We report on high-power solid-state lighting facility for cultivation of greenhouse vegetables and on the results of the study of control of photosynthetic activity and growth morphology of radish and lettuce imposed by variation of the spectral composition of illumination. Experimental lighting modules (useful area of 0.22 m2) were designed based on 4 types of high-power light-emitting diodes (LEDs) with emission peaked in red at the wavelengths of 660 nm and 640 nm (predominantly absorbed by chlorophyll a and b for photosynthesis, respectively), in blue at 455 nm (phototropic function), and in far-red at 735 nm (important for photomorphology). Morphological characteristics, chlorophyll and phytohormone concentrations in radish and lettuce grown in phytotron chambers under lighting with different spectral composition of the LED-based illuminator and under illumination by high pressure sodium lamps with an equivalent photosynthetic photon flux density were compared. A well-balanced solid-state lighting was found to enhance production of green mass and to ensure healthy morphogenesis of plants compared to those grown using conventional lighting. We observed that the plant morphology and concentrations of morphologically active phytohormones is strongly affected by the spectral composition of light in the red region. Commercial application of the LED-based illumination for large-scale plant cultivation is discussed. This technology is favorable from the point of view of energy consumption, controllable growth, and food safety but is hindered by high cost of the LEDs. Large scale manufacturing of high-power red AlInGaP-based LEDs emitting at 650 nm and a further decrease of the photon price for the LEDs emitting in the vicinity of the absorption peak of chlorophylls have to be achieved to promote horticulture applications.
Combined doping of PbWO4 scintillation crystals with antimony and ytterbium as well as molybdenum and ytterbium has been carried out and the influence of the doping on optical transmittance, radiation hardness, intensity and spectrum of X-ray excited luminescence has been studied. We demonstrate that simultaneous doping with Sb (100 ppm) and Mo (500 ppm) shifts the band peak of X-ray luminescence from the region 430 - 450 nm to 520 nm and increases the intensity and decay time of the luminescence. The highest luminescence intensity is observed in crystals simultaneously doped with all 3 dopants (Mo, Sb, Y). The origin of the modifications is interpreted by formation of microinclusions of tungstenite phase in doped PWO crystals. The results indicate means for further development of scintillation characteristics of PbWO4 single crystals in order to effectively utilize the crystals as scintillators in radiation detectors for application in high-energy physics experiments.
Photoluminescence properties of short-period asymmetric GaAs/AlAs superlattices with the well and barrier thickness varied from 10 to 3 monolayers were studied at high optical excitation. It was shown that an asymmetric structure of the superlattice, in which the well layers are at least twice wider than the barrier ones, allows us to maintain the direct band gap and, hence, to improve emission properties for any well width. This is important for utilization of such structures in light-emitting devices. The stimulated emission at 80 K was observed for a GaAs/AsAs superlattice with the well and barrier thickness of 6 and 3 monolayers, respectively. At the same time, investigations of the dependences of the emission intensity on the pump intensity for different superlattices revealed an enhancement of nonradiative recombination with decreasing the well thickness due to an enhanced influence of interface roughness.
Transient and quasi-steady-state photoluminescence of a dense electron-hole plasma was studied in GaN epilayers under high photoexcitation at room-temperature. High initial carrier heating up to 1100 K was observed. Decay of nonthermalized electron-hole plasma was analyzed both in homo- and heteroepitaxial GaN layers. The heating is shown to significantly influence the luminescence peak position and the rate of spontaneous and stimulated recombination. After the thermalization process is completed, the luminescence decay is exponential and the room-temperature carrier lifetime can be extracted. The lifetime in the heteroepitaxial layer grown on sapphire was found to be 190 ps, while the homoepitaxial layer exhibited an essentially higher value of 890 ps, which is one of the highest reported for free-carrier recombination in GaN. Additionally, optical gain spectra were studied using variable-stripe method. The threshold for stimulated emission was found to be considerably lower and the gain at a certain pump intensity was shown to be much higher in the homoepitaxial layer than in the heteroepitaxial one. Maximum net gain value of 300 cm-1 was observed.
Transient changes in absorption of PbS-doped glasses are investigated by means of absorption pump-probe spectroscopy. Two opposite nonlinear effects, bleaching and induced absorption, are observed and their dynamics are investigated in the vicinity of the lowest optical transition of carriers quantum-confined in PbS nanocrystals of different size. Applicability of this composite material as saturable absorber is studied and the nonlinear absorption of the PbS- doped glass is compared with that of solutions of dyes commonly used as saturable absorbers. A potential of utilization of PbS-doped glass in mode locking is discussed and long-time stability of the medium under intense laser irradiation is demonstrated.
Strain Energy Band Engineering of Group III-N heterostructures should allow us to prevent defect formation at the heterointerfaces ad to reduce the built-in electric field in the quantum wells. The strain, caused by lattice mismatch, may be decreased by incorporation of In into AlGaN. To monitor structural perfection of the quaternary compound AlInGaN and to evaluate electronic potential profile, we employed optical methods: reflectivity, site- selectively excited photoluminescence, photoluminescence excitation and time-resolved luminescence. AlGaN with the molar fraction of Al of 9% and two samples with the lattice mismatch reduced by partial substitution of Al by 1% and 2% of In were investigated. In AlGaN, the luminescence excited resonantly with the exciton position is red shifted. The photoluminescence excitation spectra indicate that the mobility edge is above the optical band gap, and the localization vanishes. These results show that the incorporation of approximately equals 2% indium into AlGaN leads to the disappearance of the band tail states and smoothing of the potential profile.
Instability and chaotic behavior of an optical ring cavity is studied when a x(2) nonlinear medium is considered. The effects of the phase-mismatching A k and of the difference A E between the amplitudes of the input fields at the entry of the medium are considered and discussed.
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