Terahertz Excitation of Chiral Phonons to Induce Dynamical Multiferroicity

Circular motion of ions in solid materials has been posited as a means to dynamically induce or control magnetic properties. This circular motion fundamentally requires the excitation of “chiral” phonon modes, and the ability to excite and control transient crystal chirality could have a significant impact on condensed matter physics.<br/><br/>Pairs of phonon modes in ferroelectric materials can be excited to create circular or elliptical motion of ions. Depending on the charge of the ions and magnitude of their motions, these ion loops with right or left handedness can induce a magnetic moment, even in originally non-magnetic materials. Therefore, exciting ion loops in ferroelectric solids to dynamically induce magnetic ordering, will make them transiently multiferroic.<br/><br/>We have recently had much success in controlling the waveform and polarization of high-field, broadband terahertz (THz) pulses. This waveform control is driven by the capability to perform two-dimensional (2D) THz spectroscopy, including the ability to create circularly polarized THz pump pulses. In this presentation, we report how we generated intense pulses of circularly polarized THz light to excite pairs of degenerate phonon modes. As the phonon modes are excited with proper relative phase (provided by the circularly polarized THz pulses), circular phonon motion results. The transient magnetic moment induced by these excited ions loops is then measured with the ultrafast Faraday effect.<br/>To generate circularly polarized light, we combine two linearly polarized THz pulses with adjustable relative delay. A differential chopping scheme enables us to readily measure the sample response from a single (vertically polarized) THz pulse, the other individual (horizontally polarized) THz pulse, and the combined THz pulse with controllable polarization state, all in a succession of four laser shots; the probe response in the absence of a THz pump pulse is measured with the fourth laser shot.<br/><br/>To probe the sample response, we direct an 800-nm probe pulse through the excited region of the sample and measure the modification of the probe polarization with a transient birefringence detection scheme. This allows us to measure the transient Faraday effect to detect the creation of an ultrafast magnetic moment. However, it can also measure other ultrafast birefringence signals, like the THz Kerr effect. Fortunately, by controlling the relative polarization angles between THz and probe pulses, we can isolate the Faraday signal from the Kerr signal.<br/><br/>With circularly polarized THz pulses, we excite a z-cut sample of LiNbO₃. The lowest frequency E symmetry phonon modes are doubly-degenerate with perpendicular mode effective charge vectors that can be driven with THz pulses polarized along the crystallographic x and y directions. Exciting pairs of E-modes with circularly polarized THz pump pulses induces ion-loop motion, leading to a transient magnetic moment in LiNbO₃. While the chiral phonon motion takes place, a transient magnetic moment is induced that can be detected with the Faraday rotation of a probe pulse. We observe a transient signal persisting for ~3 ps, which is longer than the THz pump pulse duration (&lt;1 ps), confirming that this isn’t just the THz inverse Faraday effect. The magnetic moment signal also switches sign as the THz polarization and subsequent chiral phonon motion changes handedness.<br/><br/>In summary, using a 2D THz experimental setup, we can create high-field circularly polarized THz pulses. These circularly polarized THz pulses excite chiral phonon motion in LiNbO₃, transiently inducing a magnetic moment that persists longer than the THz excitation pulses. Isolating the Faraday signal from the Kerr signal confirms that the excited chiral phonon motion, so-called ion loops, induces magnetic ordering in non-magnetic LiNbO₃. The ability to use circularly polarized THz pulses to control chiral phonon motion will provide many new avenues in the study of condensed matter systems.