1,902 research outputs found
Parametric attosecond pulse amplification far from the ionization threshold from high order harmonic generation in He
Parametric amplification of attosecond coherent pulses around 100 eV at the
single-atom level is demonstrated for the first time by using the 3D
time-dependent Schr{\"o}dinger equation in high-harmonic generation processes
from excited states of He. We present the attosecond dynamics of the
amplification process far from the ionization threshold and resolve the physics
behind it. The amplification of a particular central photon energy requires the
seed XUV pulses to be perfectly synchronized in time with the driving laser
field for stimulated recombination to the He ground state and is only
produced in a few specific laser cycles in agreement with the experimental
measurements. Our simulations show that the amplified photon energy region can
be controlled by varying the peak intensity of the laser field. Our results
pave the way to the realization of compact attosecond pulse intense XUV lasers
with broad applications
Direct Observation of the Hyperfine Transition of the Ground State Positronium
We report the first direct measurement of the hyperfine transition of the
ground state positronium. The hyperfine structure between ortho-positronium and
para-positronium is about 203 GHz. We develop a new optical system to
accumulate about 10 kW power using a gyrotron, a mode converter, and a
Fabry-P\'{e}rot cavity. The hyperfine transition has been observed with a
significance of 5.4 standard deviations. The transition probability is measured
to be s for the first time, which
is in good agreement with the theoretical value of
s
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