THz Polarization Research Articles

Laser printing of silver and silver oxide

We show that direct laser writing in aqueous silver nitrate with a λ = 1030 nm femtosecond laser results in the deposition of a mixture of silver oxide and silver, in contrast to the pure silver deposition previously reported with 780 nm femtosecond direct laser writing. However, adding photoinitiator prevents silver oxide formation in a concentration-dependent manner. As a result, the resistivity of the material can also be controlled by photoinitiator concentration with resistivity being reduced from approximately 9e-3 Ωm to 3e-7 Ωm. Silver oxide peaks dominate the X-ray diffraction spectra when no photoinitiator is present, while the peaks disappear with photoinitiator concentrations above 0.05 wt%. A THz polarizer and metamaterial are printed as a demonstration of silver oxide printing.

Highly Sensitive Quad-Ray shaped Polarization Insensitive THz Meta-Biosensor

Abstract Metasurface-based sensors are now becoming crucial for label-free and rapid-detection technologies in biomedical applications, leading to a growing demand for new highly sensitive meta-biosensors. This paper demonstrates a perfectly symmetrical quad-ray X-shaped THz meta-absorber (QRXMA), enabling narrowband and polarization insensitivity performance. The theoretical modelling and performance investigation focuses on its potential for improved refractive index (RI) sensing for multiple biological samples. The simulation outcomes reveal that the proposed optimal QRXMA achieves an absorption rate of 99.98% at 1.3 THz for both TE and TM polarized incidences. Furthermore, we highlight the tunability of absorptive characteristics of the proposed device by altering the different geometric parameters. These findings underscore the QRXMA potential in RI sensing applications, achieving a sensitivity of approximately 120 GHz RIU-1 and a Figure of Merit (FoM) of about 2 with an 8 μm thick non-destructive analyte layer. This research paves the way for the development of highly efficient meta-biosensors with promising applications in THz detection, sensing, and imaging.

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.

Versatile platform for electrically reconfigurable THz devices based on silicon Schottky-metasurfaces

We propose a versatile platform to design tunable metasurface devices based on Au/n-Si Schottky diodes embedded in a split-ring resonator (SRR) devised on a Si-on-insulator (SOI) wafer. The horizontally formed diodes are connected in the SRR radial direction, reducing the overall junction capacitance of the metasurface array compared to its counterparts with vertically formed Schottky junctions. This reduction in the junction capacitance has an essential role in the switching speed of the metasurface between the On and Off states. By carefully varying the externally applied bias voltage to the Schottky diodes, one can manipulate the incident THz signal at the metasurface resonance frequencies by converting its resonance mode by switching states. We use the forenamed platform to design three fundamental THz devices: a modulator, a polarization switch, and a polarizing beam splitter. A reverse bias of V R =5V excites two LC resonances at 0.3 THz and 0.89 THz in the modulator, which fade away by switching the gate voltage to V F =0.49V, exciting a dipole resonance in the metasurface at 0.75 THz. The numerical results show that this THz modulator enjoys modulation depths of ≥92% at the LC resonances and a phase modulation of ∼1.16rad at 0.86 THz. An identical electric bias change of the Schottky diodes in the polarization switch alters the resonators from anisotropic to isotropic, changing the output wave polarization from circular with nearly 99% of the circular polarization percentage to linear or quasi-linear at four frequencies simultaneously. Additionally, the proposed THz polarization splitter can deflect the cross-polarized transmitted component from the normally outgoing co-polarized one with an angle of 70° at 0.56 THz. The splitting ratio is switched from 1:1 in reverse bias to 14:1 in forward bias by changing the bias to forward bias. We expect that the proposed designs in the THz frequency domain, benefiting from the several hundred GHz switching speed of the Schottky diodes array, will be beneficial in applications such as analysis of the complex organic structures or polarization modulation and polarization-dependent multiplexing/demultiplexing in wireless communication systems.

Application of dust acoustic waves as THz polarizer: Numerical investigations

Application of dust acoustic waves as THz polarizer: Numerical investigations

Giant Terahertz Birefringence in an Ultrathin Anisotropic Semimetal.

Manipulating the polarization of light at the nanoscale is key to the development of next-generation optoelectronic devices. This is typically done via waveplates using optically anisotropic crystals, with thicknesses on the order of the wavelength. Here, using a novel ultrafast electron-beam-based technique sensitive to transient near fields at THz frequencies, we observe a giant anisotropy in the linear optical response in the semimetal WTe2 and demonstrate that one can tune the THz polarization using a 50 nm thick film, acting as a broadband wave plate with thickness 3 orders of magnitude smaller than the wavelength. The observed circular deflections of the electron beam are consistent with simulations tracking the trajectory of the electron beam in the near field of the THz pulse. This finding offers a promising approach to enable atomically thin THz polarization control using anisotropic semimetals and defines new approaches for characterizing THz near-field optical response at far-subwavelength length scales.

Detection of optical properties of chiral substances by a photoconductive THz polarization detector

Chiral enantiomers have significant differences in biochemical functions. The use of THz wave polarization detection to characterize the optical properties of chiral substances is of great significance to the development of life science and the identification and application of chiral substances. However, the traditional polarization detection procedures of THz waves are complex, which limits the study of chiral substances. Herein, we proposed a high-sensitivity THz polarization detector, which can simultaneously obtain the change information of amplitude, phase, and polarization state through a single measurement. The optical rotation and elliptical angle of solid and liquid D/L-Glutamic acid 5-methyl ester in the THz band are studied. Then it is verified that anisotropic interference may occur in the preparation of solid samples. Finally, the effects of sample content and thickness on polarization are obtained. The experimental results show that different chirality has the opposite effect on the state of polarization, and the difference between chiral enantiomers can be detected by this method. This work is of great significance for understanding the optical properties of chiral substances and promoting the development of chiral recognition.

THz graphene-integrated metasurface for electrically reconfigurable polarization conversion

Abstract Terahertz (THz) waves have been widely hailed as a key enabling technology for future sixth generation (6G) wireless networks. Dynamic modulation of their polarization states is of great attraction for high-capacity communications and anisotropic sensing. The development of such technology is, however, still in very early stage owing to the difficulties of realizing electrical reconfigurability for THz devices. Artificially constructed metasurfaces and new nanomaterials, such as graphene, have been shown to provide revolutionary platforms for manipulating and controlling the wave properties, especially at THz frequencies. This work leverages the light–matter interaction in a graphene-integrated metasurface functioning as an electrically reconfigurable THz polarization converter. A novel graphene-gold bilayer topology is applied to construct such a metasurface which enables wide-range electrical tunability of the polarization conversion. Under a y-polarized illumination, the reflected components of x- and y-polarizations are tuned dynamically through an external bias voltage across the metasurface, thereby producing an elliptically polarized wave with tuneable ellipticity and angle. By changing the voltage from 0 V to 12 V, the reflected polarization ellipticity has been tuned from −0.94 to −0.5 at around 240 GHz, featuring linear-to-circular and linear-to-elliptical polarization conversions. Meanwhile, the polarization angle has been modulated from 12° to −23° at around 236 GHz. This work provides an experimentally validated THz graphene-integrated metasurface with wide polarization modulation depths, low biasing voltages and simple configuration. It promises great potential for applications in future THz communications and sensing.

Design and simulation of an achromatic phase controller for THz polarization imaging system

Design and simulation of an achromatic phase controller for THz polarization imaging system

Tunable and switchable multifunctional terahertz meta-mirror based on graphene and vanadium dioxide.

We design a multifunctional THz polarization modulation meta-mirror integrated with polarization conversion and dichroism functions switched by temperature and voltage. The meta-mirror is composed of two-layered graphene metasurfaces and a layer of vanadium dioxide (VO2) on a gold film substrate. Linear-to-linear polarization conversion and linear dichroism (LD) can be switched by temperature control in the VO2 film and Fermi level adjustments in the graphene metasurfaces, where the polarization conversion ratio (PCR) is higher than 0.9 in the range of 2.89 THz to 4.02 THz, LD value reached a maximum of 0.6 at 3.84 THz, and linear-to-circular polarization conversion and circular dichroism (CD) can also be tuned with ellipticity higher than 0.9 in the range of 2.32 THz to 2.69 THz and CD value as high as 0.71 at 2.45 THz. The proposed meta-mirror is the first THz metamaterial device integrating four switchable functions, including linear-to-linear polarization conversion, linear-to-circular polarization conversion, linear dichroism and circular dichroism. The meta-mirror is a promising design for compact system integration in THz imaging, sensing and biological detection applications.

Terahertz inverse spin Hall effect in spintronic nanostructures with various ferromagnetic materials

Bilayers of ferromagnetic and heavy metal nanolayers excited with femtosecond laser pulses emit subpicosecond bursts of electromagnetic radiation with spectral frequencies up to several THz. We fabricated such spintronic THz emitters containing various ferromagnetic materials with vastly different magnetic remanence, saturation magnetization, and coercive fields. In all cases, the THz amplitude versus magnetic-field dependence follows the independently measured magnetization hysteresis loops and the THz polarization direction is perpendicular to the sample’s magnetization M, both consistent with the inverse spin Hall effect (ISHE) as the physical origin. The mV-level signals of observed THz transients also favor the ISHE over the anomalous Nernst effect (ANE). The M(T) variation governs the temperature dependence of the THz generation. Emitters with weakly remanent ferromagnets are magnetic-field tunable, while moderately and strongly remanent ferromagnets, once magnetized, allow intense THz generation even without an external field, nevertheless exhibiting low susceptibility to magnetic perturbations if the hysteresis loop is square. Hence, exploiting the magnetic properties of the ferromagnetic layer enables tailoring of weakly temperature-dependent spintronic THz emitters. Finally, we explored the applicability of perovskite oxide materials such as La-Sr-Mn-O (LSMO) for THz transient generation. We detected weak (<40-µV amplitude) THz emissions from both LSMO/Au and pure LSMO nanostructures with no sign flip upon the sample reversal. Thus, we excluded the ISHE and believe it was likely governed by the ANE in the thick-film regime, although we cannot exclude the transient demagnetization mechanism.

Performance analysis of 160 Gbps-60 GHz OFDM-MIMO RoFSO transmission with WDM-PDM dual multiplexing

Abstract An orthogonal frequency division multiplexed-radio over free space optics (OFDM-RoFSO) communication system with dual multiplexing is proposed. Eight input data streams are transmitted simultaneously by using wavelength division multiplexing (WDM) at level 1 and polarization division multiplexing (PDM) at level 2. Input data streams 1–4 and 5–8 are assigned with same set of carrier frequencies i.e. 193.1 THz, 193.3 THz, 193.5 THz and 193.7 THz and separate polarization levels (X and Y polarized) before transmitting through RoFSO link. A total data rate of 160 Gbps (4 × 2 × 20 Gbps) is achieved in this design. With proposed system, channel performance is evaluated under the influence of atmospheric attenuation and turbulence conditions while measuring system BER, Q-factor, SNR and received power etc. The effect of channel crosstalk is analysed for WDM-PDM dual multiplexed design while considering single input single output (SISO) and multiple input multiple output (MIMO) FSO configurations. The overall analysis predicts that a higher transmission rate and improved capacity levels can easily be achieved with dual multiplexed system.

Tunable linear-to-circular terahertz polarization convertor enabled by a plasmonic nanocomposite metasurface.

We proposed and demonstrated a metasurface based terahertz polarizer consisting of an optically responsive nanocomposite and a flexible base body, which fulfilled the function of linear-to-circular polarization conversion in transmission mode. Meanwhile, as the dynamic and stretchable materials enable the active manipulation of conversion points, evident frequency shifts for circular polarization transformation were discovered by applying laser irradiation and tension. Hence the modulation of conversion points covered a broadband with combination of those two external excitations. This THz polarization convertor may find its applications in polarization controls and beam steering, which also provides a low-cost and large-scale manufacturable method to achieve versatile active THz devices.

A THz fiber polarization splitter based on anti-resonant hollow-core fiber with the asymmetric dual-suspended cores

In this paper a novel THz fiber polarization splitter based on anti-resonant hollow-core fiber with asymmetric dual-suspended cores has been proposed. For the proposed polarization splitter, the ultra-low confinement loss and great polarization splitting performance in the THz band can be achieved, which benefits from the good combination of anti-resonance mechanism with refractive index guidance mechanism. In order to confine the THz wave well in the central large air hole, 8 sets of anti-resonance rings are introduced into the cladding of the fiber. Moreover, the asymmetric dual-suspended cores are used to control the polarization state of the guiding light for enhancing the polarization splitting effect, which exists in the central large air hole of anti-resonant hollow-core fiber. One of the dual cores is designed with a symmetrical structure to reduce the birefringence effect. The other core is designed with an asymmetric structure to increase that in the THz band. The numerical simulations results show that the THz polarization splitter with a length of 0.37 cm is realized in the operating band around 1 THz by optimizing the geometric structure of the dual-suspended-core fiber, and the confinement losses of both x and y polarization modes are lower than 6 × 10−3 dB/cm. Furthermore, the working bandwidth at the center frequency of 1 THz can reach ∼0.06 THz, and the extinction ratio is better than 20 dB. The proposed THz polarization splitter has the characteristics of special short length, low loss and broad bandwidth. In conclusion, it has some potential applications of polarization devices in the field of optical fiber sensing and optical fiber communications.

Reconfigurable graphene elliptic patch antenna

In this paper, a triple band graphene elliptic patch antenna is designed for operation in 4 THz, 4.95 THz and 7.75 THz. The main idea of this paper is to partition the graphene patch into pieces to have more degrees of freedom to reconfigure the polarization of the antenna. The antenna’s polarization is controlled by partitioning the patch into 9 elements, polar griding, resizing them, and changing their chemical potential. The main operation mode of the antenna has linear polarization in its boresight in 4 THz and 7.75 THz but circular polarization in 4.95 THz. Circular polarization in 4 THz and 7.75 THz is reached in four different modes; each mode has its own advantages. The main purpose is to achieve some modes that have minimum radiation efficiency loss. In these modes, LHCP and RHCP are reached in a single pattern.

Flexible Metamaterial Quarter-Wave Plate and Its Application in Blocking the Backward Reflection of Terahertz Waves.

A terahertz flexible metamaterial quarter-wave plate (QWP) is designed and fabricated using polyimide as the substrate in this paper, with a 3 dB axial ratio bandwidth of 0.51 THz and high polarization conversion efficiency and transmittance. The effect of the incidence angle on the polarization conversion performance of the QWP is discussed by measuring the transmissions at multiple incidence angles. The blocking effect of this QWP combined with a polarizer on the backward reflection of terahertz waves is investigated by terahertz time-domain spectral transmission experiments. By adjusting the angle of the QWP and polarizer with respect to the incident light in the optical path, a blocking efficiency of 20 dB can be achieved at a 20° incidence angle, with a bandwidth of 0.25 THz, a maximum blocking efficiency of 58 dB at 1.73 THz, and an insertion loss of only 1.4 dB. Flexible terahertz metamaterial QWPs and polarizers can effectively block harmful reflected waves in terahertz communication and other systems. They have the advantages of a simple structure, ultra-thinness and flexibility, easy integration, no external magnetic field, and no low-temperature and other environmental requirements, thus having broad application prospects for terahertz on-chip integrated systems.

Electrically tunable THz graphene metasurface wave retarders

Abstract Anisotropic materials with chirality or birefringence can be used to manipulate the polarization states of electromagnetic waves. However, the comparatively low anisotropy of natural materials hinders the miniaturization of optical components and devices at terahertz frequencies. In this study, we experimentally demonstrate that the relative phase retardation of a THz wave can be electrically controlled by integrating patterned mono- and bilayer graphene onto an otherwise isotropic metasurface. Specifically, we show that a refractive index for one of the orthogonal polarization states can be electrically controlled by modulating graphene’s conductivity, thereby weakening the capacitive coupling between adjacent meta-atoms in an anisotropic manner. With monolayer graphene, phase retardation of 15° to 81° between two orthogonal polarization states can be achieved. Maximum phase retardation of 90° through a metasurface with bilayer graphene suggests its use as a tunable quarter-wave plate. Continuous control from linear- to circular-polarization states may provide a wide range of opportunities for the development of compact THz polarization devices and polarization-sensitive THz technology.

Equivalent circuit model for a graphene-based high efficiency tunable broadband terahertz polarizer.

An equivalent circuit model for a graphene-based high-efficiency tunable broadband THz polarizer is presented. The conditions for linear-to-circular polarization conversion in the transmission mode are utilized to derive a set of closed-form design formulas. Given a set of target specifications, the key structural parameters of the polarizer are directly calculated using this model. The proposed model is rigorously validated by comparing the circuit model and full-wave electromagnetic simulation results, from which it is found that the model is accurate and effective, accelerating the analysis and design processes. This offers a further step in developing a high-performance and controllable polarization converter with potential applications in imaging, sensing, and communications.

A study on angular distribution of THz radiation driven by two-colour laser-induced microplasma

We present an analytical method to study the angular distribution of THz radiation driven by two-colour laser-induced microplasma in nitrogen gas. Directionally dependent temporal waveforms of the radiated THz pulse is calculated. To do so, we have extended the photo-current model to calculate three-dimensional distribution of the photo-induced current from which the far-field THz radiation is computed. We have also studied the effect of optical polarization state and phase difference on the angular distribution of radiation. Our study shows that for two orthogonally-polarized colours, not only THz polarization state, but also the angular distribution of THz radiation is mostly influenced by the second optical harmonic. The possibility of rotating the angular radiation pattern around the axis of the laser propagation is illustrated for circularly-polarized laser drivers by changing the optical phase difference.

A 220 GHz-1.1THz continuous frequency and polarization tunable quasi-optical electron paramagnetic resonance spectroscopic system.

Here, we describe a custom-designed quasi-optical system continuously operating in the frequency range 220GHz to 1.1THz with a temperature range of 5-300K and magnetic fields up to 9T capable of polarization rotation in both transmitter and receiver arms at any given frequency within the range through a unique double Martin-Puplett interferometry approach. The system employs focusing lenses to amplify the microwave power at the sample position and recollimate the beam to the transmission branch. The cryostat and split coil magnets are furnished with five optical access ports from all three major directions to the sample sitting on a two-axes rotatable sample holder capable of performing arbitrary rotations with respect to the field direction, enabling broad accessibility to experimental geometries. Initial results from test measurements on antiferromagnetic MnF2 single crystals are included to verify the operation of the system.