Reconfiguring structured light beams using nonlinear metasurfaces.

Ultra-compact, low-loss, fast, and reconfigurable optical components, enabling manipulation of light by light, could open numerous opportunities for controlling light on the nanoscale. Nanostructured all-dielectric metasurfaces have been shown to enable extensive control of amplitude and phase of light in the linear optical regime. Among other functionalities, they offer unique opportunities for shaping the wave front of light to introduce the orbital angular momentum (OAM) to a beam. Such structured light beams bring a new degree of freedom for applications ranging from spectroscopy and micromanipulation to classical and quantum optical communications. To date, reconfigurability or tuning of the optical properties of all-dielectric metasurfaces have been achieved mechanically, thermally, electrically or optically, using phase-change or nonlinear optical materials. However, a majority of demonstrated tuning approaches are either slow or require high optical powers. Arsenic trisulfide (As2S3) chalcogenide glass offering ultra-fast and large χ(3)nonlinearity as well as a low two-photon absorption coefficient in the near and mid-wave infrared spectral range, could provide a new platform for the realization of fast and relatively low intensity reconfigurable metasurfaces. Here, we design and experimentally demonstrate an As2S3 chalcogenide glass based metasurface that enables reshaping of a conventional Hermite-Gaussian beam with no OAM into an OAM beam at low intensity levels, while preserves the original beam's amplitude and phase characteristics at high intensity levels. The proposed metasurface could find applications for a new generation of optical communication systems and optical signal processing.

High-Efficiency All-Dielectric Metasurfaces for Ultracompact Beam Manipulation in Transmission Mode.

Metasurfaces are two-dimensional structures enabling complete control on light amplitude, phase, and polarization. Unlike plasmonic metasurfaces, silicon structures facilitate high transmission, low losses, and compatibility with existing semiconductor technologies. We experimentally demonstrate two examples of high-efficiency polarization-sensitive dielectric metasurfaces with 2π phase control in transmission mode (45% transmission efficiency for the vortex converter and 36% transmission efficiency for the beam steering device) at telecommunication wavelengths. Silicon metasurfaces are poised to enable a versatile platform for the realization of all-optical circuitry on a chip.

Confinement-guided shaping of semiconductor nanowires and nanoribbons: "writing with nanowires".

To fully exploit their full potential, new semiconductor nanowire building blocks with ab initio controlled shapes are desired. However, and despite the great synthetic advances achieved, the ability to control nanowire's geometry has been significantly limited. Here, we demonstrate a simple confinement-guided nanowire growth method that enables to predesign not only the chemical and physical attributes of the synthesized nanowires but also allows a perfect and unlimited control over their geometry. Our method allows the synthesis of semiconductor nanowires in a wide variety of two-dimensional shapes such as any kinked (different turning angles), sinusoidal, linear, and spiral shapes, so that practically any desired geometry can be defined. The shape-controlled nanowires can be grown on almost any substrate such as silicon wafer, quartz and glass slides, and even on plastic substrates (e.g., Kapton HN).

Knocking down highly-ordered large-scale nanowire arrays.

The large-scale assembly of nanowire elements with controlled and uniform orientation and density at spatially well-defined locations on solid substrates presents one of the most significant challenges facing their integration in real-world electronic applications. Here, we present the universal "knocking-down" approach, based on the controlled in-place planarization of nanowire elements, for the formation of large-scale ordered nanowire arrays. The controlled planarization of the nanowires is achieved by the use of an appropriate elastomer-covered rigid-roller device. After being knocked down, each nanowire in the array can be easily addressed electrically, by a simple single photolithographic step, to yield a large number of nanoelectrical devices with an unprecedented high-fidelity rate. The approach allows controlling, in only two simple steps, all possible array parameters, that is, nanowire dimensions, chemical composition, orientation, and density. The resulting knocked-down arrays can be further used for the creation of massive nanoelectronic-device arrays. More than million devices were already fabricated with yields over 98% on substrate areas of up, but not limited to, to 10 cm(2).

High-yield fabrication of molecular vertical junctions.

High-yield fabrication and characterization of a ferrocene-based molecular device is reported. This device, fabricated with the use of modified nanocavity architecture shows very high yield, thus providing a template for exploring the transport properties of molecular junctions. The ferrocene-based devices show multiple negative differential peaks with high reproducibility and temperature stability. We use the multiple arrays to investigate the top-contact effect on the NDR signal at different locations in the wafer.

Robust topologically protected transport in photonic crystals at telecommunication wavelengths.

Photonic topological insulators offer the possibility to eliminate backscattering losses and improve the efficiency of optical communication systems. Despite considerable efforts, a direct experimental demonstration of theoretically predicted robust, lossless energy transport in topological insulators operating at near-infrared frequencies is still missing. Here, we combine the properties of a planar silicon photonic crystal and the concept of topological protection to design, fabricate and characterize an optical topological insulator that exhibits the valley Hall effect. We show that the transmittances are the same for light propagation along a straight topological interface and one with four sharp turns. This result quantitatively demonstrates the suppression of backscattering due to the non-trivial topology of the structure. The photonic-crystal-based approach offers significant advantages compared with other realizations of photonic topological insulators, such as lower propagation losses, the presence of a band gap for light propagating in the crystal-slab plane, a larger operating bandwidth, a much smaller footprint, compatibility with complementary metal-oxide-semiconductor fabrication technology, and the fact that it allows for operation at telecommunications wavelengths.

Author Correction: Robust topologically protected transport in photonic crystals at telecommunication wavelengths.

In the version of this Letter originally published, Fig. 5g in the Supplementary Information was missing the scale bar. This has now been corrected.

Experimental measurement of quality factor enhancement using slow light modes in one dimensional photonic crystal.

We experimentally investigate the effects of slow light modes within a one dimensional photonic crystal resonator. We show that the slow light mode leads to significant increase in the quality factor of the resonator. We provide a theoretical analysis explaining our experimental results. We also include the effect of disorder to simulate the fabrication imperfection. Further results regarding the properties of the one dimensional photonic crystal are discussed.

Current induced polycrystalline-to-crystalline transformation in vanadium dioxide nanowires.

Vanadium dioxide (VO2) exhibits a reversible insulator-metal phase transition that is of significant interest in energy-efficient nanoelectronic and nanophotonic devices. In these applications, crystalline materials are usually preferred for their superior electrical transport characteristics as well as spatial homogeneity and low surface roughness over the device area for reduced scattering. Here, we show applied electrical currents can induce a permanent reconfiguration of polycrystalline VO2 nanowires into crystalline nanowires, resulting in a dramatically reduced hysteresis across the phase transition and reduced resistivity. Low currents below 3 mA were sufficient to cause the local temperature in the VO2 to reach about 1780 K to activate the irreversible polycrystalline-to-crystalline transformation. The crystallinity was confirmed by electron microscopy and diffraction analyses. This simple yet localized post-processing of insulator-metal phase transition materials may enable new methods of studying and fabricating nanoscale structures and devices formed from these materials.

Real-time single-molecule imaging of quantum interference.

The observation of interference patterns in double-slit experiments with massive particles is generally regarded as the ultimate demonstration of the quantum nature of these objects. Such matter-wave interference has been observed for electrons, neutrons, atoms and molecules and, in contrast to classical physics, quantum interference can be observed when single particles arrive at the detector one by one. The build-up of such patterns in experiments with electrons has been described as the "most beautiful experiment in physics". Here, we show how a combination of nanofabrication and nano-imaging allows us to record the full two-dimensional build-up of quantum interference patterns in real time for phthalocyanine molecules and for derivatives of phthalocyanine molecules, which have masses of 514 AMU and 1,298 AMU respectively. A laser-controlled micro-evaporation source was used to produce a beam of molecules with the required intensity and coherence, and the gratings were machined in 10-nm-thick silicon nitride membranes to reduce the effect of van der Waals forces. Wide-field fluorescence microscopy detected the position of each molecule with an accuracy of 10 nm and revealed the build-up of a deterministic ensemble interference pattern from single molecules that arrived stochastically at the detector. In addition to providing this particularly clear demonstration of wave-particle duality, our approach could also be used to study larger molecules and explore the boundary between quantum and classical physics.

A complete scheme for creating predefined networks of individual carbon nanotubes.

We describe and demonstrate a method of creating arrays of patterned, individual, single-walled carbon nanotubes, including the spectroscopic mapping of the array. The process consists of creating networks of nanotubes suspended between silicon pillars, which are then transferred onto other substrates by an innovative process of direct stamping. Raman spectroscopy is used to spatially map and assign the specific properties of the suspended network prior to transfer. This method provides a simple and inexpensive means for deriving nanoscale devices utilizing individually assigned carbon nanotubes in a robust and non-surface-specific technique.