M. V. Ryzhkov

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.

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.

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.

Analysis of ZnO random laser spectra under nanosecond pumping

Early observed peculiarities of ZnO random laser spectra are analyzed. By simplified simulation of lasing time behavior it is shown that plausible origin of lasing spectra variations from shot to shot can be the fluctuations of the number of spontaneous photons that can serve as a seed for laser effect creation in a certain mode. Such fluctuations are very significant for spatially overlapping modes. Analysis of a statistics of different line appearance in the lasing spectra showed that the number of lasing modes in monodisperse sample is less than in polydisperse ones. This finding can explain why lasing spectra variations are more pronounced in polydisperse samples. We associate observed large line widths with the existence of several lasing acts during every pumping shot and with the slight changes of some modes frequencies in the course of each lasing act. Preliminary experimental result confirms such supposition.

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.

Study of the physical origin of a resonant feedback in random lasers

By expanding the spatial distribution of a dielectric constant in random media into Fourier series, we analyze the assumption that the resonant feedback is provided by the presence of components with spatial periods satisfying the condition of the Braff diffraction. Numerical simulations of random media allow us to predict specific features of eigenfrequencies of lasing modes, which are similar to those obtained in the literature from the numerical solution of the Maxwell equations. Thus, a plausible explanation of the physical nature of frequency selection in random lasers is revealed. Furthermore, using a simplified description of the Bragg feedback, we show why the threshold gain decreases with the increase in the sizes of the active region.

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.

Interpretation of the effect of dielectric spacer on the ZnO/Ag structure luminescence intensity

For the analysis of ZnO luminescence and the influence of surface plasmon resonance (SPR) on it the simplified approach is proposed. This approach is based on the set of rate equations (SRE), which describes processes taking part in the luminescence. The SRE includes the set of parameters that describe processes determining luminescence of an investigated sample. The proposed approach gives an opportunity for modeling the dependence of radiation intensity on pumping level and to estimate the values of parameters in SRE. As a result it is possible to make conclusions about peculiarities of samples and investigated processes. A number of experimental facts can be explained using this SRE, in particular the proposed approach was applied to consideration of insulating spacer role in ZnO/Ag system. It was shown that it is possible to interpret experimental results using SRE where values of some parameters depend on the spacer thickness. The proposed approach can be applied not only to ZnO-based structures but also to other emitters.

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

For the analysis of ZnO luminescence the system of rate equations (SRE) was proposed. It contains a set of parameters that characterizes processes participating in luminescence: zone-zone excitation, excitons formation and recombination, formation and disappearance of photons and surface plasmons (SP). It is shown that experimental ZnO microstructure radiation intensity dependence on photoexcitation level can be approximated by using SRE. Thus, the values of these parameters can be estimated and used for luminescence analysis. This approach was applied for the analysis of ZnO microfilms radiation with different thickness of Ag island film covering. It was revealed that the increase of cover thickness leads to the increase of losses and decrease of probability of photons to SP conversion. In order to take into account visible emission, rate equations for levels populations in band-gap and for corresponding photons and SP were added to SRE. By using such SRE it is demonstrated that the form of visible luminescence intensity dependence on excitation level (P) like P1/3, as obtained elsewhere [1], is possible only in case of donor-acceptor pairs existence. The proposed approach was applied for consideration of experimental results obtained in [5-8] taking into account their interpretation of these results based on assumption about transfer of electrons from defect level in ZnO band-gap to metal and then to conduction band in ZnO. Results of performed calculations using modified SRE revealed that effects observed in these papers can exist under only low pumping level. This result will be experimentally checked later.