|
This animation is a series of
photographs of parametric down-conversion (PDC) light from an
experimental setup. PDC light is created as photons of a beam of light,
in effect, decay into pairs of photons. The medium for this conversion
process is a nonlinear crystal (not shown). The output directions of PDC
light are governed by the constraints of energy and momentum
conservation, usually referred to as "phase matching." In the photo
series above, the nonlinear crystal is potassium dihydrogen phosphate (KDP) and the input beam is a laser at 351 nm. Also in this case, the
input light has extraordinary polarization (polarization in the plane
containing the pump beam direction and the crystal optic axis), while
the output light has ordinary polarization (polarization orthogonal to
the crystal optic axis). This configuration is known as Type-I phasematching. The view of the photo series is looking directly at the
output end of the crystal. The little blue dot in the center is a small
amount of the input laser beam leaking past a spectral filter designed
to reject light at the wavelength of the input light. The PDC light
exiting the crystal at range of angles from the input light beam is seen
as rings, because this process is symmetric about the input beam
direction. Note that because the PDC process is not resonant, it creates
a broad range of output wavelengths. The photo series was taken as the
optic axis of the KDP crystal was varied relative to the input light
direction (see the icon in the upper left corner). This changes the
directions of the PDC output light. While it is clear the output angles
increase as the optic axis angle increase, it is also seen that the
angles of the red wavelengths increase faster than the blue. This can be
seen as the final frames show the color order reversing, with the red
ring ending up outside of the blue ring. (Note - The faint ring that is
seen moving across the screen is a just a reflection of the rings from
an intermediate surface.)
|
