arises from both excitation-induced contraction of cardiac cells and the deformation-induced generation of intra-cellular currents. The respective bidomain reaction-diffusion and the momentum balance equations are recast into the corresponding weak forms through a conventional isoparametric Galerkin approach. The resultant set of non-linear residual equations is consistently linearized. The monolithic scheme is employed to avoid stability issues that may arise due to the strong coupling between excitation and deformation. The performance of the put forward framework is further assessed through three-dimensional representative electromechanical initial-boundary value problems. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time-resolved spectroscopy of soantaneous luminescence of CdSSe1−x quantum dots

Picosecond time-resolved spectroscopy of the edge luminescence band of CdS _x Se_1–x quantum dots with crystallite diameters as small as a few nanometers under band-to-band excitation reveals strong enhancement of the radiative recombination rate compared to bulk CdS owing to quantum confinement. The splitting of the luminescence band into two lines originates from near-band-gap absorption. Analysis of the temperature as well as the spectral dependence of the decay time (leading to a red shift of the luminescence with increasing time) and of the total-light-decay law result in a new model for the dominant radiative recombination channel: donor-acceptor pair recombination instead of an excitonic mechanism as claimed in previous publications.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

THz generation via optical rectification with ultrashort laser pulse focused to a line

We report on efficient THz pulse generation via optical rectification with femtosecond laser pulses focused to a line by a cylindrical lens. This configuration provides phase-matched conditions in the superluminal regime. 35 pJ THz pulses have been generated with this technique in a stoichiometric LiNbO₃ crystal pumped by 2 μJ femtosecond laser pulses at room temperature. An unusual superquadratic rise of the THz pulse energy with the laser pulse energy has been observed at high laser energies. This extraordinary energy dependence of the THz generation efficiency is explained by self-focusing of the laser beam in the crystal. Z-scan measurements and comparison of the THz pulse spectra created with laser pulses having different energies confirm this interpretation.