Double-cavity Fabry-Perot resonators based on one-dimensional silicon photonic crystals

This is the final version. Available from AIP Publishing via the DOI in this record.The 3rd Joint International Conference on Energy Engineering and Smart Materials (ICEESM-2018) and International Conference on Nanotechnology and Nanomaterials in Energy (ICNNE-2018)In this work we report on optical properties of Fabry-Pérot (FP) resonator based on Si-air one-dimensional photonic crystal (1D PC) with coupled double-cavity modes (or defects). These defects obtained by infiltration of the air-cavities of refractive index (nAir) with the filler of tunable refractive index. In the periodic structure of 1D PC, the filling of two defined grooves with a filler with a refractive index different from nAirleads to the appearance of two resonant modes, the position of which can be adjusted purposefully with changing of n filler. In comparison with (λ / 2) air resonators, the splitting of the doublet increases, which is explained by the increase in the coupling between the resonant modes due to the decrease in reflection R of the internal mirror. The coupled FP resonators design is CMOS compatible and has potential for application in tuning of individual transmission bands in wave-division multiplexing systems as well as for multiple narrow filters in the wide infrared spectral range.We acknowledge financial support from: The Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom via the EPSRC Grant No. EP/NO35569/1, and the Royal Society International Exchange Grant 2015/R3. The part of this research at the Ioffe Institute was supported by the Russian Federal Agency of Scientific Organizations

Experiments on Sonoluminescence: Possible Nuclear and QED Aspects and Optical Applications

Experiments aimed at testing some hypothesis about the nature of Single Bubble Sonoluminescence are discussed. A possibility to search for micro-traces of thermonuclear neutrons is analyzed, with the aid of original low-background neutron counter operating under conditions of the deep shielding from Cosmic and other sources of background. Besides, some signatures of QED-contribution to the light emission in SBSL are under the consideration, as well as new approaches to probe a temperature inside the bubble. An applied-physics portion of the program is presented also, in which an attention is being paid to single- and a few-pulse light sources on the basis of SBSL.Comment: 4 pages; to be published by AIP in the Proc. ISNA-1

Transient Cherenkov radiation from an inhomogeneous string excited by an ultrashort laser pulse at superluminal velocity

An optical response of one-dimensional string made of dipoles with a periodically varying density excited by a spot of light moving along the string at the superluminal (sub-luminal) velocity is theoretically studied. The Cherenkov radiation in such system is rather unusual, possessing both transient and resonant character. We show that under certain conditions, in addition to the resonant Cherenkov peak another Doppler-like frequency appears in the radiation spectrum. Both linear (small-signal) and nonlinear regimes as well as different string topologies are considered.Comment: accepted to Phys. Rev.

Learning analytics in massive open online courses as a tool for predicting learner performance

Learning analytics in MOOCs can be used to predict learner performance, which is critical as higher education is moving towards adaptive learning. Interdisciplinary methods used in the article allow for interpreting empirical qualitative data on performance in specific types of course assignments to predict learner performance and improve the quality of MOOCs. Learning analytics results make it possible to take the most from the data regarding the ways learners engage with information and their level of skills at entry. The article presents the results of applying the proposed learning analytics algorithm to analyze learner performance in specific MOOCs developed by Ural Federal University and offered through the National Open Education Platform. © 2018, National Research University Higher School of Economics.This study was support- ed by financial assis- tance provided under the Resolution of the Government of the Rus sian Federation No. 211, Contract No. 02. A03.21.0006. Translated from Russian by I. Zhuchkova

All-optical dc nanotesla magnetometry using silicon vacancy fine structure in isotopically purified silicon carbide

We uncover the fine structure of a silicon vacancy in isotopically purified silicon carbide (4H-$^{28}$SiC) and find extra terms in the spin Hamiltonian, originated from the trigonal pyramidal symmetry of this spin-3/2 color center. These terms give rise to additional spin transitions, which are otherwise forbidden, and lead to a level anticrossing in an external magnetic field. We observe a sharp variation of the photoluminescence intensity in the vicinity of this level anticrossing, which can be used for a purely all-optical sensing of the magnetic field. We achieve dc magnetic field sensitivity of 87 nT Hz$^{-1/2}$ within a volume of $3 \times 10^{-7}$ mm$^{3}$ at room temperature and demonstrate that this contactless method is robust at high temperatures up to at least 500 K. As our approach does not require application of radiofrequency fields, it is scalable to much larger volumes. For an optimized light-trapping waveguide of 3 mm$^{3}$ the projection noise limit is below 100 fT Hz$^{-1/2}$.Comment: 12 pages, 6 figures; additional experimental data and an extended theoretical analysis are added in the second versio

Determination of the dimensions of the heat-affected zone in welding gas pipeline components

Analytical decisions supported by experimental data were used to determine the dependences for calculating the size of the heat-affected zone (HAZ) in multilayer welding of circumferential joints in transmission gas pipelines. Data on the dimensions of this zone are essential for evaluating the possibility of applying cold cutting in the rejection of elements of gas pipelines because of defects in circumferential welded joints or welded joints in transition rings in the vicinity of circumferential welded joints. © 2013 Copyright Taylor and Francis Group, LLC

Synthesis and reactivity of 5-polyfluoroalkyl-5-deazaalloxazines

Reaction of 6-arylamino-1,3-dialkyluracils with anhydrides of polyfluorocarboxylic acids in the presence of pyridine and subsequent cyclization with concentrated H2SO4 gave the corresponding 1,3-dialkyl-5-(polyfluoroalkyl)pyrimido[4,5-b]quinoline-2,4(1H,3H)-diones (5-polyfluoroalkyl-5-deazaalloxazines). The reactivity of these compounds towards nucleophilic reagents, such as sodium cyanoborohydride, acetophenone, nitromethane, potassium cyanide, indole and p-thiocresol, as well as Suzuki and Sonogashira couplings are described. The nucleophilic addition takes place at the 5-position of the 5-deazaalloxazine system and is in many cases irreversible to give 5,10-dihydropyrimido[4,5-b]quinoline-2,4(1H,3H)-dione derivatives in good to excellent yields.© 2013 The Royal Society of Chemistry

Liquid Crystal WDM filter in Si photonic crystal technology with individual channel fine-tuning capability

We demonstrate a simple, low-cost solution for a single multi-channel WDM (Wavelength Division Multiplexing) filter with fine–tuning capability of individual channels. The filter is based on Si photonic crystal technology and can be integrated with CMOS processes. Although, fabrication technologies of Si integrated WDM systems have significantly advanced over the last decade, the most difficult challenges are posed by wavelength accuracy control as well as wavelength drifts and optical switching time. The proposed novel design of a multichannel integrated filter is based on the 1D silicon photonic crystal (PhC) model. By infiltration of the certain grooves of 1D PhC with matching filler, an efficient coupled Fabry-Pérot microresonator can be realized in which the wide-band stop band (SB) is used for frequency channel separation. By using the commercial nematic liquid crystal 5CB [1], we demonstrated electro-optical switching in the range of 30-50 nanoseconds and the continuous tuning of the individual channels up to 30 % of the channel-spacing. The fabricated multichannel filters have bandwidth of 0.1-0.9 nm with high extinction ration of 20dB at high modulation of reflection/ transmission coefficient. Using the gap map approach as a core engineering tool allows to predict formation and separation of transmission channels within the SBs and, thus, effectively determine the exact design parameters of the optical device. The obtained experimental spectral characteristics in the NIR range around 1.31 and 1.55 μm validated the proposed method and its applicability for the wavelength selective switching (WSS) as well as for the WDM in Si chip optical interconnects. [1] M. W. Geis, T. M. Lyszczarz, R. M. Osgood, and B. R. Kimball, " 30 to 50 ns liquid-crystal optical switches", Opt. Express 18, 18886-18893 (2010)The authors acknowledge the EPSRC Centre for Doctoral Training in Metamaterials, Exeter, Devon for the continue support in this project

Sun-Sized Water Vapor Masers in Cepheus A

We present the first VLBI observations of a Galactic water maser (in Chepeus A) made with a very long baseline interferometric array involving the RadioAstron Earth-orbiting satellite station as one of its elements. We detected two distinct components at -16.9 and 0.6 km/s with a fringe spacing of 66 microarcseconds. In total power, the 0.6 km/s component appears to be a single Gaussian component of strength 580 Jy and width of 0.7 km/s. Single-telescope monitoring showed that its lifetime was only 8~months. The absence of a Zeeman pattern implies the longitudinal magnetic field component is weaker than 120 mG. The space-Earth cross power spectrum shows two unresolved components smaller than 15 microarcseconds, corresponding to a linear scale of 1.6 x 10^11 cm, about the diameter of the Sun, for a distance of 700 pc, separated by 0.54 km/s in velocity and by 160 +/-35 microarcseconds in angle. This is the smallest angular structure ever observed in a Galactic maser. The brightness temperatures are greater than 2 x 10^14K, and the line widths are 0.5 km/s. Most of the flux (about 87%) is contained in a halo of angular size of 400 +/- 150 microarcseconds. This structure is associated with the compact HII region HW3diii. We have probably picked up the most prominent peaks in the angular size range of our interferometer. We discuss three dynamical models: (1) Keplerian motion around a central object, (2) two chance overlapping clouds, and (3) vortices caused by flow around an obstacle (i.e., von Karman vortex street) with Strouhal number of about~0.3.Comment: 15 pages, 9 figures. Accepted for publication in ApJ, February 16, 201