Large effective magnetic fields from chiral phonons in rare-earth halides

Moiré collective vibrations in atomically thin van der Waals superlattices

Collective vibration is pivotal for materials' thermal, electrical, phase transition and topological properties. Lately, the rising of moiré superlattices, characterized by overarching periodicity of moiré pattern, generates highly tunable interfacial structures that manipulate collective excitations in material at the atomic scale. Here, we experimentally demonstrate moiré collective vibrations, the mechanical counterparts of moiré excitons, at heterointerfaces of twisted tungsten diselenide/tungsten disulfide heterobilayers. Using helicity-resolved inelastic Raman scattering, we find chiral interfacial phonons carrying angular momentum analogous to that of chiral bulk phonons in quartz, enabling unprecedented spectral resolution of rich vibrational modes at heterointerface in a few atomic layers. Upon mutual torsion of heterobilayers, we observe terahertz interlayer vibrations proportional to moiré periodicity as a periodic function of rotation angles, demonstrating moiré-tuned interlayer modes which couple to Coulomb-bound electron-hole pairs in interlayer moiré excitons. In low-angle strong coupling regime, interlayer dynamics exhibit a distinct long-lived breathing mode with zero angular momentum and pronounced high energy, highlighting phonon-hybridization character wherein intralayer breathing vibrations are folded into moiré mini-Brillouin zone by spatial periodicity and hybridize with interlayer vibrations. Our findings establish moiré collective vibrations as candidates for exploitation in energy-efficient thermal management, strongly correlated electrical engineering, and new emergent topological phononics.

Origin of Large Effective Phonon Magnetic Moments in Monolayer MoS2

Recent helicity-resolved magneto-Raman spectroscopy measurement demonstrates large effective phonon magnetic moments of ∼2.5 μB in monolayer MoS2, highlighting resonant excitation of bright excitons as a feasible route to activate Γ-point circularly polarized phonons in transition metal dichalcogenides. However, a microscopic picture of this intriguing phenomenon remains lacking. In this work, we show that an orbital transition between the split conduction bands (Δ0 = 4 meV) of MoS2 couples to the doubly degenerate E″ phonon mode (Ω0 = 33 meV), forming two hybridized states. Our phononic and electronic Raman scattering measurements capture these two states: (i) one with predominantly phonon contribution in the helicity-switched channels and (ii) one with primarily orbital contribution in the helicity-conserved channels. An orbital-phonon coupling model successfully reproduces the large effective magnetic moments of the circularly polarized phonons and explains their thermodynamic properties. Strikingly, the Raman mode from the orbital transition is superimposed on a strong quasi-elastic scattering background, indicating the presence of spin fluctuations. As a result, the electrons excited to the conduction bands through the exciton exhibit paramagnetic behavior although MoS2 is generally considered as a nonmagnetic material. By depositing nanometer-thickness nickel thin films on monolayer MoS2, we tune the electronic structure so that the A exciton perfectly overlaps with the 633 nm laser. The optimization of resonance excitation leads to pronounced tunability of the orbital-phonon hybridized states. Our results generalize the orbital-phonon coupling model of effective phonon magnetic moments to material systems beyond the paramagnets and magnets.

Ultrafast Infrared Plasmonics

Ultrafast plasmonics represents a cutting-edge frontier in light-matter interactions, providing a unique platform to study electronic interactions and collective motions across femtosecond to picosecond timescales. In the infrared regime, where energy aligns with the rearrangements of low-energy electrons, molecular vibrations, and thermal fluctuations, ultrafast plasmonics can be a powerful tool for revealing ultrafast electronic phase transitions, controlling molecular reactions, and driving subwavelength thermal processes. Here, the evolution of ultrafast infrared plasmonics, discussing the recent progress in their manipulation, detection, and applications is reviewed. The future opportunities, including their potential to probe electronic correlations, investigate intrinsic ultrafast plasmonic interactions, and enable advanced applications in quantum information are highlighted, which may be promoted by multi-physical field integrated ultrafast techniques.

Unidirectional chiral scattering from single enantiomeric plasmonic nanoparticles

Controlling scattering and routing of chiral light at the nanoscale is important for optical information processing and imaging, quantum technologies as well as optical manipulation. Here, we introduce a concept of rotating chiral dipoles in order to achieve unidirectional chiral scattering. Implementing this concept by engineering multipole excitations in helicoidal plasmonic nanoparticles, we experimentally demonstrate enantio-sensitive and highly-directional forward scattering of circularly polarised light. The intensity of this highly-directional scattering is defined by the mutual relation between the handedness of the incident light and the chirality of the structure. The concept of rotating chiral dipoles offers numerous opportunities for engineering scattering from chiral nanostructures and optical nano-antennas paving the way for innovative designs and applications of chiral light-matter interactions.

Photo-induced chirality in a nonchiral crystal

Chirality, a pervasive form of symmetry, is intimately connected to the physical properties of solids, as well as the chemical and biological activity of molecular systems. However, inducing chirality in a nonchiral material is challenging because this requires that all mirrors and all roto-inversions be simultaneously broken. Here, we show that chirality of either handedness can be induced in the nonchiral piezoelectric material boron phosphate (BPO4) by irradiation with terahertz pulses. Resonant excitation of either one of two orthogonal, degenerate vibrational modes determines the sign of the induced chiral order parameter. The optical activity of the photo-induced phases is comparable to the static value of prototypical chiral α-quartz. Our findings offer new prospects for the control of out-of-equilibrium quantum phenomena in complex materials.

Magnetophononics and the chiral phonon misnomer

The direct, ultrafast excitation of polar phonons with electromagnetic radiation is a potent strategy for controlling the properties of a wide range of materials, particularly in the context of influencing their magnetic behavior. Here, we show that, contrary to common perception, the origin of phonon-induced magnetic activity does not stem from the Maxwellian fields resulting from the motion of the ions themselves or the effect their motion exerts on the electron subsystem. Through the mechanism of electron-phonon coupling, a coherent state of circularly polarized phonons generates substantial non-Maxwellian fields that disrupt time-reversal symmetry, effectively emulating the behavior of authentic magnetic fields. Notably, the effective fields can reach magnitudes as high as 100 T, surpassing by a factor of α - 2 ≈ 2 × 10 4 the Maxwellian fields resulting from the inverse Faraday effect; α is the fine-structure constant. Because the light-induced nonreciprocal fields depend on the square of the phonon displacements, the chirality the photons transfer to the ions plays no role in magnetophononics.

Phonon Inverse Faraday Effect from Electron-Phonon Coupling

The phonon inverse Faraday effect describes the emergence of a dc magnetization due to circularly polarized phonons. In this work we present a microscopic formalism for the phonon inverse Faraday effect. The formalism is based on time-dependent second order perturbation theory and electron phonon coupling. While our final equation is general and material independent, we provide estimates for the effective magnetic field expected for the ferroelectric soft mode in the oxide perovskite SrTiO_{3}. Our estimates are consistent with recent experiments showing a huge magnetization after a coherent excitation of circularly polarized phonons with THz laser light. Hence, the theoretical approach presented here is promising for shedding light into the microscopic mechanism of angular momentum transfer between ionic and electronic angular momentum, which is expected to play a central role in the phononic manipulation of magnetism.

A semiclassical non-adiabatic phase-space approach to molecular translations and rotations: Surface hopping with electronic inertial effects

We demonstrate that working with a correct phase-space electronic Hamiltonian captures electronic inertial effects. In particular, we show that phase space surface hopping dynamics do not suffer (at least to very high order) from non-physical non-adiabatic transitions between electronic eigenstates during the course of pure nuclear translational and rotational motion. This work opens up many new avenues for quantitatively investigating complex phenomena, including angular momentum transfer between chiral phonons and electrons as well as chiral-induced spin selectivity effects.

Chiral Phonon, Valley Polarization, and Inter/Intravalley Scattering in a van der Waals ReSe2 Semiconductor

Exploring valley manipulatable layered semiconductors is highly significant for valleytronic devices. Here, we report the phonon chirality and resulting inter/intravalley scattering in valley polarized van der Waals (vdW) layered ReSe2 by linearly/circularly polarized Raman (L/CPR), transmission (L/CPT), and photoluminescence (L/CPL) spectroscopic techniques. L/CPR combined with scanning transmission electron microscopy determines the Re chains' direction and displays the existence of chiral phonons. The LPT discloses the energetic valley polarization between the Re chains and its perpendicular crystal axis directions. Intriguingly, the valley polarization strength depends on the layer thickness even in a micrometer scale and abnormaly increases with temperature increase. Further, CPT manifests the optical rotation in ReSe2 due to strong chiral phonon-photon coupling. More essentially, L/CPL unveils a strong exciton-like effect (Stokes shift) that can be interpreted from the inter/intravalley scattering related to the (chiral) phonon-carrier coupling. This investigation suggests a promising platform based on a low-symmetry vdW ReSe2 semiconductor for exploring valley physics and fabricating valley(opto)tronic nanodevices.

Phonon-Induced Geometric Chirality

Chiral properties have seen increasing use in recent years, leading to the emerging fields of chiral quantum optics, plasmonics, and phononics. While these fields have achieved manipulation of the chirality of light and lattice vibrations, controlling the chirality of materials on demand has yet remained elusive. Here, we demonstrate that linearly polarized phonons can be used to induce geometric chirality in achiral crystals when excited with an ultrashort laser pulse. We show that nonlinear phonon coupling quasistatically displaces the crystal structure along phonon modes that reduce the symmetry of the lattice to that of a chiral point group corresponding to a chiral crystal. By reorienting the polarization of the laser pulse, the two enantiomers can be induced selectively. Therefore, geometric chiral phonons enable the light-induced creation of chiral crystal structures and therefore the engineering of chiral electronic states and optical properties.

Light-induced magnetization from magnonic rectification

Rectification describes the conversion of an oscillating field or current into a quasi-static one and the most basic example of a rectifier is an AC/DC converter in electronics. This principle can be translated to nonlinear light-matter interactions, where optical rectification converts the oscillating electric field component of light into a quasi-static polarization and phononic rectification converts a lattice vibration into a quasi-static structural distortion. Here, we present a rectification mechanism for magnetism that we call magnonic rectification, where a spin precession is converted into a quasi-static magnetization through the force exerted by a coupled chiral phonon mode. The transiently induced magnetic state resembles that of a canted antiferromagnet, opening an avenue toward creating dynamical spin configurations that are not accessible in equilibrium.

Real-time dynamics of angular momentum transfer from spin to acoustic chiral phonon in oxide heterostructures

Chiral phonons have recently been explored as a novel degree of freedom in quantum materials. The angular momentum carried by these quasiparticles is generated by the breaking of chiral degeneracy of phonons, owing to the chiral lattice structure or the rotational motion of ions of the material. In ferromagnets, a mechanism for generating non-equilibrium chiral phonons has been suggested, but their temporal evolution, which obeys Bose-Einstein statistics, remains unclear. Here we report the real-time dynamics of thermalized chiral phonons in an artificial superlattice composed of ferromagnetic metallic SrRuO3 and non-magnetic insulating SrTiO3. Following the photo-induced ultrafast demagnetization in the SrRuO3 layer, we observed the appearance of a magneto-optic signal in the superlattice, which is absent in the SrRuO3 single films. This magneto-optic signal exhibits thermally driven dynamic properties and a clear correlation with the thickness of the non-magnetic SrTiO3 layer, implying that it originates from thermalized chiral phonons. We use numerical calculations considering the magneto-elastic coupling in SrRuO3 to validate our experimental observations and the angular momentum transfer mechanism between the lattice and spin systems in ferromagnetic systems and also to the non-magnetic system.

Ultrafast Polarization-Resolved Phonon Dynamics in Monolayer Semiconductors

Monolayer transition metal dichalcogenide semiconductors exhibit unique valleytronic properties interacting strongly with chiral phonons that break time-reversal symmetry. Here, we observed the ultrafast dynamics of linearly and circularly polarized E'(Γ) phonons at the Brillouin zone center in single-crystalline monolayer WS2, excited by intense, resonant, and polarization-tunable terahertz pulses and probed by time-resolved anti-Stokes Raman spectroscopy. We separated the coherent phonons producing directional sum-frequency generation from the incoherent phonon population emitting scattered photons. The longer incoherent population lifetime than what was expected from coherence lifetime indicates that inhomogeneous broadening and momentum scattering play important roles in phonon decoherence at room temperature. Meanwhile, the faster depolarization rate in circular bases than in linear bases suggests that the eigenstates are linearly polarized due to lattice anisotropy. Our results provide crucial information for improving the lifetime of chiral phonons in two-dimensional materials and potentially facilitate dynamic control of spin-orbital polarizations in quantum materials.

Nonreciprocal magnetoacoustic waves with out-of-plane phononic angular momenta

Surface acoustic wave (SAW) can carry phononic angular momentum, showing great potential as an energy-efficient way to control magnetism. Still, out-of-plane phononic angular momentum in SAW and its interaction with magnetism remain elusive. Here, we studied the SAW-induced magnetoacoustic waves and spin pumping in Ni-based films on LiNbO3 with selected SAW propagation direction. The crystal inversion asymmetry induces circularly polarized phonons with large out-of-plane angular momenta so that up to 60% of the SAW power attenuates nonreciprocally controlled by the out-of-plane magnetization component. The SAW propagation direction dependence of the nonreciprocity verifies the crystal origin of the phononic angular momentum, and a chiral spin pumping demonstrates that the circular polarization can control the spin current generation efficiency. These results provide an additional degree of freedom for the acoustic control of magnetism and open an avenue for applying circularly polarized phonons.

Epsilon-near-zero regime for ultrafast opto-spintronics

Over the last two decades, breakthrough works in the field of non-linear phononics have revealed that high-frequency lattice vibrations, when driven to high amplitude by mid- to far-infrared optical pulses, can bolster the light-matter interaction and thereby lend control over a variety of spontaneous orderings. This approach fundamentally relies on the resonant excitation of infrared-active transverse optical phonon modes, which are characterized by a maximum in the imaginary part of the medium's permittivity. Here, in this Perspective article, we discuss an alternative strategy where the light pulses are instead tailored to match the frequency at which the real part of the medium's permittivity goes to zero. This so-called epsilon-near-zero regime, popularly studied in the context of metamaterials, naturally emerges to some extent in all dielectric crystals in the infrared spectral range. We find that the light-matter interaction in the phononic epsilon-near-zero regime becomes strongly enhanced, yielding even the possibility of permanently switching both spin and polarization order parameters. We provide our perspective on how this hitherto-neglected yet fertile research area can be explored in future, with the aim to outline and highlight the exciting challenges and opportunities ahead.

Theoretical study of the nonlinear magnon-phonon coupling in CoF2

The coupling and interplay between magnon and phonon are important topics for spintronics and magnonics. In this work we studied the nonlinear magnon-phonon coupling in CoF2. First-principles calculations demonstrate that the antiferromagnetic resonance magnon drives a phonon with B1gcharacter; the oscillating driving force has a frequency twice of that of the magnon. Comparing with similar materials indicates a strong correlation between the strength of nonlinear magnon-phonon coupling and the orbital magnetic moment of the magnetic ion. This work pave the way for theoretical study of nonlinear magnon-phonon coupling.

Gate-tunable circular phonon dichroism effect in bilayer graphene

Circular phonon dichroism effect has been proposed in two-dimensional materials; however, the lack of tunability hinders the exploration of the effect. Here, we investigate the role of dual-gating-induced inversion symmetry breaking in the circular phonon dichroism effect in bilayer graphene. We find that the introduction of inversion symmetry breaking modifies the response in the layer-symmetric and layer-antisymmetric channels, and results in the occurrence of phonon dichroism in the cross-channel. In the layer representation, the inversion symmetry breaking breaks the equality of intralayer circular phonon dichroism and enhances the interlayer response. Our results suggest that layer degree of freedom provides possibilities to tune phonon dynamics, which paves a way toward different physics and applications of two-dimensional acoustoelectronics and layertronics.

Chiral Phonons: Prediction, Verification, and Application

Chirality as an asymmetric property is prevalent in nature. In physics, the chirality of the elementary particles that make up matter has been widely studied and discussed, and nowadays, the concept has developed into the field of phonons. As an important fundamental excitation in condensed matter physics, phonons are traditionally considered to be linearly polarized and nonchiral. However, in recent years, the chirality of phonons has been revealed and further experimentally verified. The discovery has triggered a series of new explorations and developments in phonon-related physical processes. This Mini-Review provides an overview of the theoretical prediction of chiral phonons and multiple experimental detection methods and highlights the current key issues in the application of chiral phonons in different fields.

Terahertz metamaterials for light-driven magnetism

We describe the design of two types of metamaterials aimed at enhancing terahertz field pulses that can be used to control the magnetic state in condensed matter systems. The first structure is a so-called "dragonfly" antenna, able to realize a five-fold enhancement of the impinging terahertz magnetic field, while preserving its broadband features. For currently available state-of-the-art table top sources, this leads to peak magnetic fields exceeding 1 T. The second structure is an octopole antenna aimed at enhancing a circularly-polarized terahertz electric field, while preserving its polarization state. We obtain a five-fold enhancement of the electric field, hence expected to exceed the 1 MV/cm peak amplitude. Both our structures can be readily fabricated on top of virtually any material.

Light-Driven Spontaneous Phonon Chirality and Magnetization in Paramagnets

Spin-phonon coupling enables the mutual manipulation of phonon and spin degrees of freedom in solids. In this study, we reveal the inherent nonlinearity within this coupling. Using a paramagnet as an illustration, we demonstrate the nonlinearity by unveiling spontaneous symmetry breaking under a periodic drive. The drive originates from linearly polarized light, respecting a mirror reflection symmetry of the system. However, this symmetry is spontaneously broken in the steady state, manifested in the emergence of coherent chiral phonons accompanied by a nonzero magnetization. We establish an analytical self-consistency equation to find the parameter regime where spontaneous symmetry breaking occurs. Furthermore, we estimate realistic parameters and discuss potential materials that could exhibit this behavior. Our findings shed light on the exploration of nonlinear phenomena in magnetic materials and present possibilities for on-demand control of magnetization.

Creating chirality in the nearly two dimensions

Structural chirality, defined as the lack of mirror symmetry in materials' atomic structure, is only meaningful in three-dimensional space. Yet two-dimensional (2D) materials, despite their small thickness, can show chirality that enables prominent asymmetric optical, electrical and magnetic properties. In this Perspective, we first discuss the possible definition and mathematical description of '2D chiral materials', and the intriguing physics enabled by structural chirality in van der Waals 2D homobilayers and heterostructures, such as circular dichroism, chiral plasmons and the nonlinear Hall effect. We then summarize the recent experimental progress and approaches to induce and control structural chirality in 2D materials from monolayers to superlattices. Finally, we postulate a few unique opportunities offered by 2D chiral materials, the synthesis and new properties of which can potentially lead to chiral optoelectronic devices and possibly materials for enantioselective photochemistry.

Observation of interplay between phonon chirality and electronic band topology

The recently demonstrated chiral modes of lattice motion carry angular momentum and therefore directly couple to magnetic fields. Notably, their magnetic moments are predicted to be strongly influenced by electronic contributions. Here, we have studied the magnetic response of transverse optical phonons in a set of Pb1-xSnxTe films, which is a topological crystalline insulator for x > 0.32 and has a ferroelectric transition at an x-dependent critical temperature. Polarization-dependent terahertz magnetospectroscopy measurements revealed Zeeman splittings and diamagnetic shifts, demonstrating a large phonon magnetic moment. Films in the topological phase exhibited phonon magnetic moment values that were larger than those in the topologically trivial samples by two orders of magnitude. Furthermore, the sign of the effective phonon g-factor was opposite in the two phases, a signature of the topological transition according to our model. These results strongly indicate the existence of interplay between the magnetic properties of chiral phonons and the topology of the electronic band structure.

Circling back to magnetism

Ultrafast experiments unveil control of magnetization with atomic rotations.