Bulk plasmon‐polaritons in hyperbolic nanorod metamaterial waveguides

Hyperbolic metamaterials comprised of an array of plasmonic nanorods provide a unique platform for designing optical sensors and integrating nonlinear and active nanophotonic functionalities. In this work, the waveguiding properties and mode structure of planar anisotropic metamaterial waveguides are characterized experimentally and theoretically. While ordinary modes are the typical guided modes of the highly anisotropic waveguides, extraordinary modes, below the effective plasma frequency, exist in a hyperbolic metamaterial slab in the form of bulk plasmon‐polaritons, in analogy to planar‐cavity exciton‐polaritons in semiconductors. They may have very low or negative group velocity with high effective refractive indices (up to 10) and have an unusual cut‐off from the high‐frequency side, providing deep‐subwavelength (λ₀/6–λ₀/8 waveguide thickness) single‐mode guiding. These properties, dictated by the hyperbolic anisotropy of the metamaterial, may be tuned by altering the geometrical parameters of the nanorod composite.

     

Enhancement of single‑photon emission from nitrogen‑vacancy centers with TiN/(Al,Sc)N hyperbolic metamaterial

The broadband enhancement of single‑photon emission from nitrogen‐vacancy centers in nanodiamonds coupled to a planar multilayer metamaterial with hyperbolic dispersion is studied experimentally. The metamaterial is fabricated as an epitaxial metal/dielectric superlattice consisting of CMOS‐compatible ceramics: titanium nitride (TiN) and aluminum scandium nitride (Al_xSc_1‐xN). It is demonstrated that employing the metamaterial results in significant enhancement of collected single‑photon emission and reduction of the excited‐state lifetime. Our results could have an impact on future CMOS‐compatible integrated quantum sources.

     

Broadband and high‐efficiency conversion from guided waves to spoof surface plasmon polaritons

The conversion from spatial propagating waves to surface plasmon polaritons (SPPs) has been well studied, and shown to be very efficient by using gradient‐index metasurfaces. However, feeding energies into and extracting signals from functional plasmonic devices or circuits through transmission lines require the efficient conversion between SPPs and guided waves, which has not been reported, to the best of our knowledge. In this paper, a smooth bridge between the conventional coplanar waveguide (CPW) with 50 Ω impedance and plasmonic waveguide (e.g., an ultrathin corrugated metallic strip) has been proposed in the microwave frequency, which converts the guided waves to spoof SPPs with high efficiency in broadband. A matching transition has been proposed and designed, which is constructed by gradient corrugations and flaring ground, to match both the momentum and impedance of CPW and the plasmonic waveguide. Simulated and measured results on the transmission coefficients and near‐filed distributions show excellent transmission efficiency from CPW to a plasmonic waveguide to CPW in a wide frequency band. The high‐efficiency and broadband conversion between SPPs and guided waves opens up a new avenue for advanced conventional plasmonic integrated functional devices and circuits.     

Waveguide sub‐wavelength structures: a review of principles and applications

Periodic structures with a sub‐wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high‐precision lithography techniques that sub‐wavelength structures have found widespread application in the field of photonics. This review first provides an introduction to the physics of sub‐wavelength structures. An overview of the applications of sub‐wavelength structures is then given including: anti‐reflective coatings, polarization rotators, high‐efficiency fiber–chip couplers, spectrometers, high‐reflectivity mirrors, athermal waveguides, multimode interference couplers, and dispersion engineered, ultra‐broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub‐wavelength structures and their impact in photonics.

     

ZnCdO/ZnO – a new heterosystem for green‐wavelength semiconductor lasing

We report on our efforts to cultivate the ternary compound ZnCdO as a semiconductor laser material. Molecular beam epitaxy far from thermal equilibrium allows us to overcome the standard solubility limit and to fabricate alloys with band gaps ranging from 3.4 down to 2.1 eV. Optimized structures containing well‐defined quantum wells as active zones are capable of low‐threshold lasing under optical pumping up to room temperature. The longest lasing wavelength achieved so far is 510 nm.     

Retrieval of the atomic displacements in the crystal from the coherent X‐ray diffraction pattern

The retrieval of spatially resolved atomic displacements is investigated via the phases of the direct(real)‐space image reconstructed from the strained crystal's coherent X‐ray diffraction pattern. It is demonstrated that limiting the spatial variation of the first‐ and second‐order spatial displacement derivatives improves convergence of the iterative phase‐retrieval algorithm for displacements reconstructions to the true solution. This approach is exploited to retrieve the displacement in a periodic array of silicon lines isolated by silicon dioxide filled trenches.     

X‐ray third‐order nonlinear plane‐wave Bragg‐case dynamical diffraction effects in a perfect crystal

Two‐wave symmetric Bragg‐case dynamical diffraction of a plane X‐ray wave in a crystal with third‐order nonlinear response to the electric field is considered theoretically. For certain diffraction conditions for a non‐absorbing perfect semi‐infinite crystal in the total reflection region an analytical solution is found. For the width and for the center of the total reflection region expressions on the intensity of the incidence wave are established. It is shown that in the nonlinear case the total reflection region exists below a maximal intensity of the incidence wave. With increasing intensity of the incidence wave the total reflection region's center moves to low angles and the width decreases. Using numerical calculations for an absorbing semi‐infinite crystal, the behavior of the reflected wave as a function of the intensity of the incidence wave and of the deviation parameter from the Bragg condition is analyzed. The results of numerical calculations are compared with the obtained analytical solution.     

Utilizing broadband X‐rays in a Bragg coherent X‐ray diffraction imaging experiment

A method is presented to simplify Bragg coherent X‐ray diffraction imaging studies of complex heterogeneous crystalline materials with a two‐stage screening/imaging process that utilizes polychromatic and monochromatic coherent X‐rays and is compatible with in situ sample environments. Coherent white‐beam diffraction is used to identify an individual crystal particle or grain that displays desired properties within a larger population. A three‐dimensional reciprocal‐space map suitable for diffraction imaging is then measured for the Bragg peak of interest using a monochromatic beam energy scan that requires no sample motion, thus simplifying in situ chamber design. This approach was demonstrated with Au nanoparticles and will enable, for example, individual grains in a polycrystalline material of specific orientation to be selected, then imaged in three dimensions while under load.     

Formation of Calcium Carbonate Sub‐Micron Particles in a High Shear Stress Three‐Phase Reactor

The production of precipitated calcium carbonate (PCC) was investigated experimentally under industrially relevant conditions, i.e. at high solid concentrations and increasing amount of solid product in the slurry. Temperature is an important parameter since it determines the crystal structure, the particle shape and, as a consequence, the viscosity of the slurry. Of course, the mass concentration of the raw material also has an important influence on the viscosity. From the particle size distributions of primary particles and agglomerates, it can be concluded that the nucleation process is governed by primary nucleation. Also, heterogeneous nucleation occurs on solid calcium hydroxide particles that are present in the slurry. Especially if the raw material contains impurities heterogeneous nucleation occurs and large and unwanted particles are formed. If the slurry is not stabilized, strong agglomeration occurs that can be influenced by the shear stress introduced to the slurry: a high shear stress which is linked to the viscosity of the slurry limits the upper particle diameter and leads to a steep particle size distribution of the product.     

Spatial structure of a focused X‐ray beam diffracted from crystals

The spatial structure of a beam focused by a planar refractive lens and Bragg diffracted from perfect silicon crystals was experimentally studied at the focal plane using a knife‐edge scan and a high‐resolution CCD camera. The use of refractive lenses allowed for a detailed comparison with theory. It was shown that diffraction leads to broadening of the focused beam owing to the extinction effect and, for a sufficiently thin crystal, to the appearance of a second peak owing to reflection from the back surface. It was found that the spatial structure of the diffracted beam depends on whether the crystal diffracts strongly (dynamically) or weakly (kinematically). The results help to understand the physical origin of the diffracted intensity recorded in a typical microbeam diffraction experiment.     

High‐energy transmission Laue micro‐beam X‐ray diffraction: a probe for intra‐granular lattice orientation and elastic strain in thicker samples

An understanding of the mechanical response of modern engineering alloys to complex loading conditions is essential for the design of load‐bearing components in high‐performance safety‐critical aerospace applications. A detailed knowledge of how material behaviour is modified by fatigue and the ability to predict failure reliably are vital for enhanced component performance. Unlike macroscopic bulk properties (e.g. stiffness, yield stress, etc.) that depend on the average behaviour of many grains, material failure is governed by `weakest link'‐type mechanisms. It is strongly dependent on the anisotropic single‐crystal elastic–plastic behaviour, local morphology and microstructure, and grain‐to‐grain interactions. For the development and validation of models that capture these complex phenomena, the ability to probe deformation behaviour at the micro‐scale is key. The diffraction of highly penetrating synchrotron X‐rays is well suited to this purpose and micro‐beam Laue diffraction is a particularly powerful tool that has emerged in recent years. Typically it uses photon energies of 5–25 keV, limiting penetration into the material, so that only thin samples or near‐surface regions can be studied. In this paper the development of high‐energy transmission Laue (HETL) micro‐beam X‐ray diffraction is described, extending the micro‐beam Laue technique to significantly higher photon energies (50–150 keV). It allows the probing of thicker sample sections, with the potential for grain‐level characterization of real engineering components. The new HETL technique is used to study the deformation behaviour of individual grains in a large‐grained polycrystalline nickel sample during in situ tensile loading. Refinement of the Laue diffraction patterns yields lattice orientations and qualitative information about elastic strains. After deformation, bands of high lattice misorientation can be identified in the sample. Orientation spread within individual scattering volumes is studied using a pattern‐matching approach. The results highlight the inability of a simple Schmid‐factor model to capture the behaviour of individual grains and illustrate the need for complementary mechanical modelling.     

Design and development of a controlled pressure/temperature set‐up for in situ studies of solid–gas processes and reactions in a synchrotron X‐ray powder diffraction station

A novel set‐up has been designed and used for synchrotron radiation X‐ray high‐resolution powder diffraction (SR‐HRPD) in transmission geometry (spinning capillary) for in situ solid–gas reactions and processes in an isobaric and isothermal environment. The pressure and temperature of the sample are controlled from 10^?3 to 1000 mbar and from 80 to 1000 K, respectively. To test the capacities of this novel experimental set‐up, structure deformation in the porous material zeolitic imidazole framework (ZIF‐8) by gas adsorption at cryogenic temperature has been studied under isothermal and isobaric conditions. Direct structure deformations by the adsorption of Ar and N₂ gases have been observed in situ, demonstrating that this set‐up is perfectly suitable for direct structural analysis under in operando conditions. The presented results prove the feasibility of this novel experimental station for the characterization in real time of solid–gas reactions and other solid–gas processes by SR‐HRPD.     

The role of the dielectric environment in surface‐enhanced Raman scattering on the detection of a 4‐nitrothiophenol monolayer

Nanotechnology enables the generation and characterization of novel surface‐enhanced Raman scattering (SERS) substrates. In this study, we focus on the impact of the carrier material of the SERS active layer and hence the dielectric environment to the enhancement. Therefore, a self‐assembled monolayer of 4‐nitrothiophenol is immobilized on silver and gold particles substrates on a quartz carrier. The detection of the monolayer occurs through the quartz carrier and through air. For the former, an increase of the intensity of the SERS bands in the spectrum is observed compared to the latter. The magnitude of the increase is larger for gold than for silver. Calculations according to the theoretical model of the electromagnetic enhancement agree with our experimental data. The presented detection mode will stimulate the fabrication of novel SERS sensors. Copyright © 2013 John Wiley & Sons, Ltd.     

Radiation damage to protein specimens from electron beam imaging and diffraction: a mini‐review of anti‐damage approaches,with special reference to synchrotron X‐ray crystallography

Recent research progress using X‐ray cryo‐crystallography with the photon beams from third‐generation synchrotron sources has resulted in recognition that this intense radiation commonly damages protein samples even when they are held at 100 K. Other structural biologists examining thin protein crystals or single particle specimens encounter similar radiation damage problems during electron diffraction and imaging, but have developed some effective countermeasures. The aim of this concise review is to examine whether analogous approaches can be utilized to alleviate the X‐ray radiation damage problem in synchrotron macromolecular crystallography. The critical discussion of this question is preceded by presentation of background material on modern technical procedures with electron beam instruments using 300–400 kV accelerating voltage, low‐dose exposures for data recording, and protection of protein specimens by cryogenic cooling; these practical approaches to dealing with electron radiation damage currently permit best resolution levels of 6 Å (0.6 nm) for single particle specimens, and of 1.9 Å for two‐dimensional membrane protein crystals. Final determination of the potential effectiveness and practical value of using such new or unconventional ideas will necessitate showing, by experimental testing, that these produce significantly improved protection of three‐dimensional protein crystals during synchrotron X‐ray diffraction.     

Structural variation in a synchrotron‐induced contamination layer (a‐C:H) deposited on a toroidal Au mirror surface

A carbon layer deposited on an optical component is the result of complex interactions between the optical surface, adsorbed hydrocarbons, photons and secondary electrons (photoelectrons generated on the surface of optical elements). In the present study a synchrotron‐induced contamination layer on a 340 mm × 60 mm Au‐coated toroidal mirror has been characterized. The contamination layer showed a strong variation in structural properties from the centre of the mirror to the edge region (along the long dimension of the mirror) due to the Gaussian distribution of the incident photon beam intensity/power on the mirror surface. Raman scattering measurements were carried out at 12 equidistant (25 mm) locations along the length of the mirror. The surface contamination layer that formed on the Au surface was observed to be hydrogenated amorphous carbon film in nature. The effects of the synchrotron beam intensity/power distribution on the structural properties of the contamination layer are discussed. The I(D)/I(G) ratio, cluster size and disordering were found to increase whereas the sp²:sp³ ratio, G peak position and H content decreased with photon dose. The structural parameters of the contamination layer in the central region were estimated (thickness ? 400 Å, roughness ? 60 Å, density ? 72% of bulk graphitic carbon density) by soft X‐ray reflectivity measurements. The amorphous nature of the layer in the central region was observed by grazing‐incidence X‐ray diffraction.     

Short‐wavelength infrared defect emission as a probe of degradation processes in 980 nm single‐mode diode lasers

Infrared emission from 980‐nm single‐mode high power diode lasers is recorded and analyzed in the wavelength range from 0.8 to 8.0 μm. A pronounced short‐wavelength infrared (SWIR) emission band with a maximum at 1.3 μm originates from defect states located in the waveguide of the devices. The SWIR intensity is a measure of the non‐equilibrium carrier concentration in the waveguide, allowing for a non‐destructive waveguide mapping in spatially resolved detection schemes. The potential of this approach is demonstrated by measuring spatially resolved profiles of SWIR emission and correlating them with mid‐wavelength infrared (MWIR) thermal emission along the cavity of devices undergoing repeated catastrophic optical damage. The enhancement of SWIR emission in the damaged parts of the cavity is due to a locally enhanced carrier density in the waveguide and allows for an analysis of the spatial damage patterns. The figure shows a side view of a diode laser during catastrophic degradation as recorded by a thermocamera within 5 successive current pulses. The geometry of the device is given in grayscale. The position of the laser chip is indicated by the dotted line. The thermal signatures of the internal degradation of the diode laser are overlaid in color. The bi‐directional spread of the damage along the laser cavity is clearly visible.     

Polarization‐independent absorption enhancement in a graphene square array with a cascaded grating structure

The polarization‐independent enhanced absorption effect of graphene in the near‐infrared range is investigated. This is achieved by placing a graphene square array on top of a dielectric square array backed by a two‐dimensional multilayer grating. Total optical absorption in graphene can be attributed to critical coupling, which is achieved through the combined effect of guided‐mode resonance with the dielectric square array and the photonic band gap with the two‐dimensional multilayer grating. To reveal the physical origin of such a phenomenon, the electromagnetic field distributions for both polarizations are illustrated. The designed graphene absorber exhibits near‐unity polarization‐independent absorption at resonance with an ultra‐narrow spectrum. Moreover, the polarization‐independent absorption can be tuned simply by changing the geometric parameters. The results may have promising potential for the design of graphene‐based optoelectronic devices.     

Design and application of a high‐temperature microfurnace for an in situ X‐ray diffraction study of phase transformation

Thermal treatment of mineral ores such as ilmenite can initiate phase transformations that could affect their activation or deactivation, subsequently influencing their ability to dissolve in a leaching agent. Most laboratory‐based X‐ray diffraction (XRD) studies were carried out ex situ in which realistic diffraction patterns could not be obtained simultaneously with occurring reactions and were time‐consuming. The availability of synchrotron‐radiation‐based XRD not only allows in situ analysis, but significantly shortens the data recording time. The present study details the design of a robust high‐temperature microfurnace which allows thermal processing of mineral ore samples and the simultaneous collection of high‐resolution synchrotron XRD data. In addition, the application of the manufactured microfurnace for in situ study of phase transformations of ilmenite ore under reducing conditions is demonstrated.     

Simultaneous brightness enhancement and wavelength conversion to the eye‐safe region in a high‐power diamond Raman laser

Brightness enhancement in an external cavity diamond Raman laser designed for high power conversion of a neodymium (1064 nm) laser to the eye‐safe spectral region is reported. Using a multimode input beam pulsed at 36 kHz pulse repetition frequency, 16.2 W with 40% overall conversion efficiency was obtained at the second Stokes wavelength of 1485 nm. The output beam had a quality factor of which is a factor of 2.7 times lower than that of the input beam, resulting in a higher overall brightness. The output power, brightness, and brightness enhancement obtained represent significant advances in performance for Raman lasers as well as other competing kHz‐pulsed eye‐safe technologies.     

A first low‐resolution difference Fourier map of phosphorus in a membrane protein from near‐edge anomalous diffraction

Crystal diffraction of three membrane proteins (cytochrome bc₁ complex, sarcoplasmic reticulum Ca²⁺ ATPase, ADP‐ATP carrier) and of one nucleoprotein complex (leucyl tRNA synthetase bound to tRNAleu, leuRS:tRNAleu) was tested at wavelengths near the X‐ray K‐absorption edge of phosphorus using a new set‐up for soft X‐ray diffraction at the beamline ID01 of the ESRF. The best result was obtained from crystals of Ca²⁺ ATPase [adenosin‐5′‐(β,γ‐methylene) triphosphate complex] which diffracted out to 7 Å resolution. Data were recorded at a wavelength at which the real resonant scattering factor of phosphorus reaches the extreme value of ?20 electron units. The positions of the four triphosphates of the monoclinic unit cell of the ATPase have been obtained from a difference Fourier synthesis based on a limited set of anomalous diffraction data.