All-Metal Terahertz Metamaterial Absorber and Refractive Index Sensing Performance

All-Metal Metamaterial-Based Sensor with Novel Geometry and Enhanced Sensing Capability at 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.

Metamaterial enhanced sensor for powder material classification

In this paper, a simple and efficient approach is presented to classify different power materials based on a one port microwave sensor in X-band. This classification focuses on powder materials, unlike prior studies that focused on liquids (castor oil, neem oil, sunflower oil, sesame oil, and mahua oil), this classification represents a shift towards powdered materials. The response of the proposed sensor is enhanced by adding a metamaterial (MTM) unit cell of F-shape to focus electromagnetic waves on the sample under test. This metamaterial-based sensor is designed to differentiate between different types of materials based on the corresponding reflection coefficient. The sample under test is included inside a dielectric box inserted inside a rectangular waveguide. The MTM unit cell is added on the front face of this box towards the direction of the incident wave. The resonance frequency depends on the characteristics of the powder material inside the box. The MTM unit cell enhances this resonance to simplify the process of classification of different materials. The measured results show that the proposed sensor can detect a wide range of powder materials, including clay, cement, sand, and mixtures: cement & sand, and clay & sand. The designed sensor can be used in various applications, including detection and classification of different powder materials in industrial applications.

Highly sensitive refractive index sensing with a dual-band optically transparent ITO-based perfect metamaterial absorber for biomedical applications

In this paper, a dual-band optically transparent square-shaped perfect metamaterial absorber operating in the frequency range from 2 to 4 terahertz (THz) is proposed. The structure consists of an indium tin oxide (ITO)-based split ring resonator (SRR) structure with additional splits and rectangular inner strips to form the top layer over the lead glass substrate. Perfect absorption is attained in the frequencies of 2.089 and 3.892 THz with absorbances of 99.99% and 99.98% in TE polarization mode, respectively. Perfect absorption is also achieved in TM polarization mode at 2.23 THz. Broadband absorption is found in TM polarization mode with full width half maximum (FWHM) of 1.1742. The proposed structure has one polarization-insensitive band in TE polarization mode. Absorbance is greater than 80% and 90% in the successive absorption peaks even at 60° and 75° of incidence, respectively. The resonance frequency is sensitive to the refractive index of the medium. As a result, the proposed metamaterial structure may be implemented as a refractive index (RI) sensor with a high sensitivity of 1109 GHz/RIU and 1954 GHz/RIU in both absorption bands for a refractive index range of 1.34 to 1.40. It's interesting to note that the refractive index of most biological samples ranges from 1.3 to 1.39. The figure of merit (FOM) of the proposed sensor can reach as high as 10 and 14 for the 1st and 2nd frequency bands. As a result, the proposed sensor has a high sensitivity and can be employed in medical applications. Potential applications of the proposed absorber include imaging, biomedical sensing, etc.

Ultrasensitive refractive index sensor based on stainless steel metamaterial

Terahertz metamaterial technology, as an efficient nondestructive testing method, has shown great development potential in biological detection. This paper presents a stainless steel terahertz metamaterial absorber that achieves a near-perfect absorption of incident metamaterial waves with a 99.99% absorption at 2.937 THz. We demonstrate the theoretical discussion about the absorber and the application in sensing. The effect of the metamaterial absorber's structural parameters on the sensing performance is also analyzed. Simulation results show that the sensor can detect analytes with a refractive index between 1.0 and 1.8. Additionally, the performance of the sensor in detecting analytes in three states (solid, liquid, and gas) is analyzed in detail, and the sensitivity and the FoM of the sensor to detect methane are 22.727 THz/RIU and 568.175R I U -1, respectively. In addition, the terahertz sensor has the advantage of wide incident angle insensitivity, maintaining a good sensing performance within a wide manufacturing tolerance range of -10% to 10%. Compared to metal-dielectric-metal or dielectric-metal structures, the proposed sensor adopts stainless steel as the only manufacturing material, which has the advantages of simple structure, low manufacturing costs, and high sensitivity, and has potential application prospects in label-free high-sensitivity biomedical sensing.

A high Q-factor dual-band terahertz metamaterial absorber and its sensing characteristics

In this paper, a dual-band metamaterial absorber in the terahertz frequencies is proposed and its refractive index sensing characteristics is analyzed. The metamaterial structure is designed using a square metal ring with four T-shaped strips loaded outside of the ring, where the metal periodic array is on top of a silicon wafer backed with a metal ground plane. The resonant frequencies of the absorber are at 0.89 and 1.36 THz, whose absorption rates are both over 99% under normal TE and TM polarized incidences. The full widths at half maximum of them are 4.4 and 11.2 GHz, respectively, resulting in high quality factors (Q-factors) for these two frequency bands. The absorption rate of the absorber remains stable as the incident and polarized angles are changed. Several proposed metamaterial absorbers are experimentally fabricated and electron beam lithography (EBL) technology is employed. Good measurement results of the dual-band absorption performance are obtained using a terahertz time-domain spectroscopy system based on photoconductive antennas. Furthermore, the metamaterial absorber also shows sensing properties for analytes with different refractive indices or thicknesses. This work provides a new choice for the design of high-Q dual-band terahertz metamaterial absorbers and their application to refractive index sensing.

Dual-band terahertz absorber based on square ring metamaterial structure

In this paper, a dual-band terahertz absorber based on metamaterial structure is designed, fabricated, and measured. The metal periodic array is located on the upper surface of a silicon wafer with a metal ground plane, while the metamaterial structure is created utilizing a square metal ring with four T-shaped metal strips loaded inside of the ring. Two absorption peaks are realized at 0.715 and 1.013 THz with high Q-factors of 152.1 and 98.3, respectively, under normal TE and TM polarized incidence. A prototype of the proposed metamaterial absorber is fabricated by electron beam lithography (EBL) and electron beam evaporation (EBE) technology. Furthermore, a terahertz time-domain spectroscopy (TDS) measurement system is employed to test the absorber sample, with good measurement results obtained. This work provides a new option for the design of multi-band terahertz metamaterial absorbers.

Actively MEMS-Based Tunable Metamaterials for Advanced and Emerging Applications

In recent years, tunable metamaterials have attracted intensive research interest due to their outstanding characteristics, which are dependent on the geometrical dimensions rather than the material composition of the nanostructure. Among tuning approaches, micro-electro-mechanical systems (MEMS) is a well-known technology that mechanically reconfigures the metamaterial unit cells. In this study, the development of MEMS-based metamaterial is reviewed and analyzed based on several types of actuators, including electrothermal, electrostatic, electromagnetic, and stretching actuation mechanisms. The moveable displacement and driving power are the key factors in evaluating the performance of actuators. Therefore, a comparison of actuating methods is offered as a basic guideline for selecting micro-actuators integrated with metamaterial. Additionally, by exploiting electro-mechanical inputs, MEMS-based metamaterials make possible the manipulation of incident electromagnetic waves, including amplitude, frequency, phase, and the polarization state, which enables many implementations of potential applications in optics. In particular, two typical applications of MEMS-based tunable metamaterials are reviewed, i.e., logic operation and sensing. These integrations of MEMS with metamaterial provide a novel route for the enhancement of conventional optical devices and exhibit great potentials in innovative applications, such as intelligent optical networks, invisibility cloaks, photonic signal processing, and so on.

Biosensing on a Plasmonic Dual-Band Perfect Absorber Using Intersection Nanostructure

Optical absorbers with multiple absorption channels are required in integrated optical circuits and have always been a challenge in visible and near-infrared (NIR) region. This paper proposes a perfect plasmonic absorber (PPA) that consists of a closed loop and a linked intersection in a unit cell for sensitive biosensing applications. We elucidate the physical nature of finite element method simulations through the absorptance spectrum, electric field intensity, magnetic flux density, and surface charge distribution. The designed PPA achieves triple channels, and the recorded dual-band absorptance reaches 99.64 and 99.00% nm, respectively. Besides, the sensitivity can get 1000.00 and 650 nm/RIU for mode 1 and mode 2, respectively. Our design has a strong electric and magnetic field coupling arising from the mutual inductance and the capacitive coupling in the proposed plasmonic system. Therefore, the designed structure can serve as a promising option for biosensors and other optical devices. Here, we illustrated two examples, i.e., detecting cancerous cells and diabetes cells.

A Theoretical Terahertz Metamaterial Absorber Structure with a High Quality Factor Using Two Circular Ring Resonators for Biomedical Sensing

Metamaterial absorbers, on account of their inherent property of electromagnetic radiation absorption, have become a center of attraction for many researchers in recent times. This paper proposes a unique design of a terahertz metamaterial absorber that can be used to sense biomedical samples. The proposed design consists of two identical circular ring resonators (CRRs) made of aluminum on top of a gallium arsenide (GaAs) substrate. On account of its high field confinement in the sensing regime, a near-to-perfect absorption rate of 99.50% is achieved at a frequency of 2.64 THz, along with a large quality factor (Q-Factor) of 44. The design is highly sensitive to the refractive index changes in the encompassing medium. Hence, the proposed absorber can be used as a refractive index sensor exhibiting a reasonable sensitivity of 1500 GHz/RIU and a figure of merit (FoM) of 25. The refractive index range has been varied in the range of 1.34 to 1.39. As many biomedical samples, including cancerous cells, reside within this range, the proposed sensor can be used for biomedical sensing applications.