In this paper, a whole general design and optimization process is detailedly demonstrated by taking the design and optimization of a 55mm diameter variable curvature mirror(VCM) with a cycloid-like thickness distribution as example. The finite-element analysis to the VCM under each change of main structure parameter is done and analyzed to choose the proper parameter value of each structure to obtain the optimum surface figure accuracy. Finally, the designed VCM can achieve 0.386mm central deflection and RMS 82.84nm within the effective aperture 28.4mm.
A variable curvature mirror (VCM) fabricated by 3D printing technique which is thickness optimized in structure design to reduce spherical aberration and supposed to be used in zoom imaging system is investigated. First, measurement and parameters fix of the mirror blank printed by 3D printing of AlSi10Mg are done for its precision deviation introduce by the manufacturing method. Second, elementary optical polishing is done for the purpose of Nickel plated. Fine optical polishing is applied on the VCM after the Nickel plated. Third, an actuation test experiment is built and tested by piezoelectric actuators of PI with nanometer precision and Zygo interferometer. The original surface figure accuracy of 90% radius is 2.225 λ / 0.394 λ (λ = 632.8 nm). As a result, within the ultimate testing range of the interferometer, the VCM achieve about 8.68μm deformation with the corresponding position change of actuator is 18μm, which is about 50% of it. Finally, an experiment of zoom imaging effect is done. The experiment shows that it does have effect to the zoom imaging which can compensate the defocus within 230.7μm. From the performance of the VCM at this stage, it can be used in infrared imaging. For the following work, its structure will be further optimized and the precision problems will be solved through using more proper manufacture method to improve its radius change performance during actuation process. Therefore, it can be used in visible light imaging in the future.
Laser ablation of zircon can be used to analyze its composition for geological history. However, the effect of laser properties on nanoparticle size has not been studied extensively. The effect of laser fluence and pulse duration on the diameter of zircon nanoparticles was analyzed using field-emission scanning electron microscopy and energy dispersive spectroscopy. The results showed that the diameters of the zircon nanoparticles induced by a femtosecond laser increased with increasing laser fluence, and that these particles were smaller than those induced by a nanosecond laser with the same laser fluence. Furthermore, the mechanism of zircon nanoparticle formation induced by laser ablation has been discussed. The explosion mechanism is the primary mechanism of nanoparticle generation. In particular, the zircon nanoparticles induced by the femtosecond laser were the result of Coulomb explosion, while phase explosion contributed to the zircon nanoparticles induced by the nanosecond laser. Therefore, the nature of zircon nanoparticles induced by laser ablation is mainly determined by the pulse duration.
A periodic two-dimensional metamaterial with plasmon induced transparency effect in terahertz region, composed of polyimide layer covered by a monolayer graphene ring-rod structure, is presented and numerically investigated. The structure shows a remarkable polarization-sensitive transparent window in terahertz band due to the interference phenomenon caused by mode coupling. The results demonstrate that the transparent window of plasmon induced transparency can be flexibly controlled by adjusting the Fermi level of graphene and the polarization angle of incident terahertz wave, which are verified by numerical simulation and dipole-dipole interaction model. Moreover, the maximum group delay time corresponding to the polarization angle at 0 degree is 136.57 fs, exhibiting obvious slow light characteristics. The proposed metamaterial may give rise to practical application in terahertz switches and slow light devices.
Femtosecond laser-induced air plasma can generate terahertz waves. The sawtooth-like asymmetric photoelectric field can be made by the superposition of three-color laser with appropriate amplitude ratio, frequency ratio and phase difference. Based on the transient photocurrent model and numerical simulation, it is found that intense terahertz waves can be generated by the sawtooth-like asymmetric photoelectric field with different frequency ratios of three-color femtosecond laser, such as 1:2:4 and 1:2:6. By investigating time-varying parameters, such as electron density, electron velocity and photocurrent density, it was further demonstrated that the shape of the asymmetric photoelectric field plays an important role in the formation of net photocurrent and the generation of terahertz waves. The investigation will be helpful to the development of key techniques on intense terahertz waves generated by laser-induced air plasma.
The Ti-6Al-4V polished by femtosecond laser processing is first investigated. The surface nanoparticles of Ti-6Al-4V induced by femtosecond laser have been characterized by scanning electron microscopy and energy dispersive spectrometry, and the influence of laser fluence and the overlapping rate of laser beam on surface roughness have also been analyzed. Moreover, the relationship between the laser parameters and the surface roughness of Ti-6Al-4V has been revealed, and the fine surface roughness of Ti-6Al-4V is obtained based on the optimized femtosecond laser processing parameters.
In this paper, we deployed a home-made THz time-domain system and experimentally investigated the influence of impairment on photoconductive antenna. The low temperature grown GaAs photoconductive antenna (PCA) was used as THz emitter and a <110< ZnTe crystal modulating the probe beam to detect the THz field based on electro-optic sampling. By adjusting the position of laser spot on PCA gap, we found that the generated THz signal was quite different as the laser beam irradiating on different position of the antenna’s gap. Moreover, we found that the small burned holes on the edge of both metal electrodes may affect the electric field of the antenna. Furthermore, we simulated the electric field of the PCA with a DC biased voltage applied across the electrodes, and found that the simulation results agreed with the experimental phenomenon, which can well demonstrated that the impairments on the antenna electrodes have a great influence on THz generation.
A route of delivering ultra-short laser pulses through a polarization-maintaining single-mode fiber has been investigated experimentally. The group velocity dispersion was well compensated, and the self-phase modulation was depressed in fiber. Finally, a 40-fs laser pulse with the energy of 70 pJ was obtained at the output of the transmitted fiber with the length up to 4 m in our fiber-delivery system. The route exhibits distinguishing features of compactness and flexibility, and this 40-fs laser pulse can be used not only to generate broadband terahertz (THz) radiation but also to detect broadband THz radiation, which is much better than the 150-fs laser pulse used previously.
Many materials of interest to the forensic and security services, such as explosives, drugs and biological agents, exhibit
characteristic spectral features in the terahertz (THz) frequency range. These spectral features originate from
inter-molecular interactions, involving collective motions of molecules. Broadband THz time-domain spectroscopy
(THz-TDS) system have been used to analyze a number of drugs-of-abuse and explosives that are of interest to the
forensic and security services. These samples ranged from crystalline powders, pressed into pellets, to thin sheets of
plastic explosives, and all being measured in transmission geometry in the frequency range 0.1 - 8 THz.
To well understand the nature of the observed spectral features and the effects of thermal broadening on these
far-infrared signatures, temperature-dependent THz-TDS measurements have also been performed at temperatures as
low as 4 K, especially for two types of cocaine. Well-resolved low-frequency absorption peaks were observed in the
frequency range 0.1 - 3 THz with high resolution. Some of absorption peaks were found clearly to become more intense
and shift to higher frequencies as the temperature was reduced. The results confirm that the low-frequency collective
modes are highly sensitive to the structural and spatial arrangement of molecules. Furthermore, a number of common postal packaging materials made from paper, cardboard, even several types of plastic,
have been tested with drug sample to assess the ability of THz-TDS in a hostile detection environment.
Broadband terahertz time-domain spectroscopy (THz-TDS) has been shown to be a valuable technique for the detection and analysis of explosives. In this paper, we present recent work on the use of this technique to analyse two pure explosives, 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) and 1,3-dinitrato-2,2-bis(nitratomethyl)propane (PETN) and three plastic explosives (Semtex, SX2 and Metabel). For each explosive, a clear and unique spectral absorption fingerprint is found, which can be attributed to oscillations of the molecular lattice. Temperature dependent terahertz absorption measurements of RDX show that the observed vibrational modes are anharmonic in nature. Terahertz spectra are compared with Raman spectra, and the similarities and differences between the spectra produced by the two techniques are discussed. A molecular mechanics calculation approach is used to calculate vibrational intensities and frequencies, and these results are compared with experimental results at both room temperature and 4 K.
We demonstrate that, through coherent measurement of the transmitted terahertz frequency electric fields, broadband (0.3 - 8 THz) time-domain spectroscopy can be used to measure far-infrared vibrational modes of a range of drugs-of-abuse and high explosives that are of interest to the forensic and security services. Our results indicate that absorption features in these materials are highly sensitive to the structural and spatial arrangement of the molecules. Terahertz frequency spectra are also compared with high-resolution low-frequency Raman spectra to assist in understanding the low-frequency inter- and intra-molecular vibrational modes of the molecules.
We demonstrate that, through coherent measurement of the transmitted terahertz electric fields, broadband (0.3-8THz) time-domain spectroscopy can be used to measure far-infrared vibrational modes of a range of illegal drugs and high explosives that are of interest to the forensic and security services. Our results show that these absorption features are highly sensitive to the structural and spatial arrangement of the molecules. Terahertz frequency spectra are also compared with high-resolution low-frequency Raman spectra to assist in understanding the low frequency inter- and intra-molecular vibrational modes of the molecules.
Electron trapping thin films CaS:Eu,Sm have been deposited by pulsed laser deposition in vacuum and simultaneous sulfur coevaporation. The films prepared by doubly rare-earth ions doped alkaline earth sulfides CaS have the characteristics of infrared upconversion and optical storage. Its phases and microstructures were identified by X-ray diffraction. The chemical composition of the films was determined by secondary ion mass spectrum. The studies on the optical properties of the films show they can convert the infrared light (800 - 1600 nm) to red light (approximately 672 nm). Infrared upconversion efficiency of CaS:Eu,Sm thin films with different thickness has been investigated by using the ultrashort infrared laser with different FWHM from microsecond(s) to ps. It is shown the upconversion efficiency of CaS:Eu,Sm thin films not only depends on the growth conditions and the post annealing process, but also has the `exhaustion' phenomenon. By means of measuring transmittance and spatial resolution of CaS:Eu,Sm thin films, the post annealing process was found to promote grain growth which could obviously improve upconversion efficiency of CaS:Eu,Sm thin films, even though it had negative influence on transmittance and spatial resolution of CaS:Eu,Sm thin films.
The paper describes our research on infrared-sensitive streak camera that adopts a streak tube with a complex photocathode which consists of a membrane made of up-converting material (one kind of ET material) and a S-20 photocathode. The up- converting screen is able to convert 800 nm - 1600 nm infrared radiation to visible light. The peak values of the converting efficiency are located at 1165 nm for CaS:Eu, Sm and 1060 nm for CaS:Ce, Sm. We obtained a novel result of time resolution of the ET-materials and the streak camera attains time resolution of 12.3 ps for CaS:Eu, Sm screen and 8.4 ps for CaS:Ce, Sm screen.
A 256 X 256 cell color ACPDP panel and its prototype display unit are developed based upon the development of low melting point barrier, dielectric layer and seal materials which reduces the deformation and thermal shrinkage of soda- lime glass substrate. A special technology is used in high voltage drive of color ACPDP.
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