Terahertz Frequency Range Research Articles

3D Architectural MXene‐based Composite Films for Stealth Terahertz Electromagnetic Interference Shielding Performance

AbstractThe terahertz frequency range is gaining popularity in security, stealth technology, and the future 6G network communication. For the control of severe terahertz electromagnetic interference (EMI) pollution, frequency‐selective stealth‐capable shielding materials are being explored to mask terahertz signals. For the realization of masking terahertz signals, the robustness, lightweight, and shape‐conformable materials with excellent terahertz EMI shielding/absorption are crucial. Here, the study reports the fabrication of 3D symmetric pyramidal architectural MXene composite films with frequency‐selective stealth performance characteristics via the facile drop casting method. With the high absorption capability of 2D MXene layers, the MXene composite films exhibit substantial terahertz stealth performance. 3D pyramidal microstructure design leads to frequency selective surface‐assisted reflection resonance in the frequency range of 0.6–1.1 THz. The MXene composite film demonstrates an outstanding maximum terahertz shielding effectiveness (SE) of up to 70.4 dB and a specific SE of 0.55 dB µm−1. These terahertz SE values exceed all of those for MX‐based shielding material designs reported in the literature. The investigation will open a new direction toward developing terahertz EMI shielding thin films with easy integration into any surface for stealth capabilities.

Electrically Tunable Nonlinearity at 3.2 Terahertz in Single-Layer Graphene.

Graphene is a nonlinear material in the terahertz (THz) frequency range, with χ(3) ∼ 10-9 m2/V2 ∼ 15 orders of magnitude higher than that of other materials used in the THz range, such as GaAs or lithium niobate. This nonlinear behavior, combined with ultrafast dynamic for excited carriers, proved to be essential for third harmonic generation in the sub-THz and low (<2.5 THz) THz range, using moderate (60 kV/cm) fields and at room temperature. Here, we show that, for monochromatic high peak power (1.8 W) input THz signals, emitted by a quantum cascade laser, the nonlinearity can be controlled using an ionic liquid gate that tunes the graphene Fermi energy up to >1.2 eV. Pump and probe experiments reveal an intense absorption nonlinearity at 3.2 THz, with a dominant 3rd-order contribution at EF > 0.7 eV, hence opening intriguing perspectives per engineering novel architectures for light generation at frequencies > 9 THz.

Multiple designs with broad applicability for enhancing birefringence in low-loss terahertz HC-ARF

The use of hollow-core anti-resonant fiber (HC-ARF) as a transmission medium for the terahertz (THz) frequency range is attracting increasing attention from researchers. This paper introduces a low-loss, high birefringence anti-resonant fiber. The proposed structure achieves a maximum birefringence of 2.72 × 10−3 around 1 THz and maintains a birefringence above the 10−3 level over a wide frequency band (0.7∼1.12 THz). It has a minimum loss of 0.2 dB/m and a minimum bend radius of 0.3 m, and the paper validates that this structure has good manufacturing error tolerance. Furthermore, three methods for introducing high birefringence into low-loss ARFs are proposed and three structures are selected for verification. These findings contribute to the development of more efficient and reliable THz transmission systems.

Near-Field Spectroscopy of Individual Asymmetric Split-Ring Terahertz Resonators.

Metamaterial resonators have become an efficient and versatile platform in the terahertz frequency range, finding applications in integrated optical devices, such as active modulators and detectors, and in fundamental research, e.g., ultrastrong light-matter investigations. Despite their growing use, characterization of modes supported by these subwavelength elements has proven to be challenging and it still relies on indirect observation of the collective far-field transmission/reflection properties of resonator arrays. Here, we present a broadband time-domain spectroscopic investigation of individual metamaterial resonators via a THz aperture scanning near-field microscope (a-SNOM). The time-domain a-SNOM allows the mapping and quantitative analysis of strongly confined modes supported by the resonators. In particular, a cross-polarized configuration presented here allows an investigation of weakly radiative modes. These results hold great potential to advance future metamaterial-based optoelectronic platforms for fundamental research in THz photonics.

Electrically controllable broadband reflective linear cross-polarization conversion based on liquid crystals.

This paper presents an electrically controllable reflective broadband linear polarization (LP) converter based on liquid crystals (LCs) for cross-polarization conversion (CPC) in the terahertz frequency range. The proposed structure achieves a high polarization conversion ratio (PCR) exceeding 0.9 within the frequency range of 236.8 - 269.6 GHz. A vital feature of this design is the dynamic control of polarization conversion by re-orienting the nematic liquid crystal molecules through voltage bias switching between 'on' and 'off' states, allowing for precise manipulation of cross-polarized and co-polarized reflected waves. Experimental results validate the simulation outcomes, demonstrating excellent agreement. In contrast to conventional reflective polarization converters with fixed frequency responses, the proposed electrically controllable polarization conversion offers significant potential in imaging and optical communications.

Terahertz broadband near-perfect absorber with a single-layer coating on doped semiconductor

We demonstrate that a single-layer coating on a doped GaAs or Si substrate enables broadband antireflection and, hence, broadband perfect absorption in the terahertz frequency range. This broadband behavior can be generally expected when the substrate material has a Drude-type dispersion. Our mathematical analyses show that the reflection from Drude-type material may have an anomalously dispersive phase shift. The anomalous dispersion of the reflection phase is used to compensate for the normal dispersion of the accumulation phase in the single-layer coating film. Consequently, the antireflection conditions are satisfied in a wide frequency range, and broadband antireflection is achieved. Thus, broadband perfect absorption is realized with only a single-layer coating film on the substrate. Our method provides a simple and efficient approach to achieving broadband perfect absorption, which is critical in many applications such as radar stealth techniques and solar cells.

High-performance terahertz polarization filter based on the anti-resonant fiber

A novel terahertz polarization filter based on the anti-resonant fiber is designed and analyzed. The polarization extinction ratio (PER) is improved by using high-resistivity silicon (HRS) as the bulk material in conjunction with a double conjoined tube cladding structure. The effects of the structure and material of the anti-resonant fiber on the polarization filtering characteristics are analyzed by the finite element method. The PER of the anti-resonant fiber reaches 100 dB and the insertion loss of y-polarization fundamental mode (YPFM) is less than 0.66 dB at 1 THz when the anti-resonant fiber length is 5.58 cm. The bandwidth can be expanded by changing the fiber length and for a fiber length of 8 cm, the frequency bandwidth is 0.92–1.14 THz and the insertion loss is less than 3 dB. The polarization filter is demonstrated to deliver excellent performance in the terahertz frequency range and has large commercial potential in terahertz communication and biomedical spectroscopy.

Review of Bioplastics Characterisation by Terahertz Techniques in the View of Ensuring a Circular Economy

The increasing scarcity of natural resources, worsening global climate change, environmental degradation, and rising demand for food are forcing the biotechnology and plastics industries to seek and apply circular economy models that would lead to a sustainable transition in the production and use of bioplastics. Circular economy models can improve the economic productivity of bio-based plastics and have a positive impact on the environment by reducing conventional plastic waste and the consumption of petrochemical feedstocks for plastic production. In addition, some agricultural wastes that have the potential to be used as bioplastics can be reused. Terahertz (THz) systems are already used in the plastics and rubber industries for non-destructive testing, detection, imaging, and quality control. Several reports have highlighted the potential applications of THz spectroscopy and imaging in polymer analysis and plastics characterisation. This potential is even greater with chemometric methods and artificial intelligence algorithms. In this review, we focus on applications that support the transformation of the biotechnology sector to the circular economy, particularly via the transition from conventional plastics to bioplastics. In this review, we discuss the potential of THz systems for the characterisation and analysis of bioplastics and biopolymers. The results of previous studies on biopolymers in the THz frequency range are summarised. Furthermore, the potential of using artificial intelligence approaches such as machine learning as advanced analytical methods in THz spectroscopy and imaging, in addition to the conventionally used chemometric methods, is discussed. The results of this review highlight that THz technology can contribute to closed technological circles in important areas of biotechnology and the related plastics and rubber industries.

Dispersive Propagation of Terahertz Pulses in a Plasmonic Fiber

The dispersion properties of surface plasmon polaritons (SPPs) during propagation on metal wires with a dielectric coating in the terahertz frequency range were investigated theoretically. An analytical expression was obtained for a pulsed electric field using the solution of Maxwell equations taking into account high-order dispersion terms. The influence of the dielectric coating on the distortion of the pulse shape was investigated. Unlike uncoated wire, the propagation of surface plasmon pulses along a coated wire is highly dispersive. It was shown that the coating leads to the appearance of a long-chirped signal with a propagation of only a few millimeters, i.e., when a terahertz pulse propagates along a coated wire, it acquires a long oscillatory tail, the frequency of which depends on time.

Design of a novel graphene buzzle metamaterial refractometer for sensing of cancerous cells in the terahertz regime

This paper introduces a novel biosensor with enhanced sensitivity for detecting the dielectric constant. The biosensor utilizes a metamaterial buzzle resonator incorporated with a thin graphene layer measuring 35 nm, operating in the terahertz (THz) frequency range. The biosensor demonstrates remarkable performance, featuring a high average sensitivity of 495 GHz/RIU (7.64 dB/RIU) and a quality factor of 82. These attributes enable the biosensor to effectively differentiate between various types of cancer cells, including basal cell, breast cell, and cervical cell, Jurkat, MCF-7, and PC12. The biosensor's resonance frequency and peak attenuation (S11 dB) display considerable sensitivity to changes in the refractive index of the sample, thereby achieving excellent linear performance. As a result of developing these characteristics, the biosensor demonstrates to be highly effective in detecting and distinguishing cancer cells built on their dielectric properties. The sensor's sensitivity, combined with its capacity to distinguish between different cell types, makes it a promising tool for cancer finding and characterization. This Research presents a state-of-the-art biosensor capable of detecting the dielectric constant with remarkable sensitivity. This utilization of a metamaterial buzzle resonator and thin graphene layer in the THz regime, coupled with high average sensitivity, quality factor, and linear performance, enables effective discrimination of various cancer cell types. This biosensor holds great potential for advancing cancer detection and characterization techniques.

Obtaining the Effective Dielectric Permittivity of a Conducting Surface in the Terahertz Range via the Characteristics of Surface Plasmon Polaritons

With the intensive development of data transmitting and processing devices in the terahertz (THz) frequency range, an important part of which are integrated plasmonic components and communication lines, it becomes necessary to measure correctly the optical constants of their conductive surfaces. In this paper, we describe a reliable method for determining the effective permittivity εm of a metal surface from the measured characteristics (refractive and absorption indices) of THz surface plasmon polaritons (SPPs). The novelty of the method is the conduction of measurements on a metal surface with a dielectric layer of subwavelength thickness, suppressing the radiative losses of SPPs, which are not taken into account by the SPP dispersion equation. The method is tested on a number of flat “gold sputtering–zinc sulfide layer–air” structures with the use of the THz radiation (λ0 = 141 μm) from the Novosibirsk free electron laser (NovoFEL). The SPP characteristics are determined from interferograms measured with a plasmon Michelson interferometer. It is found that the method allows a significant increase in the accuracy of the εm in comparison with measurements on the same metal surface without a dielectric layer.

Phonon Polaritonics in Broad Terahertz Frequency Range with Quantum Paraelectric SrTiO3.

Photonics in the frequency range of 5-15terahertz (THz) potentially open a new realm of quantum materials manipulation and biosensing. This range, sometimes called "the new terahertz gap", is traditionally difficult to access due to prevalent phonon absorption bands in solids. Low-loss phonon-polariton materials may realize sub-wavelength, on-chip photonic devices, but typically operate in mid-infrared frequencies with narrow bandwidths and are difficult to manufacture on a large scale. Here, for the first time, quantum paraelectric SrTiO3 enables broadband surface phonon-polaritonic devices in 7-13THz. As a proof of concept, polarization-independent field concentrators are designed and fabricated to locally enhance intense, multicycle THz pulses by a factor of 6 and increase the spectral intensity by over 90 times. The time-resolved electric field inside the concentrators is experimentally measured by THz-field-induced second harmonic generation. Illuminated by a table-top light source, the average field reaches 0.5GVm-1 over a large volume resolvable by far-field optics. Theseresults potentially enable scalable THz photonics with high breakdown fields made of various commercially available phonon-polariton crystals for studying driven phases in quantum materials and nonlinear molecular spectroscopy.

Diffraction of Electromagnetic Waves on Graphene Metasurfaces in the Terahertz Frequency Range

Diffraction of Electromagnetic Waves on Graphene Metasurfaces in the Terahertz Frequency Range

Near-field radiative heat transfer in three-body system based on topological insulator Bi2Se3

This study investigates the near-field radiative heat flux in a three-body system made of Bi2Se3, a topological insulator that supports hyperbolic phonon polaritons (HPPs) in the terahertz frequency range. We demonstrate that the near-field radiative heat flux in the three-body system made of Bi2Se3 is remarkably enhanced compared to the two-body counterpart. The thickness of the intermediate body or the chemical potential of Bi2Se3 can be used to control the radiative heat flux. Our numerical results reveal that the heat flux initially increases and then decreases with the thickness of the intermediate body, indicating that an optimal heat flux can be obtained by selecting an appropriate chemical potential and intermediate body thickness. Moreover, increasing the chemical potential of Bi2Se3 shifts the hybrid surface plasmon phonon polaritons, which arise from the coupling of surface plasmons and HPPs, toward higher frequencies. These findings have important implications for developing thermal devices such as thermal transistors and offer a promising approach to modulating near-field radiative heat transfer and thermal management.

Electrically-pumped compact topological bulk lasers driven by band-inverted bound states in the continuum

One of the most exciting breakthroughs in physics is the concept of topology that was recently introduced to photonics, achieving robust functionalities, as manifested in the recently demonstrated topological lasers. However, so far almost all attention was focused on lasing from topological edge states. Bulk bands that reflect the topological bulk-edge correspondence have been largely missed. Here, we demonstrate an electrically pumped topological bulk quantum cascade laser (QCL) operating in the terahertz (THz) frequency range. In addition to the band-inversion induced in-plane reflection due to topological nontrivial cavity surrounded by a trivial domain, we further illustrate the band edges of such topological bulk lasers are recognized as the bound states in the continuum (BICs) due to their nonradiative characteristics and robust topological polarization charges in the momentum space. Therefore, the lasing modes show both in-plane and out-of-plane tight confinements in a compact laser cavity (lateral size ~3λlaser). Experimentally, we realize a miniaturized THz QCL that shows single-mode lasing with a side-mode suppression ratio (SMSR) around 20 dB. We also observe a cylindrical vector beam for the far-field emission, which is evidence for topological bulk BIC lasers. Our demonstration on miniaturization of single-mode beam-engineered THz lasers is promising for many applications including imaging, sensing, and communications.

Near-infrared light-induced superconducting-like state in underdoped YBa2Cu3Oy studied by c -axis terahertz third-harmonic generation

The recent observation of the light-induced superconducting (SC)-like transient response in $c$-axis optical conductivity far above the SC transition temperature ${T}_{\mathrm{c}}$ in underdoped $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ (YBCO) has attracted a great deal of attention in the field of high-${T}_{\mathrm{c}}$ superconductors. Since then, various theoretical and experimental studies have been devoted to elucidating its microscopic origin. One prominent fingerprint of light-induced superconductivity is the emergence of $1/\ensuremath{\omega}$-like spectral behavior in the imaginary part of the optical conductivity in the terahertz (THz) frequency range. However, the spectral profile can also be described by the Drude response of the quasiparticles (QPs) with a substantially low scattering rate. To circumvent this critical ambiguity, we investigated the light-induced nonequilibrium state in an underdoped YBCO sample with ${T}_{\mathrm{c}}$ of 61 K using the nonlinear THz optical response originating from the SC collective excitation of the ac-driven Josephson current. Upon the near-infrared (NIR) photoexcitation above ${T}_{\mathrm{c}}$ in the YBCO sample, the $1/\ensuremath{\omega}$-like spectral behavior in the imaginary part of the optical conductivity emerges, consistent with previous studies. However, the THz third-harmonic generation arising from the ac-driven Josephson current along the $c$-axis was absent in the NIR photoexcited state. These results indicate that the NIR pump-induced state exhibiting the $1/\ensuremath{\omega}$-like response above ${T}_{\mathrm{c}}$ is distinct from the long-range-ordered SC state in equilibrium. Based on these observations, the possible origins of the irregularly coherent charge-carrier response along the $c$-axis induced by photoexcitation are discussed.

Dispersion of optical constants of Si:PbGeO crystal in the terahertz range

Objectives. Advances in laser physics over the last decade have led to the creation of sources of single-period electromagnetic pulses having a duration of about 1 ps, corresponding to the terahertz (THz) frequency range and a field amplitude of several tens of MV/cm. This allows the electrode-free application of an electric field to a ferroelectric for observing not only the excitation of coherent phonons, but also ultrafast (at the sub-picosecond timescale) dynamic polarization switching. To detect polarization switching, a pump-probe technique is used in which a THz pulse is used with an optical probe. Since its intensity is proportional to the square of the polarization, the signal of the optical second harmonic is used to measure polarization switching under the action of a THz pulse. To evaluate switching efficiency, both linear (refractive index and absorption coefficient) and non-linear optical characteristics (quadratic and cubic susceptibilities) are required. For any application of ferroelectric crystals in the THz range, knowledge of the relevant linear optical characteristics is also necessary.Methods. The technique of THz spectroscopy in the time domain was used; here, a picosecond THz pulse transmitted through the crystal is recorded by strobing the detector with a femtosecond optical pulse. The THz-induced dynamics of the order parameter in a ferroelectric was studied by detecting the intensity of a nonlinear optical signal at the frequency of the second optical harmonic.Results. The transmission of a THz wave and the intensity of second harmonic generation on a lead germanate crystal doped with silicon in the time and spectral domains were measured. On this basis, the absorption coefficient dispersion and cubic nonlinear susceptibility were calculated in the range of 0.5-2.0 THz. The presence of a region of fundamental absorption near the phonon modes was confirmed along with a resonant enhancement of the cubic nonlinear susceptibility for two phonon modes Ω1 = 1.3 THz and Ω2 = 2.0 THz.Conclusions. The proposed technique is effective for analyzing the dispersion of the optical characteristics of ferroelectric crystals. The significantly improved spectral resolution (0.1 THz) increases the accuracy of determining nonlinear susceptibility due to the detailed analysis of the linear and nonlinear contributions to the second harmonic intensity.

Simulation of a System of Nanoantennas Located in a TSV Channel as a System for Receiving and Transmitting Data

The results of a theoretical study of the behavior of a system of nanophotonic devices, consisting of receiving and transmitting plasmonic metal antennas, are presented. Based on the finite element method, the main parameters of antennas located in the TSV channel and receiving a signal in the terahertz frequency range are calculated. The limiting range of signal transmission and the coefficient of its amplification are determined. Conclusions are drawn on the suitability of the presented configuration as a system for wireless data transmission and reception in three-dimensional integrated circuits.

Sub-terahertz scanning near-field optical microscope using a quartz tuning fork based probe.

We report a sub-terahertz scattering-type scanning near-field microscope (sub-THz s-SNOM) which uses a 6 mm long metallic tip driven by a quartz tuning fork as the near-field probe. Under continuous-wave illumination by a 94 GHz Gunn diode oscillator, terahertz near-field images are obtained by demodulating the scattered wave at both the fundamental and the second harmonic of the tuning fork oscillation frequency together with the atomic-force-microscope (AFM) image. The terahertz near-field image of a gold grating with a period of 2.3 µm obtained at the fundamental modulation frequency agrees well with the AFM image. The experimental relationship between the signal demodulated at the fundamental frequency and the tip-sample distance is well fitted with the coupled dipole model indicating that the scattered signal from the long probe is mainly contributed by the near-field interaction between the tip and the sample. This near-filed probe scheme using quartz tuning fork can adjust the tip length flexibly to match the wavelength over the entire terahertz frequency range and allows for operation in cryogenic environment.

ოპტიკურ ბოჭკოში წარმოქმნილი პარაზიტული სიხშირეების გამოთვლა LabVIEW პროგრამით საწყისი ხუთი ტალღის შემთხვევაში

The calculation of the parasitic frequencies generated by the four-wave mixing method during the passage of the five main frequencies in the optical fiber using the LabVIEW program is discussed. The virtual tool used for this is built using mathematical expressions and a program for calculating the approximate values of parasitic frequencies given in the article. It counts the number of parasitic frequencies and calculates their corresponding values that will be placed in the 185-189 terahertz frequency range of the first five frequencies. An algorithm has been compiled for the construction of the virtual tool, which consists of four options. According to each option, subtools are built, by combining which a virtual tool is obtained. This tool allows us to clearly visualize the values of parasitic frequencies in a given range and the total combinations of those frequencies by which these values are obtained.