Ruiqi Zhao

Metamaterials (MMs) enable the manipulation of terahertz (THz) waves, which are vital for numerous applications due to their unique spectral fingerprints, non-ionizing nature, sensitivity to weak resonances, and strong penetration through non-polar materials. However, detecting minor disturbances and subtle features in biological and chemical sensing is challenging when the object’s scale is smaller than the THz wavelength. This limits the development of THz bio-chemical sensing and detection technology. THz MMs offer a solution, with recent research combining these materials with new structures and mechanisms to enhance sensitivity. This article reviews the progress in THz MMs for bio-chemical sensing, focusing on key performance parameters and mechanisms like spoof surface plasmon polaritons (SSPPs), bound states in the continuum (BIC), and electromagnetically induced transparency (EIT). It explores future directions in biomedical diagnostics, non-destructive testing, security, and environmental sensing, highlighting the transformative impact of MMs on THz technology.

Electromagnetically induced transparency (EIT) metamaterials (MTMs) based on the bright-dark mode theory have gained great interest in slow light, sensing, and energy storage in recent years. Typically, various split ring resonators with magnetic response have been proposed as dark resonators in EIT MTMs. Here, we have employed a cut-wire (CW) and two electric-field-coupled inductor-capacitor (ELC) resonators with a pure electrical response on a liquid crystal polymer (LCP) substrate with a low loss tangent to fulfill the EIT effect in the terahertz (THz) region. The former works as the bright mode, and the latter functions as the dark mode. The EIT phenomenon results from the destructive interference between these two modes, which can be verified by numerical simulation and near field distribution. In addition, a Lorentz oscillator model was studied to quantitatively analyze the relationship between the coupling strength and the coupling distance. As a demonstration, an EIT MTM device with 5000 units was fabricated and characterized, which showed a transmission window with a peak value of 0.75 at 0.414 THz. This work may inspire new multifunctional EIT MTMs, especially the flexible applications at THz frequencies.

The spoof surface plasmon polaritons (SSPPs) structure can be used as a sensor in THz region for the biosensing. The accuracy of resonance and amplitude for sensor is very important for biosensing. The momentum matching of SSPPs determines the resonance position and the gap distance determines the amplitude. For the biomolecular sensing, the sample is positioned between the prism base and the SSPPs structure. The momentum matching condition at the current study does not consider the effect of sample refractive index and the resonance position has a significant error. Here the correction is made to the momentum matching condition which considers the effect of the sample refractive index. A comparative study of surface plasmon resonance (SPR) sensing performance based on frequency and angle variations shows that the sensing sensitivity for frequency region is superior to that of angle region; in the meanwhile, as an application of biosensors, we have detected different types of brain lesions in the frequency range. Furthermore, the reflection amplitude is related to gap size between the prism and SSPPs. The relationship of gap size and reflection amplitude is studied. By using the relationship between gap size and reflection amplitude, the amplitudes at different frequencies or incident angles for different refractivities have the same reflection dips compared to the other published results. The simulation is performed and the results proved the theory.

Surface plasmon resonance sensing technology has many advantages, such as high sensitivity, no interference to samples and real-time monitoring. It has been widely used in biomedical detection, food safety testing and environmental protection etc. How to increase the measurement sensitivity is a key technology in these applications. In this paper, a stepped surface plasmon resonance sensing structure is proposed, which is easy to be machined. The influence of its structure parameters on the sensing sensitivity is simulated and analyzed. The results show that the sensitivity can be increased to 2.44THz/RIU by optimizing the structure parameters when the refractive index of the liquid to be measured is between 1.33 and 1.36.

High-resolution microscopy technique is of significant importance for studying nanomaterials. It is necessary to understand the near-field interaction between the probe and substrate materials in order to get the fine structure of the nanomaterial in the subwavelength scale. The numerical methods such as FDTD, FEM, and MoM are inefficient for the SNOM problems because of the illness of the impedance matrix. The analytic method can only be used for some simple objects such as sphere. Here, a quasianalytical method is developed, in which the analytic formula is refined to adapt to various shapes of the probe approaching the curve of SNOM. By this way, it is helpful in comparing the performance of different probes and giving us a direction to design a new type probe in SNOM. As an application, the developed method is used to study the contrast in the SNOM for the interface between the two different surfaces that have different materials and topography.