Time-reversal symmetry (TRS) is pivotal for materials’ optical, magnetic, topological, and transport properties. Chiral phonons, characterized by atoms rotating unidirectionally around their equilibrium positions, generate dynamic lattice structures that break TRS. Here, in our recent work published on Science, we report that coherent chiral phonons, driven by circularly polarized terahertz light pulses, will polarize the paramagnetic spins in cerium fluoride in a manner similar to that of a quasi-static magnetic field on the order of 1 tesla. Through time-resolved Faraday rotation and Kerr ellipticity, we found that the transient magnetization is only excited by pulses resonant with phonons, proportional to the angular momentum of the phonons, and growing with magnetic susceptibility at cryogenic temperatures. The observation quantitatively agrees with our spin-phonon coupling model and may enable new routes to investigating ultrafast magnetism, energy-efficient spintronics, and nonequilibrium phases of matter with broken TRS.

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Mature optical technologies from the microwave frequencies to the X-ray frequencies have dramatically changed our life in the modern society, but there is a notable gap between the mid- and far-infrared light, roughly the frequencies of 5 – 15 terahertz, where there are no good commercial solutions to control light. In our new work published on Advanced Materials, we use quantum paraelectric perovskite SrTiO3, whose atoms couple with terahertz light so strongly that form a new particle called “phonon-polaritons” to realize sub-wavelength THz field focus. Unlike other materials that also support phonon-polaritons in other frequencies and usually in a very narrow range, SrTiO3 works for the entire “new terahertz gap” due to its quantum paraelectricity property. We prove the concept of SrTiO3 phonon-polariton devices in the frequency range of 7 – 13 terahertz by designing and fabricating ultrafast THz field concentrators to achieve a strong transient electric field around gigavolts per meter. Such a strong field may be used to change the materials’ structure to create new electronic properties, or create new nonlinear optical response from trace amounts of specific molecules to be detected by a common optical microscope. Our methodology framework is also applicable for a broad range of phonon-polaritonic materials to work for photonic devices in 3 – 19 terahertz, opening a new route to control light in one of the last under-explored territories. More details in Rice news.

Check out our new results on Nano Letters! We have observed coherent hybridization of magnons and phonons first ever in monolayer antiferromagnet FePSe₃ by cavity-enhanced magneto-Raman spectroscopy. The coupling remains robust even in zero magnetic field down to 2D limit in a single atomic layer. We also discovered theoretically that such magnon-phonon hybridization enables nontrivial band inversion between longitudinal and transverse optical phonons and guarantee magnetic-field-controlled topological phase transition. The 2D topological magnon–phonon hybridization would potentially offer a new route toward quantum phononics and magnonics with an ultrasmall footprint.

Phase boundaries can exhibit new properties that neither of the two sides possess. In ultrathin 2D crystals of MoTe₂, both the 2H and 1T’ phases have very weak, if any, inversion symmetry breaking, and thus do not have strong piezoelectricity. However, the metal-semiconductor junction between these two phases has a build-in electric field, so the wavefunctions are asymmetric at the boundary. In this work, we extracted the tiny displacement of these boundary atoms as a result of interfacial piezoelectricity in the CVD-grown 2H-1T’ MoTe2 homojunctions by vector piezoresponse force microscopy. The additional torque from the atoms was distinguished from electrostatic background, and qualitatively agreed with the first-principle calculations. Our findings demonstrate the versatility of boundary engineering in nanoscale electromechanical devices. See more news and details of this work in collaboration with Ajayan group at Rice MSNE.