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Agnuslims in the Middle East

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Coherence X 3.3

In the previous investigations, the retinal blood perfusion was examined applying techniques such as fluorescein angiography, confocal scanning laser ophthalmoscopic angiography with fluorescein dye, color Doppler imaging, Canon laser blood flowmetry, scanning laser Doppler flowmetry, and retinal photographic oximetry15,16,17,18. All these techniques had disadvantages such as limitations in spatial resolution, relatively low validity, or only indirectly assessing parameters of blood flow, to mention a few. Split-spectrum amplitude-decorrelation angiography (SSADA) associated with optical coherence tomography angiography (OCTA) is a new method that can visualize the vascular networks in separate layers of the retina in the macular region and in the ONH19,20,21. Recent studies confirmed a high intra-visit repeatability and inter-visit reproducibility of the measurements taken with OCTA, a high spatial resolution of the images, and the possibility of the OCTA technique to assess for the first time the retinal blood circulation system at different levels of the retina and ONH22. In view of these new technical possibilities, we conducted the present study to re-assess the relationship between the blood circulation in the retina and optic nerve in eyes at a normal IOP and at an elevated level of IOP. In contrast to previous investigations in which artificial means, such as a suction cup, were applied to increase the IOP, we used the physiological model of IOP elevation by performing a dark room provocative test23,24,25.

Coherence X 3.3

The phase retrieval of diffraction with oversampling strategies was introduced by Sayre in 1952 (Sayre, 1952), though several decades would pass before coherent diffractive imaging (CDI) would be demonstrated with X-rays on a fabricated test pattern (Miao et al., 1999). A few years later, CDI techniques were applied to a gold nanoparticle in Bragg geometry to image 3D structure (Robinson et al., 2001) and the strain introduced by re-crystallization (Williams et al., 2003). In the Bragg coherent diffractive imaging (BCDI) experiment, fully coherent X-rays illuminate a single crystal that is within the coherence volume of the incident beam. Diffracted X-rays interfere about the Bragg peaks, and form coherent diffraction features about the center of the Bragg peak. One can measure the 3D coherent diffraction pattern in the vicinity of the Bragg peak, by rocking the crystal over a fraction of a degree in orientation relative to the incident beam. The 3D crystal structure and lattice distortion field can then be retrieved via phase retrieval to produce an image of the crystal. BCDI is highly sensitive to strains or the disorder/deviation in the highly periodic crystalline system. Transmission electron microscopy (TEM) has also been used to study strains in materials with very high spatial resolution (Cherkashin et al., 2017; Hÿtch et al., 2008). However, due to the penetration power of X-rays, BCDI allows one to probe 3D strain information non-destructively in operando materials with little specific sample preparation required. The white-beam Laue scanning microscope (Larson et al., 2002) and the three-dimensional X-ray diffraction microscope (Schmidt et al., 2004) have also demonstrated the capabilities of strain imaging, though the spatial resolution of these methods has not yet reached below the size of typical battery cathode particles.

OCT is based on low coherence interferometry [4, 15]. The time delay and the intensity of backscattered light from the sample are measured by evaluating the interference of a light beam with low temporal coherence from a reference arm and the sample arm. Technically, the experiment is realized by splitting an illumination beam into a reference and a signal beam using, e.g., a Michelson-type interferometer, where one of the mirrors is replaced by the sample. The rescattered light from the sample is superimposed with the light from the reference arm. Due to a broad bandwidth of the illumination and as a consequence of its short coherence length, interference only occurs if the path difference of the two beams is smaller than the coherence length. Thus, by scanning the path difference, the axial structure of the sample in the direction of the optical axis can be probed (time-domain OCT). Alternatively, the axial information can be obtained for a fixed path-difference by analyzing the spectrum of the backscattered light at the output of the interferometer (frequency-domain OCT). Additionally, via conventional imaging, the lateral structure of the sample can be obtained by scanning a focused illumination beam across the sample.

One of the main advantages of OCT is that the axial and the lateral resolutions are decoupled. The lateral resolution is limited by the focus spot size provided by the conventional imaging, whereas the axial resolution is equivalent to the coherence length of the radiation source. For a light source with a Gaussian spectral distribution, the coherence length is given by:

where \(\lambda _0\) is the central wavelength and \(\Delta \lambda\) is the spectral width of the light source [16]. In the near-infrared regime with a central wavelength of \(\lambda _0= 1\, \mu \text m\) and a bandwidth of \(\Delta \lambda =400\,\text nm\), the axial resolution is limited to \(1.1\,\mu \text m\). Although this resolution is sufficient to investigate, for example, retinal structures, imaging of nanoscaled objects like semiconductor structures is out of reach. However, the coherence length and thus the axial resolution of OCT can significantly be improved using radiation with a shorter wavelength.

XUV coherence tomography (XCT) extends optical coherence tomography (OCT) into the extreme ultraviolet (XUV) and soft X-ray (SXR) range, which enables approaching axial resolutions in the nanometer regime [17]. XCT has been demonstrated for the first time with a synchrotron radiation source [18]. Although the applicability of synchrotron-based imaging methods is limited due to troublesome accessibility of the large-scale facilities, it has been shown that three-dimensional imaging of axial nanostructures is feasible [18]. However, in recent years, the rapid development of extremely broad bandwidth laser-driven XUV and SXR sources using the high-harmonic generation (HHG) [19] and laser-plasma sources (LPS) [20, 21] has facilitated the realization of XCT at a laboratory scale [22, 23].

In Table 1, we summarize the most important features of different realizations of the coherence tomography experiments in the XUV/SXR regime. The features are grouped according to the type of the radiation source used in the experiments. The table-top laser-based sources based on the HHG process (XCT in Si window) and LPS (XCT in the water window) are compared with results obtained using the synchrotron radiation (PETRA III). The following discussion concentrates on the most essential features of each approach.

However, like in OCT, lateral imaging can be added to XCT by repeating the depth measurement with a focused illumination in a scanning approach. The lateral resolution is therefore limited by the NA of the focusing optics, but the axial resolution remains only dependent on the coherence length of the light source and is thus independent on the focusing. This is a considerable advantage especially in the XUV range, where high-NA focusing is extremely demanding and expensive. Although not shown in this article, the XCT implementations at synchrotrons and HHG sources already included lateral imaging by using toroidal mirrors. These reflective grazing-incidence optics provide a lateral resolution of a few tens of micrometers. Further improvement of the lateral resolution of XCT requires better focusing optics with more advanced components such as multiple toroidal mirrors [47], Kirkpatrick-Baez Optics [48], or zone plates [49]. Alternatively, a mask with a small hole in front of the sample can be used to limit the area of interaction and thus increasing the lateral resolution at the expense of the usable photon flux.

These experiments break a new frontier in the coherence of hybrid spin-superconducting qubit devices. The new coupling regime achieved can enable long-range two-qubit gates between distant spin qubits or improve qubit readout.

Our analysis shows that the obscuring gas in NGC 3783 varies on timescales in the range between one hour and ten hours. From a physical point of view, the observed variations could be due to a fast response of the obscuring gas to variations of the illuminating continuum, possibly combined with variability associated with an inhomogeneous structure crossing our line of sight. In any event, changes in the properties of the obscurer are expected to produce non-linearly correlated (i.e. incoherent) variability (e.g. Rybicki & Lightman 1991). To verify this hypothesis, we carried out a comparative analysis of the rms and covariance spectra of the source, and of the coherence spectra.

To this aim we first measured the intrinsic coherence (Vaughan & Nowak 1997; Uttley et al. 2014) of the source as a function of energy, in the two epochs. While the Fvar (Sect. 3.2) shows the distribution of variable flux over energy, the coherence spectrum picks out only the coherently variable (i.e. linearly correlated with the broad band X-ray continuum) fraction, and is not influenced by the presence of constant components.

We obtained energy-dependent light curves of the transmitted plus scattered emission by integrating each synthetic spectrum within the energy bins used to compute the coherence spectrum (Fig. 5, right). Fluxes were then converted to observed count rate, and randomised in order to include the effects of counting noise. We used these light curves to compute the expected coherence spectrum resulting from variability of the ionisation state of the obscuring gas in response to variations of the ionising continuum. This is shown in Fig. 5 (right), where the corresponding 90% confidence contours (red shaded area) are overplotted on the data. 041b061a72

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