Lanju Liang

Superconducting terahertz metamaterials mimicking electromagnetically induced transparency

We designed and fabricated planar terahertz (THz) metamaterials made from superconducting NbN films to mimic electromagnetically induced transparency (EIT) system. They are characterized using THz time domain spectroscopy over a temperature range from 8 to 300 K. High transmittance and large delay-bandwidth product at transparency window are demonstrated, which mainly arise from the enhanced coupling and decreased damping in superconducting state. The EIT-like spectral response could be tuned in a wide frequency range. By applying two dark resonators with different resonance frequencies coupled with a radiative resonator, we experimentally demonstrated the planar metamaterials mimicking four-level EIT system.

Nonlinear response of superconducting NbN thin film and NbN metamaterial induced by intense terahertz pulses

We present the nonlinear response of superconducting niobium nitride (NbN) thin film and NbN metamaterial with different thicknesses under intense terahertz pulses. For NbN thin film, nonlinearity emerges and superconductivity is suppressed with increasing incident terahertz electric field, and the suppression extent weakens as the film thickness increases from 15 to 50?nm. As the variation in intense terahertz fields alters the intrinsic conductivity in NbN, a consequent remarkable amplitude modulation in NbN metamaterial is observed due to the strong nonlinearity. Absorbed photo density in either NbN film or NbN metamaterial is estimated and used to understand the mechanism of nonlinear response. With a thicker NbN film element of 200?nm, the resonance of the metamaterial shows similar nonlinear modulation accompanied by a lower loss and a higher quality factor compared with a thinner NbN film element of 50?nm, which demonstrates the innovative implementation of strongly enhanced nonlinearity with thick superconducting film elements and the potential for novel applications using nonlinear metamaterial.

Broadband diffuse terahertz wave scattering by flexible metasurface with randomized phase distribution.

Suppressing specular electromagnetic wave reflection or backward radar cross section is important and of broad interests in practical electromagnetic engineering. Here, we present a scheme to achieve broadband backward scattering reduction through diffuse terahertz wave reflection by a flexible metasurface. The diffuse scattering of terahertz wave is caused by the randomized reflection phase distribution on the metasurface, which consists of meta-particles of differently sized metallic patches arranged on top of a grounded polyimide substrate simply through a certain computer generated pseudorandom sequence. Both numerical simulations and experimental results demonstrate the ultralow specular reflection over a broad frequency band and wide angle of incidence due to the re-distribution of the incident energy into various directions. The diffuse scattering property is also polarization insensitive and can be well preserved when the flexible metasurface is conformably wrapped on a curved reflective object. The proposed design opens up a new route for specular reflection suppression, and may be applicable in stealth and other technology in the terahertz spectrum.

Label-free measurements on cell apoptosis using a terahertz metamaterial-based biosensor

Label-free, real-time, and in-situ measurement on cell apoptosis is highly desirable in cell biology. We propose here a design of terahertz (THz) metamaterial-based biosensor for meeting this requirement. This metamaterial consists of a planar array of five concentric subwavelength gold ring resonators on a 10 μm-thick polyimide substrate, which can sense the change of dielectric environment above the metamaterial. We employ this sensor to an oral cancer cell (SCC4) with and without cisplatin, a chemotherapy drug for cancer treatment, and find a linear relation between cell apoptosis measured by Flow Cytometry and the relative change of resonant frequencies of the metamaterial measured by THz time-domain spectroscopy. This implies that we can determine the cell apoptosis in a label-free manner. We believe that this metamaterial-based biosensor can be developed into a cheap, label-free, real-time, and in-situ detection tool, which is of significant impact on the study of cell biology.

Broadband and wide-angle RCS reduction using a 2-bit coding ultrathin metasurface at terahertz frequencies

A novel broadband and wide-angle 2-bit coding metasurface for radar cross section (RCS) reduction is proposed and characterized at terahertz (THz) frequencies. The ultrathin metasurface is composed of four digital elements based on a metallic double cross line structure. The reflection phase difference of neighboring elements is approximately 90° over a broadband THz frequency. The mechanism of RCS reduction is achieved by optimizing the coding element sequences, which redirects the electromagnetic energies to all directions in broad frequencies. An RCS reduction of less than −10 dB bandwidth from 0.7 THz to 1.3 THz is achieved in the experimental and numerical simulations. The simulation results also show that broadband RCS reduction can be achieved at an incident angle below 60° for TE and TM polarizations under flat and curve coding metasurfaces. These results open a new approach to flexibly control THz waves and may offer widespread applications for novel THz devices.

Electrically tunable terahertz metamaterials with embedded large-area transparent thin-film transistor arrays

Engineering metamaterials with tunable resonances are of great importance for improving the functionality and flexibility of terahertz (THz) systems. An ongoing challenge in THz science and technology is to create large-area active metamaterials as building blocks to enable efficient and precise control of THz signals. Here, an active metamaterial device based on enhancement-mode transparent amorphous oxide thin-film transistor arrays for THz modulation is demonstrated. Analytical modelling based on full-wave techniques and multipole theory exhibits excellent consistent with the experimental observations and reveals that the intrinsic resonance mode at 0.75 THz is dominated by an electric response. The resonant behavior can be effectively tuned by controlling the channel conductivity through an external bias. Such metal/oxide thin-film transistor based controllable metamaterials are energy saving, low cost, large area and ready for mass-production, which are expected to be widely used in future THz imaging, sensing, communications and other applications.

Nonlinear terahertz superconducting plasmonics

Nonlinear terahertz (THz) transmission through subwavelength hole array in superconducting niobium nitride (NbN) film is experimentally investigated using intense THz pulses. The good agreement between the measurement and numerical simulations indicates that the field strength dependent transmission mainly arises from the nonlinear properties of the superconducting film. Under weak THz pulses, the transmission peak can be tuned over a frequency range of 145 GHz which is attributed to the high kinetic inductance of 50 nm-thick NbN film. Utilizing the THz pump-THz probe spectroscopy, we study the dynamic process of transmission spectra and demonstrate that the transition time of such superconducting plasmonic device is within 5 ps.

Terahertz narrow bandstop, broad bandpass filter using double-layer S-shaped metamaterials

In this study, double-layer S-shaped metamaterials (MMs) are analyzed by terahertz time-domain spectroscopy. These materials exhibit narrow bandstop and broad bandpass transmission properties at both horizontal and vertical electric-field polarizations. A 117% increase in the unloaded quality factor is experimentally observed for these materials. The center frequency is approximately 0.45 THz, with a 3-dB bandwidth of 0.52 THz from 0.20 to 0.72 THz at normal incidence. The measured average insertion loss is 0.5 dB with a ripple of 1 dB. These results show that double-layer S-shaped MMs are effective in designing tunable terahertz devices.

Effect of loss and coupling on the resonance of metamaterial: An equivalent circuit approach

In this work we establish an equivalent circuit model to analyze the resonace of the metamaterial considering the loss of the unit cell and coupling effect between them. From this model, we find that metamaterial can be divided into three categories: weak, critical and strong couplings, depending on the values of the loss and coupling strength, where the different resonant properties are presented. The physical reason of the division is whether the loss in each unit cell can be offset by energy coupling from the adjunct unit cells. Full-wave electromagnetic simulations have also been carried out to verify the equivalent circuit analysis. Our circuit analysis provides a simple and effective way to understand the coupling of the metamaterial and gives guidance for the analysis and design of the metamaterial.概要创新点人工电磁材料的结构之间存在电磁耦合, 特别是结构距离很近的时候. 这种电磁耦合以及结构的损耗对其谐振特性起很重要的影响. 本文建立了人工电磁材料的等效电路模型, 在模型中分别考虑了电场耦合和磁场耦合以及结构损耗, 并根据耦合强度和损耗值的关系, 把人工电磁材料中的谐振特性分成三类: 强耦合, 临界耦合和弱耦合. 通过对等效电路模型的分析, 得出了人工电磁材料的谐振频率的劈裂和这三类耦合之间的关系, 并从能量的角度解释了其物理机理.

Tailoring electromagnetically induced transparency effect of terahertz metamaterials on ultrathin substrate

Electromagnetically induced transparency (EIT) is a fascinating phenomenon in optical physics and has been employed in slow light technology. In this work, we use terahertz (THz) metamaterials to mimic EIT phenomenon and study their spectral dependence on the coupling strength between bright and dark resonators. In these metamaterials, two kinds of resonators are located on two different layers separated by a 10-µm-thick polyimide (PI) film. The whole sample is supported by a 5-µm-thick flexible PI film, so the Fabry-Perot resonance at THz can be avoided. The coupling strength is tuned by the translational offset of symmetry axes between two different kinds of resonators, resulting in the change of EIT-like spectra.