Backscattering Configuration Research Articles

Spectral intensity correlations of backscattered diffuse light: the effect of scattering anisotropy

In this work, we report experiments and a theoretical scheme of photon transport in the frequency domain of rigid turbid media. We have employed spectral multi-speckle intensity correlations to estimate optical properties as the transport mean free path and the absorption length of turbid systems. We propose a scheme based on the photon diffusion model using an effective path-length distribution in the backscattering configuration and take explicitly into account the particles scattering anisotropy parameter g. By studying rigid Teflon slabs and polymer matrices doped with polystyrene particles of different degrees of scattering anisotropy, we find that the proposed model adequately describes our experimental results. Our hypothesis for the diffuse transport of backscattered photons in the weak multiple scattering regime is further validated using a numerical simulation scheme of speckle dynamics, based on the Copula method.

Evaluation of photoluminescence from milk with various vitamin B2 concentrations

Concentrations of vitamin B2 in milk were evaluated based on photoluminescence (PL) peak intensities at around λ = 545 nm. PL was measured in a back-scattering configuration using a 3.2 mm diameter optical fibre bundle irradiated by ultra-violet LEDs, violet LEDs or a blue LED as the excitation light. The tip of the optical fibre bundle was dipped into commercially available whole milk, low-fat milk and non-fat milk. PL peaking at 545 nm was observed under LED irradiation. Vitamin B2 concentrations were measured using a standard addition method and a calibration curve method. Deviation between the vitamin B2 concentration measured by PL using a standard addition method and those by HPLC technique varied with wavelength of excitation light and was smallest with violet LED (405 nm) excitation. The standard deviation of results measured by PL using a calibration curve method was better than those using a standard addition method.

Quantitative analysis of fat and protein concentrations of milk based on fibre-optic evaluation of back scattering intensity

Optical back-scattering from milk with various fat and protein concentrations were evaluated in a back-scattering configuration using white, violet, blue, green, red or near infrared (NIR, λ = 850 and 940 nm) LEDs as the light source. The tip of the optical fibre bundle was dipped into the milk with fat and protein concentrations from 8.2 to 39 g L−1 and 32 to 37.7 g L−1, respectively. The back-scattering intensities varied linearly with the fat and protein concentrations of the milk. The slope (G) of calibration lines varied with the wavelength of the incident light. The G of the fat concentrations became zero at wavelength around λ = 520 nm and that of the protein concentrations was much smaller than that of the fat concentrations at NIR wavelengths. The fat and protein concentrations can therefore be measured separately based on the scattering intensities by selecting the wavelength for measurement.

Sea Ice Roughness Overlooked as a Key Source of Uncertainty in CryoSat‐2 Ice Freeboard Retrievals

AbstractESA's CryoSat‐2 has transformed the way we monitor Arctic sea ice, providing routine measurements of the ice thickness with near basin‐wide coverage. Past studies have shown that uncertainties in the sea ice thickness retrievals can be introduced at several steps of the processing chain, for instance, in the estimation of snow depth, and snow and sea ice densities. Here, we apply a new physical model to CryoSat‐2, which further reveals sea ice surface roughness as a key overlooked feature of the conventional retrieval process. High‐resolution airborne observations demonstrate that snow and sea ice surface topography can be better characterized by a lognormal distribution, which varies based on the ice age and surface roughness within a CryoSat‐2 footprint, than a Gaussian distribution. Based on these observations, we perform a set of simulations for the CryoSat‐2 echo waveform over “virtual” sea ice surfaces with a range of roughness and radar backscattering configurations. By accounting for the variable roughness, our new lognormal retracker produces sea ice freeboards that compare well with those derived from NASA's Operation IceBridge airborne data and extends the capability of CryoSat‐2 to profile the thinnest/smoothest sea ice and thickest/roughest ice. Our results indicate that the variable ice surface roughness contributes a systematic uncertainty in sea ice thickness of up to 20% over first‐year ice and 30% over multiyear ice, representing one of the principal sources of pan‐Arctic sea ice thickness uncertainty.

High contrast dental imaging using a random fiber laser in backscattering configuration

In this work, a backscattering imaging method based on near infrared random fiber laser is shown to provide a high contrast optical image between carious and sound enamel. The obtained contrast is 0.70, which is more than 8 times higher than the contrast obtained from radiographic imaging. Caries and cracks in enamel could clearly be identified against healthy enamel using the optical system. The near infrared wavelength, high spectral density and low coherence of random fiber laser contribute to its deep penetration, high brightness and low speckle contrast, using the method in a backscattering configuration opens potential clinical use.

Theoretical Study on Microwave Scattering Mechanisms of Sea Surfaces Covered With and Without Oil Film for Incidence Angle Smaller Than 30°

This article is devoted to investigating the microwave scattering mechanisms of oil-free and oil-covered sea surfaces for an incidence angle smaller than 30° in a backscattering configuration. The Elfouhaily spectrum is used to simulate an oil-free sea surface, whereas the Elfouhaily spectrum combined with the Jenkins damping model is applied to the simulation of an oil-covered sea surface. Then, the Kirchhoff approximation-stationary phase approximation (KA-SP) and the first order of small-slope approximation (SSA-1) are employed to simulate the scattering coefficients induced by specular scattering and total scattering, respectively. Importantly, a new parameter defined as specular scattering to total scattering ratio (STR) is proposed in this article, which can be used to measure the ratio of specular backscattered power to total backscattered power. The dependencies of the scattering coefficient and the STR on incidence angles, wind speeds, wind directions, oil thicknesses, and so on, are investigated. This article provides new insights for a better understanding of the evolution of microwave scattering mechanisms from oil-free and oil-covered sea surfaces in the transition region of incidence angles (from about 15° to 30°).

Through-bottle whisky sensing and classification using Raman spectroscopy in an axicon-based backscattering configuration.

Non-intrusive detection systems have the potential to characterise materials through various transparent glass and plastic containers. Food and drink adulteration is increasingly problematic, representing a serious health risk as well as an economic issue. This is of particular concern for alcoholic spirits such as Scotch whisky which are often targeted for fraudulent activity. We have developed a Raman system with a novel geometry of excitation and collection, exploiting the beam propagation from an axicon lens, which results in an annular beam at the bottle surface before focusing within the sample. This facilitates the efficient acquisition of Raman signals from the alcoholic spirit contained inside the bottle, while avoiding the collection of auto-fluorescence signals generated by the bottle wall. Therefore, this technique provides a way of non-destructive and non-contact detection to precisely analyse the contents without the requirement to open the bottle.

Band Structure Effects on the Charge Exchange Processes in H+ Colliding with a Cu(111) Surface

The low-energy ion scattering (LEIS) technique was used to experimentally determine the formation of positive and negative ions in the scattering of protons by a Cu(111) surface for a large scattering angle in the backscattering configuration and a wide range of incoming energies (2–8 keV). Two different collisional geometries were analyzed for a 135° fixed scattering angle: 45°/90° and 67.5°/67.5° incoming/exit angles measured with respect to the target surface. The total fraction of backscattered ions ranges from 10% to 25%, and a peculiarly high yield of negative ions, which always exceeds that of positive ions, was detected for the whole energy range analyzed. A strong dependence of the measured ion fractions with the geometrical conditions was experimentally found. On the theoretical side, a first-principles quantum-mechanical formalism that takes into account the three possible final charge states of the H+ in a correlated way and the fine details of the band structure of the Cu(111) surface was app...

Acoustic phonons in unfilled tetragonal tungsten-bronze crystals

ABSTRACT We have studied several unfilled tetragonal tungsten-bronze crystals (SBN-35, CBN-28, CBN-30, CBN-32) by differential scanning calorimetry, Brillouin spectroscopy and piezoresponse-force microscopy. The Brillouin backscattering configuration in the c-plates revealed the longitudinal acoustic phonon, which has a frequency near 50 GHz and displays softening when approaching the phase transition in all crystals. The ferroelectric domain structure and the domain size are dependent on the Sr/Ba or Ca/Ba ratios, and on the occupation rate in the channels as shown by the piezoresponse-force microscopy images. A splitting of the longitudinal acoustic phonon was found in SBN-35 in the multidomain sample, but poling of the crystal removed this splitting and just left the phonon with higher frequency.

Hydrogen analysis in nominally anhydrous minerals by transmission Raman spectroscopy

We present a new approach for the analysis of water in nominally anhydrous minerals using transmission Raman spectroscopy. Using this approach, the laser was shone through thin, nearly transparent samples of clinopyroxene, garnet and synthetic rhyolite glass. To remove mineral-induced background and to improve the quality of the OH spectral region, specifically for clinopyroxene, a reference spectrum of a dehydrated crystal was measured and subtracted. Water contents of all clinopyroxene samples were previously determined by Fourier transformed infrared spectroscopy (FTIR). The application of transmission Raman spectroscopy and a reference spectrum of a dry sample revealed a noticeable improvement in the quality of spectra and thus the detection limit, compared to the standard backscattering configurations. We show that the quality of transmission spectra and the detection limit depend on the sample thickness, and that the thickness has to be taken into account when measuring and comparing OH-integrated intensity and water content if the results are used for OH quantification.

On the Amplitude Distributions of Bistatic Scattered Fields From Rough Surfaces

Non-Rayleigh distributed radar clutter is widely reported in studies of radar scattering from sea and land surfaces. Existing models of scattered field amplitude distributions have been developed primarily through empirical fits to the statistics of radar backscatter measurements. In contrast, this paper investigates a physics-based approach to determine the amplitude distributions of fields scattered from rough surfaces using Monte Carlo simulations and analytical methods, for both backscattering and bistatic configurations. The rough surface is represented using a “two-scale” model. An individual surface facet contains “small-scale” roughness, for which scattered fields are evaluated using the second-order small slope approximation. Individual surface facets are tilted by the slopes of the “large-scale” roughness in a given observation. The results show that non-Rayleigh amplitude distributions are obtained when tilting is performed, and that the departure from the Rayleigh distribution becomes more significant as the variance of the tilting slope increases. Further analysis shows that this departure results from variations in the mean scattering amplitude from a facet (the texture) as tilting occurs. The distribution of the texture is studied and compared with existing models. Finally, the distribution of the scattered field amplitude is modeled through the compound Gaussian model, first using the distribution of the texture, and then in terms of the probability density function of tilting slopes (which avoids the requirement of the knowledge of the texture distribution). The results from the above two methods are in good agreement and both agree well with the Monte Carlo simulation.

Raman spectrum study of δ-doped GaAs/AlAs multiple-quantum wells**Project supported by Shandong Province Natural Science Foundation, China (Grant No. ZR2017MF018) and the National Natural Science Foundation of China (Grant No. 61675223).

Three samples of GaAs/AlAs multiple-quantum wells with different quantum well widths and δ-doped with Be acceptors at the well center were grown on (100) GaAs substrates by molecular beam epitaxy. Polarized Raman spectra were recorded on the three samples at temperatures in a range of 4–50 K in a backscattering configuration. The two branches of coupled modes due to the interaction of the hole intersubband transitions and the quantum-well longitudinal optical (LO) phonon were observed clearly. The evaluation formalism of the Green function was employed and each lineshape of the Raman spectrum of the coupled modes was simulated. The dependence of the peak position of Raman shifts of the two coupled modes as well as the quantum-well LO phonon on the quantum-well size and measured temperature were given, and the coupling interaction mechanism between the hole subband transitions and the quantum-well LO phonon was researched.

Fe–Mn Mixed Oxide Catalysts Synthesized by One-Step Urea-Precipitation Method for the Selective Catalytic Reduction of NO x with NH3 at Low Temperatures

A range of Fe–Mn mixed oxide catalysts, FeαMn1−αO x (α = 1, 0.25, 0.33, 0.50, 0 mol%) were prepared via one-step urea-precipitation method and applied to the selective catalytic reduction (SCR) of nitric oxide (NO x ) with NH3. The Fe0.33Mn0.66O x catalyst showed the highest activity in the NH3-SCR process within a broad operation temperature range (75–225 °C) with 90% NO x conversion. Series of characterization have been taken to investigate the physical and chemical properties of the catalysts. The BET results evidenced that the doping of iron species increased the surface area of the catalyst effectively, and this structure could provide more acid sites and active sites on the surface of catalysts for SCR reaction. X-ray powder diffraction results and Raman spectroscopy indicate that the active Mn and Fe species were in poorly crystalline or amorphous states which could increase lattice defects and oxygen vacancies on the surface of Fe0.33Mn0.66O x catalyst. The X-ray photoelectron spectra results suggested that more surface-adsorbed oxygen (OA), Fe3+, Mn4+ species existed on the surface of the Fe0.33Mn0.66O x catalyst compared with that of MnO x and FeO x catalysts, which is favorable to the NH3-SCR performance. Analysis by in situ Fourier transform infrared spectroscopy (FTIR) suggested that Fe-doping can enhance the absorption and the activation ability of NO which could promote the catalytic performance in the SCR process. Catalyst characterization Powder X-ray diffraction (XRD) was carried out on a PAN alytical powder X-ray diffractometer (Model EMPYREAN) with a monochromatic Cu Kα1 radiation (λ = 0.154056 nm) within a 2θ range of 10°–90° in the step of 0.02° at room temperature. The Brunauer–Emmett–Teller (BET) surface areas of the samples were computed by physical adsorption of N2 at − 196 °C using a NOVA 1200 (Quanta Chrome). The samples were pretreated at 300 °C for 5 h in a vacuum state previous to BET measurement. The high-resolution transmission electron microscopy (HR-TEM) determinations were conducted on a JEOL-2100 microscope. The Raman spectrums of samples were computed by a confocal Raman microscope (Labram DILOR) equipped with an Olympus BX-41 microscope (objective ×100) and TEcooled CCD detector (Andor), in a back-scattering configuration using an He–He laser (532 nm excitation line) with power of 510 mW at a sample and spectral resolution of 0.8 cm−1. X-ray photoelectron spectra (XPS) over the samples were by captured Al-Kα radiation (1486.7 eV). Binding energies of Mn 2p and O 1s were calibrated using C 1s (BE = 285.0 eV) as a standard. The in situ FTIR experiments of NO adsorption over MnO x and Fe0.33Mn0.66O x catalysts were performed on a VERTEX70-FTIR. Prior to NO adsorption, the samples (approximately 20 mg each) was pretreated at 250 °C in the He atmosphere for 1 h. The spectrum of NO adsorption at different temperatures were selected over each catalyst. The reaction condition was controlled as follows: 500 ppm NO and He as balance, and the flow rate was 100 mL/min.

Facet-Based Investigation on Microwave Backscattering From Sea Surface With Breaking Waves: Sea Spikes and SAR Imaging

A complete facet model for the backscattering from rough sea surface with breaking waves is proposed in this paper. In consideration of the spatial distribution of breaking waves on sea surface, the model is able to give a good interpretation to the “super events” under high sea states, such as sea spikes and high polarization ratios. Based on the proposed model, normalized radar cross section plots of sea surface under backscattering configuration are calculated and compared with measured data. The comparisons show that the proposed model is tractable to estimate the scattering from electrically large ocean surface under high sea states with accuracy and efficiency. In addition, the non-Gaussian statistics and spatial correlation properties of sea clutter are analyzed under different range resolutions and incident angles. The results show that high kurtosis value, which means a sea spike phenomenon, mostly happens in lower grazing angle and higher range resolution cases. The comparisons of simulated SAR images of sea surface with and without breaking waves also reveal that sea spikes and high polarization ratios are caused by breaking waves. All the simulation results prove that our model not only is able to explain the physical mechanism of the scattering but also can be applied to the analyses of statistical properties of sea clutter under high sea states.

Localizing scatterers from surf noise cross correlations.

The backscattered travel-time structure is obtained by cross-correlating air-acoustic ocean surf noise recorded on microphone pairs (separation ∼2 m) on the beach. The scatterer is a 20 cm radius Polyvinyl chloride pipe 2.5 m landside of the microphone array. Arrivals corresponding to the time-difference (travel-time difference between two scatterer-receiver paths) and scattered (travel time for receiver-scatterer-receiver path) waves emerge in the cross-correlation functions in a backscattering configuration. Theoretically, only a microphone pair is needed to locate the scatterer using the time-difference and scattered travel times. Localization of the scatterer is demonstrated with the microphone array on the beach.

Temperature dependence of the Raman-active modes in the semipolar (112¯2) plane GaN Film

Temperature dependences of the polarized Raman scattering spectra in the backscattering configuration of the semipolar (112¯2) plane GaN thin film are analyzed in the range from 83 K to 563 K. The semipolar GaN film is cut at a tilted angle from polar GaN wafer grown by hydride vapor phase epitaxy. The spectral features of the frequency shift and linewidths of the Raman-active phonon modes Quasi-TO, E1 (TO), E2 (high), and Quasi-LO are prominently revealed, and the temperature coefficients corresponding to the lattice thermal expansion and phonon anharmonic effect, as well as defects and impurities in crystals, are well deduced by the theoretical equations. With the increasing temperature, the Raman scattering peaks would substantially shift to lower frequencies and the linewidths gradually broaden. Our studies will lead to a better understanding of the fundamental physical characteristics of the semipolar (112¯2) plane GaN film.

Vibrational spectra and nonlinear optical coefficients of rhombohedralCsGeX3halide compounds withX=I, Br, Cl

The zone-center phonons are calculated for rhombohedral $\mathrm{CsGe}{X}_{3}$ compounds with $X=\mathrm{I}$, Br, Cl using density functional perturbation theory in a plane-wave pseudopotential method. The infrared absorption and reflection spectra are simulated and show that the absorption has a strong contribution from the LO as well as TO modes. Both the lowest and highest IR active modes have strong oscillator strengths for each symmetry (${A}_{1}$ and $E$) and correspond to motions of either the Cs or Ge ions relative to the halogen ions. respectively. The intermediate modes have low oscillator strength because their mode pattern shows less clearly a dipole pattern. The polarization dependent Raman spectra for various backscattering configurations are simulated. Only the highest modes of each symmetry have a strong Raman intensity. The results for the Raman spectra are found to be in good agreement with the available experimental data for polycrystalline films when averaging over directions. The Born effective charges and high and low frequency dielectric tensors as well as the second-order nonlinear optical coefficients are calculated. The ratio of the static to the high-frequency dielectric constants are extremely high and analyzed in terms of the contribution of each mode to the Lyddane-Sachs-Teller relation. The nonlinear optical coefficients decrease strongly from I to Br to Cl in reverse order of the band gaps and are found to be consistent with available experimental results.

In-Plane Biaxial Strain Evaluation Induced in Ge1-X Sn x Films Using Oil-Immersion Raman Spectroscopy

Introduction Germanium Tin (GeSn) has a potential to supersede Si as channel material for next generation LSI. The hole mobility of GeSn with 3% Sn concentration is higher than that of pure Ge [1]. Strain or strain relaxation is certainly induced in the channel region through the device process. In addition, it is well known that carrier mobility is varied by the strain. Therefore, we believe that it is important to evaluate the precise strain states. In our previous reports, we have evaluated the anisotropic strain in Si and Si1-x Ge x using oil-immersion Raman spectroscopy, which can excite both longitudinal optical (LO) mode and transverse optical (TO) mode. In this study, we evaluated the in-plane biaxial strain induced in Ge1-x Sn x films using oil-immersion Raman spectroscopy. Experiment We evaluated the strain induced in the Ge1-x Sn x films which were epitaxially grown on (001) Ge substrate by the metal-organic chemical vapor deposition (MOCVD) [2]. Sn concentrations in Ge1-x Sn x films were 1.8, 2.2, and 3.2%, respectively, which were defined by Rutherford backscattering spectrometry. From transmission electron microscopy measurements, thicknesses of Ge1-x Sn x films were 104, 17, and 34 nm, respectively. In the oil-immersion Raman spectroscopy, the excitation light with wavelength of 532 nm irradiated the Ge1-x Sn x films under the conventional (001) backscattering configuration. The light path was immersed by the high refractive index oil. The numerical aperture (NA) of the immersion lens and refractive index of the oil were 1.4 and 1.5, respectively. In the XRD measurements, synchrotron radiation was used for the X-ray source with the energy of 10 or 12 keV. Out-of-plane strain εL in the Ge1-x Sn x films was measured from 004 diffraction. Results and discussion Fig. 1 shows the LO and TO mode Raman spectra from the GeSn0.018 film. These spectra were calibrated by the strain free bulk Ge peak at 300 cm-1. From Raman spectroscopy and XRD results, we derived phonon deformation potentials (PDPs) of the strained Ge1-x Sn x , which is indispensable to evaluate stress. The PDPs (p and q) can be derived by solving equation 1, where Δω, ω0, εL, and ε|| are Raman wavenumber shift, strain-free Raman shift, in-plane strain, and out-of-plane strain, respectively. The derived p and q were plotted in figure 2. In this figure, p and q for pure Ge (x=0) are reported by S. C. Jain et al. [3]. As shown in this figure, the p and q suggest almost constant for the Ge1-x Sn x (x < 0.032). Using the derived PDPs, we calculated the anisotropic stresses (σ xx and σ yy ) induced in Ge1-x Sn x films. As a result, we obtained σ xx = -0.40 and σ yy = -0.40 GPa for the strained GeSn0.018, σ xx = -0.47 and σ yy = -0.47 GPa for the strained GeSn0.022, and σ xx = -0.68 and σ yy = -0.68 GPa for the strained GeSn0.032. From these results, we can confirm that the stresses induced in Ge1-x Sn x films were almost isotropic states. In this work, the PDPs for Ge1-x Sn x were derived for the first time. Acknowledgements This study was partially supported by the Japan Society for the Promotion of Science through a Grant-in-Aid for Science Research B (No. 24360125). XRD measurements were performed at BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2014B1613, 2014B1892, 2015A1971, 2015B1925, and 2016A1529).

Enhanced Light Emission from N-Doped Ge Microdisks by Thermal Oxidation

Germanium (Ge) has been considered a promising material candidate for Optoelectronic Integrated Circuit (OEIC) on silicon (Si) substrates [1, 2, 3]. Although Ge is an indirect-band-gap semiconductor, the energy difference between its direct and indirect band gaps is rather small. Tensile strain and heavy n-type doping have been proposed and demonstrated to be able to compensate for this difference and significantly enhance the direct-gap light emission [4]. To fabricate device structures for optical resonators, lithography and dry etching are desirable. However, defects on the sidewall induced by dry etching would have deleterious effects on the devices performance. In this study, we fabricate circular mesa structures (microdisks) from Ge grown on Si substrate and observe enhanced light emission from device by surface passivation via thermal oxidation. A 740-nm-thick Ge film with a tensile strain of about 0.22% was grown on a p-type Si(100) substrate by solid source molecular beam epitaxy (SSMBE) using two-step growth method [5]. The samples were heavily n-type doped to a concentration up to about 4×1019 cm-3 by phosphorus diffusion from a spin-on-dopant (SOD) source [6]. Electron-beam lithography was used to define circular patterns with different radii on resist. The patterns were then transferred to the Ge by inductively coupled plasma reaction ion etching. After resist removal and cleaning, the samples were oxidized at atmospheric pressure in a furnace. The light emission was then measured by micro-photoluminescence (PL) under a normal-incidence backscattering configuration, immediately after oxidation, to avoid degradation of oxide film in atmosphere. The PL spectra of a blank Ge film and microdisks with radii of 0.8 µm, before and after thermal oxidation at 550 °C for 1 hour, are shown in figure 1(a) and (b). The pump laser power was fixed at 5 mW in all cases. For blank Ge film, the light emission intensity is enhanced by a factor of 1.31 (with respect to the integrated PL intensity) after oxidation. And the emission peak is slightly blue-shifted, which might be attributed to the reduced doping concentration due to out diffusion of phosphorus during oxidation under such a high temperature. The PL intensity of the microdisks with radii of 0.8 µm after thermal oxidation is significantly enhanced compared with that without thermal oxidation. The enhancement factor is about 3.42. We thus attribute this to reduced surface recombination or defects induced by dry etching. Compared with blank Ge film, the enhancement factor for microdisk is much larger. This indicates that the effect of surface passivation of the etched sidewall is dominant over the top surface. We also investigated the light emission properties of microdisks with different radii, as shown in figure 1(c) and 1(d). The enhancement factors are 1.42 and 1.27 for disk with radii of 1.8 and 3.0 µm, respectively, e.g, enhancement factor decreases with increasing disk size. During PL measurement, the pump laser spot was fixed at the same size (a diameter of about 2 µm) and at the same position (the center of the microdisks). For lager microdisks, the sidewall is further from the pumped area where carriers are generated. The contribution of nonradiative recombination induced by sidewall is less dominant for lager microdisks. Therefore, under the same surface passivation condition, the enhancement of the light emission intensity is more pronounced for smaller microdisks. In summary, the effect of thermal oxidation on the light emission properties of dry-etched n-type doped Ge-on-Si microdisks was investigated. The direct-gap light emission was significantly enhanced owing to surface passivation, up to 3.42 times for microdisks with radii of 0.8 µm. The enhancement factor increased as the disk size decreased, indicating that a sidewall-related nonradiative recombination process was an important limiting factor achieving a high injected carrier density and it could be mitigated by thermal oxidation. With this simple method, it is thus highly promising to realize low-threshold Ge lasers monolithically integrated on Si substrates by a mature and low-cost complementary metal-oxide-semiconductor-compatible process.[1] J. Liu et al, Semicond. Sci. Technol. 27. 094006 (2012) [2] P. Boucaud et al, Photo. Res. 1, 102 (2013) [3] J. Liu et al, Opt. Express. 15. 11272 (2007) [4] X. Sun et al, Appl. Phys. Lett. 95. 011911 (2009) [5] K. Nishida et al, Thin Solid Films 557, 66 (2014) [6] X. Xu et al, Appl. Phys. Express 8, 092101 (2015) Figure 1

Structural displacement measurements using DC coupled radar with active transponder

Summary Continuous wave (CW) radar sensors are a promising technology for structural displacement measurement. However, the commonly employed passive backscattering measurement configuration has some limitations for low-power applications of CW radar. To improve displacement measurement performance using the low-power CW radar, an active transponder configuration is introduced. Compared with passive backscattering, the active transponder configuration is capable of increasing the radar's signal power, thereby increasing the signal to noise ratio for more accurate displacement measurement. A series of laboratory experiments to demonstrate the performance of the transponder configuration was conducted, and the experimental results were compared with the results of using the backscattering configuration. The investigation revealed that the signal improvement provided by the transponder is a function of the distance between the transponder and the radar. In addition, a full-scale bridge test with the active transponder configuration was conducted to verify the improved performance. An oblique angle test using the transponder was conducted in the laboratory to demonstrate the potential for flexible installation strategies when deploying the radar/transponder pair for bridge displacement measurements. Copyright © 2016 John Wiley & Sons, Ltd.