Driven-dissipative dynamics underlie a wide range of nonequilibrium phenomena in quantum materials, yet reduced descriptions beyond the quasi-equilibrium picture remain difficult to establish. Here, we experimentally demonstrate that a resonantly driven phonon mode admits a generalized thermodynamic description in which coherence and energy jointly organize the nonequilibrium evolution. Beyond a threshold driving field strength, we observe a delayed ultrafast response of a coherently driven phonon mode. Combined with experimentally constrained Lindblad dynamics, we show that this delay reflects the finite-time spreading of excitations across many phonon levels. At the same time, the full density-matrix trajectories for three driving conditions collapse onto a common surface defined by energy and coherence. Our results establish a coherence-extended thermodynamic regime for driven phonons and provide a framework for broader state engineering in driven-dissipative bosonic excitations.

Spatial photonic crystals (SPCs) are unique structures for light-matter interactions because they achieve a large and spatially periodic dielectric contrast on wavelength scales1-4. Their temporal analogues, photonic time crystals (PTCs), promise similar advances by periodically modulating optical properties in time5-11, but require strong, ultrafast modulation, which is challenging to obtain experimentally5,12-15. Driven metamaterials have been considered as a route to realize PTCs, yet all-optical implementations have remained unknown because of the challenge of achieving modulation on such short timescales. Here we demonstrate the all-optical realization of a photonic time crystal, achieved with a surface plasmon cavity metamaterial operating at terahertz frequencies. We demonstrate strong (near-unity) and coherent (sub-optical cycle) periodic driving of the plasmonic metamaterial enabled by field-induced dynamical modulation of the kinetic energy of the carriers and effective mass reaching up to 80% of their rest mass. Our spectroscopic measurements show a transition into the PTC regime mediated by an exceptional point, at which two Floquet-driven optical eigenmodes coalesce. In the PTC regime, emergent gain is shown to reduce plasmonic losses by more than 50% (refs. 16,17), and we predict plasmonic lasing to be within experimental reach. These results establish a robust platform for time-domain photonics in plasmonic systems.

Ultrafast magnetization switching is essential for next-generation memory and logic devices operating at terahertz (THz) frequencies. Presently, efficient picosecond spin-orbit torque (SOT) switching relies on heavy metals (HMs), which are costly and environmentally burdensome. Self-generated SOT in nanostructures comprising a single conducting ferromagnetic layer offers a promising, cost-effective alternative, but ultrafast SOT has not been demonstrated in such systems and the underlying torque mechanisms remain unclear. Here we demonstrate SOT-driven picosecond magnetization dynamics in oxide-capped Ni 81 Fe 19 with efficiencies comparable to benchmark Ni 81 Fe 19 /HM heterostructures. By probing spin-charge interconversion on the picosecond timescale, we show that SOT is active only at off-stoichiometric oxide interfaces and negligible at stoichiometric ones, identifying interfacial atomic-scale oxidation of Ni 81 Fe 19 as the key ingredient for SOT efficiency. Moreover, the magnitude and sign of the torques are tunable via the oxidation level and oxide type. These results establish atomic-layer oxidation engineering as a viable, heavy-metal-free route to ultrafast SOT for THz spintronic technologies.

High harmonic generation (HHG) is a sensitive probe for investigating electronic structures and dynamics of materials and a source for attosecond pulses. In particular, HHG with terahertz (THz) light can enable probing of nonlinear responses in correlated materials arising from low-energy many-body interactions. However, THz HHG studies have so far largely focused on topological materials and superconductors, leaving out other potential material systems which could also become efficient THz HHG sources. Here, we report THz third harmonic generation (THG) in rare-earth nickelates – a prototype material for exploring the Mott insulator-metal transition and related technological applications. We find that the THG amplitude is highly sensitive to the strengths of electronic and magnetic phases of nickelates. In films with sharp phase-transitions, the local maximum and minimum in the temperature-dependent THG amplitude coincide with insulator-metal and magnetic transition temperatures, respectively. While in films with weaker transitions, these features shift toward lower temperatures or even monotonous THG enhancement is observed down to low temperatures. We developed a generalized theory for THz harmonic generation in negative charge-transfer insulators and outlined strategies to enhance the THz nonlinearities further. Our study broadens the scope of THz HHG studies and related applications to strongly correlated materials.

High harmonic generation (HHG) in the terahertz (THz) regime is an emerging field that has already provided new insights into fundamental low-energy processes in Dirac materials and high-TC superconductors. Here, we demonstrate THz harmonic generation across the Mott insulator-metal transition in rare-earth nickelates (RNiO3, R = rare-earth atom). The THz harmonic signal shows distinct characteristics in the three different accessible phases: the intensity of harmonics increases upon cooling in both the low-temperature antiferromagnetic (AFM) insulating and high-temperature paramagnetic (PM) metallic phases, while this trend is reversed in the intermediate PM insulating phase. Using single-and two-band Hubbard models, we find different dominant origins of the THz harmonics in different phases: strong spin-charge and orbital-charge coupling in the AFM insulating phase, intraband currents from renormalized quasiparticles with temperature-dependent scattering rate in the PM metallic phase, and the reduction of the charge carrier density due to the opening of the Mott gap in the PM insulating phase. These results and mechanisms significantly differ from those observed upon excitation in the optical regime. Our study lays the foundation for THz HHG physics in Mott and other strongly correlated systems and offers design principles for efficient THz HHG from these systems.

We report on direct measurement of rise (τr) and fall times (τf) of 22.5 ps and 45.04 ps, respectively, with a inhouse developed ZBSD detector whose intermediate frequency (IF) video bandwidth is up to ~ 50 GHz. The maximum extracted peak voltage responsivity is ℜV = 13.73 µV/W for a broadband pulse with a width of 1.2 THz. Primarily, these results are benchmarks for future detector development and their commissioning at accelerator facilities. The detector was evaluated with Keysights’ UXR1104A Infiniium UXR-Series Oscilloscope with 110 GHz bandwidth.

The interplay between superconductivity and charge density wave has often been studied from an equilibrium point of view. For example, using static tuning knobs such as doping, magnetic field and pressure, superconductivity can be enhanced or suppressed. The resulting effect on the co-existing charge density wave order, if any, is judged by variations in its ground state properties such as the ordering temperature or the spatial correlation. Such an approach can be understood as coordinated static displacements of two coupled order parameters within a Ginzburg-Landau description, evincing their interplay as either co-operative or competing but does not provide further microscopic information about the interaction. In order to assess such information, we dynamically perturb both orders from equilibrium and observe their coupling directly in the time-domain. We show that high-field multicycle terahertz pulses drive both the Higgs amplitude fluctuations of the superconducting order as well as collective fluctuations of the charge order in an electron-doped cuprate, resulting in characteristic third harmonic generation. A notable time delay is manifested between their respective driven dynamics. We propose that this may signify the important energy scale describing their coupling or imply a terahertz field-depinned charge density wave that destroys macroscopic superconductivity. Our work demonstrates a holistic approach for investigating coupled superconducting and charge density wave orders, which may shed novel light on their intertwined presence and widespread fluctuations in many classes of unconventional superconductors.

The ultrafast control of magnetisation states in magnetically ordered systems poses significant technological challenges yet is vital for the development of memory devices that operate at picosecond timescales or terahertz (THz) frequencies. Despite considerable efforts achieving convenient ultrafast readout of magnetic states remains an area of active investigation. For practical applications, energy-efficient and cost-effective electrical detection is highly desirable. In this context, unidirectional spin-Hall magnetoresistance (USMR) has been proposed as a straightforward two-terminal geometry for the electrical detection of magnetisation states in magnetic heterostructures. In this work, we demonstrate that USMR is effective at THz frequencies, enabling picosecond time readouts initiated by light fields. We observe ultrafast USMR in various ferromagnet/heavy metal thin film heterostructures via THz second-harmonic generation. Our findings, along with temperature-dependent measurements of USMR, reveal a substantial contribution from electron-magnon spin-flip scattering, highlighting the potential for all-electrical detection of THz magnon modes.

We demonstrate a highly efficient method for upconverting broadband sub-terahertz (sub-THz) signals to multiple terahertz (THz) bands using a photoconductive antenna coupled with the TELBE radiation source, leveraging the unique properties of HgTe-based Dirac materials. HgTe heterostructures known for their robust third-order nonlinearities, enhance frequency mixing and signal amplification across THz bands. We achieved a field conversion efficiency over 2% for room temperature.

We demonstrate THz second harmonic generation (SHG) in magnetic heterostructures arising from ultrafast spin Hall magnetoresistance (USMR). The interference between USMR and thermally driven contributions leads to an anisotropic dependence of the THz SHG. The USMR contribution can be separated from the others by analysing the angular dependence between the THz excitation pulse polarisation and the sample magnetisation direction on the THz SHG.

The use of the THz frequency domain in future network generations offers an unparalleled level of capacity, which can enhance innovative applications in wireless communication, analytics, and imaging. Communication technologies rely on frequency mixing, enabling signals to be converted from one frequency to another and transmitted from a sender to a receiver. Technically, this process is implemented using nonlinear components such as diodes or transistors. However, the highest operating frequency of this approach is limited to sub-THz bands. Here, we demonstrate the upconversion of a weak sub-THz signal from a photoconductive antenna to multiple THz bands. The key element is a high-mobility HgTe-based heterostructure with electronic band inversion, leading to one of the strongest third-order nonlinearities among all materials in the THz range. Due to the Dirac character of electron dispersion, the highly intense sub-THz radiation is efficiently mixed with the antenna signal, resulting in a THz response at linear combinations of their frequencies. The field conversion efficiency above 2% is provided by a bare tensile-strained HgTe layer with a thickness below 100 nm at room temperature under ambient conditions. Devices based on Dirac materials allow for high degree of integration, with field-enhancing metamaterial structures, making them very promising for THz communication with unprecedented data transfer rate.

Hydration water is vital for the stabilization of protein structure and function. The strong interaction of hydration water with the protein surface brings into question how dynamics and asymmetry of hydrogen bonds are perturbed for hydration water compared to bulk water. Here, z-scan transmission measurements at 0.5 Terahertz (THz) were performed for dilute and concentrated lysozyme solutions. A giant nonlinear absorption coefficient was found for dilute lysozyme solutions that is ten times greater than previous studies. This giant nonlinear response stems from the high average THz power generated by the TELBE free electron laser source, which drives the formation of a persistent thermal lens. In contrast, concentrated lysozyme solutions did not demonstrate a nonlinear response, revealing that crowding annihilates the thermal lensing effect. These results indicates that the THz nonlinear transmission of aqueous proteins solutions depends on the amount of hydration water present, and opens to the door to understanding the nonlinear optical properties of biologically relevant systems.

Spintronic terahertz (THz) frequency conversion in ferromagnet/heavy metal (FM/HM) heterostructures has the potential to enhance high-speed data communication and advance ultrafast magnetic memory applications. By leveraging ultrafast spin currents and spin-orbit interactions in FM/HM systems, broadband THz generation can be achieved, with recent studies demonstrating spintronic THz second harmonic generation (TSHG) and optical rectification. We introduce concepts for controlling the frequency conversion and temporal characteristics of TSHG through active manipulation of FM magnetization, providing flexibility in second harmonic emission and waveform shaping. The TSHG valve is realized by employing THz metamaterials, consisting of hybrid FM/HM structures combined with subwavelength gold periodic arrays. Additionally, using microstructured gold periodic arrays, we investigate the TSHG field enhancement capability as a function of grating filling factor and explore the potential for TSHG cavity enhancement. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

The interplay of electronic charge, spin, and orbital currents, coherently driven by picosecond long oscillations of light fields in spin-orbit coupled systems, is the foundation of emerging terahertz lightwave spintronics and orbitronics. The essential rules for how terahertz fields interact with these systems in a nonlinear way are still not understood. In this work, we demonstrate a universally applicable electronic nonlinearity originating from spin-orbit interactions in conducting materials, wherein the interplay of light-induced spin and orbital textures manifests. We utilized terahertz harmonic generation spectroscopy to investigate the nonlinear dynamics over picosecond timescales in various transition metal films. We found that the terahertz harmonic generation efficiency scales with the spin Hall conductivity in the studied films, while the phase takes two possible values (shifted by π), depending on the d-shell filling. These findings elucidate the fundamental mechanisms governing non-equilibrium spin and orbital polarization dynamics at terahertz frequencies, which is relevant for potential applications of terahertz spin- and orbital-based devices.

An important strategy for understanding the microscopic physics of strongly correlated systems and enhancing their technological potential is to selectively drive the fundamental degrees of freedom out of equilibrium. Intense terahertz (THz) pulses with photon energies of a few meV, can not only serve this purpose, but also unravel their electronic and quantum nature. Here, we demonstrate THz‐driven ultrafast dynamics of rare‐earth nickelates (RNiO 3 , R = rare‐earth atom) – a prototype system to study the Mott insulator‐metal transition (IMT). The THz drive of the Mott insulating state induces instantaneous IMT via quantum tunneling of valence electrons across the bandgap while the THz drive of the correlated metallic state leads to overall heating of the conduction electrons. The subsequent relaxation of excited electrons in these two states occurs via a two‐step process (electron‐phonon thermalization and recovery of the charge‐ordered insulating state) and a one‐step process (electron‐phonon scattering), respectively. The relaxation dynamics of the electrons and the absence of acoustic phonon modes, in particular, suggest a purely electronic and highly non‐thermal nature of the THz‐induced dynamics that is predominantly controlled by the charge degree of freedom. Such THz‐induced IMT may find its applications in opto‐electronics with enhanced performance and minimal device size and heat dissipation.

The nonlinear Hall effect with time-reversal symmetry is a second-order electronic transport phenomenon-seen as a quadratic voltage transverse to an applied electric field-that induces frequency doubling and occurs in non-centrosymmetric crystals with large Berry curvature. Optoelectronic devices based on this effect are limited because it typically appears at low temperatures and in complex compounds characterized by Dirac or Weyl electrons. Here we report a room-temperature nonlinear Hall effect in polycrystalline thin films of the centrosymmetric elemental material bismuth. The electrons at the (111) surface possess a Berry curvature triple that activates side jumps and skew scatterings, which generate nonlinear transverse currents. We show that the zero-field nonlinear transverse voltage can be boosted in arc-shaped bismuth stripes due to an extrinsic geometric classical counterpart of the nonlinear Hall effect. The electrical frequency doubling in curved geometries can be extended to optical second-harmonic generation in the terahertz spectral range. We also demonstrate efficient third-harmonic generation in polycrystalline bismuth films and bismuth-based heterostructures across a broad range of terahertz frequencies. Polycrystalline thin films of elemental bismuth exhibit a room-temperature nonlinear transverse voltage due to geometric effects of surface electrons that is tunable and can be extended to efficient high-harmonic generation at terahertz frequencies.

Abstract Understanding spin-lattice interactions in antiferromagnets is a critical element of the fields of antiferromagnetic spintronics and magnonics. Recently, coherent nonlinear phonon dynamics mediated by a magnon state were discovered in an antiferromagnet. Here, we suggest that a strongly coupled two-magnon-one phonon state in this prototypical system opens a novel pathway to coherently control magnon-phonon dynamics. Utilizing intense narrow-band terahertz (THz) pulses and tunable magnetic fields up to μ 0 H ext = 7 T, we experimentally realize the conditions of magnon-phonon Fermi resonance in antiferromagnetic CoF2. These conditions imply that both the spin and the lattice anharmonicities harvest energy from the transfer between the subsystems if the magnon eigenfrequency f m is half the frequency of the phonon 2f m = f ph. Performing THz pump-infrared probe spectroscopy in conjunction with simulations, we explore the coupled magnon-phonon dynamics in the vicinity of the Fermi-resonance and reveal the corresponding fingerprints of nonlinear interaction facilitating energy exchange between these subsystems.

In this paper, we demonstrate the successful implementation of reconfigurable field-programmable gate array technology into a pulse-resolved data acquisition system to achieve a femtosecond temporal resolution in ultrafast pump-probe experiments in real-time at large scale facilities. As proof of concept, electro-optic sampling of terahertz waveforms radiated by a superradiant emitter of a quasi-cw accelerator operating at a 50 kHz repetition rate and probed by an external laser system is performed. Options for up-scaling the developed technique to a MHz range of repetition rates are discussed.

In this Letter, we demonstrate terahertz (THz) magnetic field detection in fused silica with sensitivity that can be easily controlled by sample tilting (for both amplitude and polarization). The proposed technique remains in the linear regime at magnetic fields exceeding 0.3 T (0.9 MV/cm of equivalent electric field) and allows the use of low-cost amorphous materials. Furthermore, the demonstrated effects should be present in a wide variety of materials used as substrates in different THz-pump laser-probe experiments and need to be considered in order to disentangle different contributions to the measured signals.

Finding efficient ways to generate and manipulate spin currents on ultrafast time scales is one of the hot topics in spintronics. Near-infrared (NIR) femtosecond pulses have demonstrated their perfect performance in this regard when interacting with ferromagnet/heavy metal heterostructures. The generated spin-transfer current can be converted into a charge current in a non-magnetic heavy metal layer emitting terahertz (THz) waves. Here we present our results showing that similar processes can also be observed using electromagnetic pulses of spectral ranges far beyond previous reports. The results show that THz pulses irradiating the heterostructure can not only be efficiently down-converted by an optical rectification mechanism (OR), as in case of NIR, but also up-converted, resulting in THz second harmonic generation (SHG) [1]. In the case of extreme-ultraviolet (XUV) pumping, we show that the XUV pulse parameters are encoded in the parameters of the emitted THz pulse, allowing a robust single-shot monitoring technique for the arrival time, energy, and duration of the XUV pulse [2].