Single photons can be detected by several types of detectors without nonlinear crystals, E.g., specially designed photomultipliers produce 100 mV pulse from one photon absorbed by photocathode. Concerning nonlinear crystals, they may help in detecting weak photon flux in the spectral region poorly detected by existing single photon detectors, converting it into another part of spectrum via sum frequency mixing in the presence of strong laser pump. Growth method itself is not important, but the crystal must be transparent at all three wavelength and phase matchable for SFM required. Special case is SFM of entangled photonic states in nonlinear crystals. This process ir more sensitive even in the absence of strong pump..