Explore the vast range of titles available.

AbstractA study of the optical-reflection spectra (250–2500 nm) for the surface of lead sulfide crystals in the initial state and after the formation of a homogeneous ensemble of nanostructures is conducted. Single crystals of PbS are grown using the vertical-zone-melting method, with the [100] orientation along the growth axis. Surface nanostructuring is realized in a reactor of high-density argon plasma with a low-pressure high-frequency inductive discharge (13.56 МHz) at the ion energy ~200 eV. The uniform array of stepped lead sulfide nanostructures formed due to plasma treatment is up to 140 nm in height, with cruciform bases having ❬100❭-oriented lateral orthogonal elements 20–60 nm long. It is found that the specular-reflection- and diffuse-reflection spectra for the initial surface of the (100) PbS crystals and for that nanostructured in argon plasma differ significantly. Using the Kubelka–Munk theory of diffuse reflection and the Kumar theory of specular reflection, the band-gap value for the nanostructured surface of (100) PbS crystals is determined as 3.45–3.47 eV, exceeding the value for the initial surface of lead sulfide ~0.4 eV.

Optical reflection spectra in the photon energy range of 1.5-6 eV have been studied after the growth of ultrathin iron films on silicon (001) in the 0-1.2 nm thickness range. It has been noted that the reflection coefficient values vary nonmonotonically in thickness near the limiting energy of 1.5 eV and 6 eV. Moreover, they are abnormally large one at 1.5 eV and small one at 6 eV. It is shown that these phenomena can not be explained by simple models of the formation of a metallic, silicide film or amorphous silicon layer at the interface and correspond to structural-phase transformations in the film and in the substrate interface region.

Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface, which are in excellent agreement with generalized laws derived from Fermat's principle. Phase discontinuities provide great flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer metallic interfaces.

When the Coulomb repulsion between electrons dominates over their kinetic energy, electrons in two-dimensional systems are predicted to spontaneously break continuous-translation symmetry and form a quantum crystal 1 . Efforts to observe 2 – 12 this elusive state of matter, termed a Wigner crystal, in two-dimensional extended systems have primarily focused on conductivity measurements on electrons confined to a single Landau level at high magnetic fields. Here we use optical spectroscopy to demonstrate that electrons in a monolayer semiconductor with density lower than 3 × 10 11  per centimetre squared form a Wigner crystal. The combination of a high electron effective mass and reduced dielectric screening enables us to observe electronic charge order even in the absence of a moiré potential or an external magnetic field. The interactions between a resonantly injected exciton and electrons arranged in a periodic lattice modify the exciton bandstructure so that an umklapp resonance arises in the optical reflection spectrum, heralding the presence of charge order 13 . Our findings demonstrate that charge-tunable transition metal dichalcogenide monolayers 14 enable the investigation of previously uncharted territory for many-body physics where interaction energy dominates over kinetic energy. The signature of a Wigner crystal—the analogue of a solid phase for electrons—is observed via the optical reflection spectrum in a monolayer transition metal dichalcogenide.

The CHIRON optical high-resolution echelle spectrometer was commissioned at the 1.5 m telescope at CTIO in 2011. The instrument was designed for high throughput and stability, with the goal of monitoring radial velocities of bright stars with high precision and high cadence for the discovery of low-mass exoplanets. Spectral resolution of R = 79 000 is attained when using a slicer with a total (including telescope and detector) efficiency of 6% or higher, while a resolution of R = 136 000 is available for bright stars. A fixed spectral range of 415-880 nm is covered. The echelle grating is housed in a vacuum enclosure and the instrument temperature is stabilized to ± 0.2°. Stable illumination is provided by an octagonal multimode fiber with excellent light-scrambling properties. An iodine cell is used for wavelength calibration. We describe the main optics, fiber feed, detector, exposure-meter, and other aspects of the instrument, as well as the observing procedure and data reduction.

One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional \"striped\" spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La₁.₆₇₅Eu₀.₂Sr₀.₁₂₅CuO₄, into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.

Biological communication by means of structural color has existed for at least 500 million years. Structural color is commonly observed in the animal kingdom, but has been little studied in plants. We present a striking example of multilayer-based strong iridescent coloration in plants, in the fruit of Pollia condensata . The color is caused by Bragg reflection of helicoidally stacked cellulose microfibrils that form multilayers in the cell walls of the epicarp. We demonstrate that animals and plants have convergently evolved multilayer-based photonic structures to generate colors using entirely distinct materials. The bright blue coloration of this fruit is more intense than that of any previously described biological material. Uniquely in nature, the reflected color differs from cell to cell, as the layer thicknesses in the multilayer stack vary, giving the fruit a striking pixelated or pointillist appearance. Because the multilayers form with both helicoidicities, optical characterization reveals that the reflected light from every epidermal cell is polarized circularly either to the left or to the right, a feature that has never previously been observed in a single tissue.

Two-dimensional graphene monolayers and bilayers exhibit fascinating electrical transport behaviors. Using infrared spectroscopy, we find that they also have strong interband transitions and that their optical transitions can be substantially modified through electrical gating, much like electrical transport in field-effect transistors. This gate dependence of interband transitions adds a valuable dimension for optically probing graphene band structure. For a graphene monolayer, it yields directly the linear band dispersion of Dirac fermions, whereas in a bilayer, it reveals a dominating van Hove singularity arising from interlayer coupling. The strong and layer-dependent optical transitions of graphene and the tunability by simple electrical gating hold promise for new applications in infrared optics and optoelectronics.

In November and December 2012, the Hubble Space Telescope (HST) imaged Europa's ultraviolet emissions in the search for vapor plume activity. We report statistically significant coincident surpluses of hydrogen Lyman-a and oxygen OI 130.4-nanometer emissions above the southern hemisphere in December 2012. These emissions were persistently found in the same area over the 7 hours of the observation, suggesting atmospheric inhomogeneity; they are consistent with two 200-km-high plumes of water vapor with line-of-sight column densities of about 10²⁰ per square meter. Nondetection in November 2012 and in previous HST images from 1999 suggests varying plume activity that might depend on changing surface stresses based on Europa's orbital phases. The plume was present when Europa was near apocenter and was not detected close to its pericenter, in agreement with tidal modeling predictions.

Imagers that use their own illumination can capture three-dimensional (3D) structure and reflectivity information. With photon-counting detectors, images can be acquired at extremely low photon fluxes. To suppress the Poisson noise inherent in low-flux operation, such imagers typically require hundreds of detected photons per pixel for accurate range and reflectivity determination. We introduce a low-flux imaging technique, called first-photon imaging, which is a computational imager that exploits spatial correlations found in real-world scenes and the physics of low-flux measurements. Our technique recovers 3D structure and reflectivity from the first detected photon at each pixel. We demonstrate simultaneous acquisition of sub-pulse duration range and 4-bit reflectivity information in the presence of high background noise. First-photon imaging may be of considerable value to both microscopy and remote sensing.