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Observation of evolution dynamics from bound states to single-pulse states in a passively mode-locked fiber laser
摘要： We report what is, to our knowledge, the first experimental observation of the ultrafast evolution dynamics from bound states (BSs) to single-pulse states (SPSs) by using the dispersive Fourier-transform (DFT) technique. The evolutions from three categories of initial BSs to SPSs are spectrally resolved in real time. Usually, accompanied by complex soliton–soliton interaction and competition, one of the two bound pulses weakens to disappearance, and the other one evolves into SPS. During the transition, the two bound pulses ordinarily depart away from each other with complex changes of relative phase. However, it is found that not all the evolutions are accompanied by the increase of temporal separation between two bound pulses. The obtained results would facilitate a deep understanding of complex dynamics in nonlinear systems and provide valuable data for further theoretical studies.
关键词： single-pulse states,bound states,fiber lasers,Ultrafast evolution dynamics
更新于2025-11-28 14:24:03
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A tunable external cavity laser operating at excited states of bimodal-sized quantum-dot
摘要： In this paper, a tunable external cavity diode laser (ECDL) based on the first excited states of two independent quantum dot (QD) ensemble has been demonstrated. The device exhibits stabilized lasing with high power of 120 mW. We attribute the excellent output characteristics of the QD ECDL to carrier radiation recombination in high energy states, which has higher degeneracy than low energy states and can accommodate more carriers. The linewidth of ECDL is less than 0.2 nm, which is also much narrower than most of QD ECDLs as reported. Preliminary discussion attributes the spectra feature to the lower QD density and independent carrier transitions. The tunable range is 28.9 nm (970.1 ~ 999 nm) and the wavelength shift with the injection current is 0.7 nm/A.
关键词： tunable external cavity laser,bimodal-sized,quantum-dot,excited states
更新于2025-11-28 14:23:57
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Cell Imaging Using Two-Photon Excited CdS Fluorescent Quantum Dots Working within the Biological Window
摘要： In recent years, two-photon excited semiconductor quantum dots (QDs) have been the subject of intense investigation due to their long excitation wavelength which helps to achieve deeper penetration and higher image resolution in optical bioimaging. In this paper, water-soluble CdS QDs were synthesized using a hydrothermal method and applied to human liver hepatocellular carcinoma (HepG2) cells. The first-principles calculation suggested that the S-rich defected structure contributes to a narrower band gap compared to the pristine structure. The resulting fluorescence wavelength was significantly red shifted, which was attributed to the deep defect states emission. The large Stokes shifts (> 200 nm) of the QDs can eliminate the possible cross-talk between the excitation light and the emission light. Two-photon induced red fluorescence emission can avoid overlapping with the autofluorescence emission of biological samples. The uptake and cell viability measurements of the HepG2 cells showed a good biocompatibility and a low toxicity of CdS QDs. Two-photon excited scanning microscopy images revealed that the HepG2 cells incubated with CdS QDs emitted bright red upconversion fluorescence and the fluorescence brightness was 38.2 times of that of the control group. These results support CdS QDs as a good candidate for application in cellular imaging.
关键词： two-photon absorption,CdS quantum dots,deep defect states,HePG2 cells,biological imaging
更新于2025-11-21 11:24:58
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Synthesis, Morphology, Optical and Electrical Properties of Cu <sub/> 1? <i>x</i> </sub> Fe <sub/><i>x</i> </sub> O Nanopowder
摘要： The pure and Fe-doped CuO nanoparticles of the series Cu1?xFexO (x = 0, 0.027, 0.055, 0.097 and 0.125) were synthesized by a simple low temperature sol–gel method. Synthesized samples were characterized by a series of techniques including Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive X-ray electron spectroscopy (EDX), Diffuse Reflectance Spectroscopy (DRS), Fourier Transform Infrared Spectroscopy (FTIR), Hall Effect Set-up and Current–Voltage (I–V) characteristics. FESEM analysis shows formation of disc type structure increasing in grain size with Fe concentration in CuO. EDX confirmed the incorporation of iron in CuO. FTIR results of pure and Fe doped CuO samples have confirmed the formation of monoclinic CuO. The optical band gap estimated using Diffuse Reflectance Spectroscopy (DRS) shows the increment in the band gap values with Fe substitution. The Hall measurements show predominantly p-type conduction in all the samples and carrier densities decrease with increased Fe substitution. I–V characteristics of pure and Fe doped CuO nanoparticles show rectification behaviour of Schottky diodes.
关键词： Defect States,Hall Effect,Schottky Diode,Cation Vacancies,Fe-Substituted CuO
更新于2025-11-19 16:56:35
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High lying energy of charge-transfer states and small energetic offsets enabled by fluorinated quinoxaline-based alternating polymer and alkyl-thienyl side-chain modified non-fullerene acceptor
摘要： Significant driving forces are the prerequisite to achieve fast and efficient charge separation in fullerene derivatives-based polymer solar cells to achieve high power conversion efficiency (PCE). However, the large driving forces both in photo-induced hole transfer (PHT) and in photo-induced electron transfer (PET) processes lead to significant energy losses, resulting in low open-circuit voltage in the devices. Recent studies indicate the driving forces in non-fullerene acceptors-based devices can be reduced to very low values but still with high PCE and low energy losses. Herein, we report a new donor:acceptor system with high lying energy of charge-transfer excitons (ECT) of 1.50 eV and very small driving forces (PHT of 0.28 eV and PET of 0.11 eV), in which a fluorinated quinoxaline-based alternating polymer (FTQ) and an alkyl-thienyl side-chain modified small molecule (ITIC-Th) are taken as the donor material and non-fullerene acceptor material, respectively. A high power conversion efficiency (PCE) of 8.19% with maximal external quantum efficiency of 71% are achieved successfully in FTQ:ITIC-Th-based device after appropriate thermal annealing treatment, indicating FTQ can be further applied as donor materials with other highly efficient NF-acceptors to achieve enhanced performances and low energy losses.
关键词： Power conversion efficiency,Driving forces,Energy of charge-transfer states,Polymer solar cells
更新于2025-11-14 17:28:48
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Determination of physical mechanism responsible for the capacitance-voltage weak inversion “hump” phenomenon in n-InGaAs based metal-oxide-semiconductor gate stacks
摘要： Weak inversion capacitance-voltage (C-V) “hump” is a widely observed phenomenon at n-InGaAs based metal oxide semiconductor (MOS) structures. The mechanism responsible for this phenomenon is still under discussion. The C-V hump can be explained as an interaction of interface states with either one or both semiconductor energy bands. Each of the proposed mechanisms leads to a different interpretation of C-V hump. Simulating the mechanisms by relevant equivalent circuits, the capacitance and conductance characteristics of the MOS structure were calculated and compared with experimental results. The mechanism responsible for the C-V hump was determined.
关键词： interface states,equivalent circuits,n-InGaAs,metal-oxide-semiconductor,capacitance-voltage hump
更新于2025-11-14 17:28:48
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Mechanical analysis of the interface bonding state of a TiO2 film/Si substrate
摘要： TiO2 film is prepared on the surface of a natural oxide layer of a monocrystalline silicon substrate via RF magnetron sputtering. HRTEM (High Resolution Transmission Electron Microscope) imaging and EDX spectroscopy are performed on the sample interface, demonstrating that the film sample has a two-layer structure. Combined with XRD, the analysis shows that the upper film is a crystalline TiO2 film with a thickness of approximately 30 nm and that the lower film is a natural amorphous SiO2 oxide film with a thickness of approximately 22 nm. A geometric phase analysis (GPA) and Hooke's law are used to analyse the stress and bonding state at the interfaces between the monocrystalline silicon substrate and the natural oxide layer, between rutile TiO2 and the natural oxide layer, and between anatase TiO2 and the natural oxide layer. It is concluded that the interface bonding state of the monocrystalline silicon/natural oxide layer is good and that the interface bonding state at the interface between rutile and the natural oxide layer is better than that between anatase and the natural oxide layer.
关键词： geometric phase analysis,Hooke's law,natural oxidation,interface bonding states,TiO2
更新于2025-11-14 17:04:02
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Experimental Observation of Bound States of 2D Dirac Electrons at Surface Steps of the Topological Insulator Bi <sub/>2</sub> Se <sub/>3</sub>
摘要： Topologically protected surface states of three-dimensional topological insulators provide a model framework for studying massless Dirac electrons in two dimensions. Usually a step on the surface of a topological insulator is treated as a scatterer for the Dirac electrons, and the study of its effect is focused on the interference of the incident and scattered electrons. Then a major role plays the warping of the Dirac cone far from the Dirac point. The existence of another significant effect near the Dirac point brought about by the presence of steps is experimentally demonstrated here. Namely the band bending in the vicinity of steps leads to formation of 1D bound states in the corresponding potential wells. The observation of bound states in such potential wells in our scanning tunneling microscopy and spectroscopy investigation of the surface of the topological insulator Bi2Se3 is reported. Numerical simulations support our conclusion and provide a recipe for the identification of such states.
关键词： density of states,Dirac electrons,surface states,edge states,bound states,scanning tunneling microscopy,Bi2Se3,topological insulators,scanning tunneling spectroscopy
更新于2025-09-23 15:23:52
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Electron properties of topological insulators. The structure of edge states and photogalvanic effects
摘要： Integrating the ideas of topology and topological transitions into solid-state physics has led to the theoretical prediction and subsequent experimental discovery of topological insulators, a new class of three- or quasi-two-dimensional dielectric crystalline systems exhibiting stable conducting surface states. This paper briefly reviews the electronic properties of topological insulators. The structure of edge and bulk electronic states in two- and three-dimensional HgTe-based topological insulators is described in particular detail. Recent theoretical and experimental results on the interaction of an electromagnetic field with topological insulators and on edge and surface photogalvanic effects are presented.
关键词： photogalvanic effects,topological insulators,edge and surface states
更新于2025-09-23 15:23:52
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Current linearity and operation stability in Al <sub/>2</sub> O <sub/>3</sub> -gate AlGaN/GaN MOS high electron mobility transistors
摘要： To investigate current linearity and operation stability of metal–oxide–semiconductor (MOS) AlGaN/GaN high electron mobility transistors (HEMTs), we have fabricated and characterized the Al2O3-gate MOS-HEMTs without and with a bias annealing in air at 300 °C. Compared with the as-fabricated (unannealed) MOS HEMTs, the bias-annealed devices showed improved linearity of ID–VG curves even in the forward bias regime, resulting in increased maximum drain current. Lower subthreshold slope was also observed after bias annealing. From the precise capacitance–voltage analysis on a MOS diode fabricated on the AlGaN/GaN heterostructure, it was found that the bias annealing effectively reduced the state density at the Al2O3/AlGaN interface. This led to efficient modulation of the AlGaN surface potential close to the conduction band edge, resulting in good gate control of two-dimensional electron gas density even at forward bias. In addition, the bias-annealed MOS HEMT showed small threshold voltage shift after applying forward bias stress and stable operation even at high temperatures.
关键词： current linearity,interface states,MOS-HEMT,AlGaN/GaN,operation stability,bias annealing
更新于2025-09-23 15:23:52