9 research outputs found
Atom focusing by far-detuned and resonant standing wave fields: Thin lens regime
The focusing of atoms interacting with both far-detuned and resonant standing
wave fields in the thin lens regime is considered. The thin lens approximation
is discussed quantitatively from a quantum perspective. Exact quantum
expressions for the Fourier components of the density (that include all
spherical aberration) are used to study the focusing numerically. The following
lens parameters and density profiles are calculated as functions of the pulsed
field area : the position of the focal plane, peak atomic density,
atomic density pattern at the focus, focal spot size, depth of focus, and
background density. The lens parameters are compared to asymptotic, analytical
results derived from a scalar diffraction theory for which spherical aberration
is small but non-negligible (). Within the diffraction theory
analytical expressions show that the focused atoms in the far detuned case have
an approximately constant background density
while the peak density behaves as , the focal distance or
time as , the focal spot size as
, and the depth of focus as .
Focusing by the resonant standing wave field leads to a new effect, a Rabi-
like oscillation of the atom density. For the far-detuned lens, chromatic
aberration is studied with the exact Fourier results. Similarly, the
degradation of the focus that results from angular divergence in beams or
thermal velocity distributions in traps is studied quantitatively with the
exact Fourier method and understood analytically using the asymptotic results.
Overall, we show that strong thin lens focusing is possible with modest laser
powers and with currently achievable atomic beam characteristics.Comment: 21 pages, 11 figure
Diamond (111) surface: a dilemma resolved
A dilemma due to the experimental observation of a 'clean' unreconstructed elemental semiconductor surface without band gap states is resolved. Results from photon stimulated ion description, high resolution low energy electron loss spectroscopy and photoemission spectroscopy find that the conventionally polished diamond (111) 1 multiplied by 1 surface is atomically terminated and electronically stabilized by hydrogen. Thermal desorption of hydrogen upon heating results in a reconstructed 2 multiplied by 2/2 multiplied by 1 surface with filled electronic surface states in and near the fundamental gap. Exposure of the reconstructed surface to atomic hydrogen (or dueterium) is found to again terminate the surface and remove the near band gap surface states. Apparent inconsistencies in the understanding of the diamond: hydrogen interaction are resolved