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.

Generation of unipolar pulses in a circular Raman-active medium excited by few-cycle optical pulses

We study theoretically a new possibility of unipolar pulses generation in Raman-active medium excited by a series of few-cycle optical pulses. We consider the case when the Raman-active particles are uniformly distributed along the circle, and demonstrate a possibility to obtain a unipolar rectangular video pulses with an arbitrarily long duration, ranging from a minimum value equal to the natural period of the low frequency vibrations in the Raman-active medium

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

High-efficiency optical pumping of nuclear polarization in a GaAs quantum well

The dynamic polarization of nuclear spins by photoexcited electrons is studied in a high quality GaAs/AlGaAs quantum well. We find a surprisingly high efficiency of the spin transfer from the electrons to the nuclei as reflected by a maximum nuclear field of 0.9 T in a tilted external magnetic field of 1 T strength only. This high efficiency is due to a low leakage of spin out of the polarized nuclear system, because mechanisms of spin relaxation other than the hyperfine interaction are strongly suppressed, leading to a long nuclear relaxation time of up to 1000 s. A key ingredient to that end is the low impurity concentration inside the heterostructure, while the electrostatic potential from charged impurities in the surrounding barriers becomes screened through illumination by which the spin relaxation time is increased compared to keeping the system in the dark. This finding indicates a strategy for obtaining high nuclear spin polarization as required for long-lasting carrier spin coherence.Comment: 6 pages, 4 figure

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

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

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

Mathematical description of electromechanical processes with energy-efficient electric induction motor in transient conditions

Для энергосберегающего асинхронного электропривода с компенсационной обмоткой получена преобразованная схема замещения с эквивалентными параметрами намагничивающего контура. Показано, что при определенных значениях емкости компенсирующего конденсатора cosφ ≤ 1. Дано математическое описание динамических режимов преобразованного двигателя с использованием метода пространственных векторов. Разработанная структурная схема и модель электропривода позволяет произвести качественную и количественную оценку динамических режимов.For energy-efficient asynchronous electric drive with compensation winding get converted to the equivalent circuit equivalent parameters of the magnetizing circuit. It is shown that for certain values of the compensating capacitor capacitance cosφ ≤ 1. The mathematical description of dynamical regimes Engine converted using the space vector. The block diagram of the model and allows the drive to produce qualitative and quantitative assessment of dynamic modes