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RECENT ADVANCES IN ULTRAFAST TIME-RESOLVED SCANNING TUNNELING MICROSCOPY
摘要： Making smaller and faster functional devices has led to an increasing demand for a microscopic technique that allows the investigation of carrier and phonon dynamics with both high spatial and temporal resolutions. Traditional optical pump–probe methods can achieve femtosecond temporal resolution but fall short in the spatial resolution due to the diffraction limit. Scanning tunneling microscopy (STM), on the contrary, has realized atomic-scale spatial resolution relying on the high sensitivity of the tunneling current to the tip-sample distance. However, limited by the electronics bandwidth, STM can only push the temporal resolution to the microseconds scale, restricting its applications to probe various ultrafast dynamic processes. The combination of these two methods takes advantages of optical pump–probe techniques and highly localized tunneling currents of STM, providing one viable solution to track atomic-scale ultrafast dynamics in single molecules and low-dimensional materials. In this review, we will focus on several ultrafast time-resolved STM methods by coupling the tunneling junctions with pulsed electric waves, THz, near-infrared and visible laser. Their applications to probe the carrier dynamics, spin dynamics, and molecular motion will be highlighted. In the end, we will present an outlook on the challenges and new opportunities in this field.
关键词： terahertz technique,femtosecond laser,Scanning tunneling microscopy,pump-probe technique,ultrafast dynamics.
更新于2025-09-19 17:15:36
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Sol-Gel Synthesis and Spectral Characterizations of (35-x)B2O3-65Bi2O3-x Fe2O3 Glass System
摘要： Sol-gel technique was used to prepare glasses of the (35-x)B2O3-65Bi2O3-x Fe2O3 (0.1≤ x ≤ 0.4) system (A1-A4: x = 0.1, 0.2, 0.3, 0.4). The local structure and symmetry around trivalent iron were determined by studying X-band powder EPR spectra at room temperature. The EPR spectra of trivalent iron in glass samples are characterized by a more intense line at g = 4.2 and a less intense line at g = 2.0. The EPR line at g = 4.2 is attributed to trivalent iron in rhombic octahedral environment. The line at g = 2.0 is because of two or more trivalent iron coupling through dipole-dipole interactions in distorted octahedral symmetry. The intensity of EPR lines is dependent of Fe2O3 content in the glass samples. At higher concentration of Fe2O3, EPR line at g = 4.2 is less intense whereas the line at g = 2.0 is more intense which is ascribed to the formation of clusters of trivalent iron. The electronic spectra of glass samples show two broad bands corresponding to d-d transition in the range 410-450 nm and in the range 530-570 nm, respectively which are assigned to trivalent iron in distorted octahedral environment.
关键词： Glass samples,Dipole-dipole interaction,Electronic spectra,Sol-gel technique
更新于2025-09-19 17:15:36
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Investigations on single crystal growth and nonlinear optical studies of 2-Hydroxy-1-naphthaldehyde
摘要： The optically good quality single crystals of 2-Hydroxy-1-naphthaldehyde (2H1N) were grown by vertical Bridgman method. The thermal behavior of the grown crystal was studied by thermogravimetric and differential thermal analysis (TG-DTA). The cell parameters of the grown 2H1N crystal were confirmed by single crystal X-ray diffraction (SXRD). The powder XRD (PXRD) was carried out to identify the crystalline planes. The nature of bonding and structure of 2H1N was analyzed using 1HNMR and 13CNMR. Functional groups of 2H1N crystal were confirmed by Fourier transform infrared (FTIR) spectral analysis. The optical transmittance was determined by the UV–Vis NIR spectral analysis and the grown crystal has good optical transparency in the visible region (300-900 nm). To analyze the third order nonlinear optical properties, z-scan study was carried out and the nonlinear optical parameters like refractive index (1.30 x 10-7cm2/W), absorption coefficient (0.0410-4cm/W) and the third order nonlinear optical susceptibility (1.90 x 10-6 esu) was determined. The optical limiting behavior and clamping measurements of 2H1N was 37.3 mW and 1.08 mW respectively. The dielectric constant (~12) and low dielectric loss as a function of frequency were measured to ensure the 2H1N has better optical application.
关键词： A2. Vertical Bridgman technique,A1. Crystal structure,B3. Nonlinear optics,B1. organic compounds,B2. Nonlinear optic materials
更新于2025-09-19 17:15:36
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Growth, Thermal and Spectroscopic Studies of Bis-thiourea Nickel Barium Chloride Single Crystals
摘要： Bis-thiourea nickel barium chloride was synthesized and crystals were grown by slow aqueous solvent evaporation technique. The powder XRD analysis of grown crystal was suggested to have orthorhombic crystal structure. The unit cell parameters of grown crystal as: a = 9.70 ?, b = 10.68 ? and c = 17.95 ?. The contents of nickel and barium in the grown crystals was analyzed using EDAX. The presence of various functional groups was confirmed by FTIR spectroscopy studies. From the UV-visible spectrum, the material has about 90 % optical transparency in the entire visible region. From the thermogravimetric analysis, it was found that the crystals remained stable up to 170 oC. The endothermic reactions were identified from the differential thermal analysis.
关键词： Crystal growth,Slow evaporation technique,Bis-thiourea nickel barium chloride,Powder X-ray diffraction
更新于2025-09-19 17:15:36
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Measuring of nonlinear properties of spatial light modulator with different wavelengths
摘要： The non-linear optical properties of Spatial Light Modulator(SLM) represented by Nonlinear Refractive Index (NLR) and nonlinear Absorption coefficient has been measured in this work using highly sensitive method known as Z-scan technique for different wavelengths (red and green). The capability to do instant measurements of different nonlinear optical parameters lead to consider these techniques as a one of the most desired and effective methods that could apply for different materials. The results showed that the NLR were in the same power for the different wavelengths while the nonlinear absorption is higher in case of green laser.
关键词： Z-scan technique,Nematic Liquid Crystal,SLM
更新于2025-09-19 17:15:36
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Studying the Effect of Deposition Time on Optical Properties of CdS Thin Films
摘要： We have studied in this work the effect of deposition time on the absorption spectra and the optical energy gap of CdS thin film. That prepared by chemical bath deposition method (CBD) on glass substrate at temperature of (80 ± 5)°C. The structural features were examined by X-ray diffraction (XRD) is wurtzite structure for all samples. Transmittance spectra have been recorded in order to determine absorbance, reflectivity, absorption coefficient and the optical constants such as refractive index, extinction coefficient, real and imaginary part of dielectric constant. It was seen that all the parameters under investigation affected by deposition time (1,2 3, and 4) hours.
关键词： Cadmium sulfide,optical constants,chemical bath technique
更新于2025-09-19 17:15:36
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Many-body States Description of Single-molecule Electroluminescence Driven by Scanning Tunneling Microscope
摘要： Electron transport and optical properties of a single molecule in contact with conductive materials have attracted considerable attention owing to their scientific importance and potential applications. With recent progresses of experimental techniques, especially by the virtue of scanning tunneling microscope (STM)-induced light emission, where the tunneling current of the STM is used as an atomic-scale source for induction of light emission from a single molecule, it becomes possible to investigate single-molecule properties at sub-nanometer spacial resolution. Despite extensive experimental studies, the microscopic mechanism of electronic excitation of a single molecule in STM-induced light emission is yet to be clarified. Here we present a formulation of single-molecule electroluminescence driven by electron transfer between a molecule and metal electrodes based on a many-body state representation of the molecule. The effects of intra-molecular Coulomb interaction on conductance and luminescence spectra are investigated using the nonequilibrium Hubbard Green's function technique combined with first-principles calculations. We compare simulation results with experimental data and find that the intra-molecular Coulomb interaction is crucial for reproducing recent experiments for a single phthalocyanine molecule. The developed theory provides a unified description of both electron-transport and optical properties of a single molecule in contact with metal electrodes driven out of equilibrium, and thereby it contributes to a microscopic understanding of optoelectronic conversion in single molecules on solid surfaces and in nanometer-scale junctions.
关键词： Single molecule luminescence,exciton formation,nonequilibrium Hubbard Green's function technique,time-dependent density functional theory (TDDFT),scanning tunneling microscope-induced light emission,Vibronic interaction
更新于2025-09-19 17:15:36
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Effect of Mg dopant on SnO <sub/>2</sub> thin films grown by spray pyrolysis technique
摘要： The undoped and magnesium (Mg)-doped Tin (IV) oxide (SnO2) thin films were grown on glass substrate by spray pyrolysis technique. In order to observe the effect on the optical, structural, morphological, and hydrogen (H2) gas answer properties of SnO2 by Mg doping, X-ray diffractometer (XRD), ultraviolet–visible (UV) spectrometer, scanning electron microscope (SEM) and hydrogen (H2) gas, measurements were taken. The absorption measurements of undoped and Mg-doped SnO2 thin films demonstrated that band gaps varied with the changing Mg dopant ratio and this variation may be from Burstein–Moss (BM) effect. XRD measurements showed that the samples were tetragonal structures and have (110), (101), (200), (211), (220), (310) planes. The surface morphology of SnO2 showed that samples was affected considerably by Mg dopant. The H2 gas sensor response improves with the increase of 1%, 2% and 3% Mg doping ratio in SnO2.
关键词： H2 sensor.,Mg-doped SnO2,spray pyrolysis technique
更新于2025-09-19 17:15:36
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New Evolutionary-Based Techniques for Image Registration
摘要： The work reported in this paper aims at the development of evolutionary algorithms to register images for signature recognition purposes. We propose and develop several registration methods in order to obtain accurate and fast algorithms. First, we introduce two variants of the firefly method that proved to have excellent accuracy and fair run times. In order to speed up the computation, we propose two variants of Accelerated Particle Swarm Optimization (APSO) method. The resulted algorithms are significantly faster than the firefly-based ones, but the recognition rates are a little bit lower. In order to find a trade-off between the recognition rate and the computational complexity of the algorithms, we developed a hybrid method that combines the ability of auto-adaptive Evolution Strategies (ES) search to discover a global optimum solution with the strong quick convergence ability of APSO. The accuracy and the efficiency of the resulted algorithms have been experimentally proved by conducting a long series of tests on various pairs of signature images. The comparative analysis concerning the quality of the proposed methods together with conclusions and suggestions for further developments are provided in the final part of the paper.
关键词： hybrid techniques,image recognition,image registration,firefly technique,evolutionary computing,affine perturbation,evolution strategies,mutual information
更新于2025-09-19 17:15:36
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A multi verse optimization based MPPT controller for drift avoidance in solar system
摘要： This paper proposes Multi verse optimization (MVO) based MPPT controller to mitigate the possibility of losing tracking direction in solar photovoltaic system under variable irradiance. Generally conventional Perturb & Observe and hill climbing MPPT techniques are used due to effortless implementation. However, these techniques are not capable of handling oscillations in power at MPP and exhibit drift under variable irradiance conditions which leads to power loss. Therefore a hybrid of standard MVO and direct duty cycle control is proposed to minimize the inadequacies occurring in conventional controllers. Three cases i.e. constant irradiance, rapid and step changes in irradiance are considered for the analysis. The supremacy of proposed method is justified by comparing it with traditional P&O, Particle swarm optimization (PSO) based MPPT and Grey wolf optimization (GWO) based MPPT techniques. It is observed from the results that MVO based MPPT controller is capable of avoiding drift and offers fast convergence. Therefore proposed controller outperforms in terms of tracking efficiency, settling time, peak overshoot, and integral absolute error.
关键词： multi verse optimization,MPPT,optimization technique,Solar PV system
更新于2025-09-19 17:15:36