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-Broad Dual-band Metamaterial Filter with Sharp Out-of-band Rejections

Limei Qi and Syed Mohsin Ali Shah

https://www.osapublishing.org/copp/abstract.cfm?uri=copp-2-6-629

A broad dual-band terahertz metamaterial filter with sharp out-of-band rejections is designed and demonstrated. The center frequencies of the first and the second bands occur at 0.35 THz and 0.96 THz with 3 dB relative bandwidth of 31% and 17%, respectively. Results are measured using a THz time-domain spectroscopy system that shows agreement with simulations. Physical mechanisms of the broad dual-band resonance are clarified based on transmissions of different structures and surface current density distributions. Influence of structure parameters on the transmission characteristics are discussed. Symmetry of the structure ensures the filter polarization independence at normal incidence. These results supported by the design of the filter could find applications in broad multi-band sensors, terahertz communication systems, and other emerging terahertz technologies.

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-One-dimensional multiband terahertz graphene photonic crystal filters

Yizhe Li, Limei Qi, Junsheng Yu, Zhijiao Chen, Yuan Yao, and Xiaoming Liu

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-7-4-1228&origin=search

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.

© 2017 Optical Society of America

Abstract-Multi-Band Terahertz Filter with Independence to Polarization and Insensitivity to Incidence Angles

Limei Qi, Chao Li,
http://link.springer.com/article/10.1007/s10762-015-0202-4

Multi-band terahertz filters with independence to polarization and insensitivity to incidence angles are designed, fabricated, and measured, respectively. The promoted multi-band terahertz filters consist of two and three concentric ring complementary structure. Compared with the dual-ring structure, the triple-ring structure not only increases numbers of the pass bands but also heightens the out-of-band rejection of the middle band. The physical mechanisms of the multi-band resonant responses are clarified using three different configurations and distribution of magnetic fields and current surfaces. At normal incidence, the triple-ring structure is independent to polarization due to the symmetry. At oblique incidences, transmission magnitudes as a function of the frequency are firstly demonstrated for the multi-band filter; it is found that this structure shows great insensitivity to incidence angles. The multi-band filter also has advantages in easy fabrication. The encouraging results afforded by the design of the filters could find applications in multi-band sensors, terahertz communication systems, and other emerging terahertz technologies.

Abstract-Terahertz dual-resonance bandpass filter using bilayer reformative complementary metamaterial structures

Feng Lan, Ziqiang Yang, Limei Qi, Xi Gao, and Zongjun Shi  »View Author Affiliations

Optics Letters, Vol. 39, Issue 7, pp. 1709-1712 (2014)
http://dx.doi.org/10.1364/OL.39.001709

http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-39-7-1709

A dual-resonance frequency selective surface filter in the THz range that uses bilayer modified complementary metamaterial structures is proposed in this Letter. The bandpass filter, with dual bands centered at 0.315 and 0.48 THz, uses a single crystal quartz substrate and is simulated, fabricated, and measured. To minimize the manufacturing risks of working with fragile and thin quartz substrates, efforts have been made to improve the transmission frequency response features at realizable substrate thicknesses. Experimental results from 0.1 to 0.6 THz measured by THz time-domain spectroscopy show excellent agreement with the simulation results.

© 2014 Optical Society of America