High Incident Light Intensities Research Articles

Saturation Mechanisms in Common LED Phosphors.

Commercial lighting for ambient and display applications is mostly based on blue light-emitting diodes (LEDs) combined with phosphor materials that convert some of the blue light into green, yellow, orange, and red. Not many phosphor materials can offer stable output under high incident light intensities for thousands of operating hours. Even the most promising LED phosphors saturate in high-power applications, that is, they show decreased light output. The saturation behavior is often poorly understood. Here, we review three popular commercial LED phosphor materials, Y3Al5O12 doped with Ce3+, CaAlSiN3 doped with Eu2+, and K2SiF6 doped with Mn4+, and unravel their saturation mechanisms. Experiments with square-wave-modulated laser excitation reveal the dynamics of absorption and decay of the luminescent centers. By modeling these dynamics and linking them to the saturation of the phosphor output intensity, we distinguish saturation by ground-state depletion, thermal quenching, and ionization of the centers. We discuss the implications of each of these processes for LED applications. Understanding the saturation mechanisms of popular LED phosphors could lead to strategies to improve their performance and efficiency or guide the development of new materials.

Effects of incident light intensity and light path length on cell growth and oil accumulation in Botryococcus braunii (Chlorophyta).

Botryococcus braunii was cultured in different light path length under different incident light intensity to investigate the effect of light on alga growth as well as hydrocarbon and fatty acid accumulation. Results indicated that longer light path length required higher incident light intensity in order to meet the light requirement of algal growth and hydrocarbon accumulation during the course of cultivation. However, hydrocarbon profile was only affected by the incident light intensity and not influenced by the light path length. High incident light intensity enhanced the accumulation of hydrocarbons with longer carbon chains. Besides, the fatty acid content and profiles were significantly influenced by both incident light intensity and light path. Higher fatty acid content and higher percentage of C18 and monounsaturated fatty acid components were achieved at the higher incident light intensity and lower light path length. Taken together, these results are benefit to improve its biomass and oil productivity through the optimization of light and photobioreactor design.

A Charge Compensation Phototransistor for High-Dynamic-Range CMOS Image Sensors

This paper presents a charge compensation phototransistor for high-dynamic-range CMOS image sensors (CIS). With a built-in charge compensation mechanism, the photoresponse of the proposed device can switch between the linear mode and the logarithmic mode automatically, according to the incident light intensity. In particular, the device does not saturate until approximately 0.5 W/cm2, almost 5 times the brightness of the sun. Therefore, CIS with this proposed phototransistor demonstrates a measured dynamic range of 167 dB, and an output swing of more than 2 V without amplification. The nonideal behavior of the proposed device due to the thermal effect and the parasitic resistances is investigated when exposed with high incident light intensity. In addition, the nonuniform response of the pixel array is exhibited and analyzed.

Saturable Absorption in Suspensions of Single-Digit Detonation Nanodiamonds

Highly nonlinear optical properties of nanocarbon materials make them perspective candidates for saturable absorption (SA) applications, characterized by a short-term decrease in the optical medium absorption at high incident light intensity. The nonlinear optical properties and SA of aqueous suspensions of 5 nm detonation nanodiamonds (DNDs) have been studied by the z-scan technique under irradiation with nanosecond laser pulses at wavelengths of 532 and 1064 nm. The saturation intensity and nonlinear absorption coefficient describing the SA were obtained for concentrations of DND suspensions ranging between 0.5 and 5 wt %. It was concluded that the single-digit DND suspensions are suitable for optical limiting only at relatively high intensities of incident radiation, with saturation intensities of ∼5.9 and 14.1 MW/cm2 for 532 and 1064 nm incident excitation, respectively.

Saturation of intraband absorption in a self-assembled double-quantum-dot molecule

Saturation absorption induced by interaction between the double-quantum-dot molecule and the intense-laser-field has been studied theoretically by using the effective mass and adiabatic approximations. Saturation absorption arising from the transition between bonding and antibonding states is strongly dependent on the dot-separation, the dot size and the incident light intensity. Absorption spectra appeared at THZ frequency-domain are bleached significantly and broadened under the high incident light intensity. Saturation intensities are of the order of MW/cm2 and the calculation results coincide with that of the experimental measurement. Finally, the influence of the relaxation time on the saturation absorption has been also studied.

Avoiding Diffusion Limitations in Cobalt(III/II)‐Tris(2,2′‐Bipyridine)‐Based Dye‐Sensitized Solar Cells by Tuning the Mesoporous TiO2 Film Properties

Dye-sensitized solar cells based on electrolytes containing cobalt complexes as redox shuttles typically suffer a major limitation in terms of slow diffusion of those couples through the mesoporous TiO(2) film. This results in a drop of the photocurrent density, particularly at high incident light intensities, reducing the overall cell performance. This work illustrates how tuning the four characteristic parameters of the mesoporous TiO(2) layer, namely film thickness, particle size, pore size and porosity, by simply optimizing the TiCl(4) post-treatment, completely eliminates diffusion problems of cobalt(III/II) tris(2,2'-bipyridine) and at the same time maximizes the short-circuit photocurrent density. As a result, a power conversion efficiency of 10.0% at AM 1.5 G 100 mW cm(-2) was reached in conjunction with an organic sensitizer.

Determination of kinetic constants of a photocatalytic reaction in micro-channel reactors in the presence of mass-transfer limitation and axial dispersion

The main objective of this article is to evaluate quantitatively the kinetic constants of a photocatalytic reaction in micro-channel reactors despite the presence of a radial concentration profile (mass-transfer limitation) and axial dispersion. Photocatalytic micro-channel reactors with immobilized titanium dioxide as photocatalyst have been fabricated using stereolithography process. The photocatalytic degradation of salicylic acid is investigated as a function of rectangular micro-channel size, contaminant concentration, flow rate and incident UV light intensity. All the micro-channel reactors exhibit the same tendency. Higher degradation is observed for high incident light intensities, low pollutant concentrations and flow rates. A simple equation for the determination of the kinetic constants during the photocatalytic degradation is reported. The equation includes the hydrodynamic properties and a surface reaction model for the photocatalytic reaction (monomolecular Langmuir–Hinshelwood kinetics). In the experimental system, we demonstrate that some external mass-transfer limitation and axial dispersion occur. They are included in the modeling with calculated values of the mass-transfer coefficient of salicylic acid from the solution to the catalyst surface and the axial dispersion coefficient. A single couple of values of the reaction rate constant k and the adsorption equilibrium constant K represent properly the experimental degradation ratios for all reactor dimensions, flow rates and pollutant concentrations. The major parameter that controls the values of the reaction rate constant is the incident light intensity. The dependence of the reaction rate on the incident light intensity is first order.

Maximizing Algal Growth in Batch Reactors Using Sequential Change in Light Intensity

Algal growth requires optimal irradiance. In photobioreactors, optimal light requirements change during the growth cycle. At low culture densities, a high incident light intensity can cause photoinhibition, and in dense algal cultures, light penetration may be limited. Insufficient light supply in concentrated algae suspensions can create zones of dissimilar photon flux density inside the reactor, which can cause suboptimal algal growth. However, growth of dense cultures can also be impaired due to photoinhibition if cells are exposed to excessively high light intensities. In order to simultaneously maintain optimal growth and photon use efficiency, strategies for light supply must be based on cell concentrations in the culture. In this study, a lipid-producing microalgal strain, Neochloris oleoabundans, was grown in batch photobioreactors. Growth rates and biomass concentrations of cultures exposed to constant light were measured and compared with the growth kinetic parameters of cultures grown using sequentially increasing light intensities based on increasing culture densities during batch growth. Our results show that reactors operated under conditions of sequential increase in irradiance levels yield up to a 2-fold higher biomass concentration when compared with reactors grown under constant light without negatively impacting growth rates. In addition, this tailored light supply results in less overall photon use per unit mass of generated cells.

The Effect of Atmosphere and ZnO Morphology on the Performance of Hybrid Poly(3-hexylthiophene)/ZnO Nanofiber Photovoltaic Devices

We present detailed investigations of the fabrication and characterization of photovoltaic devices consisting of poly(3-hexylthiophene) (P3HT) intercalated into a mesoporous structure of ZnO nanofibers. ZnO nanofibers were grown via a low-temperature hydrothermal route from a solution of zinc nitrate precursor. P3HT was spin-coated on top of the structure, and intercalation into the voids between the nanofibers was induced with annealing. A silver electrode was used as the top contact. Spin-coating, storage, and testing of the device were performed in air. We discuss the effects of atmosphere and ZnO nanofiber morphology on device performance. Optimized nanofiber devices exhibited a 4-fold increase in the short circuit current (2.17 mA/cm2) as compared to that of a planar ZnO−P3HT bilayer device (0.52 mA/cm2) as a result of the increased donor−acceptor interfacial area. The efficiency of the nanofiber based device under 1 sun-simulated solar illumination was 0.53% and was found to increase at higher incident light intensities, reaching a value of 0.61% at 2.5 suns. Additionally, we found that for these devices fabrication in and exposure to air is required to obtain good diode characteristics. We also show that the morphology of the ZnO nanostructures in the nanocomposite directly impacts device performance. Treatment of the ZnO surface using surfactants increased the open circuit voltage at the expense of the short circuit current; however, there was little effect on overall device efficiency. Because of the inverted geometry of this device that allowed for the use of a silver top contact, the device was not susceptible to oxidative degradation when stored in the dark.

The response of a planktonic microbial community to experimental simulations of sudden mixing conditions in temperate coastal waters: Importance of light regime and nutrient enrichment

The effect of light and nutrients on a temperate water planktonic microbial community was investigated via experiments conducted in Scottish coastal waters during summer. Experimental water was collected from the depth of maximum chlorophyll concentration in the Clyde Sea on the 7th and 9th of August 2001. For each experiment, the microbial community was incubated for 36 h under one of two differing natural light regimes: continuous 3% solar irradiation or alternating 1-hour periods of 80% solar irradiation and darkness. Water incubated with the alternating light regime was also subjected to three initial nutrient manipulations: added nitrate, added nitrate and phosphate, or no nutrient addition. The four treatments were designed to simulate stratified or mixed water column conditions; the latter with or without nutrient entrainment. During the first experiment, the photosynthetic capacity and efficiency of phytoplankton were significantly reduced after 36 h incubation under the alternating light regime, regardless of nutrient addition; however phytoplankton in the second experiment showed not such response. Higher incident light intensities, recorded towards the end of the first experiment and throughout the second, may have resulted in photoinhibition of phytoplankton during the last hours of the first experiment, with cells acclimatised by the end of the second experiment, despite low nitrogen availability. In general, the four treatments had relatively little effect on the taxonomic composition and abundance of the microbial community; however, significant differences were observed in the abundance and activity of some microbial groups and species. Bacterial production increased during incubation across all treatments and a positive relationship was observed between bacterial specific growth and photosynthetic capacity. This suggests strong coupling between phytoplankton and heterotrophic bacteria via dissolved organic material allowing bacteria and phytoplankton to sustain their growth under fluctuating environmental conditions. By contrast, the dinoflagellates Alexandrium sp. exhibited highest abundance when exposed to the alternating light regime but only during the first experiment. Reduced competition for ammonium, due to photoinhibition of low-light adapted diatoms, may explain this observation suggesting that growth rate of potentially toxic dinoflagellates, such as Alexandrium spp., can be limited in temperate waters by ammonium availability rather than irradiance.

Dynamics and toxicity of Anabaena aphanizomenoides (Cyanobacteria) waterblooms in the shallow brackish Oued Mellah lake (Morocco)

The structure and abundance of phytoplankton communities were investigated during 1997 to 1999 in Oued Mellah, a shallow brackish and hypertrophic lake, with particular regard to Anabaena aphanizomenoides dynamics. Important events of algal blooms were observed mostly by the cyanobacteria Microcystis ichthyoblabe, Anabaena aphanizomenoides and Oscillatoria chlorina and by the ichthyotoxic haptophyceae Prymnesium parvum. Anabaena aphanizomenoides proliferated during late summer after the Microcystis ichthyoblabe blooms. The percentage of Anabaena aphanizomenoides of the phytoplankton biomass varied from 88 to 94 percent during bloom periods. Maximum biomass was 146 and 120 mg fresh weight l−1 during the 1997 and 1999 summer periods, respectively. The main environmental factors leading to the ecological success of A. aphanizomenoides were high temperature (25–28ˆC), high incident light intensities (1488–1912 μ E m−2 s−1, high nutrient deficiency (0 μ g P-PO−4 l−1; 0–0.18 mg N-NO3 l−1) and decrease of alkalinity (329–494 mg HCO3− l−1). The toxicity of the Anabaena aphanizomenoides bloom was evaluated by bioassays and analyses. The lethal dose50 of the bloom sample tested in mice was 254 mgDW kg−1 body weight while toxicity (24 h LC−50) in the brine shrimp Artemia salina was 3.68 mgDW ml−1. The low microcystin content (3.28 μ g gDW−1) determined by ELISA was not consistent with the tested bioassays and is suggestive of the presence of other toxic compounds in the bloom extracts. Four toxic fractions were separated by HPLC-PDA and identified as microcystins according to their UV spectra. The production of microcystins by Anabaena aphanizomenoides bloom was confirmed by the analysis of the isolated strain which, in Z8 medium under controlled laboratory conditions, produced three variants of microcystins, two of them being similar to those produced by the natural bloom.

Monte Carlo simulation of impact ionization in photodetectors

Impact ionization in optical detectors has been studied using a self-consistent Monte Carlo method. Current multiplication was found to be sensitive to both the applied potential and the incident light intensity. The amount of current multiplication was significantly higher than might be expected under the assumption that the field in the device is unchanged by the photogenerated carrier distribution. The potential within the device was also found to be significantly affected by the impact-ionization process and this in turn gave rise to interesting oscillations in the current at higher incident light intensities and potentials.

Monte Carlo simulation of impact ionisation in photodetectors

The authors have studied impact ionisation in optical detectors using a self-consistent Monte Carlo method. Current multiplication was found to be sensitive to both the applied potential and the incident light intensity. The amount of current multiplication was significantly higher than might be expected from a naïve interpretation of the nominally applied field and this was attributed to screening effects within the device. The potential within the device was also significantly affected by the impact ionisation process and this in turn gave rise to an interesting structure (oscillations) in the current at higher incident light intensities and potentials.

Depressed photoemission from Görlich cathodes at high laser light intensities

The current density of electrons emitted from surfaces irradiated by light is known to follow either a linear or quadratic dependence with intensity for single- or double-photon absorption respectively or, at high incident light intensities, a square root dependence (depressed photoemission). In separate experiments, a Görlich (Cs3Sb) cathode was irradiated with the green (λ=5140 Å) and blue (λ=4880 Å) lines of an Ar+ laser at various intensities in an effort to determine over what range depressed photoemission continued above the current density threshold of about 10−5 A cm−2. The results have confirmed and extended the existence of depressed photoemission to intensities of up to five orders of magnitude higher than previously observed threshold values with no apparent evidence of saturation of the effect. The consequences of this are discussed in relation to surface traps and two-photon-plasmon excitation in the photocathode.

Microalgal photosynthesis and spectral scalar irradiance in coastal marine sediments of Limfjorden, Denmark

Scalar irradiance and oxygenic photosynthesis were measured simultaneously at 100-µm spatial resolution by a fiber-optic scalar irradiance microsensor and an oxygen microelectrode spaced 120 µm apart. Marine microbial mats on sandy sediments along the coast of Limfjorden, Denmark, were dominated by cyanobacteria with a surface layer populated by pennate diatoms. In dim light Oscillatoria sp. migrated upward and a dense surface film of cyanobacteria developed. The spectral distribution of scalar irradiance showed absorption peaks at 430 and 675 nm (Chl a), 630 (phycocyanin), and 800 and 860 nm (bacteriochlorophyll a). Infrared scalar irradiance reached 200% of incident light intensity at 0.0–0.3-mm depth and IR penetration was independent of the development of a cyanobacterial surface film. At high incident light intensity, 740 µEinst m−2 s−1, the photosynthetic efficiency at 1.0-mm depth was 10-fold higher than in the uppermost 0.0–0.6 mm of the sediment. The lower boundary of the euphotic zone (detectable gross photosynthesis) was at a mean light level of ≥7.5 µEinst m−2 s−1.

Energetic and temporal characteristics of picosecond pulse self‐diffraction in silicon

AbstractSelf‐induced diffraction of ultrashort pulses in silicon crystals at high incident light intensities is studied. A theoretical model is presented which takes into account the band‐filling effect as a major mechanism of light‐induced grating anharmonicity. Experimental results on the energetic and temporal characteristics of four orders of diffraction are in good agreement with this theoretical model.

Time-delayed photoelectric effect

The Einstein photoelectric relation1 initially explained the photoelectric effect, hν=Tmax + Eb where hν is the energy of a photon incident on a material, Eb is the binding energy of the electron to be excited, and Tmax is the maximum kinetic energy of the emitted electron. This work has generated considerable research2–5 because of its multidisciplinary interest. We describe here photoelectric emission (PE) experiments using very low-intensity nanosecond light pulses with energies near the PE threshold. Signal correlated, time-delayed pulses of emitted electrons were observed for single light pulses incident on a photosensitive material. However, the energies of the delayed emitted electrons were not correlated to the wavelength of the incident light, which is not consistent with Einstein's relation. At higher incident light intensities, the delayed pulses were not observed. These pulses were not due to electrical noise because they disappeared when the laser light was blocked from entering the cell. They were not due to single photon luminescence from a particular species because the pulses were observed using another experimental apparatus with an alternate source of nanosecond light. We now describe these delayed pulses and give a preliminary interpretation of them.

Performance of a new high-intensity silicon solar cell

A new silicon solar cell, designed to have improved electrical, optical, and thermal transfer characteristics at very high incident light intensities, has been fabricated and provides experimental verification of the basic design concepts. The AM1 efficiency for nonoptimized cells is 12.8% at 25 °C. At 300 suns the efficiency increases to 19%. It is shown that efficiencies of over 25% are possible for this type of cell in a more-optimized form at intensities of ∼500–1000 suns.

PICOSECOND TIME RESOLVED FLUORESCENCE OF CHLOROPHYLL IN VIVO

Abstract. The published data concerning the fluorescence kinetics of chlorophyll a in various photosynthetic species are reviewed. The effects of singlet‐singlet and singlet‐triplet annihilation induced by excessively high incident light intensities are discussed and related to the changes produced in the fluorescence lifetimes and quantum yields. We also review the fluorescence lifetimes of Chlorella pyrenoidosa and spinach chloroplast fragments under a variety of experimental conditions; these measurements were performed at single pulse excitation intensities of less than 5 × 1013 photons cm–2 where distortion due to annihilation processes is negligible. Evidence for and against a time dependent rate equation for energy migration will be discussed with reference to the authors' work on in vitro systems.

High light-level blooming in the silicon vidicon

Blooming occurs in silicon-vidicon targets because more hole-electron pairs are created at a given point by the incident light signal than can be removed by the action of the scanning electron beam. The excess carriers move laterally in the target through diffusion and, as a result, the blooming has the following properties- 1) the Slope of the blooming curve on semilog paper does not depend on the size of the incident light spot, i.e., at a given overload intensity, all circular spot sizes add the same excess amount to their radii; 2) the radius of a bloomed spot increases linearly with the log of the incident intensity; 3) the Slope of the blooming curve does not depend on beam current; however, the intensity at which the blooming begins does depend directly on the beam current. At very high incident-light intensities, a dramatic increase in the blooming is observed. This can be attributed to diffusion under conditions where the minority carrier level has become equal to the majority carrier level.