THz field-driven phonons and magnons probed with x-rays and 2D THz polarimetry

Keith A. Nelson

Department of Chemistry, MIT

THz electric and magnetic field components have been used to drive nonlinear responses in a wide variety of systems. Here we report recent measurements in which THz-driven phonon responses were probed by femtosecond time-resolved x-ray diffuse scattering and THz-driven magnon responses were probed by polarimetric 2D THz.

Figure 1: (a) THz alternate-polarity excitation of SrTiO₃ using two LiNbO₃ crystals with opposite c-axis orientations. (b) (3,3,3) Bragg diffraction peak and off-Bragg region of interest. (c) Time-resolved x-ray scattering from region of interest shows soft optical phonon and transverse acoustic modes in signals that are inverted with opposite THz field polarities.

In earlier results [1], we reported the use of a single-cycle THz field to drive SrTiO₃ (STO) from its low-temperature quantum paraelectric (QPE) phase into a transient ferroelectric configuration. XFEL measurements have revealed that the THz-driven response was highly nonuniform in character even though the THz driving field was essentially uniform spatially. The time-dependent lattice motion is observed in diffuse x-ray scattering at wavevectors that are distinct from the primary Bragg peaks. (See Fig. 1.) The time-dependence reveals soft optical phonon and transverse acoustic phonon displacements, and the signals induced by THz fields of opposite polarity are inverted. The heterogeneous responses result from pre-existing polar nanoregions in the STO QPE phase. The mesoscale responses, which could not be inferred from optical measurements, highlight the value of x-ray probes of photoinduced collective dynamics, especially in quantum phases which often include nanoscale features.

In 2D THz spectroscopy measurements, nonlinear magnonic responses of canted antiferromagnetic materials have been characterized [2,3]. In the first measurements of this kind [4], a single 2D THz spectrum took several days to collect as the time between two THz excitation pulses and the time of electro-optic (EO) measurement of the coherent signal field both were scanned. The use of single-shot EO measurements leaves only one time delay to be scanned, reducing the data acquisition time from days to minutes. This makes systematic variation of key experimental parameters possible on practical time scales. In some measurements, the crystalline orientation was varied with respect to the incident THz polarization direction in 5-degree steps, covering the complete 360-degrees with two different signal polarizations for a total of 144 2D spectra, in addition to spectra recorded as a function of THz field strength, temperature, and other parameters. The results have revealed the full set of second-order couplings between ferromagnetic and antiferromagnetic magnon modes in YFeO3. Magnon 2-quantum coherences with sum and difference frequencies (including second harmonic and rectified signals), magnon up-conversion, and magnon parametric amplification have all been observed. Higher-order signals and couplings have been observed as well.

References
[1] X. Li, T. Qiu, J. Zhang, E. Baldini, J. Lu, A. M. Rappe, and K. A. Nelson, “Terahertz field–induced ferroelectricity in quantum paraelectric SrTiO3,” Science 364, 1079-1082 (2019); https://doi.org/10.1126/science.aaw4913
[2] Z. Zhang, F. Y. Gao, J. B. Curtis, Z.-J. Liu, Y.-C. Chien, A. von Hoegen, T. Kurihara, T. Suemoto, P. Narang, E. Baldini, and K. A. Nelson, “Terahertz field-induced nonlinear coupling of two magnon modes in an antiferromagnet,” Nat. Phys. (2024); https://doi.org/10.1038/s41567-024-02386-3
[3] Z. Zhang, F. Y. Gao, Y.-C. Chien, Z.-J. Liu, J. B. Curtis, E. Sung, X. Ma, W. Ren, S. Cao, P. Narang, A. von Hoegen, E. Baldini, and K. A. Nelson, “Terahertz-field-driven magnon upconversion in an antiferromagnet,” Nat. Phys. (2024); https://doi.org/10.1038/s41567-023-02350-7
[4] J. Lu, X. Li, H. Y. Hwang, B. K. Ofori-Okai, T. Kurihara, T. Suemoto, and K. A. Nelson, “Coherent two-dimensional terahertz magnetic resonance spectroscopy of collective spin waves,” Phys. Rev. Lett. 118, 207204 (2017); doi.org/10.1103/PhysRevLett.118.207204