Influence of piezoelectricity, doping and magnetostatic field on Brillouin amplification in compound (AIIIBV and AIIBVI) semiconductors

A theoretical formulation followed by numerical analysis describing Brillouin amplification in compound (AIIIBV and AIIBVI) semiconductors is explored. The threshold condition for the onset of Brillouin amplification is determined. Well above the threshold intensity, the influence of piezoelectricity, doping concentration, and external magnetostatic field on the parameters characterizing Brillouin amplification viz. Brillouin amplification coefficient, transmitted intensity of Brillouin-scattered Stokes mode (BSSM), and Brillouin cell efficiency of the Brillouin cell isestimated. Numerical analysis is made for three different Brillouin cells consisting of [Formula: see text]-InSb, [Formula: see text]-GaAs, and [Formula: see text]-CdS, at 77[Formula: see text]K duly irradiated by a pulsed CO2 laser. Efforts are directed towards to determine appropriate values of doping concentration and magnetostatic field to enhance the parameters characterizing Brillouin amplification, at lower excitation intensity, and to establish the suitability of compound semiconductors as hosts for fabrication of efficient Brillouin amplifiers.

The supernova remnant populations of the galaxies NGC 45, NGC 55, NGC 1313, NGC 7793: luminosity and excitation functions

ABSTRACT We present a systematic study of the supernova remnant (SNR) populations in the nearby galaxies NGC 45, NGC 55, NGC 1313, and NGC 7793 based on deep H $\rm {\alpha }$ and [S ii] imaging. We find 42 candidate and 51 possible candidate SNRs based on the [S ii]/H $\rm {\alpha }$>0.4 criterion, 81 of which are new identifications. We derive the H $\rm {\alpha }$ and the joint [S ii]–H $\rm {\alpha }$ luminosity functions after accounting for incompleteness effects. We find that the H $\rm {\alpha }$ luminosity function of the overall sample is described with a skewed Gaussian with a mean equal to $\rm \log (L_{H\alpha }/10^{36}\, erg\, s^{-1})=0.07$ and $\rm \sigma (\log (L_{H\alpha }/10^{36}\, erg\, s^{-1}))=0.58$. The joint [S ii]–H $\rm {\alpha }$ function is parametrized by a skewed Gaussian along the log([S ii]$\rm /10^{36}\, erg\, s^{-1}) = 0.88 \times \log (L_{H\alpha }/10^{36}\, erg\, s^{-1}) - 0.06$ line and a truncated Gaussian with $\rm \mu (\log (L_{[S\, II]}/10^{36})) = 0.024$ and $\rm \sigma (\log (L_{[S\, II]}/10^{36})) = 0.14$, on its vertical direction. We also define the excitation function as the number density of SNRs as a function of their [S ii]/H $\rm {\alpha }$ ratios. This function is represented by a truncated Gaussian with a mean at −0.014. We find a sub-linear [S ii]–H $\rm {\alpha }$ relation indicating lower excitation for the more luminous objects.

Optimized photoluminescence quantum yield in upconversion composites considering the scattering, inner-filter effects, thickness, self-absorption, and temperature

Optimizing upconversion (UC) composites is challenging as numerous effects influence their unique emission mechanism. Low scattering mediums increase the number of dopants excited, however, high scattering mediums increase the UC efficiency due to its non-linear power dependency. Scattering also leads to greater thermal effects and emission saturation at lower excitation power density (PD). In this work, a photoluminescence quantum yield (PLQY) increase of 270% was observed when hexagonal NaYF4:(18%)Yb3+,(2%)Er3+ phosphor is in air compared to a refractive index-matched medium. Furthermore, the primary inner-filter effect causes a 94% PLQY decrease when the excitation focal point is moved from the front of the phosphor to 8.4 mm deep. Increasing this effect limits the maximum excitation PD, reduces thermal effects, and leads to emission saturation at higher excitation PDs. Additionally, self-absorption decreases the PLQY as the phosphor’s thickness increases from 1 to 9 mm. Finally, in comparison to a cuboid cuvette, a 27% PLQY increase occurs when characterizing the phosphor in a cylindrical cuvette due to a lensing effect of the curved glass, as supported by simulations. Overall, addressing the effects presented in this work is necessary to both maximize UC composite performance as well as report their PLQY more reliably.

Modelling the response of structure-tuned liquid damper systems under large amplitude excitation using SPH

The tuned liquid damper (TLD) is a system used to reduce the response of tall structures. Numerical modelling is a very important tool when designing TLDs. Many existing numerical models are capable of accurately capturing the structure-TLD system response at serviceability levels, covering the range where TLDs are primarily intended to perform. However, these models often have convergence issues when considering more extreme structural excitations. The goal of this study is to develop a structure-TLD model without convergence limitations at large amplitude excitations. A structure-TLD numerical model where the TLD is represented by a 2D incompressible SPH scheme is presented. The TLD contains damping screens which are represented by a force term based on the Morison equation. The performance of the model is assessed by comparing to experimental data for a structure-TLD system undergoing large amplitude excitations consisting of four-hour random signals and shorter transient signals. The model shows very good agreement with the experimental data for the structural response. The free surface response of the TLD is captured accurately by the model for the lower excitation forces considered, however as the excitation force is increased there are some discrepancies. The large amplitude excitations also result in SPH fluid particles penetrating the boundaries, resulting in degradation of the model performance over the four-hour simulations. Overall, the model is shown to capture the response of a structure-TLD system undergoing large amplitude excitations well.

Vibroacoustical Performance Analysis of a Rigid Device Casing with Piezoelectric Shunt Damping

Noise and vibration are common issues that may have a negative impact on human’s’ health. To minimize their consequences, several vibroacoustical methods may be employed. One well-known method is Piezoelectric Shunt Damping (PSD). Over the years, many approaches have been investigated, from passive, state switching circuits to active pulse-switching. In this paper, the authors propose three PSD implementations—passive Synchronized Switch Damping on Inductor (SSDI), semi-active SSDI and active Synchronized Switch Damping on Voltage source (SSDV)—for a single-panel structure mounted on a rigid-frame casing. The nine Macro Fiber Composite (MFC) elements were mounted on the plate based on preliminary simulations in FreeFEM. Then, the theoretical results were validated by an identification experiment. The main research is concentrated on the Sound Pressure Level (SPL) and structural vibrations reduction for selected frequencies. The active method provided the highest reduction of vibration—up to 5.5 dB for maximal possible loudspeaker level without overdrive and up to 7.5 dB for lower excitation levels.

Performance Assessment of Fluidic Oscillators Tested on the NASA Hump Model

Flow separation control over a wall-mounted hump model was studied experimentally to assess the performance of fluidic oscillators (sweeping jet actuators). An array of fluidic oscillators was used to control flow separation. The results showed that the fluidic oscillators were able to achieve substantial control over the separated flow by increasing the upstream suction pressure and downstream pressure recovery. Using the data available in the literature, the performance of the fluidic oscillators was compared to other active flow control (AFC) methods such as steady blowing, steady suction, and zero-net-mass-flux (ZNMF) actuators. Several integral parameters, such as the inviscid flow comparison coefficient, pressure drag coefficient, and modified normal force coefficient, were used as quality metrics in the performance comparison of the AFC methods. These quality metrics indicated the superiority of the steady suction method, especially at lower excitation amplitudes that is followed by the fluidic oscillators, steady blowing, and the ZNMF actuators, respectively. An aerodynamic figure of merit (AFM) was also constructed using the integral parameters and AFC power usage. The AFM results revealed that, for this study, steady suction was the most efficient AFC method at lower excitation amplitudes. The steady suction loses its efficiency as the excitation amplitude increases, and the fluidic oscillators become the most efficient AFC method. Both the steady suction and the fluidic oscillators have an AFM > 1 for the range tested in this study, indicating that they provide a net benefit when the AFC power consumption is also considered. On the other hand, both the steady blowing and ZNMF actuators were found to be inefficient AFC methods (AFM < 1) for the current configuration. Although they improved the flow field by controlling flow separation, the power requirement was more than their benefit.

Observations and analysis of absorption lines including J = K rotational levels of CH3CN: the envelope of Sagittarius B2(M)

ABSTRACT Molecules in diffuse and translucent clouds experience cooling as a result of radiation and less excitation from collisions. However, rotation around a molecular axis of acetonitrile, CH3CN, cannot be cooled by radiation, causing rotational populations to concentrate at the J = K levels. We aim to search for absorption lines of CH3CN having J = K level concentrations in diffuse and translucent clouds. The JK = 43–33 transition at 73.6 GHz was investigated toward Sgr B2(M) in the Galactic Centre region and other sources, using the Nobeyama 45-m telescope. Based on the absorption lines detected toward Sgr B2(M), a radiation temperature of 2.8 ± 0.5 K, kinetic temperature of 88 ± 29 K and column density of (1.35 ± 0.14) × 1014 cm−2 were derived for this molecule, revealing extremely concentrated J = K levels due to the lower excitation temperature and higher kinetic temperature. The absorption lines occurred at a velocity of 64 km s−1. The results confirm that CH3CN with J = K level concentrations exists in the envelope of Sgr B2(M).

Experimental Rotordynamic Force Coefficients for a Diffusion Bonded Compliant Hybrid Journal Gas Bearing Utilizing Fluid-Filled Hermetic Dampers

Dynamic force coefficients are presented from experimental results of a radial gas bearing with hermetically sealed squeeze film dampers (HSFDs) in the bearing support. HSFDs are a relatively new technology aimed to increase damping levels in gas bearings while sustaining an oil-free bearing sump. Past HSFD designs proved bulky and contained many components making it difficult to employ in size-limited environments such as jet engines, while the diffusion bonded bearing discussed in this paper provides a compact integral design. Details of the design are found in a companion paper by Ertas (Ertas, B. H., 2019, “Compliant Hybrid Gas Bearing Using Integral Hermetically-Sealed Squeeze Film Dampers,” ASME Paper No. GT2018-76312). Test results for a 3 in. (76.2 mm) diameter bearing using a test rig providing static loads up to 80 lbs (356 N), controlled-dynamic orbital motion, and speeds up to 27 krpm are shown. Results include frequency- and speed-dependent direct and cross-coupled rotordynamic force coefficients. Dynamic testing showed little dependence on rotor speed or static load and exhibited frequency dependency at lower excitation frequencies. Cross-coupled terms are generally an order of magnitude lower than direct terms. Results show the direct stiffness coefficients increasing with frequency, while direct damping decays radically with frequency. Comparison of the overall gas bearing coefficients with the companion paper (Ertas, B. H., 2019, “Compliant Hybrid Gas Bearing Using Integral Hermetically-Sealed Squeeze Film Dampers,” ASME Paper No. GT2018-76312), showing bearing support coefficients, reveals a drastic reduction in damping when engaging the gas film. The results also indicate that the bearing can withstand vibration levels representative of a large rotor system critical speed at lower excitation frequencies.

"Double-W-shaped" temperature dependence of emission linewidth in an InGaN/GaN multiple quantum well structure with intense phase separation

The temperature dependence of the spectra of photoluminescence (PL) from a blue InGaN/GaN multiplequantum-well (MQW) structure is investigated at lower excitation power. Two emission peaks, related to InGaN and assigned to In-rich quasi-quantum dots (QDs) and InGaN-matrix in the full PL spectrum, were observed. Upon increasing the temperature, both PL peak linewidths exhibited "double-W-shaped" (narrowing–broadening–narrowing–broadening–narrowing–broadening) temperature dependence. Combined with the observed features of the temperature dependences of the PL intensities, the temperature-dependent behaviors in this case can be interpreted as the relaxation and thermalization of carriers inside respective phase structures and the transfer of carriers between two phase structures, because of the strong phase separation and significant component fluctuation in the InGaN well layers.