The development of transparent radio-frequency electronics has been limited, until recently, by the lack of suitable materials. Naturally thin and transparent graphene may lead to disruptive innovations in such applications. Here, we realize optically transparent broadband absorbers operating in the millimetre wave regime achieved by stacking graphene bearing quartz substrates on a ground plate. Broadband absorption is a result of mutually coupled Fabry-Perot resonators represented by each graphene-quartz substrate. An analytical model has been developed to predict the absorption performance and the angular dependence of the absorber. Using a repeated transfer-and-etch process, multilayer graphene was processed to control its surface resistivity. Millimetre wave reflectometer measurements of the stacked graphene-quartz absorbers demonstrated excellent broadband absorption of 90% with a 28% fractional bandwidth from 125–165 GHz. Our data suggests that the absorbers' operation can also be extended to microwave and low-terahertz bands with negligible loss in performance.

Numerous studies have been made to design switchable terahertz absorber for the application of amplitude modulator. However, it is still a challenge to achieve large modulation range while maintaining broad bandwidth. Here, we propose a switchable broadband absorber/reflector in the low-terahertz regime. By utilizing a hybrid graphene-gold metasurface on SiO₂/pSi/PDMS substrate with an aluminum back, an excellent absorption across 0.53–1.05 THz with a wide incident angles for both TE and TM polarizations is achieved. By controlling the voltage across gold electrode and pSi, the chemical potential of graphene can be changed correspondingly. When the chemical potential of graphene varied from 0eV to 0.3eV, the state of the proposed structure can be switched from absorption (>90%) to reflection (>82%) over the whole operation bandwidth. Electric field intensity and surface loss density of the proposed absorber under different chemical potential are given to have a physical insight of the mechanisms. The switchable absorber/reflector can enable a wide application of high performance terahertz devices, such as active camouflage, imaging, modulators and electro-optic switches.

A tri-band bandpass filter using stub loaded resonator (SLR) and defected ground structure (DGS) resonator is proposed in this letter. The DGS resonator on the lower plane constructs the first pass-band, and the SLR on the upper plane forms the second and third pass-bands, in which a DGS loop is properly used to provide the coupling between the SLRs. The three pass-bands are combined together with a common T-shaped feed line, and a reasonable tuning method is adopted to tune the three pass-bands with smooth responses. A compact tri-band filter with three pass-bands centered at 2.45, 3.5, and 5.25 GHz is designed and fabricated. The measurement results agree well with the full-wave electromagnetic designed responses.

In this communication, a millimeter-wave (MMW) pattern reconfigurable Vivaldi antenna utilizing graphene-based tunable resistor is proposed. First, a modified Vivaldi antenna with high front-to-back ratio is achieved by equal proportion slotting on the ground patch. Then, a pattern reconfigurable antenna consisting of a pair of back-to-back modified Vivaldi antennas with two graphene nanoplate pads symmetrically loaded on the feed line is proposed to realize dynamical manipulation of gain. By applying bias voltage from 0 to 4 V, the resistance of the graphene pad can be gradually reduced from 200 to 20Ω. Two slots located on the ground allow two graphene pads to be dc interruption for independent resistance control. By varying the resistance of two graphene pads, the radiation pattern of Vivaldi antenna can be manipulated from two opposite beams to a single beam. Finally, a prototype antenna centering at 30 GHz with a bandwidth of 16.6% is fabricated and measured, which realizes reconfigurable radiation beams at 90° and/or 270° with three states.

Radar-absorbing materials are used in stealth technologies for concealment of an object from radar detection. Resistive and/or magnetic composite materials are used to reduce the backscattered microwave signals. However, nontunable characteristics or the required complex structure hampered the application of these materials. Here, multilayer graphene-based frequency selective surfaces (MLGFSS), which reach a size of 150 mm $x$ 150 mm, are designed and fabricated. By properly changing the growth temperature of MLG using the chemical vapor deposition approach and designing the pattern of graphene layer, the impedance matching condition can be satisfied at different frequencies. As a result, two kinds of absorbers with different working bandwidths are realized. The performances of the proposed absorbers are analyzed using full-wave simulation and are also tested with experimental results. Our method of fabricating large-area MLGFSS avoids the direct contact between the stencil mask and graphene, and guarantees the integrity and quality of patterned graphene structure. A good agreement between simulation and measurement results demonstrates that such ultrathin MLGFSS is very useful in the design of graphene-based functional devices at microwave frequencies.

A low-profile circularly polarized wearable antenna with flexible performance and reduced specific absorption rate (SAR) based on graphene is presented. The antenna adopts highly conductive graphene film and polydimethylsiloxane substrate, which have good flexibility, mechanical stability, and light weight. The wearable antenna operating at 5.8 GHz with a profile of 0.05 λ is designed and fabricated, which retains a reflection coefficient of less than -10 dB when loaded on the human body. The measured results indicate the axial-ratio (AR) bandwidth for AR<; 3 dB covers 5.75-5.83 GHz with the realized gain varying from 5.0-6.1 dBic. The SAR peak is 1.02 W/kg when the human tissue is 1 g and 0.947 W/kg when the human tissue is 10 g, which meets the international SAR standards.