查看更多>>摘要：? 2022The knowledge of the optical properties of coating materials at high temperatures is important for understanding the dynamic process of high-power laser-material interactions. In this paper, the variations of refractive index and physical thickness of single layer coatings were studied by ellipsometric spectroscopy at different temperatures. From 23 °C to 320 °C, a decrease and then an increase of the refractive index of SiO2, and HfO2 single layer was observed, while the thickness of these layers increased first and then decreased. The inflection points of different coating materials occurred at different temperatures. Water evaporation processes, densification and hydrophilicity of films were used to explain the temperature dependent properties of the dielectric coatings. Results of the variation of refractive index and thickness of single layer coatings at different temperatures in the vacuum proved the mentioned theory. Moreover, HfO2 single layer shows better resistance to both temperature change and vacuum change, indicating that it is promising for high-reflective coatings in photoelectric devices. Besides, the formation of transient defects during high power laser irradiation was interpreted considering the optical properties variation with temperature. Three confusing and debated issues concerning laser-coating interaction are interpreted and explained.
查看更多>>摘要：? 2022 Elsevier B.V.Light emitting diodes (LED) is a promising device in visible light communication (VLC), enabling simultaneous illumination and data transmission via visible light. However, the generation of modulated visible light signals causes variation to the LED intensity in intensity modulation/direct detection (IM/DD) transmission, which may cause flickering — the fast blinking of the LED (when signals are below 200 Hz). Flicker may cause migraines, headaches, and even repetitive behavior among persons with autism. Therefore, flicking mitigation approaches must be applied for VLC with simultaneous illumination scenarios. While the optimization of the LED driving circuit and encoding can be employed to reduce flickering, driving circuit dramatically increase the system cost and communication requirements are usually excluded. As to encoding methods, they will result in reduced data rate and hardware overhead, leading to performance bottlenecks. In this paper, we proposed a signal processing method in the frequency domain to mitigate LED flickering. Low-frequency components (3–70 Hz) of modulated signals are extracted by using Fourier transform and inversely restored for transmission. Besides flickering, it is shown that interference caused by other artificial light sources can also be mitigated with the proposed method. Simulation results show that by applying the proposed frequency domain signal processing technique, communication performance of the system is enhanced with the cancellation of interference while flicker is reduced significantly. Under the condition of a 13 dB signal-to-noise (SNR) ratio, the bit error rate can reach the order of 10?3.
查看更多>>摘要：? 2022 Elsevier B.V.High harmonic generation in semiconductor materials with ultra-short pulses of the femtosecond order provides a promising approach to generate attosecond pulses and control the electron dynamics. In this study, an accurate time dependent density functional theory model describing high-order harmonic generation in WS2 bulk is investigated. The effect of driving laser specifications such as ellipticity and chirp rate on producing attosecond pulse is studied. The results show that using a nonlinearly chirped laser can lead to generation of harmonics up to 50 eV, and consequently, production of an attosecond pulse with duration of 440 as. Moreover, a two-color chirped driving laser is proposed which is capable of increasing harmonics up to 63 eV with a slight decrease in the intensity, and reducing the output attosecond pulse duration to 310 as. Our results offer a valuable route towards development of solid-state high harmonic generation spectroscopy and high-performance attosecond source applications.
查看更多>>摘要：? 2022 Elsevier B.V.An approach to evolving the photonics-based real-time Fourier transform or microwave frequency measurement system into a time–frequency analysis (TFA) system is proposed by adding a new measurement dimension to the system. Combining the above approach with stimulated Brillouin scattering (SBS)-assisted frequency-to-time mapping, two TFA links, namely the parallel SBS link and the time-division SBS link, are proposed. The parallel SBS link can be utilized to perform real-time TFA of microwave signals. A simulation is made to verify its feasibility by analyzing signals in multiple formats. The time-division SBS link has a simpler and reconfigurable structure, which can realize a high-resolution TFA of periodic signals using the time segmentation and accumulation technique. An experiment is performed for the time-division SBS link. The multi-dimensional reconfigurability of the system is experimentally demonstrated. An analysis bandwidth of 3.9 GHz, an analysis frequency up to 20 GHz, and a frequency resolution of 15 MHz are demonstrated, respectively.
查看更多>>摘要：? 2022 Elsevier B.V.Polarization modulation (PoM) provides an additional degree of freedom for the modulation of carrier waves and may be an alternative to conventional wireless communication techniques that use amplitude, frequency, and phase modulations. Metasurfaces have provided unprecedented freedom for modulating the polarization state of electromagnetic waves. In this study, we introduce the Smith chart method to guide the design of metasurface-based modulator that transforms a linearly polarized wave into a circularly polarized wave. The proposed method bridges the metasurface pattern and its equivalent circuit impedance. We show that the parameter changes in the pattern can be directly displayed in the Smith chart, which provides clear physical image guidance to designers. According to the Smith chart analysis, we obtained a single-layer modulator with 50% efficiency and a three-layer modulator with 100% efficiency, all of which are verified using the full-wave simulation method. The Smith chart method will also be helpful in the design of other metasurface-based devices, such as metasurface absorbers.
查看更多>>摘要：? 2022 Elsevier B.V.A terahertz reconfigurable metasurface with multifunction based on vanadium dioxide (VO2) is proposed, which can realize dynamic switching of reflection beam splitting, reflection focusing, vortex beam control and narrowband absorption functions. The designed structure is composed of VO2-aluminum hybrid pattern, polyimide dielectric layer and metal plate. Making use of the phase-changed material, programmable terahertz metasurface with active, switchable controls can be used to realize the flexible functionalities. Depending on predesigned arrangements, the reflected terahertz waves perform diverse states of deflection beams, vortex beams and focused beams, whose angles can be modulated. In addition, the designed structure realizes a high absorption rate over 99.5% at the frequency of 1.69 THz. The proposed metasurface is verified by simulation results for its functionality and feasibility, which has potential application characteristics for terahertz imaging, terahertz communication and sensing.
查看更多>>摘要：? 2022 Elsevier B.V.The combination of Gray code and phase-shifting fringes is widely used in the 3D structured light measurement. However, an error in the period order of the phase-shifting fringe obtained by the Gray code results in a period jump error approximately equal to an integer multiple of 2π. This error causes a gross measurement error, and even a measurement failure for a measured surface whose height changes drastically. In order to effectively reduce this error, a 3D measurement method, based on the unequal-period combination of shifting Gray code and dual-frequency phase-shifting fringes, is proposed by analyzing the generation mechanism of the period jump error. In the proposed method, on the basis of the unequal-period combination of shifting Gray code and single-frequency phase-shifting fringe, a strategy of additionally projecting a set of low-frequency phase-shifting fringe patterns is adopted to effectively correct the period jump error. A measurement model of this method is established, its applicable conditions are given, the experimental equipment is built, and verification experiments are carried out. The experimental results show that the proposed method can effectively reduce the period jump error, and can measure multiple isolated objects and objects with drastic changes in surface height such as plaster heads.
查看更多>>摘要：? 2022 Elsevier B.V.In this paper, by applying the Collins integral formula, we obtain the analytical solution of the superimposed chirped Bessel–Gaussian vortex beam (SCBGVB) in strongly nonlocal nonlinear medium (SNNM). We investigate the SCBGVB's propagation behaviors as well as the distribution of the intensity and the phase of the periodic self-focusing light field generated by the SNNM. And we can modulate its focusing times and the depth of the focus by choosing the appropriate factors. In this way, we can achieve a one-dimensional flat-topped light field with controllable length. In addition, we can manipulate the pattern of beam transmission, change its centroid position and regulate its focal intensity by choosing different components of SCBGVB's vortex structure. What important is, we can also adjust its autofocusing position and control the ratio of two focal light intensity to form a series of high and low fluctuations without changing input power.
查看更多>>摘要：? 2022 Elsevier B.V.An investigation was made of the features of the competing effects in gaseous stimulated Raman scattering (SRS) under single and di-harmonic pumping. Firstly, the one aim of this work was to observe the variations of forward and backward SRS generation in CH4 gas under di-harmonic pumping composed of 1064 nm and 532 nm lasers. At low pressure di-harmonic pumping suppressed backward SRS of 1064 nm beam, and enhanced its output energy of forward SRS. Secondly, di-harmonic pumping of 1064 nm and 1.54μm radiations obviously suppressed stimulated rotational Raman scattering, and enhanced its stimulated Raman vibrational scattering at high pressure of H2 gas. A 13.6 mJ 4.30μm mid-infrared Raman laser under di-harmonic pumping was obtained with the H2 gas pressure of 3.4 MPa. The corresponding photon conversion efficiency was 41.9%, and peak power reached 9.4 MW.
查看更多>>摘要：? 2022 Elsevier B.V.The flexible control of femtosecond laser filamentation is crucial for many practical and promising applications. We have demonstrated a simple method to control arbitrarily the onset and the terminal positions of femtosecond laser filament in the fused silica by tailoring the profile of the annular Bessel beam. The tailoring of the inner and outer radius of the annular beam is realized by amplitude mask and iris diaphragm respectively before the axicon. Our experiments have shown that the onset of the filamentation can be modulated by changing the inner radius of the laser beam, and the filamentation termination is determined by the outer radius of the annular beam. Moreover, the results show that the onset and terminal positions of the filaments are positively correlated with the inner and outer radius of the beam, respectively, which facilitate the arbitrary start and length control of the filament in the experiment. In addition, this arbitrary regulation of the onset and termination of the filamentation is being negligibly affected by incident laser power that is lower than the damage threshold of fused silica. The flexible control of filamentation enabled by the simple method is beneficial to many applications.
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