Twisting between layered materials in close proximity causes reconstruction of the electron and crystal structures. The resulting reconstruction has received increasing attention due to the realization of magic-angle superconductivity, moiré excitons, and moiré ferroelectricity. However, demonstrations of such concepts for phonon polarization have been very limited. Here, we reported on the systematic angle-dependent polarized Raman (ADPR) investigations on the twisted bilayer (tBL) α-MoO3 with each layer in a thickness of ∼14 nm. Under 532 and 633 nm excitation, (i) the Raman intensity (I) of representative Mo–Oc symmetric stretching vibration phonon mode (SVPM) at 818 cm–1 and rigid MoO4 translational vibration phonon mode (TVPM) at 156 cm–1 decreases monotonically with the twist angle (φ) increase and (ii) the polar plots of both modes were tuned from axial symmetry to central symmetry by φ increase. The dependence of I and symmetry of both modes on φ were described semiquantitatively by applying the classical Placzek approximation and the interference effect. Our findings unravel the complex interaction between the twisted layer configurations and phonon behaviors of α-MoO3, providing valuable insights into the anomalous phonon polarization responses in low symmetric two-dimensional materials and potentially facilitating the development of technologies based on tunable phonon polarization.
Kaizhen Liu,Guangyan Huang,Xiang Li,Guangpeng Zhu,Wei Du,Tao Wang
Vibrational strong coupling (VSC), the strong coupling between optical resonances and the dipolar absorption of molecular vibrations at mid-infrared frequencies, holds the great potential for the development of ultrasensitive infrared spectroscopy, the modification of chemical properties of molecules, and the control of chemical reactions. In the realm of ultracompact VSC, there is a need to scale down the size of mid-infrared optical resonators and to elevate their optical field strength. Herein, by using single quartz micropillars as mid-infrared optical resonators, the strong coupling is demonstrated between surface phonon polariton (SPhP) resonances and molecular vibrations from far-field observation. The single quartz micropillars support sharp SPhP resonances with an ultrasmall mode volume, which strongly couples with the molecular vibrations of 4-nitrobenzyl alcohol (C 7 H 7 NO 3 ) molecules featuring pronounced mode splitting and anticrossing dispersion. The coupling strength depends on the molecular concentration and reaches the strong coupling regime with only 7300 molecules. The findings pave the way for promoting the VSC sensitivity, miniaturing the VSC devices, and will boost the development of ultracompact mid-infrared spectroscopy and chemical reaction control devices.
The strong coupling between photons and phonons in polar materials gives rise to phonon-polaritons that encapsulate a wealth of physical information, offering crucial tools for the ultrafast terahertz sources and the topological engineering of terahertz light. However, it is still quite challenging to form and manipulate the terahertz phonon-polaritons under the ultrastrong coupling regime till now. In this work, we demonstrate the ultrastrong coupling between the phonon (at 0.95 THz) in a MAPbI 3 film and the metallic bound states in the continuum (BICs) in Au metasurfaces. The Rabi splitting can be continuously tuned from 28% to 48.4% of the phonon frequency by adjusting the parameters (size, shape and period) of Au metasurfaces, reaching the ultrastrong coupling regime. By introducing wavelet transform, the mode evolution information of the terahertz phonon-polariton is successfully extracted. It indicates that the phonon radiation intensity of the MAPbI 3 film is enhanced as the coupling strength is increased. This work not only establishes a new platform for terahertz devices but also opens new avenues for exploring the intricate dynamics of terahertz phonon-polaritons.
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