Question

Instructions

Identify a current problem in physics by searching for news articles and current events. One reputable source of news in physics is Phys.org. Choose one article, and in two pages, describe how the scientific method is being used to solve the problem mentioned in the article. Identify the initial observations that identified the problem, the hypothesis, tests, and any revisions of the original hypothesis. Cite the article in APA format as well as other references you might use.

LINK:
https://phys.org/physics-news/

Answer 1

Refered article: https://phys.org/news/2019-02-virtual-lens-x-ray-microscopy.html

Wakonig et al., Sci. Adv. 2019;5: eaav0282

Our electromagnetic spectrum consists of a broad band of wavelengths ranging from the micro waves to the gamma rays. Of them X-rays having a broad range of energy (100-50,000eV, hard and soft X-rays) plays a significant role in medical diagnosis, material science and condensed matter problems, archaeology and many more. It is especially a boon for the medical society, since X-rays have huge applications in imaging, radiography and many more. However, since in X-ray region the refractive index is less than unity, simple lens based optics as used for the visible light cannot be used because of its high absorption. Thus, in X-ray region optics may be of three different types- (a) diffractive optics which consists of Fresnel's zone plate (b) reflective optics, mirrors & multilayers and (c) refractive optics.

There are several literature available which shows the applications of various types of X-ray optics in imaging. It is known that one can obtain ideal image only in presence of high resolution, high numerical aperture and high contrast. However, the X-ray optics used till now for imaging purpose contains many flaws. The reflective as well as the refractive optics can create distortions in the image. Though the diffractive optics are less prone to abberations but still they are less efficient.

A new technique is thus developed which provide near to ideal image and is based on lensless technique. In this technique various mathematical algothrims are used to produce image. This technique is known as "Ptychography".

The experiments were carried out at Swiss light source where the high resolution ideal images were obtained using two different techniques. In the first technique a full field Transmission X-ray microscopic (TXM) image was acquired. During the experiment, the beam was passed through a condenser lens with a diameter of 1 mm, a subfield size of 50 mm, and an outermost zone width of 70 nm . In this set up the numerical aperture (NA) is found to be 10⁻³ at the wavelength of 0.14. A 150 mm central stop was fixed close to the condenser in order to avoid the direct beam. Every optical element was placed in combination with the stepper motors in order to allow three dimensional (3D) movements. The images were collected with a X-ray camera, based on a full-frame Kodak CCD having an optical resolution of 4 mm. The image obtained has a magnification of 150 and the effective pixel size was 27 nm in the sample plane. It was assumed that the highest intensities occur at the center of the subfield and thus with this assumption a 3 s frame was acquired at each such local maximum.

After conduction of experiment, the next step was to reconstruct the images. Of the total measurements, 142 number of acquisitions were used for the Fourier ptychographic reconstruction. Another challenge was to the tackle the artifact arising from the artifacts which results from camera’s fiber coupling. To treat this the raw data resulting from the camera’s fiber coupling acquisitions were intensity-thresholded and pixels below this threshold were replaced by nearest-neighbor interpolation. The reconstructed spectrum was being in Fourier space where transformed back to the direct space. To estimate image resolution using FRC, the data were split into two disjoint sets, which were analyzed independently. Having thereby divided the flux by two, we compared the FRC to the 1 /2-bit criterion.

In the second technique the full field TXM image was acquired, however the condenser lens, the central stop, and the order-sorting aperture were removed. Instead of those a 30 mm pinhole close to the object plane was placed. The same objective lens as was used in 1st technique created a magnified image of the sample at a distance of 7.9 m, where the direct beam was blocked by central stop. The pinhole objective was mounted on a 3D piezo-electric stage with a maximum range of 100 mm. The data were collected using a 400 × 400 pixel prototype MOENCH detector with a physical pixel size of 25 mm.

In this technique also reconstruction of images were carried out. In this case also the artifacts may be introduced in the reconstructed data which can affect the resolution as calculated by FRC. To prevent this, a second Fermat’s scan was performed, with an average step size of 5 mm for the Fresnel Zone Plate while keeping the same overall scan range of 80 mm. At each of the 255 scan points, 30,000 acquisitions of 0.25 ms were taken. From the results obtained from these two independent measurements the FRC obtained was compared with the 1-bit criterion.

As mentioned previously, here also near-neighbour data interpolation were carried out on the detector which again introduced periodic artifacts. Proper alignment was carried out to avoid the directional dependence of magnification. The inner part of the scan pattern was discarded since it was affected by direct beam. The different frames obtained in scan pattern differ in spatial-frequency content, thus the alignment was performed sequentially on frames with optimum overlap in the frequency domain. Accumulation of error was removed by repeating the process in reverse order and the process was continued until convergence was obtained. The Fourier ptychographic reconstruction took 600 iterations of the difference map to converge. Finally the solution was refined by 1800 iterations of likelihood optimization, before the reconstructed spectrum was transformed back to direct space. Two coherent modes were reconstructed for the pupil function, of which the 2nd mode was discarded since it contains approximately 33% of energy and displaying essentially no structure, as background contribution.

Both of these measurements illustrate the utility of modifications in data acquisition strategy enhance TXM imaging capabilities in terms of resolution, contrast, and image distortions. These techniques will be utilized for in situ or in operando measurements having the requirement of large working distances between sample and objective lens. With the advent of availability of higher brilliance sources, Fourier ptychography pioneers in achieving high coherence.