Quasi-Phase Matching (QPM) is one of the most important tools to achieve efficient second-order nonlinear frequency conversion. Generally, it uses the periodic electric filed poling technique to inverse χ^((2) ) of ferroelectric crystals, thus the phase mismatch can be compensated by an additional reciprocal vector. In recent years, with the development of commercially available thin-film lithium niobate and the miniaturization of optical devices, integrated on-chip QPM nonlinear devices have received increasing attention. However, electric field poling at the chip scale is still extremely challenging, with high cost, complex process, and difficulty in large-scale processing at the wafer level. How to reduce the difficulty of implementation of QPM for efficient integration with other on-chip functional devices has become an urgent problem.

In this paper, we utilize the anisotropic characteristics of crystals with the design of the orientation and the geometry parameters of the micro racetrack resonator on X-cut thin film lithium niobate, making the maximum effective nonlinear coefficient, d_33, spontaneously reverse with the cyclic propagation of the fundamental-frequency light inside the cavity, so that the energy conversion in the nonlinear process can be controlled, and finally realizing the poling-free spontaneous QPM second harmonic generation.

In this paper, we utilize the anisotropic characteristics of crystals with the design of the orientation and the geometry parameters of the micro racetrack resonator on X-cut thin film lithium niobate, making the maximum effective nonlinear coefficient, d_33, spontaneously reverse with the cyclic propagation of the fundamental-frequency light inside the cavity, so that the energy conversion in the nonlinear process can be controlled, and finally realizing the poling-free spontaneous QPM second harmonic generation.

Figure 1  (a-c) Schematic of spontaneous quasi phase matching (SQPM). (b) SQPM racetrack resonator on X-cut thin film lithium niobate, and (c) corresponding effective nonlinear coefficient and SH intensity varying with θ.

In our experiments, in order to obtain an effective SQPM second harmonic generation, we further design the waveguide geometrical parameters of the racetrack resonator.  First, the straight waveguide length needs to be equal to an odd multiple of the coherence length (L_c) to meet arbitrary-order QPM condition; second, in order to realize an effective second harmonic increase, the radius of the half-circle waveguide needs to be adjusted to a suitable value, so that the phase-mismatch delay between the fundamental-frequency light and the second harmonic light therein satisfies the integer multiples of 2π; and lastly, the fundamental-frequency and second-harmonic fields in the microcavity can resonate. Combining the above conditions, we designed the micro racetrack resonator satisfying 37th and 111th order SQPM, with the on-chip conversion efficiencies of 1.01×10^(-4)/W and 0.43×10^(-4)/W, respectively. By optimizing the racetrack structure, improving the Q factor and the coupling condition, a significant improvement of the efficiency is expected.

This work proposes a poling-free SQPM second harmonic generation on thin film lithium niobate. With the increasing maturity and stability of semiconductor processing, precise control of each parameter of the micro racetrack resonator can be achieved with the assistance of electro-optical and thermo-optical modulation of dispersion and phase. And this is of great significance of future on-chip high-efficiency nonlinear frequency conversion devices on quantum light source on thin film lithium niobate.

Figure 2 (a) Schematic of conformal transformation. (b) Design of half-circle waveguide radius and (c) straight waveguide length. (d) Measured SH intensity pumped by different resonant modes of FW. (e) SH detected by OSA with the FW wavelength at 1540.2 nm. (f) On-chip SH power versus square of the on-chip FW power.

This research is published in “Tingge Yuan, Jiangwei Wu, Yi’an Liu, Xiongshuo Yan, Haowei Jiang, Hao Li, Zhaokang Liang, Qiang Lin, Yuping Chen, and Xianfeng Chen, Chip-scale spontaneous quasi-phase matched second harmonic generation in a micro-racetrack resonator, Sci. China-Phys. Mech. Astron. 66, 284211 (2023)”.

Link:https://doi.org/10.1007/s11433-023-2145-6