Tunable terahertz absorber based on transparent and flexible metamaterial

We demonstrate a tunable terahertz(THz) absorber based on an indium tin oxide(ITO) metamaterial. The upper ITO cross-shaped metasurface with different arm lengths is fabricated by direct femtosecond laser etching.The thickness of the middle dielectric layer is only 60 μm, which makes the absorber very transparent and flexible. The experimental results show that the THz resonant peaks have a high performance near 1 THz. By setting spacers of different thicknesses between the middle layer and the ITO mirror, a new type of tunable THz absorber is proposed. Its absorption peak frequency can be continuously adjusted from 0.92 to 1.04 THz between TE and TM polarization. This transparent THz metamaterial absorber is expected to be widely used in THz imaging, sensing, and biological detection.

Misalignments among stacked layers of metamaterial terahertz absorbers

Misalignment among stacked layers of absor- bers is inevitable in practice. Adverse effects induced by this undesired factor was investigated and analyzed in this paper. The absorption responses of thin terahertz metama- terial （MM） absorber with different degree of misalign- ment were simulated by finite-difference time-domain （FDTD） method under both transverse magnetic （TM） and transverse electric （TE） polarization. Results show that slight misalignment deteriorates absorption response due to the decreased spatial resolution. The analyses are given in terms of the magnetic field distribution in the cross section. In addition, the depravation is changed with polarization, which depends on the direction of excursion.

Bridging the terahertz near-field and far-field observations of liquid crystal based metamaterial absorbers

Metamaterial-based absorbers play a significant role in applications ranging from energy harvesting and thermal emitters to sensors and imaging devices.The middle dielectric layer of conventional metamaterial absorbers has always been solid.Researchers could not detect the near field distribution in this layer or utilize it effectively.Here,we use anisotropic liquid crystal as the dielectric layer to realize electrically fast tunable terahertz metamaterial absorbers.We demonstrate strong,position-dependent terahertz near-field enhancement with sub-wavelength resolution inside the metamaterial absorber.We measure the terahertz far-field absorption as the driving voltage increases.By combining experimental results with liquid crystal simulations,we verify the near-field distribution in the middle layer indirectly and bridge the nearfield and far-field observations.Our work opens new opportunities for creating high-performance,fast,tunable,terahertz metamaterial devices that can be applied in biological imaging and sensing.

Dual-function terahertz metasurface based on vanadium dioxide and graphene

A dual-function terahertz metasurface based on VO_(2) and graphene is proposed in this paper.It consists of a gold layer embedded with VO_(2) patches,a SiO_(2) spacer layer,a VO_(2) layer,graphene and a SiO_(2) spacer substrate.When the bottom VO2 layer is in the metallic state,the designed metasurface can achieve absorption.When the top VO_(2) patches are in the metallic state,the proposed metasurface can be used as a single-band absorber with terahertz absorptance of 99.7% at 0.736 THz.When the top VO2 patches are in the insulating state,the designed structure behaves as a dual-band absorber with an absorptance of 98.9%at 0.894 THz and 99.9% at 1.408 THz.In addition,the absorber is polarization insensitive and keeps good performance at large angles of incidence.When the bottom VO_(2) is in an insulating state,the metasurface shows electromagnetically induced transparency.The transparent window can be dynamically regulated by controlling the chemical potential of graphene.The proposed metasurface exhibits the advantages of terahertz absorption,electromagnetically induced transparency and dynamic control,which provides more options for the design of terahertz devices in the future.

Tunable and multichannel terahertz perfect absorber due to Tamm surface plasmons with graphene

In this paper, we have shown that perfect absorption at terahertz frequencies can be achieved by using a composite structure where graphene is coated on one-dimensional photonic crystal(1 DPC) separated by a dielectric. Due to the excitation of optical Tamm states(OTSs) at the interface between the graphene and 1 DPC, a strong absorption phenomenon occurs induced by the coupling of the incident light and OTSs. Although the perfect absorption produced by a metal–distributed Bragg reflector structure has been researched extensively, it is generally at a fixed frequency and not tunable. Here, we show that the perfect absorption at terahertz frequency not only can be tuned to different frequencies but also exhibits a high absorption over a wide angle range. In addition,the absorption of the proposed structure is insensitive to the polarization, and multichannel absorption can berealized by controlling the thickness of the top layer.

Enhancing sensing capacity of terahertz metamaterial absorbers with a surface-relief design

Metamaterial absorbers(MAs)serve as important electromagnetic wave-absorbing devices that have captured the attention of researchers for a long term.Functioning as sensitive detectors to determine perturbations in an ambient environment is another significant subsidiary function.Here,we theoretically propose an optimized fabrication method to implement terahertz MAs with fewer steps and also evaluate both absorption and sensing performances of such MAs realized by the new method.Simulation findings demonstrate that such MAs can basically maintain the original absorption features perfectly,including near-complete absorption at resonance as well as strong robustness to wide incident angles.Specifically,the full width at half-maximum and quality factor of the absorption resonances attenuate less than 26%and 8%with this new method,remaining in the ranges of^0.03–0.04 THz and^20–27 for two selected example MAs.More significantly,sensing capacities of this type of MA,in terms of maximum detection range(enhancing at least 9%),observable spectral modulation(increasing at least 6.3%),and refractive index sensitivity,are improved to a large extent because of more intense coupling between resonant field and matter in the case of surface-relief MAs.This stronger coupling results from exposing more spots of the resonantly high field to direct contact with an approaching analyte,which is illustrated by field profiles of the MAs at resonance in this work.Additionally,other desirable absorber features are also explored with such MAs,like functioning as building blocks to configure multiband MAs and strong robustness against fabrication errors.Such new-style terahertz MAs shown in the paper,acting as good examples,not only prove that terahertz MAs can be fabricated by the proposed time-and cost-saving route in contrast to the traditional MA fabrication process,but also can serve as novel platforms to explore other intriguing terahertz photonic effects,such as the field enhancement effect.

Nonlinear terahertz metamaterial perfect absorbers using GaAs[Invited]

We investigate the nonlinear response of terahertz(THz) metamaterial perfect absorbers consisting of electric split ring resonators on GaAs integrated with a polyimide spacer and gold ground plane. These perfect absorbers on bulk semi-insulating GaAs are characterized using high-field THz time-domain spectroscopy. The resonance frequency redshifts 20 GHz and the absorbance is reduced by 30% as the incident peak field is increased from 30 to 300 kV/cm. The nonlinear response arises from THz field driven interband transitions and intervalley scattering in the GaAs. To eliminate the Fresnel losses from the GaAs substrate, we design and fabricate a flexible metamaterial saturable perfect absorber. The ability to create nonlinear absorbers enables appealing applications such as optical limiting and self-focusing.

Graphene-metasurface for wide-incident-angle terahertz absorption

We demonstrate a graphene-metasurface structure for tunable wide-incident-angle terahertz wave absorption,which involves depositing planar arrays of Omega-shaped graphene patterns on a silicon dioxide substrate.We also discuss how the graphene Fermi-level layer and various substrates affect the absorption characteristics.The absorption of the proposed terahertz absorber is above 80%at an incident angle of 0°–60°in frequencies ranging from 0.82 to 2.0 THz.Our results will be very beneficial in the application of terahertz wave communications and biomedical imaging/sensing systems.

A spectrum-adjusted white organic light-emitting diode for the optimization of luminous efficiency and color rendering index

High luminous efficiency and high color rendering index(CRI) are both the foremost factors for white organic lightemitting diodes(WOLEDs) to serve as next generation solid-state lighting sources. In this paper, we show that both luminous efficiency and CRI can be improved by adjusting the green/red spectra of WOLEDs. With green emission spectra matching with the human photopic curve, the WOLEDs exhibit higher luminous efficiency and higher CRI. Theoretical calculation shows that by tuning the white emission spectra to maximally match with the human photopic curve, the luminous efficiency can be improved by 41.8% without altering the color coordinates, the color correlated temperature(CCT) and the external quantum efficiency(EQE) of the WOLEDs.