Dynamically tunable terahertz passband filter based on metamaterials integrated with a graphene middle layer

The dynamic tunability of a terahertz（THz） passband filter was realized by changing the Fermi energy（EF） of graphene based on the sandwiched structure of metal-graphene-metal metamaterials（MGMs）. By using plane wave simulation, we demonstrated that the central frequency（ f0） of the proposed filter can shift from 5.04 THz to 5.71 THz; this shift is accompanied by a 3 dB bandwidth（ f） decrease from 1.82 THz to 0.01 THz as the EFincreases from 0 to 0.75 eV.Additionally, in order to select a suitable control equation for the proposed filter, the curves of f and f0 under different graphene EFwere fitted using five different mathematical models. The fitting results demonstrate that the Dose Resp model offers accurate predictions of the change in the 3 dB bandwidth, and the Quartic model can successfully describe the variation in the center frequency of the proposed filter. Moreover, the electric field and current density analyses show that the dynamic tuning property of the proposed filter is mainly caused by the competition of two coupling effects at different graphene EF, i.e., graphene-polyimide coupling and graphene-metal coupling. This study shows that the proposed structures are promising for realizing dynamically tunable filters in innovative THz communication systems.

Hybrid metasurface using graphene/graphitic carbon nitride heterojunctions for ultrasensitive terahertz biosensors with tunable energy band structure

Integrating novel materials is critical for the ultrasensitive,multi-dimensional detection of biomolecules in the terahertz(THz)range.Few studies on THz biosensors have used semiconductive active layers with tunable energy band structures.In this study,we demonstrate three THz biosensors for detecting casein molecules based on the hybridization of the metasurface with graphitic carbon nitride,graphene,and heterojunction.We achieved lowconcentration detection of casein molecules with a 3.54 ng/m L limit and multi-dimensional sensing by observing three degrees of variations(frequency shift,transmission difference,and phase difference).The favorable effect of casein on the conductivity of the semiconductive active layer can be used to explain the internal sensing mechanism.The incorporation of protein molecules changes the carrier concentration on the surface of the semiconductor active layer via the electrostatic doping effect as the concentration of positively charged casein grows,which alters the energy band structure and the conductivity of the active layer.The measured results indicate that any casein concentration can be distinguished directly by observing variations in resonance frequency,transmission value,and phase difference.With the heterojunction,the biosensor showed the highest response to the protein among the three biosensors.The Silvaco Atlas package was used to simulate the three samples'energy band structure and carrier transport to demonstrate the benefits of the heterojunction for the sensor.The simulation results validated our proposed theoretical mechanism model.Our proposed biosensors could provide a novel approach for THz metasurface-based ultrasensitive biosensing technologies.

Ultra-sensitive Dirac-point-based biosensing on terahertz metasurfaces comprising patterned graphene and perovskites

Biosensors are a focus of research on terahertz metasurfaces. However, reports of ultra-sensitive biosensors based on Dirac points are rare. Here, a new terahertz metasurface is proposed that consists of patterned graphene and perovskites. This serves as an ultra-sensitive Dirac-point-based biosensor for qualitative detection of sericin.Theoretically, sericin may make graphene n-doped and drive the Fermi level to shift from the valence band to the Dirac point, causing a dramatic decrease in conductivity. Correspondingly, the dielectric environment on the metasurface undergoes significant change, which is suited for ultra-sensitive biosensing. In addition, metal halide perovskites, which are up-to-date optoelectronic materials, have a positive effect on the phase during terahertz wave transmission. Thus, this sensor was used to successfully detect sericin with a detection limit of 780 pg/m L, achieved by changing the amplitude and phase. The detection limit of this sensor is as much as one order of magnitude lower than that of sensors in published works. These results show that the Dirac-pointbased biosensor is a promising platform for a wide range of ultra-sensitive and qualitative detection in biosensing and biological sciences.

Microfluidic integrated metamaterials for active terahertz photonics

A depletion layer played by aqueous organic liquids flowing in a platform of microfluidic integrated metamaterials is experimentally used to actively modulate terahertz(THz)waves.The polar configuration of water molecules in a depletion layer gives rise to a damping of THz waves.The parallel coupling of the damping effect induced by a depletion layer with the resonant response by metamaterials leads to an excellent modulation depth approaching 90%in intensity and a great difference over 210°in phase shift.Also,a tunability of slow-light effect is displayed.Joint time-frequency analysis performed by the continuous wavelet transforms reveals the consumed energy with varying water content,indicating a smaller moment of inertia related to a shortened relaxation time of the depletion layer.This work,as part of THz aqueous photonics,diametrically highlights the availability of water in THz devices,paving an alternative way of studying THz wave–liquid interactions and developing active THz photonics.

High-sensitivity modulation of electromagnetically induced transparency analog in a THz asymmetric metasurface integrating perovskite and graphene

Active control of the electromagnetically induced transparency(EIT)analog is desirable in photonics development.Here,we theoretically and experimentally proposed a novel terahertz(THz)asymmetric metasurface structure that can possess high-sensitivity modulation under extremely low power density by integrating perovskite or graphene.Using the novel metasurface structure with the perovskite coating,the maximum amplitude modulation depth(AMD)of this perovskite-based device reached 490.53%at a low power density of 12.8037 mW/cm^(2).In addition,after the novel THz metasurface structure was combined with graphene,this graphene-based device also achieved high AMD with the maximum AMD being 180.56%at 16.312 mW/cm^(2),and its transmission amplitude could be electrically driven at a low bias voltage.The physical origin of this modulation was explained using a two-oscillator EIT model.This work provides a promising platform for developing high-sensitivity THz sensors,light modulators,and switches.