Photos of Type I PDC Light (Experimental) Series of photos 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.)

Simulations of Type II PDC (Calculated) Series of simulated images of PDC light created using the calculational capability of the Fortran program described on the previous web page. This calculation was done for a beta-barium borate (BBO) crystal pumped at 702 nm using Type-II phasematching. Type-II means one of the emitted PDC photons has ordinary polarization, while its twin has extraordinary polarization. As a result, each photon of a pair is emitted into separate, nonconcentric and orthogonally polarized output cones of light. See how the output light cones change as the crystal optic axis varies. The curves at the bottom show the wavelength of the PDC light as a function of its output angle. The upper curve represents the larger output cone and the lower curve represents the smaller cone. These particular curves are for light emitted in the horizontal plane. False color was used for the simulated image to map the 0.6 µm to 1.2 µm wavelength range into the visible. The gaps in the curves and the central holes in each light cone are shown to aid the eye in associating the light cones with the appropriate curves.