Momentum alignment and the optical valley Hall effect in low-dimensional Dirac materials

We study the momentum alignment phenomenon and the optical control of valley population in gapless and gapped graphene-like materials. We show that the trigonal warping effect allows for the spatial separation of carriers belonging to different valleys via the application of linearly polarized light. Valley separation in gapped materials can be detected by measuring the degree of circular polarization of band-edge photoluminescence at different sides of the sample or light spot (optical valley Hall effect). We also show that the momentum alignment phenomenon leads to the giant enhancement of near-band-edge interband optical transitions in narrow-gap carbon nanotubes and graphene nanoribbons independent of the mechanism of the gap formation. A detection scheme to observe these giant interband transitions is proposed which opens a route for creating novel terahertz radiation emitters.Comment: 28 pages, 9 figure

Tuning terahertz transitions in a double-gated quantum ring

We theoretically investigate the optical functionality of a semiconducting quantum ring manipulated by two electrostatic lateral gates used to induce a double quantum well along the ring. The well parameters and corresponding inter-level spacings, which lie in the THz range, are highly sensitive to the gate voltages. Our analysis shows that selection rules for inter-level dipole transitions, caused by linearly polarized excitations, depend on the polarization angle with respect to the gates. In striking difference from the conventional symmetric double well potential, the ring geometry permits polarization-dependent transitions between the ground and second excited states, allowing the use of this structure in a three-level lasing scheme.Comment: 7 pages, 6 figure

Electro-absorption of silicene and bilayer graphene quantum dots

We study numerically the optical properties of low-buckled silicene and AB-stacked bilayer graphene quantum dots subjected to an external electric field, which is normal to their surface. Within the tight-binding model, the optical absorption is calculated for quantum dots, of triangular and hexagonal shapes, with zigzag and armchair edge terminations. We show that in triangular silicene clusters with zigzag edges a rich and widely tunable infrared absorption peak structure originates from transitions involving zero energy states. The edge of absorption in silicene quantum dots undergoes red shift in the external electric field for triangular clusters, whereas blue shift takes place for hexagonal ones. In small clusters of bilayer graphene with zigzag edges the edge of absorption undergoes blue/red shift for triangular/hexagonal geometry. In armchair clusters of silicene blue shift of the absorption edge takes place for both cluster shapes, while red shift is inherent for both shapes of the bilayer graphene quantum dots.Comment: 7 pages, 7 figure

2N+4-rule and an atlas of bulk optical resonances of zigzag graphene nanoribbons

Development of on-chip integrated carbon-based optoelectronic nanocircuits requires fast and non-invasive structural characterization of their building blocks. Recent advances in synthesis of single wall carbon nanotubes and graphene nanoribbons allow for their use as atomically precise building blocks. However, while cataloged experimental data are available for the structural characterization of carbon nanotubes, such an atlas is absent for graphene nanoribbons. Here we theoretically investigate the optical absorption resonances of armchair carbon nanotubes and zigzag graphene nanoribbons continuously spanning the tube (ribbon) transverse sizes from 0.5(0.4) nm to 8.1(12.8) nm. We show that the linear mapping is guaranteed between the tube and ribbon bulk resonance when the number of atoms in the tube unit cell is 2 N+ 4 , where N is the number of atoms in the ribbon unit cell. Thus, an atlas of carbon nanotubes optical transitions can be mapped to an atlas of zigzag graphene nanoribbons

Dynamics of Cryogenic Jets: Non-Rayleigh Breakup and Onset of Nonaxisymmetric Motions

We report development of generators for periodic, satellite-free fluxes of mono-disperse drops with diameters down to 10 mikrometers from cryogenic liquids like H_2, N_2, Ar and Xe (and, as reference fluid, water). While the breakup of water jets can well be described by Rayleigh's linear theory, we find jet regimes for H_2 and N_2 which reveal deviations from this behavior. Thus, Rayleigh's theory is inappropriate for thin jets that exchange energy and/or mass with the surrounding medium. Moreover, at high evaporation rates, axial symmetry of the dynamics is lost. When the drops pass into vacuum, frozen pellets form due to surface evaporation. The narrow width of the pellet flux paves the way towards various industrial and scientific applications.Comment: 4 pages, 4 figures, 1 table; final version to appear in Phys.Rev.Lett (minor changes with respect to v1

Adenosine thiamine triphosphate and adenosine thiamine triphosphate hydrolase activity in animal tissues

Adenosine thiamine triphosphate (AThTP), a vitamin B1 containing nucleotide with unknown biochemi­cal role, was found previously to be present in various biological objects including bacteria, yeast, some human, rat and mouse tissues, as well as plant roots. In this study we quantify AThTP in mouse, rat, bovine and chicks. We also show that in animal tissues the hydrolysis of AThTP is catalyzed by a membrane-bound enzyme seemingly of microsomal origin as established for rat liver, which exhibits an alkaline pH optimum of 8.0-8.5 and requires no Mg2+ ions for activity. In liver homogenates, AThTP hydrolase obeys Michaelis-Menten kinetics with apparent Km values of 84.4 ± 9.4 and 54.6 ± 13.1 µМ as estimated from the Hanes plots for rat and chicken enzymes, respectively. The hydrolysis of AThTP has been found to occur in all samples examined from rat, chicken and bovine tissues, with liver and kidney being­ the most abundant in enzyme activity. In rat liver, the activity of AThTP hydrolase depends on the age of animals

Работа кремниевых фотоэлектронных умножителей со структурой p+–p–n+ в режиме одноквантовой регистрации

The conditions for realizing the single-quantum detection mode for silicon photomultiplier tubes with the p+–p–n+ structure are studied and data on their characteristics in this mode are obtained. The structure of the experimental setup and the research technique are presented. Measurements of the counting characteristics of the photodetectors, such as the dependences of the counting rate of single-photon pulses, the speed of dark pulses, and the signal-to-noise ratio, have been performed. The dependences of the counting rate of one-photon pulses on the intensity of optical radiation recorded by a silicon photomultiplier tube are presented. It was found that these dependences had a linear section, the length of which increased with increasing overvoltage of silicon photomultiplier tubes. Also, with an increase in overvoltage, the angle of inclination of the linear section increased. The dependences of the count rate of one-photon and dark pulses, as well as the signal-to-noise ratio on overvoltage, are given. It was found that the counting rate of dark pulses increased with increasing overvoltage. It was found that the dependence of the signal-to-noise ratio on the overvoltage for these silicon photomultiplier tubes has a maximum. To obtain the maximum sensitivity of the studied silicon photomultiplier tubes, it is necessary to select the overvoltage corresponding to this maximum. As a result of comparing the sensitivity of the investigated silicon photomultiplier tubes and avalanche photodiodes, it was found that silicon photomultiplier tubes operating in the single-quantum detection mode have a higher sensitivity compared to avalanche photodiodes in the same operating mode. With a decrease in temperature, this superiority persisted. Also, a decrease in temperature led to a decrease in the minimum value of the intensity of the recorded optical radiation. Thus, the possibility of operation of silicon photomultiplier tubes in the single-quantum registration mode has been proved. These results can be applied in quantum cryptography systems when receiving an optical signal.Изучены условия реализации режима одноквантовой регистрации для кремниевых фотоэлектронных умножителей со структурой p+–p–n+ и получены данные об их характеристиках в этом режиме. Приведены структура экспериментальной установки и методика исследований. Выполнены измерения счетных характеристик фотоприемников, таких как зависимости скорости счета однофотонных импульсов, скорости темновых импульсов и отношения сигнал/шум. Представлены зависимости скорости счета однофотонных импульсов от интенсивности оптического излучения, регистрируемого кремниевым фотоэлектронным умножителем. Установлено, что данные зависимости имеют линейный участок, длина которого увеличивается с ростом перенапряжения кремниевых фотоэлектронных умножителей. Также с ростом перенапряжения увеличивается угол наклона линейного участка. Приведены зависимости скорости счета однофотонных и темновых импульсов, а также отношения сигнал/шум от перенапряжения. Получено, что скорость счета темновых импульсов возрастает с увеличением перенапряжения. Установлено, что зависимость отношения сигнал/шум от перенапряжения для этих кремниевых фотоэлектронных умножителей имеет максимум. Для получения максимальной чувствительности исследованных кремниевых фотоэлектронных умножителей необходимо выбирать перенапряжение, соответствующее этому максимуму. В результате сравнения чувствительности исследуемых кремниевых фотоэлектронных умножителей и лавинных фотодиодов установлено, что кремниевые фотоэлектронные умножители, работающие в режиме одноквантовой регистрации, имеют более высокую чувствительность по сравнению с лавинными фотодиодами в этом же режиме работы. С уменьшением температуры данное превосходство сохраняется. Также понижение температуры приводит к уменьшению минимального значения интенсивности регистрируемого оптического излучения. Тем самым доказана возможность работы кремниевых фотоэлектронных умножителей в режиме одноквантовой регистрации. Данные результаты могут применяться в системах квантовой криптографии при приеме оптического сигнала

Детектор среднего и дальнего ИК излучения на основе плоских массивов графеновых нанолент

A concept of a middle- and far-infrared detector has been proposed. The detector is built as a planar collection of parallel graphene strips of different length and width. The feature of the detector scheme is the concurrent utilization of two different detection mechanisms: excitation in the given frequency range of low-frequency interband transitions inherent in armchair graphene strips and antenna resonances of strongly slowed-down surface waves (plasmon polaritons). It has been shown that matching these two resonances results in the essential detector signal amplification, thus providing an alternative way how to solve the problem of the low efficiency of resonant graphene antennas. An approach is proposed to analyze the design of such detectors, as well as to discuss the ways of tuning the both mechanisms.Предложена принципиальная схема детектора электромагнитных волн среднего и дальнего ИК диапазона частот, основанная на использовании плоских массивов графеновых нанолент различной ширины и длины. Особенностью рассматриваемой схемы является использование для детектирования двух различных механизмов: возбуждение межзонных переходов, присущих графеновым лентам типа «armchair» в данной частотной области, и антенных резонансов поверхностных волн (плазмон-поляритонов). Показано, что совпадение двух резонансов, достигаемое путем соответствующего подбора геометрических параметров нанолент и настройки химического потенциала графена, позволяет существенно усилить сигнал, тем самым обеспечивая альтернативное решение проблемы низкой эффективности резонансных графеновых антенн. В работе предлагается возможный подход к проектированию и анализу таких детекторов, а также обсуждаются способы настройки обоих механизмов

Зависимости характеристик кремниевых фотоумножителей от температуры

The characteristics dependence on the ambient temperature for three types of silicon photoelectronic multipliers have been studied in this research. The prototypes of Si-photoelectronic multipliers with a p+–p–n+ structure produced by JSC Integral (Republic of Belarus), serially produced silicon photoelectronic multipliers KETEK РМ3325 and ON Semi FC 30035 have been used as objects of research. We present the setup diagram and research technique. Measurements of the photocurrent magnitude versus the illumination intensity, calculations of the critical and threshold intensities, and the dynamic range have been performed. We also present the photocurrent dependences on the illumination intensity at different ambient temperatures. As it was found, these dependences have a linear section, the length of which characterizes the critical intensity value, and the inclination angle of the linear section to the intensity axis characterizes the photodetector sensitivity to optical radiation. It has been determined that the temperature increase leads to an increase in the critical intensity value and to a decrease in the sensitivity value. We present the dependences of the threshold intensity on the overvoltage at different ambient temperatures. The dependence of the threshold intensity on overvoltage is most strongly pronounced when the supply voltage is below the breakdown voltage. It was found that the threshold intensity is increased with the temperature increase and the threshold intensity dependence on the temperature is the same for all investigated photodetectors. It was found that the dynamic range value is decreased with the temperature increase, which is caused by a more significant change in the threshold intensity as compared to the critical one. The results given in this article can be applied when developing and designing the tools and devices for recording optical radiation based on silicon photoelectronic multipliers.Изучены зависимости характеристик от температуры окружающей среды трех типов кремниевых фотоумножителей. В качестве объектов исследования использовались опытные образцы Si-ФЭУ со структурой p+–p–n+ производства ОАО «Интеграл» (Республика Беларусь), серийно выпускаемые Si-ФЭУ KETEK РМ3325 и ON Semi FC 30035. Приведена схема установки и методика исследования. Выполнены измерения величины фототока от интенсивности засветки, расчеты критической и пороговой интенсивности, динамического диапазона регистрируемого оптического излучения. Представлены зависимости фототока от интенсивности засветки при разных температурах окружающей среды. Установлено, что данные зависимости имеют линейный участок, длина которого характеризует значение критической интенсивности излучения, а угол наклона линейного участка к оси интенсивности – чувствительность Si-ФЭУ к оптическому излучению. Определено, что рост температуры приводит к ро- сту величины критической интенсивности и снижению чувствительности. Представлены зависимости пороговой интенсивности излучения от перенапряжения при разных температурах окружающей среды. Наиболее сильно зависимость пороговой интенсивности от перенапряжения проявляется при напряжении питания ниже напряжения пробоя. Установлено, что пороговая интенсивность излучения повышается с ростом температуры и зависимость пороговой интенсивности от температуры одинакова для всех исследуемых Si-ФЭУ. Определено, что значение динамического диапазона с ростом температуры уменьшается, что вызвано более значительным изменением пороговой интенсивности по сравнению с критической. Результаты исследований могут найти применение при разработке и конструировании приборов и устройств для регистрации оптического излучения на основе Si-ФЭУ