Diffraction-free beam propagation in free space can keep the light intensity distribution constant, but the complex system and bulky devices bring inconvenience to the experiment. In this paper, we propose an effective approach to generate a terahertz diffraction-free beam from diffractive transmitted deformed axicon. The main research methods are phase folding technique and three-dimensional (3D)-printing technique. Based on the existing research, the refractive transmitted axicon is compressed into a lighter diffractive element to implement a 0.1THz beam with 344.75mm diffraction-free length. The effect of element thickness on the phase folding efficiency is investigated, and the results of the refractive axicon and diffractive deformed axicon are compared to demonstrate the advantages of diffractive optical elements in generating diffraction-free beams.
We numerically generate the one-dimensional terahertz quasi-perfect vortex beam arrays by using encoding holographic grating. Further, we use a focal hyperbolic lens to detect the topological charge of these vortex beam arrays.
We built a 300G terahertz (THz) frequency modulated continuous wave(FMCW) imaging system. This paper describes the construction and imaging experiment of a FMCW system with a sweep bandwidth of 275~325GHz. Quasi-optical module of the system can focus the spot radius of the system from 16.35 mm to 7.61 mm. The signal-to-noise ratio and resolution of the system were greatly improved. At the same time, the cage structure design of industrial application-grade lens group is implemented, which greatly increases its applicability. We conducted both one-sided and three-dimensional imaging experiments on the built imaging experimental system. The results show that the resolution of the designed imaging system can reach 3 mm, which is of tremendous value for the application of THz nondestructive testing.
We design a simple optical element to generate a monochromatic plate-like diffraction-free terahertz (THz) beam. By using 3D-printing technology, this element can be fabricated easily and quickly. Numerical and experimental results show that the output beam of such element has a one-dimensional (1D) invariant transverse profile, i.e. it is a plate-like THz beam in free space. In order to make this kind of beam more practical, we design a lens group system to increase the diffraction-free length from 5cm to more than 20cm. In addition, a fast numerical algorithm based on diffraction integral has been built to analyze the measured results.
Art conservation terahertz (THz) diagnostics is an increasing field since THz imaging systems are commercially available. Since most of these experiment are done using femtosecond laser base time domain systems, we present in this paper how we can use frequency modulated continuous wave system to evaluate painting during restoration process.
Inspired by the extensive application of terahertz (THz) imaging technologies in the field of aerospace, we exploit a THz frequency modulated continuous-wave imaging method with continuous wavelet transform (CWT) algorithm to detect a multilayer heat shield made of special materials. This method uses the frequency modulation continuous-wave system to catch the reflected THz signal and then process the image data by the CWT with different basis functions. By calculating the sizes of the defects area in the final images and then comparing the results with real samples, a practical high-precision THz imaging method is demonstrated. Our method can be an effective tool for the THz nondestructive testing of composites, drugs, and some cultural heritages.
Terahertz (THz) waves can penetrate many optically opaque dielectric materials such as plastics, ceramics and colorants. It is effective to reveal the internal structures of these materials. We have built a THz Computed Tomography (CT) system with 0.3 THz zero-order Bessel beam to improve the depth of focus of this imaging system for the non-diffraction property of Bessel beam. The THz CT system has been used to detect a paper cup with a metal rod inside. Finally, the acquired projection data have been processed by the filtered back-projection algorithm and the reconstructed image of the sample has been obtained.
We present the design, numerical simulations and experimental measurements of an asymmetric cross terahertz metamaterial absorber (MPA) on ultra-flexible polyimide film. The perfect metamaterial absorber composed of two structured metallic layers separated with a polyimide film with a total thickness of functional layers much smaller than the operational wavelength. Two distinct absorption peaks are found at resonance frequencies of 0.439THz and 0.759 THz with resonance amplitude of near unity, which are in good agreement with the simulation results. The sample is also measured by a THz-TDS imaging system to illustrate the absorption characterization. The scanning images show that the sample could act as a perfect absorber at specific resonance frequencies while a perfect reflector at off resonance frequencies. To illustrate the physical mechanism behind these spectral responses, the distribution of the power loss and surface current are also presented. The result shows that the incident wave is trapped and absorbed by the polyimide dielectric layer at different vicinities of the proposed asymmetric cross MPA for the two absorption peaks. Furthermore, the index sensing performance of the structure is also investigated, and the calculated sensitivity is 90GHz/RIU for f1 mode and 154.7GHz/RIU for f2 mode, indicating that the higher frequency resonance absorption peak has better potential applications in sensing and detection. The ultra-flexible, low cost, high intensity dual band terahertz absorbers may pave the way for designing various terahertz functional devices, such as ultrasensitive terahertz sensors, spatial light modulators and filters.
Terahertz wave carrying OAM would boost the capacity of free-space communication with a high carrier frequency and additional degrees of freedom. In this work, we present a experimental demonstration of THz orbital angular momentum (OAM) encoding via 3D printed spiral phase plates(SPPs). By using four “coding” spps with toplogical values -2, - 1,+1,+2, we can encode OAM information onto THz waves and generate temporal varying OAM states with controlled sequences (i.e., 2-bit coding). By using interference between OAM beam and a reference gaussian beam, the OAM information can be detected from the interference fork pattern.
Terahertz (THz) imaging technology shows great advantage in nondestructive detection (NDT), since many optical opaque materials are transparent to THz waves. In this paper, we design and fabricate dielectric axicons to generate zeroth order-Bessel beams by 3D printing technology. We further present an all-electric THz imaging system using the generated Bessel beams in 100GHz. Resolution targets made of printed circuit board are imaged, and the results clearly show the extended depth of focus of Bessel beam, indicating the promise of Bessel beam for the THz NDT.
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