Measuring the Magnetization of Circular Phonon with an Ultrafast Gaussmeter

MPSD Exchange

Date: Sep 8, 2026
Time: 10:15 AM - 11:45 AM (Local Time Germany)
Speaker: Zhou Shen
PhD Student, QCMD Group
Location: MPSD Bldg. 900
Room: Seminar Room 136

Driving degenerate infrared-active phonons with circularly polarized laser pulses is predicted to generate magnetization [1]. Estimates based on the circular motion of the mode’s Born effective charge typically yield values on the order of one nuclear magneton per unit cell. Recent work has revealed large effective magnetic fields from circularly driven phonons in materials showing large phonon Zeeman effect [2], and – more surprisingly – phonon magnetizations up to four orders of magnitude larger than predicted by calculations even in systems without magnetic ions [3,4]. In most cases, this induced magnetization has been probed optically, via the Faraday rotation in the photo-excited volume, making it challenging to unambiguously separate the Faraday signal from other contributions arising by nonlinear optical interactions [5,6]. A promising, artefact-free approach is to quantify the phonon-induced magnetization by the fringing field it generates. Recently, we developed an ultrafast magnetometry technique that measures magnetic fields with sub-picosecond time resolution and sub-microtesla sensitivity [7,8]. Here, we study 6H-SiC, driven with circularly polarized pulses resonant with a doubly degenerate E1 phonon. By combining measurements of the fringing field from phonon-induced magnetization with polarization rotation measurements within the photoexcited volume, we find that the induced magnetization is at least an order of magnitude smaller than inferred from Faraday rotation alone [3]. This approach yields a more accurate estimate of the induced magnetization and helps clarify the origin of the large magnetization values reported in literature [3, 4].

In most cases, this induced magnetization has been probed optically, via the Faraday rotation in the photo-excited volume, making it challenging to unambiguously separate the Faraday signal from other contributions arising by nonlinear optical interactions [5,6]. A promising, artefact-free approach is to quantify the phonon-induced magnetization by the fringing field it generates. Recently, we developed an ultrafast magnetometry technique that measures magnetic fields with sub-picosecond time resolution and sub-microtesla sensitivity [7,8].

Here, we study 6H-SiC, driven with circularly polarized pulses resonant with a doubly degenerate E1 phonon. By combining measurements of the fringing field from phonon-induced magnetization with polarization rotation measurements within the photoexcited volume, we find that the induced magnetization is at least an order of magnitude smaller than inferred from Faraday rotation alone [3]. This approach yields a more accurate estimate of the induced magnetization and helps clarify the origin of the large magnetization values reported in literature [3, 4].

References

[1] D. M. Juraschek et al., Orbital magnetic moments of phonons, Phys. Rev. Mater. 3, 064405 (2019).

[2] J. Luo et al., Large effective magnetic fields from chiral phonons in rare-earth halides, Science 382, 698 (2023).

[3] M. Basini et al., Terahertz electric-field-driven dynamical multiferroicity in SrTiO3, Nature 628, 534 (2024).

[4] C. S. Davies et al., Phononic switching of magnetization by the ultrafast Barnett effect, Nature 628, 540 (2024).

[5] M. Basini et al., Terahertz ionic Kerr effect: Two-phonon contribution to the nonlinear optical response in insulators, Phys. Rev. B 109, 024309 (2024).

[6] R. Merlin, Magnetophononics and the chiral phonon misnomer, PNAS Nexus 4, pgaf002 (2025).

[7] S. Fava et al., Magnetic field expulsion in optically driven YBa₂Cu₃O_6.48, Nature 632, 75 (2024).

[8] G. De Vecchi et al., Generation of ultrafast magnetic steps for coherent control, Nat. Photon. 1 (2025).