Effect of Noise on Frequency-Resolved Optical Gating Measurements of Ultrashort Pulses

We study the effects of noise in Frequency-Resolved Optical Gating measurements of ultrashort pulses. We quantify the measurement accuracy in the presence of additive, muliplicative, and quantization noise, and discuss filtering and pre-processing of the data

A coded aperture with sub-mean free-path thickness for neutron implosion geometry imaging on inertial confinement fusion and inertial fusion energy experiments

Inertial confinement fusion and inertial fusion energy experiments diagnose the geometry of the fusion region through imaging of the neutrons released through fusion reactions. Pinhole arrays typically used for such imaging require thick substrates to obtain high contrast along with a small pinhole diameter to obtain high resolution capability, resulting in pinholes that have large aspect ratios. This leads to expensive pinhole arrays that have small solid angles and are difficult to align. Here, we propose a coded aperture with scatter and partial attenuation (CASPA) for fusion neutron imaging that relaxes the thick substrate requirement for good image contrast. These coded apertures are expected to scale to larger solid angles and are easier to align without sacrificing imaging resolution or throughput. We use Monte Carlo simulations (Geant4) to explore a coded aperture design to measure neutron implosion asymmetries on fusion experiments at the National Ignition Facility (NIF) and discuss the viability of this technique, matching the current nominal resolution of 10 µm. The results show that a 10 mm thick tungsten CASPA can image NIF implosions with neutron yields above 1014 with quality comparable to unprocessed data from a current NIF neutron imaging aperture. This CASPA substrate is 20 times thinner than the current aperture arrays for fusion neutron imaging and less than one mean free-path of 14.1 MeV neutrons through the substrate. Since the resolution, solid angle, and throughput are decoupled in coded aperture imaging, the resolution and solid angle achievable with future designs will be limited primarily by manufacturing capability

A two-dimensional, two-electron model atom in a laser pulse: exact treatment, single active electron-analysis, time-dependent density functional theory, classical calculations, and non-sequential ionization

Owing to its numerical simplicity, a two-dimensional two-electron model atom, with each electron moving in one direction, is an ideal system to study non-perturbatively a fully correlated atom exposed to a laser field. Frequently made assumptions, such as the ``single active electron''- approach and calculational approximations, e.g. time dependent density functional theory or (semi-) classical techniques, can be tested. In this paper we examine the multiphoton short pulse-regime. We observe ``non-sequential'' ionization, i.e.\ double ionization at lower field strengths as expected from a sequential, single active electron-point of view. Since we find non-sequential ionization also in purely classical simulations, we are able to clarify the mechanism behind this effect in terms of single particle trajectories. PACS Number(s): 32.80.RmComment: 10 pages, 16 figures (gzipped postscript), see also http://www.physik.tu-darmstadt.de/tqe

Coherent control using adaptive learning algorithms

We have constructed an automated learning apparatus to control quantum systems. By directing intense shaped ultrafast laser pulses into a variety of samples and using a measurement of the system as a feedback signal, we are able to reshape the laser pulses to direct the system into a desired state. The feedback signal is the input to an adaptive learning algorithm. This algorithm programs a computer-controlled, acousto-optic modulator pulse shaper. The learning algorithm generates new shaped laser pulses based on the success of previous pulses in achieving a predetermined goal.Comment: 19 pages (including 14 figures), REVTeX 3.1, updated conten

Fringe-free, Background-free, Collinear Third Harmonic Generation FROG Measurements for Multiphoton Microscopy

Collinear pulse measurement tools useful at the full numerical aperture (NA) of multiphoton microscope objectives are a necessity for a quantitative characterization of the femtosecond pulses focused by these systems. In this letter, we demonstrate a simple new technique, for characterizing the pulse at the focus in a multiphoton microscope. This technique, a background-free, fringe-free, form of frequency-resolved optical gating, uses the third harmonic signal generated from a glass coverslip. Here it is used to characterize 100 fs pulses (typical values for a multiphoton microscope) at the focus of a 0.65 NA objective

Correlation dynamics between electrons and ions in the fragmentation of D2_2 molecules by short laser pulses

We studied the recollision dynamics between the electrons and D$_2^+$ ions following the tunneling ionization of D$_2$ molecules in an intense short pulse laser field. The returning electron collisionally excites the D$_2^+$ ion to excited electronic states from there D$_2^+$ can dissociate or be further ionized by the laser field, resulting in D$^+$ + D or D$^+$ + D$^+$, respectively. We modeled the fragmentation dynamics and calculated the resulting kinetic energy spectrum of D$^+$ to compare with recent experiments. Since the recollision time is locked to the tunneling ionization time which occurs only within fraction of an optical cycle, the peaks in the D$^+$ kinetic energy spectra provides a measure of the time when the recollision occurs. This collision dynamics forms the basis of the molecular clock where the clock can be read with attosecond precision, as first proposed by Corkum and coworkers. By analyzing each of the elementary processes leading to the fragmentation quantitatively, we identified how the molecular clock is to be read from the measured kinetic energy spectra of D$^+$ and what laser parameters be used in order to measure the clock more accurately.Comment: 13 pages with 14 figure

Collinear helium under periodic driving: stabilization of the asymmetric stretch orbit

The collinear eZe configuration of helium, with the electrons on opposite sides of the nucleus, is studied in the presence of an external electromagnetic (laser or microwave) field. We show that the classically unstable "asymmetric stretch" orbit, on which doubly excited intrashell states of helium with maximum interelectronic angle are anchored, can be stabilized by means of a resonant driving where the frequency of the electromagnetic field equals the frequency of Kepler-like oscillations along the orbit. A static magnetic field, oriented parallel to the oscillating electric field of the driving, can be used to enforce the stability of the configuration with respect to deviations from collinearity. Quantum Floquet calculations within a collinear model of the driven two-electron atom reveal the existence of nondispersive wave packets localized on the stabilized asymmetric stretch orbit, for double excitations corresponding to principal quantum numbers of the order of N > 10.Comment: 13 pages, 12 figure

Stability Measurements for Alignment of the NIF Neutron Imaging System Pinhole Array

The alignment system for the National Ignition Facility's neutron imaging system has been commissioned and measurements of the relative stability of the 90-315 DIM, the front and the back of the neutron imaging pinhole array and an exploding pusher target have been made using the 90-135 and the 90-258 opposite port alignment systems. Additionally, a laser beam shot from the neutron-imaging Annex and reflected from a mirror at the back of the pinhole array was used to monitor the pointing of the pinhole. Over a twelve hour period, the relative stability of these parts was found to be within {approx} {+-}18 {micro}m rms even when using manual methods for tracking the position of the objects. For highly visible features, use of basic particle tracking techniques found that the front of the pinhole array was stable relative to the 90-135 opposite port alignment camera to within {+-}3.4 {micro}m rms. Reregistration, however, of the opposite port alignment systems themselves using the target alignment sensor was found to change the expected position of target chamber center by up to 194 {micro}m

Atomic excitation during recollision-free ultrafast multi-electron tunnel ionization

Modern intense ultrafast pulsed lasers generate an electric field of sufficient strength to permit tunnel ionization of the valence electrons in atoms. This process is usually treated as a rapid succession of isolated events, in which the states of the remaining electrons are neglected. Such electronic interactions are predicted to be weak, the exception being recollision excitation and ionization caused by linearly-polarized radiation. In contrast, it has recently been suggested that intense field ionization may be accompanied by a two-stage `shake-up' reaction. Here we report a unique combination of experimental techniques that enables us to accurately measure the tunnel ionization probability for argon exposed to 50 femtosecond laser pulses. Most significantly for the current study, this measurement is independent of the optical focal geometry, equivalent to a homogenous electric field. Furthermore, circularly-polarized radiation negates recollision. The present measurements indicate that tunnel ionization results in simultaneous excitation of one or more remaining electrons through shake-up. From an atomic physics standpoint, it may be possible to induce ionization from specific states, and will influence the development of coherent attosecond XUV radiation sources. Such pulses have vital scientific and economic potential in areas such as high-resolution imaging of in-vivo cells and nanoscale XUV lithography.Comment: 17 pages, 4 figures, original format as accepted by Nature Physic

High Bandwidth Data Recording Systems for Pulsed Power and Laser Produced Plasma Experiments.

We present two high bandwidth data transmission and recording systems for the measurement of transient signals during pulsed power and laser produced plasmas. These systems use fiber optic cables to transmit analog data over long distances to high bandwidth digitizing oscilloscopes. One system is based on the direct modulation of a laser diode and has a bandwidth of 1.5 GHz. The other system is based upon a fiber-optic Mach-Zehnder modulator and has a bandwidth of 12 GHz, and is limited by the photo receiver. The signals are recorded on commercial digitizing oscilloscopes that have approximately 6 effective bits. The transmission systems use many off-the-shelf components from the telecommunications industry and thus have a high reliability and a moderate cost. Results from recent measurements will be presented. Investigation of the reduction in optical transmission by the fibers during exposure to high dose radiation will also be discussed