Arc Sec Research Articles

Heteroepitaxy mechanisms of AlN on nitridated c- and a-plane sapphire substrates

We investigate the metalorganic vapor phase epitaxy of c-oriented AlN on c- and a-plane sapphire substrates, focusing on the effect of sapphire nitridation on the AlN structure. Prior to AlN growth, the sapphire surface is subjected to nitridation via an in-situ NH3 treatment. We demonstrate that nitridation without H2 thermal etching treatment realizes high quality AlN on both c- and a-plane sapphires, indicating that a reaction between NH3 and oxygen on the sapphire surface is a critical factor in the material growth. It is proposed that nitridation initially creates nanometer-scale inversion domains in the AlN epilayer, but as growth proceeds, the N-polar domains are annihilated, leaving voids. Such growth behaviors can be regarded as spontaneous selective area growth with strain-adsorbing void formation, and lead to crack-free, ∼5 μm thick AlN layers, which produce x-ray line widths as narrow as 180 and 483 arc sec for the (0002) and (101¯2) reflections, respectively, on c-plane sapphire, and 237 and 433 arc sec for these reflections on a-plane sapphire.

Effect of Yb doping on the refractive index and thermo-optic coefficient of YVO4 single crystals.

Single crystals of YVO4 with different doping concentrations of Yb (1.5, 3.0, 8.0, and 15.0 at. %) and with good crystalline quality (FWHM ∼43-55 arc sec of rocking curve) were grown by the optical floating zone technique. Refractive index measurements were carried out at four wavelengths as a function of temperature. The measurements show that as the doping concentration of Yb is increased, the refractive index varies marginally for ne whereas there is a significant change in the value of no. The thermo-optic coefficient (dn/dT) was found to be positive with a value ∼10-5/°C, which is 1 order higher than that for the undoped YVO4 crystal. The thermo-optic coefficient is higher for ne compared to that of no. Also, a set of relations describing the wavelength dependence of the thermo-optic coefficient were established that are useful for calculating the thermo-optic coefficient at any temperature in the range 30°C-150°C and at any wavelength in the range 532-1551 nm.

Comparison of the structural properties of Zn-face and O-face single crystal homoepitaxial ZnO epilayers grown by RF-magnetron sputtering

Homoepitaxial ZnO growth is demonstrated from conventional RF-sputtering at 400 °C on both Zn and O polar faces of hydrothermally grown ZnO substrates. A minimum yield for the Rutherford backscattering and channeling spectrum, χmin, equal to ∼3% and ∼12% and a full width at half maximum of the 00.2 diffraction peak rocking curve of (70 ± 10) arc sec and (1400 ± 100) arc sec have been found for samples grown on the Zn and O face, respectively. The structural characteristics of the film deposited on the Zn face are comparable with those of epilayers grown by more complex techniques like molecular beam epitaxy. In contrast, the film simultaneously deposited on the O-face exhibits an inferior crystalline structure ∼0.7% strained in the c-direction and a higher atomic number contrast compared with the substrate, as revealed by high angle annular dark field imaging measurements. These differences between the Zn- and O-face films are discussed in detail and associated with the different growth mechanisms prevailing on the two surfaces.

Development of a XYθz 3-DOF nanopositioning stage with linear displacement amplification device

The main purpose of this study is to develop a XYθ Z 3-DOF nanopositioning stage with linear displacement amplification device. This stage is used by three groups of linear displacement amplification device to composition, and thus achieves precise XYθ Z 3-DOF movement. The linear displacement amplification device makes use of a symmetrical layer mechanism, toggle amplification mechanism and parallel guiding spring to composition, and with amplification capability. Then it uses an equilateral triangle way to set the three groups of linear displacement amplification device. Overall design uses the finite element software ANSYS 12.0 to analysis the displacement, stress and dynamic response of nanopositioning stage. Experiment demonstrated takes the laser interferometer as the standard of displacement measurement. The result of experiment measurement shows that the maximum XY axis displacement and θ Z rotational angle travel of the stage is 36.5 μm, 32 μm and 265 arc sec.

Prospective IS-MST radar. Potential and diagnostic capabilities

In the next few years, a new radar is planned to be built near Irkutsk. It should have capabilities of incoherent scatter (IS) radars and mesosphere-stratosphere-troposphere (MST) radars [Zherebtsov et al., 2011]. The IS-MST radar is a phased array of two separated antenna panels with a multichannel digital receiving system, which allows detailed space-time processing of backscattered signal. This paper describes characteristics, configuration, and capabilities of the antenna and transceiver systems of this radar. We estimate its potential in basic operating modes to study the ionosphere by the IS method at heights above 100 km and the atmosphere with the use of signals scattered from refractive index fluctuations, caused by turbulent mixing at heights below 100 km. 
 
 The modeling shows that the radar will allow us to regularly measure neutral atmosphere parameters at heights up to 26 km as well as to observe mesosphere summer echoes at heights near 85 km in the presence of charged ice particles (an increase in Schmidt number) and mesosphere winter echoes at heights near 65 km with increasing background electron density. Evaluation of radar resources at the IS mode in two height ranges 100–600 and 600–2000 km demonstrates that in the daytime and with the accumulation time of 10 min, the upper boundaries of electron density and ionospheric plasma temperature are ~1500 and ~1300 km respectively, with the standard deviation of no more than 10 %. The upper boundary of plasma drift velocity is ~1100 km with the standard deviation of 45 m/s. The estimation of interferometric capabilities of the MST radar shows that it has a high sensitivity to objects of angular size near 7.5 arc min, and its potential accuracy in determining target angles can reach 40 arc sec.

Перспективный радар НР-МСТ: потенциал и диагностические возможности

In the next few years, a new radar is planned to be built near Irkutsk. It should have capabilities of incoherent scatter (IS) radars and mesosphere-stratosphere-troposphere (MST) radars [Zherebtsov et al., 2011]. The IS-MST radar is a phased array of two separated antenna panels with a multichannel digital receiving system, which allows detailed space-time processing of backscattered signal. This paper describes characteristics, configuration, and capabilities of the antenna and transceiver systems of this radar. We estimate its potential in basic operating modes to study the ionosphere by the IS method at heights above 100 km and the atmosphere with the use of signals scattered from refractive index fluctuations, caused by turbulent mixing at heights below 100 km. The modeling shows that the radar will allow us to regularly measure neutral atmosphere parameters at heights up to 26 km as well as to observe mesosphere summer echoes at heights near 85 km in the presence of charged ice particles (an increase in Schmidt number) and mesosphere winter echoes at heights near 65 km with increasing background electron density. Evaluation of radar resources at the IS mode in two height ranges 100–600 and 600–2000 km demonstrates that in the daytime and with the accumulation time of 10 min, the upper boundaries of electron density and ionospheric plasma temperature are ~1500 and ~1300 km respectively, with the standard deviation of no more than 10 %. The upper boundary of plasma drift velocity is ~1100 km with the standard deviation of 45 m/s. The estimation of interferometric capabilities of the MST radar shows that it has a high sensitivity to objects of angular size near 7.5 arc min, and its potential accuracy in determining target angles can reach 40 arc sec.

Surgical Effect of Medial Rectus Posterior pulley fixation in Esotropia greater at near fixation

PurposeTo determine the surgical effect of medial rectus posterior pulley fixation in esotropia greater at near fixation.MethodsMedical records of consecutive patients who underwent medial rectus posterior pulley fixation for esotropia greater at near fixation and followed up at least 1 year were reviewed retrospectively. Surgical success was defined as orthotropic or ≤8 prism diopters (PD) esotropia at distance at 1 year after surgery. Preoperative and postoperative deviation, distance‐near disparity and the sensory status were evaluated. Stereoacuity was measured with Titmus test and normal stereoacuity was defined as ≤100 arc sec at one year after surgery.ResultsSixteen patients were included the study. The mean age at surgery was 6.29 ± 2.17 years (range 3.00–9.92) and mean follow up period was 18.8 ± 4.79 months (range 12.00–27.00). Preoperative angle of deviation was 23.31 ± 12.37 PD at distance, 33.81 ± 10.95 PD at near and distance‐near disparity was 10.50 ± 4.77 PD. Twelve patients underwent bilateral medial rectus recession and 4 underwent unilateral medial rectus recession. Fourteen (85.8%) patients achieved surgical success and 2(14.2%) patients were undercorrected. The mean distance‐near discrepancy was 5.81 ± 4.48 PD at 1 year after surgery, which was significantly decreased (p = 0.018, Wilcoxon signed ranked test). Normal stereoacuity at near fixation was achieved in 10 (32.5%) patients at 1 year after surgery.ConclusionsMedial rectus posterior pulley fixation may be the effective method in esotropia greater at near fixation.

An integral field spectrograph utilizing mirrorlet arrays

AbstractAn integral field spectrograph (IFS) has been developed that utilizes a new and novel optical design to observe two spatial dimensions simultaneously with one spectral dimension. This design employs an optical 2‐D array of reflecting and focusing mirrorlets. This mirrorlet array is placed at the imaging plane of the front‐end telescope to generate a 2‐D array of tiny spots replacing what would be the slit in a traditional slit spectrometer design. After the mirrorlet in the optical path, a grating on a concave mirror surface will image the spot array and provide high‐resolution spectrum for each spatial element at the same time; therefore, the IFS simultaneously obtains the 3‐D data cube of two spatial and one spectral dimensions. The new mirrorlet technology is currently in‐house and undergoing laboratory testing at NASA Goddard Space Flight Center. Section 1 describes traditional classes of instruments that are used in Heliophysics missions and a quick introduction to the new IFS design. Section 2 discusses the details of the most generic mirrorlet IFS, while section 3 presents test results of a lab‐based instrument. An example application to a Heliophysics mission to study solar eruptive events in extreme ultraviolet wavelengths is presented in section 4 that has high spatial resolution (0.5 arc sec pixels) in the two spatial dimensions and high spectral resolution (66 mÅ) across a 15 Å spectral window. Section 4 also concludes with some other optical variations that could be employed on the more basic IFS for further capabilities of this type of instrument.

A Global Prospective of the Indian Optical and Near-Infrared Observational Facilities in the Field of Astronomy and Astrophysics: A Review

A review of modernization and growth of ground based optical and near-infrared astrophysical observational facilities in the globe attributed to the recent technological developments in optomechanical, electronics and computer science areas is presented. Hubble Space Telescope (HST) and speckle and adaptive ground based imaging have obtained images better than 0.1 arc sec angular resolution bringing the celestial objects closer to us at least by a factor of 10 during the last two decades. From the light gathering point of view, building of large size (more than 5 meter aperture) ground based optical and nearinfrared telescopes based on latest technology have become economical in recent years. Consequently, in the world, a few 8-10 meter size ground-based optical and near-infrared telescopes are being used for observations of the celestial objects, three 25-40 meter size are under design stage and making of a ~ 100 meter size telescope is under planning stage. In India, the largest sized optical and near-infrared telescope is the modern 3.6-meter located at Devasthal, Nainital. However, the existing Indian moderate size telescopes equipped with modern backend instruments have global importance due to their geographical location. Recently, participation of India in the Thirty Meter Telescope (TMT) project has been approved by the Government of India.

Note: Autocollimation with ultra-high resolution and stability using telephoto objective together with optical enlargement and beam drift compensation.

An autocollimation (AC) setup with ultra-high resolution and stability for micro-angle measurement is presented. The telephoto objective, which is characterized in long focal length at a compact structure size, and the optical enlargement unit, which can magnify the image displacement to improve its measurement resolution and accuracy, are used to obtain an ultra-high measurement resolution of the AC. The common-path beam drift compensation is used to suppress the drift of measurement results, which is evident in the high-resolution AC, thus to obtain a high measurement stability. Experimental results indicate that an effective resolution of better than 0.0005 arc sec (2.42 nrad) over a measurement range of ±30 arc sec and a 2-h stability of 0.0061 arc sec (29.57 nrad) can be achieved.

A fiber-coupled displacement measuring interferometer for determination of the posture of a reflective surface.

A fiber-coupled displacement measuring interferometer capable of determining of the posture of a reflective surface of a measuring mirror is proposed. The newly constructed instrument combines fiber-coupled displacement and angular measurement technologies. The proposed interferometer has advantages of both the fiber-coupled and the spatially beam-separated interferometer. A portable dual-position sensitive detector (PSD)-based unit within this proposed interferometer measures the parallelism of the two source beams to guide the fiber-coupling adjustment. The portable dual PSD-based unit measures not only the pitch and yaw of the retro-reflector but also measures the posture of the reflective surface. The experimental results of displacement calibration show that the deviations between the proposed interferometer and a reference one, Agilent 5530, at two different common beam directions are both less than ±35 nm, thus verifying the effectiveness of the beam parallelism measurement. The experimental results of angular calibration show that deviations of pitch and yaw with the auto-collimator (as a reference) are less than ±2 arc sec, thus proving the proposed interferometer's effectiveness for determination of the posture of a reflective surface.

Growth and characterization of metamorphic InAs/GaSb tunnel heterojunction on GaAs by molecular beam epitaxy

The structural, morphological, optical, and electrical transport characteristics of a metamorphic, broken-gap InAs/GaSb p-i-n tunnel diode structure, grown by molecular beam epitaxy on GaAs, were demonstrated. Precise shutter sequences were implemented for the strain-balanced InAs/GaSb active layer growth on GaAs, as corroborated by high-resolution X-ray analysis. Cross-sectional transmission electron microscopy and detailed micrograph analysis demonstrated strain relaxation primarily via the formation of 90° Lomer misfit dislocations (MDs) exhibiting a 5.6 nm spacing and intermittent 60° MDs at the GaSb/GaAs heterointerface, which was further supported by a minimal lattice tilt of 180 arc sec observed during X-ray analysis. Selective area diffraction and Fast Fourier Transform patterns confirmed the full relaxation of the GaSb buffer layer and quasi-ideal, strain-balanced InAs/GaSb heteroepitaxy. Temperature-dependent photoluminescence measurements demonstrated the optical band gap of the GaSb layer. Strong optical signal at room temperature from this structure supports a high-quality material synthesis. Current–voltage characteristics of fabricated InAs/GaSb p-i-n tunnel diodes measured at 77 K and 290 K demonstrated two bias-dependent transport mechanisms. The Shockley–Read–Hall generation–recombination mechanism at low bias and band-to-band tunneling transport at high bias confirmed the p-i-n tunnel diode operation. This elucidated the importance of defect control in metamorphic InAs/GaSb tunnel diodes for the implementation of low-voltage and high-performance tunnel field effect transistor applications.

ICESat‐derived inland water surface spot heights

AbstractAccurate measurement of water surface height is key to many fields in hydrology and limnology. Satellite radar and laser altimetry have been shown to be useful means of obtaining such data where no ground gauging stations exist, and the accuracy of different satellite instruments is now reasonably well understood. Past validation studies have shown water surface height data from the ICESat instrument to have the highest vertical accuracy (mean absolute errors of ∼10 cm for ICESat, compared, for example, with ∼28 cm from Envisat), yet no freely available source of processed ICESat data currently exists for inland water bodies. Here we present a database of processed and quality checked ICESat‐derived inland water surface heights (IWSH) for water bodies greater than 3 arc sec (∼92 m at the equator) in width. Four automated methods for removing spurious observations or outliers were investigated, along with the impact of using different water masks. We find that the best performing method ensures that observations used are completely surrounded by water in the SRTM Water Body data. Using this method for removing spurious observations, we estimate transect‐averaged water surface heights at 587,292 unique locations from 2003 to 2009, with the number of locations proportional to the size of the river.

Corneal Transplantation in Disease Affecting Only One Eye: Does It Make a Difference to Habitual Binocular Viewing?

BackgroundClarity of the transplanted tissue and restoration of visual acuity are the two primary metrics for evaluating the success of corneal transplantation. Participation of the transplanted eye in habitual binocular viewing is seldom evaluated post-operatively. In unilateral corneal disease, the transplanted eye may remain functionally inactive during binocular viewing due to its suboptimal visual acuity and poor image quality, vis-à-vis the healthy fellow eye.Methods and FindingsThis study prospectively quantified the contribution of the transplanted eye towards habitual binocular viewing in 25 cases with unilateral transplants [40yrs (IQR: 32–42yrs) and 25 age-matched controls [30yrs (25–37yrs)]. Binocular functions including visual field extent, high-contrast logMAR acuity, suppression threshold and stereoacuity were assessed using standard psychophysical paradigms. Optical quality of all eyes was determined from wavefront aberrometry measurements.Binocular visual field expanded by a median 21% (IQR: 18–29%) compared to the monocular field of cases and controls (p = 0.63). Binocular logMAR acuity [0.0 (0.0–0.0)] almost always followed the fellow eye’s acuity [0.00 (0.00 –-0.02)] (r = 0.82), independent of the transplanted eye’s acuity [0.34 (0.2–0.5)] (r = 0.04). Suppression threshold and stereoacuity were poorer in cases [30.1% (13.5–44.3%); 620.8arc sec (370.3–988.2arc sec)] than in controls [79% (63.5–100%); 16.3arc sec (10.6–25.5arc sec)] (p<0.001). Higher-order wavefront aberrations of the transplanted eye [0.34μ (0.21–0.51μ)] were higher than the fellow eye [0.07μ (0.05–0.11μ)] (p<0.001) and their reduction with RGP contact lenses [0.09μ (0.08–0.12μ)] significantly improved the suppression threshold [65% (50–72%)] and stereoacuity [56.6arc sec (47.7–181.6arc sec)] (p<0.001).ConclusionsIn unilateral corneal disease, the transplanted eye does participate in gross binocular viewing but offers limited support to fine levels of binocularity. Improvement in the transplanted eye’s optics enhances its participation in binocular viewing. Current metrics of this treatment success can expand to include measures of binocularity to assess the functional benefit of the transplantation process in unilateral corneal disease.

Focused gas beam injection for efficient ammonia-molecular beam epitaxial growth of III-nitride semiconductors

A focused gas beam injection is proposed for high-efficiency ammonia molecular beam epitaxial growth of III-nitride. This new injector design is based on a double, coaxial radial high-conductance geometry, which allows rotation-free growth with fast gas switching. The injection profile is characterized through a mobile ion gauge and is then compared to simulations, where experimental results show that up to 27% of the injected molecules reach the surface of the substrate. The injector is tested for the growth of GaN layers, and high-resolution x-ray diffraction rocking curves of a 1 μm-thick GaN layer grown on a commercial GaN template (1 μm-thick layer of GaN on Si) was measured around the 002 Bragg condition and a full width at half maximum of 594 arc sec was obtained. Low-temperature photoluminescence for the same layer shows intense band edge emission and a low yellow luminescence. Hall measurements of the silicon-doped layers show high carrier concentrations up to 2 × 1019 cm−3 and a corresponding mobility of 204 cm2/V s.

Nine-parameter curvilinear transformation between datums

One major challenge, in geodesy, is the conversion of geodetic coordinates related to a geodetic datum to geodetic coordinates related to another datum, due to differences between datums in size and position. Essentially, there are two approaches, where the first, and most well known, is conversion via three-dimensional Cartesian coordinate space. The second, curvilinear transformation, is a direct transformation of latitude-longitude coordinates between two datums. The method proposed in this paper is named nine-parameter curvilinear transformation between datums and may be considered a new curvilinear transformation method. This method has good accuracy, since the difference between suggested transformation and seven-parameter transformation does not exceed 0.0015 arc sec. Moreover, it replaces several steps in seven-parameter transformation methods by one direct step.

Two-step heating synthesis of sub-3 millimeter-sized orthorhombic black phosphorus single crystal by chemical vapor transport reaction method

A facile and green strategy to synthesize orthorhombic black phosphorus ( o -BP) single crystals with high yield (~90%) and large size (sub-3 millimeter) is presented. The strategy was based on a two-step heating chemical vapor transport (CVT) reaction method, in which tin and iodine (Sn/I2) was used as mineralization additives and red phosphorus as precursor. Tin phosphide was the only by-product captured at the end of reaction, which greatly simplified the subsequent separation and purification processes of o -BP single crystals. The full width at half maximum (FWHM) of X-ray rocking curve of the as-grown o -BP was 21.65 arc sec, indicating its respectable crystalline quality. A bottom electrode structure field-effect transistor (FET) based on the multilayer phosphorene mechanically exfoliated from the as-grown o -BP single crystal was successfully fabricated through an all-dry transfer technique. Impressively, the FET based on a 6 nm thick multilayer (approximate 12 layers) phosphorene exhibited a record high hole mobility ( μ p) of 1744 cm2 V-1 s-1 and an admirable on/off current switching ratio ( I on/ I off) of ~104, which further proved the high-quality of the o -BP single crystals synthesized by the twostep heating CVT reaction method using the simple Sn/I2/red phosphorus system.

Exoplanet detection and characterization via parallel broadband nulling coronagraphy

The contrast and angular resolution required to directly image and characterize mature exoplanetary systems place stringent requirements on the space-based telescopes and starlight suppression systems needed to study spatial distributions of debris disks, exozodiacal dust, and individual planets at multiple epochs in their orbits. A nulling interferometer (nuller) is a coronagraphic suppression system that can be used with all telescope types, including those with obscured and segmented apertures envisioned for upcoming and future observatories. One of the challenges for detection and characterization of exoplanetary signals is achieving high contrast with broad spectral coverage. This work presents design concepts for broadband nulling over four parallel ∼20% bandpasses spanning the visible spectrum. Contrast-limiting effects of stellar angular extent, residual chromaticity of broadband phase shifters, and aperture diffraction are considered to reach simultaneous ≲2×10−8 contrast over separations spanning 0.2 to 0.9 arc sec for a 2.4-m telescope observing a Sun-like star at 10 pc. With added dark hole wavefront control and postprocessing point spread function subtraction techniques to further reduce scattered starlight, such a system could be capable of detecting the very the nearest Earth-like exoplanets and spectral characterization of several nearby extrasolar gas giants.

Dynamic analysis of the Green Bank Telescope structure and servo system

The Green Bank Telescope is a 100-m aperture single-dish radio telescope. For high-frequency observations (above 100 GHz), it needs a tracking error below 1.5 arc sec rms. The present system has a tracking error of 1 arc sec rms for very low wind speeds of ≤1 m/s, which increases well above 1.5 arc sec for wind speeds above 4 m/s. Hence, improvements in the servo control system are needed to achieve pointing accuracy goals for high-frequency observations. As a first step toward this goal, it is necessary to evaluate the dynamic response of the present servo system and the telescope, which forms a large flexible structure. We derive the model of the telescope dynamics using finite element analysis data. This model is further tuned and validated using system identification experiments performed on the telescope. A reduced model is developed for controller design by using modes with the highest Hankel singular value for frequencies up to 2 Hz. We quantify the uncertainty in azimuth axis dynamics with a change in elevation angle by varying the zeros of the model. We discuss the effects of transient response, wind disturbances, and azimuth track joint disturbances on telescope tracking performance.

Probing the low surface brightness dwarf galaxy population of the virgo cluster

We have used public data from the Next Generation Virgo Survey (NGVS) to investigate the dwarf galaxy population of the Virgo cluster beyond what has previously been discovered. We initially mask and smooth the data, and then use the object detection algorithm Sextractor to make our initial dwarf galaxy selection. All candidates are then visually inspected to remove artefacts and duplicates. We derive Sextractor parameters to best select low surface brightness galaxies using g band central surface brightness values of 22.5 to 26.0 mag sq arc sec and exponential scale lengths of 3.0 - 10.0 arc sec to identify 443 cluster dwarf galaxies - 303 of which are new detections. These new detections have a surface density that decreases with radius from the cluster centre. We also apply our selection algorithm to 'background', non-cluster, fields and find zero detections. In combination, this leads us to believe that we have isolated a cluster dwarf galaxy population. The range of objects we are able to detect is limited because smaller scale sized galaxies are confused with the background, while larger galaxies are split into numerous smaller objects by the detection algorithm. Using data from previous surveys combined with our data, we find a faint end slope to the luminosity function of -1.35+/-0.03, which does not significantly differ to what has previously been found for the Virgo cluster, but is a little steeper than the slope for field galaxies. There is no evidence for a faint end slope steep enough to correspond with galaxy formation models, unless those models invoke either strong feedback processes or use warm dark matter.