Entire Visible Wavelength Range Research Articles

A Search for Laser Emission with Megawatt Thresholds from 5600 FGKM Stars

Abstract We searched high-resolution spectra of 5600 nearby stars for emission lines that are both inconsistent with a natural origin and unresolved spatially, as would be expected from extraterrestrial optical lasers. The spectra were obtained with the Keck 10 m telescope, including light coming from within 0.5 arcsec of the star, corresponding typically to within a few to tens of astronomical units of the star, and covering nearly the entire visible wavelength range from 3640 to 7890 Å. We establish detection thresholds by injecting synthetic laser emission lines into our spectra and blindly analyzing them for detections. We compute flux density detection thresholds for all wavelengths and spectral types sampled. Our detection thresholds for the power of the lasers themselves range from 3 kW to 13 MW, independent of distance to the star but dependent on the competing “glare” of the spectral energy distribution of the star and on the wavelength of the laser light, launched from a benchmark, diffraction-limited 10 m class telescope. We found no such laser emission coming from the planetary region around any of the 5600 stars. Because they contain roughly 2000 lukewarm, Earth-size planets, we rule out models of the Milky Way in which over 0.1% of warm, Earth-size planets harbor technological civilizations that, intentionally or not, are beaming optical lasers toward us. A next-generation spectroscopic laser search will be done by the Breakthrough Listen initiative, targeting more stars, especially stellar types overlooked here including spectral types O, B, A, early F, late M, and brown dwarfs, and astrophysical exotica.

Fabrication of a transparent and super-hydrophilic window by depositing WOX nanoparticles via magnetron sputtering onto a glass

Tungsten trioxide (WOX) films were prepared on a glass substrate by depositing WOX particles using a magnetron sputtering source, which imparted a nano-scale surface roughness to the glass. The chemical and geometric structures of the films were characterized with X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The AFM analysis revealed that the WOX nanoparticles, with a lateral size of 65nm, were randomly distributed on the glass substrate at room temperature. Only a slight rearrangement of particles was observed with no increase of mean particle size after annealing at 100°C under air, while the annealing at 300°C induced aggregation of the WOX, forming larger WOX particles (∼85nm). The formation of micro-sized 2-D islands of WOX was observed upon increasing the annealing temperature to 500°C. The adsorption capabilities of these WOX/glass samples toward methylene blue from an aqueous solution decreased as the annealing temperature increased, which corresponded with the changes of the geometric structure (or number of surface adsorption sites) as annealing temperature increased. Lastly, the optical transmittance and water wettability of WOX/glass samples was evaluated and it was found that the WOX films with nano-scale surface roughness provided super-hydrophilic functionality to the glass substrate while maintaining good transparency over the entire visible light wavelength range. It is suggested that the WOX/glass could be used in fog resistant windows.

Controlling color emission of InGaN/AlGaN nanowire light-emitting diodes grown by molecular beam epitaxy

The authors report on the achievement of full-color nanowire light-emitting diodes (LEDs), with the incorporation of InGaN/AlGaN nanowire heterostructures grown directly on the Si (111) substrates by molecular beam epitaxy. Multiple color emission across nearly the entire visible wavelength range can be realized by varying the In composition in the InGaN quantum dot active region. Moreover, multiple AlGaN shell layers are spontaneously formed during the growth of InGaN/AlGaN quantum dots, leading to the drastically reduced nonradiative surface recombination, and enhanced carrier injection efficiency. Such core–shell nanowire structures exhibit significantly increased carrier lifetime and massively enhanced photoluminescence intensity compared to conventional InGaN/GaN nanowire LEDs. A high color rendering index of ∼98 was recorded for white-light emitted from such phosphor-free core–shell nanowire LEDs.

Design of electro-optic photonic crystal structure for subdiffraction imaging over the entire visible wavelength range

Design of electro-optic photonic crystal structure for subdiffraction imaging over the entire visible wavelength range

White‐Light Emitters: (Al,Ga)Ox Microwire Ensembles on Si Exhibiting Luminescence over the Entire Visible Wavelength Range (Advanced Optical Materials 12/2016)

White‐Light Emitters: (Al,Ga)O<i>_x</i> Microwire Ensembles on Si Exhibiting Luminescence over the Entire Visible Wavelength Range (Advanced Optical Materials 12/2016)

Self-assembled hemispherical nanowell arrays for superhydrophobic antireflection coatings

The present study reports an inexpensive and simple bottom-up technology for fabricating superhydrophobic antireflection coatings with highly ordered hemispherical nanowell structures, which are assembled by a scalable Langmuir-Blodgett technology. The subwavelength gratings lead to a gradual change in the effective refractive index, substantially suppressing specular reflection over the entire visible wavelength range. After surface modification with fluorinated silane, the resulting nanowell arrays exhibit superhydrophobic surface with high static water contact angle (154°) and low water contact angle hysteresis (7°). The experimental results on the wettability can be qualitatively interpreted by adopting the Cassie’s dewetting model. Moreover, the effect of the nanowell shape on the antireflective and superhydrophobic properties has also been investigated in the study.

Broadband full-color monolithic InGaN light-emitting diodes by self-assembled InGaN quantum dots

We have presented broadband full-color monolithic InGaN light-emitting diodes (LEDs) by self-assembled InGaN quantum dots (QDs) using metal organic chemical vapor deposition (MOCVD). The electroluminescence spectra of the InGaN QDs LEDs are extremely broad span from 410 nm to 720 nm with a line-width of 164 nm, covering entire visible wavelength range. A color temperature of 3370 K and a color rendering index of 69.3 have been achieved. Temperature-dependent photoluminescence measurements reveal a strong carriers localization effect of the InGaN QDs layer by obvious blue-shift of emission peak from 50 K to 300 K. The broadband luminescence spectrum is believed to be attributed to the injected carriers captured by the different localized states of InGaN QDs with various sizes, shapes and indium compositions, leading to a full visible color emission. The successful realization of our broadband InGaN QDs LEDs provide a convenient and practical method for the fabrication of GaN-based monolithic full-color LEDs in wafer scale.

(Al,Ga)Ox Microwire Ensembles on Si Exhibiting Luminescence over the Entire Visible Wavelength Range

(Al,Ga)O<i>_x</i> Microwire Ensembles on Si Exhibiting Luminescence over the Entire Visible Wavelength Range

Lithography-free wide-angle antireflective self-cleaning silicon nanocones.

Low-cost, wideband, and wide-angle antireflective layers are of prime importance to photovoltaic and other optoelectronic applications. We report a novel fabrication methodology of random textured silicon nanocones (SiNCs) array through metal-assisted chemical etching combined with oxidation. The optical properties of the fabricated structure are studied theoretically and experimentally. The random textured SiNCs array showed very promising broadband antireflective properties through the entire visible wavelength range at different incident angles up to ±60°. In addition, the nanostructures inherently could become self-cleaning due to the high contact angle. This random cheap textured SiNCs array increases the absorption efficiency of photodetectors and reduces its cost.

Chemically Responsive Polymer Inverse-Opal Photonic Crystal Films Created by a Self-Assembly Method

The synthesis of poly-2-hydroxyethyl methacrylate inverse-opal hydrogel (IOHGPHEMA) was realized by capillary-force-induced in situ polymerization in a polystyrene colloidal crystal template. The created IOHGPHEMA films show brilliant blue-violet color when they are immersed in deionized water and reach swelling equilibrium. The stop band of the IOHGPHEMA films can be tuned within the entire visible wavelength range by immersing them into different chemical solutions, such as aldehydes, ketones, amides, dimethyl sulfoxide, and alcohols. The extent of the reflective peak shift not only depends on the number of hydrogen band donors but also on the chain length and structure of the chemicals and their concentration. Since the IOHGPHEMA films have different reflectance spectra and structural colors in response to different compounds of the same series, this provides a potential way to visually detect homologues and other compounds with similar structure and properties. This simple, yet effective, method also ...

The IAG solar flux atlas: Accurate wavelengths and absolute convective blueshift in standard solar spectra

We present a new solar flux atlas with the aim to understand wavelength precision and accuracy in solar benchmark data. The atlas covers the wavelength range 405--2300 nm and was observed at the Institut f\"ur Astrophysik, G\"ottingen (IAG) with a Fourier Transform Spectrograph. In contrast to other FTS atlases, the entire visible wavelength range was observed simultaneously using only one spectrograph setting. We compare the wavelength solution of the new atlas to the Kitt Peak solar flux atlases and to the HARPS frequency-comb calibrated solar atlas. Comparison reveals systematics in the two Kitt Peak FTS atlases resulting from their wavelength scale construction, and shows consistency between the IAG and the HARPS atlas. We conclude that the IAG atlas is precise and accurate on the order of $\pm 10$ m s$^{-1}$ in the wavelength range 405--1065 nm while the Kitt Peak atlases show deviations as large as several ten to 100 m s$^{-1}$. We determine absolute convective blueshift across the spectrum from the IAG atlas and report slight differences relative to results from the Kitt Peak atlas that we attribute to the differences between wavelength scales. We conclude that benchmark solar data with accurate wavelength solution are crucial to better understand the effect of convection on stellar RV measurements, which is one of the main limitations of Doppler spectroscopy at m s$^{-1}$ precision.

Organic–inorganic hybrid inverted photodiode with planar heterojunction for achieving low dark current and high detectivity

In this study, the strategy of using an organic–inorganic hybrid planar heterojunction consisting of polymeric semiconductors and inorganic nanocrystals is introduced to realize a high-performance hybrid photodiode (HPD) with low dark current and high detectivity. To prevent undesired charge injection under the reverse bias condition, which is the major dark current source of the photodiode, a well-defined planar heterojunction is strategically constructed via smart solution process techniques. The optimized HPD renders a low dark current of ∼10−5 mA cm−2 at −5 V and ∼10−6 mA cm−2 at −1 V, as well as a high detectivity ∼1012 Jones across the entire visible wavelength range. Furthermore, excellent photocurrent stability is demonstrated under continuous light exposure. We believe that the solution-processed planar heterojunction with inverted structure can be an attractive alternative diode structure for fabricating high-performance HPDs, which usually suffer from high dark current issues.

Two-dimensional metal-dielectric hybrid-structured film with titanium oxide for enhanced visible light absorption and photo-catalytic application

Two-dimensional design based on ultrathin TiO2 film for enhanced visible light absorption and photo-catalytic applications is reported, which mainly consists of three layers of gold film, TiO2 film, and gold nanoparticles (Au NPs). The Au and TiO2 films are produced by e-beam evaporation and atomic layer deposition, respectively, in a carefully controlled way to minimize surface roughness. As compared with bare TiO2 film, the Au NPs/TiO2/Au film significantly increased the photoactivity over the entire UV and visible wavelength range. The Au film increases the light absorption in the UV region with TiO2 acting as an impedance-matching layer, while the Au NPs increase the light absorption in the visible region due to the plasmonic resonance effects, increasing the photocurrent under visible light. 3D numerical simulation results suggest that the Au film also plays an important role in enhancing the electrical field intensity at the TiO2 film in contact with Au NPs, by efficient excitation of localized surface plasmon resonances, thereby, contributing to the enhanced photoactivity of the film in the visible range. This simple system may serve as an efficient platform for solar energy conversion utilizing the whole UV-visible range of solar spectrum based on two-dimensional plasmonic photoelectrodes.

Visible-wavelength two-photon excitation microscopy for fluorescent protein imaging

The simultaneous observation of multiple fluorescent proteins (FPs) by optical microscopy is revealing mechanisms by which proteins and organelles control a variety of cellular functions. Here we show the use of visible-light based two-photon excitation for simultaneously imaging multiple FPs. We demonstrated that multiple fluorescent targets can be concurrently excited by the absorption of two photons from the visible wavelength range and can be applied in multicolor fluorescence imaging. The technique also allows simultaneous single-photon excitation to offer simultaneous excitation of FPs across the entire range of visible wavelengths from a single excitation source. The calculation of point spread functions shows that the visible-wavelength two-photon excitation provides the fundamental improvement of spatial resolution compared to conventional confocal microscopy.

The Pillars of Creation revisited with MUSE: gas kinematics and high-mass stellar feedback traced by optical spectroscopy

Integral field unit (IFU) data of the iconic Pillars of Creation in M16 are presented. The ionisation structure of the pillars was studied in great detail over almost the entire visible wavelength range, and maps of the relevant physical parameters, e.g. extinction, electron density, electron temperature, line-of-sight velocity of the ionised and neutral gas are shown. In agreement with previous authors, we find that the pillar tips are being ionised and photo-evaporated by the massive members of the nearby cluster NGC 6611. They display a stratified ionisation structure where the emission lines peak in a descending order according to their ionisation energies. The IFU data allowed us to analyse the kinematics of the photo-evaporative flow in terms of the stratified ionisation structure, and we find that, in agreement with simulations, the photo-evaporative flow is traced by a blueshift in the position-velocity profile. The gas kinematics and ionisation structure have allowed us to produce a sketch of the 3D geometry of the Pillars, positioning the pillars with respect to the ionising cluster stars. We use a novel method to detect a previously unknown bipolar outflow at the tip of the middle pillar and suggest that it has an embedded protostar as its driving source. Furthermore we identify a candidate outflow in the leftmost pillar. With the derived physical parameters and ionic abundances, we estimate a mass loss rate due to the photo-evaporative flow of 70 M$_{\odot}$ Myr$^{-1}$ which yields an expected lifetime of approximately 3 Myr.

Shape-dependent light scattering properties of subwavelength silicon nanoblocks.

We explore the shape-dependent light scattering properties of silicon (Si) nanoblocks and their physical origin. These high-refractive-index nanostructures are easily fabricated using planar fabrication technologies and support strong, leaky-mode resonances that enable light manipulation beyond the optical diffraction limit. Dark-field microscopy and a numerical modal analysis show that the nanoblocks can be viewed as truncated Si waveguides, and the waveguide dispersion strongly controls the resonant properties. This explains why the lowest-order transverse magnetic (TM01) mode resonance can be widely tuned over the entire visible wavelength range depending on the nanoblock length, whereas the wavelength-scale TM11 mode resonance does not change greatly. For sufficiently short lengths, the TM01 and TM11 modes can be made to spectrally overlap, and a substantial scattering efficiency, which is defined as the ratio of the scattering cross section to the physical cross section of the nanoblock, of ∼9.95, approaching the theoretical lowest-order single-channel scattering limit, is achievable. Control over the subwavelength-scale leaky-mode resonance allows Si nanoblocks to generate vivid structural color, manipulate forward and backward scattering, and act as excellent photonic artificial atoms for metasurfaces.

Nematic liquid crystal nanocomposite with scattering-free, microsecond electro-optic response

We report a microsecond electro-optic response in an anisotropic-polymer/liquid-crystal composite, which forms a homogeneously mixed structure in the nanoscale range owing to the high miscibility between them. The nanocomposite was fabricated by photopolymerizing a nematic liquid crystal (NLC) mixture doped with a cross-linkable mesogenic monomer at a concentration of 30 wt%. Our system is inherently different from polymer-dispersed liquid crystals in that the LC molecules are almost miscible in the anisotropic polymer matrix and do not form observable domains. When an electric field is applied to such a nanocomposite, the molecular alignment of the polymer matrix is retained, while the non-polymerizable NLC reorients along the electric field, leading to a shift in the birefringence. Furthermore, the reorientation of the NLC molecules in a space sufficiently smaller than the wavelength of visible light results in scattering-free characteristics over the entire visible wavelength range and a short decay response time of 15 μs.

Tunable hyperchromatic lens system for confocal hyperspectral sensing

A new approach for confocal hyperspectral sensing based on the combination of a diffractive optical element and a tunable membrane fluidic lens is demonstrated. This highly compact lens system is designed to maximize the longitudinal chromatic aberration and select a narrow spectral band by spatial filtering. Changing the curvature of the fluidic lens allows the selected band to be scanned over the whole given spectrum. A hybrid prototype with an integrated electro-magnetic micro-actuator has been realized to demonstrate the functionality of the system. Experimental results show that the spectrum transmitted by the system can be tuned over the entire visible wavelength range, from 450 to 900 nm with a narrow and almost constant linewidth of less than 15 nm. Typical response time for scanning the spectrum by 310 nm is less than 40 ms and the lens system shows a highly linear relationship with the driving current.

Efficient absorption of visible radiation by gap plasmon resonators

We demonstrate experimentally a periodic array of differently-sized and circularly-shaped gap plasmon resonators (GPRs) with the average absorption ~94% for unpolarized light in the entire visible wavelength range (400-750 nm). Finite-element simulations verify that the polarization insensitive broadband absorption originates from localized gap surface plasmons whose resonant excitations only weakly depend on the angle of incidence. Arrays of GPRs also exhibit enhanced local field intensities (~115) as revealed by scanning two-photon photoluminescence microscopy, that are spectrally correlated with the minima in corresponding linear reflection spectra.

Complementary cellophane optic gate and its use for a 3D iPad without glasses

A complementary cellophane optic gate was fabricated using a birefringent cellophane sheet. Previous versions of the optic gate required the retardance of the cellophane to be as close to 180° as possible throughout the entire visible wavelength range, which meant it was often difficult to find a cellophane sheet with the right thickness and dispersion characteristics to meet this requirement. The complementary optic gate reported in this paper has no restriction on the thickness, composition, or wavelength range of the cellophane sheet except that the cellophane must have some birefringence. Even with an arbitrary retardance, an extinction ratio of 5 × 10(-3) was achieved at λ = 0.63 μm. The optic gate was used to convert an iPad into a 3D display without the need for the observer to wear glasses. The high extinction ratio of the optic gate resulted in a 3D display of supreme quality.