Ultrafast electron dynamics in solids under strong optical fields has recently found particular attention [1][2][3][4][5][6][7][8][9] .In dielectrics and semiconductors, various light-field-driven effects have been explored, such as high-harmonic generation 1-4 , sub-optical-cycle interband population transfer 5,6 and nonperturbative increase of transient polarizability 7 . In contrast, much less is known about field-driven electron dynamics in metals because charge carriers screen an external electric field in ordinary metals 7,8,10 . Here we show that atomically thin monolayer Graphene offers unique opportunities to study light-field-driven processes in a metal. With a comparably modest field strength of up to 0.3 V/Å, we drive combined interband and intraband electron dynamics, leading to a light-fieldwaveform controlled residual conduction current after the laser pulse is gone. We identify the underlying pivotal physical mechanism as electron quantum-path interference taking place on the 1-femtosecond (10 −15 second) timescale. The process can be categorized as Landau-ZenerStückelberg interferometry 11 . These fully coherent electron dynamics in graphene take place on a hitherto unexplored timescale faster than electron-electron scattering (tens of femtoseconds) and electron-phonon scattering (hundreds of femtoseconds) 12-15 . These results broaden the scope of light-field control of electrons in solids to an entirely new and eminently important material class -metals -promising wide ramifications for band structure tomography 3,6 and light-fielddriven electronics 8 .Graphene is an ideal platform to extend the concept of light-field-driven current control to metals. Even though the metallic nature of graphene is reflected in its excellent carrier mobilities 16,17 , the carrier concentration is low compared with conventional metals and thus screening due to free carriers is negligible at optical frequencies 18 . Therefore, strong optical fields can be generated in graphene. In addition, graphene, in particular epitaxial graphene on SiC (0001), is one of the most robust materials available 17,19 , and can thus withstand high laser intensities. Moreover, the optical response of graphene is broadband and ultrafast 17 . Earlier photocurrent studies in graphene revealed that photocarriers are generated on an ultrashort timescale of tens of femtoseconds 12,13 , associated with efficient and fast carrier heating 14,15,20 . Still, the timescale of these experiments is limited by the duration of the laser pulse (envelope) because the photocarrier generation is driven by optical absorption, which is governed by the cycleaveraged light intensity.Here we show that a current induced in graphene by few-cycle laser pulses is sensitive to the electric-field waveform, i.e., the exact shape of the optical carrier field of the pulse, which is controlled by the carrier-envelope phase (Fig. 1a). As will be shown, the main mechanism of this waveform-dependent current generation is based on a large modulation of the interband coup...

Mechanisms of high-harmonic generation from crystals are described by treating the electric field of a laser as a quasistatic strong field. Under the quasistatic electric field, electrons in periodic potentials form dressed states, known as Wannier-Stark states. The energy differences between the dressed states determine the frequencies of the radiation. The radiation yield is determined by the magnitudes of the interband and intraband current matrix elements between the dressed states. The generation of attosecond pulses from solids is predicted. Ramifications for strong-field physics are discussed.

Application of coherent light–matter interactions has recently been extended to the ultrafast control of magnetization. An important but unrealized technique is the manipulation of magnetization vector motion to make it follow an arbitrarily designed multidimensional trajectory. Here we demonstrate a full manipulation of two-dimensional magnetic oscillations in antiferromagnetic NiO with a pair of polarization-twisted femtosecond laser pulses. We employ Raman-type nonlinear optical processes, wherein magnetic oscillations are impulsively induced with a controlled initial phase. Their azimuthal angle follows well-defined selection rules that have been determined by the symmetries of the materials. We emphasize that the temporal variation of the laser-pulse polarization angle enables us to control the phase and amplitude of the two degenerate modes, independently. These results lead to a new concept of the vectorial control of magnetization by light.

The discrete rotational symmetry of nanostructures provides a powerful and simple guiding principle for designing the second-harmonic generation process in nonlinear metamaterials. We demonstrate that, in achiral nanostructures with threefold rotational symmetries, a circularly polarized fundamental beam produces a countercircularly polarized second-harmonic beam. In this case, the polarization state of the second harmonic is determined in a very simple manner. We also demonstrate how rotational symmetries in nonlinear metamaterials manifest themselves in SHG selection rules.

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An all-solid-state redox device composed of Fe3O4 thin film and Li(+) ion conducting solid electrolyte was fabricated for use in tuning magnetization and magnetoresistance (MR), which are key factors in the creation of high-density magnetic storage devices. Electrical conductivity, magnetization, and MR were reversibly tuned by Li(+) insertion and removal. Tuning of the various Fe3O4 thin film properties was achieved by donation of an electron to the Fe(3+) ions. This technique should lead to the development of spintronics devices based on the reversible switching of magnetization and spin polarization (P). It should also improve the performance of conventional magnetic random access memory (MRAM) devices in which the ON/OFF ratio has been limited to a small value due to a decrease in P near the tunnel barrier.

The electronic structure of p-type SrTiO 3 has been studied by photoemission spectroscopy ͑PES͒. Comparing with the PES of n-type SrTiO 3 , the Fermi level of p type is lower by about 0.7 eV and a prominent feature of Ti 3d character within the band gap, which is formed in the n-type SrTiO 3 , cannot be found in p-type SrTiO 3. It is suggested that the band structure of p-type SrTiO 3 follows the rigid-band model. The resonant photoemission study shows that the Ti 3d partial density of states in the valence band of p-type SrTiO 3 is much larger than that of n-type SrTiO 3. Furthermore, it is also found that the satellite intensities of several core lines in p-type SrTiO 3 are stronger than those in n-type SrTiO 3. These facts suggest that the hybridization effect between the Ti 3d and O 2p states becomes stronger in p-type SrTiO 3. ͓S0163-1829͑98͒03312-8͔

An all-solid-state electric-double-layer transistor (EDLT) with a Y-stabilized ZrO₂ (YSZ) proton conductor/SrTiO₃ (STO) single crystal has been fabricated to investigate ionic conductivity effect on the response speed, which should be a key parameter for development of next-generation EDLTs. The drain current exhibited a 4-order-of-magnitude increment by electrostatic carrier doping at the YSZ/STO interface due to ion migration, and the behavior strongly depended on the operation temperature. An Arrhenius-type plot of the ionic conductivity (σ(i)) in the YSZ and t(c)⁻¹, which is a current-rise time needed for charge accumulation at the YSZ/STO interface, shows a synchronized variation, indicating a proportional relationship between the two parameters. Analysis of the σ(i)-t(c) diagram shows that, in contrast to conventional EDLTs, the response speed should reach picosecond order at room temperature by using extreme miniaturization and superionic conductors. Furthermore, the diagram indicates that plenty of solid electrolytes, which have not been used due to the lack of criteria for evaluation, can be a candidate for all-solid-state EDLTs exceeding the carrier density of conventional EDLTs, even though the response speed becomes comparably lower than those of FETs.