Terahertz (THz) wave is generally defined as the place of the electromagnetic spectrum in the frequency range from 100 GHz (\(\lambda\)=3 mm) to 10 THz (\(\lambda\) =30 µm). The terahertz wavelength is longer than the infrared wavelength. Therefore, terahertz waves have less dispersion and better penetration depth compared to infrared. However, terahertz waves have a shorter wavelength than microwaves, making them a better choice for high-resolution spatial imaging [1]. Due to the characteristics of waves in this frequency range, many applications are feasible for this area, including wave absorption, terahertz imaging, spectroscopy, and wireless communications [2–5]. Terahertz wave absorption has significant applications and received great attention from researchers in this field. Consequently, various structures for terahertz wave absorption have been designed and proposed in the literature. Classical terahertz absorbers consist of two metallic layers (usually gold), whereas a dielectric layer is placed between them. Various examples of these structures can be found in [6–9].
Noteworthy, the recently emerged structures of absorbers are based on metamaterial structures. A metamaterial is a material whose properties do not commonly exist in nature and have abnormal features. Among the applications that can be mentioned from metamaterials is the negative refractive index, reverse Doppler effects, invisibility cloak, perfect lens, negative refraction, artificial magnetism, transparency, perfect imaging, hyperbolic dispersion, sensing [10–12], perfect absorption [13–16], antennas, super lensing, switches [17, 18], modulators, polarization rotation, filters [19], energy harvesting, multiplexers, [20]. However, with the help of special engineering of natural materials, it is possible to achieve metamaterial-like behaviors. The newly developed metamaterials can be used to control electromagnetic waves by filtering, absorbing, amplifying, or removing unwanted waves. Metamaterial based terahertz absorbers have been designed in the recent years. Single, multi-band, broad band, and Graphene based terahertz metamaterial absorbers have been proposed by the researchers [21]. An important application of terahertz metamaterial absorbers is in sensing. The absorption spectrum of the absorbers varies with the changes in the physical parameters such as, refractive index of the analyte [22], analyte depth [23], and temperature of the surrounding medium [24]. Thus, refractive index sensors, temperature sensors, hybrid sensors, etc. have been proposed based on terahertz metamaterials [25–27].
Refractive index sensing using terahertz metamaterial absorbers has applications in bio-medical sensing, gas sensing, etc. So several researchers have proposed unique metamaterial based absorbers for refractive index sensing. In the initial years, the refractive index sensors proposed by the researchers had low sensitivity [28, 29]. Subsequently, researchers designed highly sensitive refractive index sensors [30–34]. However, these designs were polarization dependent and angle dependent. An ideal absorber should be able to absorb the incident electromagnetic wave irrespective of its polarization and angle of incidence. So, apart from being highly sensitive, the design should offer absorption characteristics that are independent of polarization angle and incidence angle. Recently, in [35–37] polarization and incident angle independent refractive index sensors have been proposed that can also detect the changes in the analyte depth. However, they operate at high terahertz frequencies, well beyond the defined 10 THz boundary, also making their fabrication difficult. Moreover, the achieved peak sensitivity is not significant. In this paper, four different structures are proposed that operate with the 0.1 THz – 10 THz band, and are insensitive to changes in incident electromagnetic wave’s polarization angle and incidence angle. Also, the proposed structure is analyzed in terms of manufacturing feasibility. The sensitivity of four different structures is numerically investigated by changing the thickness of the analyte layer. Finally, field distributions at resonance frequency are investigated.