On-chip efficient and compact frequency converters are promising to be one of the key components in the wavelength-division-multiplexing (WDM) systems, , and have a wide range of applications including tunable coherent radiation, telecommunications, spectroscopy, and quantum optics. Nonlinear optics provides a convenient route to access the frequencies we are interested in. Optical whispering gallery-mode (WGM) microcavities owing to their strong confinement of a light field (ultrahigh quality factors and small mode volumes) could considerably enhance the light-matter interaction, making it an ideal platform for studying a broad range of nonlinear optical effects, ultrahigh sensitivity sensors, and other physical applications.

Our work demonstrates a method to access near-infrared, visible, and other wavelengths of interest using on-chip nonlinear effects based on z-cut insulator thin-film lithium tantalate (LTOI). Lithium tantalate has a higher laser-radiation-induced damage threshold, higher photorefractive damage threshold, and a wider UV transparency. It may pave the way to achieve an on-chip wide range of wavelengths coverage via direct optical transition. Cascaded three- and four-wave mixing generation simultaneously were achieved in the LTOI microdisk cavity (Fig.1), which heralds its application potential in integrated nonlinear photonics.

Fig. 1. Schematic of the multiple nonlinear processes generated in the LTOI microdisk and the corresponding energy-level diagrams.

In the experiment, when the pump light is increased to 36.19 mW, we can observe an obvious near-infrared signal (1037.75 nm). Fig.2(b) shows the power dependence of the nearinfrared signal on the different pumps in the telecom band, by keeping increasing the input power to around 78.89 mW. The wavelength of the generated near-infrared signal fluctuated slightly with the pump power increasing that may be caused by the thermal effect and photorefractive effect under the high input power condition. Then we further calculated the effective refractive indices of the input pump wave (FW), its SH signal (SHG), and the generated near-infrared signal (cFWM) as shown in Fig. 2c, respectively. It shows that the mode phase matching was nearly satisfied too. The cubic relationship between the near-infrared signal and the pump power is also in agreement with the experimental data.(Fig. 2d)

Fig. 2 (a) The recorded spectrum of the visible signal (SHG, cSFG/THG, and cFWM signal with 36.19-mW pump power. (b) cFWM near-infrared signal generation with different input power. (c) Effective indices as functions of wavelength in the telecom (FW), visible (SHG), and near-infrared (cFWM). (d) The near-infrared signal intensity with the pump-power cubic.

One can improve the efficiency by optimizing the microcavity geometry, pump wavelength, power, phase-matching conditions, etc. And more optical nonlinear processes can be generated by designing periodically poled domain structures and the device dispersion in the LTOI microdisk. More broad spectral span frequency combs and even UV signals may also be achieved by careful dispersion design combining the relatively high damage threshold of LTOI microresonators. All these nonlinear process generations show the great application potential for LTOI in integrated nonlinear photonics. And its outstanding physical properties make it can be an excellent nonlinear photonics and hybrid photonics platform.

The work was published in “Xiongshuo Yan, Miao Xue, Tingge Yuan, Jiangwei Wu, Rui Ge, Yuping Chen, and Xianfeng Chen. Cascaded degenerate four-wave mixing generation in thin-film lithium tantalate microdisk cavity. Physical Review Applied, 21(2), 024033 (2024)”.

Link: https://doi.org/10.1103/PhysRevApplied.21.024033