Bessel vortex beams with topological charges of $l=\pm 1$ and $l=\pm 2$ were produced in the terahertz spectral range from a free electron laser Gaussian beam ($\lambda =141\mu \mathrm{m}$) transformed using silicon binary diffractive optical elements. The spatial characteristics of the beams were obtained using a microbolometer array. A radius to path length ratio of $1:100$ was achieved for nondiffractive beams with the average power of 30 W. Surface plasmon polaritons (SPPs) on gold–zinc-sulphide–air interfaces were generated due to diffraction of vortex beams on a sample edge. A new effect, a dependence of the efficiency of SPP generation on the direction of the azimuthal component of incident-radiation Poynting vector, was revealed.

Green catalyst derived from plants, a cheap and abundant natural source, is used for the synthesis of multi-walled carbon nanotubes (MWNTs). The concept is unconventional and practically realized into existence by simple CVD growth while keeping away the potential hazards caused by metal catalyst on environment and living organisms. The notable points to mention of such growth are: (1) grown CNTs are free from toxic metal catalyst, (2) low growth temperature (575 °C) required and produced high yield vis-à-vis any other catalyst used so far reported, and (3) no need of expensive and complex systems for its synthesis. Besides, growth of SWNT as well as carbon nano-belts with hollow rectangular cross-section is observed when growth temperature increased to 800 °C, specifically, for the wall-nut extract. The samples were characterized by microscopic and spectroscopic analysis and the results verified our study. The present work provides innovative technique and may open up new avenues for CNTs synthesis and its applications.

In this paper, we have performed experimental, analytical, and numerical studies of beams with topological charges of $\pm 1$ and $\pm 2$ formed by silicon binary phase axicons (BPAs) with spiral zone structures. The axicons were illuminated with the Novosibirsk free electron laser radiation (a continuous stream of 100-ps pulses at $f=5.6$ MHz). The cw power of the beams produced reached 30 W and can by doubled via antireflection coating of the axicons. The intensity distribution in the beam cross sections was in good agreement with the Bessel functions and was kept constant within a distance of about $L/r\approx 190$ and 100, where the first ring radii of the beams r were 0.9 and 1.5 mm for the Bessel beams of the first and second orders, respectively. Although the characteristics of the beams (Bessel cross section, “diffraction-free” propagation, self-recovery after passing obstacles, and randomly inhomogeneous media) corresponded to the properties of ideal Bessel beams, their spatial Fourier spectrum (the image in the focal plane of the lens) was, instead of an ideal ring, intertwined segments of arcs with phases shifted by $\pi$, the number of which was equal to the double value of the topological charge. This feature can be used, for example, in a demultiplexing unit of a free vortex-wave communication system or for identification of beam topological charge. We also revisited Young's double-slit diffraction and rotation of beams obstructed by a half-plane, previously applied to Laguerre-Gaussian beam characterization, in the case of the Bessel beams. The Young diffraction pattern demonstrated in this case a complicated intensity-phase distribution. It was shown that the Bessel beams formed by BPAs have two important advantages, which can be used in applications, in comparison with other methods of generation, e.g., a combination of an axicon lens with a spiral phase plate. Although the phase jumps of the axicons are designed for a determined wavelength (141 $\mu \mathrm{m}$ in our case), the BPAs can form the beams at incident radiation with any wavelength, albeit with a reduced diffraction efficiency, and their cross section is the same for any wavelength.

In the modern cosmopolitan world, flexible sensors are essential components of the intelligence and information industry. We analyze the latest advancement and improvement of research into the pliable or flexible material based carbon nanotubes sensors. Linear cables kind and flexible materials based on carbon nanotubes are used for developing intelligent flexible equipment for applications in electronics, artificial skin, biomedicine, robotics, energy, healthcare products and most importantly for sensing hazardous gases. In the present analysis, we outlined the development of flexible sensing platforms based on carbon nanotubes and polymers for sensing various environmental polluting gases such as CO ₂ , H ₂ S and NH ₃ . In addition, we also discussed the device structure, materials and fabrication techniques for developing flexible sensors. The conclusion exhibits the viewpoint and the challenges faced by the materials and methods of the flexible sensor.