Efficient enhancement of nonlinear optical processes in a microcavity demands simultaneous fulfillment of several conditions: tight mode confinement, optimal mode overlap, long interaction length, large effective nonlinear coefficient and phase matching condition. Moreover, resonances of the involved optical fields must occur simultaneously. Due to material and geometric dispersion, precise design and fabrication of resonators’ structure are essential for ensuring simultaneous resonances at desired frequencies, which can be difficult. In fact, tuning mechanisms such as the thermo-optic and electro-optic effects are commonly employed to compensate resonance frequency shifts caused by fabrication imperfections. In second-harmonic generation (SHG) experiments, reaching optimized nonlinear conversion is widely regarded as an indicator of the condition where both the waves simultaneously satisfy the resonance requirements. However, the condition can be vague with frequency detunings, and direct experimental measurement of the double-resonance detuning, which is important, has yet to be reported. The specific impact of double-resonance detuning on nonlinear effects has not been discussed.

We demonstrate an approach to visually observe the effect of double-resonance detuning on the cavity-enhanced SHG spectrum by synchronously scanning the fundament wave (FW) and its second harmonic (SH) to obtain the transmission spectrum of quadratic microcavities. The cavity-enhanced efficient SHG process is carried out in a periodically poled lithium niobate (PPLN) microring on the thin-film lithium niobate (TFLN) platform. Through SHG in another PPLN nonlinear waveguide, we can scan the whispering-gallery-mode (WGM) microcavity simultaneously in the FW and SH bands, achieving strict one-to-one correspondence between the FW and SH WGMs in the frequency domain and precisely determine the resonance detuning of each wave. The experimental setup is shown in Fig. 1. We also leverage the thermal-optical effect to precisely manipulate the double-resonance detuning of the microcavity, directly revealing the double-resonance dynamics of SHG in the quadratic microcavity, as shown in Fig. 2. This work paves a way for a deeper understanding of nonlinear processes in nonlinear microcavities, which may also be useful in non-Hermitian nonlinear photonics investigation.

FIG. 1. Experimental setup for FW-SH dual-band transmission measurement of the TFLN microring. EDFA, erbium-doped fiber amplifier; BS, beam splitter; PC, polarization controller; PPLNWG array: PPLN ridge waveguide array, VOA, variable optical attenuator; WDM, wavelength-division multiplexer; TEC, thermoelectric cooler; PD, photodetector; OSC, oscilloscope.

FIG. 2. (a-i) Transmission spectra at FW-SH dual bands and generated SH signal spectra with different double-resonance detuning. Upper curve shows the transmission spectra of the FW (blue) and SH (red) WGMs under synchronous scanning, with dashed lines be Lorentzian fitting. Lower curve is the generated SH signal spectra, with dashed lines corresponding to the calculation. (Intensity not to scale.)

This work is published in “Jing Qiu, Hao Li, Yongzhi Tang, Zhuoran Hu, Wenjun Ding, Shijie Liu, Juanjuan Lu, Wenjie Wan, Yuanlin Zheng, Xianfeng Chen, Directly revealing double-resonance dynamics of second-harmonic generation in a quadratic microcavity, APL Photonics, 10, 076102 (2025).”

Link: https://doi.org/10.1063/5.0276408