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Femtosecond pump-probe spectroscopy investigation of carrier dynamics in GaAlAs photocathods
摘要： Based on the study of GaAs photocathode, the carrier dynamics of re?ection-mode GaAlAs photocathode has been investigated. In this paper, the re?ection-mode GaAlAs/GaAs photocathode is grown by metal organic chemical vapor deposition (MOCVD) epitaxial technology. The femtosecond pump-probe spectroscopy is performed to measure carrier dynamics in GaAlAs and GaAs photocathods. After laser excitation the surfaces of GaAlAs/GaAs photocathode, the re?ectivity of photocathode surface is changed. At the same time, the dynamic processes of the non-equilibrium carrier distribution in semiconductor material is obtained by the transient re?ectance spectrum. Compared with GaAs photocathode, the Al element in GaAlAs photocathode is unfavorable to photoemission under some conditions, while it solves the problem that the spectrum response range of binary compounds is not adjustable. The change trend of initial relaxation transient of GaAs photocathode in the 22 ps range is the same as GaAlAs, while the change of GaAs and GaAlAs relaxation state is signi?cantly di?erent with the change of delay time after 22 ps. The results show that the GaAlAs increases band gap due to a signi?cantly slower than GaAs photocathode in initial process. Meanwhile, this paper provides a reference for the preparation of the GaAlAs photocathode that only respond to blue and green light range.
关键词： Re?ectivity,Dynamic processes,Femtosecond,Re?ection-mode,Photocathodes
更新于2025-09-23 15:23:52
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Transient optics of gold during laser irradiation: From first principles to experiment
摘要： Intense femtosecond laser pulses can induce dramatic changes on different properties of materials. Laser induced changes of the optical properties are particularly relevant, since they can lead to a modi?cation of the amount of energy that the material absorbs from the laser pulse. In noble metals, changes of re?ectivity upon femtosecond laser illumination are expected to be strong due to the excitation of d electrons. In this work we perform measurements of the re?ectivity of laser excited gold in the infrared and in the ultraviolet range, respectively. We ?nd a remarkable dependence of the re?ectivity on laser ?uence, which is in turn different in both ranges of photon energy. In order to understand the behavior of the re?ectivity in laser excited solids and to explain our measured re?ectivity curves we develop a theoretical scheme in the framework of the two-temperature model with the electronic temperature as the key parameter. Our approach is based on all-electron calculations of the interband contribution to the re?ectivity and a careful determination of the intraband, Drude-like terms and a realistic model for the space and time resolved energy transfer from a Gaussian laser pulse into the electronic system. We obtain very good agreement between experiment and theory and identify the main mechanisms for re?ectivity changes as a function of laser ?uence.
关键词： intraband,Drude-like terms,interband contribution,electronic temperature,Gaussian laser pulse,re?ectivity,two-temperature model,femtosecond laser pulses,optical properties,gold
更新于2025-09-23 15:19:57
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A Basic Complete Numerical Toolbox for Picosecond Ultrasonics
摘要： A complete numerical complete toolbox is proposed concerning the simulation of photo-induced propagative mechanical wave, and concerning the optical re?ectometric measured response of the material, which is initially exposed to a ?rst pump laser beam that photo-induces the acoustic wavefronts. The deformation ?eld and its propagation into a bulk material are simulated. Based on this ?eld expression, the complex transient re?ectivity is given for a medium considered as homogeneous. The real part of this quantity permits afterwards to propose a numerical simulation of the transient re?ectivity, which corresponds to the optical signal measured during experimental works. The frequency acoustic spectrum is simulated and successfully compared to the measured frequency spectrum. For the ?rst time, numerical complete developments are explicitly proposed and fully-developed under the SciLab R(cid:13) environment, related to the simulation of laser-induced picosecond acoustic wavefront photogenerated through an opto-acoustic transduction process (ultrasonics and pretersonics).
关键词： simulation,computational acoustics,laser ultrasonics,acoustic waves,optical re?ectivity,pretersonics
更新于2025-09-19 17:15:36
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Strain Improving the Performance of a Flexible Monolayer MoS <sub/>2</sub> Photodetector
摘要： Electromagnetic wave absorbing foamed ceramics (FCs) with high closed porosity and gradient SiC distribution were prepared from the powder blend containing quartz, talc, CaCO3, Si3N4 and SiC by sintering at 1200 °C for 1 h in air. The retention in the temperature holding process of the continuous and gradual SiC distribution arising from the inward oxidation of SiC in the temperature rising process is the formation mechanism of the gradient SiC distribution in FCs. As to the FC prepared from the powder blend with 3 wt% of SiC, it shows the best comprehensive performance with low bulk density of 0.48 g/cm3 and relatively high compressive strength of 9.6 MPa, good thermal insulation performance for its high total porosity of 82%, good water resistance performance for its high closed porosity of 80%, and excellent electromagnetic wave absorption performance for its low mean re?ectivity of ?11.9 dB in the frequency range of 8.0–18.0 GHz and especially low minimum re?ectivity of ?14.2 dB at 12.7 GHz. More importantly, it is also an excellent high temperature electromagnetic wave absorbing material for its little increased mean re?ectivity of ?11.3 dB after heat treating at 1000 °C for 20 h in air.
关键词： Electromagnetic wave re?ectivity,High closed porosity,Electromagnetic wave absorption performance,Foamed ceramics,Gradient distribution
更新于2025-09-19 17:13:59
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Study of Optical Properties of Surface Layers Produced by Laser Surface Melting and Laser Surface Nitriding of Titanium Alloy
摘要： This study measured optical properties, such as specular, di?use, and total re?ection for 808 nm wavelength, characteristic for high power diode lasers radiation, from the surface of titanium alloy Ti6Al4V at delivery conditions, polished, and oxidized. Moreover, the optical properties of surface layers produced by high power direct diode laser (HPDDL) melting and nitriding were determined. Additionally, a methodology for determining the value of absorption for 808 nm wavelength of the HPDDL radiation on the surface of a melt pool during laser surface melting and nitriding of titanium alloy was proposed. The results show that the distinct di?erences in absorption a?ect the heat transfer, thermal conditions of laser heating and thereby the penetration depth during laser melting and nitriding of the titanium alloy.
关键词： high power diode laser,absorption,titanium nitrides,re?ectivity
更新于2025-09-16 10:30:52
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The characteristics of laser scattered by droplets
摘要： Droplets are the primary factors which in?uence laser transmitting in bad weather. This paper researches the analytic expression of droplets in?uencing the laser, which is based on the geometric optic theory and distribution of raindrop size and shape. In this paper, the relationship between the incidence and emergence angle of droplets is discussed. The e?ects of an individual raindrop on Ladar divergence angle, illuminance and optical path di?erence are investigated, and then the cumulated in?uence of a large quantity of raindrops is analyzed. The illuminance of scanning the targets and laser radar received by the detector is presented.
关键词： illuminance,re?ectivity,incidence angle,divergence angle,Laser ladar
更新于2025-09-11 14:15:04
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The mechanism of photocatalyst and the effects of co-doping CeO2 on refractive index and reflectivity from DFT calculation
摘要： In this paper, the structural, electronic and optical properties of (N, P), (N, S), (C, P) and (C, S) co-doped CeO2 are studied by using the method of density functional theory (DFT) + U method. The calculated lattice constant, the charge, the bond order, the cell volume and the bond length of CeO2 are consistent with previous experimental and theoretical results. For co-doped CeO2 systems, the calculated results show that the lattice constants, the bond order of CeeO as well as the cell volume increase, while the charge of Ce decrease. Due to co-doping, the energy gap (Eg) between valence band and conduction band becomes narrower than that of pure CeO2. Meanwhile, the electron structures of the co-doped CeO2 are changed, which may lead to transformation of optical properties. Nonmetallic atomic orbital (C 2p, N 2p, P 3p, S 3p) and metal atomic orbital (Ce 4f and 5d) are hybridized, except for the hybridization between Ce (4f and 5d) and O (2p). Compared with CeO2, Eg between Ce 4f and Ce 5d becomes smaller. Further, the optical absorption, refractive index and re?ectivity of them are investigated. The calculated results reveal that their absorption edges have a red shift. Co-doping enhances visible light photocatalytic activity, which is related to the transition of valence band → conduction band, and Ce 4f → Ce 5d. The co-doped CeO2 has high refractive index and re?ectivity. Therefore they may be becoming an important materials in single (or multilayered) optical coatings and optical re?ective pigments.
关键词： Refractive index,Density functional theory,Photocatalytic activity,Re?ectivity,Electronic transition
更新于2025-09-11 14:15:04
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Optical correlation-domain reflectometry without optical frequency shifter
摘要： To achieve distributed re?ectivity measurements along an optical ?ber, we developed a simpli?ed cost-effective con?guration for optical correlation- (or coherence-) domain re?ectometry based on a synthesized optical coherence function using sinusoidal modulation. By excluding the conventional optical heterodyne detection that accompanies the frequency shift (without using an acousto-optic modulator) and by exploiting the foot of the electrical Fresnel re?ection spectrum, the electrical bandwidth required for signal processing is lowered to several megahertz. We evaluate the basic system performance and demonstrate its high-speed operation (10 ms for one scan) by tracking a moving re?ection point in real time.
关键词： optical correlation-domain re?ectometry,distributed re?ectivity measurements,optical coherence function,sinusoidal modulation
更新于2025-09-09 09:28:46
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Plasma emission correction in reflectivity spectroscopy during sputtering deposition
摘要： Surface di?erential re?ectivity spectroscopy is a fast non-destructive in situ and real-time measurement technique which allows following the ?rst stages of thin ?lm deposition. However, when applied to sputtering technique, spectra can strongly be distorted by residual light coming from plasma in a way, as shown herein, that depends on sample re?ectivity. Thanks to suitable measurements, before and after growth with and without plasma or illumination lights, a protocol of signal correction is proposed to get rid of the spurious plasma contribution. The interest of the method is illustrated in the case of silver deposition on a silicon substrate.
关键词： sputtering deposition,Surface Di?erential Re?ectivity Spectroscopy,UV-visible
更新于2025-09-09 09:28:46