We present enhancement of operational bandwidths of planar terahertz metasurfaces by incorporating a complex unit cell that consists of a pair of concentric ring resonators. The inner resonator is a closed ring, while the outer resonator contains a pair of split gaps introduced symmetrically into the two opposite arms. The enhancement of the bandwidth is attributed to the spectral overlapping of the resonant responses arising from both the resonators. A total of five metasurface samples (MM1 to MM5) have been designed, fabricated, and characterized using terahertz time domain spectroscopy (THz-TDS). Resonance broadening along with the blue shifting of higher order resonance mode is being observed while introducing the split gaps in the outer ring and increasing the split gap length (MM1 to MM5) successively. We have further performed detailed numerical investigations in order to explain the experimental observations which support the experimental results. Such metasurfaces can pave the way to realize versatile photonic applications including broadband modulators and band stop filters in the comparatively lesser explored yet technologically relevant terahertz region.

通过结合由一对同心环形谐振器组成的复杂晶胞，我们提出了平面太赫兹超颖表面的工作带宽增强。内部谐振器是一个闭合的环，而外部谐振器包含一对对称引入两个相对臂中的裂隙。带宽的增加归因于两个谐振器产生的谐振响应的频谱重叠。总共使用太赫兹时域光谱仪（THz-TDS）设计，制造和表征了五个超表面样品（MM1至MM5）。在外圈引入裂隙并依次增加裂隙长度（MM1至MM5）的同时，观察到了随着高阶共振模式的蓝移而发生的共振变宽。为了说明支持实验结果的实验​​观察结果，我们进一步进行了详细的数值研究。这样的超表面可为在相对较少探索但技术上相关的太赫兹区域中实现多功能光子应用（包括宽带调制器和带阻滤波器）铺平道路。