Threshold Irradiance Research Articles

Threshold characteristics of epitaxial Al(Ga,As) surface-emitting lasers with integrated quarter-wave high reflectors

We report a study of threshold characteristics of as-grown surface-emitting lasers fabricated with molecular-beam epitaxy by the monolithic integration of two quarter-wave high reflectors (mirrors) of AlAs/Al0.4Ga0.6As surrounding an active spacer layer. The spacer was either a multiple quantum well of GaAs/Al0.4Ga0.6As (100 Å/200 Å) or GaAs. Several structures were grown corresponding to different mirror reflectance and different spacer thicknesses from ultrashort 0.9 to 10 μm. One of the structures was chemically etched to form a two-dimensional array of microlasers. All of the structures were photopumped at room temperature, and the lasing threshold was determined. Without any lateral confinement, the threshold irradiance was as low as 2×105 W/cm2. Near-field images of the light emitted slightly above threshold reveal several competing filaments. This competition broadens the lasing linewidth, but can be controlled by lateral confinement schemes.

Dose-response relationship between light irradiance and the suppression of plasma melatonin in human volunteers

This study tested the capacity of different irradiances of monochromatic light to reduce plasma melatonin in normal humans. Six healthy male volunteers, 24–34 years old, were exposed to 0.01, 0.3, 1.6,5, or 13 μW/cm 2 of 509 nm monochromatic light for 1 h during the night on separate occasions. Light irradiance depressed plasma melatonin in a dose-response pattern. The data indicate that the mean threshold irradiance for suppressing melatonin is between 1.6 and 5μW/cm 2. Individual variations in threshold responses to monochromatic light were observed among the volunteers.

Light-induced stimulation of retinal dopamine: a dose-response relationship

Light stimulates dopamine (DA) release in the retina. The purpose of this was to determine the threshold and dose-response relationship between ocular light exposure and retinal DA synthesis in vivo. Groups of dark-adapted rats were exposed to 0, 1, 3, 5, 10, 25, 50, 100 or 1000 microwatts per square centimeter (μW/cm 2 of white light for 15 min. Retinal DA and dihydroxyphenylalanine (DOPA) were subsequently quantified by liquid chromatography with electrochemical detection. Both the DA and DOPA data fit hyperbolic curves significantly ( Pˇ0.01). Exposure to white light at 25 μW/cm 2 or greater appears to elicit the maximum response of these neurons. Threshold irradiance is calculated to be 3–5 μW/cm 2. These results indicate that retinal DA synthesis and presumably DA neuron activity have a graded response to increasing irradiances of white light.

THE MECHANISM OF DAYLENGTH PERCEPTION IN THE RED ALGA ACROSYMPHYTON PURPURIFERUM1

ABSTRACTThe red alga Acrosymphton purpuriferum (J. Ag.) Sjöst. (Dumontiaceae) is a short day plant in the formation of its tetrasporangia. Tetrasporogenesis was not inhibited by 1 h night‐breaks when given at any time during the long (16 h) dark period (tested at 2 h intervals). However, tetrasporogenesis was inhibited when short (8 h) main photoperiods were extended beyond the critical daylength with supplementary light periods (8 h) at an irradiance below photosynthetic compensation. The threshold irradiance below photosynthetic compensation. The threshold irradiance for inhibition of tetrasporogenesis was far lower when supplementary light periods preceded the main photoperiod than when they followed it (< 0.05 μmol.m−2. s−1 vs. 3 μmol.m−2.s−1. The threshold level also depended on the irradiance given during the main photoperiod and was higher after a main photoperiod in bright light than after one in dim light (threshold at 3 μmol.m−2.s−1 after a main photoperiod at ca. 65 μmol.m−2.s−1 vs. threshold at <0.5 μmol.m−2.s−1 after a main photoperiod at ca. 35 μmol.m−2.s−1. The spectral dependence of the response was investigated in day‐extensions (supplementary light period (8 h) after main photoperiod (8 h) at 48 μmol. m−2.s−1) with narrow band coloured light. Blue light (λ= 420 nm) was most effective, with 50% inhibition at a quantum‐dose of 2.3 mmol.m−2. However, yellow (λ= 563 nm) and red light (λ= 600 nm; λ= 670 nm) also caused some inhibition, with ca. 30% of the effectiveness of blue light. Only far‐red light (λ= 710 nm; λ= 730 nm) was relatively ineffective with no significant inhibition of tetrasporogenesis at quantum‐doses of up to 20 mmol. m−2.

Visible, room-temperature, surface-emitting laser using an epitaxial Fabry–Perot resonator with AlGaAs/AlAs quarter-wave high reflectors and AlGaAs/GaAs multiple quantum wells

We report operation of a new surface-emitting laser. This epitaxial laser is fabricated with molecular beam epitaxy by the growth of quarter-wave high reflectors of AlAs/Al0.4Ga0.6As (710 Å/630 Å) which surround a 4.5-μm-thick multiple quantum well of GaAs/Al0.4Ga0.6As (100 Å/200 Å). We characterize the structure with cw spectroscopy (absorption, reflection, and luminescence) and investigate stimulated emission spectra under pulsed photopumping. When photopumped, the structure lases in its as-grown condition without need of substrate removal, cleaving, or heatsinking. The lasing wavelength is as short as 7400 Å and can be tuned to as long as 8400 Å by positioning the pump spot to different regions across the wafer. The pulsed threshold irradiance has a very weak temperature dependence varying from 6×105 W/cm2 at 4.2 K to 1.6×106 W/cm2 at 295 K.

THE MECHANISM OF DAYLENGTH PERCEPTION IN THE RED ALGA ACROSYMPHYTON PURPURIFERUM1

ABSTRACTThe red alga Acrosymphyton purpuriferum (J. Ag.) Sjöst. (Dumontiaceae) is a short day plant in the formation of its tetrasporangia. Tetrasporogenesis was not inhibited by 1 h night‐breaks when given at any time during the long (16 h) dark period (tested at 2 h intervals). However, tetrasporogenesis was inhibited when short (8 h) main photoperiods were extended beyond the critical daylength with supplementary light periods (8 h) at an irradiance below photosynthetic compensation. The threshold irradiance for inhibition of tetrasporogenesis was far lower when supplementary light periods preceded the main photoperiod than when they followed it (<0.05 μmol·m−2·s−1 vs. 3 μmol·m−2·s−1). The threshold level also depended on the irradiance given during the main photoperiod and was higher after a main photoperiod in bright light than after one in dim light (threshold at 3 μmol·m−2·s−1 after a main photoperiod at ca. 65 μmol·m−2·s−1 vs. threshold at <0.5 μmol·m−2·s−1 after a main photoperiod at ca. 35 μmol·m−2·s−1). The spectral dependence of the response was investigated in day‐extensions (supplementary light period (8 h) after main photoperiod (8 h) at 48 μmol·m−2·s−1) with narrow band coloured light. Blue light (λ= 420 nm) was most effective, with 50% inhibition at a quantum‐dose of 2.3 mmol·m−2. However, yellow (λ= 563 nm) and red light (λ= 600 nm; λ= 670 nm) also caused some inhibition, with ca. 30% of the effectiveness of blue light. Only far‐red light (λ= 710 nm; λ= 730 nm) was relatively ineffective with no significant inhibition of tetrasporogenesis at quantum‐doses of up to 20 mmol·m−2.

Energy deposition in ultraviolet laser desorption mass spectrometry of biomolecules

The important role of the laser wavelength vis à vis wavelength-dependent sample and/or substrate absorption is demonstrated for high irradiance, uv laser desorption mass spectrometry of organic molecules. Tryptophane (trp) and alanine (ala) samples, of ∼ 10 Å to 1 μm in thickness, were prepared by a molecular beam technique on silver and quartz substrates. Mass spectra were taken with wavelengths of 265 and 355 nm, respectively. At 265 nm the absorbing trp samples have a low threshold irradiance (2×10 7 W cm −2) independent of absorption properties of the substrate. Lack of absorption for trp at 355 nm and ala at 265 nm and 355 nm results in considerably higher threshold irradiances. Lower thresholds for absorbing metal substrates evidence substrate participation in energy transfer in these cases, but qualitatively comparable spectra obtained with non-absorbing quartz substrates demonstrate the dominating role of non-linear absorption. Dominance of non-linear absorption for energy transfer even to samples with strong linear absorption at the wavelength used is concluded from the fact that at 265 nm trp layers as thin as 100 Å on silver completely shield the substrate, despite their nominal 95% transmission. The results indicate the potential of laser desorption for surface analysis of just a few monolayers.

Visual Prey Detection and Foraging in Sympatric Cutthroat Trout (Salmo clarki clarki) and Dolly Varden (Salvelinus malma)

In sympatric lacustrine cutthroat trout (Salmo clarki clarki) and Dolly Varden (Salvelinus malma), reaction distance to artificial and natural prey increased with increases in irradiance from a visual irradiance threshold (VIT, the maximum quantity of irradiance resulting in zero reaction distance to prey) to a saturation irradiance threshold (SIT, the minimum quantity of irradiance that maximizes reaction distance to prey). The VIT and SIT for Dolly Varden were approximately one and two orders of magnitude lower, respectively, than those for trout. At all irradiance levels >4.2 × 1017 photons m−2∙s−1 the reaction distance of trout exceeded that of Dolly Varden. Reaction distance to prey in both species was greatest for red irradiance followed in descending order by green, yellow and blue irradiance. The midsummer ambient irradiance regime in a lake cohabited by trout (concentrated in surface waters) and Dolly Varden (inhabiting deeper waters) showed that during daylight hours a much larger portion of the water column had irradiance levels [Formula: see text] the SIT of Dolly Varden than the SIT of trout and that irradiance levels were always [Formula: see text] the VST of Dolly Varden although only in the upper 5 m at night. Estimates of reaction distance to Diaptomus kenai and foraging velocity at several irradiance levels showed that trout have the potential to visually search a volume of water seven times greater than that searched by Dolly Varden for Diaptomus kenai on a summer day.

Environmental factors and aquatic macrophyte response in regulated lakes - a statistical approach

In regulated lakes, it is necessary to distinguish between depth and relative elevation. Depth is the height of water overlaying the plants at any time, thus the instantaneous value may be nil. The optimal “depth” and elevation datum is shown to be the median water level. However, depth in its time-averaged sense is not equivalent to relative elevation when the water level fluctuates throughout time. The magnitude of the discrepancy is generally large in shallow waters. Environmental stress factors, e.g. ice-scour, influence aquatics over a much larger part of the “depth” gradient with a variable water level. Factors operating through threshold statistics, e.g. subsurface irradiance, are most adversely influenced by a time-varying water level. In general, regulation impact results in a compressed vertical niche. Simultaneously, the potential niche is shifted into deeper waters. Vegetation data from ninne oligotrophic, regulated Norwegian lakes show significant correlation to the reduced vertical niche defined by ice-scour and threshold irradiance. Water-level schedules determine the actual vegetational response; thus the total probability distribution of water levels should replace the regulation height or mean annual range of water level variation in an analysis of response features.

Metabolic energy for stomatal opening. Roles of photophosphorylation and oxidative phosphorylation

The supply of energy for stomatal opening was investigated with epidermal peels of Commelina communis L. and Vicia faba L., under white, blue and red irradiation or in darkness. Fluencerate response curves of stomatal opening under blue and red light were consistent with the operation of two photosystems, one dependent on photosynthetic active radiation (PAR) and the other on blue light, in the guard cells. The PAR-dependent system was 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-sensitive and KCN-resistant and showed a relatively high threshold irradiance for its activation; its activity was most prominent at moderate to high irradiances. The blue-light-dependent photosystem was KCN-sensitive, was active at low irradiances, and interacted with the PAR-dependent photosystem at high blue irradiances. Stomatal opening in darkness, caused by CO2-free air, fusicoccin or high KCl concentrations, was KCN-sensitive and DCMU-resistant. These data indicate that stomatal opening in darkness depends on oxidative phosphorylation for the supply of high-energy equivalents driving proton extrusion. Light-dependent stomatal opening appears to require photophosphorylation from guard-cell chloroplasts and the activation of the blue-light photosystem which could rely either on oxidative phosphorylation or a specific, membrane-bound electron-transport carrier.

Influence of the Static Sample Temperature on the Mass Spectra of Alkali Halides and Metals Obtained with Pulsed Laser Induced Ion Generation

Abstract Experimental investigations of the interdependence between the ion yield and the static sample temperature at a fixed laser irradiance. between the ion yield and the laser irradiance at different static sample temperatures and between the ion detection threshold irradiance and the static sample temperature are reported. Laser wavelengths of 532 nm. 355 nm and 266 nm have been used to obtain the data. The laser pulse duration was 10 ns. Experimental results for alkali halides and metals are reported and compared. A strong dependence of the experimental parameters mentioned above on the static sample temperature has been observed for alkali halides. In the case of metals only minor effects have been found. In any case the different laser irradiation wavelengths do not significantly effect the experimental results.

Regulation of Gametogenesis, Formation of the Zygotes and their Maturation and Germination by Light in Chlamydomonas geitleri E TTL

Summary The present experiments enabled us to know whether various number of the cells in a volume unit or the average irradiance of the cells was the determining factor for the dependence of gametogenesis, formation of zygotes and their maturation and germination on the culture density. Almost the whole effect of the changes in culture density on the above mentioned characteristics was primarily due to the influence of light. The cultures grown at considerably different levels of irradiance differentiated in the above mentioned characteristics as did the cultures grown at considerably different densities. There existed a certain threshold irradiance level that made it possible to induce gametogenesis and clumping reaction could be observed. But, no conjugation occurred and no zygotes were formed. Accordingly, the size of the zygotes, their physiological condition and probably, in consequence of that, also their maturation and germination were different from different culture densities or irradiance levels.

Backscattered radiation at 0/2 from neodymium laser plasma interactions

A line at the subharmonic frequency omega 0/2 has been detected in the radiation backscattered from plasmas produced by irradiation of plane targets with a neodymium laser. The dependence of the threshold irradiance on the density scale length over the range 1012 to 1015 W cm-2 has been determined.

Chlorophyll b accumulation and grana formation in low intensities of red light

Abstract When dark grown leaves of wheat (Triticum aesivum L.) were given a brief irradiation, there was an immediate onset of chlorophyll (Chl) b synthesis in the dark. This synthesis led to a rather slow accumulation of Chl b, which ceased when the Chl b/Chl a ratio had reached a value of about 0.1. The Chl b synthesis occurred also when the seedlings were treated with the herbicide SAN 9789. Leaves grown under different intensities of red light accumulated Chl b and Chl a, resulting in a ratio Chl b/Chl a which depended on the light intensity. If the light intensity was low, Chl a accumulated to a level about ten times the level of PChlide of the dark grown leaves. This occurred without any increase in the Chl b/Chl a ratio. There was no difference between SAN 9789‐treated seedlings and water controls in this respect. Above a certain threshold of irradiance, the Chl b/Chl a ratio in the control leaves increased rapidly with the irradiation intensity. The increase in Chl b/Chl a ratio coincided with formation of grana in the plastids. This increase was not found and grana formation was completely absent in the seedlings treated with SAN 9789. The possibility of two different stages in the Chl b synthesis is discussed.

Predicting the canopy resistance of a mixed hardwood forest

Extensive measurements of stomatal resistance using a porometer in a maple-beech hardwood forest over two growing seasons and at two different canopy levels are described. The analysis permits a generalization that yields a final equation relating overall canopy resistance to solar irradiance and soil moisture content. Threshold irradiance initiating stomatal opening in both the maple and beech leaves was around 0.01–0.02 cal. cm −2 min −1, rising to about 0.15 cal. cm −2 min −1, when the leaves were touched by frost and senescing. Extrapolated values of minimum leaf resistance varied between 1.0 and about 7.0 sec cm −1, and were strongly dependent on soil moisture deficit. Using measured leaf area indices, overall canopy resistance was then derived from the weighted values of leaf resistance of the sunlit and shaded regions of the canopy for the whole summer season.

MECHANISMS OF RAPID PHOTOSYNTHETIC ADAPTATION IN NATURAL PHYTOPLANKTON COMMUNITIES. I. REDISTRIBUTION OF EXCITATION ENERGY BETWEEN PHOTOSYSTEMS I AND II1

ABSTRACTAn inverse linear relationship between chlorophyll fluorescence yield (R) and light intensity was recorded in the near‐surface waters of six lakes (New Zealand, England) of greatly different trophic status and phytoplankton species composition. This surface depression of R values could be removed by incubation of samples in dim light or darkness and was not observed in situ below a threshold irradiance (146 μEin ·m−2·s−1 for Lake Taupo, New Zealand). The time course of chlorophyll fluorescence depression and recovery in response to light treatment was measured in samples from Lake Windermere (England). Fluorescence exponentially decreased upon exposure to bright light and the response was 100% (5 m samples) or 83% (dim light‐adapted 0 m samples) complete within 2 min. An increase in R values in the dim light occurred after a lag of 60 s and the rate of increase decreased exponentially with time. Full recovery took 15 min or more. Deep (6.5 m) populations from Lake Windermere exhibited large, time‐dependent variations in chlorophyll fluorescence over the first 25 s of exposure to 450 nm light, whereas surface populations did not. These data were interpreted in terms of decreased spillover from PSII to PSI with increasing depth, to a minimum at the threshold light intensity below which cells are in light state 1.

Study of nonionizing microwave radiation effects upon the central nervous system and behavior reactions.

The biologic effect of an electromagnetic field of a frequency of 2375 +/- 50 MHz was studied in rats and rabbits in specially constructed absorbant chambers. The results of the investigations have shown that microwave radiation of 10, 50, 500 mu W/cm2 for 30 days, 7 hr/day, causes a number of changes in bioelectric brain activity and also in behavioral immunological, and cytochemical reactions. It was found that levels of 10 and 50 mu W/cm2 stimulate the electric brain activity at the initial stage of irradiation, while a level of 500 mu W/cm2 causes its suppression, as seen from the increase of slow, high amplitude delta-waves. At 500 mu W/cm2 a decrease in capacity of work, in value of unconditioned feeding stimulus, in investigating activity, electronic irradiation threshold, and in inhibition of cellular and humoral immunity were also observed.

Threshold conditions for the formation of surface plasmas by HF and DF laser radiation

Energy thresholds for the formation of HF and DF laser-supported absorption waves at target surfaces have been measured for several target materials. Chemical laser pulses with a FWHM of 4 μsec and energies up to 150 J were focused to an effective target spot diameter of 0.94 cm. Thresholds were determined by observing the cutoff of incident laser radiation transmitted through a pinhole in the target. The observed variation of the plasma ignition time with incident pulse energy Ei showed that neither the threshold fluence es nor the threshold irradiance qm is invariant under variation of Ei. The measured dependence of es and qm on Ei is compared to calculations based on two simple threshold models. The minimum values of fluence es (in J/cm2) for which surface plasmas are formed are as follows: in the format ’’material [es (HF), es (DF)]’’: aluminum (65, 71); titanium (51, 43); stainless steel (65, 51); nickel (94, 43); copper (110, 46); Grafoil (94, 43); Lucite (100, 100).

Measurement of CO2-laser-induced shock pressures above and below LSD-wave thresholds

TEA-CO2-laser-induced shock pressures in aluminum, cellulose acetate, and polymethyl methacrylate (PMMA) targets were measured by both backsurface and in-material techniques as functions of peak laser power density and ambient air pressure. We report the first experimental observation of shock-pressure profiles in metallic and nonmetallic targets above and below the thresholds for laser-supported-detonation-wave (LSD-wave) initiation. For aluminum irradiations above threshold conditions, the measured peak pressure is consistently below that predicted by cylindrical blast-wave theory, which can be explained in terms of LSD initiation at local surface defect sites and imperfections. Surface-vapor blow-off dominates the shock-pressure response below threshold conditions for both aluminum and the plastics. LSD-wave blockage of the surface vaporization process leads to a decrease in the peak pressure above the initiation threshold for cellulose acetate. The largest peak pressures (accompanied by severe backsurface spallation) are thus recorded just below threshold irradiance conditions for LSD-wave initiation in plastics. LSD initiation times inferred from these shock-pressure measurements are in reasonable agreement with values determined by other methods and support the contention that initiation on practical metallic and nonmetallic surfaces is a highly local phenomenon.

Laser damage in triglycine sulfate: Experimental results and thermal analysis

An investigation of the effects of 10.6-μm laser radiation on triglycine-sulfate (TGS) crystals determined that the primary irreversible damage mechanisms in TGS pyroelectric detectors are cracking and thermal decomposition (charring). Irradiation thresholds for cracking and charring were determined for TGS crystals of typical detector dimensions as a function of laser power density and irradiation time. These energy density thresholds exhibit two distinct regions of behavior: For short irradiation times E0 is independent of τ while for long times E0 varies as τ1/2. The thresholds are given empirically by E0 (crack) = [0.65 + 22(τ)1/2] J/cm2 and E0(char) = [4.0 + 34 (τ)1/2] J/cm2. A theoretical model describing thermally induced damage in irradiated crystals is presented. The energy density damage thresholds obtained from this model are compared to the experimental results. These results on material damage are used to predict damage thresholds in operating TGS detectors. These predictions are in good agreement with experimentally determined detector thresholds.