Graphene Terahertz Research Articles

Tunable and polarization-dependent electromagnetically induced transparency analogy in graphene-based terahertz metasurfaces

In this paper, we theoretically and numerically demonstrate a tunable and polarization-dependent electromagnetically induced transparency analogy based on graphene terahertz metasurfaces. The unit cell of the metasurface consists of three-layer graphene strips embedded in a silicon grating. The dynamic adjustment of the transparent window can be achieved by changing the coupling distance between the graphene layers and the polarization direction of the incident lights. The operation mechanism behind the phenomenon can be attributed to the near-field interaction and electromagnetic coupling of modes in graphene strips. Furthermore, the full wave electromagnetic simulations obtained by the finite-difference time-domain method agree well with the theoretical fitting results based on the three-harmonic oscillator model. In addition, by changing the Fermi levels, it can not only realize the outstanding slow-light effects with a maximum group index of 3750 but also obtain the four-frequency asynchronous optical switch function in terahertz regions. Therefore, our proposed metamaterial device may have potential applications in image switching, optical switches, slow-light device, optical communication, and optical storage.

A tunable, dual band graphene terahertz absorber with equivalent circuit modeling

A tunable, dual band graphene terahertz absorber with equivalent circuit modeling

Graphene Terahertz Devices for Sensing and Communication.

Graphene-based terahertz (THz) devices have emerged as promising platforms for a variety of applications, leveraging graphene's unique optoelectronic properties. This review explores recent advancements in utilizing graphene in THz technology, focusing on two main aspects: THz molecular sensing and THz wave modulation. In molecular sensing, the environment-sensitive THz transmission and emission properties of graphene are utilized for enabling molecular adsorption detection and biomolecular sensing. This capability holds significant potential, from the detection of pesticides to DNA at high sensitivity and selectivity. In THz wave modulation, crucial for next-generation wireless communication systems, graphene demonstrates remarkable potential in absorption modulation when gated. Novel device structures, spectroscopic systems, and metasurface architectures have enabled enhanced absorption and wave modulation. Furthermore, techniques such as spatial phase modulation and polarization manipulation have been explored. From sensing to communication, graphene-based THz devices present a wide array of opportunities for future research and development. Finally, advancements in sensing techniques not only enhance biomolecular analysis but also contribute to optimizing graphene's properties for communication by enabling efficient modulation of electromagnetic waves. Conversely, developments in communication strategies inform and enhance sensing capabilities, establishing a mutually beneficialrelationship.

Design and Simulation of High-Efficiency Broadband Terahertz Graphene Composite Waveguide Modulators

Design and Simulation of High-Efficiency Broadband Terahertz Graphene Composite Waveguide Modulators

Design of optically pumped ultrahigh-performance terahertz sensors based on graphene metamaterials

The surface conductivity of graphene can be precisely adjusted to approach the sigma-near-zero (SNZ) point by manipulating the quasi-Fermi level via optical pumping. This capability holds great promise for advanced graphene metamaterials-based terahertz (THz) sensors. In this study, we present a rational approach for designing two distinct types of patterned graphene THz sensors, each tailored to specific requirements regarding the quasi-Fermi level, resonance frequency, and Q-factor. The first sensor type operates in transmission mode and is well-suited for analytes with refractive indices ranging from 1 to 2. With a fixed analyte thickness of 40 nm, it achieves a frequency sensitivity of 1.036 THz/RIU, a Q-factor between 230 and 360, and a figure of merit (FOM) ranging from 51/RIU to 74/RIU. The second sensor type operates in reflection mode and is optimized for detecting biomolecules with refractive indices between 1.5 and 1.7. It exhibits a significant frequency shift of approximately 0.16 THz and a reflectance variation as high as 0.941 when the bio-analyte thickness is 100 nm. Numerical simulations of the sensing performances confirm the potential of these designed sensors for applications in detecting thin dielectric films and biosensing.

Terahertz absorption properties of different graphene layers based on the Salisbury effect.

Terahertz (THz) absorbers based on the Salisbury screen have attracted significant attention for high absorption performance and simple structure. Graphene is suitable for high-performance THz absorbers due to its extraordinary electronic and optical properties. The study of graphene THz absorbers based on Salisbury screens has attracted great interest, where the number of graphene layers significantly affects the interface impedance matching and absorption efficiency. In this work, we proposed a sandwich-structured graphene/Polyimide (PI) /Au THz absorber based on the Salisbury screen. The results show that the absorption peak tended to increase and then decrease with the increase in the number of graphene layers. The simulation demonstrates that the real and imaginary parts of the relative impedance of the 3-5 layer graphene absorber were 1.02 and 0.01, which achieved a better impedance matching with the free space. Meanwhile, the measured sheet resistance value of 426 Ω/sq was closest to the free-space impedance value of 377 Ω, consistent with the simulation results. The corresponding absorption reached a maximum value of 98.7% at 0.82 THz (measured). In addition, the absorption peak decreased from 98.7% to 86.7% as the angle of incidence increased from 0° to 60°. This demonstrates the advantage of wide-angle absorption. The proposed device is suitable for applications in electromagnetic shielding and imaging, while the suggested method can be employed for the fabrication of other graphene-based devices.

Simulation of ageing and wear effect on graphene THz passive components using finite element method

In the growing scenario of 2D material-based metamaterials and metasurfaces for Terahertz (THz) applications, assessing the impact of ageing and wear due to environmental stressors on the components’ performance is becoming mandatory to understand the long-term reliability of novel technologies. This paper introduces approaches to assess the ageing and wear effects on THz passive components through numerical simulations. For this purpose, common techniques for introducing 2D materials and thin metal layers in numerical models are discussed. As a case study, this work explores the effects of graphene degradation and reflective metal ageing on the electromagnetic response of a graphene-enhanced reflective grating for THz absorption and modulation by finite element (FE) analysis. The developed FE model is validated against experimental data obtained through THz Time-Domain Spectroscopy. By computing the device’s transmission, reflection, and absorption spectra for degrading graphene and metal conductive properties, this work provides insights into the influence of ageing and wear on THz passive components.

High sensitivity terahertz biomedical sensing with graphene metamaterial

Identification of biomedical molecules by terahertz (THz) sensing is a quite promising noninvasive technique. Here, we present a THz graphene metamaterial with plasmon-induced transparency (PIT) for high sensitivity biosensing. The strong PIT effect is generated by the destructive interference between the plasmon modes excited by two graphene strips. For testing different biomedical analytes, a distinct frequency shift at PIT resonance can be observed due to the effect of surrounding refractive index variation. The metamaterial scheme for noninvasive biomedical sensing can achieve the sensitivity as high as 2.6 THz/RIU. Particular, it can not only detect the phase of red blood cell infected by malaria but also the concentration of ethanol aqueous solution. The underlying mechanism is discussed for understanding the THz sensing operation.

Tunable broadband terahertz graphene metasurface for complex-amplitude vortex beam generator and hologram

The independent controls of phase and amplitude, known as complex-amplitude modulation, play a crucial role for modulating the electromagnetic wavefronts. In this paper, a tunable broadband terahertz (THz) graphene metasurface with complex-amplitude modulation is proposed. A novel hybrid modulation mechanism that combines the absorption, polarization conversion, and Pancharatnam–Berry (PB) phase is employed to achieve a wide complex-amplitude modulation range over a broad frequency band. In addition, the metasurface can be dynamically tuned from the electrically tunable nature of the graphene in the THz range. To reveal the tunability characteristics of the metasurface as the graphene chemical potential varies, we study the Kubo formula of the graphene conductivity model and combine it with the equivalent circuit model. Two THz graphene meta-devices are designed for complex-amplitude vortex beam generator and hologram. By varying the graphene chemical potentials of 0 and 1 eV as the ON and OFF states, a THz quad-vortex-beam generator with switchable modes is designed and verified. We also demonstrate a THz hologram with complex-amplitude modulation by adjusting the chemical potential, exhibiting low imaging noise and high image quality. The proposed method is promised to be applied to the design of more complex-amplitude THz graphene meta-devices.

Tunable terahertz absorption of ion gel-graphene hybrids based on the Salisbury effect.

The gate-tunable absorption properties of graphene make it suitable for terahertz (THz) absorbers. However, the realization of a graphene-based THz absorber faces challenges between the difficulty of patterning graphene for processing and the intrinsically low absorbance of graphene with the high electric field needed to change the conductivity of graphene. This report presents an electrically tunable graphene THz absorber where a single-layer graphene film and a gold reflective layer are separated by a polyimide (PI) dielectric layer to form an easily fabricated three-layer Salisbury screen structure. The carrier density of the graphene layer can be efficiently tuned by a small external electrical gating (-5V-5 V) with the assistance of an ion gel layer. The voltage modulation of the Fermi energy level (EF) of graphene was confirmed by Raman spectra, and the variation of the device absorbance was confirmed using a THz time-domain spectroscopy system (THz-TDS). The measurements show that the EF is adjusted in the range of 0-0.5 eV, and THz absorbance is adjusted in the range of 60%-99%. The absorber performs well under different curvatures, and the peak absorbance is all over 95%. We conducted further analysis of the absorber absorbance by varying the thickness of the PI dielectric layer, aiming to examine the correlation between the resonant frequency of the absorber and the dielectric layer thickness. Our research findings indicate that the proposed absorber holds significant potential for application in diverse fields such as communication, medicine, and sensing.

Electrolyte gated graphene terahertz amplitude modulators

Active manipulation of the amplitude of terahertz (THz) frequency waves, through electrical tuning, is key for next-generation THz imaging and essential for unlocking strategic applications, from wireless communication to quantum technologies. Here, we demonstrate high-performance THz amplitude modulators based on an electrolyte-gated single-layer graphene. Broadband modulation in the 1.5–6 THz range is achieved by optimizing the electric field coupling by carefully controlling the spacer thickness in a quarter-wavelength cavity structure, with a maximum modulation depth of 40% at around 2 THz. Raman characterization confirms a Fermi-level tuning of 0.39 eV via electrolyte gating of graphene. A test 2 × 2 modulator array with independent control of sub-millimeter regions is then developed and tested, with no crosstalk between pixels. The reported results highlight the potential of electrolyte-gated graphene for efficient THz modulation. The single-chip design offers compactness and ease of integration with other electronic components, making it a promising platform for THz spatial light modulators and adaptive optical components.

Tunable Cherenkov Terahertz Graphene Surface Plasmon Generation in Graphene Sheet by a Moving Relativistic Electron Beam

Tunable Cherenkov Terahertz Graphene Surface Plasmon Generation in Graphene Sheet by a Moving Relativistic Electron Beam

A multi-functional tunable terahertz graphene metamaterial based on plasmon-induced transparency

A graphene terahertz metamaterial composed of multiple square split ring resonators is proposed to realize the excellent functions of sensing and switching based on plasmon-induced transparency (PIT). The switch, operating at multiple frequencies can be easily achieved by adjusting the Fermi level of graphene, corresponding to a maximum amplitude modulation of 89.5 %. When the device is employed as a sensing platform, the multi-PIT effect can improve the sensitivity due to greater interactions between the metamaterial and analytes, leading to a maximum sensitivity of 380 GHz/RIU. These findings suggest that the proposed metamaterial can serve as a multipurpose device with broad applicability.

Graphene-based terahertz antenna with polarization reconfiguration

This article presents the design of a terahertz antenna with the ability to reconfigure its polarization using graphene’s anisotropic properties. By applying a DC magnetic flux, the graphene conductivity tensor can be transformed into a non-diagonal tensor. Conversely, the magnetic bias can be switched off to revert the tensor to a diagonal state. This approach allows the antenna to switch between linear and circular polarizations, making it highly versatile for terahertz applications. Additionally, by altering the direction of the applied DC magnetic bias, one can select the rotation direction of circular polarization between left and right hand. The feeding line and ground plane of the antenna are made of silver metal, and its behavior in the terahertz band is described using the Modified Relaxation-Effect model, which is more accurate than the skin effect model for this band. The finite element numerical method is utilized to analyze the antenna, which yields results consistent with the design predictions. The antenna’s impedance bandwidth spans 2.8 to 3.22 terahertz for linear polarization and 2.9 to 3.18 THz for circular polarization. The antenna’s maximum gain is 6 dB for linear polarization and 5 dB for circular polarization. The axial ratio for circular polarization is 1.36 in the middle of the band, which is below the 3 dB threshold for circular polarization. The antenna produces directive patterns for both linear and circular polarizations. The key innovation of this antenna design lies in its unique utilization of a magnetostatic field to achieve reconfiguration in the terahertz graphene dielectric resonator antenna, a method that has not been previously documented in literature.

Dynamic tunable multiple plasmon induced transparency sensor and optical switch in dual-polarization excitation in a terahertz graphene metamaterial

This paper proposes the development of a simplified graphene-based terahertz device that exhibits multiple plasmon induced transparency (PIT) effects in x-polarized and y-polarized waves. The PIT device is composed of “Z-shaped” and “parallel strips” structures, which demonstrate multiple transparency windows under x-polarized and y-polarized incident light within the frequency range of 2-15 THz. Additionally, under x-polarized incident, the device demonstrates wideband transparency at 3.4 THz. By comparing the transmission spectra of individual and combined structures, we observe that the initial transmission valley of the single two horizontal stripes structure transforms into a PIT curve in the combined structure, while the transmission valley of the other individual structures remains transmission. The PIT device demonstrates a peak sensitivity of 3THz/RIU for x-polarized waves and 2.9THz/RIU for y-polarized waves when employed as a light sensor. PIT as a light switch can attain a maximum modulation depth of 81.4% for x-polarized waves and 71.2% for y-polarized waves at Fermi energy levels ranging from 0.5eV to 1.2eV. The presented PIT device shows promise for applications in high-sensitivity optical sensing and optical switching.

Direct printing of graphene terahertz closed-ring resonator array from periodic single droplets via enhanced coffee-ring effect

Graphene terahertz (THz) devices have emerged as a pivotal player in the THz area, and drop-on-demand (DOD) printing technology enables cost-effective patterning of THz microstructures. However, fast printing of THz microstructures and devices suffers from low efficiency owing to the drop-by-drop overlapping process. Here, we have developed a facile direct printing method to fabricate graphene THz closed-ring resonator (CRR) arrays from periodic single droplets ascribing to the enhanced coffee-ring effect. The construction of a complete graphene micro-ring was achieved using small reduced graphene oxide sheets, enabling the strategy of a graphene CRR from one single droplet. We attributed the mechanism of strengthened coffee-ring effect to the swift pinning, gapless between the contact line and the solute line, and close packing. Finally, we prepared a graphene THz CRR array featuring fine rings with widths of ∼12.3 μm, a high THz extinction ratio of 66.9%, and the highest measurement λ (relative THz response) of 9.8 documented so far for graphene-based THz microstructures and devices. Such an array from the expeditious DOD printing process exhibits a pronounced THz response and possible fast switching speed, which suggests new possibilities in building graphene THz microstructures, benefiting the future development of graphene THz devices.

Enhancing terahertz wave modulation by metallic structures in optical pumped graphene layer

The terahertz (THz) wave modulation in graphene with the metallic structure and optical pump is analyzed theoretically and experimentally in this paper. The modulation depth is enhanced with the combination of optical pump and metallic structure localization. The subwavelength metallic structure enhances the interaction between the THz wave and graphene with the excitation of the extraordinary transmission resonant. The optical pump of continuous waved (CW) laser provides an enhanced interact electric field on the surface of graphene. The tuning range of the Fermi level of the graphene is enhanced by optical pump on the basis of modulation voltage. The maximum amplitude modulation depth of 54% is achieved by changing the modulation voltage from -25 V to +25 V. The behavior of the modulation enhancement provides a candidate for improving the performance of graphene based THz modulator.

Quadruple Plasmon-Induced Transparency and Dynamic Tuning Based on Bilayer Graphene Terahertz Metamaterial.

This study proposes a terahertz metamaterial structure composed of a silicon-graphene-silicon sandwich, aiming to achieve quadruple plasmon-induced transparency (PIT). This phenomenon arises from the interaction coupling of bright-dark modes within the structure. The results obtained from the coupled mode theory (CMT) calculations align with the simulations ones using the finite difference time domain (FDTD) method. Based on the electric field distributions at the resonant frequencies of the five bright modes, it is found that the energy localizations of the original five bright modes undergo diffusion and transfer under the influence of the dark mode. Additionally, the impact of the Fermi level of graphene on the transmission spectrum is discussed. The results reveal that the modulation depths (MDs) of 94.0%, 92.48%, 93.54%, 96.54%, 97.51%, 92.86%, 94.82%, and 88.20%, with corresponding insertion losses (ILs) of 0.52 dB, 0.98 dB, 1.37 dB, 0.70 dB, 0.43 dB, 0.63 dB, 0.16 dB, and 0.17 dB at the specific frequencies, are obtained, achieving multiple switching effects. This model holds significant potential for applications in versatile modulators and optical switches in the terahertz range.

Tunable terahertz graphene metamaterial optical switches and sensors based on plasma-induced transparency

A periodic metamaterial comprising three graphene-strips and four C-type graphene square-ring is proposed herein to realize plasmon-induced transparency (PIT). Based on the characteristics of graphene dynamic modulation, we effectively broaden of the transparent window the relative bandwidth to realize the broadband PIT. The influence of the polarized light angle on PIT is investigated, demonstrating that the polarization angle affects PIT significantly, the dynamic change from multiple plasma-induced transparency to single-PIT can be realized. By tuning the Fermi levels in the graphene, our proposed metamaterial structure allows preparing not only a multi-frequency optical switch with modulation degrees of the amplitude (MDA) up to 95.6% and insertion loss (IL) down to 0.315 dB at 5.35THz but also preparing optical switches with MDA > 90% and IL < 0.35 dB in bands ranging from 5.28 to 5.48THz. The refractive index sensitivity of metamaterials is also investigated, and the simulation results show that a sensitivity of 0.7 THz/RIU is achieved, meaning an excellent sensors can be prepared. Thus, the proposed structure provides a new idea for the design of terahertz multi-frequency switches and refractive index sensors.

Enhanced polarization conversion and giant Faraday rotation in patterned terahertz graphene metamaterials with combined electrical and magnetic tuning

Graphene metamaterials (MMs) have the potential to reconfigure and dynamically control terahertz (THz) waves. In this study, we conducted numerical investigations to explore the effects of externally applied magnetic fields up to 20 Tesla on the transmission properties of graphene patterned split ring resonator (GSRR) MMs in the THz region. We quantitatively compared the tunability of resonance amplitude and frequency in the co-polarized transmission component between the magnetic method and the traditional electrical approach. Our results demonstrate that magnetic tuning can effectively modulate the resonant properties of GSRR MMs. Furthermore, when combining electrical and magnetic tuning, we observed an enhancement in the polarization conversion ratio, as well as the achievement of a significant Faraday rotation angle of nearly 90 degrees in GSRR MMs. These findings indicate the potential of functional graphene-based THz devices, including switches, modulators, polarization converters, and sensors.