Terahertz Filter Research Articles

Multi-stopband filter based on frequency selective surface

This paper presents the design of a terahertz multi-stopband filter based on a metasurface structure which consisting of multiple metallic strips. By adjusting the dimensions and arrangement of these metallic strips placed on a dielectric substrate, the precise control of the filter performance is achieved. This structure exploits the electromagnetic resonances of metallic strips at terahertz frequencies to generate stopbands, thereby enabling selective filtering. Numerical studies demonstrates that these configurations can produce multiple stopbands within the 4.2 to 15.5 THz frequency range, with tunable center frequencies and bandwidths. Through the research of the transverse and longitudinal filter model structure, the band-stop filter with more stopbands can be realized, and the transmission valley value is kept within 5 %, which is crucial for enhancing filter performance. Moreover, the study reveals that tuning the dimensions of the metallic strips can modulate the resonances, resulting in a redshift of the filter's central frequency and an expansion of the passband width. These findings provide a new method for the performance improvement of terahertz filters, advanced filtering technology and electromagnetic wave control technology.

Easy-to-design wide stop-band multi-layer metamaterial filter based on symmetrical modified circular resonators for terahertz communication applications and electromagnetic interference shielding

Terahertz filters represent an essential element in modern communications system engineering. Designing a stopband filter with unique electrical qualities remains a relatively complicated task to achieve. In this context, the present paper suggests an easy-to-design bandstop filter to exploit it in terahertz communications and electromagnetic interference (EMI) shielding. The proposed filter is based on the multi-layer metamaterials stacking technique; it is constituted by a five-layer metal-insulator-metal-insulator-metal (MIMIM) microstructure. The two dielectric layers in polyimide with the three metallic layers in gold represent a total thickness of 43 μm. Each metallic layer is represented by a modified metamaterial resonator (MTMR) of the same symmetrical circular shape. The filter cell dimensions are optimized as 120 × 120 μm2 to have the desired stopband feature. The electrical qualities of the proposed filter have been analyzed by the finite element method (FEM) via the ANSYS high-frequency structure simulator software (HFSS). Through the simulation outcomes, salient filter features were highlighted. A wide −20 dB bandwidth of the order of 1.062 THz centered at 1.872 THz with a low transmission rate in the stopband which is less than 0.87 %, a significant square ratio of 0.86 and a relative bandwidth of around 57 %. Moreover, the absorption rate of the stopband has reached more than 97 % with excellent flat-top characteristics. To give more credibility to our results, we have presented a parametric analysis that included several components. In addition, the designed filter was insensitive to the polarization angle thanks to the symmetrical metamaterial resonators. Based on all these results and due to the simplicity of its design combined with all these unique features, our filter can be a prominent contender in the field of THz communications. It can be also required for terahertz imaging and EMI shielding.

Tunable terahertz filter based on graphene photonic crystals with defective layers

In this paper, we design a high-precision tunable terahertz filter by using transfer matrix method. The filter structure mainly consists of graphene embedded photonic crystals (GPCs). The front part of the GPCs contains artificial synthetic material and air layer, the back part of the GPCs is composed by and periodic stack of isotropic dielectric slabs (MgF2) embedded with graphene sheets, where air defect layer is located in the middle of the GPC as a central layer. Our simulation reveals that graphene layer and air defective layer strongly affect the filter performance. And we can get a relatively pure transmission peak in a wide frequency region. Additionally, the influence of incidence angle of terahertz wave, thickness of air layer, the unit number of front periodic structure and chemical potential of the graphene sheets can also modulate the function of the filter. And the filter has strong stability when the temperature changes from 150 K to 350 K.The results indicate that single channel, dual and multiple channels filter in a narrow frequency can be obtained by optimizing the structure parameter.

Terahertz metamaterials for spectrum modulation: structural design, materials and applications

Metamaterials, an artificial electromagnetic (EM) material, have exhibited unique characteristics, enabling innovations in terahertz (THz) wavefront shaping, polarization modulation, surface wave manipulation, and spectrum modulation. In this review, we focus on the advancements in THz metamaterials for spectrum modulation, emphasizing their structural design, special materials, and applications. The review discusses various structural designs, including metal-dielectric composite structures, all-metal structures, and all-dielectric structures, each offering distinct advantages for THz applications. Key special materials such as graphene, vanadium dioxide (VO2), molybdenum disulfide (MoS2), and flexible materials are highlighted for their significant contributions to enhancing the performance and functionality of THz metamaterials. The applications of THz metamaterials in EM stealth and sensing are thoroughly introduced, with a particular focus on biosensing, pesticide detection, and other sensing applications. The review emphasizes the potential of THz metamaterials in developing highly sensitive and selective sensors, as well as efficient THz absorbers and filters. Future perspectives include the continuous development of novel materials, advanced fabrication technologies, and the integration of multifunctional capabilities to further expand the applications and efficiency of THz metamaterials. These advancements are expected to drive significant progress in THz technology, impacting fields such as medical diagnostics, environmental monitoring, food safety, wireless communication, and security.

Tunable band-stop fiber filter based on laser-induced graphene metamaterial in THz frequency.

As an important device in the application of terahertz (THz) technology, a THz filter has broad application prospects in the fields of THz communication, imaging, and sensing. In this paper, a THz filter based on grating structure laser-induced graphene (LIG)/ side polishing terahertz fiber composite structure is proposed. In the experiment, we achieved the maximum Q factor of 23.83 at the central resonant frequency of 0.715 THz. By modifying the grating structure, a tunable operational span of 269 GHz was achieved, along with a tunable range of 21 GHz through laser stimulation. In testing, we found that LIG materials prepared with circular filling are more sensitive to relatively high-power pump lasers, while LIG samples prepared with line filling exhibit better linear response to laser power. Furthermore, the compact and highly integrated nature of the device suggests its broad potential utility in the realm of THz frequency selection.

Terahertz dual-band bandpass filter based on spoof surface plasmon polaritons with wide upper stopband suppression.

In this paper, a terahertz (THz) dual-band bandpass filter based on spoof surface plasmon polaritons (SSPPs) with wide upper stopband suppression is proposed. The filter utilizes two types of SSPP unit cells loaded on a double-sided quasi-SSPPs transmission line to achieve dual-band filtering responses and wide upper stopband suppression simultaneously. The performance and bandwidth of the filter's passbands can be adjusted by modifying the SSPP unit cells within the dual-band filtering part. The detailed design method is provided to enhance understanding of the operating principle of the presented filter. The simulation results of the designed THz filter demonstrate its excellent dual-band filtering performance. It features a wide stopband spanning from 0.91 to 1.45 THz with |S21| < -35 dB. To validate the proposed approach, a microwave filter with a similar design is implemented. Finally, the simulated and measured S-parameter responses of the filter are provided for comparison and evaluation.

On-demand design of dual-band electromagnetically induced transparency metamaterials based on improved convolutional neural network

Metamaterials (MMs) with dual electromagnetically induced transparency (EIT)-like windows possess more powerful capacity in Terahertz filtering than their single-EIT counterparts, but the design difficulty normally increases as the MM structure becomes more complicated. This work proposes a deep-learning method using a long short-term memory (LSTM) assisted convolutional neural network (LSTM-CNN) to simplify the design of dual-EIT MMs. Compared to the traditional CNN, the LSTM-CNN exhibits higher prediction accuracy for the geometric parameters of the MM according to the input spectrum, which can be ascribed to enhanced capability of extracting the spectral characteristics after adding the LSTM network. Further adding a feature-transforming network before the LSTM-CNN model to build a link between the MM performance and structure, a customizable MM design is realized. Two different dual-EIT metamaterials are quickly designed with this strategy, and the design accuracy has been fully demonstrated by simulations and experiments. The proposed deep learning method can achieve the rapid and efficient design of dual-EIT MMs, which offers a new pathway for the on-demand design of complex MM-based devices.

Noncollinear electro-optic detection of terahertz waves: Advantages and limitations

Electro-optic sampling of terahertz waves by noncollinearly propagating femtosecond laser pulses in electro-optic crystals can provide high efficiency and high spectral resolution of terahertz detection with various types of crystals and laser wavelengths, unlike the conventional collinear scheme. We develop an analytical theory of noncollinear electro-optic sampling detection technique that describes the modulation of the probe laser beam polarization as a result of nonlinear interaction between the optical and terahertz fields. The theory accounts for finite widths of the terahertz and probe beams. It is found that noncollinear scheme operates as a low-pass terahertz filter with the frequency cut-off determined by the width of the probe beam and the crossing angle of the terahertz and probe beams. We apply the theory to two practical situations: sampling of terahertz waves by fiber laser pulses (1.55 μm wavelength) in a GaAs crystal and sampling by Ti:sapphire laser pulses (800 nm wavelength) in a LiNbO3 crystal.

Terahertz narrow-band filter based on 3D-printed periodic waveguides

Terahertz (THz) devices, especially waveguide-type functional devices related to transmission and control, are severely scarce due to the lack of effective design and fabrication methods. Here, we experimentally demonstrate a waveguide type of THz narrow-band filter based on 3D-printed technology, which is realized by a cylindrical hollow metal structure with corrugated tube walls. The semi-cylindrical periodic corrugations are 3D printed on a photosensitive resin substrate material, followed by sputtering a layer of gold film on its surface to endow the structure with THz filtering functions. A hollow cylindrical corrugated waveguide is obtained by assembling two identical semi-circular corrugations together. The periodic structure with Bragg resonances can produce a frequency stop band, in which the propagation of THz waves is significantly suppressed. We print a wider section of corrugations in the middle of the waveguide, which destroys the perfect periodicity of the structure and forms a defect. Due to the local resonance caused by the defect, we observe an additional narrow-band transmission peak within the former stop band, which is a good candidate for THz filtering. The filtering bandwidth and extinction ratio are 1.8 GHz and 28 dB, respectively, and the Q-factor reaches 234. The proposed 3D-printed THz filter has the advantages of the simple structure, excellent performance, and easy integration, which can improve the existing THz systems in various applications.

Monolayer actively tunable dual-frequency switch based on photosensitive silicon metamaterial

We propose a monolayer actively tunable dual-frequency switch in the terahertz (THz) range based on photosensitive silicon (PSi) metamaterial. The designed unit cell consists of hollow cross-shaped slots and four L-shaped slots made of a perfect electric conductor (PEC), with PSi embedded in both the cross-shaped and four L-shaped slots of identical dimensions. The PEC serves as the dielectric substrate. By varying the intensity of the pump beam to adjust the conductivity of the PSi, tuning of the transmittance at the resonant frequencies of the entire structure is achieved, enabling the switch to function in both ON and OFF states. Under vertical incidence of THz waves in the range of 1 THz to 3 THz, in the absence of pump beam, the PSi is in an insulating state with a conductivity of approximately 1 S/m. The proposed terahertz dual-frequency switch is in the ON state, with transmittances of 97.0% at the first resonance frequency (f1=1.736THz) and 98.3% at the second resonance frequency (f2=2.176THz), and group delays of 4.397 ps and 2.540 ps, respectively. When the pump beam intensity is 294.6 μj/cm2, the PSi is in a metallic state with a conductivity of approximately 1 × 105 S/m. The switch is in the OFF state, with zero transmittance in the range of 1 THz to 3 THz and a group delay of 0.327 ps. The calculated modulation degrees of amplitude at f1 and f2 for this terahertz dual-frequency switch are both 100%, demonstrating a high-performance switch. The two resonant frequencies f1 and f2 of the terahertz dual-frequency switch are generated by weakly coupled superposition between the embedded cross-shaped and four L-shaped PSi resonators (CSPR and FLSPR), exhibiting polarization-insensitive characteristics under vertical incidence of THz waves. We believe that this compact structure of terahertz metamaterial dual-frequency switch holds great application prospects in terahertz filtering, slow light devices, terahertz modulation, and the future 6G communication field.

THz active modulation device by electric field turned modulation characteristics of titanium sulfide nanofilm device

AbstractAn effective approach to manipulate terahertz (THz) waves involves the utilization of a device structure. In this particular investigation, we have successfully fabricated a SnS2 nanofilm device using femtosecond laser direct ablation. The passive and active polarization‐sensitive characteristics of the SnS2 nanofilm device were examined within the THz frequency range, employing different line spaces of 500, 600, and 700 μm. The combination of the device effect and SnS2 absorption led to an enhanced passive polarization modulation. Moreover, when an external electric field was applied to the SnS2 nanofilm device along the device line, persistent switching and linear modulation of THz wave transmission were observed. These findings indicate that the integration of material properties and device structure in this SnS2 device renders it highly suitable for applications in ultrafast THz switches, filters, and modulation devices.

Design and analysis of a highly uniform five-band terahertz filter based on frequency selective surface

In this paper, a highly uniform five-band terahertz filter is presented, achieved by deforming two nested square ring structures. The metal pattern layer of the filter comprises a nested square ring structure with an inner and outer opening, and a metal strip connecting the inner and outer rings. Symmetrical openings divide nested square rings, breaking the current path and generating multiple metal bars of different lengths to excite dipole oscillations. Using metal strips to connect the inner and outer rings, the inner and outer resonant structures are integrated. Compared to a single resonant particle, the coupling of different single resonant particles amplifies the resonance of the combined structure, resulting in a wider stopband width and enhanced isolation between multiple passbands. A metal strip is also added to both ends of the outer ring opening to increase the equivalent capacitance of the outer ring, enhance the resonance, and improve out-of-band suppression. The transmission spectrum has five uniform passbands in the range of 1.3807–1.9917 THz, and the maximum passband ratio (MPR) of 3 dB bandwidth is about 1.16, and the ratio of 10 dB is about 1.17. The transmission coefficient of transmission zero is about 0.1, and the peak transmission coefficient of transmission peak is close to 1. The polarization of TE and TM incident waves is stable, and the filtering characteristics are good. Besides, the rectangular coefficients of the five passbands are all less than 1.33, showing excellent band selectivity. The resonance mechanism is explained by the Q value and electric field current distribution. The influence of structural parameter changes on filtering characteristics is also analyzed, and the main control parameters are found by using correlation analysis tools. From the changes in the MPR curve, it is verified that the structure parameters of the designed filter are in good condition. The multi-passband filter designed in this paper has uniform filtering characteristics, which is helpful to the development of terahertz multi-channel communication.

Design and numerical simulation of modified tunable THz filter based on periodic graphene-PVC ribbons

In this article, we propose a tunable terahertz (THz) filter composed of graphene layers and a PVC substrate. A novel approach utilizing a static magnetic field is introduced to tune the resonance frequency. The presence of a magnetic field and gyromagnetic materials like graphene enables the proposed configuration to exhibit both TE and TM polarizations in the output structure. Additionally, a remarkable tunability of about 5 THz bandwidth is achieved, a considerably high value compared to existing works. All results are obtained through numerical simulations using MATLAB software based on the transfer matrix method (TMM), and the accuracy is verified using COMSOL software. With nearly zero transmission, this exceptionally tunable THz filter holds great potential for various applications, including THz spectrometry.

Tunable properties of graphene loaded waveguide surrounded by magnetic materials

Theoretically analysis has been accomplished for the propagating electromagnetic surface waves (EMSWs) at planar ferrite-graphene-ferrite waveguide structure. The characteristics curves are analyzed for the normalized phase and attenuation phase constant against the operating frequency. The impact of different parameters of ferrite and graphene are observed on the normalize phase and attenuation phase constant. In response to these parameters the structured waveguide exhibits the convenient propagation of electromagnetic surface waves with minimal propagation loss in the terahertz frequency region. The proposed waveguide avails position in nanophotonic devices, terahertz filters, highly integrated terahertz devices and communication systems.

A Design Method and Application of Meta-Surface-Based Arbitrary Passband Filter for Terahertz Communication.

A meta-surface-based arbitrary bandwidth filter realization method for terahertz (THz) future communications is presented. The approach involves integrating a meta-surface-based bandstop filter into an ultra-wideband (UWB) bandpass filter and adjusting the operating frequency range of the meta-surface bandstop filter to realize the design of arbitrary bandwidth filters. It effectively addresses the complexity of designing traditional arbitrary bandwidth filters and the challenges in achieving impedance matching. To underscore its practicality, the paper employs silicon substrate integrated gap waveguide (SSIGW) and this method to craft a THz filter. To begin, design equations for electromagnetic band gap (EBG) structures were developed in accordance with the requirements of through-silicon via (TSV) and applied to the design of the SSIGW. Subsequently, this article employs equivalent transmission line models and equivalent circuits to conduct theoretical analyses for both the UWB passband and the meta-surface stopband portions. The proposed THz filter boasts a center frequency of 0.151 THz, a relative bandwidth of 6.9%, insertion loss below 0.68 dB, and stopbands exceeding 20 GHz in both upper and lower ranges. The in-band group delay is 0.119 ± 0.048 ns. Compared to reported THz filters, the SSIGW filter boasts advantages such as low loss and minimal delay, making it even more suitable for future wireless communication.

Tri-channel independent switching terahertz filter based on metal-graphene hybrid coding metasurface

Terahertz (THz) filters are the core devices for practical application in THz communication technology. Recently, design of multi-channel filters with high modulation rates remains to be a challenge. In this paper, a three channels switchable THz filter based on metal-graphene hybrid metasurface is proposed. The unit cell is consisted of "n"-shaped and "u"-shaped split-ring resonators (SRRs) with different sizes, and the graphene bar is embedded at the gap of two SRRs. The four bandpass modes with single transparency windows, double two transparency windows, and three transparency windows can be freely switched by applying voltage to the left/right electrodes of the filters. Its central frequencies of transparency windows are located at 0.80 THz with modulation depth up to 49%, 1.14 THz (constant transparency windows), and 1.58 THz with modulation depth up to 68%, respectively. In addition, an equivalent circuit model is proposed to demonstrate the control mechanism. This research result not only provides a new method for the design of reconfigurable multifunctional THz modulation devices, but also give theoretical model to promote the development of multi-channel filter technology in THz communication technology.

Switchable ultra-broadband absorption and polarization conversion terahertz metasurface

Metasurfaces can realize flexible modulation of electromagnetic waves at the wavelength level. However, the reported functions of metasurface are usually fixed and cannot be changed, once its structural design is completed. The designed metasurface cannot meet the requirements for flexible regulation of terahertz waves. We find that the phase change material of vanadium dioxide can achieve a transition from insulating state to metallic state through thermal, electrical, or light excitation, and the phase transition of this material is reversible. Therefore, using vanadium dioxide to form a composite metasurface can achieve dynamic modulation of terahertz waves. In this study, we propose a terahertz metasurface with switchable broadband absorption and polarization conversion. The proposed metasurface is composed of a 9-layer structure stacked from bottom to top with a combination pattern of different dielectric layers. By adjusting the conductivity of vanadium dioxide, the designed metasurface can achieve flexible switching between terahertz wave absorption function and polarization conversion function. When the vanadium dioxide is in the metal state, the designed metasurface behaves as a broadband absorber with an absorption rate of more than 90% in a range of 6.32–18.06 THz and a relative bandwidth of 96.3%. When the vanadium dioxide is in the insulated state, the designed structure acts as a polarization converter in a frequency range of 2.41–3.42 THz, 4.78–7.48 THz, and 9.53–9.73 THz, respectively, with a polarization conversion rate of over 90%. We believe that this metasurface structure will have good applications in the fields of terahertz wave detection, terahertz switches, terahertz filtering, terahertz communication, and terahertz sensing.

Core structure of terahertz filter fabricated by femtosecond laser with high repetition rate

Core structure of terahertz filter fabricated by femtosecond laser with high repetition rate

DUAL-BAND BANDSTOP FILTERS BASED ON ULTRA THIN FREQUENCY SELECTIVE SURFACES

In this paper, we present designs for terahertz filters with two independent stopbands based on periodic metallic resonant structures patterned on the dielectric substrate. Two simple and thin structures are proposed to realize required characteristics for single-polarized and dual polarized filter designs. To realize single polarized one, a unit cell of the proposed periodic structure consists of two sets rectangular shape conducting metal patches patterned on the top of the dielectric substrate, and the dual-band response is due the lattice resonances observed in the unit cell. To realize dual polarized filter design, a modified unit cell is considered with additional patches on the bottom of the dielectric substrate. The patches on the bottom layer are the same ones as on the top layer but orthogonally oriented. For this case, polarization-independent frequency response can be obtained after geometric optimization of the unit cell because the top and the bottom layers resonantly interact with only the TE or TM incident plane wave to minimize cross coupling effects between the layers. In order to intensively understand the transmission performance of the proposed filters, a large number of simulations using integral equation method are performed based on the different values for permittivity, period of the unit cell, dielectric thickness, and geometric dimensions. The electric field distributions are analyzed to understand the mechanism of the resonance behavior.

Controlling Thermal Radiation in Photonic Quasicrystals Containing Epsilon-Negative Metamaterials

The transfer matrix approach is used to study the optical characteristics of thermal radiation in a one-dimensional photonic crystal (1DPC) with metamaterial. In this method, every layer within the multilayer structure is associated with its specific transfer matrix. Subsequently, it links the incident beam to the next layer from the previous layer. The proposed structure is composed of three types of materials, namely InSb, ZrO2, and Teflon, and one type of epsilon-negative (ENG) metamaterial and is organized in accordance with the laws of sequencing. The semiconductor InSb has the capability to adjust bandgaps by utilizing its thermally responsive permittivity, allowing for tunability with temperature changes, while the metamaterial modifies the bandgaps according to its negative permittivity. Using quasi-periodic shows that, in contrast to employing absolute periodic arrangements, it produces more diverse results in modifying the structure’s band-gaps. Using a new sequence arrangement mixed-quasi-periodic (MQP) structure, which is a combination of two quasi periodic structures, provides more freedom of action for modifying the properties of the medium than periodic arrangements do. The ability to control thermal radiation is crucial in a range of optical applications since it is frequently unpolarized and incoherent in both space and time. These configurations allow for the suppression and emission of thermal radiation in a certain frequency range due to their fundamental nature as photonic band-gaps (PBGs). So, we are able to control the thermal radiation by changing the structure arrangement. Here, the We use an indirect method based on the second Kirchoff law for thermal radiation to investigate the emittance of black bodies based on a well-known transfer matrix technique. We can measure the transmission and reflection coefficients with associated transmittance and reflectance, T and R, respectively. Here, the effects of several parameters, including the input beam’s angle, polarization, and period on tailoring the thermal radiation spectrum of the proposed structure, are studied. The results show that in some frequency bands, thermal radiation exceeded the black body limit. There were also good results in terms of complete stop bands for both TE and TM polarization at different incident angles and frequencies. This study produces encouraging results for the creation of Terahertz (THz) filters and selective thermal emitters. The tunability of our media is a crucial factor that influences the efficiency and function of our desired photonic outcome. Therefore, exploiting MQP sequences or arrangements is a promising strategy, as it allows us to rearrange our media more flexibly than quasi-periodic sequences and thus achieve our optimal result.