Contents I. Introduction
Electromagnetic metamaterials (MMs) consisting of highly conducting sub-wavelength metallic resonators enable many unusual electromagnetic properties at user defined frequencies which are not permissible with the naturally occurring materials [1]. Metamaterials have exhibited very interesting properties such as negative refractive index [2], perfect focusing [3], cloaking [4], and resonance modulation in the active and passive modes [5]–[10]. In the unit cell level, metamaterials are composed of array of small structured elements called split ring resonators (SRRs) [11]. Now a day's terahertz radiation ranging from 0.2 to 5 THz have become an interesting research area due to the scientific and technological aspects [12]. In recent times, terahertz metamaterials (MMs) have been explored to be significant in manipulating electromagnetic radiations and capable of building antennas, modulators, next generation high speed communication devices, ultra-sensitive detectors, sensors etc. [12]–[18]. In order to get suitable spectral response from the metamaterial structures, numerous approaches have been employed which include modification of substrate parameters [19], [20], lumped capacitors or varactors, the use of liquid crystals [21], ferromagnetic and ferroelectric techniques [21], microelectromechanical (MEMS) switches [22], and tuning based on near field interactions between nearest neighbor SRRs [23], [24] etc. In terahertz metamaterials, the coupling between the split ring resonators is crucial to determine its role and performance in the device design and its construction. There are several studies which has been recently reported including sensing, polarization rotation, antennas etc. where near field coupling between the resonators is a crucial parameter. Roy Chowdhury et al. have experimentally investigated the ultrafast manipulation of near field coupling between the bright and dark modes in terahertz metamaterials [12]. Singh et al. have reported investigations on coupling between the dark and bright Eigen modes in a terahertz metamaterial [7]. Zhang et al. have studied the terahertz metamaterials with asymmetric transmission where near field coupling was extremely crucial [25]. Although investigations on near field coupling in terahertz metamaterials have been dealt successfully with several hurdles, but still there are other questions which required to be addressed in order to actualize terahertz devices. In this paper, we specifically explore the gap to gap near field capacitive coupling between the coupled meta-resonators in a unit cell leading to shift of fundamental resonance mode. This is achieved by changing the top resonator gap (G2=2, 6, 10, 14, 18, 22, 26 um) while keeping the bottom resonator gap (Gl = 2 um) constant for two separation between top and bottom SRRs (Fig. 1).
