Single Light Pulse Research Articles

Magnetization manipulation in (Ga,Mn)As by subpicosecond optical excitation

We demonstrate complete reversal of a full magnetic hysteresis loop of the magnetic semiconductor (Ga,Mn)As by ultrashort optical excitation with a single subpicosecond light pulse, with obvious implications for ultrafast magneto-optical recording. Our approach utilizes the fourfold magnetic anisotropy of (Ga,Mn)As, in combination with the magnetic linear dichroism of the material.

Imaging of single light-responsive clock cells reveals fluctuating free-running periods

Zebrafish tissues and cell lines contain circadian clocks that respond directly to light. Using fluorescence-activated cell sorting, we have isolated clonal cell lines that contain the reporter construct, zfperiod4-luciferase. Bioluminescent assays show that oscillations within cell populations are dampened in constant darkness. However, single-cell imaging reveals that individual cells continue to oscillate, but with widely distributed phases and marked stochastic fluctuations in free-running period. Because these cells are directly light responsive, we can easily follow phase shifts to single light pulses. Here we show that light acts to reset desynchronous cellular oscillations to a common phase, as well as stabilize the subsequent free-running period.

PRECISION STRESSING BY UV-B RADIATION TO IMPROVE QUALITY OF SPINACH UNDER PROTECTED CULTIVATION

Plants possess a number of defence systems against plant-stress related inflictions such as natural, but at times excessive UV-B radiation. One protection mechanism is the altered synthesis of antioxidative substances like flavonoids, carotenoids, and ascorbic acid. Antioxidants have gained wide recognition for their health promoting actions in human body metabolism. Plants grown under protected cultivation do not receive any UV-B light leading to reduced amounts of secondary metabolites as compared to field crops. We concluded that plants will generate higher amounts of valuable antioxidants when exposed to UV-B light from fixtures installed in a greenhouse. Therefore, seven-week old spinach plants were treated with doses of 0, 1, 2 and 6 kJ m d UV-BBE (biologically effective) radiation for two weeks and compared to PAR-radiated (photosynthetically active radiation) and non-radiated (daylight only) plants. The sum of PAR was maintained at 14.9 mmol m d for all radiated treatments. Biomass was increased by PAR and by increasing UV-B doses. Chlorophyll fluorescence measurements indicated improved photosynthetic capacities due to PAR and moderate UV-B, while it was reduced at 6 kJ m d UVBBE. The content of carotenoids, flavonoids and ascorbic acid was increased by PAR and 1 and 2 kJ m d UV-BBE but was reduced at 6 kJ m d UV-BBE. Antioxidative capacity (AC) of carotenoid-extracts was investigated: Moderate UV-B treatment lead to higher AC compared to PAR treatment. Under conditions of high PAR, low and moderate UV-B radiation improved plant growth and the content of valuable secondary plant substances. INTRODUCTION Quality of vegetables plays an increasing role in human diet and for marketing of horticultural produce. As important determinants of internal quality we consider flavor, texture, minerals, vitamins (e.g. vitamin C) as well as bioactive substances (e.g. carotenoids). Plants are the major source of ascorbate (Vitamin C) in the human diet as humans are unable to synthesize this substance. Ascorbate is the most abundant soluble antioxidant in plant cells among secondary plant metabolites which also possess antioxidative properties. Since many diseases have been related to the occurrence of reactive oxygen species, plant derived antioxidants have gained importance for human nutrition in recent years (Kauer and Kapoor, 2001). Plant metabolism and as a consequence synthesis of vitamins and antioxidants is affected by many environmental parameters. Light conditions under cultivated protection are different from open field due to UV-B and other light/wavelength absorption of the plant coverage. In order to improve the nutritional value of greenhouse vegetables we started an initial experiment on UV-B exposure to spinach since little is known about the effect of dose and duration of UV-B on growth and metabolism in vegetables. We postulate that plants will generate higher amounts of valuable compounds when exposed to UV-B light from lamps installed in a greenhouse. MATERIALS AND METHODS Spinach (Spinacea oleracea L.) ‘Poncho‘ was cultivated in the glasshouse in pots Proc. VII IS on Prot. Cult. Mild Winter Climates Eds. D.J. Cantliffe, P.J. Stoffella & N. Shaw Acta Hort. 659, ISHS 2004 202 (12 cm, 2 plants pot) with peat, ebb-flood irrigated with complete nutrient solution (Flory 9 specifications, EC 2 mS cm) and temperatures set at 19/17°C. On November 7, nine weeks after sowing, the plants were subjected to differing radiation treatments for two weeks (Table 1). Treatments were no additional lighting (standard growing practise, SGP), additional photosynthetically active radiation (PAR), 1 kJ md UV-BBE (field grown condition, FGC), 2 kJ md UV-BBE (UV intensive, INT), 6 kJ md UV-BBE (UV permanent, PER). The treatments contained 150 plants and were not replicated. The UV lamp was a high-pressure metal-halide gas lamp (400 W, MHL400/E40, BLV Licht) emitting UV-B at >294 nm (Figure 1) and the daily doses were obtained by 4, 8 and 24 hours exposure at equal intensity. Since the UV lamp also emits PAR, the resulting different daily PAR sums were supplemented to 14.9 mol md by horticultural PAR lamps (400 W, SON-T AGRO, Philipps) from 7-17 h daily which is equal in quantity to the PAR treatment. The lighted treatments were separated from each other and from SGP by non-transmitting black PE-sheet walls. Plant weight and quality parameters were assessed before and at the end of the treatment period. Fresh weight was cumulated from 10 pots and dry matter content was determined in four replicates by oven drying. Chlorophyll fluorescence (CF) evaluation during growth was done before radiation, after one and two weeks of radiation, using two middle aged leaves/plant of the same 10 plant pots each time. Additionally, chlorophyll fluorescence of 20 detached leaves was measured after one week of storage at 1°C. The fluorescence parameters Fv/Fm and Fo were determinated with PAM-fluorometer (Walz) by emitting a single saturation light pulse on dark adapted tissue of the spinach plants at temperature of 18°C.

Entrainment Properties of the Locomotor Activity Rhythm of Drosophila melanogaster Under Different Photoperiodic Regimens

In this paper we report the results of an experiment to assess how closely repeated brief light pulses (LPs) mimic the effects of 12:12 h light/dark (LD) cycles (PPc). The locomotor activity rhythm of individual fruit flies from a laboratory population of Drosophila melanogaster was monitored under four different photoperiodic regimens, created using 12 h of light and 12 h of darkness or brief light pulses (LPs). The phase relationship (Ψ) and the stability (precision) of the locomotor activity rhythm during entrainment were estimated in order to compare the state of the circadian clocks under the four different photoperiodic regimens. The flies (n = 72) were subjected to four different LD cycles: (i) 12 h of light and 12 h of darkness (complete photoperiod, PPc); (ii) a single brief LP of 15 min duration presented close to the onset of activity (SLP-1); (iii) a single brief LP of 15 min duration administered close to the offset of activity (SLP-2); and (iv) two brief LPs administered 12 h apart (skeleton photoperiod, PPs). The locomotor activity rhythm of the flies was first monitored under constant darkness (DD) for about 10 days and then under the four different photoperiodic regimens for about 10 days, and finally in DD for the remainder of the experiment. The Ψ of the locomotor activity rhythm and its precision under PPc and PPs did not differ significantly, but they were significantly different from the SLP-1 and SLP-2 conditions. The results provide interesting insights into photoentrainment mechanisms of circadian clocks in D. melanogaster, and suggest that skeleton photoperiods, but not single brief LPs, mimic the actions of complete photoperiods.

Modeling the mammalian circadian clock: Sensitivity analysis and multiplicity of oscillatory mechanisms

We extend the study of a computational model recently proposed for the mammalian circadian clock (Proc. Natl Acad. Sci. USA 100 (2003) 7051). The model, based on the intertwined positive and negative regulatory loops involving the Per, Cry, Bmal1, and Clock genes, can give rise to sustained circadian oscillations in conditions of continuous darkness. These limit cycle oscillations correspond to circadian rhythms autonomously generated by suprachiasmatic nuclei and by some peripheral tissues. By using different sets of parameter values producing circadian oscillations, we compare the effect of the various parameters and show that both the occurrence and the period of the oscillations are generally most sensitive to parameters related to synthesis or degradation of Bmal1 mRNA and BMAL1 protein. The mechanism of circadian oscillations relies on the formation of an inactive complex between PER and CRY and the activators CLOCK and BMAL1 that enhance Per and Cry expression. Bifurcation diagrams and computer simulations nevertheless indicate the possible existence of a second source of oscillatory behavior. Thus, sustained oscillations might arise from the sole negative autoregulation of Bmal1 expression. This second oscillatory mechanism may not be functional in physiological conditions, and its period need not necessarily be circadian. When incorporating the light-induced expression of the Per gene, the model accounts for entrainment of the oscillations by light–dark (LD) cycles. Long-term suppression of circadian oscillations by a single light pulse can occur in the model when a stable steady state coexists with a stable limit cycle. The phase of the oscillations upon entrainment in LD critically depends on the parameters that govern the level of CRY protein. Small changes in the parameters governing CRY levels can shift the peak in Per mRNA from the L to the D phase, or can prevent entrainment. The results are discussed in relation to physiological disorders of the sleep–wake cycle linked to perturbations of the human circadian clock, such as the familial advanced sleep phase syndrome or the non-24h sleep–wake syndrome.

Counting photoelectrons in the response of a photomultiplier tube to single picosecond light pulses

We demonstrate the capability of a commercial photomultiplier to produce distinguishable anodic-pulse charge values when 1, 2, or more photoelectrons leave the cathode within a time shorter than the pulse-response duration. We propose a method for precise reconstruction of the photoelectron statistics from the measured pulse-height spectra and discuss applications to the characterization of quantum states in the continuous-variable regime.

Green light stimulates early stem elongation, antagonizing light-mediated growth inhibition.

During the transition from darkness to light, the rate of hypocotyl elongation is determined from the integration of light signals sensed through the phototropin, cryptochrome, and phytochrome signaling pathways. In all light conditions studied, from UV to far-red, early hypocotyl growth is rapidly and robustly suppressed within minutes of illumination in a manner dependent upon light quality and quantity. In this study, it is shown that green light (GL) irradiation leads to a rapid increase in the growth rate of etiolated Arabidopsis seedlings. GL-mediated growth promotion was detected in response to constant irradiation or a short, single pulse of light with a similar time course. The response has a threshold between 10(-1) and 10(0) micromol m(-2), is saturated before 10(2) micromol m(-2) and obeys reciprocity. Genetic analyses indicate that the cryptochrome or phototropin photoreceptors do not participate in the response. The major phytochrome receptors influence the normal amplitude and timing of the GL response, yet the GL response is normal in seedlings grown for hours under constant dim-red light. Therefore, phytochrome activation enhances, but is not required for, the GL response. Seedlings grown under green, red, and blue light together are longer than those grown under red and blue alone. These data indicate that a novel GL-activated light sensor promotes early stem elongation that antagonizes growth inhibition.

A mathematical model of the Pyrosequencing reaction system

The Pyrosequencing™ technology is a newly developed DNA sequencing method that monitors DNA nucleotide incorporation in real-time. A set of coupled enzymatic reactions, together with bioluminescence, detects incorporated nucleotides in the form of light pulses, yielding a characteristic light profile. In this study, a biochemical model of the Pyrosequencing reaction system is suggested and implemented. The model is constructed utilizing an assumption of irreversible Michaelis–Menten rate equations and a constant incorporation efficiency. The kinetic parameters are studied and values are chosen to obtain as reliable simulation results as possible. The results presented here show strong resemblance with real experiments. The model is able to capture the dynamics of a single light pulse with great accuracy, as well as the overall characteristics of a whole pyrogram™. The plus- and minus-shift effects observed in experiments are successfully reconstructed by two constant efficiency factors. Furthermore, pulse broadening can partly be explained by apyrase inhibition and successive dilution.

Light-regulated asexual reproduction in Paecilomyces fumosoroseus.

The entomopathogenic fungus Paecilomyces fumosoroseus has been successfully used in the control of several insect pests. Asexually produced spores (conidia) are the means for dispersal and transmission of the entomopathogen; upon contact with the insect cuticle they germinate and penetrate the host. In model fungal systems it has been found that phototropism, resetting of the circadian rhythm, the induction of carotenogenesis and the development of reproductive structures are controlled by blue light. The effect of light quality on conidial yield of P. fumosoroseus was investigated. Incubation in total darkness resulted in continued vegetative growth and lack of reproductive structures. In contrast, growth of the fungus in continuous illumination or under a night-day regime resulted in prolific formation of conidiophores bearing abundant mature conidia. Conidiation was photoinduced in competent mycelia by a single pulse of blue light and colonies were competent only after they had grown at least 72 h under total darkness. The fluence-response curves generated with blue light indicated that the minimal fluence required for the photomorphogenetic response was 180 micro mol m(-2) and the half-maximal response was at 400 micro mol m(-2). A fluence of 540 micro mol m(-2) was enough to saturate the system, inducing the maximum production of 2.12x10(8) conidia per colony. Higher light intensities markedly decreased conidiation, suggesting the occurrence of a process of adaptation. The authors propose the existence of a dual light-perception system with at least two photoreceptors in P. fumosoroseus, one promoting and one inhibiting conidiation.

Optimum Light Intensity Distribution for Growing Large Si Grains by Phase-Modulated Excimer-Laser Annealing

Characteristics have been investigated for both KrF excimer-laser light and KrF excimer-laser crystallization of Si thin films. The results were applied to design an optical system for growing densely packed and large grains. A high-resolution beam profiler confirmed that the laser light intensity distribution on the sample surface had a nearly ideal triangular form with a maximum-to-minimum intensity ratio of approximately 2, as designed. This distribution could grow 5-µm-long grains with a packing efficiency close to 100% by a single laser light pulse.

Light-Induced Phase-Shifting of the Peripheral Circadian Oscillator in the Hearts of Food-Deprived Mice

In the present study, we investigated the effect of fasting on photoentrainment of the peripheral circadian oscillator in the mammalian heart. Northern blotting showed that a single light pulse applied at an appropriate time in constant darkness, caused obvious phase-shifting in the circadian expression rhythm of the mammalian clock gene Period2 (mPer2) even in the hearts of food-deprived mice. Fasting did not significantly affect either the phase or the light-induced phase-shifts of the mPer2 rhythm. Although several studies of temporal feeding restriction have indicated that feeding is the dominant timing cue for mammalian peripheral oscillators, our findings suggest that feeding is not essential for mammals to induce phase resetting of the circadian oscillator in the heart.

Effect of Single Low Intensity Light Pulse Exposure and Melatonin Treatment on the Circadian Variation of Melatonin in Indian Palm Squirrel Funambulus pennanti

The endogenous circadian rhythm of melatonin in mammals provides information regarding the resetting response of the mammalian circadian timing system in response to the changes in light dark cycle. Photoperiodic changes are reported to have acute and chronic effect on melatonin rhythm. Our aim in present experiment was to study the effect of single light pulse of low intensity on the circadian variation of melatonin in Indian palm squirrel. A short pulse of 5min was given to the animals at 22:55 h on day 16th in natural photoperiodic condition of long day length (LD ~ 13.55:10.05) and melatonin levels were estimated at every 4-h interval on ZT scale on day 17th (DD). Observations suggest that the light pulse given on day 16th suppressed the melatonin level on day 17th (DD). Besides this, it was also found that there was phase delay in the peak value of melatonin. Further, we tested the ability of single melatonin injection on the light pulse induced phase shift of acrophase of melatonin in this species F. pennanti. We injected the single physiological dose of melatonin (25 microgram/100 g body wt.) just 5 min prior to the commencement of light pulse (22:50 h) on day 16 and melatonin levels were estimated on day 17th as above. Injection of melatonin prior to light pulse altered the suppressing and phase shifting effect of light in terms of peak concentration of melatonin in squirrels. Above data may lead us to conclude that the biological clock mechanism controlling circadian rhythm of melatonin in this rodent is in response to the phase shifting effect of light and acute melatonin treatment. Further, we may suggest that single melatonin injection has the capability to entrain melatonin rhythm but a dose dependent study is required to facilitate the suggestion.

Single photon detector on the dark side

Researchers from the National Institute of Standards and Technology (NIST) and Boston University have a detector that counts single pulses of light, while simultaneously reducing false or “dark counts” to virtually zero. The photon detector is part of a multi-disciplinary NIST effort to develop the sophisticated measurement methods needed to make quantum communication and cryptography possible. This is a short news story only. Visit www.three-fives.com for the latest advanced semiconductor industry news.

Ion antagonism between calcium and magnesium in phytochrome-mediated degradation of storage starch in Spirodela polyrhiza

Light and calcium regulation were studied in two processes running parallel in turions of Spirodela polyrhiza (L.) Schleiden: in germination and starch degradation. When germination of turions is induced by a single red light (R) pulse the response depends on exogenously applied Ca 2+. This Ca 2+ dependence is absent in continuous and intermittent R (hourly applied short R pulses that can substitute for continuous R). Phytochrome-mediated degradation of storage starch in the turions starts after the onset of germination. The degradation proceeds only in continuous or intermittent R. Contrary to germination, a single R pulse is ineffective in inducing starch degradation. The process, categorized as a special low-fluence phytochrome response, requires Ca 2+. Moreover, a Ca 2+/Mg 2+ ion antagonism was detected: significant starch degradation proceeds in the absence of both Ca 2+ and Mg 2+. Inhibitors of Ca 2+-uptake (La 3+, verapamil) and of calmodulin (chlorpromazine, trifluoperazine) diminish the light-induced starch degradation. This suggests that the antagonistic activity of Ca 2+ and Mg 2+ in this process is associated with the signal transduction chain, similar to that in the R pulse-induced turion germination. Because the Ca 2+ requirement is different in germination and starch degradation, the Ca 2+-sensitive step cannot be the same in the two responses. This Ca 2+/Mg 2+ antagonism can be used to clarify the relationship between biochemical reactions of the composed response of starch degradation and the mode of their regulation.

Optical Manipulation and Fusion of Liposomes as Microreactors

We present an all-optical method to manipulate and fuse giant liposomes, typically 10 μm in diameter. Optical tweezers are used to trap two individual liposomes, which are then brought into contact. A single pulse of ultraviolet laser light focused on the adjoining membranes induces the fusion between the liposomes. As a consequence, the chemical reagents encapsulated in the two liposomes mix and a chemical reaction takes place.

Demonstration of a low-noise near-infrared photon counter with multiphoton discrimination

We have demonstrated a system capable of directly measuring the photon-number state of a single pulse of light using a superconducting transition-edge sensor microcalorimeter. We verify the photon-number distribution of a weak pulsed-laser source at 1550 nm. Such single-photon metrology at telecommunication wavelengths provides the foundation for ensuring the security of photon sources used in implementations of quantum cryptography. Additionally, this system has the lowest noise equivalent power of any single-photon detector and combines high efficiency near-infrared photon counting with the ability to resolve multiphoton absorption events.

Brassinosteroid Mutants Uncover Fine Tuning of Phytochrome Signaling

Phytochromes (phy) A and B provide higher plants the ability to perceive divergent light signals. phyB mediates red/far-red light reversible, low fluence responses (LFR). phyA mediates both very-low-fluence responses (VLFR), which saturate with single or infrequent light pulses of very low fluence, and high irradiance responses (HIR), which require sustained activation with far-red light. We investigated whether VLFR, LFR, and HIR are genetically coregulated. The Arabidopsis enhanced very-low-fluence response1 mutant, obtained in a novel screening under hourly far-red light pulses, showed enhanced VLFR of hypocotyl growth inhibition, cotyledon unfolding, blocking of greening, and anthocyanin synthesis. However, eve1 showed reduced LFR and HIR. eve1 was found allelic to the brassinosteroid biosynthesis mutant dim/dwarf1. The analysis of both the brassinosteroid mutant det2 in the Columbia background (where VLFR are repressed) and the phyA eve1 double mutant indicates that the negative effect of brassinosteroid mutations on LFR requires phyA signaling in the VLFR mode but not the expression of the VLFR. Under sunlight, hypocotyl growth of eve1 showed little difference with the wild type but failed to respond to canopy shadelight. We propose that the opposite regulation of VLFR versus LFR and HIR could be part of a context-dependent mechanism of adjustment of sensitivity to light signals.

Brassinosteroid Mutants Uncover Fine Tuning of Phytochrome Signaling

Phytochromes (phy) A and B provide higher plants the ability to perceive divergent light signals. phyB mediates red/far-red light reversible, low fluence responses (LFR). phyA mediates both very-low-fluence responses (VLFR), which saturate with single or infrequent light pulses of very low fluence, and high irradiance responses (HIR), which require sustained activation with far-red light. We investigated whether VLFR, LFR, and HIR are genetically coregulated. The Arabidopsis enhanced very-low-fluence response1mutant, obtained in a novel screening under hourly far-red light pulses, showed enhanced VLFR of hypocotyl growth inhibition, cotyledon unfolding, blocking of greening, and anthocyanin synthesis. However,eve1 showed reduced LFR and HIR. eve1 was found allelic to the brassinosteroid biosynthesis mutantdim/dwarf1. The analysis of both the brassinosteroid mutant det2 in the Columbia background (where VLFR are repressed) and the phyA eve1 double mutant indicates that the negative effect of brassinosteroid mutations on LFR requires phyA signaling in the VLFR mode but not the expression of the VLFR. Under sunlight, hypocotyl growth of eve1 showed little difference with the wild type but failed to respond to canopy shadelight. We propose that the opposite regulation of VLFR versus LFR and HIR could be part of a context-dependent mechanism of adjustment of sensitivity to light signals.

Broadband ringdown spectral photography

A new technique that enables frequency-resolved cavity ringdown absorption spectra to be obtained over a large optical bandwidth by a single laser shot is described. The technique, ringdown spectral photography (RSP), simultaneously employs two key principles to record the time and frequency response of an optical cavity along orthogonal axes of a CCD array detector. Previously, the principles employed in RSP were demonstrated with narrow-band laser light that was scanned in frequency [Chem. Phys. Lett. 292, 143 (1998)]. Here, the RSP method is demonstrated using single pulses of broadband visible laser light. The ability to obtain broad as well as rotationally resolved spectra over a large bandwidth with high sensitivity is demonstrated.

Light-Induced resetting of the circadian pacemaker: quantitative analysis of transient versus steady-state phase shifts.

The suprachiasmatic nuclei of the hypothalamus contain the major circadian pacemaker in mammals, driving circadian rhythms in behavioral and physiological functions. This circadian pacemaker's responsiveness to light allows synchronization to the light-dark cycle. Phase shifting by light often involves several transient cycles in which the behavioral activity rhythm gradually shifts to its steady-state position. In this article, the authors investigate in Syrian hamsters whether a phase-advancing light pulse results in immediate shifts of the PRC at the next circadian cycle. In a first series of experiments, the authors aimed a light pulse at CT 19 to induce a phase advance. It appeared that the steady-state phase advances were highly correlated with activity onset in the first and second transient cycle. This enabled them to make a reliable estimate of the steady-state phase shift induced by a phase-advancing light pulse on the basis of activity onset in the first transient cycle. In the next series of experiments, they presented a light pulse at CT 19, which was followed by a second light pulse aimed at the delay zone of the PRC on the next circadian cycle. The immediate and steady-state phase delays induced by the second light pulse were compared with data from a third experiment in which animals received a phase-delaying light pulse only. The authors observed that the waveform of the phase-delay part of the PRC (CT 12-16) obtained in Experiment 2 was virtually identical to the phase-delay part of the PRC for a single light pulse (obtained in Experiment 3). This finding allowed for a quantitative assessment of the data. The analysis indicates that the delay part of the PRC-between CT 12 and CT 16-is rapidly reset following a light pulse at CT 19. These findings complement earlier findings in the hamster showing that after a light pulse at CT 19, the phase-advancing part of the PRC is immediately shifted. Together, the data indicate that the basis for phase advancing involves rapid resetting of both advance and delay components of the PRC.