Terahertz wave, as a special electromagnetic wave which lie in the frequency gap between infrared and microwave, have attracted more and more attentions in recent years because of its great practical applications in the field of terahertz imaging, wireless communication and detection. In this paper, a broadband and angle independent active dynamically tunable absorber is numerically demonstrated using the VO2 phase-change material in terahertz regime. As the conductivity of VO2 varies from 2×105S/m to 20S/m, the absorption efficiency of our device in the frequency range of 2.73 - 9.95 THz has dramatically decreased from averaged 94.5% to almost 0%, and the modulation depth is nearly as high as 100%. Moreover, the proposed structure exhibit an angle and polarization independent in the range of 3.4 – 8.4THz. The stacked disk structure can be fabricated with a simple e-beam lithographic and etching process. Comparaed with the proposed metamaterials wideband tunbale absorbers, our designed structure has more advantages in practical applications, such as sensing, modulating and imaging technology
A broadband and reconfigurable multilayer absorber is demonstrated in terahertz regime by using the VO2 phase-change material. During the VO2’s insulator-metal transition (IMT), the wide absorption band in 4.5-10THz is dramatically decreased from averaged 95.5% to 0.01%. Moreover, this high absorption band shows a good polarization insensitive property. Our proposed device exhibits the merits of wideband reconfigure absorbance in terahertz, and the absorber can be easily fabricated without involving any lithography process. Comparaed with metamaterials wideband tunbale absorbers, our proposed structure have more advantages in practical applications, such as sensing, modulating and imaging technology.
PIV (Particle Image Velocimetry) plays an important role in the technology of flow field display and measurement. The traditional 2D PIV technique is well-developed, though limited by the performance of the system hardware, like lasers and cameras, its sampling rate is quite low. The time-resolved PIV works at KHz, which however is not quite fit for large test area or high spatial resolution due to the low laser energy per pulse and the low pixel resolution of cameras. In fact, it is not easy to make out a PIV system with high pixel resolution and high sampling rate at the same time. But people are still work on this research field. Here in, a four-pulse PIV system for achieving high resolution particle images at extremely short time intervals is proposed, which is arranged with a four-pulse laser, two double exposure CCD cameras and DG645 synchronizer. The laser emits a series of four laser sheets with a time interval from 400 ns to any long, the laser energy is 500 mJ per sheet, pulse width is 8 ns and the wavelength is 532 nm. The cameras and laser are exactly synchronized by DG645 to get four time-resolved particle images. In this paper, the four-pulse PIV system is optimized by the liquid crystal optical shutter that controls the light transmittance by an externally applied drive voltage. Besides, the system is applied in supersonic flows to measure the shock train flow structure. The time-resolved particle images with high spatial resolution (2K×2K@10ns, time interval=3 μs) are quite important for the study of unsteadiness and evident oscillation of the shock train, which corresponds to intensive shockwave/boundary layer interaction.
The present paper is a study of laser Schlieren technique and its applications in kinds of flows from subsonic flow to high speed. Laser Schlieren technique has been investigated recently, which emitted 20mJ laser pulses at 5Hz. Owning to the benefits of light spot homogenization technique, laser Schlieren system was qualified to reveal the flow structures with high resolution and sensitivity. Benefited from these features, the laser Schlieren can be applied widely in fluid mechanism studies, including high speed flows, jets, butterfly flow field and so on.
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