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.

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.

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.

Water possesses strong absorption in the THz range due to intermolecular vibrational modes in a network of hydrogen-bonded water molecules. Its THz response is also sensitive to the coupling of water to other molecules, i.e. the hydration shell of a protein. Probing the nonlinear properties of hydration water can provide insight into protein solvent dynamics, and in the case of intrinsically disordered proteins, its subsequent role in the liquid-liquid phase separation (LLPS). Such characterization at low THz frequencies (< 3 THz) remains yet limited, due to the scarcity of brilliant light sources in this range.

Understanding spin-lattice interactions in antiferromagnets is one of the most fundamental issues at the core of the recently emerging and booming fields of antiferromagnetic spintronics and magnonics. Recently, coherent nonlinear spin-lattice coupling was discovered in an antiferromagnet which opened the possibility to control the nonlinear coupling strength and thus showing a novel pathway to coherently control magnon-phonon dynamics. Here, utilizing intense narrow band terahertz (THz) pulses and tunable magnetic fields up to 7 T, we experimentally realize the conditions of the Fermi magnon-phonon resonance in antiferromagnetic $CoF_{2}$. These conditions imply that both the spin and the lattice anharmonicities harvest energy transfer between the subsystems, if the magnon eigenfrequency $f_{m}$ is twice lower than 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 an impulsive THz-induced response. This study focuses on the role of nonlinearity in spin-lattice interactions, providing insights into the control of coherent magnon-phonon energy exchange.

The third harmonic generation (THG) of graphite layers on paper substrate upon excitation with intense (up to 100 kV cm −1 ) narrowband terahertz (THz) pulses is studied. Highest THG efficiencies are comparable with those of chemical vapor deposition‐grown single‐layer graphene. Samples are hand drawn, using commercially available pencils. The THG response shows high sensitivity regarding the hatching direction relative to the THz polarization orientation. Using Raman spectroscopy, the occurrence of graphene‐like structures in the samples is confirmed. The findings demonstrate the feasibility of virtually no‐cost and easy‐to‐fabricate materials for THz nonlinear optics.

Several technologies, including photodetection, imaging, and data communication, could greatly benefit from the availability of fast and controllable conversion of terahertz (THz) light to visible light. Here, we demonstrate that the exceptional properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nano-second time scale. We show a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V. We also demonstrate that a grating-graphene metamaterial leads to an increase in THz-induced emitted power in the visible range by 2 orders of magnitude. The experimental results are in agreement with a thermodynamic model that describes blackbody radiation from the electron system heated through intraband Drude absorption of THz light. These results provide a promising route toward novel functionalities of optoelectronic technologies in the THz regime.

We present a versatile high repetition rate, optical-parametric chirped-pulse amplifier system (OPCPA) in combination with a high-harmonic-generation (HHG) source. Tuning of the fundamental OPCPA driver wavelength allows for high harmonic generation within the full range between 25 and 50 eV. All energies between two adjacent odd harmonics can be addressed, making the system a powerful, gaplessly tunable extreme-ultraviolet (XUV) light source for spectroscopy.

Laser Produced Plasma (LPP) was employed as an ion source for the modifications in surface, electrical and mechanical properties of poly methyl (methacrylate) PMMA. For this purpose Nd:YAG laser (532nm, 6ns, 10Hz) at a fluence of 12.7J/cm2 was employed to generate Fe plasma. The fluence and energy measurements of laser produced Fe plasma ions were carried out by employing Thomson Parabola Technique in the presence of magnetic field strength of 0.5T, using CR-39 as Solid State Nuclear Track Detector (SSNTD). It has been observed that ion fluence ejecting from ablated plasma was maximum at an angle of 5° with respect to the normal to the Fe target surface. PMMA substrates were irradiated with Fe ions of constant energy of 0.85MeV at various ion fluences ranging from 3.8×106ions/cm2 to 1.8×108ions/cm2 controlled by varying laser pulses from 3000 to 7000. Optical microscope and Scanning Electron Microscope (SEM) were utilized for the analysis of surface features of irradiated PMMA. Results depicted the formation of chain scission, crosslinking, dendrites and star like structures. To explore the electrical behavior, four probe method was employed. The electrical conductivity of ion irradiated PMMA was increased with increasing ion fluence. The surface hardness was measured by shore D hardness tester and results showed the monotonous increment in surface hardness with increasing ion fluence. The increasing trend of surface hardness and electrical conductivity with increasing Fe ion fluence has been well correlated with the surface morphology of ion implanted PMMA. The temperature rise of PMMA surface due to Fe ion irradiation is evaluated analytically and comes out to be in the range of 1.72×104 to 1.82×104K. The values of total Linear Energy Transfer (LET) or stopping power of 0.8MeV Fe ions in PMMA is 61.8eV/Å and their range is 1.34μm evaluated by SRIM simulation.

The effect of transverse magnetic field on laser-induced breakdown spectroscopy of graphite plasma as a function of fluence has been investigated. Graphite targets were exposed to Nd:YAG (1064 nm, 10 ns) laser pulses at various laser fluences ranging from 0.4 to 2.9 J cm−2 under two different environment of air and Ar at a pressure of 150 and 760 torr. A transverse magnetic field of strength 0.5 tesla was employed by using permanent magnets. It is revealed that due to the presence of the magnetic field the emission intensity, electron temperature and number density of graphite plasma have been increased at all fluences and for all environmental conditions. The enhancement in plasma parameters is attributed to magnetic confinement effect and Joule heating effect. Initially by increasing the fluence from 0.4 to 1.5 J cm−2 (in air) and 0.4 to 1.8 J cm−2 (in Ar), the emission intensity, electron temperature and number density have been increased and have attained their maximum values. Further increase in fluence was responsible for the decreasing trend in all plasma parameters. More increase in fluence (beyond 1.8 J cm−2 in case of air and 2.2 J cm−2 in case of Ar) up to a maximum value of 2.9 J cm−2, the saturation or self-sustained regime was achieved, which is responsible for insignificant changes in plasma parameters. The value of plasma parameter “β” was also evaluated analytically, and it was less than one for all conditions (fluences as well as environments), which confirmed the existence of confinement effect.