Carsten Müller

Pair production in laser fields oscillating in space and time.

The production of electron-positron pairs from vacuum by counterpropagating laser beams of linear polarization is calculated. In contrast with the usual approximate approach, the spatial dependence and magnetic component of the laser field are taken into account. We show that the latter strongly affects the creation process at high laser frequency: the production probability is reduced, the kinematics is fundamentally modified, the resonant Rabi-oscillation pattern is distorted, and the resonance positions are shifted, multiplied, and split.

Cable Properties of Dendrites in Hippocampal Neurons of the Rat Mapped by a Voltage-sensitive Dye

Dendrites of pyramidal neurons from embryonic rat hippocampus are investigated in culture using a voltage‐sensitive fluorescent dye. The electrical response to somatic stimulation is observed as a time‐resolved map with a resolution of 0.9 μM at a time constant of 0.4 ms without signal averaging. The data are interpreted in terms of a tapering cable with Hodgkin‐Huxley parametrization. The spread of short hyperpolarizing transients is damped by capacitive shunting. The invasion of an action potential is boosted by voltage‐gated conductances of a low density. No irregularity is observed at a bifurcation. The passive cable parameters of internal resistance and membrane resistance at resting voltage are Ri= 300 ωcm and Rm= 40 kωcm2 respectively, at a maximum sodium conductance of ˜4.4 mS/cm2. The electrotonic length constant and the dynamic length constant at 1 kHz are 580 and 90 μm respectively. These results are compatible with electrophysiological data of dendrites in slices of adult hippocampus and with optical data of narrow processes of leech neurons in culture. The functional implications of boosting an action potential by voltage‐gated channels of low density are considered.

Voltage-sensitive fluorescence of amphiphilic hemicyanine dyes in neuron membrane

Fluorescent amphiphilic hemicyanine dyes were adsorbed to the plasma membrane of isolated Retzius neurons of the leech. Voltage steps were applied to the neuron by the patch-clamp technique in whole-cell configuration. The change of fluorescence was observed as induced by the voltage jump. The relative changes of the excitation spectrum and of the emission spectrum of fluorescence were recorded. The complete set of spectral data for each dye was fitted by five parameters: shifts of the emission and the excitation spectrum, a change of fluorescence quantum yield and changes of the widths of the excitation and of the emission spectrum. The only common feature for all dyes was a blue-shift of the excitation spectrum and a drop of the yield when the neuron was stained from the outside and a positive voltage was applied to the inside. With respect to the shift of the emission spectrum and the changes of width qualitatively different results were obtained for different dyes. It is not attempted to assign a physical mechanism--probably a superposition of several mechanisms--of voltage-sensitivity.

Photoemission of a Single-Electron Wave Packet in a Strong Laser Field

The radiation emitted by a single-electron wave packet in an intense laser field is considered. A relation between the exact quantum formulation and its classical counterpart is established via the electrons Wigner function. In particular, we show that the wave packet, even when it spreads to the scale of the wavelength of the driving laser field, cannot be treated as an extended classical charge distribution, but rather behaves as a pointlike emitter carrying information on its initial quantum state. We outline an experimental setup dedicated to put this conclusion to the test.

Particle physics with a laser-driven positronium atom

Abstract A detailed quantum-electrodynamic calculation of muon pair creation in laser-driven electron–positron collisions is presented. The colliding particles stem from a positronium atom exposed to a superintense laser wave of linear polarization, which allows for high luminosity. The threshold laser intensity of this high-energy reaction amounts to a few 10 22 W / cm 2 in the near-infrared frequency range. The muons produced form an ultrarelativistic, strongly collimated beam, which is explicable in terms of a classical simple-mans model. Our results indicate the experimental feasibility of the process at high positronium densities in counterpropagating laser beams with the help of present-day technology.

Microscopic laser-driven high-energy colliders

The concept of a laser-guided e+e− collider in the single-femtosecond and high-energy regime is presented and its feasibility discussed. Ultra-intense laser pulses and strong static magnetic fields are employed to unite in one stage the electron and positron acceleration and their microscopic coherent collision in the GeV regime. We show that such coherent collisions yield a very large enhancement of the luminosity as compared to conventional incoherent colliders.

Lepton pair production in high-frequency laser fields

The production of electron-positron and muon-antimuon pairs in high-frequency laser fields via few-photon absorption is considered. It is assumed that an intense X-ray laser beam collides either with a relativistic ion beam or with a second, equally intense laser beam. We study the generation of free e+e− pairs, free μ+μ− pairs, and bound-free e+e− pairs where in the latter case the electron is born in a low-lying atomic orbit of the projectile nucleus. Effects resulting from the finite nuclear size, the laser’s polarization state, and its magnetic field component are examined, which are testable experimentally by virtue of upcoming X-ray free-electron laser (XFEL) devices.