Radiating twisted beams, Orbital Angular Momentum (OAM) based radar provides a new perspective for present radar techniques. However, estimation methods now used has a demerit of resolution. Thus, we raised Multiple Signal Classification (MUSIC) algorithm to improve resolution ability based on this innovative concept. The echo model based on uniform circular array (UCA) for MUSIC was first built. In contrast to uncorrelated signals in classical MUSIC algorithm, echo signals from targets are fully coherent with each other. Spatial smoothing technique was subsequently utilized in OAM regime to tackle it. Simulation results showed the super-resolution capacity of MUSIC to detect objectives compared to the traditional Fast Fourier Transform (FFT) method. Introduction: Orbital Angular Momentum (OAM) has been widely studied in optic regime regarding imaging, microscopic particle and communication [1,2]. While radio OAM had not been explored until Bo. Thide et al. first proved the effectiveness of OAM generating in a low-frequency band in [3]. The numerous orthogonal helical beams entitle OAM a larger freedom degree, benefiting high capacity of communication and enriching radar targets more information. Recent work on radio OAM consists of two parts, i.e. antennas design to produce twisted beams and OAM multiplexing in communication system. Several approaches including circular array [4-6], spiral phase plate [7-8], higher resonate mode [9] are employed to engender electromagnetic vortex. Meanwhile, OAM multiplexing from simulations to outdoor experiments [10-12] were conducted to attain a high-capacity communication. Radar based on this concept had not attracted much attention until Guo et al. [13] first proposed OAM based target detection model, opening a new perspective for existing radar techniques. Such system generates various twisted beams to illuminate targets as shown in Fig.1 and receive echo signals in a reverse way. By making a Fast Fourier Transform (FFT) or back projection of received signals, this scheme manifests azimuthal resolution ability without relative moves or beams scanning. Additionally, no complex waveform is needed for this scenario. Later, [14] further extended the detection model to Multiple-in-multiple-out (MIMO) and Multiple-in-single-out (MISO) modes. Besides, two-dimension imaging method by exploiting OAM and frequency diversity was studied as well. Both papers utilized traditional methods like FFT and black projection to make estimations of azimuth angle. To gain a high resolution, numerous samples in OAM domain are required, thus increasing complexity of the system. Therefore, super-resolution techniques based on OAM radar detection are in demand. This paper built the model and derived spatial smoothing multiple signal classification (MUSIC) [15] algorithm to gain super-resolution estimations of targets.

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A terahertz metamaterial with switching characteristics from optical buffering to absorbing performance is realized by incorporating a phase-change film of vanadium dioxide. By introducing the electromagnetically induced transparency behavior based on simple strip pairs, the slow light effect with group delay up to 3.5 ps is obtained. When vanadium dioxide is in the insulator state, the remarkable delay can be observed as the incident pulse transmits through the designed structure. Once the vanadium dioxide film is tuned to the metallic state, the metamaterial is switched to a terahertz absorber and the maximum absorption rate of 94% is observed at 1.04 THz. The switching mechanism is discussed by analyzing the electric field and power loss distributions, as well as the impedance matching principle. Moreover, the buffering capability and the absorption performance both remain noticeable within a wide range of the incidence angle. This work offers a strategy for the functionswitching metamaterial which provides potential applications in terahertz detecting, switching and slow light devices.

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