A split-ring-resonator-based polarization-insensitive ultra broadband filter in terahertz range

Abstract

Split-ring-resonators (SRRs) are one of the most common unit cell designs for metamaterials. Though extensively studied and well understood, such devices are often used as narrow-band filters due to SRRs' sharp resonance to electromagnetic wave. In this work, based on the idea of patterning metal SRRs on both sides of a dielectric substrate while rotating patterns on one side 90 o to the other, we show that simple circular SRR can be a building block for broadband filters in terahertz (THz) range. The design principle is detailed with simulation results, showing that such fabricated devices essentially equates to two narrow-band filters on both sides of the substrate connected in series. By changing the unit cell from single SRR to double SRR, we effectively expanded the stop band width of the broadband filter. Devices were created on two types of substrates, 1 mm thick quartz and 100 µm thick polyethylene terephthalate (PET), demonstrating the ease and wide applicability of the fabrication process, while a bandwidth of as large as 1.40 THz has been achieved. Metamaterials with many unique optical properties [1][2][3][4][5] are made of periodically arranged sub-wavelength metallic structures that are able to couple to external electromagnetic (EM) waves. One of such structures is the commonly used split-ring-resonators (SRR) [6][7][8]. Due to their simplicity and the close resemblance of their first order resonance to a conventional inductor-capacitor (LC) circuit [7], both single and double SRRs (DSRRs) have been extensively studied and understood as how to tune the resonant frequency by changing structural designs [9][10], the coupling effect between the SRRs [11-13] and so on. Moreover, tunability brought about by microelectromechanical systems (MEMS) technologies [14] and the flexibility of the devices enabled by using flexible substrates [15] have extended their applications even further.The SRR-based metamaterials are known to have sharp and narrow resonances that are polarization-dependent [7,16]. While such narrow resonances are good for optical applications like filters, sensors and detectors, there is an increasing demand for broadband filters with easier fabrication process to give wide margins for manufacturing parameters and allow multi-frequency operations [17]. Most of previous efforts have shunned SRRs for such applications and turned to isotropic patterns like metal patch [17,18], cross patterns [19,20] and closed rings [20,21] instead, especially when designing polarization-insensitive devices. These designs relied on closely stacked metal layers with very thin dielectric layers sandwiched in between. As a result, the control of dielectric layer thickness [17] and perfect alignment of each layer [18,20] are of critical importance to device performance. Han et al. pioneered the exploration of SRR-based broadband metamaterials [22]. Their work cascaded multiple layers of SRRs of gradually changing sizes, and merged the resonances of these layers that lie at gradually shifting frequencies to form ...

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