| Metadata | Details |
|---|
| Publication Date | 2025-01-16 |
| Journal | Sensors |
| Authors | Sagnik Banerjee, Ishani Ghosh, C. Santini, Fabio Mangini, Rocco Citroni |
| Institutions | Sapienza University of Rome |
| Citations | 8 |
| Analysis | Full AI Review Included |
- Novel Design for THz Sensing: An all-metal metamaterial absorber (MMA) featuring a novel step-pyramidal geometry is proposed for enhanced refractive index sensing in the Terahertz (THz) frequency range.
- Structure and Material: The unit cell consists of four concentric gold square resonators placed on a 2 ”m thick gold ground plate, with resonator heights increasing in 2 ”m steps (6 ”m to 12 ”m) from the outer to the inner ring.
- Hexa-Narrowband Absorption: The design achieves six distinct, ultra-narrow resonant absorption peaks between 5 THz and 8 THz, with the highest peak reaching 99.98% absorptivity at 6.977 THz.
- High Sensitivity: The sensor demonstrates a high average sensitivity of 7.57 THz/RIU (Refractive Index Unit), significantly outperforming many conventional triple-layered metamaterial sensors.
- Excellent Stability: The structure is confirmed to be polarization-insensitive (stable up to 90° polarization angle) and maintains stable absorption performance up to a 60° angle of incidence.
- Target Applications: The high sensitivity and non-invasive probing capabilities make it ideal for detecting trace amounts of samples, including harmful gases (e.g., Benzene, Chlorine) and potential use in THz spectroscopy for biomolecular fingerprint detection.
| Parameter | Value | Unit | Context |
|---|
| Unit Cell Periodicity (u) | 86 | ”m | Optimized dimension |
| Ground Plate Thickness (t) | 2 | ”m | Gold base layer |
| Resonator Material | Gold (Au) | N/A | Conductivity: 4.56 x 107 Sm-1 |
| Ring Thickness (a) | 2 | ”m | Uniform thickness for all rings |
| Outer Ring Height (b) | 6 | ”m | First ring height |
| Inner Ring Heights (b1, b2, b3) | 8, 10, 12 | ”m | Increasing height steps (step-pyramidal) |
| Frequency Range Analyzed | 5 to 8 | THz | Terahertz gap |
| Number of Absorption Peaks | 6 | N/A | Hexa-narrowband response |
| Peak Absorption (Max) | 99.98 | % | At 6.977 THz |
| Average Sensitivity (s) | 7.57 | THz/RIU | Average across all six peaks |
| Max Sensitivity (s) | 11.03 | THz/RIU | At 7.934 THz peak |
| Max Quality Factor (Q) | 793.4 | N/A | At 7.934 THz peak |
| Max Figure of Merit (FoM) | 1103.57 | N/A | At 7.934 THz peak |
| Polarization Stability | Insensitive | N/A | Stable up to 90° angle |
| Incidence Angle Stability | Stable | N/A | Stable up to 60° angle |
- Metamaterial Design and Geometry: A unit cell was designed featuring four concentric square gold resonators placed on a gold ground plane. The key novelty is the step-pyramidal structure, where the height of the resonators increases sequentially (6 ”m, 8 ”m, 10 ”m, 12 ”m) from the outer to the inner ring.
- Material Modeling: The optical characteristics of the gold (Au) resonators and ground plate were modeled using the Drude model, defining the complex permittivity based on the plasma angular frequency (Ïm) and damping constant (Îł).
- Simulation Environment: Numerical simulations were executed using CST Microwave Studio, employing the Finite Integration Technique (FIT) within the frequency domain solver to analyze electromagnetic wave responses.
- Boundary Conditions: Periodic boundary conditions were applied to the lateral sides of the unit cell to simulate an infinite array, while wave propagation was analyzed along the z-axis.
- Parametric Optimization: Parametric sweeps were conducted on critical geometrical parametersâunit cell periodicity (u = 85 to 87 ”m), ground plate thickness (t = 1 to 3 ”m), and outermost ring height (b = 5 to 7 ”m)âto achieve optimal impedance matching with free space (377 Ω) and maximize absorptivity.
- Performance Characterization: The absorption (A = 1 - Rw) was calculated, and sensing performance was quantified by measuring the frequency shift (Îf) versus refractive index change (În) to determine sensitivity (s = Îf/În) and Figure of Merit (FoM).
- Stability Analysis: The structureâs robustness was tested by simulating absorption spectra across varying polarization angles (0° to 90°) and incidence angles (0° to 60°).
- Environmental and Safety Monitoring: High sensitivity enables the detection of minute traces of harmful gases (e.g., Benzene, Chloroform, Carbon Monoxide) whose refractive indices are highly correlated and closely placed.
- Biomedical Diagnostics and Biosensing: The THz frequency range is non-ionizing and sensitive to weak resonances, making the sensor suitable for label-free probing of micro-samples, such as cancer cells and biomolecular fingerprint detection.
- THz Spectroscopy: The multi-band, high-Q factor response provides high spectral resolution, useful for advanced THz spectroscopy applications.
- Next-Generation Communications (6G): The underlying metamaterial technology is relevant for developing high-performance THz components, including antennas for Integrated Sensing and Communications (ISAC) and massive MIMO (m-MIMO) systems.
- Energy Harvesting: Metamaterial absorbers, particularly those operating at optical frequencies, are utilized in energy harvesting applications, including solar energy conversion.
View Original Abstract
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 ”m 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.
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