Phonon-tunable THz magnonic emission in multiferroic heterostructures

Abstract: Collective excitations such as magnons and polar phonons provide natural access to the terahertz (THz) regime, but efficient generation and tunability remain elusive. Multiferroic BiFeO3 combines both orders at room temperature, offering a unique platform for narrowband THz emission. Here, we achieve efficient sub-bandgap optical rectification of coupled phonon-polaritons near 2 THz in bare epitaxial thin films. In Pt/BiFeO3 bilayers, we demonstrate that coupling the electromagnon branch with ultrafast strain waves, optically generated in Pt layers with various thicknesses, can produce tunable and narrowband emission between 0.4-0.8 THz. These results uncover the intertwined role of phonons, magnons, and magneto-acoustic dynamics in antiferromagnetic multiferroics, and establish these hybrid platforms as versatile engineered narrowband THz sources.

• The paper introduces a novel method for tuning THz magnonic emission in BFO heterostructures via phonon-polariton and strain-coupled mechanisms.
• It details dual emission processes: optical rectification yielding a 2.1 THz polar phonon mode and magneto-acoustic resonance enabling 0.4–0.8 THz tunability with ~100 GHz resolution.
• The study emphasizes the critical role of interface quality and transducer material, identifying Pt as optimal for enhancing coherent THz output.

Phonon-Tunable THz Magnonic Emission in Multiferroic Heterostructures

Introduction

The paper "Phonon-tunable THz magnonic emission in multiferroic heterostructures" (2511.21596) presents a systematic investigation into the generation and tunability of terahertz (THz) emission via collective excitations in multiferroic thin films and heterostructures. The study leverages BiFeO₃ (BFO), a prototypical room-temperature multiferroic material, to explore engineered sources of narrowband THz radiation. The approach is centered on manipulating coupled phonon-polaritons and electromagnons, with emission characteristics modulated by ultrafast strain pulses generated in transition metal capping layers, primarily platinum (Pt).

Experimental Design and Observations

Epitaxial thin films of BiFeO₃, grown on DyScO₃ substrates and capped with metallic layers of varying thickness, serve as the primary system. Optical excitation is performed with sub-bandgap femtosecond pulses, and THz emission is characterized via time-domain spectroscopy. The films maintain a single ferroelectric domain and antiferromagnetic cycloidal order, ensured by controlled epitaxial growth, as evidenced by piezoresponse force microscopy and NV magnetometry.

The emission spectra reveal two distinct narrowband features: a peak at 2.1 THz in bare BFO, attributed to the E-mode polar phonon, and a tunable peak between 0.4–0.8 THz in Pt/BFO bilayers, assigned to the electromagnon branch. The latter is absent in control samples employing non-magnetic ferroelectrics, confirming that the observed magnonic mode is unique to BFO’s magnetoelectric order and is interface-dependent.

Rotation of the sample and incident light polarization demonstrate that the high-frequency (2.1 THz) mode is governed by optical rectification and impulsive stimulated Raman scattering, with clear linear polarization sensitivity. In contrast, the low-frequency magnonic emission is insensitive to laser polarization, signifying excitation by non-optical (strain and thermal) mechanisms.

Mechanistic Insights

The study delineates two mechanisms for THz emission in multiferroic heterostructures:

1. Optical Rectification of Phonon-Polaritons: Efficient sub-bandgap optical rectification in BFO thin films fosters narrowband THz emission at 2.1 THz. Enhancement arises from polar phonons amplifying second-order nonlinearities, with emission intensity scaling with film thickness up to the self-absorption limit.
2. Magneto-Acoustic Excitation via Strain Pulses: In Pt/BFO bilayers, ultrafast laser pulses generate longitudinal acoustic (LA) strain waves in the Pt. These propagate into the BFO at a duration dictated by Pt thickness, setting the spectral window for magnon-phonon coupling. The proximity of the LA phonon and electromagnon branches at high wavevectors promotes magneto-acoustic resonance, enabling tunable, narrowband THz magnonic emission. Frequency control is achieved with ~100 GHz resolution, corresponding to Pt thicknesses in the nm range.

The results contrast markedly with conventional ferroelectric emitters and control heterostructures, which do not exhibit oscillatory THz signatures, consolidating that magnetic order and the quality of the metal/BFO interface are prerequisites for magnonic THz emission.

Strong Numerical Results and Claims

• THz Emission Tunability: Demonstration of frequency tuning between 0.4–0.8 THz solely via Pt thickness adjustment, with resolution limited only by experimental setup.
• Emission Lifetime: THz oscillations persist for over 5 ps, indicating long coherence and low dissipation.
• Interface and Material Dependency: Emission amplitude and mode selectivity depend quantitatively on BFO thickness, antiferromagnetic order (cycloidal vs. G-type), and transducer material (Pt, W, Cu), with Pt identified as the superior transducer for magnonic emission.
• Selective Excitation: No THz oscillations observed in heterostructures with conventional ferroelectric layers (e.g., Pb(Zr,Ti)O₃), directly linking emission to multiferroic order.

Implications and Future Directions

Practically, the capacity to engineer THz magnonic sources with fine-tuned emission characteristics opens novel opportunities for device architectures in magnonics, THz photonics, and ultrafast spintronics. The presented magneto-acoustic mechanisms bypass the limitations of weak stray fields in antiferromagnets and exploit symmetry-breaking at the metal/multiferroic interface.

Theoretically, the work elucidates the intertwined dynamics of phonons, magnons, and strain in hybrid systems, substantiating recent models of magnon excitation via ultrafast strain pulses [14, 43]. It sets the stage for further investigation into coherent control of collective modes, possibly integrating electric, magnetic, and strain stimuli to manipulate spin-lattice dynamics at sub-THz frequencies.

In future developments, exploration of other multiferroic phases, optimization of transducer compositions, and extension to layered van der Waals systems may yield even greater spectral control and efficiency. The implications for THz spin-wave generation, domain wall dynamics, and nonlinear magnetoelectric phenomena are considerable, with direct relevance for quantum technologies and THz communication systems.

Conclusion

This study establishes a versatile framework for coherent, tunable, and interface-controlled THz magnonic emission in multiferroic heterostructures. Through phonon-polariton engineering and magneto-acoustic resonance, the authors achieve frequency-selective THz sources with performance dictated by interface physics and transducer parameters. The results decisively clarify the role of strain, magnon-phonon coupling, and multiferroicity in ultrafast THz emission processes, marking a significant advance for the fields of THz photonics and spintronics.