Phase-locked single-cycle transients with frequency components between 1 and 60THz and peak fields of up to 12MV/cm are generated as the idler wave of a parametric amplifier. To achieve broadband conversion in GaSe nonlinear crystals, we match the group velocities of signal and idler components. The influence of group-velocity dispersion is minimized by long-wavelength pumping at 1.18mum. Free-space electro-optic sampling monitors the multiterahertz waveforms with direct field resolution.

Intense multi-THz pulses are used to study the coherent nonlinear response of bulk InSb by means of field-resolved four-wave mixing spectroscopy. At amplitudes above 5 MV/cm the signals show a clear temporal substructure which is unexpected in perturbative nonlinear optics. Simulations based on a two-level quantum system demonstrate that in spite of the strongly off-resonant character of the excitation the high-field pulses drive the interband resonances into a non-perturbative regime of Rabi flopping.PACS numbers: 42.65. Re, 78.47.nj, Semiconductors form a uniquely well-defined laboratory to explore novel limits of nonlinear optics. A key parameter for interaction of a coherent light field with an electronic transition is given by the Rabi frequency Ω R = µE/ . E is the electric field strength and µ the transition dipole moment. When the dephasing rate is negligible compared to Ω R , coherent Rabi flopping governs the dynamics of electronic systems [1]. If the detuning of the driving electromagnetic field is smaller than Ω R the response of a system cannot be described by the perturbative approach which usually depicts offresonant nonlinear optics. This non-perturbative excitation regime provides access to many fascinating quantum effects in semiconductors such as Rabi splitting, selfinduced transparency and generation of high harmonics [2][3][4][5][6][7][8]. In particular, sufficiently intense and ultrashort laser pulses have been exploited to implement ultimate scenarios in which the duration of the light pulse, the Rabi cycle and the oscillation period of the carrier wave all become comparable in size [9]. Under such conditions, a detailed insight into the nonlinear optical interaction calls for complete phase and amplitude resolution of all interacting light fields. However, in most of the cases, the lack of phase-stable laser pulses and fast detectors does not allow for capturing sub-cycle polarization dynamics of a system. The development of ultraintense THz laser systems generating phase-stable transients with field amplitudes above 1 MV/cm [10][11][12] paves the way towards a coherent spectroscopy of extreme nonlinearities in condensed matter systems with absolute sampling of amplitude and phase. Recent experiments performed with high-field multi-THz pulses have demonstrated a high potential of this approach [13][14][15][16]. However, the regime of an offresonant excitation of Rabi flopping has remained almost unexplored due to the lack of sufficiently intense and phase-locked pulses. The latest breakthrough in generation and field-resolved detection of multi-THz pulses with peak fields up to 100 MV/cm [17,18] opens up the possibility to explore this highly non-perturbative regime.

The spin structure of domain walls in constrictions down to 30 nm is investigated both experimentally with electron holography and with simulations using a Heisenberg model. Symmetric and asymmetric transverse domain walls for different constriction sizes are observed, consistent with simulations. The experimentally observed asymmetric transverse walls can be further divided into tilted and buckled walls, the latter being an intermediate state just before the vortex nucleation. As the constriction width decreases, the domain wall width decreases faster than linearly, which leads to very narrow domain walls for narrow constrictions. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2779109͔Control and manipulation of magnetic domain walls ͑DWs͒ are in the focus of interest because of the associated exciting physical phenomena and the potential for applications such as magnetic logic and data storage devices. 1,2Here, reproducible and controlled switching by DW motion, induced either by an external magnetic field or a spinpolarized current, is essential. [3][4][5] The detailed DW spin structure and width play a very important role in determining the DW velocity in current-induced 5 and field-induced motion.6,7For very narrow walls, nonadiabatic contributions to the electron transport are predicted to become significant, [8][9][10] which would increase the current-induced DW velocity, which is important for applications. Reciprocal effects of the spin structure on the magnetotransport, such as DW magnetoresistance, also depend on the detailed wall spin structure. 11,12On reducing the lateral dimensions, it is the geometry rather than the material parameters which determines the DW type and spin structure.13-16 For a one dimensional chain model, it has been predicted that the reduction of the lateral dimensions leads to a reduction in the Bloch DW width. 13For 180°Néel walls in Permalloy ͑Fe 20 Ni 80 ͒ thin film rectangular structures, the reduction of the DW width has been studied using scanning electron microscopy with polarization analysis with a magnetic resolution of 20-30 nm. 17 The types of DWs confined in wires or ring elements are very different from these Néel walls, since they exhibit head-tohead wall structures with two types prevailing: transverse and vortex DWs.14,15 Using transmission electron microscopy techniques, the spin structure of head-to-head DWs in elements down to 200 nm lateral size has been measured. 18Observations of DWs in constrictions have also been carried out to determine the pinning potential, but the detailed spin structures and wall widths have not been ascertained. 19Employing micromagnetic simulations, the geometry dependence of the wall types ͑wall phase diagram͒ has been studied down to 20 nm width. In addition to the transverse and vortex walls, asymmetric transverse walls were predicted. 16 In the conventional micromagnetic approach used in Ref. 16, the exchange energy is approximated by ٌ͑ ជ · m ជ ͒ 2 ͑m ជ is the local magnetization͒, which is the first order Taylor exp...

Observations of domain wall motion and transformations due to injected current pulses in permalloy zigzag structures using off-axis electron holography and Lorentz microscopy are reported. Heating on membranes leads to thermally activated random behavior at low current densities and by backcoating the SiN membranes with Al, heating effects are significantly reduced. A set of indicators is devised to separate unambiguously spin torque effects from heating and it is shown that by using the Al layer the structures are sufficiently cooled to exhibit current-induced domain wall motion due to spin torque.

We report the direct transmission electron microscopy observation of spin structure transformations in nanoscale Permalloy zigzag wires due to Joule heating during the injection of current pulses. This heating is sufficient to overcome the energy barriers separating the different metastable domain wall spin structures. Due to the large energy barriers these are stable and observable at room temperature by off‐axis electron holography and Fresnel imaging. The interaction between different domain walls is probed and the main pinning mechanism is determined to be the edge roughness. In addition to transformations, we also report on thermally assisted domain wall hopping between two pinning sites and structural changes that occur when the samples are subjected to even higher current pulses. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)