V. M. Markushev

DEGREE OF' COHERENCE AND DIMENSIONS OF THE GENERATION REGION OF POWDER LASERS

A degree of coherence of powder-laser radiation of 10–20% is estimated by the magnitude of the image contrast for a speckle, although cases are recorded where it attains 40–50%. It is found that as the average speckle intensity increases, the contrast decreases. It is established that the minimum, dimensions of the generation region in powders are 20–30 μm. Cases of existence of several generation centers simultaneously are noted. An attempt to interpret the results obtained is made.

ZnO random laser spectra under nanosecond pumping

Spectra of ZnO random lasers were obtained using a Peltier-cooled CCD camera Videoscan-285 at a single shot of nanosecond pumping. It was demonstrated that these spectra differ essentially from lasing spectra under picosecond pumping: as a rule, the line widths are significantly larger and the spectra often change essentially from shot to shot in a random manner on the same pumping spot. We suggest that large line widths can be the result of many lasing acts appearing during a single pumping pulse and of the lasing frequency changing in every lasing act. The random variations of spectra from shot to shot can be called forth by spontaneous emission fluctuations.

Use of radiation intensity dependence on excitation level for the analysis of surface plasmon resonance effect on ZnO luminescence

Abstract. For the analysis of ZnO luminescence, a set of rate equations (SRE) is proposed. It contains a set of parameters that characterize processes participating in luminescence: zone–zone excitation, excitons formation and recombination, formation and disappearance of photons, surface plasmons (SP), and phonons. It is shown that experimental ZnO microstructure radiation intensity dependence on photoexcitation levels can be approximated by using SRE. This approach was applied for the analysis of ZnO microfilm radiation with different thicknesses of Ag island film covering. It was revealed that the increase of cover thickness leads to an increase of losses and a decrease of the probability of photon-to-SP conversion. In order to take into account visible emission, rate equations for level populations in the bandgap and for corresponding photons and SPs were added to the SRE. By using such an SRE, it is demonstrated that the form of visible luminescence intensity dependence on excitation level (P) like P1/3, as obtained elsewhere, is possible only if donor–acceptor pairs exist. The proposed approach was also applied for consideration of experimental results obtained in several papers taking into account the interpretation of these results based on assumptions about the transfer of electrons from the defect level in the ZnO bandgap to metal and then to the conduction band.

Neodymium luminescence in chalcogenides of europium-gallium: EuGaS4 and EuGa2Se4

Abstract Photoluminescence of chalcogenides of europium-gallium, EuGa2S4 and EuGa2Se4, doped with neodymium is investigated. The positions of Stark levels are determined from the spectra. The symmetry of luminescence centres is shown to be lower than cubic and the existence of nonequivalent centers is established. At 77 K the decay time of luminescence from the excited levels of Nd3+ depends on the spin of the states. That indicates a slow relaxation rate in the crystals under investigation. It is probable that these crystals can be used as effective luminophores.

UV radiation of powdered ZnO pumped by nanosecond pulses

Since the intrinsic lifetime of spontaneous recombination UV radiation in zinc oxide amounts less than ~200 ps it is of interest to obtain stimulated UV radiation of powdered ZnO with pumping pulses of nanosecond duration. This will clarify the possibility of quasi-continuous laser radiation in disordered media. At the same time this effect can open the way for working out a cathodoluminescent screen with narrow spectrum and short persistence time. Investigations of UV radiation spectra of powdered zinc oxide and some disordered films were conducted. The samples were pumped by 3-rd harmonics of the two-stage Nd:YAG-laser (λ=355 nm) with pulse duration ~10 ns. Maximum density of energy of pumping pulses was about 160 mJ/cm2. Spectra of spontaneous emission were registered at 300K and 77K. With some of our powdered samples we achieved lasing at 300K. The threshold values of pumping energy density occurred to be higher than that under picosecond pumping approximately for two orders of magnitude. Peculiarities of different samples lasing are demonstrated and discussed. In spectra of the ZnO films investigated at 300K UV band maximum is situated at ~382 nm, while in powders of ZnO this maximum was located at ~389 nm. Besides, in the films the long-wavelength part of the UV band broadens with increase of pumping power.

Electronic-vibrational spectra of nonequivalent europium luminescence centers in lanthanide oxides

The authors studied the EV laser-excitation spectra of europium (Eu/sup 3 +/) in yttrium and lanthanide seisquioxides, belonging to the A and C modifications of the structure. The EV spectra belonging to two different centers in the oxides of the C modification were separated. The electronic spectra and the symmetry of one of the centers in Y/sub 2/O/sub 3/ were refined. It is demonstrated that there is a relationship between the EV spectra and the structure of luminescence centers. Qualitative estimates of the relative contributions of the motions of the oxygen and metal atoms to the vibrations of the crystal lattice are reported. The EV spectra of Eu/sup 3 +/ in oxides of the cubic modification (C/sub 2/ and C/sub i/ centers) and EV spectra of Eu/sup 3 +/ in La/sub 2/O/sub 3/ (C/sub 3v/ center) were studied and obtained for the first time. The symmetry of one of the Eu/sup 3 +/ luminescence centers in Y/sub 2/O/sub 3/ was refined.

ZnO exciton recombination radiation in a weak magnetic field

The influence of a weak magnetic field (H ~ 1 T) on the exciton radiation of the ZnO films covered with Ag island film was investigated at room temperature. It was found that there are some areas on these films where the several-percent enhancement of the exciton radiation intensity can be observed. The value of the enhancement slightly depends on the excitation level. At the same time there are other areas on the same films where the effect is negligible or absent. It is supposed that in the first case, magnetoexcitons are possibly formed due to the increase of the exciton momentum nearby an Ag nanoparticle. The probability of magnetoexciton recombination significantly increases even in a weak magnetic field due to the strong stretch of its wave function.

Laser Spectra of the Complex Structures of Zinc Oxide

The laser spectra of the ZnO powder and complex structures and the time evolution of the spectra are studied at room temperature using low- and high-level pumping with nanosecond laser pulses. The ZnO complex structures consist of a dense core and a loose sheath formed of tetrapod crystallites. The nature of the laser modes that contribute to the spectra is discussed. A simplified model of such modes is proposed and analyzed. It is demonstrated that, in the powder, the modes are densely concentrated in the UV range of the zinc-oxide emission. In the complex structure, several laser modes are spectrally separated. We conclude that the modes are related to the tetrapods.

Spectra of ZnO random lasers under nanosecond pumping

Investigations of ZnO random lasers spectra due to single shot of nanosecond pumping were performed using CCD camera Videoscan-285. It was obtained that these spectra essentially differ from lasing spectra under picosecond pumping: as a rule the line widths are significantly larger, often spectra have quite smooth shape; often spectra essentially change from shot to shot in random manner on the same pumping spot. From our point of view it can be the result of many lasing acts appearance during single pumping pulse and of lasing lines frequency changing in every lasing act.

Interpretation of ZnO luminescence peculiarities by use of rate equation system

For the interpretation of ZnO luminescence and influence of surface plasmon resonance (SPR) on it a simplified approach is proposed. This approach is based on the set of rate equations (SRE), which describes processes taking part in luminescence. A number of experiments can be explained using this SRE, in particular, the proposed approach was applied for modeling experimental results for ZnO microstructure radiation intensity dependence on photoexcitation level and for consideration of insulating spacer role in ZnO/Ag system. In order to take into account visible emission, rate equations for level populations in band-gap and for corresponding photons and surface plasmons (SP) were added to SRE. By use of such SRE it is demonstrated that the form of visible luminescence intensity dependence on excitation level (P) like P1/3, as obtained elsewhere, is possible only if donor-acceptor pairs (DAP) exist. The proposed approach was also applied for consideration of experimental results on relation between UV and visible ZnO luminescence obtained in several works. This approach can be applied not only to ZnO-based structures but also to other semiconductors as well.