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The intensity of vertical heat and mass transfer remains among the challenging topics in the study of ice-covered lakes. Presumably, internal waves (IWs) make a significant contribution to the heat transfer in the water column. However, the mechanisms of mixing enhancement by generation, interaction, and breaking of IWs of different scales, especially short-wavelength ones, have not been sufficiently studied. Furthermore, the experimental data required for estimating the key parameters of IWs (wavelengths, propagation velocities) are rather fragmentary, which makes it difficult to quantify the turbulent transfer caused by IWs. This paper presents the estimates of these IW parameters based on data obtained in the winter months of 2014 and 2016 in a small boreal ice-covered lake. Having analyzed horizontally spaced thermistor chain data, we managed to detect the presence of short standing and propagating IWs, and to estimate their length (from several meters to several tens of meters) and phase and group velocities (from several mm/s to several tens of mm/s). Also, their vertical mode structure was detected. It was shown that IW generation events were characterized by a high degree of spatial localization, and the IW energy was unevenly distributed through the water column. [ABSTRACT FROM AUTHOR]

The relative importance of propagating and cavity mode waves remains an important question regarding the generation of Pi2 pulsations detected on the ground. To determine the wave mode, we statistically generate spatial maps of magnetospheric oscillations that are coherent with ground Pi2 pulsations. The magnetospheric observations were made by the two Van Allen Probes spacecraft over a 7‐year period. The amount and quality of the spacecraft data allow us to investigate the mode structure of Pi2 pulsation in ways that were not possible in previous studies. We use the H component of low‐latitude ground Pi2 pulsations detected in the 22–02 magnetic local time (MLT) sector as the reference signal to generate L‐MLT and meridional maps of the coherence, amplitude, and phase of the magnetospheric electric and magnetic field components defined in magnetic field aligned coordinates. We identify low‐frequency and high‐frequency components in Pi2 power spectra, and we are able to determine the mode structure of the high‐frequency events for the first time. The maps demonstrate that the poloidal components have higher coherence than the toroidal components. For each frequency component, the maps of the poloidal components agree with those of cavity mode oscillations obtained in a numerical simulation using realistic models for the magnetospheric mass density and magnetic field. This result is conclusive evidence of the cavity mode nature of Pi2 pulsations detected in the inner magnetosphere. Plain Language Summary: The magnetospheric phenomena known as Pi2 pulsations are oscillations of the electric and magnetic fields with periods around 1 min. The pulsations typically occur at the onset of nightside disturbances known as substorms, which result in sudden brightening of auroras. Pi2 pulsations propagate globally and are detected on the ground even at noon, serving as a useful monitor of substorms. This is analogous to propagation of seismic waves generated by earthquakes. Similar to the solid Earth, the magnetosphere sustains both propagating waves and normal mode oscillations (cavity modes), with the latter exhibiting discrete frequencies and standing wave structures that depend on the plasma distribution in the magnetosphere. There is debate on whether Pi2 pulsations are propagating waves or cavity modes. We construct spatial maps of the amplitude and phase of magnetospheric Pi2 pulsations using observations made by the Van Allen Probes spacecraft. The maps show theoretically predicted mode structures of cavity modes. Key Points: Van Allen Probes data are used to generate maps of the amplitude and phase of Pi2 pulsations in the inner magnetosphereA secondary frequency component of the pulsations is treated separately from the main componentThe maps for both frequency components show radial structures of cavity mode resonances [ABSTRACT FROM AUTHOR]

The intensity of vertical heat and mass transfer remains among the challenging topics in the study of ice-covered lakes. Presumably, internal waves (IWs) make a significant contribution to the heat transfer in the water column. However, the mechanisms of mixing enhancement by generation, interaction, and breaking of IWs of different scales, especially short-wavelength ones, have not been sufficiently studied. Furthermore, the experimental data required for estimating the key parameters of IWs (wavelengths, propagation velocities) are rather fragmentary, which makes it difficult to quantify the turbulent transfer caused by IWs. This paper presents the estimates of these IW parameters based on data obtained in the winter months of 2014 and 2016 in a small boreal ice-covered lake. Having analyzed horizontally spaced thermistor chain data, we managed to detect the presence of short standing and propagating IWs, and to estimate their length (from several meters to several tens of meters) and phase and group velocities (from several mm/s to several tens of mm/s). Also, their vertical mode structure was detected. It was shown that IW generation events were characterized by a high degree of spatial localization, and the IW energy was unevenly distributed through the water column.

Features of internal tides in the northern South China Sea are investigated using measurements of two moorings at 506 m (MR1) and 260 m (MR2) working between April and November of 2014. The diurnal internal tides at MR1 are significantly enhanced in June-July and display a regular fortnight modulation, whereas at shallower MR2, the June-July intensification is absent and the fortnight cycle becomes irregular. Fortnight cycle peaks of diurnal internal tides have a regular time-lag of ~ 2.8 days after those of diurnal barotropic tides (at MR1). We derive a mathematical relationship indicating that this time-lag of fortnight cycle peaks is around twice the time for the diurnal internal tides propagating from the generation place to the mooring site. Semidiurnal internal tides are dominated by the mode-1 (~ 70%), while diurnal internal tides exhibit multi-modal structures, with the contribution of ~ 20-30% for each of the first three modes. The diurnal mode-1 contribution varies fortnightly, and eddy passages are highly correlated with the increasing contribution of semidiurnal high modes (at MR1). Furthermore, we find that the nonlinear interaction between internal tides and near-inertial motions is weak in summer, even during periods of strong near-inertial motions (at MR2), because in summer, the shear of near-inertial currents is maximum at a depth shallower than the depth of maximum internal tide shear. [ABSTRACT FROM AUTHOR]

Ultralow frequency (ULF) waves play a fundamental role in the dynamics of the inner magnetosphere and outer radiation belt during geomagnetic storms. Broadband ULF wave power can transport energetic electrons via radial diffusion, and discrete ULF wave power can energize electrons through a resonant interaction. Using observations from the Magnetospheric Multiscale mission, we characterize the evolution of ULF waves during a high‐speed solar wind stream (HSS) and moderate geomagnetic storm while there is an enhancement of the outer radiation belt. The Automated Flare Inference of Oscillations code is used to distinguish discrete ULF wave power from broadband wave power during the HSS. During periods of discrete wave power and utilizing the close separation of the Magnetospheric Multiscale spacecraft, we estimate the toroidal mode ULF azimuthal wave number throughout the geomagnetic storm. We concentrate on the toroidal mode as the HSS compresses the dayside magnetosphere resulting in an asymmetric magnetic field topology where toroidal mode waves can interact with energetic electrons. Analysis of the mode structure and wave numbers demonstrates that the generation of the observed ULF waves is a combination of externally driven waves, via the Kelvin‐Helmholtz instability, and internally driven waves, via unstable ion distributions. Further analysis of the periods and toroidal azimuthal wave numbers suggests that these waves can couple with the core electron radiation belt population via the drift resonance during the storm. The azimuthal wave number and structure of ULF wave power (broadband or discrete) have important implications for the inner magnetospheric and radiation belt dynamics. Key Points: ULF wave power is systematically characterized as discrete or broadband, and the azimuthal wave number of discrete ULF waves is determinedStorm time discrete ULF waves can couple with the core radiation belt electron population via the drift resonanceStorm time ULF waves are the result of both external (Kelvin‐Helmholtz) and internal (plasma instabilities) generation mechanisms [ABSTRACT FROM AUTHOR]

Abstract: The lasers application in the information systems have to be preceded by the development of the requirements to the laser radiation parameters. It is clear, that the lasers radiation coherence growth by means of which the holograms were registered leads the holograms quality improvement. [ABSTRACT FROM AUTHOR]

The structure of longitudinal modes of a passively Q-switched, non-planar unidirectional ring-resonator,with Nd:YAG active medium is described in this article. Two different techniques are used to study the longitudinal mode structure of the laser resonator. At first, the fast-fourier transform technique is applied for analyzing the mode beating of the optical fields by intensity frequency structure of the laser pulses to determine the number of longitudinal modes. Then, an analyzer etalon is used to observe Fabry-Perot fringes to compute the numbers of the resonator longitudinal modes. The results of two techniques are in good agreement with each other. Under the proper conditions, a reliable single longitudinal mode of the non-planar ring-resonator can be achieved with a good spatial mode profile that originates from the unidirectional travelling optical field propagation in the resonator having a very low sensitivity of the non-planar ring resonator to the optical elements misalignment.

We investigate the lateral energy flow of light bound to an OLED (i.e., light that cannot be coupled to the substrate or air mode), with a focus on the attenuation characteristics. A methodology is developed for approximating the total bound field as a superposition of the individual guided modes constructed from multilayer eigenmodes with complex wave vectors. The field distribution and surface energy flow density obtained from our approximate expressions are found to be well matched to those calculated using the Green's function formalism. The method presented in this study provides mode-based interpretation of the surface energy flow density versus distance characteristics, and can be used in designing OLEDs for applications where the optical energy transported by the bound field is of interest. [Display omitted] • The complex photonic band structure of an OLED is calculated. • The bound-mode field of an OLED is constructed from eigenmodes with complex wave vectors. • The attenuation characteristics of the surface energy flow density are quantitatively described. [ABSTRACT FROM AUTHOR]

Absorption spectra of liquid water HO are investigated in the region 4500-5600 cm at temperatures from -53.0 to +20.3°C. The absorption band is decomposed into component modes at (I) 4700, (II) 4890, (III) 5080, and (IV) 5200 cm whose centers are shifted slightly with temperature. Modes (II), (III), and (IV) are present in the spectrum of liquid water, whereas modes (I), (II), and (III) are registered in the spectrum of ice. The appearance of the ordered crystalline lattice is characterized by the occurrence of low-frequency mode (I) and by the disappearance of high-frequency component (IV) of the water absorption band in the region 4500-5600 cm. The spectral regions and temperature ranges in which the structure is changed during phase transition are determined. [ABSTRACT FROM AUTHOR]

An infrared laser propagating through a semiconductor waveguide, embedded with a density ripple of wave number q, resonantly excites third harmonic radiation. The phase matching is achieved when q equals difference between third harmonic wave number and three times the wave number of the laser. The excited third harmonic is in higher order radial Eigen mode than the pump laser. The third harmonic efficiency increases with electron concentration and decreases with the frequency of the laser. [ABSTRACT FROM AUTHOR]

We show the impact of lateral structuring on the optical properties of organic thin films in microcavities. The active material tris-(8-hydroxy quinoline) aluminium (Alq3) was deposited using shadow mask evaporation, resulting in various micron sized shapes. A simplified box model is employed to describe the steady state electric field distribution in these mesa cavities. The complex mode structure measured in transmission and photoluminescence is confirmed by a transfer matrix calculation and a Fourier transform of the internal electric field distribution. [ABSTRACT FROM AUTHOR]

An angular spectrum of plane-wave representation is employed to calculate the discrimination between the fundamental and higher order transverse modes in step-index-guided vertical-cavity surface-emitting lasers. The effect of material composition and number of layer pairs in the distributed Bragg reflectors, as well as mode size and structure, are examined with the goal of optimizing the mode discrimination for better mode stability and higher single-mode power. In particular, it is shown that decreasing the width of the distributed Bragg reflector stopband, by means of controlling the material composition, improves significantly the mode discrimination. [ABSTRACT FROM PUBLISHER]

We present results of investigations of the directionality diagram and mode structure of radiation of semiconductor lasers with electron pumping that have microrelief reflectors instead of a blind mirror and output mirrors, whose coefficient of reflection depends on the wavelength and the angle of light propagation. Application of such mirrors allows one to narrow the spectrum of output radiation of the lasers, simultaneously reducing the threshold and increasing the output power. The discovered multilobe structure of the directionality diagrams is explained on the basis of geometrical optics. The possibility of controlling the directionality of radiation depending on the spectral and angular characteristics of the output mirror is shown. [ABSTRACT FROM AUTHOR]

We have proposed that application of singular value decomposition on finite-difference time-domain simulations is best suited for modal analysis of complex real-life electromagnetic problems. This combination of two powerful techniques yields a wealth of information about the complete mode structure which cannot be otherwise obtained. This is particularly important for a real-life system having complex geometry and consisting of a range of materials from good conductors to dielectrics and this is further complicated by the existence of broad-band excitation. This hybrid approach is versatile, accurate, and computationally efficient. [ABSTRACT FROM PUBLISHER]

The structure of longitudinal modes of a passively Q-switched, non-planar unidirectional ring-resonator, with Nd:YAG active medium is described in this article. Two different techniques are used to study the longitudinal mode structure of the laser resonator. At first, the fast-fourier transform technique is applied for analyzing the mode beating of the optical fields by intensity frequency structure of the laser pulses to determine the number of longitudinal modes. Then, an analyzer etalon is used to observe Fabry-Perot fringes to compute the numbers of the resonator longitudinal modes. The results of two techniques are in good agreement with each other. Under the proper conditions, a reliable single longitudinal mode of the non-planar ring-resonator can be achieved with a good spatial mode profile that originates from the unidirectional travelling optical field propagation in the resonator having a very low sensitivity of the non-planar ring resonator to the optical elements misalignment. [ABSTRACT FROM AUTHOR]