THz Magnetic Fields Research Articles

Strongly nonlinear antiferromagnetic dynamics in high magnetic fields

Antiferromagnetic (AFM) materials possess a well-recognized potential for ultrafast data processing thanks to their intrinsic ultrafast spin dynamics, absence of stray fields, and large spin transport effects. The very same properties, however, make their manipulation difficult, requiring frequencies in THz range and magnetic fields of tens of Teslas. Switching of AFM order implies going into the nonlinear regime, a largely unexplored territory. Here we use THz light from a free electron laser to drive antiferromagnetic NiO into a highly nonlinear regime and steer it out of nonlinearity with magnetic field from a 33-Tesla Bitter magnet. This demonstration of large-amplitude dynamics represents a crucial step towards ultrafast resonant switching of AFM order.

Terahertz inverse spin Hall effect in spintronic nanostructures with various ferromagnetic materials

Bilayers of ferromagnetic and heavy metal nanolayers excited with femtosecond laser pulses emit subpicosecond bursts of electromagnetic radiation with spectral frequencies up to several THz. We fabricated such spintronic THz emitters containing various ferromagnetic materials with vastly different magnetic remanence, saturation magnetization, and coercive fields. In all cases, the THz amplitude versus magnetic-field dependence follows the independently measured magnetization hysteresis loops and the THz polarization direction is perpendicular to the sample’s magnetization M, both consistent with the inverse spin Hall effect (ISHE) as the physical origin. The mV-level signals of observed THz transients also favor the ISHE over the anomalous Nernst effect (ANE). The M(T) variation governs the temperature dependence of the THz generation. Emitters with weakly remanent ferromagnets are magnetic-field tunable, while moderately and strongly remanent ferromagnets, once magnetized, allow intense THz generation even without an external field, nevertheless exhibiting low susceptibility to magnetic perturbations if the hysteresis loop is square. Hence, exploiting the magnetic properties of the ferromagnetic layer enables tailoring of weakly temperature-dependent spintronic THz emitters. Finally, we explored the applicability of perovskite oxide materials such as La-Sr-Mn-O (LSMO) for THz transient generation. We detected weak (<40-µV amplitude) THz emissions from both LSMO/Au and pure LSMO nanostructures with no sign flip upon the sample reversal. Thus, we excluded the ISHE and believe it was likely governed by the ANE in the thick-film regime, although we cannot exclude the transient demagnetization mechanism.

Plasmonic Metasurface Resonators to Enhance Terahertz Magnetic Fields for High‐Frequency Electron Paramagnetic Resonance

Nanoscale magnetic systems play a decisive role in areas ranging from biology to spintronics. Although, in principle, THz electron paramagnetic resonance (EPR) provides high-resolution access to their properties, lack of sensitivity has precluded realizing this potential. To resolve this issue, the principle of plasmonic enhancement of electromagnetic fields that is used in electric dipole spectroscopies with great success is exploited, and a new type of resonators for the enhancement of THz magnetic fields in a microscopic volume is proposed. A resonator composed of an array of diabolo antennas with a back-reflecting mirror is designed and fabricated. Simulations and THz EPR measurements demonstrate a 30-fold signal increase for thin film samples. This enhancement factor increases to a theoretical value of 7500 for samples confined to the active region of the antennas. These findings open the door to the elucidation of fundamental processes in nanoscale samples, including junctions in spintronic devices or biological membranes.

Excitation and detection of terahertz coherent spin waves in antiferromagnetic α−Fe2O3

The efficiency of the ultrafast excitation of spins in antiferromagnetic $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}$ using a nearly single-cycle THz pulse is studied as a function of the polarization of the THz pulse and the sample temperature. Above the Morin point the most efficient excitation is achieved when the magnetic field of the THz pulse is perpendicular to the antiferromagnetically coupled spins. Using the experimental results and equations of motion for spins, we show that the mechanism of the spin excitation above and below the Morin point relies on a magnetic-dipole interaction of the THz magnetic field with spins, and the mechanism implies that the efficiency of the coupling is proportional to the time derivative of the magnetic field.

Phase-Sensitive Vector Terahertz Electrometry from Precision Spectroscopy of Molecular Ions

This article proposes a new method for sensing THz waves that can allow electric field measurements traceable to the International System of Units and to the fundamental physical constants by using the comparison between precision measurements with cold trapped HD+ ions and accurate predictions of molecular ion theory. The approach exploits the lightshifts induced on the two-photon rovibrational transition at 55.9 THz by a THz wave around 1.3 THz, which is off-resonantly coupled to the HD+ fundamental rotational transition. First, the direction and the magnitude of the static magnetic field applied to the ion trap is calibrated using Zeeman spectroscopy of HD+. Then, a set of lightshifts are converted into the amplitudes and the phases of the THz electric field components in an orthogonal laboratory frame by exploiting the sensitivity of the lightshifts to the intensity, the polarization and the detuning of the THz wave to the HD+ energy levels. The THz electric field measurement uncertainties are estimated for quantum projection noise-limited molecular ion frequency measurements with the current accuracy of molecular ion theory. The method has the potential to improve the sensitivity and accuracy of electric field metrology and may be extended to THz magnetic fields and to optical fields.

Effective tuning of isotropic and anisotropic properties of quantum dots and rings by external fields

Abstract The combined effects of intense THz laser field, magnetic field and in-plane electric field on electronic states and intraband optical properties of single isotropic quantum dots and rings are investigated using the non-perturbative Floquet theory in the high-frequency limit. Here we show that the change of the electric field direction for fixed values of intense THz laser field parameter can recover the degeneracy of the excited states and the dipole-allowed transition intensities in a quantum dot. Additionally, it is shown that in isotropic quantum rings the intense THz laser and the in-plane electric field create unusual Aharonov-Bohm oscillations. For fixed values of the intense THz laser field parameter, the amplitudes of Aharonov-Bohm oscillations can be effectively tuned by changing the electric field direction. Furthermore, for fixed values of the electric field strength and the laser field parameter the intraband optical properties can be effectively tuned by changing the electric field direction. Therefore, it can be asserted that circular quantum dots or circular rings in the intense THz laser field are equivalent to anisotropic quantum dots or rings respectively, and the electric field direction effectively tunes the single-electron energy spectrum and intraband optical properties of these structures.

Coherent and incoherent ultrafast magnetization dynamics in 3d ferromagnets driven by extreme terahertz fields

Ultrafast spin dynamics in magnetic materials is generally associated with ultrafast heating of the electronic system by a near infrared femtosecond laser pulse, thus offering only an indirect and nonselective access to the spin order. Here we explore spin dynamics in ferromagnets by means of extremely intense THz pulses, as at these low frequencies the magnetic field provides a direct and selective route to coherently control the magnetization. We find that, at low fields, the observed off-resonantly excited spin precession is phase locked to the THz magnetic field. At extreme THz fields, the coherent spin dynamics become convoluted with an ultrafast incoherent magnetic quenching due to the absorbed energy. This demagnetization takes place upon a single shot exposure. The magnetic properties are found to be permanently modified above a THz pump fluence of $\ensuremath{\approx}100\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$. We conclude that magnetization switching cannot be reached. Our atomistic spin-dynamics simulations excellently explain the measured magnetization response. We find that demagnetization driven by THz laser-field coupling to electron charges occurs, suggesting nonconducting materials for achieving coherent THz-magnetization reversal.

Extreme THz fields from two-color filamentation of midinfrared laser pulses

Nonlinear THz photonics is probably the last frontier of nonlinear optics. The strength of both the electric and the magnetic fields of these ultrashort low-frequency light bunches opens the way to exciting science and applications. Progress in the field though is slow because of the deficiency in suitable sources. Here we show that two-color filamentation of midinfrared $3.9\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ laser pulses allows one to generate single-cycle THz pulses with multimillijoule energies and record conversion efficiencies. Moreover, the focused THz peak electric and magnetic fields reach values of GV/cm and kT, respectively, exceeding by far any available quasi-dc field source today. These fields enable extreme field science, including into other, relativistic phenomena. In addition, we elucidate the origin of this high efficiency, which is made up of several factors, including a mechanism where the harmonics produced by the midinfrared pulses strongly contribute to the field symmetry breaking and enhance the THz generation.

THz-driven demagnetization with perpendicular magnetic anisotropy: towards ultrafast ballistic switching

We study THz-driven spin dynamics in thin CoPt films with perpendicular magnetic anisotropy. Femtosecond magneto-optical Kerr effect measurements show that demagnetization amplitude of about can be achieved with a peak THz electric field of 300 kV cm−1, and a corresponding peak magnetic field of 0.1 T. The effect is more than an order of magnitude larger than observed in samples with easy-plane anisotropy irradiated with the same field strength. We also utilize finite-element simulations to design a meta-material structure that can enhance the THz magnetic field by more than an order of magnitude, over an area of several tens of square micrometers. Magnetic fields exceeding 1 Tesla, generated in such meta-materials with the available laser-based THz sources, are expected to produce full magnetization reversal via ultrafast ballistic precession driven by the THz radiation. Our results demonstrate the possibility of table-top ultrafast magnetization reversal induced by THz radiation.

Model of THz Magnetization Dynamics.

Magnetization dynamics can be coherently controlled by THz laser excitation, which can be applied in ultrafast magnetization control and switching. Here, transient magnetization dynamics are calculated for excitation with THz magnetic field pulses. We use the ansatz of Smit and Beljers, to formulate dynamic properties of the magnetization via partial derivatives of the samples free energy density, and extend it to solve the Landau-Lifshitz-equation to obtain the THz transients of the magnetization. The model is used to determine the magnetization response to ultrafast multi- and single-cycle THz pulses. Control of the magnetization trajectory by utilizing the THz pulse shape and polarization is demonstrated.

Optical modulation characteristics of all-dielectric grating at terahertz frequencies

In recent years, metamaterials (MMs) have been widely investigated for their exotic electromagnetic characteristics which cannot be achieved in nature. However, one of the main limitations in traditional metallic-film MMs is a high level of radiation loss in metal and insertion loss of the high-permittivity or thick substrate. Fortunately, all-dielectric MMs with high refractive-index dielectric structures show significantly less material loss than their metallic counterparts. In this paper, an all-dielectric grating is fabricated on a 100-m-thick silicon wafer by using direct-laser-writing technique, and the properties of its Mie resonances are investigated by THz time-domain spectroscopy. Then we measure the spectral response of the all-dielectric grating under the optical modulation by a near-infrared pump-THz probe method. The modulation light source is an 808 nm continuous semiconductor laser with a maximum power (10 W). To give an insight into the underlying mechanisms of the Mie-type resonance effects on the arrayed, silicon pillars, the transmission of the all-dielectric grating is investigated numerically by the finite-element simulations through using CST Microwave Studio. In our experiment, the incident THz magnetic field is along the grating lines. The research results show that three typical Mie resonances are excited from 0 to 1 THz in the all-dielectric structure, and all the three resonant modes are different in the distributions of electric field and magnetic field. Furthermore, it is found that the resonance intensities of these three resonance peaks appear to be weakened variously with the increase of the optical power, and the first resonant peak modulation amplitude maximally reaches more than 50%. Combining the simulation results, we prove that the decrease of Mie resonance intensity under photo-excitation is caused by the absorption and the scattering of the incident THz wave by photo-generated carriers. Besides, we estimate the conductivity values of the all-dielectric grating under different optical excitations and find that the conductivity values reach 1000 S/m and 1500 S/m corresponding to 5 W and 10 W optical excitation, respectively. The estimated conductivity data will play an important role in the prospective optical modulation simulation. All the results mentioned above will provide an important reference for researches on the resonance properties of the all-dielectric metamaterials and the development of related functional devices.

Metastable nature of InN and In-rich InGaN alloys

The paper provides a thermodynamic insight into the metastable nature of InN and In-rich InGaN alloys, based on experimental studies of their plasma-assisted MBE growth and high-temperature decomposition, as well as on theoretical modeling of nitrogen vacancy behavior. This instability may easily result in occurrence of metallic In nanoparticles in the bulk of In(Ga)N films and in the vicinity of extended defects at high enough In content, which makes us consider this material as a metal–semiconductor composite. An overview of a wide set of experimental studies performed by us on the epitaxial films grown in many laboratories all around the world is given which proves an existence of such In nanoparticles in the films and shows how they affect optical and electrical properties of the epilayers. Possible applications of epitaxial InN layers for THz emitters and magnetic field sensors are discussed.

微谐振环结构体内太赫兹增强效应

Based on the rigorous electromagnetic field theory, the rigorous expression of terahertz waves in micro-ring resonators with shape and fishnet structure is given in this paper. With the boundary conditions of electromagnetic field, the enhancement effect of the spatial distribution of terahertz waves in the micro-ring resonator with shape and fishnet structure is analyzed. The numerical results show that the electric field is greater than the magnetic field near the ring resonator metal strips. The electric field near the metal strips is obviously stronger than that in other regions, especially in the opening. The THz electric and magnetic fields in the peak position of electromagnetic field in fishnet structure are symmetry with respect to x. The extremum of the electric field appears in the four up and down corners of the large cross, while the extremum of the magnetic field appears in the upper and lower ends of the small cross. The physical interpretation of these phenomena is also given in the point view of the theory of electromagnetic field transmission lines.

Spin relaxation in an InAs quantum dot in the presence of terahertz driving fields

The spin relaxation in a 1D InAs quantum dot with the Rashba spin-orbit coupling under driving THz magnetic fields is investigated by developing the kinetic equation with the help of the Floquet-Markov theory, which is generalized to the system with the spin-orbit coupling, to include both the strong driving field and the electron-phonon scattering. The spin relaxation time can be effectively prolonged or shortened by the terahertz magnetic field depending on the frequency and strength of the terahertz magnetic field. The effect can be understood as the sideband-modulated spin-phonon scattering. This offers an additional way to manipulate the spin relaxation time.

Terahertz investigation of high quality indium nitride epitaxial layers

AbstractWe report on the optical characterization of InN layers in the THz range and magnetic fields up to 13 T. The results are interpreted using the dielectric function formalism, with contributions of cyclotron resonance, phonons, plasmons and helicon wave excitations. We show how THz radiation transmission measurements can provide an optical contactless method of determining the quality (carrier density and momentum scattering rate) in the InN layers. (© 2005 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)