Giant enhancement of THz-frequency optical nonlinearity by phonon polariton in ionic crystals

Abstract

The field of nonlinear optics has grown substantially in past decades, leading to tremendous progress in fundamental research and revolutionized applications. Traditionally, the optical nonlinearity for a light wave at frequencies beyond near-infrared is observed with very high peak intensity, as in most materials only the electronic nonlinearity dominates while ionic contribution is negligible. However, it was shown that the ionic contribution to nonlinearity can be much larger than the electronic one in microwave experiments. In the terahertz (THz) regime, phonon polariton may assist to substantially trigger the ionic nonlinearity of the crystals, so as to enhance even more the nonlinear optical susceptibility. Here, we experimentally demonstrate a giant second-order optical nonlinearity at THz frequency, orders of magnitude higher than that in the visible and microwave regimes. Different from previous work, the phonon-light coupling is achieved under a phase-matching setting, and the dynamic process of nonlinear THz generation is directly observed in a thin-film waveguide using a time-resolved imaging technique. Furthermore, a nonlinear modification to the Huang equations is proposed to explain the observed nonlinearity enhancement. This work brings about an effective approach to achieve high nonlinearity in ionic crystals, promising for applications in THz nonlinear technologies.

Topologically tuned terahertz confinement in a nonlinear photonic chip

“…Over the past decades, numerous techniques have been developed to enhance THz generation efficiency, achieve narrow THz bandwidth, and decrease THz-wave decay in LN crystals 31 – 33 . In particular, it has been recently shown that giant enhancement of the optical nonlinearity can be achieved at THz-frequencies by stimulated phonon-polaritons in LN crystals pumped by a femtosecond laser pulse, where ionic contributions to light–matter interaction become significant 11 . While the underlying mechanisms under different excitation conditions merit further investigation, it is experimentally evident that tunable THz pulses can be generated in nonlinear LN crystals by means of ultrashort laser pulses in the range from a few tenths of THz to a few THz 11 , 31 , 32 .…”

“…In order to realize the proposed THz field manipulation, we perform a series of experiments with a typical pump-probe setup 11 to obtain both the dispersion curves of the structure at different z -positions (Fig. 3 , first row) and the energy distributions of the confined modes (Fig.…”

“…Therefore, our scheme can be extended to integrated topological circuits and implemented in compact THz devices. For THz wave detection, the lateral propagation of waves (along the x -direction in the chip plane) can be directly observed using time-resolved imaging by monitoring the refractive index change induced by the THz waves and using a phase contrast imaging technique 11 . We mention that there are several other ways to observe the band spectra of structures such as angle-resolved transmission measurements 43 , energy-resolved photoluminescence 44 and polarization-resolved momentum-space imaging spectroscopy 45 .…”

“…In addition, it has always been a challenge to guide and manipulate THz waves due to unavoidable losses arising from the critical features of the THz spectrum. Therefore, there have been tremendous efforts in exploring different designs and approaches for THz sources and integrated THz devices using a variety of platforms, including metamaterials 3 , 5 and nonlinear metasurfaces 9 , surface plasmonic waves 10 , nonlinear wave mixing in ionic crystals 11 , as well as time-domain integration of THz pulses 12 .…”

Topological Valley Transport of Terahertz Phonon–Polaritons in a LiNbO3 Chip

“…Research on phonon–polaritons mainly include two regimes, one in the van der Waals materials , and the other in the bulk ionic crystals, − based on the material they are explored in. In very recent studies, the topological transition of phonon–polaritons in twisted stacks of two-dimensional materials (such as α-MoO 3 ) , and gratings were experimentally studied, but the topological edge transport of phonon–polaritons in the ionic crystals still remains a fertile ground to be explored.…”