Fluorescence Enhancement of CdSe Q-Dots with Intense Femtosecond Laser Irradiation

Effects of intense femtosecond (fs) laser irradiation on the optical properties of cadmium selenide (CdSe) nanocrystals are studied. We present the changes in emission and absorption of laser (800 nm, 110 fs, Ti–Sapphire) irradiated CdSe nanocrystals dispersed in dimethylformamide (DMF). It is observed that the absorbance of CdSe nanocrystals capped with trioctylphosphine (TOP) increases with the number of laser pulses. The trap state luminescence intensity of these crystals degrades, whereas the band edge luminescence intensity shows an increase as a function of the fs laser irradiation. We also report strong two photon absorption and reduction in the trap state luminescence intensity after irradiation with the laser pulses.     

Luminescence and Stimulated Emission in Zinc Oxide Nanoparticles,Films, and Crystals

ZnO has attracted attention as a candidate material for ultraviolet light-emitting devices. Its 3.37 -eV band gap is comparable to that of GaN, and single crystal substrates can be grown. Control of p-type conductivity in ZnO is under study in several laboratories including ours. We report streak camera measurements of time-resolved luminescence and stimulated emission excited in single crystal, film, and particle samples under excitation by 300 fs laser pulses at temperatures from 17 K to 295 K. We also describe p-n junctions formed by control of oxygen pressure in reactive sputtering of ZnO films, and results of introducing nitrogen during reactive sputtering.     

Experimental and theoretical studies of polaron optical properties in KNbO3 perovskite

Time-resolved absorption and luminescence spectra have been measured in KNbO₃ perovskite crystals after pulsed band-gap excitation by 200 fs laser pulses and 10 ns electron pulses. Quantum chemical calculations using the large unit cell periodic model support the interpretation of the observed transient absorption bands at 0.8 and 1.1 eV as the self-trapped electron polarons and bound hole polarons, respectively. The activation energy for the 2.2 eV green luminescence quenching is 0.05 eV. We suggest that the short lifetime (<15 ns) of the luminescence at RT is caused by the radiative recombination of nearest electron and hole polarons.     

Synthesis of ZnO thin films by 40 ps @ 532 nm laser pulses

The synthesis by pulsed laser deposition of ZnO thin films with a Nd:YAG laser system delivering pulses of 40 ps @ 532 nm is reported. The laser beam irradiated the target placed inside a vacuum chamber evacuated down to 1.33×10⁻¹ Pa. The incident laser fluence was of 28 J/cm² in a spot of 0.1 mm². The ablated material was collected onto double face polished (111) Si or quartz wafers placed parallel at a separation distance of 7 mm. The AFM, SEM, UV-Vis, FT-IR and absorption ellipsometry results indicated that we obtained pure ZnO films with a rather uniform surface, having an average roughness of 37 nm. We observed by SEM that particulates are present on ZnO film surface or embedded into bulk. Their density and dimension were intermediary between particulates observed on similar structures deposited with fs or ns laser pulses. We noticed that the density of the particulates is increasing while their average size is decreasing when passing from ns to ps and fs laser pulses. The average transmission in the UV-Vis spectral region was found to be higher than 85%.     

Up-conversion Luminescence of Terbium(III) in SiO2 and ZnO–SiO2 Glasses Induced by Simultaneous Absorption of Three Photons and Saturation

Green up-conversion luminescence (UCL) of Tb³⁺ in SiO₂ and ZnO–SiO₂ glasses was observed with femtosecond (fs) laser (800 nm) excitation. The UCL mechanism of Tb³⁺ in both glasses contributed to the simultaneous three-photon absorption. The saturation effect of Tb³⁺ in SiO₂ glass took place. The saturation effect of excited-state levels of Tb³⁺ in SiO₂ glass was associated with the slower decay rate of ⁵D₄ level. This result implied that saturation power of Tb³⁺ in ZnO–SiO₂ glass was higher than that in SiO₂ glass.     

Luminescence and structural properties of thermally evaporated benzanthrone dyes thin films

We report optical and luminescence properties of 3-N, N-diacetylaminobenzanthrone thin films deposited on glass substrate by thermal evaporation. The structural and optical properties of organic thin films were studied by means of the confocal microscope with an input of femtosecond laser radiation, X-ray diffractometer, and scanning electron microscope (SEM). Intense luminescence with the maximum at 530 nm was observed when excited by laser radiation with the wavelengths 458, 476, 488, 496, 514 nm. In addition, the luminescence caused by two-photon absorption of femtosecond (fs) laser radiation has been investigated. Semi empirical calculations by AM1 and ZINDO/S methods and ab initio calculations using Gaussian software were carried out to estimate the electron system of structure. The calculations show planar configurations for the aromatic core and diacetylamino fragment of this compound. The study of the structure of benzanthrone derivative thin films with X-ray diffraction (XRD) methods, indicates the distance between molecular layers and ordered molecular fragments.     

Infrared femtosecond laser-induced great enhancement of ultraviolet luminescence of ZnO two-dimensional nanostructures

Two-dimensional (2D) complex nanostructures on the surface of ZnO crystal are fabricated by the interference of three 800 nm fs laser beams. The 2D nanostructures exhibit a great enhancement of UV emission excited by infrared fs laser with central wavelengths ranging from 1,200 nm to 2,000 nm. We propose that the defect states in the band gap of 2D nanostructures induced by 800 nm fs laser ablation cause the great enhancement of UV emission. We make theoretical calculations and explain well with the experimental results.     

Femtosecond laser ablation of ZnO nanorods for two-photon-pumped random lasing and optical data storage

Femtosecond laser ablation based on two-photon absorption was employed to cut ZnO nanorods into uniform ZnO nanoparticles of deep subwavelength size. The fabricated ZnO nanoparticles possess a shorter mean transport length for photons at the emission wavelength and a much smaller scattering cross section at the pump wavelength, leading to highly efficient two-photon-pumped random lasing with a low threshold of ??8?mJ/cm². It was demonstrated that the significant enhancement in two-photon luminescence after the irradiation of femtosecond laser pulses could also be utilized for realizing optical data storage.     

Simultaneous three-photon absorption induced ultraviolet upconversion in Pr:Y2SiO5 crystal by femtosecond laser irradiation

Near-infrared to ultraviolet upconversion luminescence was observed in the Pr³⁺:Y₂SiO₅ crystal with 120 fs, 800 nm infrared laser irradiation. The observed emissions at around 270 nm and 305 nm could be assigned to 5d → 4f transitions of Pr³⁺ ions. The relationship between the upconversion luminescence intensity and the pump power of the femtosecond laser reveals that the UV emission belongs to simultaneous three-photon absorption induced upconversion luminescence.     

Donor-acceptor pair luminescence in nitrogen-doped ZnO films grown on lattice-matched ScAlMgO4 (0001) substrates

We have grown nitrogen-doped ZnO (ZnO:N) films by laser molecular-beam epitaxy. The use of lattice-matched ScAlMgO₄ substrates prevented the degradation of crystallinity induced by the nitrogen incorporation to the films. Despite this improvement, we have not obtained ZnO:N films which showed p-type conductivity. We studied the optical properties of these ZnO:N films. Donor-acceptor pair (DAP) luminescence was observed. The results indicate the formation of an acceptor state. The energy position of the DAP luminescence is lower than that reported by Look et al. [Appl. Phys. Lett. 81 (2002) 1830]. The DAP luminescence band shifts to lower energy with increasing nitrogen concentration. A photoluminescence recombination possibly due to the free-electron-to-acceptor (FA) transition was observed at temperatures higher than 40 K. The acceptor ionization energy was estimated from the energy position of the FA luminescence to be 266 meV.     

Violet luminescence from ZnO nanorods grown by room temperature pulsed laser deposition

ZnO thin films were deposited at room temperature by pulsed laser deposition (PLD) varying the oxygen pressure. Morphological analysis using scanning electron microscope (SEM) and atomic force microscopy (AFM) demonstrated the formation of ZnO nanorods at a particular oxygen pressure. Room temperature violet luminescence was observed from these ZnO nanorods and temperature dependence of luminescence was studied. Influence of oxygen pressure on the growth of ZnO thin films by PLD was studied using the X-ray photoelectron spectroscopy of both post ablated targets and deposited films. The ZnO films were crystalline and the formation of crystalline phase is found to follow a pressure–temperature (P–T) scaling with increase of temperature.     

Characterization of enhanced emission from excimer laser treated ZnO ceramics using one- and two-photon luminescence spectroscopy and microscopy

The effects of excimer laser irradiation on the surface structure and luminescence of sintered ZnO ceramics were investigated. Photoluminescence spectra of three materials (as-prepared ZnO ceramic, the ceramic blackened by high-energy irradiation and the recovered ceramic treated using lower-energy irradiation) were compared in the visible region at room temperature and 10 K. Each material exhibited a green luminescence band but the blackened ZnO ceramics had the strongest emission in the visible region while the recovered ceramics had significantly shorter decay times.     

Er^3＋／Yb^3＋共掺杂ZnO粉末的上转换发光特性

采用高温氧化法制备了Er3 /Yb3 共掺杂ZnO粉。通过X射线衍射和扫描电镜对其进行了成分和组织结构分析,发现样品主要由ZnO和YbF3组成。在ZnO中测量到少量Er3 和Yb3 ,而YbF3中无Er3 ,故发光主要是由ZnO产生的。在980nm半导体激光器的激发下,观察到由处于激发态能级4F9/2,4S3/2,2H11/2和2H9/2的Er3 离子向基态4I15/2跃迁时发出的波长依次为658,538,522和409nm的上转换发光。在488nmAr 激光器的激发下,观察到了较强的409nm的紫光,466和450nm的弱蓝光以及379nm的紫外光,分别对应于Er3 离子的2H9/2→4I15/2,2P3/2→4I11/2,4F3/2/4F5/2→4I15/2,4G11/2→4I15/2等跃迁。上转换发光强度随激发功率的变化关系表明,488nm激发下紫色上转换荧光为双光子过程,主要是通过Er3 /Yb3 离子间正向和反向的能量传递来实现的。     

紫外光激发下氧化锌纳米线的发光特性研究

室温条件下,用355 nm的激光激发氧化锌纳米线,测量了其发光光谱.观察到半宽度较小、峰值波长约382 nm的紫光峰和半宽度较宽、峰值波长约507 nm的绿光峰;两峰的发光强度随激发光功率密度的变化而变化,且均存在饱和效应,但各自的变化规律及饱和值的大小不同;紫光峰的中心波长随激发光功率密度的增加而发生了明显的红移.对两峰产生的机理、强度饱和值存在的原由、强度随激发光功率密度变化及紫光峰红移的起因进行了分析.     

Up-conversion luminescence tuning in Er~(3+) -doped ceramic glass by femtosecond laser pulse at different laser powers

The up-conversion luminescence tuning of rare-earth ions is an important research topic for understanding luminescence mechanisms and promoting related applications. In this paper, we experimentally study the up-conversion luminescence tuning of Er~(3+)-doped ceramic glass excited by the unshaped, V-shaped and cosine-shaped femtosecond laser field with different laser powers. The results show that green and red up-conversion luminescence can be effectively tuned by varying the power or spectral phase of the femtosecond laser field. We further analyze the up-conversion luminescence tuning mechanism by considering different excitation processes, including single-photon absorption(SPA), two-photon absorption(TPA), excited state absorption(ESA), and energy transfer up-conversion(ETU). The relative weight of TPA in the whole excitation process can increase with the increase of the laser power, thereby enhancing the intensity ratio between green and red luminescence(I_(547)/I_(656)). However, the second ETU(ETU2) process can generate red luminescence and reduce the green and red luminescence intensity ratio I_(547)/I_(656), while the third ESA(ESA3) process can produce green luminescence and enhance its control efficiency. Moreover, the up-conversion luminescence tuning mechanism is further validated by observing the up-conversion luminescence intensity, depending on the laser power and the down-conversion luminescence spectrum under the excitation of 400-nm femtosecond laser pulse. These studies can present a clear physical picture that enables us to understand the up-conversion luminescence tuning mechanism in rare-earth ions, and can also provide an opportunity to tune up-conversion luminescence to promote its related applications.     

纳米ZnO薄膜的光致发光性质

利用溶胶－凝胶法制备了纳米ZnO薄膜,室温下测量了样品的光致发光谱(PL)、吸收谱(ABS)、X射线衍射谱(XRD)．X射线衍射(XRD)的结果表明：纳米ZnO薄膜呈多晶状态,具有六角纤锌矿晶体结构和良好的C轴取向．观察到二个荧光发射带,中心波长分别位于395 nm的紫带、524 nm的绿带和450 nm附近的蓝带．证实了纳米ZnO薄膜绿光可见发射带来自氧空位(VO)形成的浅施主能级和锌空位(VZn)形成的浅受主能级之间的复合;450 nm附近的蓝带来自电子从VO的浅施主能级到价带顶或锌填隙(Zni) 到价带顶或导带底到VZn的浅受主能级的复合．     

Optical and structural properties of ZnO nanorods grown by pulsed laser deposition without a catalyst

Pulsed laser deposition without a catalyst is used to grow ZnO nanorods less than 10 nm in diameter. The structure of the rods is studied by Raman scattering during excitation in the visible and UV regions. The temperature dependences of exciton spectra and the behavior of green luminescence are investigated in the temperature range 10–280 K. At room temperature, the luminescence intensity of the ZnO nanorods in the exciton region is higher than the green luminescence intensity by a factor of 7.8.     

The features of nonlinear absorption during resonance one- and two-photon excitation of the basic exciton transition in colloidal CdSe/ZnS quantum dots

The features of the nonlinear absorption of CdSe/ZnS quantum dots (colloidal solution) in the case of resonant one- and two-photon excitation of the basic exciton transition by powerful ultra-short laser pulses were determined. In one-photon excitation, with an increasing intensity of impulses, a decrease in absorption (bleaching) is relayed by an increase in absorption, which is associated with the process of the filling of the states (saturation) of a two-level system with the lifetime of the excited state depending on the light intensity. The arising Fresnel or Fraunhofer diffraction of the laser ray that pass through a colloidal solution with a high concentration of quantum dots is associated with the formation of the transparency channel and self-diffraction of laser ray on an induced diaphragm. In two-photon excitation, the features of the nonlinear absorption and luminescence tracks (the dependence of luminescence intensity on distance) were explained by the influence, in addition to the two-photon absorption, of the processes that are responsible for the slower growth of nonlinear absorption and luminescence quenching at high intensities of laser pulses.     

Femtosecond laser induced defects in various silica glasses

The mechanism of near infrared (IR) focused femtosecond (fs) laser induced defects in silica glasses produced by different methods is systematically investigated through measurements of absorption,fluorescence, and electronic spin resonance (ESR) spectra. The influence of impurities and hydroxyl groups on defects is discussed. The results show that ES silica glasses containing high OH and few defects are much stable under fs laser irradiation. It is also verified that Si E'δ center formation has no direct relation with chloride ions.     

Up-conversion luminescence polarization control in Er~(3+)-doped NaYF_4 nanocrystals

We propose a femtosecond laser polarization modulation scheme to control the up-conversion(UC) luminescence in Er~(3+)-doped NaYF_4 nanocrystals dispersed in the silicate glass. We show that the UC luminescence can be suppressed when the laser polarization is changed from linear through elliptical to circular, and the higher repetition rate will yield the lower control efficiency. We theoretically analyze the physical control mechanism of the UC luminescence polarization modulation by considering on- and near-resonant two-photon absorption, energy transfer up-conversion, and excited state absorption, and show that the polarization control mainly comes from the contribution of near-resonant two-photon absorption. Furthermore, we propose a method to improve the polarization control efficiency of UC luminescence in rare-earth ions by applying a two-color femtosecond laser field.