Enhanced photoinduced desorption from metal nanoparticles by photoexcitation of confined hot electrons using femtosecond laser pulses

Strong fluence dependence of photodesorption cross sections is observed in femtosecond laser photodesorption of NO from (NO)2 on silver nanoparticles, in contrast to femtosecond photodesorption on bulk metals. The time scale of excitation buildup is found to be equal or less than the pulse duration of ～100 fs; NO translational energies are independent of fluence and pulse duration. We propose a nanoparticle-specific nonlinear mechanism in which, due to confinement, strongly nonthermal hot-electron distributions are maintained during the femtosecond pulses, enhancing the normal desorption pathway.     

Two-Photon Raman scattering and high excitation luminescence in ZnTe

Two-Photon-Raman Scattering (TPRS) and the luminescence of ZnTe are investigated when the samples are highly excited with a tunable narrow-band dye-laser. In luminescence, one observes emission bands due to the well-known inelastic exciton-exciton scattering at intermediate excitation intensities, and the recombination radiation of an electronhole plasma (EHP) at the highest excitation levels. For the first time, TPRS is reported in ZnTe. From the change in the TPRS lines in magnetic fields up to 10T we deduce a diamagnetic shift of 1.2°10^?2 meV/T² of the free longitudinal exciton. This value is in good agreement with results obtained by other authors from reflection spectroscopy.     

Two-Photon Raman scattering and high excitation luminescence in ZnTe

Two-Photon-Raman Scattering (TPRS) and the luminescence of ZnTe are investigated when the samples are highly excited with a tunable narrow-band dye-laser. In luminescence, one observes emission bands due to the well-known inelastic exciton-exciton scattering at intermediate excitation intensities, and the recombination radiation of an electronhole plasma (EHP) at the highest excitation levels. For the first time, TPRS is reported in ZnTe. From the change in the TPRS lines in magnetic fields up to 10T we deduce a diamagnetic shift of 1.2°10^–2 meV/T² of the free longitudinal exciton. This value is in good agreement with results obtained by other authors from reflection spectroscopy.     

Luminous and tunable white-light upconversion for YAG (Yb3Al5O12) and (Yb,Y)2O3 nanopowders

We report on multiphoton white-light upconversion in vacuum for Pechini synthesis Yb(3)Al(5)O(12) (YbAG) and combustion synthesis (Yb,Y(2))O(3) nanopowders under IR excitation. Their intense white-light upconversion is attributed to charge transfer luminescence superimposed upon a broadband emission. Unlike common nanoscale phosphors, which show low luminescence efficiency, the intensity of white-light upconversion for nanopowders is similar to that of their bulk counterparts. The luminary efficacy of the upconversion is estimated to be 10-15 lm W(-1), and the Commission Internationale d'Eclairage (CIE) coordinates can be widely tuned by the excitation power, pressure, and codoping ratio. The nano-YbAG sample exhibits a longer buildup time for emission, a higher excitation threshold, and a wider CIE range than the oxide nanopowders.     

Formation of the electron-hole droplet cloud in germanium

We describe luminescence imaging experiments that probe the nature of the phonon wind which transports electron-hole droplets in Ge. The participation of non-equilibrium phonons emanating near the excitation region is supported by: (a) sharp features in the droplet spatial distribution (b) a dependence of the average cloud density on the excitation photon energy, and (c) a rapid initial buildup of the cloud.     

Linear and nonlinear optics,dynamics, and lasing in ZnO bulk and nanostructures

In linear optics, we report on measurements of the absolute external quantum efficiency of bulk ZnO and powders using an integrating sphere. At low temperature the near band edge emission efficiency can reach 0.15 in the best samples. For deep center luminescence this value may be even higher. When going to room temperature (RT) the quantum efficiency drops by about one order of magnitude. From time resolved luminescence measurements we deduce the lifetime of the free and bound excitons to be in the sub ns regime and find for the latter a systematic increase with increasing binding energy.Concerning lasing, we discuss the role of excitonic processes and the recombination in an inverted electron–hole plasma (EHP). While excitonic processes seem well justified at lower temperatures and densities, doubts arise concerning the concept of excitonic lasing at RT in ZnO. The densities at laser threshold at RT are frequently close to the Mott density or above but below the density at which population inversion in an EHP is reached. We suggest alternative processes which can explain stimulated emission in this density regime in an EHP at RT.     

激光分子束外延方法生长的ZnO薄膜的发光特性

研究了不同温度和不同光激发强度下激光分子束外延方法生长的ZnO薄膜样品的发光性能,发现YAG脉冲激光激发,强度超过一定值时会在长波方向上出现一个新的发光峰,此峰可能起源于电子-空穴的复合。室温下氙灯激发的光谱中可以看到峰值位于381nm的近带边紫外发射峰和位于450nm的强的蓝绿带发射,根据光致发光激发光谱的特征给出了一个简单的蓝光发射模型。对比YAG脉冲激光激发和氙灯激发得到的实验光谱,我们认为不同的光谱特征和样品发光的激发机制有关,紫外峰发射需激发强度超过一定值才能观察到,而蓝带发射则在一定的激发强度下迅速饱和。     

A mechanism of the anti-Stokes luminescence of a dye-sensitized silver halide emulsion

A two-photon stepwise mechanism of the low-temperature anti-Stokes luminescence of a sensitized AgBrI emulsion, caused by photoexcitation of dye aggregates, is proved experimentally. According to this mechanism, the photoexcitation energy is transferred from the dye to the AgHal microcrystal through silver cluster centers. It is found that the luminescence with the maximum at λ ≈ 750 nm is caused by the recombination of an electron localized at the silver cluster with a free hole in the valence band of the AgHal microcrystal. The migration of an electron from a silver cluster to a iodine pair center with a captured hole creates a nonradiative recombination channel, which is responsible for the flare buildup of the green anti-Stokes luminescence upon excitation of dye aggregates.     

Dependence of the properties of the dual luminescence of 3-hydroxyflavone on the excitation wavelength

The luminescence and luminescence excitation spectra of 3-hydroxyflavone in acetonitrile obtained at different excitation/recording wavelengths are studied. The dependences of the position of the normal luminescence band maximum and of the intensity ratio of the normal and proton-transfer bands, on the excitation wavelength are found and studied for the first time. It is found that the spectral contour of dual luminescence also depends on the excitation wavelength and the blue emission band is cut off at a sufficiently long-wavelength excitation in the region of 390 nm. In the luminescence excitation spectrum, an additional wide band is observed in the proton-transfer region at 200–260 nm. Excitation in the region of 380–440 nm allowed us to reveal a wide structureless band near 470 nm (with the maximum of its excitation near 420 nm) belonging to the anionic form of 3-hydroxyflavone. Addition of water at a concentration of ～2.2 M quenches this band almost completely.     

Spectral dependent kinetics of the EHD luminescence in As-doped Ge

Following pulsed laser excitation of As-doped Ge with impurity concentrations between 10¹⁵-10¹⁷cm^-3, we observe the electron-hole drop (EHD) and excitonic luminescence decay. The spectrum resolved no-phonon (NP) EHD luminescence kinetics are found to depend on its spectral position. “Plateaus” on the kinetic curves for the high energy side of NP-spectrum are observed at high excitation. The data suggest this luminescence is due to the states which are in dynamical equilibrium during some time after excitation pulse.     

Luminescence of a ZnO:Ga crystal upon excitation in vacuum UV region

The spectral-kinetic characteristics of a ZnO:Ga single crystal upon excitation in the vacuum UV region have been studied. At a temperature of 8 K, the exciton luminescence line peaking at 3.356 eV has an extremely small half-width (7.2 meV) and a short decay time (360 ps). In the visible range, a wide luminescence band peaking at ～2.1 eV with a long luminescence time at 8 K and a decay time in the nanosecond range at 300 K is observed. The luminescence excitation spectra of ZnO:Ga have been measured in the range of 4–12.5 eV.     

Dynamics of electron–hole plasmas and localized excitons in highly excited GaN-based ternary alloys

We have studied photoluminescence (PL) spectra of GaN crystals and InGaN ternary alloys at low temperatures as a function of the femtosecond laser excitation intensity. With an increase of the intensity, the broad PL due to electron–hole plasmas (EHP) appears below the biexciton PL in the GaN sample. On the other hand, the broad EHP PL appears above the localized exciton PL in the InGaN sample. The intensity dependence of PL properties of InGaN crystals is completely different from that of GaN crystals. The effect of alloy disorder on PL processes in ternary alloys is discussed.     

Stimulated emission processes in highly excited CdS at 80 K

The stimulated emission from CdS at 80 K under high excitation density is studied by means of quasi-resonant dye laser pumping. The evidence of exciton-exciton (P line) and exciton-electron (E line) scattering and, at the highest excitation level, of electron-hole plasma (EHP) recombination are reported and discussed also by means of optical gain measurements.     

Yb:YAG晶体的合作发光现象

研究了Yb：YAG晶体的合作发光现象。当用940nm的近红外光激发时,Yb:YAG晶体有明显的上转换蓝色发光。实验发现498 nm的蓝色发光强度与激发功率的平方成正比,而且Yb3+掺杂浓度越高,蓝色发光越强。分析表明这是Yb3+间强的相互作用导致的合作发光,是由于Yb3+在共价性的YAG基质中,它的4f13电子易于与近邻离子发生相互作用导致的。     

Enhanced sensitivity to residual dipolar couplings of elastomers by higher-order multiple-quantum NMR

The homonuclear and heteronuclear residual dipolar couplings in elastomers reflect changes in the cross-link density, temperature, the uniaxial and biaxial extension or compression as well as the presence of penetrant molecules. It is shown theoretically that for an isolated methyl group the relative changes in the intensity of the homonuclear double-quantum buildup curves in the initial time regime due to variation of the residual dipolar coupling strength is less sensitive than the changes in the triple-quantum filtered NMR signal when considering the same excitation/reconversion time. For a quadrupolar nucleus with spin I=2 the sensitivity enhancement was simulated for four-quantum, triple-quantum, and double-quantum buildup curves. In this case the four-quantum build-up curve shows the highest sensitivity to changes of spin couplings. This enhanced sensitivity to the residual dipolar couplings was tested experimentally by measuring 1H double-quantum, triple-quantum, and four-quantum buildup curves of differently cross-linked natural rubber samples. In the initial excitation/reconversion time regime, where the residual dipolar couplings can be measured model free, the relative changes in the intensity of the four-quantum buildup curves are about five times higher than those of the double-quantum coherences. For the first time proton four-quantum coherences were recorded for cross-linked elastomers.     

Current injected electron-hole plasma in GaP pin-device structures

We report first on an electrically generated electron-hole plasma (EHP) in GaP light emitting diodes having a pin-structure. For current densities above j ? 10³Acm^?2 and at T = 77 K a dense EHP is formed within the i-region whose carrier density can easily be changed by the injection level. The corresponding electron-hole pair densities n of the plasma and the resulting gap shrinkage ΔE_g have been determined using a least-squares fit of the EHP luminescence spectra. The lineshape analysis of the spectra yields a $\text{n}{}^{\mathrm{<mtext>1</mtext><mtext>4</mtext>}}$ -dependence of the gap shrinkage in quantitative agreement with findings from optically excited EHP and corresponding theoretical work. From electrical investigations it can be concluded that the EHP causes a strong nonlinear behaviour of the current-voltage characteristics.     

Mechanisms of ZnO Luminescence in the Visible Spectral Region

Existing models of zinc oxide luminescence in the visible spectral region are considered and comparatively analyzed. Experiments are performed with ceramics obtained from initial ZnO powder and ZnO powders annealed in vacuum (ZnO-vac) and in air (ZnO-air). The luminescence characteristics of the ceramics, namely, emission and excitation spectra, kinetics, and temperature stability, are studied. The ZnO-vac ceramics exhibits green luminescence caused by neutral oxygen vacancies. The luminescence of ZnO-air ceramics is shifted to the red and is presumably related to residual lithium impurity. The ceramics differ in the luminescence excitation spectra, time characteristics, and temperature stability.     

Origin of broadband near-infrared luminescence in bismuth-doped glasses

Interstitial negative-charged bismuth dimers, Bi(-)(2) and Bi(2-)(2), are suggested as a model of broadband IR luminescence centers in bismuth-doped glasses. The model is based on quantum-chemical calculations of equilibrium configurations, absorption, luminescence, and luminescence excitation spectra of the dimers in an alumosilicate network and is supported by IR luminescence observed for the first time, to our knowledge, in bismuth-doped polycrystalline magnesium cordierite.     

Electroluminescence kinetics of film structures based on manganese-doped zinc sulfide

A theoretical and experimental study is made of the growth and decay constants of the brightness as functions of the rise time and amplitude of the pulse from electroluminescent structures based on manganese-doped zinc sulfide and deposited on smooth and rough substrates, and as functions of the rise time and amplitude of the ramped (linearly rising) exciting voltage. The curves obtained are used to determine a variety of parameters and characteristics of the electroluminescence process: the lifetime of the excited luminescence centers, the excitation and relaxation probabilities of luminescence centers per unit time, and the cross section of impact excitation of luminescence centers and their dependences on the rise time and amplitude of the linearly rising exciting voltage. Explanations are given for the fact that the indicated characteristics show different behavior for structures on smooth and rough substrates. Zh. Tekh. Fiz. 69, 65–73 (May 1999)     

Persistent hole burning in semiconductor nanocrystals

The persistent spectral hole burning (PSHB) phenomenon was found to occur in many kinds of nanocrystalline semiconductors, such as CdSe, CdS, CuCl, CuBr and CuI, embedded in crystals, glass or polymers. In inhomogeneously broadened exciton absorption spectra of these nanocrystals, the spectral hole and its associated structure were created by the narrow-band laser excitation and were conserved for more than several hours at 2 K. Hole depth grew in proportion to the logarithm of the burning fluence. Thermally-annealing and light-induced hole-filling phenomena were observed. The hole burning takes place by the tunneling process through potential barriers with broadly distributed barrier height and thickness. Unusual luminescence behaviors related to the PSHB phenomena were also observed. They are luminescence elongation with increase of the light exposure and hole burning in the luminescence spectrum. The observed PSHB phenomena are explained by the exciton localization and the succeeding ionization of nanocrystals. The energy of the photoionized nanocrystal is released from the original energy and the new energies depend on the spatial arrangement of the trapped carriers. Quantum confinement of carriers and resulting strong Coulomb interaction between confined carriers and trapped carriers are essential for the energy change. Possible applications of the PSHB phenomenon is discussed.