Qiao Wu

Bio: Qiao Wu is an academic researcher from Huazhong University of Science and Technology. The author has contributed to research in topics: Terahertz radiation & Bessel beam. The author has an hindex of 2, co-authored 3 publications receiving 17 citations.

Diffractive Elements for Zero-Order Bessel Beam Generation With Application in the Terahertz Reflection Imaging

Abstract: Novel diffractive elements are presented to produce zero-order Bessel beams in the terahertz (THz) band. First, diffractive elements are designed as phase plates to change the direction of the incident Gaussian beam. Then, they are fabricated by the emerging three-dimensional printing technology. Last, zero-order Bessel beams are generated by the diffractive elements at 0.3 THz. To verify the feasibility of this approach, the generated zero-order Bessel beams are monitored and compared with the ones generated by the axicons. The comparison confirms that the diffractive elements can produce Bessel beam effectively and efficiently. In addition, the diffractive elements are applied to introduce zero-order Bessel beam to a THz reflection imaging system. A resolution test phantom is imaged by the THz reflection imaging system with zero-order Bessel beam and a conventional THz reflection imaging system. The results indicate that the THz reflection imaging system with Bessel beam offers a significantly extended depth of field and high anti-interference capability.

Generation of One-Dimensional Terahertz Airy Beam by Three-Dimensional Printed Cubic-Phase Plate

Abstract: We first present the one-dimensional (1-D) terahertz (THz) Airy beam with a 0.3-THz continuous wave. The required phase modulation and the optical Fourier transform are implemented with a 3-D printed cubic-phase plate (CPP) and a cylindrical lens, respectively. According to the parameter of the designed CPP, we numerically calculate the intensity distributions of the 1-D THz Airy beam in the free space. The experimental results show a good accordance with the theoretical ones. Additionally, by blocking part of the generated 1-D Airy beam with a metal screen, the beam exhibits a self-healing property, i.e., tends to reform during propagation.

THz computed tomography system with zero-order Bessel beam

Abstract: 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.

3-D Printed All-Dielectric Dual-Band Broadband Reflectarray With a Large Frequency Ratio

Abstract: This communication proposes a new all-dielectric broadband dual-band reflectarray with a large frequency ratio using low-cost 3-D printing. In contrast to conventional reflectarrays using metallic resonant cells or dielectric slabs as phasing elements with full metal ground, the proposed design uses air as the phasing element and a stepped dielectric mirror structure as the ground. In this way, the metal ground is removed, which makes the design an all-dielectric one. Taking advantage of the dielectric mirror that only exhibits a bandgap in the predesigned band while allowing electromagnetic (EM) waves to pass through it at the frequency out of the bandgap region, a dual-band reflectarray is obtained. By properly selecting the bandgap frequency of the dielectric mirror, the dual-band frequency ratio is scalable and can be very large. Furthermore, instead of using a metallic or dielectric resonator based on resonance, air layers with linear phase response are adopted as the phasing element. Thus, the reflectarray shows broadband and stable performance over the dual-band. Compared with the state-of-the-art works using printed circuit boards (PCBs) or microfabrication, the proposed design is low cost and lightweight, and can be rapidly prototyped. For proof-of-concept, a prototype operating at $K$ -band and $V$ -band with a frequency ratio of 2.7 is printed and measured.

Airy transform of Laguerre-Gaussian beams.

Abstract: Airy transform of Laguerre-Gaussian (LG) beams is investigated. As typical examples, the analytic expressions for the Airy transform of LG01, LG02, LG11, and LG12 modes are derived, which are special optical beams including the Airy and Airyprime functions. Based on these analytical expressions, the Airy transform of LG01, LG02, LG11, and LG12 modes are numerically and experimentally investigated, respectively. The effects of the control parameters α and β on the normalized intensity distribution of a Laguerre-Gaussian beam passing through Airy transform optical systems are investigated, respectively. It is found that the signs of the control parameters only affect the location of the beam spot, while the sizes of the control parameters will affect the characteristics of the beam spot. When the absolute values of the control parameters α and β decrease, the number of the side lobes in the beam spot, the beam spot size, and the Airy feature decrease, while the Laguerre-Gaussian characteristic is strengthened. By altering the control parameters α and β, the performance of these special optical beams is diversified. The experimental results are consistent with the theoretical simulations. The Airy transform of other Laguerre-Gaussian beams can be investigated in the same way. The properties of the Airy transform of Laguerre-Gaussian beams are well demonstrated. This research provides another approach to obtain special optical beams and expands the application of Laguerre-Gaussian beams.

Beam Propagation Factor of a Cosh-Airy Beam

Abstract: Based on the second-order moments, the analytical expression of the beam propagation factor of a cosh-Airy beam has been derived. The beam propagation factor was determined by the decay factor and the cosh parameter. Because the beam propagation factors in the x- and y-directions of the cosh-Airy beam have the same form, only the beam propagation factor in the x- direction was selected as the object of numerical calculation and analysis. The effects of the decay factor and the cosh parameter on the beam propagation factor were investigated. When the decay factor was greater than 1, the beam propagation factor first increased and then decreased with the increase of the cosh parameter, and finally, tended to a minimum value. Under the condition that the decay factor was less than 1, the beam propagation factor always increased with the increase of the cosh parameter. As the decay factor increased, the beam propagation factor decreased and tended to a minimum value. Finally, the effects of the decay factor and the cosh parameter on the squares of the beam waist and the divergence were analyzed in more detail.

The Magic of Optics-An Overview of Recent Advanced Terahertz Diffractive Optical Elements.

Abstract: Diffractive optical elements are well known for being not only flat but also lightweight, and are characterised by low attenuation. In different spectral ranges, they provide better efficiency than commonly used refractive lenses. An overview of the recently invented terahertz optical structures based on diffraction design is presented. The basic concepts of structure design together with various functioning of such elements are described. The methods for structure optimization are analysed and the new approach of using neural network is shown. The paper illustrates the variety of structures created by diffractive design and highlights optimization methods. Each structure has a particular complex transmittance that corresponds to the designed phase map. This precise control over the incident radiation phase changes is limited to the design wavelength. However, there are many ways to overcome this inconvenience allowing for broadband functioning.

Recent Advances in Terahertz Time-Domain Spectroscopy and Imaging Techniques for Automation in Agriculture and Food Sector

Abstract: Automated non-destructive testing in the agricultural and food sector gathered great interest because of higher production, lower wastage, food quality, food safety, time-saving, lower workforce requirements, and food security. There is a demand for faster, accurate, reliable, and simplified technology to classify the sound quality of food and agricultural products. Terahertz (THz) waves in a frequency interval of 0.1 to 10 THz and their unique properties such as low-energy, penetrating, rapid, and non-invasive technology encouraged the extensive application in the food and agriculture sector as a non-destructive and cost-effective method. The great application of THz imaging is due to the potentials of extraction of spectra from amplitude and phase information using THz spectroscopy. In this review, the principle and instrumentation of THz spectroscopy are discussed and focuses on the recent advances in terahertz technology used in water monitoring, soil sensing, classification of seeds, varieties/origin discrimination, residue detection, microbes, toxin and food spoilage detection, food adulteration identification, and foreign particles inspection in the food and agriculture sector.