A comparative analysis of frequency-doubling in type-II quasi-phase-matched KTP crystals*

Second harmonic generation (SHG) is a widely used nonlinear three-wave mixing process to characterize different bulk nonlinear crystals/waveguides e.g., beta barium borate (BBO), potassium titanyl phosphate (KTP), Lithium niobate (LN), etc. for its implementation in nonclassical experiments utilizing spontaneous parametric down-conversion (SPDC) for the generation of entangled-photon pair sources. Such sources are useful candidates for applications in Optical Quantum technology e.g., quantum key distribution (QKD) and quantum computing. As a characterization tool for the SPDC source, we experimentally investigated the variation of second harmonic (SH) power by tuning the crystal temperature for three (10mm, 20mm, and 30mm) periodically poled KTP (ppKTP) crystals, phase-matched for the type-II process (H+V→H). We observed an increment in the conversion efficiency of SHG from 2.96 to 12.6 (P_SH/P_pump*10⁶) and a decrement in the temperature bandwidth from 2.80°C to 1.50°C with an increase in crystal length, respectively, validated by our theoretical calculations. Moreover, the focusing of the pump beam reveals the emergence of asymmetry in the SH power for different crystal lengths due to a competition between the confocal parameter attaining the crystal length, indicating the need for an optimization of the focusing parameters. Such thorough optimization of different parameters e.g., pump focusing, and crystal temperature is crucial for its utilization in different quantum applications.