Hello everyone, I have a question about the process of spontaneous parametric down conversion (or SPDC). This process is used to create two entangled photons in the visible regime by shining a laser through a beta-barium borate (BBO) nonlinear crystal. My question is, can this process be done at ANY wavelength in the visible regime? Or is the wavelength specific to the crystal? So for example, say I wanted two photons at 800 nm. Does this mean I should choose a 400 nm laser to shine through the crystal so it splits into two 800 nm photons? Do I have the choice of wavelength (within the boundaries of the visible regime)?