Fast thermal dephasing limits macroscopic quantum phenomena to cryogenic conditions1-4 and hinders their use at ambient temperatures5,6. For electronic excitations in condensed media, dephasing is mediated by thermal lattice motion1,7,8. Therefore, taming the lattice influence is essential for creating collective electronic quantum states at high temperatures. Although there are occasional reports of high-Tc quantum effects across different platforms, it is unclear which lattice characteristics and electron-lattice interactions lead to macroscopically coherent electronic states in solids9. Here we studied intensity fluctuations in the macroscopic polarization during the emergence of superfluorescence in a lead halide perovskite10 and showed that spontaneously synchronized polaronic lattice oscillations accompany collective electronic dipole emission. We further developed an effective field model and theoretically confirmed that exciton-lattice interactions lead to a new electronically and structurally entangled coherent extended solitonic state beyond a critical polaron density. The analysis shows a phase transition with two processes happening in tandem: incoherent disordered polaronic lattice deformations establish an order, while macroscopic quantum coherence among excitons simultaneously emerges. Recombination of excitons in this state culminates in superfluorescence at high temperatures. Our study establishes fundamental connections between the transient superfluorescence process observed after the impulsive excitation of perovskites and general equilibrium phase transitions achieved by thermal cooling. By identifying various electron-lattice interactions in the perovskite structure and their respective role in creating collectively coherent electronic effects in solids, our work provides unprecedented insight into the design and development of new materials that exhibit high-temperature macroscopic quantum phenomena.

Single-shot picosecond (ps) laser induced delamination allows for the direct generation of suspended membranes from a continuous metallic film, offering a promising platform for control of ultrafast magnetization dynamics driven by acoustic waves. Using the picosecond-ultrasonics method, we demonstrate that long-lived low-frequency acoustic waves can be optically-excited in the delaminated cavities. At the same time, higher-frequency modes >60GHz exhibit a surprisingly fast damping, following a scaling law incompatible with the expected attenuation mediated by phonon-phonon scattering. Comparing measurements between delaminated cavities and a benchmark nickel film in contact with the substrate, we link our findings with structural modifications of the nickel crystal induced by the delamination process.

In this Letter, we calculate the optical and magneto- optical reflectivity in a dielectric/gap/ferromagnet excited by ap-polarized monochromatic optical beam through the prism (Otto configuration) as a function of the angle of incidence 6 and the gap thickness d . Besides the well-known surface plasmon polariton (SPP resonance at d lambda ), we find a new, to the best of our knowledge, resonance with a nanometric gap d 10 nm at a large 6 80 degrees. Both resonances display pronounced resonant behavior in the transverse magneto- optical Kerr effect (T-MOKE). (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

Femtosecond lasers are routinely used for inducing local modification, including nanostructuring, and ultrafast laser spectroscopy in solids. However, these studies are often being performed separately making the unveiling of exciting physical properties of laser-fabricated materials out of reach. Here, we present an all-optical platform combining the fabrication of nano to micrometer size single-shot "femtosecond-laser-delamination" membranes or cavities of ferromagnetic thin films and multilayers together with their quasi in situ characterization using the Abbe-limited interferometric, ultrafast scanning photo-acoustic and magneto-plasmonic microscopies. Ferromagnetic nickel and iron cavities display high-Q acoustic resonances providing access to long-lived ultrahigh frequency coherent phonon modes in the above 100 GHz frequency range. Cavities in cobalt-gold bilayers allow for magnetically controlled surface plasmon resonance experiments in the Otto configuration, which is otherwise very difficult to implement experimentally. Quantitative experimental characterization of functional magnetic cavities, supported by the numerical modeling of all experimental data, opens an avenue to design and fabricate tunable nanoscaled femtosecond-laser-delamination architectures in thin films and multilayers.

The quest for manipulation of magnetization on ultrafast timescales faces many technological challenges. Successful achievement thereof could shed light on novel fundamental phenomena, such as inertial magnetization dynamics, as well as accelerate technological advancements towards higher information processing rates. One of the recent approaches towards this end concerns excitation of magnetization dynamics via laser-induced picosecond acoustic pulses, which has given birth to the field of ultrafast magneto-acoustics. Considerable progress has been made in the field from an experimental point of view, as well as from the perspective of theoretical modelling. In this talk, we aim to review some of the aforementioned progress and propose the frequency dependent cooperativity parameter (strong coupling regime) to measure the efficiency of resonantly enhanced phonon-magnon interactions in the GHz-to-THz frequency range.

Laser nanostructuring of thin films with ultrashort laser pulses is widely used for nanofabrication across various fields. A crucial parameter for optimizing and understanding the processes underlying laser processing is the absorbed laser fluence, which is essential for all damage phenomena such as melting, ablation, spallation, and delamination. While threshold fluences have been extensively studied for single compound thin films, advancements in ultrafast acoustics, magneto-acoustics, and acousto-magneto-plasmonics necessitate understanding the laser nanofabrication processes for functional multilayer films. In this work, we investigated the thickness dependence of ablation and delamination thresholds in Ni/Au bilayers by varying the thickness of the Ni layer. The results were compared with experimental data on Ni thin films. Additionally, we performed femtosecond time-resolved pump-probe measurements of transient reflectivity in Ni to determine the heat penetration depth and evaluate the melting threshold. Delamination thresholds for Ni films were found to exceed the surface melting threshold suggesting the thermal mechanism in a liquid phase. Damage thresholds for Ni/Au bilayers were found to be significantly lower than those for Ni and fingerprint the non-thermal mechanism without Ni melting, which we attribute to the much weaker mechanical adhesion at the Au/glass interface. This finding suggests the potential of femtosecond laser delamination for nondestructive, energy-efficient nanostructuring, enabling the creation of high-quality acoustic resonators and other functional nanostructures for applications in nanosciences.

We report on measurements of the anisotropy of velocities and attenuation of GHz-frequency acoustic phonons in a cubic MgO crystal at room temperature. They are used to quantify the strong anisotropy of the Grüneisen parameter and calculate the attenuation anisotropy for Rayleigh surface acoustic waves. These observations constitute important building blocks for better understanding of ultrafast laser-based magneto-acoustic and nonlinear acoustic experiments at ultrahigh frequencies.

Ultrafast optical experiments using narrow-bandwidth tunable IR laser pulses enable nonvolatile all-optical switching of ferroelectric polarization in BaTiO3 in the epsilon-near-zero regime.

We revisit the quantitative analysis of the ultrafast magnetoacoustic experiment in a freestanding nickel thin film by Kim and Bigot [J.-W. Kim and J.-Y. Bigot, Phys. Rev. B 95, 144422 (2017)] by applying our recently proposed approach of magnetic and acoustic eigenmode decomposition. We show that the application of our modeling to the analysis of time-resolved reflectivity measurements allows for the determination of amplitudes and lifetimes of standing perpendicular acoustic phonon resonances with unprecedented accuracy. The acoustic damping is found to scale as proportional to omega 2 for frequencies up to 80 GHz, and the peak amplitudes reach 10-3. The experimentally measured magnetization dynamics for different orientations of an external magnetic field agrees well with numerical solutions of magnetoelastically driven magnon harmonic oscillators. Symmetry-based selection rules for magnon-phonon interactions predicted by our modeling approach allow for the unambiguous discrimination between spatially uniform and nonuniform modes, as confirmed by comparing the resonantly enhanced magnetoelastic dynamics simultaneously measured on opposite sides of the film. Moreover, the separation of timescales for (early) rising and (late) decreasing precession amplitudes provide access to magnetic (Gilbert) and acoustic damping parameters in a single measurement.

Femtosecond (fs) time-resolved magneto-optics is applied to investigate laser-excited ultrafast dynamics of one-dimensional nickel gratings on fused silica and silicon substrates for a wide range of periodicities Λ = 400-1500 nm. Multiple surface acoustic modes with frequencies up to a few tens of GHz are generated. Nanoscale acoustic wavelengths Λ/n have been identified as nth-spatial harmonics of Rayleigh surface acoustic wave (SAW) and surface skimming longitudinal wave (SSLW), with acoustic frequencies and lifetimes being in agreement with theoretical calculations. Resonant magnetoelastic excitation of the ferromagnetic resonance (FMR) by SAW's third spatial harmonic, and, most interestingly fingerprints of the parametric resonance at 1/2 SAW frequency have been observed. Numerical solutions of Landau-Lifshitz-Gilbert (LLG) equation magnetoelastically driven by complex polychromatic acoustic fields quantitatively reproduce all resonances at once. Thus, our results provide a solid experimental and theoretical base for a quantitative understanding of ultrafast fs-laser-driven magnetoacoustics and tailoring the magnetic-grating-based metasurfaces at the nanoscale.

In this study, we present a novel investigation into the magnetic and morphological properties of equiatomic B2-ordered FeRh thin films irradiated with single high-intensity ultrashort laser pulses. The goal is to elucidate the effect of femtosecond laser ablation on the magnetic properties of FeRh. We employed Scanning Magneto-Optical Kerr Effect (S-MOKE) microscopy to examine the magnetic phase after laser processing, providing high spatial resolution and sensitivity. Our results for the first time demonstrated the appearance of a magneto-optical signal from the bottom of ablation craters, suggesting a transition from antiferromagnetic to ferromagnetic behavior. Fluence-resolved measurements clearly demonstrate that the ablation threshold coincides with the threshold of the antiferromagnet-to-ferromagnet phase transition. The existence of such a magnetic phase transition was independently confirmed by temperature-dependent S-MOKE measurements using a CW laser as a localized heat source. Whereas the initial FeRh film displayed a reversible antiferromagnet-ferromagnet phase transition, the laser-ablated structures exhibited irreversible changes in their magnetic properties. This comprehensive analysis revealed the strong correlation between the femtosecond laser ablation process and the magnetic phase transformation in FeRh thin films.

In this Letter we demonstrate a fundamentally new, to the best of our knowledge, concept to enhance the magnetic modulation of the surface plasmon polaritons (SPPs) by using hybrid magneto-plasmonic structures consisting of hyperbolic plasmonic metasurfaces and magnetic dielectric substrates. Our results show that the magnetic modulation of SPPs in the proposed structures can be an order of magnitude stronger than in the hybrid metal-ferromagnet multilayer structures conventionally used in active magneto-plasmonics. We believe that this effect will allow for the further miniaturization of magneto-plasmonic devices.

While most ultrafast time-resolved optical pump-probe experiments in magnetic materials reveal the spatially homogeneous magnetization dynamics of ferromagnetic resonance (FMR), here we explore the magneto-elastic generation of GHz-to-THz frequency spin waves (exchange magnons). Using analytical magnon oscillator equations, we apply time-domain and frequency-domain approaches to quantify the results of ultrafast time-resolved optical pump-probe experiments in free-standing ferromagnetic thin films. Simulations show excellent agreement with the experiment, provide acoustic and magnetic (Gilbert) damping constants and highlight the role of symmetry-based selection rules in phonon-magnon interactions. The analysis is extended to hybrid multilayer structures to explore the limits of resonant phonon-magnon interactions up to THz frequencies.

Using valid experimental parameters, we quantify the magnitude of resonantly phonon-driven precession of exchange magnons in freestanding ferromagnetic nickel thin films on their thickness L. Analytical solutions of acoustically driven equations for magnon oscillators display a nonmonotonous dependence of the peak magnetization precession on the film thickness. It is explained by different L-dependence of multiple prefactors entering in the expression for the total magnetization dynamics. Depending on the ratio of acoustic and magnetic (Gilbert) damping constants, the magnetization precession is shown to be amplified by a Q-factor of either the phonon or the magnon resonance. The increase in the phonon mode amplitude for thinner membranes is also found to be significant. Focusing on the magnetization dynamics excited by the two first acoustic eigenmodes with p=1 and p=2, we predict the optimum thicknesses of nickel membranes to achieve large amplitude magnetization precession at multi 100 GHz frequencies at reasonably low values of an external magnetic field. By extending the study to the case of Ni-Si bilayers, we show that these resonances are achievable at even higher frequencies, approaching the THz range.

We analyze resonant magnetoelastic interactions between standing perpendicular spin wave modes (exchange magnons) and longitudinal acoustic phonon modes in free-standing hybrid metal-ferromagnet bilayer and trilayer structures. Whereas the ferromagnetic layer acts as a magnetic cavity, all metal layers control the frequencies and eigenmodes of acoustic vibrations. The design proposed here allows for achieving and tuning the spectral and spatial mode overlap between phonons and magnons that results in their strong resonant interaction. Realistic simulations for gold-nickel multilayers show that sweeping the external magnetic field should allow for observing resonantly enhanced interactions between individual magnon and phonon modes in a broad range of frequencies spanning from tens of gigahertz up to several hundreds of gigahertz, which can be finely tuned through the multilayer design. Our results would enable the systematic study and the deep understanding of resonantly enhanced magnetoelastic coupling between individual phonon and magnon modes up to frequencies of great contemporary fundamental and applied interest.

Magneto-optical (MO) effects, viz. magnetically induced changes in light intensity or polarization upon reflection from or transmission through a magnetic sample, were discovered over a century and a half ago. Initially they played a crucially relevant role in unveiling the fundamentals of electromagnetism and quantum mechanics. A more broad-based relevance and wide-spread use of MO methods, however, remained quite limited until the 1960s due to a lack of suitable, reliable and easy-to-operate light sources. The advent of Laser technology and the availability of other novel light sources led to an enormous expansion of MO measurement techniques and applications that continues to this day (see section 1). The here-assembled roadmap article is intended to provide a meaningful survey over many of the most relevant recent developments, advances, and emerging research directions in a rather condensed form, so that readers can easily access a significant overview about this very dynamic research field. While light source technology and other experimental developments were crucial in the establishment of today’s magneto-optics, progress also relies on an ever-increasing theoretical understanding of MO effects from a quantum mechanical perspective (see section 2), as well as using electromagnetic theory and modelling approaches (see section 3) to enable quantitatively reliable predictions for ever more complex materials, metamaterials, and device geometries. The latest advances in established MO methodologies and especially the utilization of the MO Kerr effect (MOKE) are presented in sections 4 (MOKE spectroscopy), 5 (higher order MOKE effects), 6 (MOKE microscopy), 8 (high sensitivity MOKE), 9 (generalized MO ellipsometry), and 20 (Cotton–Mouton effect in two-dimensional materials). In addition, MO effects are now being investigated and utilized in spectral ranges, to which they originally seemed completely foreign, as those of synchrotron radiation x-rays (see section 14 on three-dimensional magnetic characterization and section 16 on light beams carrying orbital angular momentum) and, very recently, the terahertz (THz) regime (see section 18 on THz MOKE and section 19 on THz ellipsometry for electron paramagnetic resonance detection). Magneto-optics also demonstrates its strength in a unique way when combined with femtosecond laser pulses (see section 10 on ultrafast MOKE and section 15 on magneto-optics using x-ray free electron lasers), facilitating the very active field of time-resolved MO spectroscopy that enables investigations of phenomena like spin relaxation of non-equilibrium photoexcited carriers, transient modifications of ferromagnetic order, and photo-induced dynamic phase transitions, to name a few. Recent progress in nanoscience and nanotechnology, which is intimately linked to the achieved impressive ability to reliably fabricate materials and functional structures at the nanoscale, now enables the exploitation of strongly enhanced MO effects induced by light–matter interaction at the nanoscale (see section 12 on magnetoplasmonics and section 13 on MO metasurfaces). MO effects are also at the very heart of powerful magnetic characterization techniques like Brillouin light scattering and time-resolved pump-probe measurements for the study of spin waves (see section 7), their interactions with acoustic waves (see section 11), and ultra-sensitive magnetic field sensing applications based on nitrogen-vacancy centres in diamond (see section 17). Despite our best attempt to represent the field of magneto-optics accurately and do justice to all its novel developments and its diversity, the research area is so extensive and active that there remains great latitude in deciding what to include in an article of this sort, which in turn means that some areas might not be adequately represented here. However, we feel that the 20 sections that form this 2022 magneto-optics roadmap article, each written by experts in the field and addressing a specific subject on only two pages, provide an accurate snapshot of where this research field stands today. Correspondingly, it should act as a valuable reference point and guideline for emerging research directions in modern magneto-optics, as well as illustrate the directions this research field might take in the foreseeable future.

The formation of coherent macroscopic states and the manipulation of their entanglement using external stimuli are essential for emerging quantum applications. However, the observation of collective quantum phenomena such as Bose–Einstein condensation, superconductivity, superfluidity and superradiance has been limited to extremely low temperatures to suppress dephasing due to random thermal agitations. Here we report room-temperature superfluorescence in hybrid perovskite thin films. This surprising discovery shows that in this material platform, there exists an extremely strong immunity to electronic dephasing due to thermal processes. To explain this observation, we propose that the formation of large polarons in hybrid perovskites provides a quantum analogue of vibration isolation to electronic excitation and protects it against dephasing even at room temperature. Understanding the origins of sustained quantum coherence and the superfluorescence phase transition at high temperatures can provide guidance to design systems for emerging quantum information technologies and to realize similar high-temperature macroscopic quantum phenomena in tailored materials.

In this Letter, a fundamentally new concept of realization of hyperbolic plasmonic metasurfaces by anisotropic gain-loss competition is proposed, and the possibility of highly directional propagation and amplification of surface plasmon polaritons is predicted. A simple realistic configuration of such a metasurface represents the periodic array of lossy metallic slabs embedded in the gain matrix. Our results may pave the way for numerous applications ranging from integrated and highly directional quantum light emitters to nonlinear-optical frequency converters.

We analyze dispersion relations of magnons in ferromagnetic nanostructures with uniaxial anisotropy taking into account inertial terms, i.e. magnetic nutation. Inertial effects are parametrized by damping-independent parameter β, which allows for an unambiguous discrimination of inertial effects from Gilbert damping parameter α. The analysis of magnon dispersion relation shows its two branches are modified by the inertial effect, albeit in different ways. The upper nutation branch starts at ω = 1/β, the lower branch coincides with FMR in the long-wavelength limit and deviates from the zero-inertia parabolic dependence ≃ ωFMR + Dk 2 of the exchange magnon. Taking a realistic experimental geometry of magnetic thin films, nanowires and nanodiscs, magnon eigenfrequencies, eigenvectors and Q-factors are found to depend on the shape anisotropy. The possibility of phase-matched magneto-elastic excitation of nutation magnons is discussed and the condition was found to depend on β, exchange stiffness D and the acoustic velocity.