High Q-factor microresonators are excellent platforms for the study of nonlinear optics by confining photons in the cavity for long periods of time through total internal reflection, thus greatly increasing the energy density of the optical field inside the cavity while enhancing the light-matter interaction. Currently, state-of-the-art nanofabrication techniques enable thin-film lithium niobate (TFLN) microresonators to reach Q values of up to 10^8orders of magnitude, which is close to their theoretical limit. This helps to realize various nonlinear effects in TFLN microresonators, such as second harmonic generation (SHG), sum-frequency generation (SFG), optical parametric oscillation (OPO) and spontaneous parametric down-conversion (SPDC).

Stimulated Raman scattering (SRS), on the other hand, as a third-order nonlinear nonparametric process, is an effective way to extend the operating spectral range of a light source to provide new wavelengths that are different from the pump. SRS has been previously demonstrated on a variety of photonic integration platforms, including silicon, silicon dioxide, diamond and aluminum nitride. LN is also a promising material with strong multi-Raman phonon branches, large second-order nonlinear coefficient and a wide transparency window. The combination of SRS and SFG processes facilitates the wavelength conversion of the light source to the short-wavelength direction, which will greatly increase the potential for the application of SRS processes in TFLN platforms.

In this paper, we demonstrate the generation of cascaded multi-phonon Raman signals and associated cascaded SFG processes by modal phase matching near the SHG peak under cw pump of x-cut high Q-factor TFLN microdisk. We fabricated TFLN microdisk with Q-factors above 8 × 10^5. The high Q of the WGM modes in the microdisk and the small mode volume compensate for the small spatial mode overlap between the interacting modes, allowing us to observe SRS effects and cascaded nonlinear effects. The multi-phonon Raman signals and its cascaded SFG signals can be modulated by varying the pump wavelength in a small range. In addition, we observed the generation of multi-color visible light in the TFLN microdisk under an optical microscope.

Fig. 1  Spectrum of SRS in the TFLN microdisk

We tuned the pump wavelength to 1543.59 nm with a fixed power of 21.78 mW, and observed the multi-phonon Raman spectral lines near the SHG peak as shown in Fig. 1. By comparing the wavelength of the Raman spectral line with that of the SHG peak, calculating the difference in wave number between the two, and referring to the Raman spectra of the LN crystals, we can correspond the spectral lines to different Raman phonon branches, and the relationship is shown in Table 1.

Table 1.  Relationship between Raman Spectral Lines and Raman Active Phonons in Fig. 1

Fig. 2  SFG process of the cascaded SRS in the TFLN microdisk. (a) (b) show the experimental spectrum. (c)-(f) are photographs of the scattered light from microdisk under light microscope.

Fig. 3  Fine tuning process of Raman peaks and cascaded Raman peaks

At the same time, we observed the SFG process associated with the Raman peaks as shown in Fig. 2, which allows the SRS spectral lines to be converted to a shorter wavelength range. Under the light microscope we also observed visible light scattered from the periphery of the microdisk. By changing the pump wavelength, we also demonstrated the fine tuning process of Raman peaks and cascaded Raman peaks as shown in Fig. 3.

Our SRS experimental results are characterized by broadband phase matching and tunability. This is mainly attributed to the unique modal phase-matching conditions and the fulfillment of multiple resonance conditions in the x-cut TFLN microdisk. However, the spatial overlap factor between these interacting modes may be small, which can be compensated by the high Q-factor of the microdisk. Our work extends the wavelength of SRS in TFLN microcavities to the SHG range, which will greatly increase the wavelength conversion potential of SRS on the TFLN platform.

This work is published in “Yuxuan He, Xiongshuo Yan, Jiangwei Wu, Xiangmin Liu, Yuping Chen, and Xianfeng Chen, Cascaded multi-phonon stimulated Raman scattering near second-harmonic generation in a thin-film lithium niobate microdisk, Opt. Lett. 49, 4863-4866 (2024)”。

Link：https://doi.org/10.1364/OL.533732