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Recent progress in ultrafast lasers based on 2D materials as a saturable absorber
摘要： Two-dimensional (2D) materials are crystals with one to a few layers of atoms and are being used in many fields such as optical modulator, photodetector, optical switch, and ultrafast lasers. Their exceptional optoelectronic and nonlinear optical properties make them as a suitable saturable absorber for laser cavities. This review focuses on the recent progress in ultrafast laser use 2D materials as a saturable absorber. 2D materials traditionally include graphene, topological insulators, transition metal dichalcogenides, as well as new materials such as black phosphorus, bismuthene, antimonene, and MXene. Material characteristics, fabrication techniques, and nonlinear properties are also introduced. Finally, future perspectives of ultrafast lasers based on 2D materials are also addressed.
关键词： saturable absorber,MXene,antimonene,bismuthene,graphene,ultrafast lasers,transition metal dichalcogenides,black phosphorus,2D materials,topological insulators
更新于2025-09-19 17:13:59
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Quantum cascade laser lives on the edge
摘要： Electromagnetic waves with frequencies in the terahertz range (300?GHz to 10?THz) have applications in many areas, from imaging and security screening to the atmospheric and biological sciences. Semiconductor devices called quantum cascade lasers (QCLs) provide the most compact and efficient way to generate terahertz radiation. In QCLs, electrons cascade down in energy through a series of discrete quantum energy levels, emitting a photon at each step1. But, as with all compact semi conducting lasers, QCLs are notoriously sensitive to fabrication imperfections, which results in device-to-device variability of the laser output frequency. Now, on page 246, Zeng et al.2 report the realization of a terahertz QCL that is insensitive to such disorder. This achievement opens the door for terahertz lasers and optoelectronics that have unprecedented stability and fabrication reproducibility.
关键词： topological protection,terahertz radiation,topological insulators,photonics,quantum cascade lasers
更新于2025-09-19 17:13:59
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Switching Quasi-Flat Modes at the Interface between Detuned Anomalous Floquet Topological Insulators
摘要： Anomalous Floquet Topological Insulators (A-FTIs) [1–3] are time-dependent arrangements with topologically non-trivial phases. These recently discovered systems are characterized by an integer Winding number W [4]. We theoretically investigate photonic A-FTI comprised of honeycomb lattices of waveguides (see Fig. 1(a)). Crucially, the couplings between adjacent lattice sites are periodically modulated in time so that, at any point in time, a site is coupled to only one of its three next neighbors. This system exhibits a number of non-degenerate bands separated by a gap centered on the quasi-energy (cid:2) = 0. Since time-periodicity translates to periodicity in quasi-frequency, another band gap appears around (cid:2) = ±π. By varying the coupling angle θc, we obtain di?erent values (W0, Wπ) for the Winding numbers of the two gaps, as well as for the band’s Chern numbers (C). We ?nd that C = 0 if topological modes appear across both gaps. In contrast, the Winding numbers ful?ll (W0 ∨Wπ) = 1 and correctly count the number of protected modes across each gap, as predicted in [4] (see Fig. 1(b)). Based on these considerations, we explore the interface between two mutually detuned realizations of the aforementioned A-FTI, placed next to one another. The detuning is implemented by a constant on site potential and results in a relative shift of the respective band structures. If, as a result, the π-gap of the second system becomes matched with the 0-gap of the non detuned system, the boundary between these two domains identi?es a transition between di?erent, albeit topologically equivalent, media that share the same band gap. While the bulk-edge correspondence principle doesn’t allow topological modes to exist on such interfaces, we can nevertheless observe a number of seemingly trivial, quasi-?at, defect modes (see Fig. 1(c) inset). Their existence is enabled by the interplay between counter-propagating modes on the inside edges of each sub-system. Both interface modes are phase-matched with the edge modes for exactly one value of the wave vector k. This behaviour is con?rmed by tight-binding simulations of the dynamics of a broad beam (i.e. a narrow wave packet in k-space), which show no coupling between the topological mode and the trivial interface state except for the phase-matching points between the two (see Fig. 1(c)). This mechanism can be harnessed to selectively extract a fraction of light that is propagating in a topologically protected edge channel by depositing it into quasi-static defect modes.
关键词： honeycomb lattices,Anomalous Floquet Topological Insulators,quasi-?at defect modes,topological modes,Chern numbers,Winding number,photonic A-FTI
更新于2025-09-16 10:30:52
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Topological Cylindrical Resonators and Optical Fibers
摘要： We propose optical resonators based on the edge modes of a cylindrical multilayer structure with nontrivial topological configuration. We calculate the field confinement and resonator dispersion by a recursive approach able to tackle with arbitrary arrangements of annular concentric layers. We apply the method to a periodical structure with Harper modulation and study the resulting cylindrical edge modes. Conventional circular optical resonators are designed exploiting the effect of total internal reflection or by Bragg structured claddings. The Bragg reflection mechanism allows to decouple the detrimental link between cavity dimensions and cavity losses present if total internal reflection is exploited. Reducing bending losses requires a large core radius, while for achieving a high quality factor small dimensions are needed. We consider photonic topological insulators, sustaining edge states, as a viable alternative for the cladding patterning to achieve optical resonators operating on protected states, immune to disorder and perturbations.
关键词： photonic topological insulators,optical resonators,topological configuration,edge modes,Harper modulation
更新于2025-09-16 10:30:52
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Detection of power line insulators on digital images with the use of laser spots
摘要： The massive growth of technologies used to register and process digital images allow for their application in evaluating the technical condition of power lines. However, it is not possible without a set of dedicated methods for obtaining diagnostic information based on registered video data. The method described here details the detection of power line insulators in digital images featuring diversified backgrounds using laser spots. The algorithm of detecting an insulator in analysed images is based on testing the digital signal of pixel intensity profiles read between subsequent pairs of laser points in the image. The method is comprised of the following stages: import the image with laser spots, detection of spots on the image, and pattern classification of each image profile that is calculated for each found laser spots pair. The evaluated profiles depicting an insulator were characterised by regular patterns that reflect the target structure. To classify profiles as either insulator containing or non-containing, several steps should be followed: averaging the signal, removing the linear trend, finding and alternating the minima and maxima. The performance of the proposed method was verified using an open-access dataset, comprised of various scenes featuring high-voltage power line insulators.
关键词： digital images,pattern classification,detection algorithm,power line insulators,laser spots
更新于2025-09-16 10:30:52
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Analysis of Pollution in High Voltage Insulators via Laser-Induced Breakdown Spectroscopy
摘要： Surface pollution deposition in a high voltage surface can reduce the surface ?ashover voltage, which is considered to be a serious accident in the transmission of electric power for the high conductivity of pollution in wet weather, such as rain or fog. Accordingly, a rapid and accurate online pollution detection method is of great importance for monitoring the safe status of transmission lines. Usually, to detect the equivalent salt deposit density (ESDD) and non-soluble deposit density (NSDD), the pollution should be collected when power cut o? and bring back to lab, time-consuming, low accuracy and unable to meet the online detection. Laser-induced breakdown spectroscopy (LIBS) shows the highest potential for achieving online pollution detection, but its application in high voltage electrical engineering has only just begun to be examined. In this study, a LIBS method for quantitatively detecting the compositions of pollutions on the insulators was investigated, and the spectral characteristics of a natural pollution sample were examined. The energy spectra and LIBS analysis results were compared. LIBS was shown to detect pollution elements that were not detected by conventional energy spectroscopy and had an improved capacity to determine pollution composition. Furthermore, the e?ects of parameters, such as laser energy intensity and delay time, were investigated for arti?cial pollutions. Increasing the laser energy intensity and selecting a suitable delay time could enhance the precision and relative spectral intensities of the elements. Additionally, reducing the particle size and increasing the density achieved the same results.
关键词： quantitatively analysis,surface pollution,high voltage insulators,laser-induced breakdown spectroscopy
更新于2025-09-16 10:30:52
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Investigations on Novel Carbon Quantum Dots Covered Nanofluid Insulation for Medium Voltage Applications
摘要： The extensive use of mineral oil for high voltage insulation applications has directed to widespread research works targeted at improving its dielectric and thermal characteristics. The present study investigates a novel nanofluid insulation prepared using carbon quantum dots (CQD) which is used for the surface treatment of SiO2 nanoparticles added to commercially available mineral oil. The proposed nanofluid referred as CQD–SiO2 nanofluid is tested for its insulation properties such as AC breakdown voltage (BDV), partial discharge (PD), lightning impulse (LI), dissipation factor (Tan-delta), volume resistivity, viscosity and thermal characteristics such as flash and fire point. The tests were conducted for three different nanofluid concentrations, that is, 0.01%wt, 0.05%wt and 0.1%wt. Weibull distribution analysis is carried out for breakdown data of BDV test and LI test. Results of CQD–SiO2 nanofluid are compared with mineral oil with and without SiO2 nanofillers. The results show that inclusion of CQD covered SiO2 nanoparticles have the capability to improve the AC breakdown strength by more than 100%, partial discharge inception voltage by 50%, lightning impulse withstand capabilities up to 20% and volume resistivity by 50%. It has also the tendency to slightly improve the flash point and fire point properties than the mineral oil. Significant reduction of PD magnitude and Tan-delta values are noticed with CQD–SiO2 nanofluid. Such results indicate that the CQD–SiO2 nanofluid can be a suitable candidate for MV insulation applications.
关键词： Carbon quantum dots,Liquid insulators,Nanofluid,Breakdown voltage,Partial discharge
更新于2025-09-12 10:27:22
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Topological strong-field physics on sub-laser-cycle timescale
摘要： The sub-laser-cycle timescale of the electronic response to strong fields enables attosecond dynamical imaging in atoms, molecules and solids, with optical tunnelling and high-harmonic generation the hallmarks of attosecond optical spectroscopy. Topological insulators are intimately linked with electron dynamics, as manifested via the chiral edge currents, but it is unclear if and how topology leaves its mark on optical tunnelling and sub-cycle electronic response. Here, we identify distinct bulk topological effects on directionality and timing of currents arising during electron injection into conduction bands. We show that electrons tunnel differently in trivial and topological insulators, for the same band structure, and identify the key role of the Berry curvature in this process. These effects map onto topologically dependent attosecond delays and helicities of emitted harmonics that record the phase diagram of the system. Our findings create new roadmaps in studies of topological systems, building on the ubiquitous properties of the sub-laser-cycle strong-field response—a unique mark of attosecond science.
关键词： high-harmonic generation,attosecond science,Berry curvature,topological insulators,strong-field physics
更新于2025-09-12 10:27:22
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Chern and Z2 topological insulating phases in perovskite-derived 4d and 5d oxide buckled honeycomb lattices
摘要： Based on density functional theory calculations including a coulomb repulsion parameter U, we explore the topological properties of (LaXo3)2/(LaAlo3)4 (111) with X = 4d and 5d cations. the metastable ferromagnetic phases of Latco3 and Lapto3 with preserved P321 symmetry emerge as Chern insulators (CI) with C = 2 and 1 and band gaps of 41 and 38 meV at the lateral lattice constant of LaAlo3, respectively. Berry curvatures, spin textures as well as edge states provide additional insight into the nature of the ci states. While for X = Tc the CI phase is further stabilized under tensile strain, for X = Pd and Pt a site disproportionation takes place when increasing the lateral lattice constant from aLAo to aLno. the ci phase of X = Pt shows a strong dependence on the Hubbard U parameter with sign reversal for higher values associated with the change of band gap opening mechanism. Parallels to the previously studied (X2o3)1/(Al2o3)5 (0001) honeycomb corundum layers are discussed. Additionally, non-magnetic systems with X = Mo and W are identified as potential candidates for Z2 topological insulators at aLAo with band gaps of 26 and 60 meV, respectively. The computed edge states and Z2 invariants underpin the non-trivial topological properties.
关键词： density functional theory,perovskite-derived oxides,Chern insulators,Z2 topological insulators,honeycomb lattices
更新于2025-09-11 14:15:04
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Routine Microscopy in Quantum Dot Industry
摘要： Heterojunctions, composed of different materials, are widely explored in optoelectronic devices thanks to their unique advantages, such as high carrier mobility and excellent photoelectronic characteristics. In this work, Bi2Se3/Te@Se heterojunctions (Bi2Se3/Te@Se) are synthesized through the epitaxial growth of Bi2Se3 nanosheets (Bi2Se3 NTs) on tellurium@selenium nanotubes (Te@Se NTs) by using a low-cost and facile solvothermal process. Bi2Se3/Te@Se are further applied in high-performance photoelectrochemical (PEC)-type photodetection due to the advantages of broadband optical response and fast carrier relaxation time. The PEC results demonstrate that the as-prepared photodetectors have pronounced photoresponse behavior from the ultraviolet to visible band with self-driven ability and excellent long-term stability. It is anticipated that this work provides a new strategy for epitaxial growth of topological insulators on semiconductors for designing new heterojunctions toward high-performance optoelectronic devices.
关键词： photodetectors,self-driven,topological insulators,epitaxial growth,heterojunctions
更新于2025-09-11 14:15:04