Actively MEMS-Based Tunable Metamaterials for Advanced and Emerging Applications

Surface-enhanced spectroscopy technology based on metamaterials

Surface-enhanced spectroscopy technology based on metamaterials has flourished in recent years, and the use of artificially designed subwavelength structures can effectively regulate light waves and electromagnetic fields, making it a valuable platform for sensing applications. With the continuous improvement of theory, several effective universal modes of metamaterials have gradually formed, including localized surface plasmon resonance (LSPR), Mie resonance, bound states in the continuum (BIC), and Fano resonance. This review begins by summarizing these core resonance mechanisms, followed by a comprehensive overview of six main surface-enhanced spectroscopy techniques across the electromagnetic spectrum: surface-enhanced fluorescence (SEF), surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption (SEIRA), terahertz (THz) sensing, refractive index (RI) sensing, and chiral sensing. These techniques cover a wide spectral range and address various optical characteristics, enabling the detection of molecular fingerprints, structural chirality, and refractive index changes. Additionally, this review summarized the combined use of different enhanced spectra, the integration with other advanced technologies, and the status of miniaturized metamaterial systems. Finally, we assess current challenges and future directions. Looking to the future, we anticipate that metamaterial-based surface-enhanced spectroscopy will play a transformative role in real-time, on-site detection across scientific, environmental, and biomedical fields.

Misalignment in Mechanical Interlocking Heterogeneous Integration: Emergent Behavior and Geometry Optimization

This paper addresses the challenge of misalignment in cantilever-based mechanical interlocking structures used for the heterogeneous integration of integrated circuits (ICs). As IC applications expand into flexible and multi-functional platforms, precise alignment becomes critical to maintaining optimal mechanical and electrical performance. We investigate the effects of X and Y misalignment on snap-through forces in cantilever arrays, focusing on their impact on mechanical integrity. The experimental results demonstrate that for X-axis misalignments below 15%, the increase in the required snap-through force is less than 5%. In contrast, Y-axis misalignment shows an even more negligible impact, with less than a 5% reduction in force for up to 20% misalignment. Additionally, through polynomial fits of the model across a range of cantilever angles, this study provides a design template for future exploration of cantilever interactions using nonlinear mechanics while minimizing computational load. These findings offer valuable insights for optimizing misalignment tolerance and improving the design of interlocking structures for IC integration, contributing to the development of robust systems for next-generation IC devices.

Broadband multi‐layered stepped cone shaped metamaterial absorber for energy harvesting and stealth applications

In this study, a broadband polarization and angle‐independent metamaterial absorber (MA) is investigated in the microwave range. It is made up of a periodic array of multi‐layered metal‐dielectric stepped cones. Since the dimensions of the layers forming the unit cell are different, each layer resonates at different frequencies with overlapping bands. The overall response of the structure, with its extremely wide bandwidth, can be obtained by summing all the overlapping frequency responses corresponding to each layer. In numerical simulation, it is observed that the absorption at normal incidence is above 90% in the frequency range between 9.68 and 17.45 GHz and 95% in the frequency range between 9.91 and 14.86 GHz. The energy harvesting ratios of the structure are also evaluated in a wide spectral band. A power ratio of around 90% is obtained in the same frequency range in accordance with absorption response. A noticeable harvesting efficiency of up to 82% is observed, which represents the energy level converted into electrical energy on resistors.

Microelectromechanical System-Based Reconfigurable Terahertz Metamaterial for Polarization Filter, Switch, and Logic Modulator Applications

The terahertz (THz) metamaterials integrated with microelectromechanical systems (MEMS) have led to the realization of dynamic control in amplitude, phase, polarization, and spin angular momentum of the THz wave. In this study, we demonstrate an MEMS-based reconfigurable THz metamaterial (RTM) composed of a split ring resonator (SRR) for real-time modulation of THz wave. By gradually increasing the polarization angle of the incident THz wave, the resonant frequency of SRR switches from 0.74 to 1.16 THz, and the maximum modulation depth is more than 70%. When the MEMS-based RTM is actuated by different DC bias voltages, the polarization-dependent transmission intensity and resonant frequency of the device can be actively tuned. MEMS-based RTM shows logical function characteristics that can be used for logic modulators by performing the driving voltages and polarization states as 2-bit input signals and quantizing the transmission response as "on" and "off" states. The logic gates of "NAND" are at 0.439 THz and "AND" is at 0.732 THz. These results offer potential applications for the proposed MEMS-based RTM in tunable and reconfigurable polarization filters, optical switches, programmable logic modulators, and so on.

Asymmetric metamaterial sandwich structure with NIM characteristics for THz imaging application

This study presented a unique, miniaturised asymmetric interconnected vertical stripe (IVS) design for terahertz (THz) frequency applications. Therefore, this research aimed to achieve a frequency response of 0 to 10 THz using a 5 × 5 µm2 Silicon (Si) substrate material. Meanwhile, various parametric examinations were conducted to investigate variations in the performance. For example, the unit cell selection process was carefully examined by using various design structures and modifying the structure by adding split gaps and connecting bars between vertical stripes. Furthermore, the proposed sandwich structure design was used to compute the absorbance and reflectance properties. All the analytical examinations were executed utilising the Computer Simulation Technology (CST) 2019 software. The introduced IVS metamaterial exhibits negative index behaviour and has a single resonance frequency of 5.23 THz with an acceptable magnitude of - 24.38 dB. Additionally, the quadruple-layer IVS structure exhibits optimised transmission coefficient behaviour between 3 and 6 THz and 7 to 9 THz, respectively. However, the magnitude of the transmission coefficient increased with the number of material layers. Besides that, the absorbance study shows that using a quadruple-layer structure obtains unique and promising results. Overall, the proposed asymmetric IVS metamaterial design achieves the required performance by using a compact structure rather than extending the dimensions of the design.

Polarization-insensitive phase-modulated metasurface using single thermally stimulated vanadium dioxide chip

Considering that typically more than two pin diodes or other tunable elements are required in the unit cell of polarization-insensitive reconfigurable metasurfaces (RMs), this paper proposes a new approach to design a polarization-insensitive RM unit using only one VO2 chip. A polarization-insensitive phase-modulated metasurface (PMM) using single VO2 chip is presented. The surface layer is composed of an outer ring and an inner cross, with a VO2 chip loaded at the connection of the cross. As the VO2 chip can be connected with the metal patch on all sides, only one VO2 chip is used in this polarization-insensitive design. By thermally controlling VO2 chips switch between low-resistance and high-resistance states, the PMM achieves a 1-bit phase shift within 180° ± 37° from 7.85 to 15 GHz. A prototype is fabricated and measured, and the measured results have verified the correction of the design and analysis of the designed PMM.

Evaluating Hyperbolic Dispersion Materials for Cancer Detection

Current biosensors have limited application in clinical diagnostics as they lack the high order of specificity needed to detect low molecular analytes, especially in complex fluids (such as blood, urine, and saliva). In contrast, they are resistant to the suppression of non-specific binding. Hyperbolic metamaterials (HMMs) offer highly sought- after label-free detection and quantification techniques to circumvent sensitivity issues as low as 105 M concentration in angular sensitivity. This review discusses design strategies in detail and compares nuances in conventional plasmonic techniques to create susceptible miniaturized point-of-care devices. A substantial portion of the review is devoted to developing low optical loss reconfigurable HMM devices for active cancer bioassay platforms. A future perspective of HMM-based biosensors for cancer biomarker detection is provided.

Electric Split-Ring Metamaterial Based Microfluidic Chip with Multi-Resonances for Microparticle Trapping and Chemical Sensing Applications

In this work, an integration of terahertz (THz) electrical split-ring metamaterial (eSRM) with microfluidic chip is presented. This eSRM-based microfluidic chip exhibits multiple resonances in the THz spectrum and trapping selectively microparticle size characteristics. The arrangement of eSRM array is dislocation. It generates the fundamental inductive-capacitive (LC) resonant mode, quadrupole, and octupolar plasmon resonant modes and then exhibits high sensitivity to the environmental refraction index. The trapping structures of microparticles are elliptical barricades on eSRM surface. Thus, the electric field energy is strongly confined within the gap of eSRM in transverse electric (TE) mode and then the elliptical trapping structures are anchored on both sides of the split gap to ensure the microparticles can be trapped and located on the gap. To imitate the microparticle sensing ambient environment qualitatively and quantitatively in the THz spectrum, the microparticles are designed different feature sizes with different refraction index from 1.0 to 2.0 in ethanol medium. The results show the proposed eSRM-based microfluidic chip possesses the trapping and sensing abilities in single microparticle and high sensitivity for fungus, microorganism, chemical and environmental applications.

GeTe2 Phase Change Material for Terahertz Devices with Reconfigurable Functionalities Using Optical Activation

The phenomenon of phase change transition has been a fascinating research subject over decades due to a possibility of dynamically controlled materials properties, allowing the creation of optical devices with unique features. The present paper unravels the optical characteristics and terahertz (THz) dielectric permittivity of a novel phase change material (PCM), GeTe2, prepared by pulsed laser deposition (PLD) and their remarkable contrast in crystalline and amorphous states, in particular, a difference of 7 orders of magnitude in conductivity. The THz spectra were analyzed using the harmonic oscillator and Drude term. Using GeTe2 PLD films, we designed and prepared a THz metasurface in the form of periodic structure and revealed a possibility of tuning the THz resonance either by a thermal control or light-induced crystallization response, thus achieving the dynamic and tunable functionality of the metastructure. We propose controlling the state of metasurface by observing the intensity characteristics of the Raman peak of 155 cm-1. Density functional theory (DFT) modeling demonstrates that in the process of crystallization the mode intensity of 155 cm-1 assigned to Te-Te stretching in amorphous chain fragments decreases and disappears at full crystallization.

Reconfigurable Split Ring Resonators by MEMS-Driven Geometrical Tuning

A novel approach for dynamic microwave modulation is proposed in the form of reconfigurable resonant circuits. This result is obtained through the monolithic integration of double split ring resonators (DSRRs) with microelectromechanical actuators (MEMS) for geometrical tuning. Two configurations were analyzed to achieve a controlled deformation of the DSRRs' metamaterial geometry by mutual rotation or extrusion along the azimuthal direction of the two constituent rings. Then, the transfer function was numerically simulated for a reconfigurable MEMS-DSRR hybrid architecture where the DSRR is embedded onto a realistic piezo actuator chip. In this case, a 370 MHz resonance frequency shift was obtained under of a 170 µm extrusion driven by a DC voltage. These characteristics in combination with a high Q factor and dimensions compatible with standard CMOS manufacturing techniques provide a step forward for the production of devices with applications in multiband telecommunications and wireless power transfer and in the IoT field.