Microstructural investigation into calcium phosphate biomaterials by spatially resolved cathodoluminescence

From cathodoluminescence (CL) investigations of synthetic and natural calcium phosphates it can be concluded that the CL of pure synthetic apatite is mainly characterized by intrinsic luminescence, whereas the luminescence of naturally occurring apatites is frequently activated by trace elements. CL revealed internal structures within plasma‐sprayed hydroxyapatite coatings which were not discernible by SEM‐BSE imaging. However, cathodoluminescence microscopy alone can presently not be used in every case to characterize synthetic calcium phosphate biomaterials because of the dominant intrinsic blue CL emission. In the future, optimum results will likely be achieved by using a combination of CL microscopy and spectroscopy with other spatially resolved analytical methods such as SEM‐BSE, SEM‐CL or micro‐Raman spectroscopy. In the present study, different types of tetracalcium phosphate dental cements could be distinguished due to varying CL colours and CL spectra that are caused by a different content of impurity Mn. These results emphasize the advantages of spectral CL measurements for spatially resolved detection of trace elements in solids.     

Imaging the hidden modes of ultrathin plasmonic strip antennas by cathodoluminescence

We perform spectrally resolved cathodoluminescence (CL) imaging nanoscopy using a 30 keV electron beam to identify the resonant modes of an ultrathin (20 nm), laterally tapered plasmonic Ag nanostrip antenna. We resolve with deep-subwavelength resolution four antenna resonances (resonance orders m = 2-5) that are ascribed to surface plasmon polariton standing waves that are confined on the strip. We map the local density of states on the strip surface and show that it has contributions from symmetric and antisymmetric surface plasmon polariton modes, each with a very different mode index. This work illustrates the power of CL experiments that can visualize hidden modes that for symmetry reasons have been elusive in optical light scattering experiments.     

Spatially and angularly resolved cathodoluminescence study of single ZnO nanorods

Single ZnO nanorods were studied with cathodoluminescence at high spatial and angular resolution. A newly developed luminescence detector consisting a fiber probe controlled by a nano-manipulator is attached to a scanning electron microscope to carry out the cathodoluminescence measurements. Excitonic emission from the sidewalls and redshifted near band edge emission guided along the nanorod axis are observed as the fiber probe axis is aligned to be perpendicular and parallel to the nanorod axis, respectively, demonstrating the angular resolving power of the experimental setup and waveguiding behavior of the nanorods. High spatial resolution cathodoluminescence measurement shows that the near band edge emission can propagate parallel and perpendicular to the nanorod axis and an increased propagation distance results in more redshift of the guided luminescence. In addition, the high spatial resolution and temperature dependent cathodoluminescence measurements demonstrate the important role of free exciton-longitudinal optical phonon interaction in the waveguiding behavior and the propagation of the near band edge emission in ZnO nanorods.     

Validation of luminescent source reconstruction using single-view spectrally resolved bioluminescence images

We characterize the capabilities and limitations of the Living Image Software 3D Analysis package (Xenogen, Alameda, California) in the reconstruction of calibrated light sources. Sources shallower than the mean free path of light propagation suffered reconstruction inaccuracy. For sources deeper than the mean free path, the average error in depth and intensity reconstruction was less than 4% and 12%, respectively, for homogeneous tissue. The reconstruction of luminescent beads implanted within an optically heterogeneous mouse abdomen proved less accurate. The ability to distinguish multiple sources decreased with increasing source depth. A number of factors influence the accuracy of light source reconstruction.     

Strong morphological dependence of luminescence efficiency and emission wavelength in hexagonal GaN crystallites directly imaged by scanning cathodoluminescence microscopy

The microstructure and morphology of hexagonal GaN crystallites grown on c-axis sapphire substrates by low pressure chemical vapor deposition is correlated with the luminescence efficiency and emission wavelength. Microscopic variation of local band gap monitored by the luminescence wavelength on the a- and c-planes of hexagonal GaN-crystallites are directly mapped by means of low temperature scanning cathodoluminescence (CL) and CL wavelength imaging (CLWI). Beside minor fluctuations from crystallite to crystallite, the a-planes show pronounced red shift of emission energy of more than 132 meV with respect to the luminescence from the c-planes. The c-plane itself shows additional inhomogeneity on a micron scale. Strongly red shifted luminescence (λ>400 nm) originates from the very center region correlated with a high dislocation density found in TEM. The CL intensity shows a reticulated structure over the c-plane visualizing the local dislocation network.     

Direct observation of plasmonic modes in au nanowires using high-resolution cathodoluminescence spectroscopy

We use cathodoluminescence imaging spectroscopy to excite and investigate plasmonic eigenmodes of Au nanowires with lengths of 500-1200 nm and approximately 100 nm width. We observe emission patterns along the Au nanowire axis that are symmetric and strongly wavelength dependent. Different patterns correspond to different resonant modes of the nanowire. From the observed patterns, we derive the spatial and spectral properties of the wire eigenmodes and determine the dispersion relation for plasmonic Au nanowire modes.     

Spectrally and spatially resolved cathodoluminescence of nanodiamonds: local variations of the NV(0) emission properties

Here we report the spectrally and spatially resolved cathodoluminescence of diamond nanoparticles using focused fast electron beams in a transmission electron microscope. We demonstrate the possibility of quickly detecting various individual colour centres of different kinds on wide areas (several micrometres square) contained in nanoparticles separated by subwavelength distances. Among them, nanoparticles containing one or more neutral nitrogen-vacancy (NV(0)) intensity maxima have been seen, attributable to individual emitters. Thanks to a spatial resolution which is solely limited by charge carrier diffusion in the case of a fast electron (80 keV) setup, the spectra of two individual NV(0) emitters separated by 80 nm inside a nanoparticle have been spatially discerned. A shift of the zero phonon line (ZPL) between the two emitters, which we attribute to internal stress, is shown to arise even within the same nanoparticle. Detailed emission spectra (ZPL, phonon lines and Huang-Rhys factor, directly linked to the relaxation energy of the colour centre) in 51 individual NV(0) centres have been measured in 39 particles. The ZPL and Huang-Rhys factor are found to be measurably dispersed, while the phonon energies keep constant.     

Experimental study of the phase-shift miscalibration error in phase-shifting interferometry: use of a spectrally resolved white-light interferometer

The white-light interferogram in a spectrally resolved white-light interferometer is decomposed in its constituent spectral components by a spectrometer and displayed along its chromaticity axis. A piezoelectric transducer phase shifter in such an interferometer can give a desired phase shift of pi/2 only at one wavelength. The phase shift varies continuously at all other wavelengths along the chromaticity axis. This situation is ideal for an experimental study of the phase error due to the phase-shift error in the phase-shifting technique, as it will be shown in this paper.     

Study of critical buckling loads and modes of cross-ply laminated annular plates

Buckling of composite annular plates under uniform internal and external radial edge loads have been investigated using energy method. Trefftez rule is used in the stability equations. The symmetric buckling of symmetric cross-ply laminates is considered. In this paper, buckling behavior for the three laminates (90/0)_2s, (90/0₂/90)_s and (90₂/0₂)_s are studied. Influence of some parameters such as thickness, stacking sequence, type of supports and the ratio of hole to sheet radius on buckling loads and modes are investigated. The results of the energy method are compared with the results of numerical method. Based on the results, in the plates with clamped boundary conditions the symmetric buckling assumption is not accurate, contrary to other boundary conditions.     

Analysis of spectrally resolved thermoluminescence of LiF:Mg,Cu,P detectors by the surface fitting method using algorithm for unrestricted peak positions

Recently developed surface fitting technique is a natural replacement for the widely used glow curve deconvolution (GCD) technique. Surface fitting can be applied for advanced spectrally resolved thermoluminescence (TL) measurements. It combines both kinetic and emission-band analysis. Owing to greater number of parameters and data points the algorithm is more time-consuming than usual GCD. However, it offers greater reliability in determination of trap parameters. This is especially important for spectrally resolved measurements that are usually performed at low-light level conditions. This paper demonstrates an application of the surface fitting method to the analysis of TL-3D data from LiF:Mg,Cu,P detectors. The spectra were analysed using two different variants of surface fitting--for restricted and unrestricted peak positions.     

GaAs/AlGaAs heterostructure nanowires studied by cathodoluminescence

In this report we explore the structural and optical properties of GaAs/A1GaAs heterostructure nanowires grown by metalorganic vapour phase epitaxy using gold seed-particles. The optical studies were done by low-temperature cathodo- luminescence （CL） in a scanning electron microscope （SEM）. We perform a systematic investigation of how the nanowire growth-temperature affects the total photon emission, and variations in the emission energy and intensity along the length of the nanowires. The morphology and crystal structures of the nanowires were investigated using SEM and transmission electron microscopy （TEM）. In order to correlate specific photon emission characteristics with variations in the nanowire crystal structure directly, TEM and spatially resolved CL measurements were performed on the same individual nanowires. We found that the main emission energy was located at around 1.48 eV, and that the emission intensity was greatly enhanced when increasing the GaAs nanowire core growth temperature. The data strongly suggests that this emission energy is related to rotational twins in the GaAs nanowire core. Our measurements also show that radial overgrowth by GaAs on the GaAs nanowire core can have a deteriorating effect on the optical quality of the nanowires. Finally, we conclude that an in situ pre-growth annealing step at a sufficiently high temperature significantly improves the optical quality of the nanowires.     

Plasmonic Amplifiers: Engineering Giant Light Enhancements by Tuning Resonances in Multiscale Plasmonic Nanostructures

The unique ability of plasmonic nanostructures to guide, enhance, and manipulate subwavelength light offers multiple novel applications in chemical and biological sensing, imaging, and photonic microcircuitry. Here the reproducible, giant light amplification in multiscale plasmonic structures is demonstrated. These structures combine strongly coupled components of different dimensions and topologies that resonate at the same optical frequency. A light amplifier is constructed using a silver mirror carrying light‐enhancing surface plasmons, dielectric gratings forming distributed Bragg cavities on top of the mirror, and gold nanoparticle arrays self‐assembled into the grating grooves. By tuning the resonances of the individual components to the same frequency, multiple enhancement of the light intensity in the nanometer gaps between the particles is achieved. Using a monolayer of benzenethiol molecules on this structure, an average SERS enhancement factor <EF> ～10⁸ is obtained, and the maximum enhancement in the interparticle hot‐spots is ～3 × 10¹⁰, in good agreement with FDTD calculations. The high enhancement factor, large density of well‐ordered hot‐spots, and good fidelity of the SERS signal make this design a promising platform for quantitative SERS sensing, optical detection, efficient solid state lighting, advanced photovoltaics, and other emerging photonic applications.     

Spatial distribution of impurities in ZnO nanotubes characterized by cathodoluminescence

Low-energy cathodoluminescence (CL) imaging and spectroscopy technique was employed to study the impurity distribution in individual ZnO hexagonal nanotubes fabricated by metalorganic chemical vapor deposition on the sapphire (0001) substrate. The CL spectra at 10 K show that acceptor and donor impurities are incorporated in the ZnO nanotubes. CL monochromatic images indicate that the concentration of donor is higher at the bottom part and the distribution of acceptors is more inhomogeneous at the surface of the nanotubes. The non-uniform defects and impurities distributions are explained by unstable growth conditions and contamination from the environment. These results indicate that the low-energy CL is a very powerful method to investigate the inhomogeneity of luminescence properties in the individual nanostructures.     

Spectroscopic cathodoluminescence studies of Mg-PSZ

Cathodoluminescence (CL) techniques have been used to examine the microstructure of a magnesia-partially stabilized zirconia. Spectroscopic analysis of the visible light emitted under electron bombardment, coupled with monochromatic imaging techniques have distinguished the luminescent monoclinic zirconia from the grain-boundary silicate phase, commonly believed to be forsterite. The CL spectrum of the grain-boundary silicate was found to differ from that of pure synthetic forsterite suggesting the presence of modifiers. Lowering the specimen temperature allows the grain-boundary silicate phase to be more easily distinguished.     

Recovery of spectral reflectances of objects being imaged by multispectral cameras

Acquisition of spectral information of objects being imaged through the use of sensor responses is important to reproduce color images under various illuminations. In the past several models have been proposed to recover the spectral reflectances from sensor responses. The accuracy of the spectral reflectances recovered by five different models is compared by using multispectral cameras. It is shown that the Wiener estimation that uses the noise variance estimated as proposed in IEEE Trans. Image Process.15, 1848 (2006) recovers the spectral reflectances more accurately than the others when the test samples are different from learning samples.