Two-photon confocal microscopy as a tool for nonequilibrium charge-carrier lifetime tomography in semiconductor materials

By the example of ZnSe crystals, the capabilities of two-photon confocal microscopy as a tool for obtaining “planar” maps of nonequilibrium charge-carrier lifetimes in semiconductor materials and for investigating other direct-gap semiconductors and semiconductor heterostructures are considered. It is shown that such maps with a depth step and an in-plane resolution of several microns can be obtained for distances from the surface up to 1 mm. This technique is used to visualize inhomogeneities in the crystals under study and to examine their structure and luminescence characteristics.     

Two-photon spectroscopy and microscopy of II–VI semiconductor nanocrystals

We review our recent nonlinear spectroscopies of nanocrystals and synthetic efforts to improve their luminescence properties. A two-photon spectroscopic study of CdSe nanocrystals as a function of size is presented and compared with predictions from the effective mass model with spherical confinement. We also detail our efforts at improving the luminescence properties of nanocrystals which have culminated in a 50% fluorescence quantum yield for inorganic capping of some sizes of CdSe nanocrystals. Finally, we present the application of two-photon microscopy to resolve fluorescence from single nanocrystals at room temperature and cryogenic temperatures.     

Distribution of Blue and Green Luminescence Centers in Barrier Regions of the Grains of Zinc Sulfide Electroluminophors

Based on the investigation of the spectra of two-photon excited luminescence of powder zinc sulfide electroluminophors, their electroluminescence spectra measured under different conditions at different stages of degradation, and also the data known from the literature dealing with the study of the microstructure of ZnS:Cu crystals, a conclusion has been drawn that the centers of green and blue luminescence are nonuniformly distributed over the luminophor grains. It is presumed that close by the surface of the grains, near the second phase–zinc–sulfide interface (the regions of energy barriers responsible for impact multiplication of charge carriers in prebreakdown electroluminescence) the centers of green luminescence predominate, while the blue luminescence centers are located at the periphery of the barriers, in deeper regions of the crystals.     

Applications of cathodoluminescence in mineralogy

Cathodoluminescence microscopy and cathodoluminescence spectrometry with the electron microprobe and the scanning electron microscope are applied to the study of some natural cassiterite, quartz and sphalerite crystals. Some examples of localization and distribution of crystalline defects revealed by cathodoluminescence microscopy are given. X-ray spectrometry and secondary ion mass analysis allow to illustrate the coincidence between the spatial variations of cathodoluminescence emission and the distribution of impurities within minerals. In order to establish a relationship between the spectral distribution of the luminescence and the nature of trace elements, comparisons are made between cathodoluminescence properties of natural and artificial zinc sulphide crystals. Particular attention is given to specimen containing copper-gallium impurities. It has been possible to distinguish between the impurities which can be responsible for the luminescence and those which are not associated with cathodoluminescence emission characteristic of the crystals. The study of cathodoluminescence intensity according to the excitation conditions led to consider the contribution of the X-ray photons generated in the specimen for the production of luminescence photons. An in-depth distribution law of generated photons similar to that used to take account of X-ray emission led to introduce an absorption function in the relation giving the measured intensity of cathodoluminescence according to the incident energy and the electron beam density.     

Polarization dependence of luminescence induced by two-photon excitation in LiF crystals with F 3 + laser color centers and the symmetry of the excited state

The energy level structure of F ₃ ⁺ laser color centers in crystals of LiF is discussed. A high-power laser (λ _ex=920 nm) is used to excite luminescence from LiF crystals with F ₃ ⁺ centers via two-photon absorption, and the dependence of the polarization and intensity of this luminescence on the polarization of the laser light is measured and calculated. It is shown that the two-photon transition involves the excitation of a previously unknown state of the F ₃ ⁺ center—a spin singlet whose wave function has ¹ A ₁ symmetry. Fiz. Tverd. Tela (St. Petersburg) 39, 1373–1379 (August 1996)     

PbWO<Subscript>4</Subscript> crystal for laser energy accumulation and transformation

The picosecond interband two-photon laser excitation of PbWO₄ crystals at a temperature of 10 K leads to electronic excitation energy accumulation, which results in almost 100% induced absorption in the 450–750 nm spectral range. The relaxation time of this induced absorption exceeds 100 min. The electronic excitation energy accumulated in the PbWO₄ crystal at T = 10 K excites the intrinsic luminescence with a decay time longer than 45 min. The decay kinetics and the spectra of the intrinsic luminescence of the PbWO₄ crystal at a temperature of 10 K were measured under two-photon and single-photon excitation. The luminescence under two-photon and single-photon excitation revealed a difference in the structure of the spectra.     

Two-photon absorption in CdS by low power CW laser

Our aim is to study the two-photon induced luminescence in CdS crystals by using a low power CW laser (5 mW). We discuss briefly the possibility given by the electron-pulse-counting technique to detect weak optical signals. This technique enables us to investigate the luminescence spectra (which show two bands at 2.4 and 2.0 eV) and the dependence of the luminescence intensity on the laser power. The discussion of the experimental results makes it clear that the level responsible for the red emission is partially saturated at a laser intensity of 10²³ photon/cm² s.     

Multicontrast nonlinear optical microscopy with a compact and rapid pulse shaper

Homodyne detection can dramatically enhance measurement sensitivity for weak signals. In nonlinear optical microscopy it can make accessible a range of novel, intrinsic, contrast like nonlinear absorption and nonlinear phase contrast. Here a compact and rapid pulse shaper is developed, implemented, and demonstrated for homodyne detection in nonlinear microscopy with high-repetition rate mode-locked femtosecond lasers. With this method we generate two-photon absorption (TPA) and self-phase modulation images of gold nanostars in biological samples. Simultaneous imaging of two-photon luminescence and TPA also enables us to produce two-photon quantum yield images.     

Two-photon optical characteristics of zinc oxide in bulk, low dimensional and nanoforms

We report the two-photon luminescence (TPL) and second-harmonic generation (SHG) characteristics of zinc oxide (ZnO) in ceramic, thick film and nanorod. Sintering, physical vapour deposition (PVD), and hydrothermal methods were used in preparing the three forms of samples, respectively. Wide-band luminescence in the visible wavelength was observed with green/yellow emission in ceramic and nano-crystal, and blue emission in the film. We attributed interstitial zinc and interstitial oxygen for the luminescence at blue and green bands. Images acquired by TPL and SHG microscopy are presented, interpretation is provided for granular structure exhibiting strong SHG at granular boundaries.     

Two-photon spectroscopy of Hg2Cl2 crystals

Two-photon absorption of Hg₂Cl₂ crystals was investigated using a pulsed dye laser at T ω 8.5 K. Both the indirect method of monitoring the luminescence produced by two-photon absorption and the two-photon attenuation measurements of the laser beam were used. The excitation spectrum of the ∼ 3.13 eV (396 nm) luminescence was measured in the energy range 4.0 ⩽ 2hv ⩽ 5.5 eV. A strong maximum at 2hv ω 5.13 eV dominates the spectrum. Comparison with previous one-photon absorption data allows us to assign the parity of electronic states involved in the transitions. The band gap of Hg₂Cl₂ is found to be direct allowed.     

Simulating resonance-mediated two-photon absorption enhancement in rare-earth ions by a rectangle phase modulation

Improving the up-conversion luminescence efficiency of rare-earth ions via the multi-photon absorption process is crucial in several related application areas. In this work, we theoretically propose a feasible scheme to enhance the resonance-mediated two-photon absorption in Er~(3+) ions by shaping the femtosecond laser field with a rectangle phase modulation. Our theoretical results show that the resonance-mediated two-photon absorption can be decomposed into the on-resonant and near-resonant parts, and the on-resonant part mainly comes from the contribution of laser central frequency components, while the near-resonant part mainly results from the excitation of low and high laser frequency components.So, the rectangle phase modulation can induce a constructive interference between the two parts by properly designing the modulation depth and width, and finally realizes the resonance-mediated two-photon absorption enhancement. Moreover, our results also show that the enhancement efficiency of resonance-mediated two-photon absorption depends on the laser pulse width(or laser spectral bandwidth), final state transition frequency, and intermediate and final state absorption bandwidths. The enhancement efficiency modulation can be attributed to the relative weight manipulation of on-resonant and near-resonant two-photon absorption in the whole excitation process. This study presents a clear physical insight for the quantum control of resonance-mediated two-photon absorption in the rare-earth ions, and there will be an important significance for improving the up-conversion luminescence efficiency of rare-earthions.     

Spectroscopic and lasing properties of disordered Yb3+-doped crystals

The luminescence properties and luminescence decay kinetics of disordered Yb³⁺-doped crystals of various oxides are investigated and the possibility of their use as active laser media in the near-IR region (in the vicinity of 1 μm) under laser diode pumping is established. Three groups of disordered oxide crystals were analyzed: calcium niobium gallium garnet, yttrium-stabilized zirconia, and double tungstates andmolybdates with scheelite structure (sodium gadolinium tungstate, sodium gadolinium molybdate, and sodium lanthanum molybdate). These compounds are characterized by large integrated cross sections of energy-level transitions of rare earth ions. The effect of degree of disorder on the spectroscopic and lasing properties of disordered oxide crystals is revealed. The results of the study show the possibility of lasing in all crystals studied and application potential of Yb³⁺-doped disordered crystals for developing new laser media.     

Pico-second spectroscopy of the secondary radiation from resonantly excited excitonic molecules in CuCl

Pico-second time-resolved measurements of the two-photon resonant Raman scattering via excitonic molecules in CuCl were carried out for the first time. The Raman scattering leaving transverse excitons decays as fast as the laser light. When the energy of the incident light falls in the vicinity of the two-photon resonant absorption, both Raman and luminescence lines are simultaneously observed. In case of just resonant excitation, the transient response indicates that the secondary radiation can be decomposed into the Raman and the luminescence components as regards its temporal and spectral behavior.     

Luminescence of the cadmium telluride quantum dots in the porous silicon oxide

The luminescence of the CdTe quantum dots deposited on glass substrate and introduced into the porous silicon oxide matrix is investigated. The experimental results on the photoluminescence with one- and two-photon excitation and cathodoluminescence indicate that the quantum dots introduced into the matrix retain the luminescence properties. The coefficient of the two-photon absorption of the CdTe quantum dots on the glass substrate is determined.     

Fabrication of a micro-optical lens using femtosecond laser 3D micromachining for two-photon imaging of bio-tissues

We report, for the first time to our knowledge, on the application of a micro-optical lens fabricated by three-dimensional (3D) femtosecond laser direct writing for two-photon fluorescence imaging of biological tissues. We show that the two-photon fluorescence images of a plant leaf tissue acquired with the micro-optical lens are comparable to that of a 5× objective lens. Our result represents an important step towards the application of micro-optical components fabricated by femtosecond laser micromachining in miniaturized nonlinear fluorescence microscopy applications, such as two-photon endoscopy.     

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.     

Thermally and optically stimulated radiative processes in LiBaF3 crystals

In LiBaF₃ crystals both valence–core transitions (5.4–6.5 eV) and so-called self-trapped exciton luminescence (about 4.3 eV) are important for practical application. Here, we present a study of 4.3 eV luminescence under photo- and thermostimulation after X-irradiation of undoped LiBaF₃ crystals at various temperatures. Optically stimulated luminescence as a result of electron recombination with both self-trapped holes and holes localized at some defects, were observed after X-irradiation below 130 K and that of electron recombination with defect-localized holes was observed after X-irradiation above 130 K. The spectra of thermo-stimulated luminescence (TSL) contain a broad band about 4 eV related to the electron (high-energy side) or hole (low-energy side) recombination depending on TSL peak temperature.     

Electronic structure calculations of lead tungstate PbWO 4 crystals with defects of molybdenum impurity

Lead tungstate PbWO 4 crystals are one of the most effective scintillation materials for calorimetric devices designed to detect elementary particles with extremely high energies [1]. The interest to PbWO 4 scintillation and luminescence properties increased noticeably in the recent years [1, v 2]. However, experimental results obtained for PbWO 4 optical properties, substantially differ for crystals, produced under different growing conditions. Such a variety led to the situation, that up to now there are no generally accepted explanations for the origin of luminescence centres in PbWO 4 . The electronic structure of possible luminescent centres in perfect lead tungstate crystals PbWO 4 and in the crystals with molybdenum impurity PbWO 4 :Mo is ab-initio calculated in order to elucidate the origin of luminescence in lead tungstate crystals. Conclusions concerning excitation of self luminescence in perfect crystals and defect luminescence in Mo-doped crystals are made on the basis of results of calculations and experimental data on luminescence and photo excitation of PbWO 4 and PbWO 4 :Mo crystals.     

低强度飞秒激光激发下CdTe和CdTe/CdS核壳量子点的荧光上转换研究

利用400 nm和800 nm不同波长的低强度飞秒激光,对CdTe和CdTe/CdS核壳量子点溶胶进行激发,研究其稳态和时间分辨荧光性质.800 nm飞秒激光激发下,CdTe和CdTe/CdS核壳量子点产生上转换发光现象,上转换荧光峰与400 nm激发下的荧光峰相比蓝移最多达15 nm,而且蓝移值与荧光量子产率有关.变功率激发确认激发光功率与上转换荧光强度间满足二次方关系,时间分辨荧光的研究表明荧光动力学曲线服从双e指数衰减.提出表面态辅助的双光子吸收模型是低激发强度上转换发光的主要机理.CdTe和CdT 关键词： CdTe量子点 CdTe/CdS核壳量子点 时间分辨荧光 上转换荧光     

加偏压中心对称双光子光折变晶体中多变量空间灰孤子

 为了得到有偏压的中心对称双光子光折变晶体中存在多变量空间灰孤子的结果,基于中心对称双光子光折变晶体中空间灰孤子的基本理论,采用数值方法推导出了中心对称双光子多变量空间灰孤子归一化包络解的积分形式,并对其特性进行研究。结果表明：这种多变量空间灰孤子是由多束偏振方向和波长都相同的互不相干光束耦合形成的。当多变量空间灰孤子只包含有1个或2个光束分量成分时,它自动退化到中心对称双光子空间灰孤子或中心对称双光子非相干耦合灰 灰空间孤子对的情况。当这一多变量空间灰孤子在有偏压的中心对称双光子光折变晶体中传播时,各分量成分光束都能稳定传播。