Spatial and spectral peculiarities of spontaneous parametric down-conversion for classical and quantum spectroscopy

Spontaneous Parametric Down-conversion (SPDC) is now routinely used to generate photon pairs for imaging, spectroscopy, and quantum information and metrology. However, the peculiar spectro-spatial properties of SPDC beams are not always known from the largest audience: they can give rise to ring-like beams, depending on the temperature, the wavelengths and pump focusing. This has to be considered for imaging or long-range applications. We will show examples of such beams created in 532-pumped PPLN or PPKTP and 1064-nm-pumped PPLN crystals. With a simple model of noncolinear phase matching we will give physical insight into the phenomena and explain the role of the signal/idler group delay. We will also explain how aperiodic crystals could be useful.