Tunable Polarization Research Articles

Reversible tuning of charge Carrier's polarity of MoTe2 FETs enabled by laser and high temperature

The transition metal dichalcogenides (TMDs) materials exhibit variety of crystal phase structures and vulnerable to extrinsic doping for phase transformation, leading diverse electronic applications. Here we demonstrate the controlled charge carrier's polarity in the phase transition of molybdenum ditelluride (MoTe2) based field effect transistors (FETs) affected by laser irradiations in ambient environment. The conductivity in MoTe2-based FETs was measured before and after laser irradiations in a range of temperatures (77, 100, 150, 200, 250, 300, 350 and 400 K). The devices achieved reversible charge polarity by changing its 2H phase to semimetal 1Tʹ phase upon laser irradiations and returned to its original state once treated with high temperature 400 K. Since, the 1Tʹ phase is confirmed by Raman spectroscopy, transmission electron microscopy (TEM) and electrical transport measurements. Thus, our finding provides an effective approach in controlling charge carrier's modulation in laser-doped 2D materials for electronic and optoelectronic applications.

Tunable circular polarization room temperature phosphorescence with ultrahigh dissymmetric factor by cholesteric liquid crystal elastomers

Tunable circular polarization room temperature phosphorescence with ultrahigh dissymmetric factor by cholesteric liquid crystal elastomers

Development of a terahertz metamaterial polarization converter with multiple tuning modes based on graphene and vanadium dioxide

A dynamically tunable terahertz metamaterial polarization converter is proposed. By manipulating the Fermi level of graphene and the conductivity of vanadium dioxide (VO2) flexibly, multiple tuning modes for polarization conversion performance are realized. First, while graphene is in the metallic or insulating state, we can achieve an ideal broadband reflective or an ideal broadband transmissive polarization converter respectively. By manipulating the conductivity of VO2, a modulation depth of 80 % for intensity of the reflected polarization conversion rate (PCRr) and that of 70 % for bandwidth of the transmissive polarization conversion rate (PCRt) can be achieved. Then, while VO2 is in the insulating state, we obtain another broadband transmissive polarization converter. Similarly, we can achieve PCRt intensity modulation by manipulating the Fermi level of graphene. While VO2 is in the metallic state, the device can switch from a broadband polarization converter to a narrow one as the Fermi level of graphene increases. Finally, by manipulating graphene and VO2 simultaneously, the central frequency modulation of PCRt can be obtained with the bandwidth keeping unchanged. Compared with the ones reported recently, our device has some advantages on tuning mode, relative bandwidth, and polarization conversion performance.

MEMS-tunable polarization management in photonic integrated circuits.

Optical fibers are generally polarization-insensitive while photonic integrated circuits (PICs) often exhibit a large polarization dependence due to the high-aspect-ratio and high-index-contrast of integrated waveguides. As PICs become more mature there is an increasing need for tunable polarization management on-chip. Although micro-electro-mechanical systems (MEMS) are increasingly finding application in PICs for optical switching and phase shifting, they have so far not found wide application for polarization management. In this work we propose two optical MEMS architectures for polarization management enabling tunable polarization splitting and rotation - key functions so far lacking in PICs. The first structure consists of a directional coupler with a MEMS-tunable gap enabling a continuously-variable polarization splitting ratio. A second architecture consists of a symmetry-breaking MEMS perturber suspended over an air-cladded waveguide enabling tunable polarization rotation. For both architectures we simulate a polarization extinction exceeding 25 dB, and the operating bandwidth can be as large as 100 nm. We conclude with a discussion of actuation schemes and examine fabrication considerations for implementation in PIC foundries.

Superconducting quantum interference device readout circuit with tunable feedback polarity.

Feedback circuits, which act as functional units in superconducting quantum interference device (SQUID) readout circuits, such as positive feedback circuits for suppressing the preamplifier noise contribution or negative feedback circuits for increasing the linear flux range of SQUID, enable SQUIDs to achieve high performance in practical applications. Integrating feedback circuits with different functions into a single SQUID chip contributes to the construction of highly compact SQUID electronics. Here, we propose a SQUID readout circuit with tunable feedback polarity (TFP). The switching of the feedback polarity is easily realized by applying a control current to the superconducting switches integrated into the SQUID chip. The enhancement of the flux-to-voltage transfer coefficient or the linear flux range depends on the choice of feedback polarity. In addition, we introduce a two-stage scheme to address the degraded noise performance induced by negative feedback. This work attempts to develop a compact and versatile architecture of SQUID readout by using a set of compatible superconducting technologies.

Tunable ultra-wideband polarization insensitivity wide-angle perfect THz absorber based on metamaterials containing vanadium dioxide

Tunable ultra-wideband polarization insensitivity wide-angle perfect THz absorber based on metamaterials containing vanadium dioxide

Tunable antiferroelectric ceramic polarization via regulating incommensurate phase and domain features

Due to the high demand for dielectric materials with high energy density, the energy storage performance of antiferroelectric ceramic capacitors has always gained much attention. Polarization intensity is a key factor that is closely related to the energy storage density. However, thus far, there has been a lack of research studies or successful methods to effectively modulate polarization intensity. The behavior of the polarization process is complex and contains domain nucleation, growth, and flip-flapping. Based on this finding, the introduction of Nb5+ at the B-site was designed to influence the three stages of antiferroelectric polarization by regulating the balance between the ferroelectric and antiferroelectric phases, and eventually realized regulation of the saturation polarization intensity in the (Pb1-1.5xLax)(Zr0.5Sn0.43Ti0.07)O3 antiferroelectric ceramics. The saturation polarization intensity has increased from 25.56 to 42 μC/cm2 with Nb5+ content increases from 0 to 4 mol% and the hysteresis was kept low, Pb0.94La0.04(Zr0.65Sn0.35)0.975Nb0.02O3 is the optimal component with a high releasable energy density of 8.26 J/cm3 and an energy storage efficiency of 90.31%. This work provides an in-depth explanation of the microscopic mechanism of antiferroelectric ceramic polarization and presents a novel approach for the composition design of high-energy storage density antiferroelectric ceramics.

Multi-channel generation of vortex beams with controllable polarization states and orbital angular momentum

Optical vortices with tunable polarization states and topological charges are widely investigated in various physical systems and practical devices for high-capacity optical communication. However, this kind of structured light beams is usually generated using several polarization and spatial phase devices, which decreases the configurability of optical systems. Here, we have designed a kind of polarized optical multi-vortices generator based on the Stokes–Mueller formalism and cross-phase modulation. In our scheme, multi-channel generation of polarized vortex beams can be realized through a single optical element and a single-input Gaussian beam. The polarization states and orbital angular momentum of the generated light beams are all-optically controllable. Furthermore, the proposed polarized optical multi-vortices generator has also been demonstrated experimentally through one-step holographic recording in an azobenzene liquid-crystalline film and the experimental results agree with theoretical analysis.

Band alignment and charge carrier transport properties of YAlN/III-nitride heterostructures

Incorporating rare earth element scandium (Sc) into III-nitride wurtzite lattice offers remarkable non-oxide ferroelectrics, tunable spontaneous polarization coefficients, and significantly enhanced linear and nonlinear optical properties. Yttrium (Y), a Sc substitute, is also capable of similar properties when alloyed with III-nitride materials, but its potential has remained largely unexplored. In this study, we present the first experimental investigation into the energy band gap alignment and transport properties of a YAlN based high-electron-mobility transistor (HEMT) structure. Our measurements revealed a type-I band alignment for the Y0.07Al0.93N/GaN heterojunction, with a small valence band offset of −0.1 eV and conduction band offset of 2.2 eV. We then developed a novel Y0.07Al0.93N/AlN/GaN heterojunction-based HEMT and evaluated its transport properties. A mobility of 1000 cm2/Vs, a sheet electron concentration of 4.2 × 1013 cm2, and a sheet resistance of 148 Ω/□ were measured at room temperature, which are among the best values reported for rare earth element-containing nitride epitaxial heterostructures. We have further performed temperature-dependent Hall measurements and identified the dominant factors affecting transport properties at different temperature ranges. The excellent electrical characteristics and compatibility with mainstream semiconductor technology represent a significant step forward in the development of next-generation multifunctional HEMTs.

Hybrid dual-mode tunable polarization conversion metasurface based on graphene and vanadium dioxide.

We present and numerically verify a functionally hybrid dual-mode tunable polarization conversion metasurface based on graphene and vanadium dioxide (VO2). The tunable polarization converter consists of two patterned graphene layers separated by grating which is composed of gold and VO2. Due to the existence of phase change material VO2, the polarization conversion mode can be switched flexibly between the transmission and reflection modes. Theoretical calculations show the proposed polarization conversion metasurface can obtain giant asymmetric transmission (AT) at 0.42 and 0.77 THz when VO2 is in the insulating state. Conversely, when VO2 is in the metallic state, the converter switches to the reflection mode, demonstrating broadband polarization conversion for both forward and backward incidences. Furthermore, the conductivity of graphene can be modulated by changing the gate voltage, which allows dynamic control polarization conversion bandwidth of the reflection mode as well as the AT of the transmission mode. The robustness of the metasurface has also been verified, the high polarization conversion efficiency and AT can be maintained over wide incidence angles up to 65° for both the xoz plane and yoz plane. These advantages make the proposed hybrid tunable polarization conversion metasurface a promising candidate for THz radiation switching and modulation.

Multifunction Tunable Piezoelectric‐Actuated Metasurface at Millimeter‐Waves

Free space (also termed quasi‐optical) metasurfaces have recently been designed and proposed as new types of polarizers that allow polarization control of an impinging wave in ways that are not possible with conventional designs. However, existing methods for tunable metasurface polarizers are not able to fully control the polarization dynamically; additionally, at mm‐wave and submillimeter bands, they typically suffer from high losses, which are predominantly produced by the inherent limitations of the tuning elements or materials. An electromechanically tunable reflection polariser that has the ability to fully control the polarization of an impinging wave has been designed, fabricated, and experimentally tested in a frequency band centered at 57 GHz. The proposed technique utilizes a variable air cavity between a periodic metasurface array and a ground plane, controlled by a piezoelectric actuator. The periodic metasurface element consists of two slightly different cross‐shaped metallic elements arranged in a periodic triangular lattice. Full‐wave simulations are presented and experimentally validated with measurements. The proposed approach is scalable from microwave up to THz frequencies with low‐loss performance due to the nature of the tuning technology whereby the tuning element is not interfering with the electromagnetic waves in the structure.

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.

Design of a Reconfigurable Ultra-Wideband Terahertz Polarization Rotator Based on Graphene Metamaterial

In this work, a reconfigurable ultra-wideband transmissive terahertz polarization rotator based on graphene metamaterial is proposed that can switch between two states of polarization rotation within a broad terahertz band by changing the Fermi level of graphene. The proposed reconfigurable polarization rotator is based on a two-dimensional periodic array of multilayer graphene metamaterial structure, which is composed of metal grating, graphene grating, silicon dioxide thin film, and a dielectric substrate. The graphene metamaterial can achieve high co-polarized transmission of a linearly polarized incident wave at the off-state of the graphene grating without applying the bias voltage. Once the specially designed bias voltage is applied to change the Fermi level of graphene, the polarization rotation angle of linearly polarized waves is switched to 45° by the graphene metamaterial at the on-state. The working frequency band with 45-degree linear polarized transmission remaining above 0.7 and the polarization conversion ratio (PCR) above 90% is from 0.35 to 1.75 THz, and the relative bandwidth reaches 133.3% of the central working frequency. Furthermore, even with oblique incidence at large angles, the proposed device retains high-efficiency conversion in a broad band. The proposed graphene metamaterial offers a novel approach for the design of a terahertz tunable polarization rotator and is expected to be applied in the applications of terahertz wireless communication, imaging, and sensing.

Electric-field driven source of photocarriers for tunable electron spin polarization in InGaAs quantum dots

Electric-field control of spin polarization of electrons during injection into InGaAs quantum dots (QDs) was studied via circularly polarized time-resolved photoluminescence. Electric-field modulation of optical spin polarization in QDs will play a key role in future progress of semiconductor opto-spintronics. The tuning of band potentials by applying external electric fields can not only affect spin-injection efficiencies but also switch dominant spin-injection layers. In this study, we developed a QD-based electric-field-effect optical spin device with two different spin-injection layers, which consisted of a GaAs and GaAs/Al0.15Ga0.85As superlattice (SL) barriers. The bias-voltage modulation of the optical spin polarization in QDs was demonstrated by changing the spin polarization degree of electrons injected from these barriers into the QD via the electric-field switching of the spin-injection layers. This was achieved by exploiting the difference in spin relaxation properties between bulk GaAs and the SL. This proposed structure, which comprised of one luminescent layer and two spin-injection layers, is highly scalable because the modulation range of optical spin polarization can be enhanced by changing the combination of spin-injection layers, as well as the material used and its layer thickness.

Tunable reflective tri-band polarization converter based on graphene composite metasurface

A graphene composite metasurface-based reflective tri-band polarization converter working in the terahertz frequency regime is proposed and investigated by CST Microwave Studio software. The polarization converter achieves high polarization conversion ratios (above 99%) for linear-to-linear polarization conversion at 6.21, 9.11, and 11.66 THz, and for linear-to-circular polarization conversion at 5.14 THz, with a fixed Fermi energy level (Ef) of 1.0 ev due to the excitation of localized surface plasmons. Subsequently, the reason for cross-polarization conversion at three resonant frequencies and the incident angle sensitive property of the converter were discussed. Based on the graphene component, a tunable tri-band polarization converter was obtained and investigated; this converter could be applied in the active modulation area of wireless communication.

Achiral and Chiral Optical Force Within Topological Optical Lattices Generated with Plasmonic Metasurfaces and Tunable Incident Beam

An optical lattice is a platform that is used in studies in multiple branches of physics, e.g., ultracold atoms and microfluidic sorting. Unlike traditional complex optical systems, surface plasmon polaritons enable the generation of optical lattices on a chip. Here, we propose an on-chip scheme to transform five topological optical lattices using tunable polarization and a phase vortex. Using the dipolar approximation and numerical simulations, we demonstrate that the spin density force is the main force for sorting chiral particles in the out-of-plane direction when their radius is more than 100 nm. A microflow system is proposed to trap particles with electric optical lattices and sort chiral particles with spin optical lattices. Our findings offer possible routes toward realization of on-chip optical lattices and enable their potential application in both biology and chemistry, e.g., in simultaneous trapping or excitation of multiple objects and identification and sorting of chiral objects.

Tunable dual-band dual-polarization terahertz polarization converter and coding metasurfaces based on Weyl semimetals

Tunable dual-band dual-polarization terahertz polarization converter and coding metasurfaces based on Weyl semimetals

Molecular Chameleon Carriers for Nucleic Acid Delivery: The Sweet Spot between Lipoplexes and Polyplexes.

Taking advantage of effective intracellular delivery mechanisms of both cationizable lipids and polymers, highly potent double pH-responsive nucleic acid carriers are generated by combining at least two lipo amino fatty acids (LAFs) as hydrophobic cationizable motifs with hydrophilic cationizable aminoethylene units into novel sequence-defined molecules. The pH-dependent tunable polarity of the LAF is successfully implemented by inserting a central tertiary amine, which disrupts the hydrophobic character once protonated, resulting in pH-dependent structural and physical changes. This "molecular chameleon character" turns out to be advantageous for dynamic nucleic acid delivery via lipopolyplexes. By screening different topologies (blocks, bundles, T-shapes, U-shapes), LAF types, and LAF/aminoethylene ratios, highly potent pDNA, mRNA, and siRNA carriers are identified, which are up to several 100-fold more efficient than previous carrier generations and characterized by very fast transfection kinetics. mRNA lipopolyplexes maintain high transfection activity in cell culture even in the presence of ≥90% serum at an ultra-low mRNA dose of 3 picogram (≈2 nanoparticles/cell), and thus are comparable in potency to viral nanoparticles. Importantly, they show great in vivo performance with high expression levels especially in spleen, tumor, lungs, and liver upon intravenous administration of 1-3µg luciferase-encoding mRNA in mice.

Strain-tunable valley polarization and localized excitons in monolayer WSe2.

Monolayer transition metal dichalcogenides (TMDs) have a crystalline structure with broken spatial inversion symmetry, making them promising candidates for valleytronic applications. However, the degree of valley polarization is usually not high due to the presence of intervalley scattering. Here, we use the nanoindentation technique to fabricate strained structures of WSe2 on Au arrays, thus demonstrating the generation and detection of strained localized excitons in monolayer WSe2. Enhanced emission of strain-localized excitons was observed as two sharp photoluminescence (PL) peaks measured using low-temperature PL spectroscopy. We attribute these emerging sharp peaks to excitons trapped in potential wells formed by local strains. Furthermore, the valley polarization of monolayer WSe2 is modulated by a magnetic field, and the valley polarization of strained localized excitons is increased, with a high value of up to approximately 79.6%. Our results show that tunable valley polarization and localized excitons can be realized in WSe2 monolayers, which may be useful for valleytronic applications.

Nanocrystal‐Enabled Perovskite Heterojunctions in Photovoltaic Applications and Beyond

AbstractHeterojunctions are used to tailor the properties of semiconductors in optoelectronic devices, yet for emerging devices composed of metal halide perovskites, fabricating perovskite/perovskite heterojunctions has proved challenging due to solvent incompatibilities and rapid homogenization due to ion migration. Recent studies have demonstrated various strategies for using perovskite nanocrystals as a component to fabricate perovskite/perovskite heterojunctions, either with a perovskite thin film or a second nanocrystal layer. Heterojunctions such as these can impart many advantages of both bulk and nanocrystalline perovskite morphologies. This perspective focuses on recent developments of solution‐processed perovskite heterojunctions for solar cells and novel optoelectronic devices, in particular, highlighting the demonstrated and potential advantages of nanocrystal‐enabled fabrication strategies. A central tenet of this perspective is that the synthesis and dispersion of perovskite nanocrystals in non‐polar organic solvents offers a key processing advantage over traditional perovskite precursor solutions in polar solvents since the former allows for layer‐by‐layer deposition without dissolving an underlying perovskite film or crystal. This processing advantage, coupled with nanocrystal size control and ligand chemistry, enables perovskite heterojunctions with highly tunable optical and electrical properties. Such heterojunctions may enable disruptive technological advances in broad classes of devices such as solar cells, photodetectors, sensors, and (in)coherent photon sources with tunable polarization.