Metamaterial absorbers are used in the terahertz frequency regime as photo-detectors, as sensing elements, in imaging applications, etc. Narrowband absorbers, on account of their ultra-slender bandwidth within the terahertz frequency spectrum, show a significant shift in the absorption peak when an extrinsic entity relative to the absorber, like refractive index or temperature of the encircling medium, is altered. This property paves the path for the narrowband absorbers to be used as potential sensors to detect any alterations in the encircling medium. In this paper, a novel design of a terahertz metamaterial (MTM) absorber is proposed, which can sense the variations in the refractive index (RI) of the surrounding medium. The effective permeability of the structure is negative, while its permittivity is positive; thus, it is a μ-negative metamaterial. The layout involves a swastika-shaped design made of gold on top of a dielectric gallium arsenide (GaAs) substrate. The proposed absorber achieved a nearly perfect absorption of 99.65% at 2.905 terahertz (THz), resulting in a quality factor (Q-factor) of 145.25. The proposed design has a sensitivity of 2.12 THz/RIU over a range of varied refractive index from n = 1.00 to n = 1.05 with a step size of 0.005, thereby achieving a Figure of Merit (FoM) of 106. Furthermore, the sensor was found to have a polarization-insensitive characteristic. Considering its high sensitivity (S), the proposed sensor was further tested for gas sensing applications of harmful gases. As a case study, the sensor was used to detect chloroform. The proposed work can be the foundation for developing highly sensitive gas sensors.

This research aims to propose a terahertz metamaterial-based absorber that can sense the alterations in the enclosing medium’s refractive index. The suggested layout comprises a pair of concentric resonators made of gold, each resembling a ring in shape, and is mounted upon a substrate comprising of gallium arsenide (GaAs). The periodicity of the unit cell in this design is only 48 μm. At 2.47 THz, it achieves a high-quality factor (Q-factor) of 61.75 and a nearly perfect absorption of 99.50%. Parametric analyses have been performed to support the selection of the parameters used in the design. Modifications in the polarization angle do not affect the design and the absorption spectra. Because numerous biomedical samples fall within this range, the refractive index has been adjusted within the range of 1.35 to 1.39. Using the proposed sensor, 560 absorption spectra are obtained for different polarization angles, incident angles, and cell specimens of normal and cancerous skin tissue. Different machine learning algorithms have been used to classify the cells based on the absorption spectrum obtained from the proposed sensor with an accuracy of 100% and a precision and recall of 100% and 100%, respectively, on the test data. The proposed work can pave the way for future research combining machine learning and sensing at the terahertz frequency.

This research proposes an all-metal metamaterial-based absorber with a novel geometry capable of refractive index sensing in the terahertz (THz) range. The structure consists of four concentric diamond-shaped gold resonators on the top of a gold metal plate; the resonators increase in height by 2 µm moving from the outer to the inner resonators, making the design distinctive. This novel configuration has played a very significant role in achieving multiple ultra-narrow resonant absorption peaks that produce very high sensitivity when employed as a refractive index sensor. Numerical simulations demonstrate that it can achieve six significant ultra-narrow absorption peaks within the frequency range of 5 to 8 THz. The sensor has a maximum absorptivity of 99.98% at 6.97 THz. The proposed absorber also produces very high-quality factors at each resonance. The average sensitivity is 7.57/Refractive Index Unit (THz/RIU), which is significantly high when compared to the current state of the art. This high sensitivity is instrumental in detecting smaller traces of samples that have very correlated refractive indices, like several harmful gases. Hence, the proposed metamaterial-based sensor can be used as a potential gas detector at terahertz frequency. Furthermore, the structure proves to be polarization-insensitive and produces a stable absorption response when the angle of incidence is increased up to 60°. At terahertz wavelength, the proposed design can be used for any value of the aforementioned angles, targeting THz spectroscopy-based biomolecular fingerprint detection and energy harvesting applications.

This study presents a cost-effective Hybrid Metamaterial Absorber (HMA) featuring a simple circular-patterned cylindrical design, comprising an indium antimonide (InSb) resonator on a thin copper sheet. Through numerical simulations, we demonstrate that the structure exhibits temperature-tunable properties and refractive index sensitivity. At 300 K (refractive index = 1), a peak absorption of 99.94% is achieved at 1.797 THz. Efficient operation is observed across a 40 K temperature range and a refractive index spectrum of 1.00–1.05, relevant for thermal imaging and spatial bio-sensing. The simulated temperature sensing sensitivity is 13.07 GHz/K, and the refractive index sensitivity is 1146 GHz/RIU. Parametric analyses reveal tunable absorption through adjustments of the InSb resonator design parameters. Owing to its high efficiency and sensitivity demonstrated in simulations, this HMA shows promise for sensing applications in biotechnology, semiconductor fabrication, and energy harvesting.

Terahertz (THz) metamaterial absorbers have become a prominent research topic in recent years. In this paper, a hexa-band metamaterial absorber is designed for bio-medical sensing applications. The design can detect changes in the surrounding medium’s refractive index and operates in the refractive index range of 1.3–1.4, with six prominent absorption peaks. The proposed structure comprises a square ring resonator made up of gold with a boss-cross structure at the center, on top of a Gallium Arsenide (GaAs) substrate having a thickness of 8 μm. When the surrounding medium’s refractive index is 1.4, it offers six absorption peaks at 0.537 THz, 2.573 THz, 3.025 THz, 3.146 THz, 3.489 THz, 3.7348 THz, with corresponding peak absorption of 75%, 92.9%, 98.4%, 98.71%, 94.1%, and 99.34%, respectively. The structure has been designed at n = 1.4 instead of n = 1, as several biological specimens, such as blood, breast cells, etc., have refractive index in the range of 1.3–1.4, and it offers 6 bands for n = 1.4. This choice was made because many biomedical applications have a refractive index around 1.4. The design parameters were selected through a parametric analysis, so as to achieve maximum absorption peaks. The design has also been tested with different polarization angles, and it has been discovered that the absorber is polarization-insensitive. This design can inspire future research on the biomedical application of THz absorbers.