Suppressing Diffraction-Related Intensity Losses in Transmissive Single-Crystal X-ray Optics

The highest-quality X-ray optics can be made of single-crystal materials such as silicon, germanium, or, even better, diamond. Unfortunately, such X-ray optics have one drawback: diffraction losses or the “glitch effect”. This effect manifests itself as follows: at some energies of X-rays, the intensity of the transmitted beam drops due to the fact that some crystalline planes have satisfied the diffraction condition. Diffraction losses are usually observed in spectroscopic experiments when the energy of the X-rays changes in a certain range. However, this effect might also influence any experiment using X-rays, especially at higher energies. In this paper, we propose a method to overcome the glitch problem in transmissive optics. This is achieved using small rotations of the optical element. We describe the algorithm for “glitch-free” measurements in detail and the theory behind it.

Transmission characteristics of terahertz Bessel vortex beams through a multi-layered anisotropic magnetized plasma slab

The transmission of terahertz (THz) Bessel vortex beams through a multi-layered anisotropic magnetized plasma slab is investigated by using a hybrid method of cylindrical vector wave functions (CVWFs) and Fourier transform. On the basis of the electromagnetic boundary conditions on each interface, a cascade form of expansion coefficients of the reflected and transmitted fields is obtained. Taking a double Gaussian distribution of the plasma density as an example, the influences of the applied magnetic field, the incident angle and polarization mode of the incident beams on the magnitude, OAM mode and polarization of the transmitted beams are analyzed in detail. The results indicate that the applied magnetic field has a major effect upon the polarization state of the transmitted fields but not upon the transmitted OAM spectrum. The incident angle has a powerful influence upon both the amplitude profile and the OAM spectrum of the transmitted beam. Furthermore, for multiple coaxial vortex beams, an increase of the maximum value of the plasma density causes more remarkable distortion of both the profile and OAM spectrum of the transmitted beam. This research makes a stable foundation for the THz OAM multiplexing/demultiplexing technology in magnetized plasma environment.

Data Link with a High-Power Pulsed Quantum Cascade Laser Operating at the Wavelength of 4.5 µm

This article is a short study of the application of high-power quantum cascade lasers and photodetectors in medium-infrared optical wireless communications (OWC). The link range is mainly determined by the transmitted beam parameters and the performance of the light sensor. The light power and the photodetector noise directly determine the signal-to-noise power ratio. This ratio could be maximized in the case of minimizing the radiation losses caused by atmospheric attenuation. It can be obtained by applying both radiation sources and sensors operated in the medium infrared range decreasing the effects of absorption, scattering or scintillation, beam spreading, and beam wandering. The development of a new class of laser sources based on quantum cascade structures becomes a prospective alternative. Regarding the literature, there are descriptions of some preliminary research applying these lasers in data transmission. To provide a high data transfer rate, continuous wave (cw) lasers are commonly used. However, they are characterized by low power (a few tens of mWatts) limiting their link range. Also, only a few high-power pulsed lasers (a few hundreds of mWatts) were tested. Due to their limited pulse duty cycle, the obtained modulation bandwidth was lower than 1 MHz. The main goal of this study is to experimentally determine the capabilities of the currently developed state-of-the-art high-power pulsed quantum cascade (QC) lasers and photodetectors in OWC systems. Finally, the data link range using optical pulses of a QC laser of ~2 W, operated at the wavelength of ~4.5 µm, is discussed.

Grazing Incidence Small-Angle Neutron Scattering: Background Determination and Optimization for Soft Matter Samples

Grazing incidence small-angle neutron scattering (GISANS) provides access to interfacial properties, e.g., in soft matter on polymers adsorbed at a solid substrate. Simulations in the frame of the distorted wave Born approximation using the BornAgain software allow to understand and quantify the scattering pattern above and below the sample horizon, in reflection and transmission, respectively. The small-angle scattering from the interfacial layer, visible around the transmitted beam, which might contribute also on the side of the reflected beam, can be understood in this way and be included into the analysis. Background reduction by optimized sample cell design is supported by simulations, paving the way for an optimized GISANS cell.

Study of two-photon absorption and optical limiting

Simultaneous absorption of two photons of identical or different frequencies to excite a system that is atom or molecule from the ground state to excited state has great importance. It has an important role as a spectroscopic tool for determining the positions of energy levels that are not connected to the atomic ground state by the one-photon transition. In this work, experimental study of two-photon absorption of fused silica, quartz crystal, and metal halides and optical limiters based on the semiconductors are studied. It was investigated that two photons absorption depletes the transmitted beam but carrier defocusing spreads the beam in space and hence density is reduced. It was realized that limiters with thin samples have low dynamic range and fluency is high on the damage-prone surface and irreversible damage occurs. Some Semiconductors with high nonlinearity are found to be useful materials for optical limiters to protect devices like sensors. BIBECHANA 18 (2) (2021) 26-31

Simulations of foil-based spin-echo (modulated) small-angle neutron scattering with a sample using McStas

For the further development of spin-echo techniques to label elastic scattering it is necessary to perform simulations of the Larmor precession of neutron spins in a magnetic field. The details of some of these techniques as implemented at the reactor in Delft are simulated. First, the workings of the magnetized foil flipper are simulated. A full virtual spin-echo small-angle neutron scattering instrument is built and tested without and with a realistic scattering sample. It is essential for these simulations to have a simulated sample that also describes the transmitted beam of unscattered neutrons, which usually is not implemented for the simulation of conventional small-angle neutron scattering (SANS) instruments. Finally, the workings of a spin-echo modulated small-angle neutron scattering (SEMSANS) instrument are simulated. The simulations are in good agreement with theory and experiments. This setup can be extended to include realistic magnetic field distributions to fully predict the features of future Larmor labelling elastic-scattering instruments. Configurations can now be simulated for more complicated combinations of SANS with SEMSANS.

Efficient high-pass filtering with practical, high-yield X-ray transmission mirror optics

Although the concept, first demonstration, and potential applications of X-ray transmission mirrors (XTMs) were initially described over 30 years ago, only a few implementations exist in the literature. This is attributed to the unsolved challenge of a thick frame supporting a thin, reflecting membrane which does not itself block the transmitted beam. Here, we introduce a novel approach to solve this problem by employing silicon microfabrication. A robust XTM frame has been fabricated by using a novel two-step etch process, which secures the thin-film membrane without blocking the transmitted beam. Specifically, we have fabricated delicate XTM optics with 90% yield, which consist of 280-nm-thick low-stress silicon nitride membrane windows that are 1.5 mm wide and 125 mm long on silicon substrates. The XTM optics have been demonstrated to be a more efficient high-pass X-ray filter; for example, when configured for 40% transmission of 11.3 keV photons, we measure the reduction of 8.4 keV photons by a factor of 56.

Diffusion Characteristics of Solar Beams Radiation Transmitting through Greenhouse Covers in Arid Climates

In hot and sunny regions, extensive solar radiation transmitted into greenhouses makes it essential to select a greenhouse cover with specific characteristics. Reflection and diffusion are the most important properties of covers for reducing solar heating load and scattering solar beams for better growth of plants. Three types of plastic films that are commonly used for covering greenhouses in arid climate were selected for this study. These were reflective-diffusive (RDF), diffusive (DF), and locally produced (CF) films. The films were used to cover three identical twin-span greenhouse models; the radiation components, air temperature (Ti), and relative humidity (RHi) were measured in each model. Transmission of solar radiation through each cover was characterized by determining: (i) The beam that is diffused during transmission, and (ii) the unscattered beam that is transmitted directly through the film. The results show that the diffuse radiation transmitted through the DF, RDF, and CF covers was enhanced, respectively by 77%, 85%, and 109% as a result of diffusing 34%, 33%, and 43% of the transmitted beam radiation by the DF, RDF, and CF covers, respectively during transmission. The diffusive nature of the tested covers increased the ratio of diffuse to direct beam radiation (D/B) from 0.3 outside the greenhouse to 0.77, 0.69, and 0.95 inside a greenhouse covered with DF, RDF, and CF, respectively. At around noon, the CF cover decreased Ti by about 5–10 °C and increased RHi by about 3%–5% compared to those under the DF and RDF covers. However, DF and RDF covers showed almost similar effects in Ti and RHi. The low-price CF cover showed higher diffusive-radiative properties than DF and RDF covers and can serve effectively in arid climate as an alternative covering material.

X-ray interference fringes from a weakly bent plane-parallel crystal with negative strain gradient

Under the anomalous transmission condition in the Bragg mode, X-ray interference fringes were observed between two beams with different hyperbolic trajectories in a very weakly bent plane-parallel perfect crystal with negative strain gradient. The origin of the fringes was analysed based on the dynamical theory of diffraction for a distorted crystal. In the reflected beam from the entrance surface, the interference fringes were observed between once- and twice-reflected beams from the back surface. In the transmitted beam from the back surface, the interference fringes were observed between the direct beam and once-reflected beam from the entrance surface. In the emitted beam from the lateral surface, the interference fringes were observed between the beams after different numbers of reflections in the crystal. The multiply reflected beams were formed by a combined result of long propagation length along the beam direction with large divergence of the refracted beams when the strain gradient was negative. The period of these interference fringes was sensitive to very weak strain, of the order of 10−7.