Limei Qi

One-dimensional multiband terahertz graphene photonic crystal filters

Properties of one-dimensional graphene photonic crystals with dual-layer defects are studied. Results show that two defect modes appear within the gaps, and the defect modes shift to the lower frequencies with the chemical potential increasing, the physical mechanism are also given based on the relative dielectric constant of the graphene. It is also found that the frequency, magnitude, and numbers of the defect modes can vary with the symmetrical changes of the dual-defect layers. For oblique incidence, the defect modes of the TE polarization follow a similar trend with the TM polarization, and all the defect modes shift to the higher frequencies and disappear while new defect modes appear at the larger incident angles. These properties of graphene photonic crystals with dual-layer defects have potential applications in tunable terahertz narrow multiband filters.

Metamaterial for polarization-incident angle independent broadband perfect absorption in the terahertz range

We propose a broadband perfect metamaterial absorber (MA) working at the terahertz (THz) frequencies based on the rectangular fish-scale structure. The averaged absorption of the MA is higher than 85% from about 0.85 to 1.95 THz with a maximum value of 99.5% at normal incidence. The broad and high absorption can also be obtained for the large oblique incidence of ±70°. The mixtures of the electric and the magnetic resonances contribute to the excellent performance of the MA. Furthermore, this MA is independent to the polarization states of the incident electromagnetic (EM) waves due to its four-folder rotational symmetry of the unit cell.

Polarization rotator of arbitrary angle based on simple slot-array

A novel polarization rotator of arbitrary angle was proposed and realized based on simple slot arrays. To achieve the rotation of an arbitrary angle α, the slots on the first layer have to be at an angle of α to the slots on the second layer. Consequently, 90° rotation can be realized using two perpendicularly oriented slot arrays, which overturns the conventional notion of that perpendicular slot arrays are not possible to pass electromagnetic wave. In addition, such structure provides the same bandwidth comparing to its counterpart utilized for frequency selective surface (FSS). Furthermore, such structure is much easier to be fabricated compared to the substrate integrated waveguide (SIW) array. Moreover, low insertion loss can be achieved based on metallic material.

Broadband graphene-based metamaterial absorbers

In this paper, a new type of broadband tunable metamaterial absorber is proposed. The structure consists of a single-layer ring graphene and a metal ground plane separated by a dielectric spacer. The graphene rings are connected each other to be easily tuned by the voltage. In addition, this absorber is polarization-independent at normal incidence due to the symmetric pattern of the unit cell. Results reveal the bandwidth of 90% absorption reaches to 2.25 THz for a single-ring structure. The bandwidth of 90% absorption increases to 3.2 THz by combining two similar rings with different sizes in one unit cell. The physical mechanism of the single-ring and dual-ring absorbers are given based on the impedance matching theory.

Ultrathin dual-functional metasurface with transmission and absorption characteristics

A dual-functional metasurface with simultaneous transmission and absorption characteristics is proposed and demonstrated by experiment. The ultrathin metasurface is composed of periodic metallic disks and split rings. The transmission band is surrounded by multiple absorption bands by properly designing the geometry of the structure. A maximum transmission of 63.8% is experimentally observed in the transmission band, and the measured absorption spectrum agrees well with numerical result. Furthermore, the designed metasurface is insensitive to the polarization states of the incident electromagnetic (EM) waves at normal incidence due to its structural symmetry.

Compact wideband circularly polarized antipodal curvedly tapered slot antenna

A method to reduce one direction width of the circularly polarized antipodal curvedly tapered slot antenna, while maintaining the wide axial ratio (AR) and impedance bandwidth and endfire radiation, is proposed in this letter. In this method, the ridge structure in the broad wall of the slotted waveguide is used to increase the current path, which makes the antenna achieving the appropriate phase difference of orthogonal electric fields within narrow antenna width. To validate the proposed method, a prototype of the antenna was fabricated and measured. Experimental results show that the proposed antenna can achieve a bandwidth of 27.6% (50–66 GHz) for both AR < 3 dB and reflection coefficient S11<–10 dB and an average gain of 8.39 dBi.

A dual-polarized wideband substrate-integrated-waveguide-fed slot antenna array for 60 GHz

A dual-polarized wideband substrate-integrated-waveguide fed aperture-coupled slot antenna array for 60 GHz is proposed. By cutting a semicircle shape on the edge of the crossed-slot antenna, the impedance bandwidth of the proposed antenna is effectively widen and achieves overlapped bandwidth of 16.7% for simulated |S11| and |S22|. In addition, over 35 dB isolation between two polarization and low cross-polarization level are achieved for dual-polarization operations. A gain up to 11.4 dB is obtained for 2×2 array. Most importantly, the proposed antenna array uses the aperture coupling method to enable the connection between PCB layers. It could be fabricated by stacking three layers of standard printed circuit board (PCB) laminates. With the advantages of low-cost, convenience of fabrication and desired performances, the proposed antenna is suitable for the millimeterwave antenna applications.

Research on terahertz reflectarray based on graphene surface and PET substrate

Complex surface conductivity at THz frequencies allowing the propagation of relatively slow plasmonic modes results from the unique electronic band structure of graphene, which is followed by a drastically reduced electrical size of the array unit cell and thereby reconfigurable performance. The reflectarray element based on silicon dioxide substrate and Boron Nitride substrate has been developed in our lab. The extraordinary properties of graphene would be damaged once coating the silicon dioxide with graphene due to its rough surface. Moreover, the processing thickness of silicon dioxide in reality should be thicker than that in theory. Thus, a design of graphene reflective cell based on PET (Polyethylene Terephthalate) substrate at terahertz would be illustrated in this paper. Ansoft HFSS software is used to simulate and optimize the proposed structure. It is manifest that a full range of 0°∼360° can be successfully achieved for the reflection phase at 1.095 Thz. In practice, the magnitude of reflection coefficient should be larger than 0.7 as a requirement to guarantee the efficiency of reflectarray. Additionally, the reflectarray has been implemented with properly arranged and biased graphene reflective cells. The method mentioned above is regarded as a qualified one to meet reflection coefficients and enable the accomplishment of beam scanning characteristics.

Enhancing reflector aperture usage efficiency using a coaxial cavity antenna: Application to compact antenna test range

Aperture usage is one of the most important problem in reflector-based compact antenna test range(CATR). Single reflector CATR provides very low aperture usage, normally 30%, because of the effects of edge diffraction. In order to solve this problem, serrated edges and rolled edges are generally chosen along the reflectors edge to reduce the edge diffraction field inside quiet zone(QZ) of the CATR. Even if edge diffraction has been handled by edge treatment, the QZ utilization rate of single reflector CATR (< 50%) is still low. Big reflector is therefore required to produce large QZ, which increases processing difficulty and construction cost. All of this means it is imperative to study reflector aperture distribution function and then design suitable feeds radiation field. In this paper, the aperture power density function of a parabolic reflector fed by an ideal point is theoretically derived and accordingly a coaxial cavity horn feed is designed to make the power density function constant on parabolic aperture surface and then on the QZ aperture, which realizes high QZ usage of the single parabolic CATR.

The design of push-broom scanning satellite antenna

This article presents the scanning mechanism of push-broom satellite antenna working at 140 GHz. Such kind of antenna can be applied into many areas, such as imaging, ocean surveillance and astronomical observation. The feeds are placed at the rotary focus with a spacing of 1.72 cm between two adjacent feeds, and the center distance from the feed to the reflector surface is 14.48 cm. Meanwhile, the reflector surface is created by the rotation of a parabola with four boundaries, which is obtained by MATLAB program. Commercial software GRASP based on Physical Optics (PO) is employed to assess the performance of antenna. The simulation results of the far-field are obtained by the superposition field of each feed, which are plotted in the same output coordinate system.