Generating Dual-Mode Dual-Polarization OAM Based on Transmissive Metasurface.

Recent advances in electromagnetic (EM) waves with helical phase wave-front carrying orbital angular momentum (OAM) has drawn great attention, since it is believed to be a promising candidate for the next generation of wireless communication technology. To make the design more practical, here, a transmissive metasurface for generating dual-mode and dual-polarization OAM has been designed, manufactured and experimentally validated. To generate EM waves carrying OAM, the element structure is well-designed and can introduce additional phase to the incident wave. The employed four-layer cascaded metasurface demonstrates a high performance of transmission and complete phase control. Dual-mode operating characterization is realized by applying the polarization-dependent physical response. Moreover, experimental results including near-field and far-field properties are conducted to validate the numerical simulations. The proposed method in this paper promotes the practical design and realization of OAM vortex waves for the next generation of wireless communication technology.

Broadband Dual-Polarized Base Station Antenna for Fifth-Generation (5G) Applications.

A broadband dual-polarized base station antenna with special designed feeding structures is investigated in this paper. The proposed antenna contains two pairs of crossed dipoles, two specially designed feeding connectors, two pieces of dielectric pads, a supporter (also a balun), and a reflector. To verify the designed antenna, a prototype is fabricated and measured. The antenna attains a bandwidth of around 46.5% operating over 3.14⁻5.04 GHz under reflection coefficient lower than -15 dB, and the port-to-port isolation is higher than 32.5 dB. It also achieves very stable radiation patterns with half power beam widths of 71.8° ± 2.5° in both the horizontal and vertical planes and gains of around 8 dBi over its operating band. Besides, the mechanism of the obtained good performances is clearly explained from the angle of current. All of the features ensure that the proposed antenna is suitable for the fifth-generation (5G) mobile communications.

Polarization Smoothing Generalized MUSIC Algorithm with Polarization Sensitive Array for Low Angle Estimation.

Direction of Arrival (DOA) estimation of low-altitude targets is difficult due to the multipath coherent interference from the ground reflection image of the targets, especially for very high frequency (VHF) radars, which have antennae that are severely restricted in terms of aperture and height. The polarization smoothing generalized multiple signal classification (MUSIC) algorithm, which combines polarization smoothing and generalized MUSIC algorithm for polarization sensitive arrays (PSAs), was proposed to solve this problem in this paper. Firstly, the polarization smoothing pre-processing was exploited to eliminate the coherence between the direct and the specular signals. Secondly, we constructed the generalized MUSIC algorithm for low angle estimation. Finally, based on the geometry information of the symmetry multipath model, the proposed algorithm was introduced to convert the two-dimensional searching into one-dimensional searching, thus reducing the computational burden. Numerical results were provided to verify the effectiveness of the proposed method, showing that the proposed algorithm has significantly improved angle estimation performance in the low-angle area compared with the available methods, especially when the grazing angle is near zero.

Reducing the effects of acoustic heterogeneity with an iterative reconstruction method from experimental data in microwave induced thermoacoustic tomography.

PURPOSE: An iterative reconstruction method has been previously reported by the authors of this paper. However, the iterative reconstruction method was demonstrated by solely using the numerical simulations. It is essential to apply the iterative reconstruction method to practice conditions. The objective of this work is to validate the capability of the iterative reconstruction method for reducing the effects of acoustic heterogeneity with the experimental data in microwave induced thermoacoustic tomography. METHODS: Most existing reconstruction methods need to combine the ultrasonic measurement technology to quantitatively measure the velocity distribution of heterogeneity, which increases the system complexity. Different to existing reconstruction methods, the iterative reconstruction method combines time reversal mirror technique, fast marching method, and simultaneous algebraic reconstruction technique to iteratively estimate the velocity distribution of heterogeneous tissue by solely using the measured data. Then, the estimated velocity distribution is used subsequently to reconstruct the highly accurate image of microwave absorption distribution. Experiments that a target placed in an acoustic heterogeneous environment are performed to validate the iterative reconstruction method. RESULTS: By using the estimated velocity distribution, the target in an acoustic heterogeneous environment can be reconstructed with better shape and higher image contrast than targets that are reconstructed with a homogeneous velocity distribution. CONCLUSIONS: The distortions caused by the acoustic heterogeneity can be efficiently corrected by utilizing the velocity distribution estimated by the iterative reconstruction method. The advantage of the iterative reconstruction method over the existing correction methods is that it is successful in improving the quality of the image of microwave absorption distribution without increasing the system complexity.

Evaluation of contrast enhancement by carbon nanotubes for microwave-induced thermoacoustic tomography.

Microwave-induced thermoacoustic tomography (MITAT) is a hybrid method which constructs images with ultrasound spatial resolution while exploiting dielectric contrast at microwave frequency. It has great potential in biomedical imaging especially in early breast cancer detection. The detection of early stage breast tumor in MITAT is challenged by the moderate endogenous dielectric contrast between malignant and healthy glandular tissues. In order to overcome this limitation, the performance of using carbon nanotubes (CNTs) as an imaging contrast enhancement agent is evaluated. First, the influences in dielectric and acoustic properties caused by CNTs are measured. Second, based on the measurements and the published data, numerical breast phantom is created and then used to explore the contrast enhancing effect of CNTs for MITAT, by an integrated simulation approach in both electromagnetic and acoustic field. With an experimental MITAT system, the thermoacoustic responses of tissue mimicking materials with different CNTs concentrations are also quantitatively investigated. Finally, the effectiveness of the contrast agent is also validated experimentally by using a MITAT system. The results show that the using of the dielectric contrast agent can effectively enhance the contrast of the MITAT image.

Plasmonic nanopatch array for optical integrated circuit applications.

Future plasmonic integrated circuits with the capability of extremely high-speed data processing at optical frequencies will be dominated by the efficient optical emission (excitation) from (of) plasmonic waveguides. Towards this goal, plasmonic nanoantennas, currently a hot topic in the field of plasmonics, have potential to bridge the mismatch between the wave vector of free-space photonics and that of the guided plasmonics. To manipulate light at will, plasmonic nanoantenna arrays will definitely be more efficient than isolated nanoantennas. In this article, the concepts of microwave antenna arrays are applied to efficiently convert plasmonic waves in the plasmonic waveguides into free-space optical waves or vice versa. The proposed plasmonic nanoantenna array, with nanopatch antennas and a coupled wedge plasmon waveguide, can also act as an efficient spectrometer to project different wavelengths into different directions, or as a spatial filter to absorb a specific wavelength at a specified incident angle.

A prototype system of microwave induced thermo-acoustic tomography for breast tumor.

Microwave-induced thermo-acoustic tomography (MITAT) is an innovative technique for tumor's detection. Due to there has high contrast in terms with permittivity and electrical conductivity of tumor versus normal tissue, even if the tumor still in the early phase it can be imaged clearly. For the proposed MITAT system, low energy microwave pulses are used as the irradiating signals, while the received signals are ultrasound, high contrast and high resolution images can be obtained. After some theoretical research and basic fundamental experiments, the first prototype of experimental system is designed and built. It includes the microwave radiator, the arrayed sensor bowl, the circular scanning platform, the system controller and the signal processor. Based on the experimental results using this integral MITAT clinic system, the images contrast can be reached higher than 383:1; while the sub-millimeter special resolution is obtained for a 1cm(3) scale tumor mimic.