Study of Type II SPDC in Lithium Niobate for High Spectral Purity Photon Pair Generation

Abstract

Recent advances of high-quality lithium niobate (LN) on insulator technology have revitalized the progress of novel chip-integrated LN-based photonic devices and accelerated application research. One of the promising technologies of interest is the generation of entangled photon pairs based on spontaneous parametric down-conversion (SPDC) in LNs. In this paper, we investigated, theoretically and numerically, Type II SPDC in two kinds of LNs—undoped and 5-mol% MgO doped LNs. In each case, both non-poled and periodically poled crystals were considered. The technique is based on the SPDC under Type II extended phase matching, where the phase matching and the group velocity matching are simultaneously achieved between interacting photons. The proposed approach has not yet been reported for LNs. We discussed all factors required to generate photon pairs in LNs, in terms of the beam propagation direction, the spectral position of photons, and the corresponding effective nonlinearities and walk-offs. We showed that the spectral positions of the generated photon pairs fall into the mid-infrared region with high potential for free-space quantum communication, spectroscopy, and high-sensitivity metrology. The joint spectral analyses showed that photon pairs can be generated with high purities of 0.995–0.999 with proper pump filtering.

Investigation of 1064-nm Pumped Type II SPDC in Potassium Niobate for Generation of High Spectral Purity Photon Pairs

“…The horizontal width of the graph falls within the full range of λ p that satisfies the Type II EPM. In this range, the d eff values span 8.40-11.13 pm/V, which are much larger than other cases using the Type II EPM approach, e.g., d eff = 1.50 pm/V for BBO under the BPM; 2.39 pm/V (=(2/π)d 24 ) for PPKTP and 2.77 pm/V (=(2/π)d 15 ) for PPLN, under the first-order QPM [10,26,27]. The largest values of d eff are 11.13 pm/V at 1062.78 nm, and the corresponding BPM direction is (θ, φ) = (63.8 • , 0 • ).…”

“…Crystals 2021, 11, x FOR PEER REVIEW 7 of 11 under the BPM; 2.39 pm/V (=(2/π)d24) for PPKTP and 2.77 pm/V (=(2/π)d15) for PPLN, under the first-order QPM [10,26,27]. The largest values of deff are 11.13 pm/V at 1062.78 nm, and the corresponding BPM direction is (θ, ϕ) = (63.8°, 0°).…”

“…Experimental results for the generation of photon pairs in thin-film PPLN have only been reported recently, but again, Type 0 SPDC were used [35,36]. The frequency-degenerate, Type II EPM approach for LN has recently been reported only as a theoretical study [26]. However, for non-poled LNs, the effective nonlinearity (and thus the SPDC efficiency) is too low as deff ≤ 0.2 pm/V, and for PPLNs, the wavelength of the photon pair is longer than 3.5 μm (with deff ~ 4 pm/V), which is far from the 2-μm band (or 1064-nm pumping).…”

Investigation of Mid-Infrared Broadband Second-Harmonic Generation in Non-Oxide Nonlinear Optic Crystals

“…As can be appreciated from the BPM conditions in Table 2, w is equal to ρ ω (for positive uniaxial crystals) or ρ 2ω (for negative uniaxial crystals), where ρ ω and ρ 2ω represent the walk-off angles of the e-waves with frequencies ω and 2ω, respectively. We note that for Type II, the relationship of ρ 2ω > ρ ω is valid in the uniaxial crystals considered in this study [59]. As expected from Equation (19), w is also given as a two-variable function for λ F and θ, and thus can be obtained by substituting a set of solutions for λ F and θ that satisfy the broadband SHG conditions in Table 2.…”

“…The d eff values calculated for the directions (θ BPM ) of the F-wave satisfying the broadband SHG conditions are listed in column 6 of Table 6. For all considered cases except CdSe, the d eff values are very large, from 7.5 pm/V to 163.6 pm/V, which are much larger than the maximum d eff for typical 5-mol% MgO-doped periodically poled LiNbO 3 (MgO:PPLN) using the first-order QPM: (2/π)d 31 = ~2.80 pm/V for Type I and II [59]. The walk-off angle w, calculated using the solution set of λ F and θ for the broadband SHG, are listed in column 7 of Table 6.…”