W. Andreas Schroeder

An efficient, selective collisional ejection mechanism for inner-shell population inversion in laser-driven plasmas

A theoretical analysis of laser-driven collisional ejection of inner-shell electrons is presented to explain the previously observed anomalous kilovolt L-shell x-ray emission spectra from atomic Xe cluster targets excited by intense sub-picosecond 248nrn ultraviolet radiation. For incident ponderomotively-driven electrons photoionized by strong above threshold ionization, the collisional ejection mechanism is shown to be highly l-state and significantly n-state (i.e. radially) selective for time periods shorter than the collisional dephasing time of the photoionized electronic wavefunction. The resulting preference for the collisional ejection of 2p electrons by an ionized 4p state produces the measured anomalous Xe(L) emission which contains direct evidence for (i) the generation of Xe{sup 27+}(2p{sup 5}3d{sup 10}) and Xe{sup 28+}(2p{sup 5}3d{sup 9}) ions exhibiting inner-shell population inversion and (ii) a coherent correlated electron state collision responsible for the production of double 2p vacancies. For longer time periods, the selectivity of this coherent impact ionization mechanism is rapidly reduced by the combined effects of intrinsic quantum mechanical spreading and dephasing--in agreement with the experimentally observed and extremely strong {minus}{lambda}{sup {minus}6} pump-laser wavelength dependence of the efficiency of inner-shell (2p) vacancy production in Xe clusters excited in underdense plasmas.

Pump laser wavelength-dependent control of the efficiency of kilovolt x-ray emission from atomic clusters

An explanation is presented for the recently reported striking differences in the kilovolt Xe L-shell x-ray emission from Xe cluster targets excited by comparable terawatt ultraviolet (248 nm) and infrared (800 nm) femtosecond laser pulses under nearly identical experimental conditions (Kondo K et al 1997 J. Phys. B.: At. Mol. Opt. Phys. 30 2707-16). A classical analysis of these results, within the framework of the first Born approximation for electron-atom collisions producing inner-shell ionization, strongly suggests that both the times stronger Xe(L) emission under ultraviolet laser excitation and the observed differences in the x-ray spectra are caused primarily by the different ultraviolet and infrared pump laser wavelengths. The kinematics of photoionized electrons in the intense laser fields (-) and the Coulomb-driven expansion of the electron distribution photoionized from the atomic cluster both indicate that the strong pump-laser wavelength scaling in the production of kilovolt x-rays from Xe clusters results from the more localized and controlled electron-cluster interactions afforded by a shorter optical period.

High-power, femtosecond, thermal-lens-shaped Yb:KGW oscillator

Thermal lens shaping for astigmatism compensation is extended to a high-power, diode-pumped, Yb:KGW laser by employing a gain crystal geometry designed for efficient polarized pumping. The 63MHz oscillator is soliton mode-locked with the aid of a saturable Bragg reflector to yield 250fs (347fs) pulses at an output power of 3.5W (5W). Frequency doubling of the 250fs pulses with an intrinsic efficiency >60% provides 1.65W of average green power.

Nanolithography using molecular optics

We explore the possibility of using an intense laser beam to focus a molecular beam onto a surface to create nanowires. We show that with a grazing angle of incidence between the laser and molecular beams, it is possible to use available technology to create wires 100 μm long with a 100 W continuous wave laser. Narrower and longer features could be created with higher power lasers. This technique is very general, and may be used to deposit any atom or molecule onto an arbitrary substrate, so long as the particles may be entrained in a molecular beam and have an adequate sticking probability. The effects of spherical and chromatic aberration and laser mode structure on the focusing properties of the molecular lens are examined in detail, and design criteria for building a practical device are discussed.

Low-threshold, dual-passive mode locking of a large mode area Nd:GdVO(4) laser.

The all solid-state combination of a saturable Bragg mirror for amplitude modulation and a cascaded chi((2)):chi((2)) nonlinearity (phase-mismatched second harmonic crystal) as an axial-mode phase locker for continuous-wave mode locking of large mode area lasers is investigated. The dual-passive mode-locking technique generates extremely stable sub-10ps sech(2) pulses at 76MHz from a ~6W, TEM(00)-mode, diode-pumped, thermal-lens-shaped, Brewster Nd:GdVO(4) laser.

DC photoelectron gun parameters for ultrafast electron microscopy.

We present a characterization of the performance of an ultrashort laser pulse driven DC photoelectron gun based on the thermionic emission gun design of Togawa et al. [Togawa, K., Shintake, T., Inagaki, T., Onoe, K. & Tanaka, T. (2007). Phys Rev Spec Top-AC 10, 020703]. The gun design intrinsically provides adequate optical access and accommodates the generation of approximately 1 mm2 electron beams while contributing negligible divergent effects at the anode aperture. Both single-photon (with up to 20,000 electrons/pulse) and two-photon photoemission are observed from Ta and Cu(100) photocathodes driven by the harmonics (approximately 4 ps pulses at 261 nm and approximately 200 fs pulses at 532 nm, respectively) of a high-power femtosecond Yb:KGW laser. The results, including the dependence of the photoemission efficiency on the polarization state of the drive laser radiation, are consistent with expectations. The implications of these observations and other physical limitations for the development of a dynamic transmission electron microscope with sub-1 nm.ps space-time resolution are discussed.

Thermal lens shaping in Brewster gain media: A high-power, diode-pumped Nd:GdVO 4 laser

A straightforward method is presented for generating a stigmatic spherical thermal lens in laser-diode-pumped, Brewster-cut solid-state gain media by shaping the aspect ratio of the elliptical pumped region. Demonstration of this laser head design with Nd:GdVO(4) as the gain medium yields a stable, efficient, high-power (>20W) diode-pumped laser at 1063nm. Analysis of the spatial mode characteristics of a 67cm-long symmetric resonator both confirms the radially symmetric nature of the pump-induced thermal lens and indicates that laser resonators incorporating this head design can readily generate a high spatial beam quality (M(2) < 2).

Thermal effects in laser pumped Kerr-lens modelocked Ti:sapphire lasers

Abstract Numerical beam propagation simulations are used to demonstrate that the distributed thermal lensing, produced by the absorption of the pump laser in the gain medium, profoundly affects the operation of hard-apertured, Kerr-lens modelocked Ti:sapphire lasers. The pump-induced thermal lensing is shown to shift and distort the resonator stability regions (even allowing the regions to overlap) and severely perturb the modelocking mechanism in one of the stability regions.

Hot electron relaxation dynamics in ZnSe

The ultrafast relaxation dynamics of hot electrons, initially photoexcited with an excess energy of 300 meV, are monitored in a high-quality ZnSe epilayer grown on a GaAs substrate by exploiting the intrinsic interferrometric asymmetric Fabry–Perot sample structure. The results are consistent with the expected characteristic electronic LO-phonon emission time of 40–50 fs and provide evidence for the influence of the “hot phonon effect”(or “LO-phonon bottleneck”) on the electron cooling dynamics at carrier densities above ∼3×1017 cm−3.

Semianalytic model of electron pulse propagation: Magnetic lenses and rf pulse compression cavities

The analytical Gaussian electron pulse propagation model of Michalik and Sipe [J. Appl. Phys. 99, 054908 (2006)] is extended to include the action of external forces on the pulse. The resultant ability to simulate efficiently the effect of electron optical elements (e.g., magnetic lenses and radio-frequency cavities) allows for the rapid assessment of electron pulse delivery systems in time-resolved ultrafast electron diffraction and microscopy experiments.