  We theoretically investigate the preparation of mid-infrared (MIR)
spectrally-uncorrelated biphotons from a spontaneous parametric down-conversion
process using doped LN crystals, including MgO doped LN, ZnO doped LN, and
In2O3 doped ZnLN with doping ratio from 0 to 7 mol%. The tilt angle of the
phase-matching function and the corresponding poling period are calculated
under type-II, type-I, and type-0 phase-matching conditions. We also calculate
the thermal properties of the doped LN crystals and their performance in
Hong-Ou-Mandel interference. It is found that the doping ratio has a
substantial impact on the group-velocity-matching (GVM) wavelengths.
Especially, the GVM2 wavelength of co-doped InZnLN crystal has a tunable range
of 678.7 nm, which is much broader than the tunable range of less than 100 nm
achieved by the conventional method of adjusting the temperature. It can be
concluded that the doping ratio can be utilized as a degree of freedom to
manipulate the biphoton state. The spectrally uncorrelated biphotons can be
used to prepare pure single-photon source and entangled photon source, which
may have promising applications for quantum-enhanced sensing, imaging, and
communications at the MIR range.
