Function Of Power Density Research Articles

Temperature and excitation power dependence of photoluminescence and electrical characterization of Ni‐passivated porous silicon

A porous silicon (PS) layer was obtained on p-Si (100) substrate by electrochemical anodization and Ni-porous silicon nanocomposites (Ni–PS) were elaborated by the electrodeposition method using nickel chloride aqueous solution. PS and Ni–PS samples were analyzed by X-ray diffraction (XRD). The XRD patterns revealed the presence of nickel elements in the PS matrix. The investigation by Photoluminescence spectroscopy (PL) enabled us to exam the evolutions of PL peak position, PL intensity, and full width at half maximum (FWHM) as a function of temperature and excitation power density. The modified Arrhenius formula, considering two activation energies, has been used to fit the temperature-dependent integration of PL intensities. PL investigations show that Ni ions cause changes in the recombination process of PS by the creation of new radiative centers as well as the reduction of non-radiative transitions. For Ni–PS nanocomposites, the laser power dependence of the integrated PL intensity shows that the most of transitions are free to bound or bound to bound where as the excitonic transitions are the most dominant in untreated PS. The current–voltage (I–V) characteristics of Ag/PS and Ag/Ni–PS Schottky diodes have been examined. The Cheung method was adopted to extract the parameters of the diode. Experimental results show that the values of ideality factor (n), resistance series (Rs), and barrier height (φb) are affected by the presence of nickel in the porous matrix.

IR Laser Induced Emission Spectra of GaN Nanoceramic

Currently, the researchers from around the world pay a lot of attention to search new white light generation sources. Such a device would have huge application potential in the lighting industry. The advantage of a broadband white light source is no necessity to mix few emission lines to obtain a white luminescence, as it is in case of the LED devices. Here, we investigate the broadband laser induced emission spectra of GaN nanoceramic measured under three different infrared excitation lines (1064, 975 and 808 nm). The GaN powder was obtained by thermal decomposition method while the nanoceramic was prepared using low temperature high pressure (LTHP) technique. The sample crystallizes in the hexagonal wurtzite structure and theoretical value of their average grain size calculated using Scherrer equation was 12 nm. The optical characterization of LIE spectra of GaN nanoceramic included the measurements of emission intensity as a function of power density, ambient pressure and luminescence kinetics. The emission spectra were recorded in both VIS (400-900 nm) and NIR (1200-2500 nm) regions (Fig. 1a,1b). It was found that the power dependence exhibit threshold behavior typical for avalanche process for all three excitation lines. The luminescence kinetics measurements show that rise and decay times of LIWE are extremely short in comparison to dielectric materials. It turned out that calculated x and y parameters of recorded emissions lie in the warm region of the CIE chromaticity diagram confirming their low color temperature regardless of the excitation wavelength (Fig. 1c). Finally, a general mechanism of LIE spectra in GaN nanoceramic is discussed.Figure 1. The LIWE intensity of GaN nanoceramic under 975 nm as a function of increasing pumping power density in VIS and NIR regions (a,b). The CIE chromaticity diagram of LIWE generated from GaN nanoceramic upon three various excitation wavelengths (c). The image of LIWE during experiment (d). Figure 1

ENZ materials and anisotropy: enhancing nonlinear optical interactions at the nanoscale.

Epsilon-near-zero materials are exceptional candidates for studying electrodynamics and nonlinear optical processes at the nanoscale. We demonstrate that by alternating a metal and a highly doped conducting-oxide, the epsilon-near-zero regime may be accessed resulting in an anisotropic, composite nanostructure that significantly improves nonlinear interactions. The investigation of the multilayer nanostructure reveals the actual role of the anisotropy, showing that high degrees of anisotropy might be necessary to effectively boost nonlinear processes. Moreover, using a microscopic, hydrodynamic approach we shed light on the roles of two competing contributions that are for the most part overlooked but that can significantly modify linear and nonlinear responses of the structure: nonlocal effects, which blueshift the resulting resonance, and the hot electrons nonlinearity, which redshifts the plasma frequency as the effective mass of free electrons increases as a function of incident power density and enhances the nonlinear signal by several orders of magnitude. Finally, we show that, even in the absence of second order bulk nonlinearity, second order nonlinear processes are also significantly enhanced by the layered structure.

Multi-function up-conversion luminescent Bi4Ti3O12 nanoparticles sensitized by Nd3+ and Yb3+

Abstract Up-conversion nanoparticles (UCNPs) stimulated by near-infrared (NIR) light have received immense interest for their potential applications in many fields. In this work, Nd3+- and Yb3+-sensitized Bi4Ti3O12 (BiTx:Nd3+/Yb3+, 0.005 ≤ x ≤ 0.3) UCNPs with thermometric and photo-thermal (PT) functions were successfully synthesized by a convenient co-precipitation approach, and the phase and morphologies were characterized. Under 980 nm excitation, BiTx:Nd3+/Yb3+ UCNPs exhibited remarkable multiple emissions in the region from 450 to 900 nm. The visible and NIR emissions were tightly dependent on the Nd3+ concentration and the strongest NIR emissions were obtained in sample BiT0.3:Nd3+/Yb3+. The possible UC mechanisms and processes were proposed. Furthermore, the capability of the UCNPs as thermometers was probed based on the thermo-responsive fluorescence intensity ratio technique in the temperature range 303–573 K. The maximum sensitivity (Smax) of BiT0.3:Nd3+/Yb3+ was determined to be as high as 0.032 K-1. Meanwhile, PT effect was evaluated by monitoring the variation of surface temperature of the sample as functions of irradiation time and power density. Conspicuous PT effect was confirmed and the maximum temperature induced by laser was about 91 °C. These results reveal that BiT0.3:Nd3+/Yb3+ UCNPs has promising potential applications as optical thermometer and heater.

Стационарное температурное поле разделительной системы с обладающей анизотропией свойств теплозащитой

We stated and solved the problem of determining the steady-state temperature field of a system simulated by a wall separating two different media. One of the wall surfaces has a thermally active layer that functions according to the feedback principle and features an anisotropic coating subjected to local heating while undergoing heat exchange with the environment. We show that the sought-after temperature field is an additive composition of two independent components, the first of which is exclusively a function of the heat exchange intensity between the separated media and the boundary surfaces of the system under consideration, and the second is a function of the heat flow power density affecting the separator system for the temperatures of the media separated being zero. We used integral transform methods in an analytically closed form to find solutions to respective steady-state heat conductivity problems. The results obtained confirm the existence of a previously found temperature field "drift" effect in an anisotropic material displaying general anisotropy of its properties.

Electrodynamics of conductive oxides: Intensity-dependent anisotropy, reconstruction of the effective dielectric constant, and harmonic generation

We study electromagnetic pulse propagation in an indium tin oxide nanolayer in the linear and nonlinear regimes. We use the constitutive relations to reconstruct the effective dielectric constant of the medium, and show that nonlocal effects induce additional absorption resonances and anisotropic dielectric response: longitudinal and transverse effective dielectric functions are modulated differently along the propagation direction, and display different epsilon-near-zero crossing points with a discrepancy that increases with increasing intensity. We predict that hot carriers induce a dynamic redshift of the plasma frequency and a corresponding translation of the effective nonlinear dispersion curves that can be used to predict and quantify nonlinear refractive index changes as a function of incident laser peak power density. Our results suggest that large, nonlinear refractive index changes can occur without the need for epsilon-near-zero modes to couple with plasmonic resonators. At sufficiently large laser pulse intensities, we predict the onset of optical bistability, while the presence of additional pump absorption resonances that arise from longitudinal oscillations of the free electron gas give way to corresponding resonances in the second and third harmonic spectra. A realistic propagation model is key to unraveling the basic physical mechanisms that play a fundamental role in the dynamics.

Helix Shape Power-Dependent Properties of Single Upconversion Nanoparticles.

Nonblinking, nonbleaching, and superbright single upconversion nanoparticles have been recently discovered with nonlinear power-dependent properties and can be switchable under dual-beam excitations, which are ideal for super-resolution microscopy, single-molecule tracking, and digital assays. Here, we report that the brightness of Nd3+-Yb3+-Er3+-doped nanoparticles displays a pair of unusual double helix shapes as the function of power densities of 976 and 808 nm excitations. We systemically analyze the power-dependent emission spectra, lifetimes, and power-intensity double-log slopes of single upconversion nanoparticles, which reveal that the dynamic roles of Nd3+ ions in the tridoped nanosystem with underlining electron population pathways are power dependent. That is, at high power 808 nm excitation, Nd3+ ions can directly emit upconverted luminescence, with their conventional role of sensitization saturated in the Nd3 → Yb3+ → Er3+ energy transfer systems. Moreover, we confirm that the universal helix shape phenomena commonly exist in a set of eight batches of core-shell nanoparticles regardless of the doping concentrations of Nd3+, Yb3+, and Er3+ ions in the sensitization shell, migration shell, and active core, though the crossing nodes occur at different excitation power ranges. This study emphasizes the important role of power-dependent properties in both improving the upconversion emission efficiency and the design of nonlinear responsive probes for imaging and sensing.

Dependence of electric power flow on solar radiation power in compact photovoltaic system containing SiC-based inverter with spherical Si solar cells

A photovoltaic power generation system suitable for mobile applications was developed. A SiC integrated converter with the maximum power point tracking circuit provided the smallest photovoltaic inverter in ~200 W level. The SiC-based inverter exhibited a peak direct current (DC)-alternating current (AC) conversion efficiency higher than that of conventional Si inverters. A Li-ion laminated battery was mounted in the same housing as the inverter. The weight of entire system containing spherical Si solar cell panels was well below 6 kg. Continuous operation measurements of this system were carried out using four solar cell modules connected in parallel under irradiation by natural sunlight. The total inverter efficiencies under realistic operation conditions were slightly decreased compared with the DC-AC converter values because of loss by the maximum power point tracking device. Even under unstable weather conditions, the system provided power stability without ripples. The behaviors of the output powers of the solar cell, storage battery, and inverter modules were analyzed as a function of the solar radiation power density. The substantial efficiencies of the solar cell modules were dependent on the weather conditions and were approximately 10% on cloudy days. The present compact photovoltaic power generation system with SiC device and spherical Si solar cells is viable for sub kW-class inverter.

Properties of Undoped Few-Layer Graphene-Based Transparent Heaters.

In many applications like sensors, displays, and defoggers, there is a need for transparent and efficient heater elements produced at low cost. For this reason, we evaluated the performance of graphene-based heaters with from one to five layers of graphene on flexible and transparent polyethylene terephthalate (PET) substrates in terms of their electrothermal properties like heating/cooling rates and steady-state temperatures as a function of the input power density. We found that the heating/cooling rates followed an exponential time dependence with a time constant of just below 6 s for monolayer heaters. From the relationship between the steady-state temperatures and the input power density, a convective heat-transfer coefficient of 60 W·m−2·°C−1 was found, indicating a performance much better than that of many other types of heaters like metal thin-film-based heaters and carbon nanotube-based heaters.

Ultrasonic modification of pectin for enhanced 2-furfurylthiol encapsulation: process optimization and mechanisms.

Pectin is an intriguing polymer, which is usually regarded as a byproduct from agricultural and biological processes. In previous studies, ultrasound treatment has been explored to improve the functionality of pectin but most of that work focused on aspects of molecular structure and the chemical properties of pectin. In this study, we utilized ultrasound treatment to modify the physiochemical properties of pectin. Using ultrasound treatment, we evaluated the emulsifying capability of pectin as a function of ultrasonic time and power density, using a response surface approach. A very potent yet unstable coffee-like aroma compound, 2-furfurylthiol, was also used for comparing the encapsulation feasibility of emulsion made with original pectin and ultrasound-treated pectin. Our results showed that the particle size of pectin was highly correlated with power density and ultrasound time. Approximately 370 nm of pectin particle size could be reached at a power density of 1.06 W mL-1 for 40 min. Ultrasound treatment increased emulsion droplet size but significantly improved emulsifying capacities, such as centrifugal stability and surface loading, although it was highly dependent upon the ultrasound treatment condition. When used as the encapsulation wall material, the ultrasound-modified pectin had significantly enhanced performance compared with the original, in terms of flavor retention over time at 45 °C and 65 °C. Ultrasound treatment was able to modify the physiochemical properties of pectin, which thus improved emulsification stability and encapsulation feasibility by forming a thicker layer at the oil / water interface to protect the core materials. © 2019 Society of Chemical Industry.

Critical Power Density: A Metric To Compare the Excitation Power Density Dependence of Photon Upconversion in Different Inorganic Host Materials

In photon upconversion (UC) based on triplet-triplet annihilation, the upconversion photoluminescent quantum yield (UC-PLQY) depends on the excitation power density in a way that can be described by a single figure of merit. This figure of merit, the threshold value, allows the excitation power density required for efficient UC-PLQY to be compared between different triplet-triplet annihilation systems. Here, we investigate the excitation power density dependence of two-photon UC processes in a series of four lanthanide-doped inorganic host materials (oxides, fluorides, and chlorides) all doped with 18 mol % Yb3+ sensitizer ions and 2 mol % Er3+ activator ions. We demonstrate that an analogous figure of merit, which we call the critical power density (CPD), accurately describes the UC power dependence of these samples. Better CPD values are obtained when the lifetime of the intermediate states is long. The UC-PLQY at the CPD is linked to the saturation UC-PLQY. Thus, a measurement of the UC-PLQY at this low power density can be used to estimate the theoretical saturation UC-PLQY in the absence of deleterious effects such as laser-induced heating. This is compared to another method to estimate the saturation based on the CPD model, namely, taking half of the level's PLQY under direct excitation. Our careful analysis of the upconversion spectra as a function of excitation power density gives several insights into the differing upconversion pathways in the hosts and proves to be a useful tool for their comparison.

Triplet Sensitization by Lead Halide Perovskite Thin Films for Efficient Solid-State Photon Upconversion at Subsolar Fluxes

Summary We investigate the rubrene triplet sensitization by perovskite thin films based on methylammonium formamidinium lead triiodide (MAFA) of varying thicknesses. The power-law dependence of both the MAFA photoluminescence (PL) intensity and upconverted emission is tracked as a function of the incident power density. Bimolecular triplet-triplet annihilation (TTA) exhibits a unique power-law dependence with a slope change from quadratic-to-linear at the threshold Ith. The underlying MAFA PL power-law dependence dictates the power law of the upconverted PL: (1) below Ith, the slope of the upconverted PL is twice the value of the MAFA PL; (2) above Ith, it follows the same power law as the underlying recombination of mobile electrons and holes in the MAFA films. We find that the Ith shifts to subsolar incident laser powers when increasing the MAFA thickness above 30 nm. For the thickest MAFA film of 380 nm we find an upconversion threshold of Ith = 7.1 mW/cm2.

Impact of the synthesis procedure on the spectroscopic properties of anti-Stokes white emission obtained from Sr2CeO4 phosphor

In the present work, the anti-Stokes white luminescence characterized by high emission intensity, low color temperature and large value of full width at half maximum (FWHM) observed from Sr2CeO4 phosphor obtained via various methods is reported. The X-ray diffraction patterns (XRD) proved that investigated host lattice was successfully prepared by solvothermal, citric-gel and solid-state techniques and their average grain sizes were calculated to be 13, 75 and 135 nm, respectively. These synthesis methods have been chosen deliberately to obtain clear differences in average grain sizes. The morphology of the samples was studied using transmission electron microscopy (TEM). In order to investigate the spectroscopic features, the anti-Stokes white emission intensity of Sr2CeO4 phosphors in a function of excitation power density and ambient pressure was measured. Significant relative photoconductance of up to six orders of magnitude was observed. In particular, the influence of the synthesis procedure on the optical properties of laser induced white emission (LIWE) generated from Sr2CeO4 was analyzed.

Analysis of efficiency curves in near-UV, blue, and green-emitting InGaN-based multiple quantum wells using rate equations of exciton recombination

The internal quantum efficiency (IQE) of near-UV, blue, and green-emitting InGaN-based multiple quantum wells was evaluated as functions of excitation power density and temperature using photoluminescence spectroscopy. The IQE curves under weak excitation densities were analyzed using a rate equation model based on the radiative and nonradiative recombination of excitons. The analysis clarified that the initial increase in the estimated IQE could be explained by the filling of nonradiative recombination centers. Moreover, the analysis elucidated that the nonradiative recombination process of excitons was independent of the emission wavelength and that the radiative recombination rate of excitons was strongly affected by the exciton localization and depended on the emission wavelength. Furthermore, the analysis suggested that the maximum IQE reached was 100% in all the samples at a low temperature.

Model of Steady-state Temperature Rise in Multilayer Tissues Due to Narrow-beam Millimeter-wave Radiofrequency Field Exposure.

The assessment of health effects due to localized exposures from radiofrequency fields is facilitated by characterizing the steady-state, surface temperature rise in tissue. A closed-form analytical model was developed that relates the steady-state, surface temperature rise in multilayer planar tissues as a function of the spatial-peak power density and beam dimensions of an incident millimeter wave. Model data was derived from finite-difference solutions of the Pennes bioheat transfer equation for both normal-incidence plane waves and for narrow, circularly symmetric beams with Gaussian intensity distribution on the surface. Monte Carlo techniques were employed by representing tissue layer thicknesses at different body sites as statistical distributions compiled from human data found in the literature. The finite-difference solutions were validated against analytical solutions of the bioheat equation for the plane wave case and against a narrow-beam solution performed using a commercial multiphysics simulation package. In both cases, agreement was within 1-2%. For a given frequency, the resulting analytical model has four input parameters, two of which are deterministic, describing the level of exposure (i.e., the spatial-peak power density and beam width). The remaining two are stochastic quantities, extracted from the Monte Carlo analyses. The analytical model is composed of relatively simple functions that can be programmed in a spreadsheet. Demonstration of the analytical model is provided in two examples: the calculation of spatial-peak power density vs. beam width that produces a predefined maximum steady-state surface temperature, and the performance evaluation of various proposed spatial-averaging areas for the incident power density.

High-repetition-rate laser-induced damage of indium tin oxide films and polyimide films at a 1064 nm wavelength

Experiments and thermal modeling of indium tin oxide transparent conductive thin film and polyimide alignment thin film coated on fused silica substrates damaged with a 1064 nm high-repetition-rate laser are described. High-repetition-rate laser irradiation results in damaged morphologies of the bulge at a low laser power density and formation of a pit in the center of the bulge at higher laser power density. The damage process that is consistent with the observations as a function of laser power density and irradiation time is related to thermal effect. Simulation of the temperature-rise by exposure to high-repetition-rate laser describes the thermal effect with different pulse oscillation.

Broadband photodetector based on 3D architect of MoS2-PANI hybrid structure for high photoresponsive properties

MoS2-PANI hybrid structure were synthesized by insitu polymerization of hydrothermally synthesized MoS2 nanosheets with PANI for its application in photodetectors. Field-Emission Scanning Electron Microscopy (FESEM), High-Resolution Transmission Electron Microscopy (HRTEM), X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and UV–vis spectroscopy delete were performed to characterize the synthesized sample. The optical sensor of MoS2-PANI hybrid structure were fabricated and altogether studied using laser excitation wavelengths (λex): 635 nm (red), 785 nm (infra-red) and 1064 nm (near infra-red). The evaluated value of photoresponsivity of hybrid structure is quite high compared to the previously reported MoS2 nanosheets based optical sensor. At 785 nm, maximum photoresponsivity of 25 A/W is observed at fixed power density of 1.4 mW/mm2. The photoresponsive characteristics of MoS2-PANI hybrid structure were examined as a function of optical power density.

Steady-state microwave conductivity reveals mobility-lifetime product in methylammonium lead iodide

Many time-resolved techniques to study charge carrier recombination involve pulsed high-power optical excitation and photo-generated carrier densities many orders of magnitude higher than present under typical solar cell operating conditions. In this report, we demonstrate a steady-state contactless microwave conductivity technique to evaluate the photoconductivity of carriers in semiconductors at low illumination intensity, as a function of optical power density. We studied characteristics of both thin films and single crystals of a hybrid halide perovskite compound, methylammonium lead iodide (MAPbI3). The aggregate mobility-lifetime product of majority and minority carriers in thin films of MAPbI3 was determined and found to be highly-dependent on incident optical power density, even at sub-1-sun illumination intensities, and attributed to trap states within the films.

Photoconductivity of tellurium-poly(methyl methacrylate) in the ultraviolet–visible-near infrared range

Fine powders composed of tellurium grains of average size <10 nm were produced by dry vibration milling combined with liquid-phase sedimentation techniques, starting from polycrystalline powders with average grain diameter of ca. 30 μm. Nanocomposite films were obtained by binding the nanosized tellurium grains with poly(methyl methacrylate). Raman spectroscopy revealed that the films were based on the coexistence of tellurium and tellurium oxide crystalline phases due to a partial oxidation in air of the grains. The optical measurements of the fabricated material showed that the absorbance was nearly constant in the 310–2200 nm range and that a typical UV absorption peak of the nanostructured tellurium was centered at around 260 nm. An extensive characterization of the photoconductivity properties was carried out by illuminating the tellurium-poly(methyl methacrylate) films with white light or radiations of different spectral composition selected from the UV–Vis-NIR region. Data analysis has allowed to demonstrate that the photoresponse is closely related to the optical absorption and is independent of the spectral composition of the incident radiation in the wavelength range from 310 to 2200 nm, while the photocurrent increases linearly as a function of the optical power density over about three orders of magnitude.

A method for correcting the excitation power density dependence of upconversion emission due to laser-induced heating

Upconversion (UC) materials enable the emission of a higher energy photon following the absorption of two or more lower-energy photons. The efficiency of UC is non-linear as it depends on the power density of the optical excitation. Hence, the measurement of the UC intensity as a function of excitation power density is a standard material characterization. Such a measurement reveals at which excitation power densities the UC material can be effectively used and hints at the underlying physical mechanisms of UC. One issue in the interpretation of these measurements is the decrease in the UC intensity due to the excitation-laser-induced heating of the sample. Here, we study the excitation-laser-induced heating of β-NaYF4:Yb3+/Er3+ (18/2%) micro-powders, ascertaining the temperature as a function of time (and the steady-state temperature reached) for a series of excitation power densities by monitoring the thermally coupled peaks in the emission of the Er3+ ion. Based on this, we demonstrate a technique to correct the excitation power density dependence of the UC in order to remove the effect of sample heating from the measured data. Correcting the reduction in UC intensity at higher power densities (>50 W/cm2), we recover the UC excitation power density dependencies that agree with theoretical predictions (that disregard thermal effects) in the high excitation power density regime. The reduction in UC intensity due to sample heating can be significant; for example, a 60 K temperature rise lead to a 24% decrease in UC intensity at an excitation power density of 250 W/cm2. However, the heating does depend on the nature of the sample. In contrast to the powder samples, β-NaYF4:Yb3+/Er3+ (18/2%) nanoparticles in toluene solution showed no heating at excitation power densities up to 250 W/cm2. Therefore, we conclude that the proposed correction can remove the effects of sample heating to allow accurate comparison to physical models, and is of especial relevance to powder samples due to their higher absorption of the excitation and poorer thermal transport.