Abstract-A broad dual-band switchable graphene-based terahertz metamaterial absorber

Limei Qi, Chang Liu, Syed Mohsin, Ali Shah

https://www.sciencedirect.com/science/article/pii/S0008622319306980

A switchable graphene-based terahertz metamaterial absorber is proposed by using nonstructured graphene loaded with simple dielectric resonators, which can achieve both the broad and dual-band absorption with polarization-independent and wide-angle characteristics. The relative bandwidth of the two bands above 80% absorption reaches 97.8% and 31% in the frequency range of 0.473-1.407 THz and 2.273-3.112 THz, respectively. By changing the chemical potential of graphene, the state of the absorber can be switched from absorption (>80%) to reflection (>91%) over the two broad bands. Physical mechanisms of the broad dual-band switchable absorber are investigated by the impedance matching theory and the coupled-mode theory. The dual bands are caused by the Fabry-Perot resonance of dielectric substrate, the broad and high absorption originates from the appropriate impedance match between the absorber and free space. As the absorber is based on a monolayer nonstructured graphene loaded with simple dielectric resonators, the processing difficulty will be reduced greatly and it is easy to tune it through the bias voltage. This structure provides a new perspective to design broad multi-band absorbers and would have promising applications in multiple amplitude modulators, imaging and sensing.

Abstract-Terahertz wide-angle metamaterial absorber fabricated by printed circuit board technique

Journal of Applied Physics

Limei Qi,  Chang Liu

https://aip.scitation.org/doi/10.1063/1.5046520

A wide-angle terahertz metamaterial absorber is designed, fabricated, and measured. The resonant structure on the top layer of the absorber consists of two metal crosses with different sizes. The wider and higher absorption is achieved when compared with the structures that only consist of a large or small cross under different incident angles. Physical mechanisms of the absorption are clarified by their electric field distributions. The influence of the permittivity and the loss tangent of the dielectric on the absorptions are also investigated. The terahertz band sample was developed based on a double-faced copper dielectric plate using the conventional printed circuit board (PCB) technique. This saves time and allows for low cost and large-area fabrication as an alternative to the expensive conventional lithography method. The reflection spectra of the sample at oblique incidence are directly obtained using a vector network analyzer in conjunction with a quasi-optical test bench, which is used instead of the most common Fourier Transform Infrared spectrometry or Terahertz (THz) time-domain spectroscopy method in the terahertz band. The measured results are in good agreement with those of the simulation. The proposed terahertz metamaterial absorber will be crucial in the implementation of future terahertz sensors, THz communication systems, and other emerging THz technologies.

Abstract-Investigating the non-radially polarized component of terahertz wave emission during single-colour femtosecond laser filamentation in air

Jiayu Zhao, Hui Gao, Shichang Li, Chang Liu, Yamin Chen, Yan Peng,  Yiming Zhu

http://iopscience.iop.org/article/10.1088/2040-8986/aadef7/pdf

Recently, simultaneous emission of radially and non-radially polarized terahertz (THz) pulses during single-colour femtosecond laser filamentation has been reported. In this work, the latter radiation has been specifically investigated, instead of the well-studied THz radial polarization. Briefly, cut-back measurements have verified that the ellipticity of the generated THz pulse with non-radial polarization decreased (became more linearly polarized) with the increasing filament length. The underlying mechanism responsible for this phenomenon is the existence of a propagation effect of THz wave along the filament plasma channel. In this case, the resulted off-axis propagation of THz wave inside the plasma column played a dominant role on the generated non-radial THz polarization, rather than the expected on-axis THz birefringence induced by the high laser intensity. This discovery will greatly renew the understanding of THz emission from plasma sources.