a & b, THz waves strongly couple with phonons in ionic crystals, taking lithium niobate for an example, resulting in a remarkable increase in the Kerr nonlinearity, which modulates the refractive index, leading to a frequency shift. For a Kerr medium, the refractive index should consist of a linear component and a nonlinear component, which indicates that the resonant frequency of a Kerr microcavity would change with the input THz power. c, Spectrum of a single-mode microcavity at different pump powers. The third-order nonlinear optical susceptibility can be calculated from the frequency shift.
Newswise — Terahertz (THz) waves and THz technologies have gradually opened a new style for communications, cloud-based storage/computing, information contest, and medical tools. With the advancement of THz technologies, studies on THz nonlinear optics have emerged and achieved considerable breakthroughs on both physics and technology. However, the studies on THz Kerr effect (TKE) cannot support such a fascinating blueprint yet. The literature TKE results are either feeble or unstable in the complicated application occasions.
In a new paper (doi: https://doi.org/10.1038/s41377-024-01509-y) published in Light Science & Applications, a team of scientists, led by Professor Qiang Wu, Yao Lu and Jingjun Xu from The Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, TEDA Applied Physics Institute and School of Physics, Nankai University, Tianjin 300071, China, and co-workers have utilized a new light-matter interaction mechanism through stimulated phonon polaritons (SPhPs), and demonstrated a giant TKE in a chip-scale lithium niobate Fabry-Pérot microcavity. Under the influence of the giant TKE, the power-dependent refractive index changes resulted in frequency shifts of the resonant modes in a single-mode microcavity, which were experimentally measured and analyzed. The experimentally SPhPs-enhanced third-order nonlinear optical susceptibility is demonstrated 4 magnitude orders larger than that in visible or infrared frequency, consistent well with the theoretical predictions from the nonlinear Huang equations. Besides, the frequency shifts caused by the hybrid modulation (TKE and cross-modulation) in the multi-mode microcavity also align well with the theoretical predictions. The reported results present an innovative platform for a range of practical THz photonic devices, which is greatly conducive to the development of high-speed THz communication, and applicable to versatile, stable and compact THz photonic chips. In the future, the study of supercontinuum spectra could possibly expand towards the THz frequency range, which could provide the SPhPs enhanced TKE with an opportunity to exhibit their great potential in broadband terahertz waves generation.
In their work, a Fabry-Pérot microcavity is fabricated on a 50 μm-thick x-cut MgO:LiNbO3 (LN) slab waveguide using the femtosecond laser direct writing technique. THz waves in the microcavity are generated by femtosecond laser pulses via nonlinear effects, ranging from 0.2-1.2 THz in frequency. The microcavity was initially single-mode designed, in which the resonant mode is located at 0.63 THz. By focusing the pump pulses in the center of the microcavity, the single-mode microcavity is excited. Since the existence of TKE, the effective resonant frequency of the microcavity would be influenced by the input THz intensity, with a power-independent frequency-shift. In addition, they also investigated the “hybrid modulation” of a similar double-mode microcavity with resonant frequencies of 0.32 THz and 0.38 THz. Both the results imply a giant chip-scale TKE, exhibiting a nonlinear coefficient 4 orders of magnitude larger than that in visible/ infrared range.
“The reported giant TKE results in the LN chip demonstrate that polar crystalline materials, especially LN, may also be a superior candidate for THz nonlinear platform, more than only an outstanding material for THz generation and detection. The SPhPs enhanced TKE presents an innovative platform for a range of practical THz photonic devices, which is greatly conducive to the development of high-speed THz communication, and applicable to versatile, stable and compact THz photonic chips.” they added.
“In the future, the study of supercontinuum spectra could possibly expand towards the THz frequency range, which could provide the SPhPs enhanced TKE with an opportunity to exhibit their great potential in broadband terahertz waves generation.” the scientists forecast.
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References
DOI
Original Source URL
https://doi.org/10.1038/s41377-024-01509-y
Funding information
This work was supported by the National Natural Science Foundation of China (62205158 and 11974192), the Foundation of State Key Laboratory of laser Interaction with Matter (SKLLIM2101), the 111 Project (B23045).
About Light: Science & Applications
The Light: Science & Applications will primarily publish new research results in cutting-edge and emerging topics in optics and photonics, as well as covering traditional topics in optical engineering. The journal will publish original articles and reviews that are of high quality, high interest and far-reaching consequence.
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