We report on a direct experimental observation of dynamic localization (DL) of light in sinusoidallycurved Lithium-Niobate waveguide arrays which provides the optical analog of DL for electrons in periodic potentials subjected to ac electric fields as originally proposed by Dunlap and Kenkre [D.H. Dunlap and V.M. Kenkre, Phys. Rev. B 34, 3625 (1986)]. The theoretical condition for DL in a sinusoidal field is experimentally demonstrated.PACS numbers: 42.82. Et, 63.20.Pw, 42.25.Bs The quantum motion of an electron in a periodic potential subjected to an external field has provided since a long time a paradigmatic model to study fascinating and rather universal coherent dynamical phenomena. These include the long-predicted Bloch oscillations (BO) for dc fields [1], i.e. an oscillatory motion of the wave packet related to the existence of a Wannier-Stark ladder energy spectrum, and the more recently-predicted dynamic localization (DL) for ac fields [2], in which a localized particle periodically returns to its initial state following the periodic change of the field. In recent years, BO have been experimentally observed in a wide variety of systems including semiconductor superlattices [3], atoms in accelerated optical lattices [4], and optical waveguide arrays with a transverse refractive index gradient [5,6,7]. DL is a phenomenon similar to BO which occurs when the electron is subjected to an ac field. The condition for DL, as originally predicted by Dunlap and Kenkre [2] in the nearest-neighbor tight-binding (NNTB) approximation and for a sinusoidal driving field E(t) = F sin(ωt), is that J 0 (Γ) = 0, where Γ = eaF/ ω and a is the lattice period. DL has been shown to be related to the collapse of the quasienergy minibands [8], and the general conditions for DL beyond the NNTB approximation and for generalized ac fields have been identified [9]; DL under the action of both ac and dc fields has been also studied [10], and the influence of excitonic and many-body effects on DL in semiconductor superlattices has been considered (see, e

The chemical stability of buckled silicene, i.e., the silicon counterpart of graphene, is investigated then resulting in a low reactivity with O 2 when dosing up to 1000 L and in a progressive oxidation under ambient conditions. The latter drawback is addressed by engineering ad hoc Al-and Al 2 O 3 -based encapsulations of the silicene layer. This encapsulation design can be generally applied to any silicene confi guration, irrespective of the support substrate, and it leads to the fabrication of atomically sharp and chemically intact Al/silicene and Al 2 O 3 /silicene interfaces that can be functionally used for ex situ characterization as well as for gated device fabrication.

Sequential infiltration synthesis (SIS) provides a successful route to grow inorganic materials into polymeric films by penetrating of gaseous precursors into the polymer, both in order to enhance the functional properties of the polymer creating an organic-inorganic hybrid material, and to fabricate inorganic nanostructures when infiltrating in patterned polymer films or in selfassembled block copolymers. A SIS process consists in a controlled sequence of metal organic precursor and co-reactant vapor exposure cycles of the polymer films in an atomic layer deposition (ALD) reactor. In this work, we present a study of the SIS process of alumina using trimethylaluminum (TMA) and H2O in various polymer films using in situ dynamic spectroscopic ellipsometry (SE). In situ dynamic SE enables time-resolved monitoring of the swelling of the polymer, which is relevant to the diffusion and retain of the metal precursor into the polymer itself. Different swelling behaviour of poly(methylmethacrylate) (PMMA) and polystyrene (PS) was observed when exposed to TMA vapor. PMMA films swell more significantly than PS films do, resulting in very different infiltrated Al2O3 thickness after polymer removal in O2 plasma. PMMA films reach different swollen states upon TMA exposure and reaction with H2O, depending on the TMA dose and on the purge duration after TMA exposure, which correspond to different amounts of metal precursor retained inside the polymer and converted to alumina. Diffusion coefficients of TMA in PMMA were extracted investigating the swelling of pristine PMMA films during TMA infiltration and shown to be dependent on polymer molecular weight. In situ dynamic SE monitoring allows to control the SIS process tuning it from an ALD-like process for long purge to a chemical vapour deposition-like process selectively confined inside the polymer films.

The process of the formation and disruption of nanometric conductive filaments in a HfO2/TiN structure is investigated by conductive atomic force microscopy. The preforming state evidences nonhomogeneous conduction at high fields through conductive paths, which are associated with pre-existing defects and develop into conductive filaments with a forming procedure. The disruption of the same filaments is demonstrated as well, according to a bipolar operation. In addition, the conductive tip of the microscopy is exploited to perform electrical operations on single conductive spots, which evidences that neighboring conductive filaments are not electrically independent. We propose a picture that describes the evolution of the shape of the conductive filaments in the processes of their formation and disruption, which involves the development of conductive branches from a common root; this root resides in the pre-existing defects that lay at the HfO2/TiN interface.

We report on a direct visualization of coherent destruction of tunneling (CDT) of light waves in a double well system which provides an optical analog of quantum CDT as originally proposed by Grossmann, Dittrich, Jung, and Hänggi [Phys. Rev. Lett. 67, 516 (1991)]. The driven double well, realized by two periodically-curved waveguides in an Er:Yb-doped glass, is designed so that spatial light propagation exactly mimics the coherent space-time dynamics of matter waves in a driven double-well potential governed by the Schrödinger equation. The fluorescence of Er ions is exploited to image the spatial evolution of light in the two wells, clearly demonstrating suppression of light tunneling for special ratios between frequency and amplitude of the driving field.PACS numbers: 42.50. Hz, 03.65.Xp, 42.82.Et Control of quantum tunneling by external driving fields is a subject of major relevance in different areas of physics [1,2]. The driven double-well potential has provided since more than one decade a paradigmatic model to investigate tunneling control in such diverse physical systems as cold atoms in optical traps, superconducting quantum interference devices, multi-quantum dots and spin systems. Depending on the strength and frequency of the driving field, suppression [3,4] or enhancement [5] of tunneling can be achieved. Tunneling enhancement is usually observed for high field strengths and driving frequencies close to the classical oscillation frequency at the bottom of each well. Since the enhancement generally involves a transition through an intermediate state which is chaotic for strong enough driving amplitudes, it is often referred to as "chaos-assisted tunneling" [1,6]. Observations of chaos-assisted tunneling have been reported in atom optics experiments [7] and in electromagnetic analogs of quantum mechanical tunneling [8,9]. In particular, tunneling enhancement has been observed in two coupled optical waveguides [9]. In the opposite limit, Grossmann, Hänggi and coworkers [3] found that, for certain parameter ratios between amplitude and frequency of the driving, tunneling can be brought to a standstill. They termed this effect "coherent destruction of tunneling" (CDT) and, since then, it has been of continuing interest. Driven tunneling is related to the problem of periodic nonadiabatic level crossing and Landau-Zener (LZ) transitions [1,10]. In particular, in the strong modulation limit CDT may be viewed as a destructive interference effect [10]. In spite of the great amount of theoretical work devoted to CDT, to date most of experimental evidences of CDT are rather indirect. In condensed-matter systems, dephasing and many-particle effects make tunneling control more involved [11]. In Ref.[12] coherent control of Rabi oscillations in Josephson-junction circuits irradiated by microwaves has been reported, however the condition for CDT was not reached. Quantum interference effects and evidences of CDT in qubit systems have been recently reported in [13,14], whereas suppression of quantum diffusion, als...