Exciting Chiral Phonons to Induce Magnetism in Non-Magnetic Materials

Materials that display magnetic properties are crucial to many electronic devices, with old and new applications in data storage. To increase the operating speed in new data storage paradigms, it is of great interest to study ways to control magnetic properties in ultrafast scales. Most current ultrafast studies of magnetic order reply on destroying the magnetic order with a visible or infrared (IR) laser pulse. On the other hand, to induce magnetic ordering, it has recently been proposed that exciting circular motion of ions using terahertz (THz) light can induce magnetic moment in materials. Light with resonant frequency can excite the IR-active phonon modes in materials; when pairs of perpendicular modes are excited, ions moving in circular (“chiral”) loops can be induced.<br/><br/>One way to create ion loops is to use a pair of perpendicularly polarized THz pump pulses with relative phase delay. The pair of perpendicular THz pulses can excite pairs of perpendicular modes; mean while the relative phase delay serves as the means to control the path of the ionic motions (being linear, elliptical or circular). We will present our work on how we induced ion loops in two non-magnetic crystals – LiNbO₃ and beta Ba(BO₂)₂ (BBO) and measured the induced magnetic moment with the ultrafast Faraday effect.<br/><br/>We pick LiNbO₃ and BBO as the samples because they both have doubly-degenerate perpendicularly E modes that are excited with our chiral THz pulses. The lowest frequency modes in LiNbO₃ form phonon-polaritons, whereas BBO has three sets of E modes that can contribute to a magnetic signal.<br/><br/>With a 2D THz spectroscopy set up, we generate circularly polarized THz pulses by directing two perpendicular THz pulses to the same spot on the sample with adjustable relative delay. When combining the two THz pulses with different delay, different polarization states of combined THz pulses are achieved (linear when the phase delay is zero or 90 degree, circular when the phase delay is 45 degree or elliptical for other delays).<br/><br/>To detect the magnetic moment induced by the combined THz pulses, we use an ultrafast Faraday effect detection scheme – we direct an 800-nm probe to the sample and detect the polarization change of the probe. Based on the strength of the induced magnetic field, the polarization of the probe is rotated. To ensure our detected signal is from polarization change due to the chiral motions, we use a differential chopping scheme to measure the sample response in consecutive four laser shots – a single vertical THz pulse, a horizontal THz pulse, the combined (circular) THz pulse and no THz pulse. This allows us to isolate the signal arised only from the combined THz pulse, and not from the linear components of that circular THz pulse.<br/><br/>We also consider rather the induced magnetic field is induced from the chiral phonon motions or the inverse Faraday effect from electronic motions. Typically, the non-resonant electronic response decays very fast and only last within the duration of the driven force. Phonon-magnetism, on the other hand, has a longer lifetime because atoms or molecules take longer time than electrons to lose energy. By modeling the electronic and ionic responses, we can account for our observed magnetic signal<br/><br/>In conclusion, we induce an apparent magnetic moment in LiNbO₃ and BBO with intense circularly polarized THz pulses. The ability to manipulate chiral phonon motion using circularly polarized light open the door to potentially control materials in ways that could not been achieved before.