UV induced thermoluminescence of benzophenone

The UV induced thermoluminescence (TL) of benzophenone has been invetigated at 77 K. Intensity dependence studies of TL induced by ultraviolet light suggest that TL is due to a biphotonic ionization of the molecule by UV. The TL excitation and emission spectra are observed to coincide with the phosphorescene excitation and emission of the molecule. It is concluded that the TL of this molecule is a result of the ion-electron recombination leading to excitation of the triplet state.     

Zur Temperaturabhängigkeit der Elektrolumineszenz von Zinksulfid-Phosphoren

The temperature dependence of brightness waves and of average emission intensity of ZnS powder phosphors excited by square pulses was studied. In certain temperature ranges brightness waves with two peaks in each half-period of the exciting field are observed. Microscopical investigations of individual luminescent areas of a ZnS-grain showed the second peak in the brightness wave to be present in that half-period which in the model of delayed recombination is called the excitation half-period. It is shown that this emission takes place after the excitation process (duration <2μs) is finished and the resulting field in the inner parts of the crystallites has almost faded. Because of this fact and on the basis of glow and built-up investigations in the excitation half-period a spontaneous recombination of electrons thermally released from traps is proposed. Therefore the brightness waves with two peaks, but also emission in the excitation half-period is observed for temperatures at which — in the time average — many traps are still filled but on the other hand the probability for their emptying is already considerable.     

Field excited phosphorescence in X-irradiated potassium halide crystals

Light emission is observed at room temperature from X-irradiated KC1, KBr and KI samples placed between insulated electrodes under high voltage excitation. The slow response of the effect to the electric field shows that metastable states are involved in the recombination process. The study of the brightness waves and the dependence of the light intensity on the temperature demonstrate that the excitation of the luminescence centers is obtained through a field assisted thermal ionization process.     

Excitation Intensity Dependence of Free-exciton Transitions in GaN Grown by Low-pressure Metalorganic Chemical Vapor Deposition

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Luminescence study of Si/Ge quantum dots

We present a photoluminescence (PL) study of Ge quantum dots embedded in Si. Two different types of recombination processes related to the Ge quantum dots are observed in temperature-dependent PL measurements. The Ge dot-related luminescence peak near 0.80 eV is ascribed to the spatially indirect recombination in the type-II band lineup, while a high-energy peak near 0.85 eV has its origin in the spatially direct recombination. A transition from the spatially indirect to the spatially direct recombination is observed as the temperature is increased. The PL dependence of the excitation power shows an upshift of the Ge quantum dot emission energy with increasing excitation power density. The blueshift is ascribed to band bending at the type-II Si/Ge interface at high carrier densities. Comparison is made with results derived from measurements on uncapped samples. For these uncapped samples, no energy shifts due to excitation power or temperatures are observed in contrast to the capped samples.     

Nonlinear Photoluminescence from ZnO Nanobelts

The nonlinear photoluminescence （PL） including second-harmonic generation （SHG） and the multiphoton luminescence （MPL） around 385 nm and 530 nm from ZnO nanobelts are investigated by using near-infrared excitations. The excitation wavelength dependence of MPL intensity reveals resonant energy transfer from SHG to MPL near the band gap excitation. The lifetime measurement of the MPL shows a much slower decay process of the defect emission, which results from the generation and recombination of both donors and acceptors on the disordered surface of the ZnO nanobelts.     

Model for degradation of band gap photo-luminescence in GaAs

Summary Degradation of near band gap photo-luminescence emission in GaAs with time of exposure to low power, c.w. laser excitation at room temperature is quantitatively described by a model based on defect reactions that are promoted by trapping and recombination of excess carriers at nonradiative recombination sites. The proposed model accurately describes the observed degradation rate, its power and temperature dependence, as well as the absence of degradation at a surface with shallow ion implantation. Summer student from the Materials Sciences Department at Massachusetts Institute of Technology, Cambridge, Mass., USA.     

Stimulated photoluminescence of ZnSe

The stimulated emission from ZnSe under one photon or two photon excitation has been investigated in the temperature range between 20 and 100 K. The spectra obtained under quasi-resonant one photon pumping show a dominant P line, ascribed to exciton-exciton scattering on the basis of its intensity dependence and its optical gain spectrum. The role of many-body processes under non-resonant one photon excitation is discussed. Under two photon pumping the LO-phonon assisted free exciton recombination is shown to be the dominant lasing mechanism.     

Correlated photons from multi-carrier complexes in GaAs quantum dots

We have studied micro-photoluminescence spectra of a self-assembled single GaAs quantum dot under 8 K. With strong pulsed excitation, the micro-photoluminescence spectrum shows bright emission lines originated from an exciton, a positively charged exciton, and a biexciton, together with weak lower energy emissions reflecting multi-excitonic structures with more carriers. We have identified the origins of these weak emission lines, and showed the existence of charged biexciton states, through single photon correlation measurements and excitation power dependence of the photoluminescence intensity. In addition, investigating the radiative recombination process of the charged biexciton, we have determined the electron–hole exchange energy in the GaAs quantum dot.     

Photon emission from sputtered nickel atoms as a function of target temperature near the Curie point

The photon emission from sputtered and excited Ni atoms was measured over the target temperature range including the ferromagnetic-paramagnetic transition point. It was found, that ion induced desorption of an adsorbed layer increases in the vicinity of the Curie point. It is also shown that excitation events and ionic desorption of adsorbates are strongly correlated. The temperature dependence of photon emission and ionic desorption indicate that points of surface and bulk magnetic transitions are identic.     

Time evolution of emission spectra from plasmas produced by irradiation of seawater surfaces by a femtosecond laser

The spectral and temporal characteristics of plasmas produced by femtosecond laser pulses acting on seawater surfaces are studied. The time dependence of the decay in the intensities of emission lines and the continuum are determined and the electron density is estimated using the three-body recombination time. It is shown that an atomic line with a low excitation potential is the most sensitive for femtosecond laser induced breakdown spectroscopy.     

Optical processes in the formation of stimulated emission from ZnO nanowires

This paper studies power dependent photoluminescence spectra, the stimulated emission occurring at ultraviolet (UV) band instead of the green emission band of ZnO nanowires, which are prepared with a chemical reduction method. The dynamics of the UV emission and green emission is given to demonstrate the reason of stimulated emission occurring at UV band but not the green emission band under high excitation, which indicates that the slow decay rate of trap state makes it easy to be fully filled and saturated, while the fast decay rate of near-band-edge exciton state makes the UV emission dominate the radiative recombination under high excitation. The UV emission, as well as the corresponding stimulated emission, occurs in competition with the green deep-trap emission. In addition, when pump fluence further increases, the multiple lasing modes appear. The dependence of these lasing modes on the pump fluence is first discussed. This diagram should be helpful to understand and design the optical nanodevices of ZnO nanowires.     

Optical properties of InGaN/GaN double quantum wells with varying well thickness

The optical properties and recombination kinetics of the InGaN/GaN double quantum well (DQW) structures with different well thickness (L_w) have been studied by means of photoluminescence (PL), time-resolved PL, and cathodoluminescence (CL) measurements. With increasing quantum well thickness up to 4 nm, the PL emission energy decreases and the blueshift of the PL emission energy increases with increasing excitation density. On the other hand, the PL emission energy of the DQWs with L_w=16 nm is higher than that of the DQWs with L_w=4 nm, and is independent of the excitation density. With increasing L_w from 1 to 4 nm, the PL decay times increase. In contrast, the decay times of 16 nm DQWs are faster than those of 4 nm DQWs. These different results for 16 nm DQWs such as the blueshift of the emission energy, the decrease of the excitation density dependence, and the increase of recombination rate can be ascribed to the relaxation of the piezoelectric field. We also observed the inhomegeneity in the CL spectra of the DQWs with L_w=1 nm on 1 μm scale.     

Pulsed laser stimulation and excitation in ZcS:Pb,Mn phosphor

Pulsed IR stimulation of UV irradiated room temperature ZnS:Pb,Mn phosphor powder was performed at 700 nm and 1.06 μm. Pulsed excitation of the phosphor at 355 nm was also studied. Transient spectra and spectrally resolved luminescence decay times are reported. The parametric dependence of the transient emission on UV excitation power and stimulation pulse energy is reported, as well as absolute stimulation efficiencies. The results are interpreted qualitatively using a model in which both intra-atomic transitions and self-activated donor-acceptor recombination occur simultaneously. Decay times range from 300 ns to 900 ns.     

Exciton binding energies in II–VI compound strained layer superlattices

II–VI strained-layer superlattices are very efficient emitters of visible light. The dependence of the luminescence intensity on the excitation power density allows us to characterise the recombination processes involved in the emission. At low temperatures excitonic processes are dominant whereas electron-hole recombinations feature at room temperature. No special evidence of the dual nature of the emission is observed at intermediate temperatures because the optical transitions are broadened by well-width fluctuations. In spite of this we may estimate the exciton binding energy from the temperature dependence of the photoluminescence intensity, as long as the photoluminescence remains excitonic. This is the case for narrow wells in CdS---ZnS superlattices over the temperature range zero to room temperature. The estimated exciton binding energy measured in this way approaches the two dimensional limit but does not exceed it.     

Linear and nonlinear photoexcitation dynamics in pi-conjugated polymers

Linear and nonlinear recombination kinetics with various lifetime distributions were identified for long-lived photoexcitations in a series of pi-conjugated polymer films using modulation frequency and excitation intensity dependencies of the photoinduced absorption. This includes monomolecular, bimolecular, and defect-limited recombination processes that lead to saturation. Using generalized kinetics parameters, we found characteristic plots for all recombination processes. Specifically, the bimolecular recombination process shows superlinear intensity dependence away from the steady state; on the contrary, dispersive bimolecular recombination leads to sublinear dependence.     

Electroluminescence and its excitation mechanism of SiOx films deposited by electron-beam evaporation

Blue electroluminescence from SiO_x films deposited by electron beam evaporation was observed. This blue emission blueshifted from 450 to 410 nm with increasing applied voltage. The dependences of blue emission on applied voltage, frequency and conduction current were studied. Our experimental data support that blue emission from SiO_x films is the result of both recombination of charge carriers injected from opposite electrodes and impact excitation of hot electrons, the recombination of carriers injected is dominant in low and medium electric fields but hot electron impact excitation is dominant under high electric fields.     

Analysis of anisotropic matrix elements in GaAs quantum wells oriented in the [112] crystal axis

We report the experimental confirmation of optical anisotropy in [112]-oriented GaAs/(Al,Ga) As quantum well structures. A theoretical calculation of the momentum matrix elements predicts the existence of this anisotropy and its dependence on the polarization of the incident radiation. It is found that recombination emission from photoluminescence excitation for the fundamental e→hh1 transition is anisotropic for incident light polarized along the two orthogonal [1¯10] and [111¯] crystallographic directions. No anisotropy is found in identically prepared [001]-oriented structures either theoretically, or experimentally.     

Influences of excitation power and temperature on photoluminescence in phase-separated InGaN quantum wells

The photoluminescence(PL) properties of a green and blue light-emitting InGaN/GaN multiple quantum well structure with a strong phase separated into quasi-quantum dots(QDs) and an InGaN matrix in the InGaN epilayer are investigated.The excitation power dependences of QD-related green emissions(P_D) and matrix-related blue emissions(P_M) in the low excitation power range of the PL peak energy and line-width indicate that at 6 K both P_m and P_D are dominated by the combined action of Coulomb screening and localized state filling effect.However,at 300 K,P_m is dominated by the non-radiative recombination of the carriers in the InGaN matrix,while P_D is influenced by the carriers transferred from the shallower QDs to deeper QDs by tunnelling.This is consistent with the excitation power dependence of the PL efficiency for the emission.     

Time-resolved luminescence of ZnIn2S4 at low temperature

Summary A detailed study of the time-resolved luminescence of ZnIn₂S₄ after pulsed-laser excitation at 30 K is presented. Results are given on the dependence of the emitted PL intensity at different emission wavelengths on the excitation intensity, on the time-resolved spectra in the range (0÷10⁻¹) s of delay time with respect to excitation and on the time decay of the luminescence intensity for different emission wavelengths. The results are discussed in the framework of a simple model for the recombination processes at low temperatures yielding a good overall agreement with the experimental data. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.