Bleaching Relaxation Research Articles

Boosting the Yield of MXene 2D Sheets via a Facile Hydrothermal-Assisted Intercalation.

Ti3C2T x (MXene) exhibits attractive properties in different applications. However, traditional synthesis leads to unsatisfactory yield of two-dimensional (2D) Ti3C2T x, e.g., lower than 20%, which stems from the strong interactions of potential Ti-Ti bonds and residual Ti-Al bonds between the adjacent Ti3C2 layers, hindering the effective intercalation and delamination. Herein, we propose a facile hydrothermal-assisted intercalation (HAI) strategy to boost the yield of 2D sheets, achieving a record high value of 74%. This HAI assists the diffusion and intercalation of reagent effectively, promoting the subsequent delamination; meanwhile, an antioxidant is applied to protect these Ti3C2T x from oxidation during the HAI process. Therefore, massive Ti3C2T x 2D sheets can be easily synthesized. Thanks to the synergistic effect of high conductivity and substantial terminated functionalities, these Ti3C2T x 2D sheets show promising application in supercapacitor, providing a high capacitance of 482 F g-1. Besides, the ultrafast carrier dynamics results of Ti3C2T x 2D sheets clearly imply the promising application in photocatalysis due to the relatively long bleaching relaxation time. Our work not only paves the way for the mass production of Ti3C2T x 2D sheets but also provides insights into their electronic and optical properties.

Plasmon-Related Modification of Spectral Kinetic Properties of Copper Phthalocyanine Thin Films in the Presence of Silver Nanoparticles*

We have studied the relaxation dynamics for hot electrons in hybrid layered nanocomposites based on island o lms of silver and copper phthalocyanine. Using femtosecond kinetic spectroscopy, we have shown that the fast component of the bleaching relaxation kinetics in the spectral region corresponding to the absorption bands of copper phthalocyanine correlates with the recovery kinetics for surface plasmon resonance absorption of the silver nanoparticles. This suggests that the fast relaxing bleaching recorded in the absorption bands of copper phthalocyanine and its dynamics are not due to a change in the populations of the energy states of copper phthalocyanine but rather to a change in the state of the plasmon nanoparticles, as a result of their excitation by femtosecond pulses and subsequent relaxation. Introduction. The ultrafast optical response of semiconductor thin o lms and nanostructures is one of the important properties explaining why these materials are in demand and promising for designing hardware components for nanophotonics and optoelectronics. Until recently, the effect of plasmon nanoparticles on the properties of semiconductor thin-o lm components were analyzed mainly with a view toward the possibility of changing their spectrally selective characteristics, in particular in order to increase the efo ciency of active elements in solar batteries. It has been established that doping functional semiconductor layers with plasmon nanoparticles can substantially change their absorption and conductivity because of the near-o eld effects arising in such systems and also as a result of additional possibilities for energy and charge carrier transfer in such structures (1, 2). At the same time, we know (3, 4) that the surface plasmon resonance (SPR) absorption band is substantially transformed when the nanoparticles are excited by ultrashort laser pulses, acting on the electronic subsystem of metallic nanoparticles. It is logical to assume that the change in state of the nanoparticles should be re? ected in fast dynamics of the near-o eld distribution pattern and consequently in fast dynamics of surface-enhanced effects, in particular the effect of modio cation of the absorption spectrum of copper phthalocyanine (CuPc) in the presence of Ag nanoparticles. This may prove to be useful in development of new types of dynamic devices: modulators, switches. The aim of this work was to study the characteristic features of the appearance of fast charge carrier relaxation processes in non-steady state absorption spectra of layered hybrid nanocomposites based on island o lms of silver and CuPc when the Ag nanoparticles are excited by femtosecond laser pulses in the SPR absorption region. Sample Preparation Procedure. The experimental samples were prepared by successive thermal vaporization under vacuum of Ag and CuPc materials on glass and quartz substrates at room temperature. The residual gas pressure was 2·10 -3 Pa. The thicknesses of the deposited layers were monitored with a quartz sensor. The spectral characteristics of the experimental samples were recorded on a Cary 500 spectrophotometer. Three groups of samples were prepared: 1) planar Ag nanostructures (close packed monolayers); 2) CuPc thin o lms; 3) two-layer Ag/CuPc nanostructures (a close packed monolayer of Ag nanoparticles, covered with a CuPc o lm). Steady-State Absorption Spectra. In the absorption spectrum of an Ag monolayer with metal surface density

Formation and Passive Q-Switch Performance of Glass-Ceramics Containing Co<sup>2+</sup>-Doped Spinel Nanocrystals

Compositions and heat treatment conditions were determined at which absorption and luminescence properties of aluminosilicate transparent glass-ceramics are defined mainly by tetrahedrally coordinated Co2+ ions located in spinel nanocrystals. Optical properties of cobaltdoped aluminosilicate transparent glass-ceramics have been studied in visible – near infrared (IR) spectral range. It was demonstrated that the concentration of tetrahedrally coordinated Co2+ ions in the spinel nanocrystals is determined by the CoO content in the initial glass and is independent of the heat-treatment schedule. Absorption saturation and bleaching relaxation under excitation of the 4A2→4T1(4F) transition of tetrahedrally coordinated Co2+ ions were studied. The value of groundstate and excited state absorption cross-sections of tetrahedrally coordinated Co2+ ions was estimated. Comparative study of output pulse parameters of Q-switched Er:glass laser using cobaltdoped glass-ceramics and Co2+:MgAl2O4 single crystal is presented.

Relaxation Mechanism of Electrochromism of Tungsten-Oxide Film for Ultra-Multilayer Optical Recording Depending on Sputtering Conditions

A tungsten-oxide single-layer film can be colored densely by using dilute sulfuric acid as a liquid electrolyte instead of the multilayer electrochromic cell. The transmissivity contrast before and after coloration reaches 70% at a wavelength of 405 nm. The bleaching relaxation time of a colored tungsten-oxide single-layer film is in the order of 1000 hours and depends on the sputtering power.

Absorption saturation properties and laser Q-switch performance of Cr5+-doped YVO4 crystal

The ground-state and excited-state absorption cross sections of a Cr5+:YVO4 single crystal were determined by the intensity-dependant transmission measurements to be(2.0±0.4)×10-18 cm2 and less than 0.1×10-18 cm2 at 1.08 μm, respectively. The bleaching relaxation time of the Cr5+:YVO4 crystal was estimated to be 3.5±1.5 ns. Passive Q-switching of a diode-pumped Yb3+:YVO4 laser with the Cr5+:YVO4 saturable absorber was realized.

Nonlinear spectroscopy of PbS quantum-dot-doped glasses as saturable absorbers for the mode locking of solid-state lasers

The bleaching relaxation characteristics of PbS quantum dots with diameters in the range of 3.8–4.3nm synthesized in silicate and phosphate glasses have been studied under the conditions of femtosecond excitation. It has been observed that increase of pump fluence from 0.1to2.2mJ∕cm2 leads to the appearance of a second (slow) component in the bleaching relaxation kinetics and to the shortening of both the fast (from several tens of picoseconds to several picoseconds) and slow relaxation times. An absorption saturation fluence of ≈15μJ∕cm2 with a nonsaturable absorption coefficient of 2.5–3cm−1 at 1.05μm has been evaluated for the PbS quantum-dot-doped glasses studied. The nature of the nonsaturable absorption and the mechanism of the bleaching relaxation in PbS quantum dots have been discussed.

Nonlinear optical properties and Q-switch performance of silica glasses doped with Cu_xSe nanoparticles

Glasses containing copper selenide nanoparticles (CuxSe) reveal an intense absorption band peaking at 1 μm (1.24 eV). The transient bleaching and intensity-dependent transmission of silica glasses with CuxSe nanoparticles of different stoichiometry are studied with 1.08 μm (1.15 eV) picosecond pulses. The bleaching relaxation time decreases with a shift in the absorption band maximum to the lower photon energies. The dependence of absorption on the input energy of the pulses is saturationlike at the beginning of the plateau at ~40 mJ/cm2. Passive Q-switching of the Nd3+:KGd(WO4)2 laser at 1.067 μm is realized with the CuxSe-doped glasses for different x.

Intensity-dependent bleaching relaxation in lead salt quantum dots

Bleaching relaxation in lead salt (PbS) quantum dots (QDs) of various sizes and under different pump intensities has been studied. The observed bleaching relaxation features (particularly, shortening of the bleaching relaxation times with a decrease in the QD size and an increase of the pump-light intensity) are explained in the context of the proposed spectroscopic model. The model takes into account transitions of the excited-charge carriers both within the system of quantum-confined energy levels and the defect states of the QDs. The characteristic times of the direct electron-hole recombination, carriers trapping to the defect states, and their subsequent relaxation from these states as well as the cross sections of the ground-state and excited-state absorption of charge carriers can be evaluated from the experimental data for PbS QDs of different sizes and under different pumping conditions using this model.

Spin‐sensitive bleaching and spin relaxation in QWs

AbstractSpin‐sensitive saturation of absorption of infrared radiation has been investigated in p‐type GaAs QWs. It is shown that the absorption saturation of circularly polarized radiation is mostly controlled by the spin relaxation time of the holes. The saturation behavior has been investigated for different QW widths and in dependence on the temperature with the result that the saturation intensity substantially decreases with narrowing of QWs. Spin relaxation times were experimentally obtained by making use of calculated (linear) absorption coefficients for inter‐subband transitions.

Linear and nonlinear optical properties of cobalt-doped zinc aluminum glass ceramics

Optical properties of cobalt-doped zinc aluminosilicate transparent glass ceramics in the visible – near-infrared (IR) spectral range have been studied. It was shown that the absorption and luminescence properties of these glass ceramics are defined mainly by tetrahedrally coordinated Co2+ ions located in ZnAl2O4 gahnite nanocrystals formed in the glass ceramic matrix. The luminescence in the visible and near IR was assigned to the transitions from the T14(4P) to the A24 and T24 energy levels of tetrahedrally coordinated Co2+ ion. Absorption saturation and bleaching relaxation under excitation of the A24→4T1(4F) transition were studied. Luminescence decay and bleaching relaxation times lie in the ranges 25–200 and 500–800 ns, respectively, depending on the Co concentration. The value of the ground-state absorption cross section of tetrahedrally coordinated Co2+ ions in zinc aluminosilicate transparent glass ceramics was estimated to be 2.0×10−19 cm2 at the wavelength of 1.54 μm.

Glass doped with PbS quantum dots as a saturable absorber for 1-μm neodymium lasers

Saturable-absorber mode locking and Q switching of neodymium-doped lasers at 1.06 µm with a PbS-doped glass were demonstrated. Q-switched pulses of 0.3 µJ in energy and 15 ns in duration from a cw diode-pumped Nd3+:KGd(WO4)2 laser and ultrashort pulses of 18 µJ in energy and 70 ps in duration from a Nd3+:Y3Al5O12 laser were obtained. The saturation intensity of the PbS-doped glass (with an average semiconductor crystallite radius of 1.7 nm) was estimated to be 2.3 MW/cm2 at 1.06 µm. The bleaching relaxation time was measured to be 23 ps.

Nonlinear Optical Properties and Laser Applications of Copper Chalcogenide Quantum Dots in Glass

Absorption saturation and picosecond pump-probe measurements on oxidized copper selenide quantum dots in sol-gel glasses were carried out. The bleaching relaxation time was found to be ∼300 ps. The saturation intensities were estimated to be 2.2 MW/cm 2 at 1.06 μm and 7 MW/cm 2 at 1.54 μm for Cu 2 Se and Cu 2+x Se-doped glasses, respectively. Passive Q-switching of 1.06, 1.34, and 1.54 μm solid state lasers with copper selenide-doped glasses as saturable absorbers were demonstrated.

Relaxation dynamics of photoinduced exciton bleaching in polymers

Presented in this paper are systematic studies of photoinduced exciton bleaching dynamics in the polymers with nondegenerate ground state, such as polydiacetylene, polythiophene and polyaniline. From a three-level model, the photoexcitation and relaxation of the exciton bleaching in the polymers are simulated. The results show that the exciton bleaching decay is composed of two components, the fast component and the slow component. For the fast component, the speed of exciton bleaching decay depends on the way in which the excitons relax. When the relaxation of the exciton to the ground state is dominant, the polymers exhibit an ultrafast initial bleaching relaxation; when the relaxation of the exciton to the self-trapped exciton state is dominant, the polymers exhibit a slower initial bleaching relaxation than that in the former case. For the slow component, the exciton bleaching decay is due to the relaxation of the self-trapped excitons to the ground state. Using femtosecond time-resolved pump-probe technology, we measured the relaxation dynamics of the photoinduced exciton bleaching in the emeraldine base form of polyaniline. Fitting the experimental data to the theoretical model indicates that the fast component arises from the relaxation of excitons to the ground state (similar 100 fs) and the slow component arises from the relaxation of self-trapped excitons to the ground state (similar 30 ps).

CuInSSe-doped glass saturable absorbers for the passive mode-locking of neodymium lasers

By using a CuInS2xSe2(1−x)-doped glass as an intracavity saturable absorber, passive mode-locking of Nd:YAG and Nd:YAlO3 lasers is obtained. Ultrashort laser pulses are achieved with duration of 16 ps (for Nd:YAlO3 at 1.08 μm) and 36 ps (for Nd:YAG at 1.064 μm), respectively. The intensity dependent transmittance and transient absorption change measurements are performed on CuInS2xSe2(1−x)-doped glasses at 1.08 μm with the use of laser pulses of 15 ps width. The bleaching relaxation time is found to be ∼11 ps.

Microstructural and optical changes induced by X-ray irradiation of single crystals of sodium chlorate

Crystals of sodium chlorate, exposed to various levels of X-ray irradiation from a synchrotron source, have been examined using X-ray topography, optical microscopy and UV–VIS spectroscopy. Two principal effects were observed during irradiation. First, a yellow coloration due to increased optical absorption with peaks at 290 and 420 nm was detected after an X-ray dose of 0.5 MRad. This has been attributed to the formation of radiation-induced colour centres. Secondly, after an accumulated dose of 3.1 MRad, significant changes in X-ray topographic images were observed. These comprised of an increase in integrated intensity and a vertical compression of the images. Both effects are due to sample curvature (convex towards the X-ray source) resulting from depth-dependant generation of point defects during irradiation. Throughout the investigation no development of new dislocations was observed and no changes were detected in the configurations of existing dislocations. Over a period of 2 to 3 weeks after irradiation, substantial (but incomplete) relaxation of the sample curvature was observed, together with partial bleaching of the optical absorption. In addition, numerous cavities formed within the crystals, particularly in the vicinity of solvent inclusions. All of these changes occurred at room temperature (25°C) and again no variation in the density or configuration of dislocations was observed. The defect-related processes potentially responsible for the observed behaviour are discussed. While some interdependence between optical bleaching and either curvature relaxation or cavity growth cannot be ruled out, it is apparent that two completely different defect types are associated with the latter two effects.