Future information technologies, such as ultrafast data recording, quantum computation or spintronics, call for ever faster spin control by light [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] . Intense terahertz pulses can couple to spins on the intrinsic energy scale of magnetic excitations 5,11 . Here, we explore a novel electric dipole-mediated mechanism of nonlinear terahertz-spin coupling that is much stronger than linear Zeeman coupling to the terahertz magnetic field 5,10 . Using the prototypical antiferromagnet thulium orthoferrite (TmFeO 3 ), we demonstrate that resonant terahertz pumping of electronic orbital transitions modifies the magnetic anisotropy for ordered Fe 3+ spins and triggers large-amplitude coherent spin oscillations. This mechanism is inherently nonlinear, it can be tailored by spectral shaping of the terahertz waveforms and its efficiency outperforms the Zeeman torque by an order of magnitude. Because orbital states govern the magnetic anisotropy in all transition-metal oxides, the demonstrated control scheme is expected to be applicable to many magnetic materials.Ultrafast magnetization control has become a key goal of modern photonics, with a broad variety of successful concepts emerging at a fast pace. Examples include light-induced spin reorientation in canted antiferromagnets 3 , vectorial control of magnetization by light 6 , photoinduced antiferromagnet-ferromagnet phase transitions 9 , optical modification of the exchange energy 4,14 and driving spin precessions via nonlinear magneto-phononic coupling 7,16 . Despite this remarkable progress, most of the photon energy in all known concepts using visible and near-infrared light is inactive with respect to the light-spin interaction, and avoiding dissipation of large excess energies requires special care.In contrast, intense electromagnetic pulses at terahertz frequencies may interface spin dynamics directly on their intrinsic energy scales 5,11 . The magnetic field component of few-cycle terahertz pulses has been used to coherently control magnons in the electronic ground state by direct Zeeman interaction 5,11 . Because magnetic dipole coupling is typically weak, however, terahertz-driven spin excitation has been confined to the linear response regime. Massive nonlinearities, such as terahertz-induced phase transitions 17,18 and terahertz lightwave electronics [19][20][21][22]
Future information technology demands ultimately fast, low-loss quantum control. Intense light fields have facilitated important milestones, such as inducing novel states of matter 1-3 , accelerating electrons ballistically 4-7 , or coherently flipping the valley pseudospin 8 . These dynamics leave unique signatures, such as characteristic bandgaps or high-order harmonic radiation. The fastest and least dissipative way of switching the technologically most important quantum attribute -the spin -between two states separated by a potential barrier is to trigger an all-coherent precession. Pioneering experiments and theory with picosecond electric and magnetic fields have suggested this possibility 9-11 , yet observing the actual dynamics has remained out of reach. Here, we show that terahertz (1 THz = 10 12 Hz) electromagnetic pulses allow coherent navigation of spins over a potential barrier and we reveal the corresponding temporal and spectral fingerprints. This goal is achieved by coupling spins in antiferromagnetic TmFeO3 with the locally enhanced THz electric field of custom-tailored antennas. Within their duration of 1 ps, the intense THz pulses abruptly change the magnetic anisotropy and trigger a large-amplitude ballistic spin motion. A characteristic phase flip, an asymmetric splitting of the magnon resonance, and a long-lived offset of the Faraday signal are hallmarks of coherent spin switching into adjacent potential minima, in agreement with a numerical simulation. The switchable spin states can be selected by an external magnetic bias. The low dissipation and the antenna's sub-wavelength spatial definition could facilitate scalable spin devices operating at THz rates.
Terahertz magnetic fields with amplitudes of up to 0.4 Tesla drive magnon resonances in nickel oxide while the induced dynamics is recorded by femtosecond magneto-optical probing. We observe distinct spin-mediated optical nonlinearities, including oscillations at the second harmonic of the 1 THz magnon mode. The latter originate from coherent dynamics of the longitudinal component of the antiferromagnetic order parameter, which are probed by magneto-optical effects of second order in the spin deflection. These observations allow us to dynamically disentangle electronic from lattice-related contributions to magnetic linear birefringence and dichroism-information so far only accessible by ultrafast THz spin control. The nonlinearities discussed here foreshadow physics that will become essential in future subcycle spin switching.
Terahertz near fields of gold metamaterials resonant at a frequency of 0.88 THz allow us to enter an extreme limit of nonperturbative ultrafast terahertz electronics: Fields reaching a ponderomotive energy in the keV range are exploited to drive nondestructive, quasistatic interband tunneling and impact ionization in undoped bulk GaAs, injecting electron-hole plasmas with densities in excess of 10 19 cm −3 . This process causes bright luminescence at energies up to 0.5 eV above the band gap and induces a complete switch-off of the metamaterial resonance accompanied by self-amplitude-modulation of transmitted few-cycle terahertz transients. Our results pave the way towards highly nonlinear terahertz optics and optoelectronic nanocircuitry with subpicosecond switching times. DOI: 10.1103/PhysRevLett.113.227401 PACS numbers: 78.20.-e, 42.65.Ky, 42.65.Sf, 72.20.Ht Intense, phase-locked light pulses in the terahertz spectral range have opened up an exciting arena for field-sensitive nonlinear optics . For a given peak electric field E, the low carrier frequency ω THz gives rise to a potentially large ponderomotive energyTHz , which quantifies the cycle-averaged quiver energy of a free electron of mass m. This situation promises a new quality of nonperturbative light-matter interaction at the boundary of terahertz optics and highspeed electronics.For frequencies between 0.5 and 3 THz, optical rectification [7][8][9][10] has enabled transients with peak field amplitudes in excess of 1 MV=cm [10]. Using such an electromagnetic pulse as an alternating bias, terahertzdriven carrier multiplication in doped semiconductors [12] and graphene [14] has been demonstrated. Strong terahertz fields have also been used to drive spectacular nonlinear intraband dynamics of quasiparticles, such as field ionization of impurity states [13] or excitons, electronhole recollisions, and high-order sideband generation [11]. In these experiments, the terahertz bias facilitates tunneling of bound electrons through potential energy barriers, which correspond to binding energies between a few meV and several 10 meV [ Fig. 1(a)] [11,13,23].A new limit of nonperturbative nonlinearities is expected if terahertz amplitudes approach atomically strong fields. As depicted in Fig. 1(b . This breakthrough has enabled ultrafast biasing of bulk solids in an unprecedented high-field limit, where a coherent interplay between nonresonant interband polarization and intraband Bloch oscillations generates terahertz high-harmonic radiation [6]. The diffraction limit of focusing, however, has precluded comparably high fields at frequencies as low as 1 THz where yet larger ponderomotive potentials U p combined with photon energies orders of magnitude below electronic interband resonances could pave the way to quasistatic biasing. Custom-tailored metamaterials are a promising concept for overcoming the diffraction limit. Indeed, field enhancement in metamaterials has been exploited to induce a metal-insulator phase transition in VO 2 by a terahertz transient with...
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