In December 2022, Optics & Photonics News, a journal run by Optica (formerly Optical Society of America (OSA)), released Optics in 2022, which included a series of research achievements on on-chip terahertz frequency conversion and regulation by the research team of Professor Wu Qiang, Professor Chen Zhigang and Professor Xu Jingjun of Nankai University (as shown in Link 1). For 40 years, Optica has annually reviewed 30 pieces of breakthroughs in optics published worldwide in the preceding year, and published them in the December issue of journal OPN. The selected "Chip-scale terahertz frequency conversion and modulation" achievements were published in world-renowned academic journals such as Nature Communications (May 2021), Light: Science & Applications (May 2022), and Laser & Photonics Reviews (May 2021).

In recent years, the NKU team has developed a terahertz integration platform based on lithium niobate chips. This report mainly introduced some of these techniques, including on-chip frequency conversion and tuning technologies, which can achieve highly efficient nonlinear terahertz frequency conversion (as shown in Link 2), peculiar phase matching and transmission of "frozen phase" THz pulses (as shown in Link 3), and dynamic topology regulation of THz pulses (as shown in Link 4). These results provide multi-functional, stable, and highly integrated terahertz photonic integration platform for applications such as terahertz computing, imaging, and fingerprint detection. 

In order to enhance the nonlinearity of THz pulses, the team proposed the nonlinear Huang equation to highlight the unique role of phonon polaritons in enhancing the terahertz nonlinear effect. In an experiment, they used phonon polaritons to realize huge terahertz nonlinear frequency conversion in a lithium niobate chip. The terahertz difference frequency produces a nonlinear polarizability of about 10-6 m/V, about five orders of magnitude higher than what was previously reported. 

Moreover, the team's latest findings revealed that the use of stimulated phonon polaritons can also achieve a light-matter interaction that goes beyond the traditional Born-Oppenheimer Approximation (as shown in Link 5). 

The research team also designed a wedge-shaped one-dimensional topological array (Su-Schrieffer-Heeger lattice) on the lithium niobate chip. Excited by femtosecond laser, it can realize the continuous regulation of THz pulses in topological domain, bulk diffusion and non-topological domain. 

These achievements open up a new avenue for THz pulse generation, localization, and detection in multifunctional integrated chip-scale devices, and are of potential value for systems with characteristic terahertz fingerprint spectroscopy. The huge nonlinear effect caused by stimulated phonon polaritons is expected to be applied to the optical control of spin quantum bits in semiconductors, as well as the regulation of various ions and ferroelectric/ferromagnetic crystal characteristics, etc. 

Link to related papers:

• Y.Lu,et al,OPN:Optics in 2022:37(2022). 

  https://www.optica-opn.org/home/articles/volume_33/december_2022/extras/chip-scale_thz_frequency_conversion_and_tuning/ 

• Y.Lu,et al,Nat.Commun.12:3183(2021).

      https://www.nature.com/articles/s41467-021-23526-w 

• Y.Lu,et al,Laser Photon.Rev.15:2000591(2021).

      https://doi.org/10.1002/lpor.202000591 

• J.Wang,et al,Light Sci.Appl.11:152(2022).

      http://dx.doi.org/10.1038/s41377-022-00823-7

• Y.Lu,et al,Commun.Phys.5:299(2022).

      https://doi.org/10.1038/s42005-022-01080-1

(Edited and translated by Nankai News Team)