Decay and polarization of luminescence pulses in the 440 nm emission band of KI:In

Model suggestions for the impurity luminescence center in KI:In crystals from the literature are found to agree with experimental results obtained by excitation of KI:In at 80 K with polarized light pulses in the A, B and C impurity absorption bands. By excitation in the C absorption band the population of the metastable M state goes not only via A state, but directly from the C state too.     

Final state effects of deep impurities in semiconductors

Current theoretical and computational models for optical excitation processes of deep level impurities in semiconductors tend to concentrate on the impurity itself, largely ignoring the effects the impurity has no final states. Using a spherical band model, we show these effects can be included in calculations on optical absorption processes. The band state is modified by the presence of the impurity and this modified state is used in the calculation of optical matrix elements. We show that final state effects can cause significant changes in the local density of states and optical matrix elements.     

Photoluminescence of a silver-doped glass

The absorption, emission and excitation spectra of Ag⁺ ions in a soda lime glass doped with two different concentration of silver are investigated. Absorption spectra exhibit a main broad band peaked at about 260 nm (4.77 eV) with a shoulder at about 227 nm (5.46 eV). The relative height of the shoulder depends on silver concentration in the glass. Emission spectra of Ag⁺ are dominated by an ultraviolet broad band at about 330 nm (3.76 eV). The excitation spectra for this emission show a preponderant broad band peaked at about 227 nm (5.46 eV) which coincides with peak position of the shoulder displayed in the optical absorption spectra. A weak broad featureless emission band centred at about 550 nm (2.25 eV) with an excitation peak at about 242 nm (5.12 eV) is tentatively related to an impurity from the host silica glass rather than originated in silver-type centres. Comparison of the luminescence decay curves for both emissions show substantial differences between them. Consequently, the emissions in the time-resolved spectra can easily be discriminated.     

Impurity Cu+ centers in LiF single crystals

LiF single crystals with copper impurity (0.0004–0.002%) have been grown by the Czochralski method and investigated. The luminescence, excitation, and optical absorption spectra have been recorded. The luminescence spectrum contains a band at 450 nm upon 250-nm excitation. This band is attributed to Cu⁺ centers in the samples grown. The mechanisms of capture and recombination during thermoluminescence are considered.     

A first-principles theoretical simulation on the electronic structures and optical absorption properties for O vacancy and Ni impurity in TiO2 photocatalysts

The band structures and optical absorption spectra of O vacancy and Ni ion doped anatase TiO₂ were successfully calculated and simulated by a plane wave pseudopotential method based on density functional theory (DFT). From the calculated results, a phenomenon of “impurity compensation” was found: the lower formation energy for O vacancy than Ni impurity indicated that introducing the intrinsic defect of O vacancy into Ni ion doped TiO₂ sample was very possible; the positive binding energy for the combination of O vacancy and Ni impurity indicated that two defects were apt to bind to each other; While Ni impurity produced the donor levels in the forbidden band of TiO₂, Ni impurity with O vacancy produced the acceptor levels upon which the excitation led to the photogenerated electrons with high energy and transferability. The combination of absorption spectra for O vacancy and Ni impurity with O vacancy models could reproduce the experimental measurement very well.     

Interband excitation induced absorption by deep impurities in GaP

Transient optical absorption at energies below the band gap in GaP crystals following pulsed interband excitation was observed. The additional absorption attained its maximum about 25 μsec after the excitation pulse and lasted about 100 μsec. The spectral dependence of the rise time and decay time of the absorption and that of the induced optical cross section were determined. The results suggest that the induced absorption is caused by several deep impurity levels, which are most probably due to Cu, being populated by the recombination of excited electrons.     

Fe-N共掺杂锐钛矿相TiO2电子性质与光学性质的第一性原理研究

近年来,Fe和N掺杂锐钛矿相TiO2半导体在实验中发现许多优异性能,本文采用基于密度泛函理论的平面波超软赝势方法研究了纯锐钛矿相TiO2、Fe和N单掺杂及Fe和N共掺杂TiO2的能带结构、电荷布居、态密度和光学性质.分析发现:Fe掺杂引起杂质能带位于禁带中央,杂质能带最高点与导带相距大约0.6 eV而最低点与价带相距大约0.2 eV;N掺杂引起的杂质能带位于价带顶部附近. Fe和N共掺杂后杂质能带由两部分组成,位于价带顶上方0.62 eV和导带底下方0.22 eV处,其中一层杂质能带主要由N原子的2p轨道和Fe原子的3d轨道杂化形成,而另一条杂质能带主要由Fe原子的3d轨道形成,由于杂质能级的出现,使锐钛矿TiO2的禁带宽度变小.对光学性质分析发现:Fe和N共掺杂会使锐钛矿TiO2光学吸收带边红移,可见光区的光吸收系数明显增大,在低能区出现了新的吸收峰,对应能量为1.82 eV,与实验结果相符.     

Electronic excitations of Sc3+ impurity in α-Al2O3 crystals

Optical properties, including luminescence, of scandium-doped α-Al₂O₃ crystals have been studied in the VUV range. An absorption band associated with the scandium impurity was observed at the fundamental-absorption edge of crystalline corundum. A strong luminescence band peaking at 5.6 eV, which is most effectively excited within the 7.7–8.8-eV interval, was found. The kinetic and polarization characteristics of this luminescence were studied within the temperature range 6–500 K. An excitation model of the impurity complex and the mechanism of its relaxation are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 653–654 (April 1998)     

Vibronic resonance in spectra of frozen solutions of the C60 fullerene derivatives

This paper reports on a study by site-selective laser spectroscopy of low-temperature broadband fluorescence spectra of N-methyl-2-(3,5-di-tert-butyl-4-hydroxyphenyl)-[C₆₀] fulleropyrrolidine (N-MBHPhFP fullerene) molecules embedded in the crystalline toluene matrix. It has been shown that monochromatic laser excitation in the region of vibrational satellites of the pure electronic absorption band initiates structurization of the emission band of a pure electronic transition, which suggests the existence of a set of impurity centers in the crystalline host matrix. An analysis has been made of the factors accounting for replacement of the wide, inhomogeneously broadened band by a series of narrow bands, with the most essential of them being the weakness of electron-phonon coupling in the impurity center, the absence of electronic excitation transfer between the centers, and the vibronic resonance in the excited state of the set of centers. Vibration frequencies of the N-MBHPhFP fullerene in the excited electronic state have been determined.     

Observation of general behaviour of Resonance Raman Spectra in the model system KBr(MnO-4)

Employing KBr(MnO^-₄) crystals as model specimens, the authors have observed for the first time the theoretically predicted peculiarities of the impurity resonance Raman scattering (RRS) resulting from excitation to a vibronic-structured absorption band, those peculiarities being the sharp irregularity of the RRS spectrum in case of excitation to Frank-Condon sublevel, the strong dependence of the spectrum on excitation frequency, the temperature dependences of the RRS line intensity which are significantly different for different orders of scattering. The observational data have been compared with calculations of cross-sections for multi-phonon RRS.     

Two- and three-photon excitation of Gd in CaAl12O19

We have employed two-photon excitation to study the higher energy levels of Gd³⁺ ions in CaAl₁₂O₁₉ and we compare the results with those obtained using conventional UV excitation techniques. Under two-photon excitation, the luminescence intensity exhibits an unusual temporal behavior, a very long build-up followed by a decrease by orders of magnitude, ascribed to a recombination-assisted luminescence excitation mechanism assuming photo-ionization of Gd³⁺ ions and trapping of free electrons on deep traps.

We also find that the two-photon excitation spectra contain an additional broadening contribution which can be attributed to homogeneous broadening of excitation levels caused by excited state absorption into the conduction band. We believe that this may be a general phenomenon whenever participating photons produce ionization of impurity ions from metastable excited states. The phenomenon can manifest itself also in two-photon ionization spectral hole burning and in up-conversion processes (in the latter case, the homogeneous broadening can be caused by an intra-ion excited-state absorption).     

Infrared absorption by pairs of group III and V impurities in silicon

Infrared absorption bands, shifted to regions of lower energies relative to narrow lines of transitions of impurities to excited states, are investigated in silicon doped with group III and V impurities in concentrations above 10¹⁶ cm^?3. It is found that the band structure is peculiar to each of the investigated impurities but independent of their concentrations, and the absorption coefficients in the bands increase approximately quadratically with concentration. This leads one to infer that the bands are caused by the excitation of charge carriers bound on impurity pairs localized within several Bohr radii.     

Spectroscopic observation of bipolaronic point defects in Ba(1-x)KxBiO3

An infrared-absorption band centered at 0.85 eV, which is below the big optical absorption at the charge-density-wave (CDW) gap energy of 1.85 eV, has been observed for semiconducting single crystalline Ba(1-x)KxBiO3. With substituting K for Ba, the spectral weight of the new band increases with x, while that of the CDW-gap excitation decreases. Since the impurity state with the K substitution is known to be nonmagnetic at low temperatures, Bi3+ the state with 6s2 electrons surrounded by the six Bi5+ ions forms a small bipolaron by losing a pair of electrons in the Rice-Sneddon model. The new band is assigned to a transition from the lower-Peierls band to a state of the bipolaronic point defect.     

2(水杨醛缩苯胺)-(1,10-邻菲罗啉)合钙的光谱特性

合成了一种新型的蓝光发射材料2(水杨醛缩苯胺)-(1,10-邻菲罗啉)合钙,并利用红外光谱、X射线衍射谱、DSC热分析、UV-vis吸收谱、荧光激发光谱和荧光发射光谱研究了其结构、晶态、热稳定性以及光学特性,分析了它的能态结构和发光机理。结果表明,2(水杨醛缩苯胺)-(1,10-邻菲罗啉)合钙的热稳定性较高,是一种多晶粉末发光材料,禁带宽度2.93eV,在紫外光的激发下,固态荧光发射峰在449.7nm处,在乙醇溶液体系中的荧光发射峰在491nm处,均为蓝色荧光,色纯度高,荧光量子效率高,其荧光发射主要来源于长波吸收带,最大波长吸收带对荧光发射贡献最大。     

Energy transfer in rare earth-doped CaWO4 after red edge excitation

Host sensitized energy transfer in CaWO₄ doped with Eu³⁺ and Sm³⁺ is investigated using pulsed laser time-resolved spectroscopy techniques. The excitation is into the long wavelength tail of the absorption edge and the results indicate that this selectively excites tungstate ions which are located next to activator impurity ions. The subsequent energy transfer is found to be consistent with a single step, nearest neighbor process. Both electric dipole-dipole and exchange interactions can predict strong enough transfer to account for the observed rates. These results are compared to those obtained previously for broad band excitation into the highest energy absorption bands of CaWO₄ crystals.     

Investigation of Mn incorporation in fifteen-period InGaAs/GaAs quantum well system

A ferromagnetic ordering with a Curie temperature of 50 K of fifteen layer of InGaMnAs/GaAs multi quantum wells (MQWs) structure grown on high resistivity (100) p-type GaAs substrates by molecular beam epitaxy (MBE) was found. It is likely that the ferromagnetic exchange coupling of sample with Curie temperature of 50 K is hole-mediated resulting in Mn substituting In or Ga sites. Temperature and excitation power dependent PL emission spectra of InGaMnAs MQWs sample grown at temperature of 170 °C show that an activation energy of Mn ion on the first quantum confinement level in InGaAs quantum well is 36 meV and impurity Mn is partly ionized. It is found that the activation energy of 36 meV of Mn ion in the QW is lower than the activation energy of 110 meV for a substitutional Mn impurity in GaAs. These measurements provide strong evidence that an impurity band existing in the bandgap due to substitutional Mn ions and it is the location of the Fermi level within the impurity band that determines Curie temperature.     

Fast luminescence processes in KI : In caused by excitation in the a absorption band

The temperature dependence of the decay time (τ) of the luminescence pulses from the 440 nm emission band as well as the rise time (τ_R) of luminescence pulses from the 575 nm emission band has been investigated for KI : In by pulsed light excitation in the A absorption band. From the agreement of τ and τ_R independent of the temperature it can be concluded that the X minimum on the ³T₁₄ adiabatic potential energy surface (APES) is mainly populated by a radiationless transition from the tetragonal T minimum of the same APES.Furthermore, a comparison of parameters for the impurity centre luminescence corresponding to the tetragonal emission band in KI : In and KI : Tl has been drawn.     

Influence of the Coulomb gap on the impurity absorption in AIIIBV semiconductors

Using the interpolation formula, which adequately describes the impurity band structure in a broad energy range regardless of the degree of compensation, an explicit expression is obtained for the light absorption coefficient related to the transitions from the impurity band to the conduction band. It is shown that at low temperatures the absorption coefficient has a clearly expressed absorption threshold. The explicit frequency dependence of the absorption coefficient is derived for a wide frequency range of the incident radiation. The absorption coefficient dependence on the degree of doping of a semiconductor is also studied.     

Localized Excitons in the Spectrum of Optical Absorption of Zinc Oxide Doped with Manganese

Physics of the Solid State - The results of the study of optical absorption and EPR signals of single crystals of zinc oxide doped with manganese are presented. A broad impurity absorption band...     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.