Terahertz Metamaterial with Multiple Resonances for Biosensing Application

A new horizon for neuroscience: terahertz biotechnology in brain research

Terahertz biotechnology has been increasingly applied in various biomedical fields and has especially shown great potential for application in brain sciences. In this article, we review the development of terahertz biotechnology and its applications in the field of neuropsychiatry. Available evidence indicates promising prospects for the use of terahertz spectroscopy and terahertz imaging techniques in the diagnosis of amyloid disease, cerebrovascular disease, glioma, psychiatric disease, traumatic brain injury, and myelin deficit. In vitro and animal experiments have also demonstrated the potential therapeutic value of terahertz technology in some neuropsychiatric diseases. Although the precise underlying mechanism of the interactions between terahertz electromagnetic waves and the biosystem is not yet fully understood, the research progress in this field shows great potential for biomedical noninvasive diagnostic and therapeutic applications. However, the biosafety of terahertz radiation requires further exploration regarding its two-sided efficacy in practical applications. This review demonstrates that terahertz biotechnology has the potential to be a promising method in the field of neuropsychiatry based on its unique advantages.

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.

Overview of Optical Biosensors for Early Cancer Detection: Fundamentals, Applications and Future Perspectives

Conventional cancer detection and treatment methodologies are based on surgical, chemical and radiational processes, which are expensive, time consuming and painful. Therefore, great interest has been directed toward developing sensitive, inexpensive and rapid techniques for early cancer detection. Optical biosensors have advantages in terms of high sensitivity and being label free with a compact size. In this review paper, the state of the art of optical biosensors for early cancer detection is presented in detail. The basic idea, sensitivity analysis, advantages and limitations of the optical biosensors are discussed. This includes optical biosensors based on plasmonic waveguides, photonic crystal fibers, slot waveguides and metamaterials. Further, the traditional optical methods, such as the colorimetric technique, optical coherence tomography, surface-enhanced Raman spectroscopy and reflectometric interference spectroscopy, are addressed.

Tunable MEMS-Based Terahertz Metamaterial for Pressure Sensing Application

In this study, a tunable terahertz (THz) metamaterial using the micro-electro-mechanical system (MEMS) technique is proposed to demonstrate pressure sensing application. This MEMS-based tunable metamaterial (MTM) structure is composed of gold (Au) split-ring resonators (SRRs) on patterned silicon (Si) substrate with through Si via (TSV). SRR is designed as a cantilever on the TSV structure. When the airflow passes through the TSV from bottom to up and then bends the SRR cantilever, the SRR cantilever will bend upward. The electromagnetic responses of MTM show the tunability and polarization-dependent characteristics by bending the SRR cantilever. The resonances can both be blue-shifted from 0.721 THz to 0.796 THz with a tuning range of 0.075 THz in transverse magnetic (TM) mode and from 0.805 THz to 0.945 THz with a tuning range of 0.140 THz in transverse electric (TE) mode by changing the angle of SRR cantilever from 10° to 45°. These results provide the potential applications and possibilities of MTM design for use in pressure and flow rate sensors.

Experimental demonstration of multiple Fano resonances in a mirrored array of split-ring resonators on a thick substrate

This work demonstrates the first experimental observation of multiple Fano resonances in the terahertz range in a system based on an array of mirror-symmetric split-ring resonators deposited on low-loss and low-refractive index polytetrafluoroethylene (PTFE) substrate. For the first time, selective surface activation induced by laser technology has been used to deposit a copper layer on a PTFE substrate with the further application of standard mask lithography for metasurface manufacturing.

Can a terahertz metamaterial sensor be improved by ultra-strong coupling with a high-Q photonic resonator?

When a metamaterial (MM) is embedded in a one-dimensional photonic crystal (PC) cavity, the ultra-strong coupling between the MM plasmons and the photons in the PC cavity gives rise to two new polariton modes with high quality factor. Here, we investigate by simulations whether such a strongly coupled system working in the terahertz (THz) frequency range has the potential to be a better sensor than a MM (or a PC cavity) alone. Somewhat surprisingly, one finds that the shift of the resonance frequency induced by an analyte applied to the MM is smaller in the case of the dual resonator (MM and cavity) than that obtained with the MM alone. However, the phase sensitivity of the dual resonator can be larger than that of the MM alone. With the dielectric perturbation theory - well established in the microwave community - one can show that the size of the mode volume plays a decisive role for the obtainable frequency shift. The larger frequency shift of the MM alone is explained by its smaller mode volume as compared with the MM-loaded cavity. Two main conclusions can be drawn from our investigations. First, that the dielectric perturbation theory can be used to guide and optimize the designs of MM-based sensors. And second, that the enhanced phase sensitivity of the dual resonator may open a new route for the realization of improved THz sensors.

Terahertz circular dichroism sensing of living cancer cells based on microstructure sensor

Terahertz (THz) waves have the advantages of being noninvasive and nonionizing because of their low radiation energy, so they have potential applications in the biomedical field, but thus far, those have been limited by the strong absorption in water and low detection sensitivity. Herein, we propose a reflective THz time-domain circular dichroism (CD) sensing system and a silicon subwavelength grating as the microstructure sensor to generate and detect the THz chiral polarization states, to realize quantitative detection of living cell numbers and qualitative identification of cell kinds in a liquid environment. Three kinds of hepatoma cell proliferation and inhibition with different concentrations of aspirin were measured by this sensing method, and the experimental results show that the sensitivities for CD resonance intensity and frequency shift can reach 3.44 dB mL/10⁶ cells and 5.88 GHz mL/10⁶ cells, respectively, and the minimum detection concentration is in the order of 10⁴ cells/mL for THz detection in a liquid environment for the first time. This new THz sensing system and sensing method are expected to become a broadband, label-free, noncontact, real-time detection technology that can be used for quantitative detection and qualitative identification of cells or other active biochemical materials.

A Review of THz Technologies for Rapid Sensing and Detection of Viruses including SARS-CoV-2

Virus epidemics such as Ebola virus, Zika virus, MERS-coronavirus, and others have wreaked havoc on humanity in the last decade. In addition, a coronavirus (SARS-CoV-2) pandemic and its continuously evolving mutants have become so deadly that they have forced the entire technical advancement of healthcare into peril. Traditional ways of detecting these viruses have been successful to some extent, but they are costly, time-consuming, and require specialized human resources. Terahertz-based biosensors have the potential to lead the way for low-cost, non-invasive, and rapid virus detection. This review explores the latest progresses in terahertz technology-based biosensors for the virus, viral particle, and antigen detection, as well as upcoming research directions in the field.

Design of Tunable Terahertz Metamaterial Sensor with Single- and Dual-Resonance Characteristic

We present two types of refractive index sensors by using tunable terahertz (THz) metamaterial (TTM) based on two concentric split-ring resonators (SRRs) with different splits. By modifying the distance between SRRs and substrate, TTM shows tunable single- and dual-resonance characteristic. The maximum tuning range of resonance is 0.432 THz from 0.958 THz to 1.390 THz. To demonstrate a great flexibility of TTM in real application, TTM device is exposed on the surrounding ambient with different refractive index (n). The sensitivity of TTM can be enhanced by increasing SRR height, which is increased from 0.18 THz/RIU to 1.12 THz/RIU under the condition of n = 1.1. These results provide a strategy to improve the sensing performance of the metamaterial-based sensing device by properly arranging the geometric position of meta-atoms. The proposed TTM device can be used for tunable filters, frequency-selective detectors, and tunable high-efficiency sensors in the THz frequency range.

Tunable Terahertz Metamaterial with Electromagnetically Induced Transparency Characteristic for Sensing Application

We present and demonstrate a MEMS-based tunable terahertz metamaterial (TTM) composed of inner triadius and outer electric split-ring resonator (eSRR) structures. With the aim to explore the electromagnetic responses of TTM device, different geometrical parameters are compared and discussed to optimize the suitable TTM design, including the length, radius, and height of TTM device. The height of triadius structure could be changed by using MEMS technique to perform active tunability. TTM shows the polarization-dependent and electromagnetic induced transparency (EIT) characteristics owing to the eSRR configuration. The electromagnetic responses of TTM exhibit tunable characteristics in resonance, polarization-dependent, and electromagnetically induced transparency (EIT). By properly tailoring the length and height of the inner triadius structure and the radius of the outer eSRR structure, the corresponding resonance tuning range reaches 0.32 THz. In addition to the above optical characteristics of TTM, we further investigate its potential application in a refraction index sensor. TTM is exposed on the surrounding ambient with different refraction indexes. The corresponding key sensing performances, such as figure of merit (FOM), sensitivity (S), and quality factor (Q-factor) values, are calculated and discussed, respectively. The calculated sensitivity of TTM is 0.379 THz/RIU, while the average values of Q-factor and FOM are 66.01 and 63.83, respectively. These characteristics indicate that the presented MEMS-based TTM device could be widely used in tunable filters, perfect absorbers, high-efficient environmental sensors, and optical switches applications for THz-wave optoelectronics.

Enhanced terahertz shielding by adding rare Ag nanoparticles to Ti3C2TxMXene fiber membranes

Polyacrylonitrile/Ti₃C₂T_xMXene/silver nanoparticles fiber membranes with different silver nanoparticles contents and thickness of porous structure have been successfully prepared by electrospinning. Through the measurement of terahertz time domain spectrum, the shielding effect of the fiber membrane with 1% silver nanoparticles content can reach up to 12 dB. Moreover, the thickness of the spinning fiber membranes is controlled by adjusting the spinning time, so as to better analyze the influence of the thickness of the shielding performance in terahertz band. We attribute this excellent phenomenon to porous structure of the spun fiber membrane and combination of Ti₃C₂T_xMXene with few-layers and silver nanoparticles to increase the absorption and conductivity of the fiber membrane, thereby enhancing the shielding effect in terahertz range. Meanwhile, the prepared polyacrylonitrile/Ti₃C₂T_xMXene/silver nanoparticles fiber membranes show good stability and little change in terahertz shielding effect after high temperature annealing. This may provide potential ideas about the development of high-performance terahertz shielding materials, which are of great significance of terahertz electromagnetic shielding.

Tunable infrared metamaterial-based biosensor for detection of hemoglobin and urine using phase change material

This paper reports about the outcomes from an investigation carried out on tunable biosensor for detection using infrared in the range of 1.5 µm and 1.65 µm. The biosensor is made of phase change material formed by different alloy combinations, Ge₂Sb₂Te₅ (GST). The nature of GST allows for the material to change phase with changes in temperature, giving the tunable sensing property for biosensing application. Sensor built with amorphous GST (aGST) and crystalline GST (cGST) in different design structures were tested on different concentrations of biomolecules: hemoglobin (10 g/l, 20 g/l, 30 g/l and 40 g/l); and urine (0-1.5 mg/dL, 2.5 mg/dL, 5 mg/dL and 10 mg/dL). The tunable response observed from the tests demonstrates the potential application of the materials in the design of switching and sensing systems.

Tunable Split-Disk Metamaterial Absorber for Sensing Application

We present four designs of tunable split-disk metamaterial (SDM) absorbers. They consist of a bottom gold (Au) mirror layer anchored on Si substrate and a suspended-top SDM nanostructure with one, two, three, and four splits named SDM-1, SDM-2, SDM-3, and SDM-4, respectively. By tailoring the geometrical configurations, the four SDMs exhibit different tunable absorption resonances spanning from 1.5 µm to 5.0 µm wavelength range. The resonances of absorption spectra can be tuned in the range of 320 nm, and the absorption intensities become lower by increasing the gaps of the air insulator layer. To increase the sensitivity of the proposed devices, SDMs exhibit high sensitivities of 3312 nm/RIU (refractive index unit, RIU), 3362 nm/RIU, 3342 nm/RIU, and 3567 nm/RIU for SDM-1, SDM-2, SDM-3, and SDM-4, respectively. The highest correlation coefficient is 0.99999. This study paves the way to the possibility of optical gas sensors and biosensors with high sensitivity.

Reconfigurable terahertz switch using flexible L-shaped metamaterial

The design of a reconfigurable terahertz (THz) switch by using flexible L-shaped metamaterial (FLM), which is composed of dual-layer L-shaped metamaterials on polydimethylsiloxane substrate, which has three resonances at 0.57, 1.05, and 1.52 THz, is presented. By stretching the FLM along the x-axis direction, the transmission intensity is increased gradually at the transverse electric mode (TE) and reduced at the transverse magnetic (TM) mode, respectively. Reversely, by stretching the FLM along the y-axis direction, the transmission intensity is reduced gradually at the TE mode and increased at the TM mode, respectively. These electromagnetic responses of FLM provide the optical-logic behaviors with programmable characteristics by stretching FLM at different polarized light. It indicates that the proposed FLM could be used for the dual/triple-band switching, polarization switching, and programmable switching applications.

Anisotropic Photonics Topological Transition in Hyperbolic Metamaterials Based on Black Phosphorus

Based on in-plane anisotropy of black phosphorus (BP), anisotropic photonics topological transition (PTT) can be achieved by the proposed hyperbolic metamaterials structure, which is composed of alternating BP/SiO₂ multilayer. Through effective medium theory and calculated iso-frequency contour, PTT can be found by carefully choosing the incident plane and other parameters. With the finite element method and transfer matrix method, a narrow angular optical transparency window with angular full width at half maximum of 1.32° exists at PTT. By changing the working wavelength, thickness of SiO₂, or electron doping of black phosphorus, the incident plane of realizing PTT can be modulated, and anisotropic PTT is achieved.

Reconfigurable Terahertz Metamaterial Using Split-Ring Meta-Atoms with Multifunctional Electromagnetic Characteristics

We propose a reconfigurable terahertz (THz) metamaterial (RTM) to investigate its multifunctional electromagnetic characteristics by moving the meta-atoms of split-ring resonator (SRR) array. It shows the preferable and capable adjustability in the THz frequency range. The electromagnetic characteristics of the proposed RTM device are compared and analyzed by moving the meta-atoms in different polarized transverse magnetic (TM) and transverse electric (TE) modes. The symmetrical meta-atoms of RTM device exhibit a resonant tuning range of several tens of GHz and the asymmetrical meta-atoms of RTM device exhibit the better tunability. Therefore, an RTM device with reconfigurable meta-atoms possesses the resonance shifting, polarization switching, electromagnetically induced transparency (EIT) switching and multiband to single-band switching characteristics. This proposed RTM device provides the potential possibilities for the use of THz-wave optoelectronics with tunable resonance, EIT analog and tunable multiresonance characteristics.

Tunable Infrared Metamaterial Emitter for Gas Sensing Application

We present an on-chip tunable infrared (IR) metamaterial emitter for gas sensing applications. The proposed emitter exhibits high electrical-thermal-optical efficiency, which can be realized by the integration of microelectromechanical system (MEMS) microheaters and IR metamaterials. According to the blackbody radiation law, high-efficiency IR radiation can be generated by driving a Direct Current (DC) bias voltage on a microheater. The MEMS microheater has a Peano-shaped microstructure, which exhibits great heating uniformity and high energy conversion efficiency. The implantation of a top metamaterial layer can narrow the bandwidth of the radiation spectrum from the microheater to perform wavelength-selective and narrow-band IR emission. A linear relationship between emission wavelengths and deformation ratios provides an effective approach to meet the requirement at different IR wavelengths by tailoring the suitable metamaterial pattern. The maximum radiated power of the proposed IR emitter is 85.0 µW. Furthermore, a tunable emission is achieved at a wavelength around 2.44 µm with a full-width at half-maximum of 0.38 µm, which is suitable for high-sensitivity gas sensing applications. This work provides a strategy for electro-thermal-optical devices to be used as sensors, emitters, and switches in the IR wavelength range.

Polarization-Sensitive Metamaterials with Tunable Multi-Resonance in the Terahertz Frequency Range

We propose two designs of polarization-sensitive metamaterials (PSMs), which are composed of face-to-face spilt-ring resonators (SRRs) and a cut-wire resonator (CWR) sandwiched by two face-to-face SRRs. For convenient description, they are denoted as PSM_1 and PSM_2, respectively. PSM_1 and PSM_2 are fabricated by tailoring Au layers with periodic configurations on silicon-on-insulator (SOI) substrates. By changing the incident polarization light, the electromagnetic responses of PSM_1 can be manipulated between single-resonance and dual-resonance, while those of PSM_2 exhibit switching behavior between single-resonance and triple-resonance. By enlarging the distance between the gap centers of the two face-to-face SRRs along the y-axis direction, the electromagnetic responses of PSM_1 show switching characteristics from single-resonance to triple-resonance at the transverse electric (TE) mode and from dual-resonance to triple-resonance at the transverse magnetic (TM) mode. PSM_2 exhibits switching characteristics from single-resonance to triple-resonance at the TE mode and from dual-resonance to quad-resonance at the TM mode. Furthermore, by changing the width of the CWR under the condition of two face-to-face SRRs with a constant gap distance, PSM_2 exhibits stable electromagnetic responses at the TE mode and tunable resonances at the TM mode, respectively. This work paves the way to the possibility of metamaterial devices with great tunability, switchable bandwidth, and polarization-dependence characteristics.

Tunable Terahertz Metamaterial Using an Electric Split-Ring Resonator with Polarization-Sensitive Characteristic

We present a tunable terahertz (THz) metamaterial using an electric split-ring resonator (eSRR), which exhibits polarization-sensitive characteristics. The proposed eSRR is composed of double symmetrical semicircles and two central metal bars. By changing the lengths of two metal bars, the electromagnetic responses can be tuned and switched between dual-band and triple-band resonances in transverse magnetic (TM) mode. Furthermore, by moving the bottom metal bar to change the gap between the two metal bars, the first resonance is stable at 0.39 THz, and the second resonance is gradually blue-shifted from 0.83 to 1.33 THz. The tuning range is 0.50 THz. This means that the free spectrum ranges (FSR) could be broadened by 0.50 THz. This proposed device exhibits a dual-/triple-band switch, tunable filter, tunable FSR and polarization-dependent characteristics. It provides an effective approach to perform tunable polarizer, sensor, switch, filter and other optoelectronics in THz-wave applications.

Tunable multiresonance using complementary circular metamaterial

We present a design of a tunable infrared (IR) resonator by using complementary circular metamaterial (CCM). CCM is composed of concentric rings. It exhibits superior characteristics of narrow multiresonance generated by the coupling between two adjacent concentric rings in the IR wavelength range. An effective modulation of reflection spectra can be realized by changing the height of each concentric ring. By slightly elevating the concentric rings, the corresponding resonances can be switched between on and off states, and the resonances become more sensitive to the surrounding refraction index. The figure of merit (FOM) is 10.91 for CCM exposed on the surrounding environment with different refraction indices. The correlation coefficient is 0.998. The proposed CCM design provides potential applications in refraction index sensors and exhibits the possibility for future multichannel IR switches, environmental sensors, bandpass filters, wavelength-division multiplexing, and so on.