Polarization Conversion Effect Research Articles

Polarization conversion effect in cholesteric liquid crystal-based polarization volume gratings

Following the recent experimental discovery of a new polarization conversion phenomenon in polarization volume gratings (PVGs), in this paper, we investigate its underlying physical mechanisms in cholesteric liquid crystal (CLC) reflectors and PVGs. We examine the transition of eigenstates from circular to linear polarization as the incident angle deviates from the helical axis, which originates such an anomalous polarization conversion effect. While this transition enables PVGs to double the in-coupling efficiency and brightness uniformity in waveguide-based augmented reality (AR) displays, it also degrades the polarization selectivity and alters the transmitted polarization state in both CLC reflectors and PVGs, which in turn narrows down their spectral and angular bandwidth in the multi-layer design. These findings help not only deepen the understanding of polarization behaviors in CLC-based optical elements but also provide valuable insights to optimize their performances for emerging AR smart glasses.

Active control of terahertz quasi-BIC and asymmetric transmission in a liquid-crystal-integrated metasurface

Quasi-bound states in the continuum (quasi-BICs) offer an excellent platform for the flexible and efficient control of light-matter interactions by breaking the structural symmetry. The active quasi-BIC device has great application potential in fields such as optical sensing, nonlinear optics, and filters. Herein, we experimentally demonstrate an active terahertz (THz) quasi-BIC device induced by the polarization conversion in a liquid crystal (LC)-integrated metasurface, which consists of a symmetrically broken double-gap split ring resonator (DSRR), an LC layer, and double graphite electrodes. In the process of LC orientation control under the external field, the device realizes the active control from the OFF state to the ON state. In the OFF state, the LC has no polarization conversion effect, and the device behaves in a non-resonant state; but for the ON state, the device exhibits obvious quasi-BIC resonance. Furthermore, we achieve asymmetric transmission based on polarization-induced quasi-BIC modulation precisely at the quasi-BIC resonance position, and its isolation can be controlled by the external field. The study on dynamic quasi-BIC by the LC-integrated metasurface introduces a very promising route for active THz devices, which guarantees potential applications for THz communications, switching, and sensing systems.

A polarization conversion metasurface for broadband and high-efficiency bidirectional filtering

Here, we demonstrate a polarization conversion metasurface which is composed of a single layer square split-ring resonator and bi-layered slotted perfect electrical conductor (PEC) to realize broadband and high-efficiency bidirectional filtering. The numerical results indicate that y-polarized waves can be converted to x-polarized transmission waves in the frequency range of 6.6 GHz to 13.9 GHz, while prevent this polarized waves with frequency beyond this band, and purity x-polarized waves can be obtained as electromagnetic (EM) waves are incident along negative z-axis, meanwhile, x-polarized waves can be converted to y-polarized transmission waves and only y-polarized waves available when EM waves are incident along positive z-axis. This designed polarization conversion metasurface has the function of bidirectional filtering in a wideband frequency range. Furthermore, the broadband property can be maintained as the incident angle up to 30°. The Fabry–Pérot-like cavity model is established to clarify the interference and enhancement effect of polarization conversion, and the electrical field and surface current distributions are used to elucidate the physical mechanism of polarization conversion and filtering. The experimental results are coincident with the numerical simulations. This metasurface can be employed in controlling the filtering signal of the polarized devices and can be functional as a bandpass filter in communication systems.

Polarization conversion and lateral shifts in multilayered structure with finitely-gapped topological surface states

Abstract The polarizatison conversion and the Goos–Hänchen (GH) shifts of the reflected electromagnetic wave for the multilayered structure made of topological insulator (TI) layers with finite surface energy gap are investigated. The transfer matrix formalism is adopted to analyze the reflection of electromagnetic wave through the multilayered structure, and the influences of surface energy gap, thickness and number of the TI layers are discussed. We find that maximum polarization conversion rate can be obtained with appropriate surface energy gap of TI, and within a certain range of finite energy gap, the polarization conversion effect is stronger than that for the case under the infinite surface energy gap limit. Greater polarization conversion rate for TI with small surface energy gap can be found than that for TI with larger energy gap in some range of layer numbers. At large incident angles the GH shifts vary considerably with the layer number for TI with relatively larger energy gap. Result of the combined influence of surface energy gap and layer number shows that, there exists both the positive and negative enhancement peaks of the GH shifts, and for smaller energy gap, fewer TI layers are required to obtain the transition between positive and negative GH shifts.

Broadband large-angle beam scanning with dynamic spin energy distribution based on liquid crystal cascaded bilayer metasurface.

Dynamic manipulation of terahertz (THz) beams plays an important role in THz application systems. The PB metasurface provides an effective scheme for space separation and deflection of the spin beam. However, mirror symmetry locking of the conjugated spin states severely limits the versatility of the device. In this work, we demonstrate a liquid crystal (LC) cascaded bilayer metasurface that includes an LC layer, anisotropic metasurface, and PB metasurface. By controlling anisotropy and polarization conversion effects, dynamic spin asymmetric transmission is realized. Meanwhile, two different dynamic energy distribution processes are realized between the L and R state with the corresponding deflection side. The results show that the device achieves a large angular spatial dispersion within the frequency-angle scanning range of ±35° to ±75° corresponding to the broadband range of 0.6-1.1 THz. Moreover, it achieves a spin beam spatial separation with a maximum proportion of energy distribution greater than 26 dB, and the active modulation rate in the energy distribution process reaches 98 %. This work provides a dynamic THz spin conversion and efficient large-angle beam scanning, with important potentials in wavelength/polarization division multiplexing and frequency-scanning antenna for large-capacity THz wireless communication, radar, and imaging systems.

Photo-Excited Metasurface for Tunable Terahertz Reflective Circular Polarization Conversion and Anomalous Beam Deflection at Two Frequencies Independently.

In this paper, a photo-excited metasurface (MS) based on hybrid patterned photoconductive silicon (Si) structures was proposed in the terahertz (THz) region, which can realize the tunable reflective circular polarization (CP) conversion and beam deflection effect at two frequencies independently. The unit cell of the proposed MS consists of a metal circular-ring (CR), Si ellipse-shaped-patch (ESP) and circular-double-split-ring (CDSR) structure, a middle dielectric substrate, and a bottom metal ground plane. By altering the external infrared-beam pumping power, it is possible to modify the electric conductivity of both the Si ESP and CDSR components. By varying the conductivity of the Si array in this manner, the proposed MS can achieve a reflective CP conversion efficiency that ranges from 0% to 96.6% at a lower frequency of 0.65 THz, and from 0% to 89.3% at a higher frequency of 1.37 THz. Furthermore, the corresponding modulation depth of this MS is as high as 96.6% and 89.3% at two distinct and independent frequencies, respectively. Moreover, at the lower and higher frequencies, the 2π phase shift can also be achieved by respectively rotating the oriented angle (αi) of the Si ESP and CDSR structures. Finally, an MS supercell is constructed for the reflective CP beam deflection, and the efficiency is dynamically tuned from 0% to 99% at the two independent frequencies. Due to its excellent photo-excited response, the proposed MS may find potential applications in active functional THz wavefront devices, such as modulators, switches, and deflectors.

Tuning Dynamics of the Acousto-Optical Tunable SOA Fiber Laser

Acoustically-induced fiber gratings (AIFGs) can be employed as the main device to achieve wide-range, time-linear, and fast wavelength tunability of various lasers, due to their fast spectral response during the high-speed tuning process. Herein, we propose a wideband fast-tunable fiber laser that consists of a semiconductor optical amplifier (SOA) and an AIFG that induces the acousto-optical polarization conversion effect. Its laser dynamics during the tuning process are revealed. The rate equation of the inversion population is used to theoretically predict the laser intensity evolution during the dynamical tuning process. The simulation results show that the relaxation oscillation and huge laser pulse during the tuning process can be suppressed by shortening the upper energy-level lifetime of the gain medium. In order to experimentally reveal the tuning dynamics, the output intensity evolution of the tunable laser is monitored. The lasing tuning response time reaches ∼200 μs, whose value has approached that of the AIFG. Due to the short energy-level lifetime of the SOA, the laser relaxation oscillation during the dynamical tuning process can be successfully suppressed, which coincides well with the simulation prediction. This work creates an avenue to understand the acousto-optical tuning dynamics profoundly. It also provides guides to maximize the speed advantage of this tuning mechanism in various tunable laser cavities.

Spinning thermal radiation from twisted two different anisotropic materials.

Thermal radiation has applications in numerous fields, such as radiation cooling, thermal imaging, and thermal camouflage. Micro/nanostructures such as chiral metamaterials with polarization-dependent or symmetry-breaking properties can selectively emit circularly (spin) polarized polarization waves. In this paper, we propose and demonstrate the spinning thermal radiation from two twisted different anisotropic materials. Taking industrial polymer and biaxial hyperbolic material α-MoO3 as an example, it is found that broadband spinning thermal radiation can be obtained from 13 µm to 18 µm. The spin thermal radiation of the proposed twisted structure originates from the combined effect of polarization conversion of circularly polarized wave and selective absorption of linearly polarized wave by the top and bottom layers of anisotropic materials, respectively. Besides, the narrowband spinning thermal radiation with 0.9 circular dichroism is achieved at wavelength of 12.39 µm and 18.93 µm for finite thickness α-MoO3 due to the epsilon-near-zero mode, and the magnetic field distribution can confirm the phenomenon. This work achieves broadband and narrowband spin thermal radiation and significantly enhances circular dichroism, which may have applications in biological sensing and thermal detection.

Conceptual radar trap model realized via polarization conversion metasurface.

General metasurfaces (MSs) can realize low observability of radar by manipulating the polarization mode and transmission direction of the electromagnetic (EM) waves. Here, we propose the radar trap model to realize EM wave imprisonment. This three-layer model is composed of the transmission polarization converter, the connected dielectric substrate and the reflection polarization converter. Using Jones calculation as a guide, we optimized the geometric parameters of the upper and lower layers to realize specific polarization conversion functions. The middle layer is regarded as the support and matching layer. On this basis, the combined radar trap model can realize the imprisonment of EM waves between upper and lower layers, which is attributed to the cooperative effect of asymmetric transmission and polarization conversion. We further verified the feasibility and correctness of our investigations through two kinds of model designs based on linear and circular polarization conversion mechanisms. Good agreements are observed between simulation and experiment. Even though the design presents a narrow operating bandwidth, it still provides novel ideas for developing radar stealth technology.

Active terahertz spin state and optical chirality in liquid crystal chiral metasurface

Dynamic control of photonic spin state and chirality plays a vital role in various applications, such as polarization control, polarization-sensitive imaging, and biosensing. Here, we present a scheme for the flexible and dynamic manipulation of terahertz spin state conversion and optical chirality by combining two achiral structures: an asymmetric metasurface and a layer of anisotropic liquid crystal. The proposed asymmetric metasurface can realize the polarization conversion effect. For the circularly polarized incidence, it exhibits the asymmetric transmission of the spin-flipped states but no spin-locked optical chirality since its geometry is mirror symmetric along with the wave propagation. The introduction of the liquid crystal makes the composite metasurface not only exhibit the spin state conversion but also spin-locked chirality and spin-flipped chirality on account of breaking mirror symmetry, which realizes an electrically active terahertz chiral device. The experimental results show that the asymmetric transmission of the terahertz spin states can be dynamically manipulated, resulting in a large controllable range 83.8% to \ensuremath{-}30.7% of spin-locked circular dichroism at 0.76 THz and \ensuremath{-}98.2% to 44.7% of spin-flipped circular dichroism at 0.73 THz. This work paves the way for the development of terahertz meta devices capable of enabling active photonic spin state and chirality manipulation.

Terahertz dual-band asymmetric transmission for a single cross-polarized linear wave

The reported dual-band asymmetric transmission is usually an effect of mutual polarization conversion, where one polarized wave is converted to its cross-polarization in the first band while the other polarized wave is converted to its cross-polarization in the second band. In this work, we experimentally demonstrate a dual-band asymmetric transmission effect only for one-polarized linear wave in the terahertz band. It is measured that the cross-polarization transmission coefficient Tyx reaches two peaks of 0.715 and 0.548 at the frequency of 0.74 THz and 1.22 THz, respectively. While the transmission coefficient Txy is lower than 0.2 in the wide-band from 0.5 THz to 1.5 THz. Firstly, the multiple interference model is used to discuss the physical mechanism of the dual-band asymmetric transmission. However, the second band of the calculated spectrum is offset due to the strong near field coupling between the two metal layers. The coupled-mode theory is then introduced and the fitting result of the coupled-mode theory is in good agreement with that of the experiment in the two bands. This research would provide new theoretical instructions in designing and analyzing multiband asymmetric transmission in the terahertz, microwave or the optical bands.

Terahertz Polarization Conversion in an Electromagnetically Induced Transparency (EIT)‐Like Metamaterial

AbstractIn this paper, a unique metamaterial (MM) design incorporating both electromagnetically induced transparency (EIT) effect and polarization conversion is proposed. It consists of a cut‐wire as a bright resonator and a side coupled split‐ring resonator (SRR) as a dark resonator, on the one hand, the dark resonator can be excited by the bright resonator via the magnetic near‐field coupling, thereby contributing to a strong cross‐polarization conversion. On the other hand, the destructive interference between the sub‐radiant (dark) SRR and the super‐radiant (bright) cut‐wire results in an EIT effect. Benefiting from the destructive interference between the radiative and dark elements in the EIT MM, radiation loss is greatly suppressed and nearly perfect cross‐polarization conversion with high transmission and low‐loss is realized. In addition, the importance of symmetry is demonstrated by the phenomenon of polarization conversion and EIT effects disappearing when an identical cut‐wire is placed on the other side of the SRR. The proposed MM design provides a new way to address the loss problem in MM polarization devices. More importantly, this design principle of taking advantage of one area to solve the challenges in others may inspire new high‐performance multifunctional MM designs.

Multi‐Band Tunable Chiral Metamaterial for Asymmetric Transmission and Absorption of Linearly Polarized Electromagnetic Waves

AbstractDue to the importance of chiral materials in realizing many exotic phenomena, a chiral metamaterial incorporated with active components of PIN diodes for the flexible control of asymmetric transmission (AT) of linearly polarized electromagnetic (EM) waves at microwave frequency is proposed. A transfer‐matrix approach and full‐wave simulations for cascading anisotropic impedance sheets is applied to analyse the physical mechanism of the structure, enabling the device with optimal multi‐band AT effect. The experimental results demonstrate that the robust real‐time controls of polarization conversion and AT effect for the linearly polarized waves can be achieved by altering the applied biasing voltage, giving rise to the tunability in three frequency bands of 9–10.5 GHz, 11–11.8 GHz, and 12.2–12.8 GHz. Furthermore, the tunable absorption can also be achieved, making it a good candidate in intelligent EM systems.

Strong Coupling to Generate Complex Birefringence: Metasurface in the Middle Etalons

We have measured the optical signatures of strong coupling between the resonance of etalons and plasmon antenna arrays in transmission and polarization. Planar etalons in the middle of which a plasmon antenna array is placed show anticrossings in transmission between the etalon resonances and plasmon antenna resonance, which we map as a function of frequency, etalon opening and oscillator strengh. We argue that the proper interpretation of strong coupling and the magnitude of the Rabi splitting requires a “metasurface-in-the-middle” cavity model and is distinct from strong coupling between a cavity and a dispersive material. Furthermore, we quantitatively connect the Rabi splitting to the electrostatic antenna polarizability, that is, the polarizability in the absence of radiative damping corrections. Finally, we demonstrate that the strong coupling brings very strong polarization conversion effects, as the hybrid modes provide for a strong retardance that can be leveraged for linear birefringence and dichroism.

Linear polarization rotation in multilayer topological insulator structures

The linear polarization conversion effect of reflected electromagnetic wave from topological insulator (TI) multilayer structure is investigated. We calculate the reflection coefficients of the anisotropic TI multilayer structure based on the transfer matrix method, and the polarization conversion effect is characterized by the polarization conversion rate and Kerr rotation angle. Polarization conversion at each interface in the TI multilayer with parallel magnetization on TI surfaces can be accumulated, and the complete polarization conversion may occur in the structure consisting of sufficiently many TI layers. The polarization conversion rate is lowered with increasing frequency and exhibits significant oscillation behavior with frequency as the layer thickness increases. Fewer TI layers are required to obtain complete polarization conversion for the structure with larger topological admittance parameter, and the rotation effect of polarization is suppressed by increasing the incidence angle. The out-of-plane uniaxial anisotropy of the TI may also affect the polarization conversion in the TI multilayer.

Terahertz polarization conversion and sensing with double-layer chiral metasurface**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0701000), the National Natural Science Foundation of China (Grant Nos. 61971242, 61831012, and 61671491), the Natural Science Foundation of Tianjin City, China (Grant No. 19JCYBJC16600), and the Young Elite Scientists Sponsorship Program by Tianjin, China (Grant No. TJSQNTJ-2017-12).

The terahertz (THz) resonance, chirality, and polarization conversion properties of a double-layer chiral metasurface have been experimentally investigated by THz time domain spectroscopy system and polarization detection method. The special symmetric geometry of each unit cell with its adjacent cells makes a strong chiral electromagnetic response in this metasurface, which leads to a strong polarization conversion effect. Moreover, compared with the traditional THz transmission resonance sensing for film thickness, the polarization sensing characterized by polarization elliptical angle (PEA) and polarization rotation angle (PRA) shows a better Q factor and figure of merit (FoM). The results show that the Q factors of the PEA and PRA reach 43.8 and 49.1 when the interval film is 20 μm, while the Q factor of THz resonance sensing is only 10.6. And these PEA and PRA can play a complementary role to obtain a double-parameter sensing method with a higher FoM, over 4 times than that of resonance sensing. This chiral metasurface and its polarization sensing method provide new ideas for the development of high-efficiency THz polarization manipulation, and open a window to the high sensitive sensing by using THz polarization spectroscopy.

Polarization conversion in terahertz planar metamaterial composed of split-ring resonators

In this paper, we numerically, experimentally and theoretically investigate the polarization conversion effects of Split-ring resonator (SRR)-based metamaterial. When the electric field (E) is not parallel to any side of the SRR, the electric dipole mode of SRR can be excited under normal incidence conditions. This electric dipole contains a component orthogonal to the incident polarization, which can radiate cross-polarized electric field thereby producing polarization conversion effects. Furthermore, a theoretical model is proposed to demonstrate the polarization conversion mechanism theoretically and the results of which are in good agreement with the simulated and experimental results.

Non-reciprocal polarization rotation using dynamic refractive index modulation

One of the most prominent classes of non-reciprocal devices relies upon the effect of non-reciprocal polarization conversion, such as those observed in Faraday isolators. This effect is usually achieved with the use of magneto-optical materials. Here, we introduce a waveguide type optical isolator based on non-reciprocal polarization conversion, without the use of magneto-optical materials. Our isolator is based on spatial-temporal dynamic refractive index modulation, which is more readily amenable for on-chip integration. We numerically demonstrate our design with both first-principle multi-frequency electromagnetic simulations and the vectorial coupled mode theory formalism.

High operative polarization converter using metasurface consisted of dual reciprocal L-shaped graphene array

A single layer dual L-shaped graphene metasurface is presented to achieve a high operative and tunable polarization converter in the mid-infrared regime. The proposed device and numerical results indicate that linearly polarized wave can be converted to its cross-polarized wave. In addition, it can be used as a converter to a circularly polarized wave. This converter is insensitive to the angle of incident wave. Working frequencies can be easily and significantly tuned within a wide frequency range by regulating the chemical potential of the graphene. The physical procedure of the polarization conversion effect is explicated via coupling of the electric field components with various plasmon resonance modes of the metasurface. The suggested structure can pave a way to development of controllable plasmonic polarization converter systems for optical communication applications.

Dual-Polarization, Tunable Breaking Window in the Polarization Conversion Pass Band in a Terahertz Dirac Semimetal-Based Metamaterial

We propose a planar THz hybrid bulk Dirac semimetal (BDS)-metal metamaterial which consists of a hybrid BDS-metal array on a dielectric layer and a back metallic mirror. The simulation results show that a spectral window that possesses high reflectance with no polarization conversion effect arises in the polarization conversion pass band due to the existence of the BDS rods. Interestingly, though the proposed structure possesses no symmetry property, the effects are nearly the same for x- and y-polarization incidences. We analyze the electric field distributions at the breaking frequency and attribute the phenomena to plasmonic analogue of electromagnetically induced reflectance (EIR-like) effect for x-polarization incidence and direct back scattering for y-polarization incidence. The dependence of the breaking window on the BDS rod length shows that, for both x- and y-polarization incidences, the breaking characteristic is mainly determined by the dipole resonance on the BDS rod. Moreover, the breaking window can be independently and dynamically manipulated by changing the Fermi energy of the BDS rods. Our results may provide potential applications in designing new polarization control devices.