Catherine Kealhofer, Seth M. Foreman, Stefan Gerlich, Mark A. Kasevich
Electron emission from hafnium carbide (HfC) field emission tips induced by a
sub-10 fs, 150 MHz repetition rate Ti:sapphire laser is studied. Two-photon
emission is observed at low power with a moderate electric bias field applied
to the tips. As the bias field and/or laser power is increased, the average
current becomes dominated by thermally-enhanced field emission due to laser
heating: both the low thermal conductivity of HfC and the laser's high
repetition rate can lead to a temperature rise of several hundred Kelvin at the
tip apex. The time delay between each electron detection event and the previous
optical pulse is measured with 30 ps time resolution, and the timing
correlation reveals that a fraction of the emission decays with a time-constant
of 470 ps following each laser pulse. This is consistent with a transient,
thermally-enhanced field emission due to pulsed heating of the tip followed by
cooling of the apex region via conduction. The contribution of current from
such a thermal transient at times shorter than the electron-phonon coupling
time is considered in the context of the two-temperature model. Under the
conditions of this experiment, the integrated current from the thermal
transient is shown to be negligible in comparison with the two-photon emission.
A finite element model of the laser heating and thermal conduction supports
these conclusions and is also used to compare the nature of thermal effects in
HfC, tungsten, and gold tips.
View original:
http://arxiv.org/abs/1104.1452
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