Terahertz Polarization Research Articles

Compact THz Polarization Splitter with Large Bandwidth Based on Cascade Hexagonal Porous Two-Core PCF

ABSTRACT A novel cyclic olefin copolymer (COC)-based terahertz (THz) polarization beam splitter (PBS) employing photonic crystal fiber (PCF) with cascade hexagonal porous two-core structure is proposed. The porous two-core is achieved by replacing the original two large air holes with three layers of small air holes. The mode coupling characteristics between the even and odd modes in the x- and y-polarization directions for the proposed THz PBS are explored using the time-domain finite difference (FDTD) method. The influence of PBS structural parameters on its coupling length is investigated. Numerical analysis results indicate that the bandwidths of 227.8 GHz (ranging from 1.8970 to 2.1248 THz) and 147 GHz (ranging from 1.928 to 2.075 THz) are acquired for the x- and y-polarization fundamental modes, respectively. The highest extinction ratios (ERs) are 114.5 dB and 72.3 dB, and the minimum insertion losses (ILs) are 0.0142 dB and 0.0369 dB for the x- and y-polarization fundamental modes, respectively. Moreover, the length of the proposed PBS is merely 11.655 mm. The proposed PBS will find potential applications in polarization-diverse THz systems, including broadband line-of-sight communication, real-time security checks, and high-resolution imaging.

Terahertz Metasurfaces for Polarization Manipulation and Detection: Principles and Emerging Applications

AbstractTerahertz frequencies locating between microwave and infrared regions can record and process optical information for next‐generation information technology such as 6G communication. Metasurfaces, as emerging planar optical components built with micro‐ or nanostructures, enable precise control, and manipulation of electromagnetic waves from near fields to far fields. These subwavelength artificial structures can be engineered to exhibit specific polarization responses, thus holding significant potential for applications in polarization detection. In the terahertz (THz) region, polarization information is crucial for identifying and analyzing the properties of materials in the terahertz. In this review, we focus on recent advances in terahertz metasurfaces for polarization manipulation and detection, including principles and emerging applications. New polarization detection methods such as polarization state conversion, polarization‐selective absorption, polarization‐selective focusing, and vector beam construction are discussed. Finally, it is concluded with a few perspectives on emerging trends and existing challenges in the fields of terahertz polarization manipulation and detection.

Design terahertz polarizers and vector polarized vortex terahertz wave generators based on the effective dielectric constant of metal gratings

Polarization and phase devices for terahertz waves have important applications in terahertz detection, imaging, communication, etc. Spatially variable metal gratings can be used for broad-spectrum, miniaturized, and low-cost terahertz polarization and phase modulation devices. Based on the effective dielectric constant and the theory of light propagation in multilayer media, we obtain the relationship between the transmittance and extinction ratio and the parameters such as the duty cycle of the metal grating, the frequency of the incident terahertz wave, the angle of incidence, the thickness of the metal grating, the refractive index of the substrate, and the thickness of the substrate. We propose a method of designing a spatially variable metal grating located on a transparent substrate. The designed spatially variable metal grating is also used to modulate the terahertz spatial polarization and phase to generate terahertz optical fields whose polarization and phase change simultaneously in space, such as azimuthally vector vortex terahertz optical fields, radially vector vortex terahertz optical fields, and so on. This will have important applications in terahertz time-domain spectroscopic detection, terahertz time-domain spectroscopic imaging, terahertz time-domain near-field microscopic imaging, terahertz communication, and so on.

Characterizing strong-field tunneling ionization with a phase-dependent THz polarization spectrum

The terahertz radiation emitted by asymmetrically ionized wavepackets in two-color strong-field tunneling ionization is essential for detecting the system's associated electron dynamics and structural properties. We propose to characterize and control tunneling ionization using a phase-dependent terahertz polarization (PTP) spectrum, analyzed through a combination of the classical trajectory Monte Carlo method, an analytical model based on the virial theorem, and the rigorous solution of the time-dependent Schrödinger equation as a benchmark. Our results demonstrate that the PTP spectrum offers a high-precision measure of the Coulomb effect through the relative phase of the two-color laser. Comparisons of PTP results calculated using different methods suggest how the electron can be manipulated by controlling the relative phase and laser intensity. In particular, the PTP spectrum can be used to calibrate the relative phase and provides a convenient and robust reconstruction of the time-averaging of tunneling positions with high precision using the analytical model. These insights reveal that the PTP spectrum as a whole can be a new and useful tool for the all-optical characterization of ultrafast atomic and molecular ionization.

Terahertz polarization conversion and asymmetric transmission based on a liquid crystal integrated EIT metasurface.

We experimentally demonstrate a liquid crystal (LC)-integrated EIT metasurface for active THz polarization conversion and asymmetric transmission. By controlling the LC orientation under static magnetic field anchoring and an adjustable electric field, the device realizes the active control from the OFF state to the ON state, corresponding to the orthogonal polarization excitation modes of the EIT metasurface. Furthermore, based on the different polarization responses at forward and backward incidences, we achieve asymmetric transmission at the EIT peak and two nearby resonances, with its isolation actively manipulated by the external electric field. This study on dynamic polarization conversion and asymmetric transmission by a LC-integrated metasurface offers a promising route for active THz devices, applicable to THz communication, switching, and sensing systems.

Terahertz wide-band wide-angle absorber and polarization converter based on VO2 metamaterial

In this study, a Terahertz wide-band wide-angle absorber and polarization converter based on VO2 metamaterial is proposed. The device is simulated by using full-wave electromagnetic computing software CST. The device functions as a wide-band absorber while VO2 is in the metallic mode. In the 1.82 THz to 4.27 THz band, the absorption efficiency exceeds 90% and the relative bandwidth reaches 81.4%. The absorption mechanism is analyzed bying equivalent circuit theory. Additionally, the absorber can work at large polarization angle and incident angle. While VO2 is in the insulated mode, the design can convert the incident y-polarized terahertz wave into the x-polarized state and realize the transformation of linear polarization. Polarization conversion efficiency (PCR) exceeds 90% in the band of 1.57 THz to 3.82 THz, with a relative bandwidth of 83.5%. The device also supports large incidence angle tolerances. These results show that the disigned metamaterial device has potential applications in absorption, polarization conversion and imaging.

Terahertz polarization beam splitter with wide bandwidth and low loss

Terahertz polarization beam splitter with wide bandwidth and low loss

Terahertz polarization sensing for influenza A virus based on plasmonic metasurface

Terahertz metasurface sensors attract extensive attention for excellent characterisics. However, most existing sensing schemes overlooked the polarization state of electromagnetic waves. Here, we propose a plasmonic metasurface sensor based on the elliptical polarization state of reflected EM wave, which can be used for the sensing of influenza A virus. The sensor achieves the conversion from linear polarization to circular polarization within 1–3 THz. By analysing the electromagnetic field distributions of the resonances at 1.43 THz and 2.16 THz, it can be concluded that the polarization conversion originates from the magnetic dipole. Besides, the sensor can characterize the changes in the complex refractive index of the test sample based on the elliptical polarization state of the reflected wave. The electromagnetic response of the metasurface sensor shows an excellent linear relationship between the rotating direction angle of polarization ellipse and the extinction coefficient (k) of the complex RI of the analyte. Furthermore, we also demonstrate the feasibility of detecting three subtypes of Influenza A viruses (H1N1, H5N2, and H9N2) at 1.39 THz though the elliptical polarization state. This sensing approach does not rely on high-precision broadband scanning, providing an alternative perspective for THz biosensing.

Ultra-broadband and wide-angle reflective terahertz polarization conversion metasurface based on topological optimization

Abstract Terahertz polarization conversion devices have significant potential applications in various fields such as terahertz imaging and spectroscopy. In this paper, we utilize genetic algorithms to topologically optimize the metasurface unit cells and design a reflective linear polarization conversion metasurface with ultra-broadband and wide-angle characteristics. By partitioning the metallic pattern layer into quadrants, the encoding length is effectively reduced, resulting in a shorter optimization time. The research results indicate that the converter possesses a polarization conversion efficiency ratio higher than 90% and a relative bandwidth ratio of 125% in a range of 0.231–0.995 THz. Meanwhile, it can maintain excellent polarization conversion properties when the incident angle of terahertz waves is less than 45° and the polarization angle is less than 15°, demonstrating excellent practicality. New insights are provided for the design of terahertz wide-angle ultra-wideband polarization conversion devices, and the proposed metasurfce has potential applications in terahertz polarization imaging, spectroscopy and communication fields.

Optical damage thresholds of single-mode fiber-tip spintronic terahertz emitters.

Spintronic terahertz emitters (STEs) are gapless, ultrabroadband terahertz sources that can be driven within a wide pump-wavelength and repetition-rate range. While STEs driven by strong pump lasers operating at kilohertz repetition rates excel in generating high electric field strengths for terahertz spectroscopy or ellipsometry, newly advancing technologies such as ultrafast modulation of terahertz polarization, scanning tunneling microscopy, laser terahertz emission nanoscopy, and fully fiber-coupled integrated systems demand an STE pumping at megahertz repetition rates. In all these applications the available terahertz power is ultimately limited by the STE's optical damage threshold. However, to date, only very few publications have targeted this crucial topic and investigations beyond the kilohertz repetition-rate regime are missing. Here, we present a complete study of our single-mode fiber-tip STEs' optical damage thresholds covering the kilohertz, megahertz, and gigahertz repetition-rate regimes as well as continuous-wave irradiation. As a very important finding, we introduce the necessity of classifying the optical damage threshold into two regimes: a low-repetition-rate regime characterized by a nearly constant fluence threshold, and a high-repetition-rate regime characterized by an antiproportional fluence dependence ("average-power threshold"). For our single-mode fiber-tip STEs, the transition between these regimes occurs around 4 MHz. Moreover, we present a cohesive theory of the damaging thermodynamical processes at play and identify temperature-driven inter-layer diffusion as the primary cause of the STE failure. These findings are substantiated by atomic force microscopy, infrared scattering-type scanning near-field optical microscopy, and scanning transmission electron microscopy measurements. This new level of understanding offers a clear optimization lever and provides valuable support for future advancements in the promising field of spintronic terahertz emission.

Tunable terahertz polarization conversion and absorption devices based on metamaterials

Metamaterials, as novel materials, have electromagnetic behaviors that can be realized by rationally designing the unit structure. It plays an important role in the research of terahertz functional devices. Switchable terahertz devices are constructed by flexibly combining the excellent properties of graphene and vanadium dioxide (VO2) with the application potential of terahertz technology. When the VO2 is in the metallic state and the graphene Fermi energy level is 0 eV, the device behaves as a broadband polarization switching device. It can realize cross-polarization conversion for the incident electromagnetic wave in the range of 2–4.2 THz. When the VO2 is in the insulating state and the graphene Fermi energy level is 0.9 eV, the device is an absorption device. The absorption rate can reach more than 99% at 4 THz. In addition, the polarization conversion ratio (PCR) as well as the absorption rate of the device can be tuned independently. This actively tunable switchable terahertz device provides ideas for dynamically controlling terahertz, bringing more possibilities for various applications such as terahertz imaging, stealth, and communication.

Terahertz Polarization Isolator Using Two-Dimensional Square Lattice Tellurium Rod Array.

A novel terahertz polarization isolator using a two-dimensional square lattice tellurium rod array is numerically investigated at the interesting band of 0.22 THz in this short paper. The isolator is designed by inserting six hexagonal tellurium rods into a fully polarized photonic crystals waveguide with high efficiency of -0.34 dB. The TE and TM photonic band gaps of the 7 × 16 tellurium photonic crystals are computed based on the plane wave expansion method, which happen to coincide at the normalized frequency domain from 0.3859(a/λ) to 0.4033(a/λ), corresponding to the frequency domain from 0.2152 to 0.2249 THz. The operating bandwidth of the tellurium photonic crystals waveguide covers 0.2146 to 0.2247 THz, calculated by the finite element method. The six hexagonal tellurium rods with smaller circumradii of 0.16a serve to isolate transverse electric waves and turn a blind eye to transverse magnetic waves. The polarization isolation function and external characteristic curves of the envisaged structure are numerically simulated, which achieves the highest isolation of -33.49 dB at the central frequency of 0.2104 THz and the maximum reflection efficiency of 98.95 percent at the frequency of 0.2141 THz. The designed isolator with a unique function and high performance provides a promising approach for implementing fully polarized THz devices for future 6G communication systems.

MEMS-actuated terahertz metamaterials driven by phase-transition materials

The non-ionizing and penetrative characteristics of terahertz (THz) radiation have recently led to its adoption across a variety of applications. To effectively utilize THz radiation, modulators with precise control are imperative. While most recent THz modulators manipulate the amplitude, frequency, or phase of incident THz radiation, considerably less progress has been made toward THz polarization modulation. Conventional methods for polarization control suffer from high driving voltages, restricted modulation depth, and narrow band capabilities, which hinder device performance and broader applications. Consequently, an ideal THz modulator that offers high modulation depth along with ease of processing and operation is required. In this paper, we propose and realize a THz metamaterial comprised of microelectromechanical systems (MEMS) actuated by the phase-transition material vanadium dioxide (VO2). Simulation and experimental results of the three-dimensional metamaterials show that by leveraging the unique phase-transition attributes of VO2, our THz polarization modulator offers notable advancements over existing designs, including broad operation spectrum, high modulation depth, ease of fabrication, ease of operation condition, and continuous modulation capabilities. These enhanced features make the system a viable candidate for a range of THz applications, including telecommunications, imaging, and radar systems.Graphical

High-precision measurements of terahertz polarization states with a fiber coupled time-domain THz spectrometer

We present a new method for high precision measurements of polarization rotation in the frequency range from 0.2 to 2.2 THz using a fiber coupled time-domain THz spectrometer. A free standing wire-grid polarizer splits THz light into orthogonal components that are then measured by two separate detectors simultaneously. We theoretically model the uncertainties introduced by optical component non-idealities and predict that we may expect to achieve accuracies of 0.8% when anti-symmetrizing the response with respect to an applied field. Anti-symmetrization improves accuracy by more than four orders of magnitude. We demonstrate this method on a 2D electron gas in magnetic field and show that we achieve a precision of 20 μrad (1.1 mdeg) for small polarization rotation angles. A detailed description of the technique and data analysis procedure is provided, demonstrating its capability to precisely measure polarization states in the 0.2 to 2.2 THz range.

Wide-angle metalens array with quadratic phase for terahertz polarization detection

Abstract With the advances of micro/nano fabrication technology, metasurface has become an alternative to design functional devices for manipulating electromagnetic wave. Metalens is one of the basic electromagnetic functional devices that can be applied in various fields. Currently, polarization measurement based on metalens arrays has been widely investigated, but most of them can only work for normal incident wave due to the limited field-of-view of metalens. Herein, a dielectric wide-angle metalens array (WMA) for the terahertz polarization detection is presented. The WMA is composed of three wide-angle metalenses, each wide-angle metalens is constructed by utilizing quadratic phase profile. The angle tolerance of meta-atoms which constitute the wide-angle metalens is elucidated in detail. The WMA can decompose incident terahertz wave into four channels, and the full Stokes parameters of incident wave is determined by intensities in these four channels. Simulated results show that the WMA proposed here has excellent performance for the polarization detection within incident angle of ±40°. In addition, this WMA can also be used to detect the phase gradient of incident terahertz wave, the detection error is less than 1.1%. This WMA is promising in the fields of terahertz polarization generation, detection and imaging.

Ultrafast Exciton Dynamics in the Atomically Thin van der Waals Magnet CrSBr.

Among atomically thin semiconductors, CrSBr stands out as both its bulk and monolayer forms host tightly bound, quasi-one-dimensional excitons in a magnetic environment. Despite its pivotal importance for solid-state research, the exciton lifetime has remained unknown. While terahertz polarization probing can directly trace all excitons, independently of interband selection rules, the corresponding large far-field foci substantially exceed the lateral sample dimensions. Here, we combine terahertz polarization spectroscopy with near-field microscopy to reveal a femtosecond decay of paramagnetic excitons in a monolayer of CrSBr, which is 30 times shorter than the bulk lifetime. We unveil low-energy fingerprints of bound and unbound electron-hole pairs in bulk CrSBr and extract the nonequilibrium dielectric function of the monolayer in a model-free manner. Our results demonstrate the first direct access to the ultrafast dielectric response of quasi-one-dimensional excitons in CrSBr, potentially advancing the development of quantum devices based on ultrathin van der Waals magnets.

Terahertz polarimetric imaging of biological tissue: Monte Carlo modeling of signal contrast mechanisms due to Mie scattering.

Many promising biomedical applications have been proposed for terahertz (THz) spectroscopy and diagnostic imaging techniques. Polarimetric imaging systems are generally useful for enhancing imaging contrasts, yet the interplay between THz polarization changes and the random discrete structures in biological samples is not well understood. In this work, we performed Monte Carlo simulations of the propagation of polarized THz waves in skin and adipose tissues based on the Mie scattering from intrinsic structures, such as hair follicles or sweat glands. We show that the polarimetric contrasts are distinctly affected by concentration, size and dielectric properties of the scatterers, as well as the frequency and polarization of the incident THz waves. We describe the experimental requirements for observing and extracting these polarimetric signals due to the low energy and small angular spread of the back-scattered THz radiation. We analyzed the spatially integrated Mueller matrices of samples in the normal-incidence back-scattering geometry. We show that the frequency-dependent degree of polarization (DOP) can be used to infer the concentrations and dielectric contents of the scattering structures. Our modeling approach can be used to inform the design of the imaging modalities and the interpretation of the spectroscopic data in future terahertz biomedical imaging applications.

Terahertz structured deep-subwavelength plasmonic grating metasurface for manipulating polarization-dependent coupling response

The complex electromagnetic anisotropy of terahertz (THz) metasurfaces with geometric symmetry breaking has attracted extensive attention. Typical effects arise from the coupling of polarization responses in orthogonal directions of various components of the metasurface structure, such as the electromagnetically induced transparency (EIT) effect. However, it is a challenge to precisely control or perfectly avoid the polarization-dependent coupling responses. In this work, deep-subwavelength plasmonic gratings (PGs) with a fine wire width of 1 μm at the order of deep subwavelengths of 1/100 THz wave are fabricated by electron beam lithography, and these wire gratings are graphically designed as a C-shaped metasurface pattern with a period of 100 μm in sub-wavelength scale. The complete anisotropic response in the single-oriented PG metasurface is demonstrated by both simulation and experiments, where the polarization-dependent coupling effect is eliminated. More interestingly, the hybrid-oriented PG metasurface exhibits narrowband and wideband EIT effects in the x and y polarization directions with the maximum polarization extinction ratio of 20 dB, respectively, indicating this mechanism can realize more flexible manipulation of polarization-dependent coupling. This patterned deep-subwavelength PG provides a new structure and mechanism for excitation, regulation, and restriction of polarization-dependent mode coupling, and has important applications in THz spectroscopic detection, polarization imaging, and wireless communication.

Electrically Active Terahertz Liquid‐Crystal Metasurface for Polarization Vortex Beam Switching

AbstractPolarized vortex waves have attracted widespread attention in investigations of light–matter interactions and the augmentation of information capacity owing to their distinctive characteristics. Nevertheless, the reconfigurable generation of vector beams, especially at terahertz (THz) frequencies, remains challenging. In this study, a tunable THz polarization vortex beam generator based on a liquid‐crystal metasurface is proposed. A unit cell featuring reconfigurable linear polarization selectivity is developed. A general methodology for designing metasurfaces to generate customized and reconfigurable polarization patterns is introduced. Furthermore, the electrically tunable generation of polarized patterns and cylinder vector beams is experimentally demonstrated. The findings of this study can open up opportunities for wireless communication and super‐resolution imaging applications.

Independently regulating linearly and circularly polarized terahertz wave metasurface

We propose a terahertz metasurface that can independently regulate linearly circularly polarized waves. It consists of the top layer “O-O” metal pattern, polyimide layer, middle layer “I” shaped metal pattern, polyimide layer, and metal substrate from top to bottom. By using the phase principle of Pancharatnam Berry (PB) for encoding and arrangement, the metasurface generates vortex beams with different topological charges under circularly polarized terahertz wave incidence, and achieves focusing shift at different positions. Combining the convolution theorem for encoding arrangement, the metasurface can achieve focused vortex function under circularly polarized terahertz wave incidence. The designed metasurface can also generate Airy beam under linearly polarized terahertz wave incidence. The simulation results indicate that under different polarization (linear/circular polarization) terahertz wave incidence, the metasurface can achieve different functions, which provides a new approach for flexible control of terahertz waves.