William Angerer - Department of Physics and Astronomy - University ...

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defined as
[max - [min
TJ-
- [max + [min'
(2.68)
where [max is the SHG intensity from a Maker fringe peak and [min is the SHG
intensity from a NIaker fringe valley. This ratio is defined in terms of the ultrafast
coefficients (equations (2.64) and (2.65)) as
(2.69)
By comparing the calculated TJ with the experimentally observed TJ, one can determine
~w of the pulse. This comparison proceeds as follows. First, the thickness of
the quartz wedge, typically 250 pm at the thinnest edge, and the angle of the quartz
wedge, typically 1 0 ,
are determined. Note that the wedge angle is directed along the
x a.'Cis of the crystal. Second, the wedge is positioned at normal incidence to the
Ti:AI 2 0 3 beam. The Ti:AI 2 0 3 beam is focussed on the quartz wedge to a beam size
of less than 100 pm. If the beam has a large spatial extent. i.e. greater than 1 mm.
each section ofthe beam travels a different path length, and the transmitted intensity
from each section of the beam is different. This latter effect also suppresses ~Iaker
fringes [38]. Therefore, a small beam diameter is required to ensure that the damped
Maker fringes result from ultrafast effects and not from finite beam effects. Third,
the quartz wedge is translated perpendicularly across the Ti:AI 2 0 3 pulse. Note that
the direction of translation is such that the beam traverses a different quartz thickness
during the translation. The second-harmonic intensity from the quartz wedge
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