Ultrafast charge and spin excitations in the elusive terahertz regime1, 2 of the electromagnetic spectrum play a pivotal role in condensed matter3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. The electric field of free-space terahertz pulses has provided a direct gateway to manipulating the motion of charges on the femtosecond timescale6, 7, 8, 9. Here, we complement this process by showing that the magnetic component of intense terahertz transients enables ultrafast control of the spin degree of freedom. Single-cycle terahertz pulses switch on and off coherent spin waves in antiferromagnetic NiO at frequencies as high as 1 THz. An optical probe pulse with a duration of 8 fs follows the terahertz-induced magnetic dynamics directly in the time domain and verifies that the terahertz field addresses spins selectively by means of the Zeeman interaction. This concept provides a universal ultrafast means to control previously inaccessible magnetic excitations in the electronic ground state
A formation process for semiconductor quantum dots based on a surface instability induced by ion sputtering under normal incidence is presented. Crystalline dots 35 nanometers in diameter and arranged in a regular hexagonal lattice were produced on gallium antimonide surfaces. The formation mechanism relies on a natural self-organization mechanism that occurs during the erosion of surfaces, which is based on the interplay between roughening induced by ion sputtering and smoothing due to surface diffusion.To date two approaches for the fabrication of semiconductor quantum dots have been pursued. In the top-down approach, lithographic methods are used for direct patterning of quantum dots, whereas the bottom-up approach relies on self-organized processes. In contrast to serial electron-beam lithography, self-organization phenomena open the way for the formation of a regular array of quantum dots on large areas in a single technological process step. Self-organized semiconductor quantum dots have been produced by the Stranski-Krastanow growth mode in molecular beam epitaxy and metal-organic vapor phase epitaxy, in which coherent island formation occurs during the growth of lattice-mismatched semiconductors (1). Here we present a controlled and costeffective method for the production of wellordered quantum dots by ion bombardment of semiconductor surfaces (2) that is based on a self-organization mechanism induced by ion sputtering of solid surfaces, where the formation kinetics is determined by etching instead of growth.There has been great effort to interpret the microscopic dynamics of surface roughness and pattern formation induced by ion sputtering in which the formation of coherent ripples has been observed on metal, semiconductor, and insulator surfaces under ion bombardment at off-normal angles of incidence (3-7). The characteristic period in the submicrometer to nanometer range is defined by the sputtering conditions (for example ion energy, ion flux, and substrate temperature) and by the material properties. An explanation of the underlying mechanism was proposed by Bradley and Harper (8) in which the sputtering yield, which is the number of surface atoms removed per incident ion, depends on the surface curvature. Under certain conditions this dependence gives rise to a surface instability where the erosion is greater in a depression than on an elevation. The ion-induced surface instability can be described by a specific term in the erosion equation that is proportional to the negative Laplacian of the surface. The proportionality factor is called negative surface tension because it tends to maximize the surface, in contrast to surface tension that minimizes the surface. It is the competition between this roughening instability and diffusive smoothing mechanisms that governs the buildup of a regular pattern with a characteristic wavelength. Under off-normal incident ions the instability is anisotropic, giving rise to characteristic ripple patterns. Their direction was found to be either parallel or perpen...
We apply ultrafast spectroscopy to establish a time-domain hierarchy between structural and electronic effects in a strongly correlated electron system. We discuss the case of the model system VO 2 , a prototypical nonmagnetic compound that exhibits cell doubling, charge localization, and a metal-insulator transition below 340 K. We initiate the formation of the metallic phase by prompt hole photo-doping into the valence band of the low-T insulator. The insulator-to-metal transition is, however, delayed with respect to hole injection, exhibiting a bottleneck time scale, associated with the phonon connecting the two crystallographic phases. This structural bottleneck is observed despite faster depletion of the d bands and is indicative of important bandlike character for this controversial insulator. Correlated electron materials exhibit remarkable effects, ranging from metal-insulator transitions to nonconventional (high temperature) superconductivity. The subtle interplay between atomic structure, charge, spin, and orbital dynamics is responsible for many of the critical phenomena observed. 1 Importantly, because "simultaneous" changes in more than one degree of freedom are often observed as chemical doping or external parameters are tuned across critical values, time-integrated spectroscopies are unable to uniquely assign cause-effect relationships.Here, we demonstrate that time-resolved spectroscopy can instead be applied to overcome such ambiguities. We study the case of nonmagnetic VO 2 , a controversial, strongly correlated compound that exhibits cell doubling in "concomitance" with electron localization and a metal-insulator transition below 340 K 2 (see Fig. 1). The issue is whether the insulating behavior in the low-T phase derives directly from the Peierls distortion 3 or from electron localization and the consequent increase in electron-electron repulsion. 4,5 Recently, a theoretical study by Wentzcovitch et al. has revived attention into this four-decade-long debate, 6 suggesting that the former mechanism may be dominant, i.e., the low-T phase may be bandlike and the transition structurally driven. New controversy has resulted 7,8 and the problem is yet to be settled experimentally.Previous time-resolved optical 9 and x-ray diffraction 10 experiments in this compound demonstrated that impulsive photoexcitation of the low-T monoclinic insulator causes an ultrafast transition in both the electronic properties and the atomic structural arrangement. However, it was not clear whether the system becomes metallic due to the change in symmetry of the unit cell or to the prompt creation of holes, causing the closure of a Mott gap. We have now performed optical experiments with 15 fs resolution, and we report evidence of a limiting structural time scale for the formation of the metallic phase. This delay is observed despite much faster hole doping into the correlated d band. Such bottleneck time originates from the coherent optical-phonon distortions in the excited state of the system, mapping onto the crystal...
High-speed asynchronous optical sampling ͑ASOPS͒ is a novel technique for ultrafast time-domain spectroscopy ͑TDS͒. It employs two mode-locked femtosecond oscillators operating at a fixed repetition frequency difference as sources of pump and probe pulses. We present a system where the 1 GHz pulse repetition frequencies of two Ti:sapphire oscillators are linked at an offset of ⌬f R = 10 kHz. As a result, their relative time delay is repetitively ramped from zero to 1 ns within a scan time of 100 s. Mechanical delay scanners common to conventional TDS systems are eliminated, thus systematic errors due to beam pointing instabilities and spot size variations are avoided when long time delays are scanned. Owing to the multikilohertz scan-rate, high-speed ASOPS permits data acquisition speeds impossible with conventional schemes. Within only 1 s of data acquisition time, a signal resolution of 6 ϫ 10 −7 is achieved for optical pump-probe spectroscopy over a time-delay window of 1 ns. When applied to terahertz TDS, the same acquisition time yields high-resolution terahertz spectra with 37 dB signal-to-noise ratio under nitrogen purging of the spectrometer. Spectra with 57 dB are obtained within 2 min. A new approach to perform the offset lock between the two femtosecond oscillators in a master-slave configuration using a frequency shifter at the third harmonic of the pulse repetition frequency is employed. This approach permits an unprecedented time-delay resolution of better than 160 fs. High-speed ASOPS provides the functionality of an all-optical oscilloscope with a bandwidth in excess of 3000 GHz and with 1 GHz frequency resolution.
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