Abstract
Among different sensing platforms, metamaterials composed of subwavelength or deep subwavelength sized metal resonance elements arrays that are etched on semiconductor substrates or dielectric substrates exhibit excellent characteristics due to the strong localization and enhancement of resonance electromagnetic fields. As a new type of detection method, metamaterial sensors can break through the resolution limit of traditional sensors for a small amount of substance and have the advantages of high sensitivity, fast response, and simple measurement. Significant enhancement of the sensing characteristics of metamaterial sensors was realized by optimizing microstructures (single split-ring, double split-ring, nested split-ring, asymmetric split-ring, three-dimensional split-ring, etc.), using ultrathin substrates or low-index substrate materials, etching away local substrate, and integrating microfluidic channel, etc. This paper mainly reviews the research advance on the improvement of sensing characteristics from optimizing resonance structures and changing substrate materials and morphology. Furthermore, the sensing mechanism and main characteristic parameters of metamaterial sensors are introduced in detail, and the development trend and challenge of metamaterial sensing applications are prospected. It is believed that metamaterial sensors will have potential broader application prospects in environmental monitoring, food safety control, and biosensing in the future.
Highlights
Optical sensors have the advantages of high sensitivity, strong resistance to electromagnetic interference, low noise, high electrical and chemical stability, etc., so they have important applications in life science, food safety, chemical monitoring, and environment monitoring [1,2,3,4,5]. e generalized optical sensor can identify the measured substance or monitor the biomedical reaction process by detecting and analyzing the changes of lightwave intensity, phase, polarization, and other related information [6, 7]. e refractive index [8] is the most typical optical parameter. e real part of the refractive index affects the phase of the light wave, the imaginary part affects the intensity of the light wave, and its anisotropic distribution determines polarization and chirality
In 2017, Srivastava et al [54] experimentally demonstrated a dual-surface terahertz metamaterial sensor based on asymmetric split-ring resonator (SRR) with double splits on an ultrathin flexible polyimide substrate with a low refractive index, as shown in Figure 9(c), which can realize sensing on both sides of the structure and reveals a highly enhanced sensitivity
In 2019, Meng et al [58] proposed a metamaterial sensor with an etched trench introduced in the inductance-capacitance gap region of a split-ring resonator, and the results showed an increase in frequency shift and sensitivity when a dielectric material of up to 18 μm thickness was deposited on the sensor surface
Summary
Optical sensors have the advantages of high sensitivity, strong resistance to electromagnetic interference, low noise, high electrical and chemical stability, etc., so they have important applications in life science, food safety, chemical monitoring, and environment monitoring [1,2,3,4,5]. e generalized optical sensor can identify the measured substance or monitor the biomedical reaction process by detecting and analyzing the changes of lightwave intensity, phase, polarization, and other related information [6, 7]. e refractive index [8] is the most typical optical parameter. e real part of the refractive index affects the phase of the light wave, the imaginary part affects the intensity of the light wave, and its anisotropic distribution determines polarization and chirality. The optical refractive index sensor will introduce a resonance mechanism based on various optical effects to enhance the interaction between the light wave and the measured substance. Metamaterials are sensitive to changes in the dielectric properties of the surrounding environment and have strong spectral characteristics for the local enhancement of the incident electromagnetic fields. When the dielectric properties (i.e., refractive index) of the surrounding environment change, the resonant characteristics (resonant amplitude, resonant frequency, phase, etc.) of the electromagnetic waves passing through the metamaterials will change . Improving the sensitivity of metamaterial sensors requires that the local field distribution of the resonant mode of the electromagnetic wave can overlap spatially to a greater extent with the substance being measured, that is, increasing the sensing area to enhance the interaction. Metamaterial sensors have potential applications in areas such as environmental sensing, homeland security, and biosensing
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