Terahertz Refractive Index Sensor Research Articles

Graphene-based tunable tri-band terahertz refractive index sensor

A tunable tri-band terahertz refractive index sensor based on graphene is proposed and designed. The structure of the sensor comprises a gold (Au) substrate layer, a dielectric layer (Topas), and a graphene pattern layer. The numerical simulation results demonstrate that the sensor exhibits three narrowband absorption peaks at 1.68, 3.82, and 4.78 THz, and the corresponding absorption rate can reach 99.9%, 98.0%, and 97.9%, respectively. By adjusting the Fermi level of graphene, the resonant frequency of the sensor can be effectively tuned. Notably, due to the rotational symmetry of the structure, the sensor shows insensitivity to transverse magnetic and transverse electric polarizations of incident light. By varying the refractive index of surrounding analytes, the sensor’s sensitivity and Figure of Merit (FOM) are studied. The sensitivities of the three resonance peaks are 0.59, 1.19, and 1.38 THz/RIU, with corresponding FOM values of 2.57, 3.40, and 6.58, respectively. Furthermore, the feasibility and effectiveness of the designed sensor in practical applications are verified by testing five types of samples. These findings indicate that the proposed sensor has superior performance in sensitivity and selectivity, which makes it have a great potential for applications in the fields of material characterization, environmental monitoring, and biomedicine detection in the terahertz band.

Terahertz refractive index sensor based on dual plasmon-induced transparency in a graphene metasurface

A novel graphene metasurface is proposed in this article, which is simulated by the finite difference time domain (FDTD) and found to exhibit a significant dual Plasmon-induced transparency (PIT) phenomenon in the terahertz frequency band. For further research, new transmission spectra are simulated using the coupled mode theory (CMT), and after comparison, it is found that the images were basically consistent and had a high degree of agreement. In addition, through observation and analysis of the PIT window, it is found that the PIT phenomenon can be effectively tuned by adjusting the Fermi level. Finally, the metasurface is proposed in this article, which has excellent sensing properties. After calculation and comparison of different refractive indices of surrounding media, it is found that the maximum sensitivity can reach 1.567 THz RIU−1 at a frequency of f = 4.8716 THz, with a figure of merit (FOM) of 23.855. Compared with other sensors of the same type, the sensor in this paper has great advantages. In summary, the graphene metasurface proposed in this article provides more theoretical support for manufacturing dynamically adjustable graphene sensors.

Ultrahigh sensitivity terahertz refractive index sensor based on four‐inscribed hole defect photonic crystal structure

AbstractWe proposed and investigated an ultrahigh sensitivity terahertz (THz) refractive index sensor based on four‐inscribed hole defect photonic crystal structure. Due to the formation of resonant modes, the sensing properties can be obtained by shifting the sharp resonance in the transmission spectrum as changing of the analyte refractive index. In addition, the influence of structure parameters on the sensing performance is explored and demonstrated numerically. The numerical results illustrate that the Q‐factor and figure of merit reach 323.71 and 167.188 can be obtained under the optimized structural parameters. In particular, an ultrahigh sensitivity of 198.8 μm/RIU can be realized in the frequency range of 0.777–0.779 THz. The proposed sensor may find significant applications in biochemical sensing systems.

Graphene-Based Refractive Index Sensor With a 1-D Topological Photonic System

A terahertz refractive index sensor, which relies on utilizing the Tamm plasmon polariton and localized topological edge state, is proposed in a one-dimensional photonic crystal based on graphene. The proposed sensor structure is an active photonic device due to graphene’s tunable Fermi energy and optical properties. High sensitivity and a high figure of merit (FOM) are the results of the topological protection that allows a perfect zero reflection. Furthermore, the study investigates the sensing performance with various structural and material parameters. This refractive index sensor has the potential to be modified for use in industrial and diagnostic applications.

Multi-resonances terahertz refractive index sensor based on a ring with embedded split rings metasurface array

Terahertz biosensors have gained significant attention due to their low energy and high penetrability, which enables them to interact with analytes without causing ionization damage. However, the weak interaction between terahertz waves and natural materials often results in poor sensing performance. To address this issue, various metasurface structures have been designed to enhance the light-matter interaction between the incident electromagnetic waves and the metasurface structure, thereby improving sensing performance. In this paper, we propose a metal–insulator–metal metasurface structure with a top metal consisting of a metasurface array that includes an outer ring with two embedded split rings. This structure generates three resonances at 0.732 THz, 1.916 THz, and 2.602 THz. Our simulation results show that the resonance formed by the mode coupling between the outer ring and the embedded split rings at 1.916 THz achieves the best sensing performance, with a sensitivity of 572 GHz/RIU, a quality factor of 22.8, and a figure of merit of 6.81. Our findings suggest that designing high-performance refractive index sensing with resonances generated from mode coupling is preferred. The remarkable sensing performance of the proposed metasurface structure enables the measurement of trace amounts of analytes in the terahertz region.

High performance tri-band terahertz refractive index sensor based on cavity resonance and Tamm plasmonic modes

In this paper, a novel terahertz refractive index sensor consisting of two asymmetric distributed Bragg reflectors (DBRs) attached to graphene in the cavity, is proposed. Based on finite difference time domain simulation, Tamm plasmon polariton mode excited at the interface between the DBR and the graphene is coupled with the resonance modes of the cavity, which helps to improve the sensing performance. In its reflection spectra, there are three dips with the bandwidths of 1.82, 3.95 and 3.86 GHz at 1.04 THz, 3.00 THz and 3.13 THz after simulation optimization. And the corresponding sensitivities and the figure of merit can achieve 1.02, 2.58, 3.00 THz/RIU, and 559.34, 652.15, 776.17 RIU−1, respectively. Due to its high sensing performance and simple structure, the tri-band sensor may pave the way for the research on the detection of multimodal microscale analytes.

High-performance terahertz refractive index sensor for cancer cells detection.

Metamaterial absorbers in the terahertz region are highly desirable in sensing and detection. In this work, a novel terahertz refractive index sensor based on metamaterial absorber for sensing biomedical samples is reported. The proposed structure consists of three asymmetrical metallic resonators made of gold placed above on a gallium arsenide (GaAs) dielectric layer. Due to high-intensity field confinement in the sensing regime, four resonance modes with nearly 100% absorption are achieved with a high Q-factor of 456.5 in the absorption spectra. The design absorber is highly sensitive to the change of refractive index of the surrounding medium. Furthermore, the proposed sensor exhibits extremely high sensitivity of 1.87THz/RIU and high FOM of 125RIU-1 in the refractive index range from 1.35 to 1.39 with fixed analyte thickness of 45µm. The design RI sensor can be fitted for biomedical sensing applications such sensing and detection of various cancerous cell as the RI varied in the range of 1.35-1.39. In addition, the design sensor has the ability to sense early stage infection of cancer cells with any of its four absorption bands due to its high quality factor, high sensitivity and high figure of merit.

Terahertz refractive index sensor based on tunable patterned graphene absorber

We introduce an ultrasensitive refractive-index sensor based on a patterned graphene dual-band absorber operating in the terahertz band. COMSOL Multiphysics software was employed to model and quantitatively assess the suggested micro-nano structure. The structure exhibits two perfect absorption peaks in the 1–9 THz band with theoretical absorptions of 99.4 % and 99.4 %. According to the properties of graphene, the positions of the absorption peaks can be adjusted using the applied bias voltage. Owing to the high symmetry of the proposed structure, it is insensitive to polarization and incident angle variations. Moreover, this study explored the effect of different parameters on the absorption peak by varying the structural parameters. Based on the simulation results, the refractive index sensor has a maximum sensitivity of 3 THz/RIU with a figure of merit (FOM) and can detect small refractive index changes. Therefore, we believe that this structure has promising applications in marine refractive index change determination. • The structure exhibits two perfect absorption peaks in the 1–9 THz band. • The positions of the absorption peaks can be adjusted using the applied bias voltage. • It is insensitive to changes in polarization and angle of incidence. • It has better sensitivity and FOM than most sensors of the same type.

High-performance terahertz refractive index sensor based on a hybrid graphene Tamm structure

In recent years, the research on tunable and high-performance terahertz devices has attracted widespread attention. In this paper, we propose a novel refractive index sensor consisting of monolayer graphene and multilayer photonic crystal with a defect layer. It is found that Tamm plasmon polaritons can be excited at the top graphene interface, which can strongly interact with the defect mode in this coupled structure. The results of numerical simulation demonstrate that the coupling can be either tuned by adjusting the geometric parameters or actively controlled by the chemical potential in graphene as well as the incident angle of light, allowing for tunable dual-band perfect absorption with strong interaction. Moreover, the resonance frequency of the defect mode is sensitive to the changes of the ambient refractive index. The sensitivity and figure of merit of this device as a sensor can reach 1.01 THz per refractive index unit (THz/RIU) and 631.2 R I U − 1 , respectively. In addition to high performance, the sensor does not require phase- or polarization-matching devices, paving the way for the research of optical devices.

Terahertz refractive index sensor with high sensitivity based on two‐core photonic crystal fiber

AbstractThis article contributes a highly sensitive novel refractive index (RI) sensor based on two‐core photonic crystal fiber (PCF) is proposed in the terahertz (THz) frequency regime. The sensing performance of the proposed RI sensor is based on the mode‐coupling theory. Two vertical cores of the PCF divided by the central air hole, act as two separate waveguides. The analyte whose refractive index being sensed is filled in the central air hole of the PCF. A full‐vector finite element method (FEM) approach is used to investigate the sensor. Numerical data reveal that the sensitivities of the proposed PCF are 672 600, 940480, 1035300, 1332200, and 1 827 300 nm/RIU for the RI sensing range of analyte from 1.30 to 1.33, 1.34 to 1.37, 1.38 to 1.40, 1.41 to 1.43, and 1.44 to 1.46, respectively. This indicates that the proposed RI sensor is highly sensitive to the refractive index of the analyte.

Multifunctional and tunable trigate graphene metamaterial with “Lakes of Wada” topology

Many plasmon-induced transparency (PIT) metamaterials previously reported had limited functions. Their tunabilities were realized by complex discrete structures, which greatly increased the difficulty and cost of device fabrication and adversely affected their resonance characteristics. It is an open question to adjust the Fermi levels of many graphene patterns with only a few in-plane electrodes. We propose and numerically study a novel electrically tunable and multifunctional trigate graphene metamaterial (TGGM) based on the concept of "Lakes of Wada". Benefiting from the trigate regulation, our proposed TGGM turns out to exhibit excellent characteristics, that can not only be used for terahertz band-stop filter, terahertz refractive index sensor, near-field optical switch, slow-light device, but also for double PIT window metamaterial with broad transparency windows and large tunable frequency range.

Self-referenced terahertz refractive index sensor based on a cavity resonance and Tamm plasmonic modes.

A self-referenced terahertz (THz) refractive index sensor is proposed. The structure consists of two opposite-facing, graphene-covered distributed Bragg reflectors (DBRs), with a cavity formed in between. The cavity is filled with the ambient medium, and its resonance frequency is sensitive to the changes of the ambient refractive index. On the other hand, Tamm-plasmonic modes, which are excited at the DBR-graphene boundaries, are insensitive to the ambient refractive index and thus provide a frequency reference. The proposed structure is studied using a semi-analytical transfer matrix method (TMM). The sensor, studied for gas sensing, achieves a sensitivity of 0.982 THz per refractive index unit (THz/RIU), and a figure of merit (FoM) of 142RIU-1 at the cavity resonance frequency of 1.1 THz. The effects of different parameters on the sensor's performance are also investigated. Compared to the previous high-performance THz refractive index sensing approaches, the proposed structure is simpler because it requires no phase- or polarization-matching devices, such as polarizers, prisms, and gratings. Moreover, it provides a self-referenced operation, which was rarely achievable using previous methods.

High-Sensitivity Terahertz Refractive Index Sensor in a Multilayered Structure with Graphene.

In this paper, we propose a high-sensitivity optical sensor at terahertz frequencies based on a composite structure containing a one-dimensional photonic crystal (1D PC) coated with a layer of monolayer graphene. Between the 1D PC and the graphene there is a sensing medium. This high-sensitivity phenomenon originates from the excitation of optical resonance between the graphene and the 1D PC. The proposed sensor is highly sensitive to the Fermi energy of graphene, the thickness and refractive index of the sensing medium, and the number of graphene layers. By selecting appropriate parameters, the maximum sensitivity () is obtained. We believe the proposed configuration is promising for fabricating graphene-based biosensor- or gas-sensor devices and other related applications in the terahertz band.

Toroidal metasurfaces integrated with microfluidic for terahertz refractive index sensing

A terahertz refractive index sensor consisting of toroidal dipole resonance metasurface integrated with microfluidic channel is proposed. The excitation of toroidal dipole resonance in this metasurface and the localized electromagnetic field enhancement in microfluidic channel are investigated comprehensively. Numerical results show that the calculated quality factor and the corresponding figure of merit can reach 1103 and 244, much higher than previously reported terahertz metamaterials sensors. Moreover, the refractive index sensing capabilities of non-polar and polar liquids are also investigated, which demonstrate the resonant frequency shift does not depend on the losses of analytes and show this device can be used for sensing various materials, including highly absorptive materials. This proposed structure can be extended to work in other frequency regions and has potential applications in high performance gases, liquids and biological materials sensing.

Terahertz refractive index sensor based on Tamm plasmon-polaritons with graphene.

A novel terahertz (THz) refractive index sensor based on Tamm plasmon-polaritons (TPPs), comprising a Bragg reflector and a graphene layer, is proposed. A semi-analytical transfer matrix method is used to study the proposed structure and its sensing performance. The sensor demonstrates a sensitivity of 0.744THz per refractive index unit (THz/RIU), or equivalently, 175.5 μm/RIU, and a figure of merit of 10.33 RIU-1 at the operating frequency of 1.132THz. The effects of structural parameters on the sensing performance are studied, offering new methods for improving TPP-based sensors. The proposed approach is a simple and practical alternative to traditional, and often more complex, THz sensing approaches, due to the ease of excitation, which lifts the requirement of phase and polarization-matching devices such as polarizers, prisms, and gratings. The proposed structure is studied for gas sensing, and its performance is compared with previous THz refractive index sensing structures.

High sensitivity terahertz refractive index sensor based on sunflower-shaped circular photonic crystal

Refractive index sensing is attracting extensive attention in biochemical sensing using terahertz technology. Various structures with strong confinements have been used to design sensors for improving the interaction between the terahertz wave field and the analytes, such as photonic crystals, nanowires, plasmonic structures, and metamaterials. Terahertz wave sensors based on two-dimensional photonic crystal have been used in various areas ranging from disease diagnostics to environmental pollution detection. For improving the performance of terahertz sensor, a sensing scheme based on high-density polyethylene sunflower-typecircular photonic crystal structure is proposed. The designed sensor contains two symmetrical sample cells surrounding a cavity in a circular photonic crystal. The transmission properties of the terahertz wave sensor are analyzed based on COMSOL Multiphysics when the central sample cells are filled with analyte with different refractive indices. The sensor characteristics depending on the structure parameters are analyzed. The choice of these parameters is discussed. Finally, a sensitivity of 10.4 μm/RIU, <i>Q</i>-factor of 62.21, and figure-of-merit of 1.46 are realized. The results in this work are expected to be able to extend the circular photonic crystal-based sensor to terahertz wave region.

Terahertz refractive index sensor based on the guided resonance in a photonic crystal slab

The optical properties of a photonic crystals slab (PCS) are highly sensitive to environmental variation. PCS thus are an attractive platform for sensing application. We numerically demonstrate a terahertz refractive index sensor based on the guided resonance in a two-dimensional PCS. The PCS consists of a square lattice of air holes in a silicon slab fabricated on a quartz substrate. By varying each structural parameter systematically, we study the dependence of the resonance located around 0.508 THz on the structural parameters of PCS, and then optimize the structural parameters of PCS for a higher sensitivity. We apply the PCS sensor to detect the refractive index of analytes. The sensitivity of the sensor reaches 23.08 GHz/RIU for the resonance located around 0.508 THz. Our results provide insight into the future terahertz spectroscopy for sensing detection.

Terahertz multi-metal-wire hybrid-cladding hollow waveguide for refractive index sensing**Project supported by the National Natural Science Foundation of China (Grant No. 51309059).

We propose a design of terahertz refractive index sensing based on the multi-metal-wire (MMW) hybrid-cladding hollow waveguide. The proposed terahertz hybrid-cladding hollow waveguide comprises one air core in the center surrounding MMW surrounded dielectric. The central air core is used for filling lossless measurands and transmitting terahertz light. In particular, the refractive index sensing is realized by measuring the mode field area (MFA) variation of radially polarized mode. The modal effective refractive index, mode field intensity distribution, and mode field area properties responding to the measurand refractive indexes for different operating frequencies and structure dimensions are investigated, respectively. Simulations show that the proposed terahertz refractive index sensor can realize easily the measurement of the measurand refractive index. Meanwhile, the effects of operating frequency and structure parameters on sensitivity and measurement accuracy are also studied. In view of the trade-off between sensitivity and measurement accuracy, the reasonable choice of the operating frequency and structure parameters can optimize appropriately the sensitivity and measurement accuracy, and the sensitivity can reach approximately 0.585 mm2/RIU (RIU is short for refraction index units) with the proper frequency and structure parameter.

Metal wire waveguide based all plasmonic refractive index sensor for terahertz frequencies

A terahertz refractive index sensor for liquid analytes consisting of a copper wire waveguide and co-axial layers of PMMA and PVDF is established numerically. The dispersion behavior of the fundamental core and surface mode and their phase matching has been demonstrated that clearly explains the working mechanism of the sensor. A core mode amplitude sensitivity of 59.9 for unit change in refractive index at 269.8μm is obtained for the proposed sensor. It has a large evanescent field penetration depth of 48μm at the phase matching point that can facilitate sensing of analyte refractive index further away from the interface. A definition for group velocity sensitivity is proposed which is then evaluated for the proposed sensor. The near dispersion free behavior of the sensor core mode except around the phase matching point gives it an excellent group velocity sensitivity of 128.9 for unit change in refractive index.

Terahertz Refractive Index Sensing Based on Photonic Column Array

Terahertz (THz) silicon photonic column (PC) array has been designed and fabricated, and its resonance effect has been experimentally and numerically investigated. Using this resonance effect, the refractive index sensing for liquids has been demonstrated in this device by the theoretical model calculation, numerical simulation, and experimental measurement. Moreover, resonance dependence on different incident angles in this device has also been shown. These results confirm that this THz PC array can be used as a THz refractive index sensor with high sensitivity and robustness. This sensing strategy shows that the structured surface of this device is compatible for liquid monitoring and easy to couple and transmit THz waves, and therefore can be more easily implemented in liquid refractive index sensing.