Controllable Polarization State Research Articles

Polarization‐Sensitive Artificial Optoelectronic Synapse Based on Anisotropic β‐Ga2O3 Single Crystal for Neuromorphic Vision Systems and Information Encryption

AbstractA novel polarization‐sensitive artificial optoelectronic synapse based on β‐Ga2O3 single‐crystal is proposed in this work, featuring reconfigurable anisotropic vision. A series of polarization‐sensitive synaptic activities and polarization‐sensitive image recognition functions are successfully simulated using this device. The intriguing performance of this device, stems from the crystal anisotropy in β‐Ga2O3, which is confirmed through polarization Raman measurements and first‐principles theoretical calculations. Furthermore, a comprehensive analysis of the persistent photoelectric properties of the device unveils that the adaptability of the optoelectronic synapse device stems from the ionization and dissociation of oxygen vacancies. Ultimately, the device is utilized in the fields of image recognition and information encryption. A four‐layer artificial neural network with two hidden layers is constructed for recognizing handwritten digits. After training, the recognition accuracy reaches over 91.5% for both unpolarized and polarized light. Information encryption is achieved by controlling polarization states. The device enables data generation and encryption to be conducted on the same platform, mitigating exposure risks during transmission and significantly enhancing data security and confidentiality. This work presents new opportunities for future applications of polarization‐based perception systems.

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.

Van der Waals Heterostructure Mid-Infrared Emitters with Electrically Controllable Polarization States and Spectral Characteristics.

Modern infrared (IR) microscopy, communication, and sensing systems demand control of the spectral characteristics and polarization states of light. Typically, these systems require the cascading of multiple filters, polarization optics, and rotating components to manipulate light, inevitably increasing their sizes and complexities. Here, we report two-terminal mid-infrared (mid-IR) emitters, in which tuning the polarity of the applied bias can switch their emission peak wavelengths and linear polarization states along two orthogonal orientations. Our devices are composed of two back-to-back p-n junctions formed by stacking anisotropic light-emitting materials, black phosphorus and black arsenic-phosphorus with MoS2. By controlling the crystallographic orientations and engineering the band profile of heterostructures, the emissions of two junctions exhibit distinct spectral ranges and polarization directions; more importantly, these two electroluminescence (EL) units can be independently activated, depending on the polarity of the applied bias. Furthermore, we show that when operating our emitter under the polarity-switched pulse mode, the time-averaged EL exhibits the characteristics of broad spectral coverage, encompassing the entire first mid-IR atmospheric window (λ: 3-5 μm), and electrically tunable spectral shapes.

Controllable high-speed rotated femtosecond cylindrical vector beam based on optical heterodyne interference.

Structured light beams that possess unique polarization distribution could offer a new degree of freedom for a variety of applications, and hence its flexible polarization manipulation is necessary. Here we experimentally report a heterodyne interference-based method for generating femtosecond cylindrical vector beam (CVB) with high-speed controllable rotated polarization states. The femtosecond CVBs are created through the superposition of two optical vortices with opposite handedness. The use of two acoustic-optical modulators (AOMs) with frequency differences allows to achieve polarization rotation in a hopping-free scheme at on demand speed. Up to 1 MHz of the rotation frequency is demonstrated by visualizing the fast rotation events through a fast-frame-rate CCD camera. Moreover, we show our method can be readily extended to produce higher order CVBs with more complex rotated polarization distributions. Such a simple yet versatile femtosecond polarization-controlled laser system has the capability to act as a nonlinear trapping platform, thus opening tremendous potential opportunities in the fields of micromachining, nanofabrication, and so force.

Controllable Polarization and Diffraction Modulated Multi‐Functionality Based on Metasurface

AbstractDiffraction gratings are crucial optical elements in various optical systems, which can realize tailored diffraction energy distributions as well as phase modulations based on Fourier series. Meanwhile, the polarization gratings can produce arbitrarily specified polarization states on a set of desired diffraction orders for realizing polarimeter. However, simultaneously and independently tailoring the encoded phase profiles and polarization states in different diffraction orders has not been investigated previously. Here, a dielectric metasurface with the capability of generating different holographic images (generalized phase profiles) in different diffraction orders with controllable polarization states is proposed. Such functionality is achieved by applying the double‐phase method into the metasurface design process for manipulating the complex amplitude distributions of two orthogonal polarization bases. Furthermore, in order to prove that the proposed design strategy is suitable to achieve complicated wavefront modulation in a straightforward and convenient manner, a vectorial holographic image with spatially continuous polarization distribution is also obtained without any iteration process. The proposed method may pave the way to achieve a variety of applications such as beam shaping, polarization imaging as well as multifunctional optical systems.

Chiro-optical fields with asymmetric orbital angular momentum and polarization.

In this paper, we proposed a flexible method for generating asymmetric chiro-optical fields. Different from most of the chiro-optical fields superimposed by vortex beams which are rotationally symmetric, the asymmetric chiro-optical field has a locally controllable orbital angular momentum (OAM) and polarization state. By using a helix phase plate (HPP) calculated based on coordinates transformation of the perfect vortex, the OAM controllability of a single chiro-optical field could be achieved. Then, by using the transformation matrix method, several discrete chiro-optical fields with different rotation angles and topological charges were stitched together as a multi-lobed chiro-optical field with asymmetric OAM on each side-lobe. Furthermore, we designed two HPPs that can be loaded into two spatial light modulators to modulate the polarization state of each side-lobe of the asymmetric chiro-optical field independently. The proposed asymmetric chiro-optical field breaks the characteristics of uniform OAM and polarization distribution of conventional chiro-optical fields, which may have potential applications in optical tweezers, communications, and enantiomer-selective sensing.

Control of polarization switching in a VCSEL via resonant feedback from a whispering-gallery-mode cavity.

We report a method for flexibly switching the dominant polarization of a vertical-cavity surface-emitting laser (VCSEL) by introducing polarization-resolved resonant optical feedback from a whispering-gallery-mode (WGM) cavity to the lasing cavity. Switching between the originally dominant mode and a side mode is experimentally demonstrated under different bias currents once one of them is locked to the resonance mode of the WGM cavity. In addition to a controllable polarization state, the reported VCSEL also demonstrates a linewidth as narrow as tens of kilohertz, which is highly desirable for many applications, including high-speed data communication, light detection and ranging (lidar), and absorption spectroscopy.

FOCUS SHAPING OF CYLINDRICALLY POLARIZED VORTEX BEAMS BY A LINEAR AXICON

Focus shaping of cylindrically polarized vortex beams (CPVBs) by linear axicon is studied theoretically. Vector diffraction theory has been used to derive the expressions of the light field in the focal region. It is shown that a different intensity distribution in the focal region can be obtained by adjusting the topological charge, the polarization rotation angle and the numerical aperture maximal angle. A focal spot, a dark channel and a flat-topped shapes are formed by choosing proper values of parameters. A controllable polarization state of dark channel is obtained. The different focal region shapes may find wide applications such as material processing and optical tweezers.

Outside Front Cover: Volume 2 Issue 1

Outside front cover image: The two-dimensional (2D) ferroelectric materials are promising candidates as synaptic weight elements in neural network hardware due to their controllable polarization states and stable ferroelectricity even at the atomic scale. In this article, 2D ferroelectric semiconductor α-In2Se3 with coupled in-plane and out-of-plane ferroelectric polarizations is used as the active channel material to emulate typical synaptic functions. Advantages like simple structure, multi-mode operations, rich functions as well as low-energy consumption make 2D ferroelectric materials promising for future neuromorphic applications. (DOI: https://doi.org/10.1002/smm2.1020)

Intrinsic polarization coupling in 2D α‐In2Se3toward artificial synapse with multimode operations

AbstractEmulation of advanced synaptic functions of the human brain with electronic devices contributes an important step toward constructing high‐efficiency neuromorphic systems. Ferroelectric materials are promising candidates as synaptic weight elements in neural network hardware due to their controllable polarization states. However, the increased depolarization field at the nanoscale and the complex fabrication process of the traditional ferroelectric materials hamper the development of high‐density, low‐power, and highly sensitive synaptic devices. Here, we report the implementation of two‐dimensional (2D) ferroelectric α‐In2Se3as an active channel material to emulate typical synaptic functions. The α‐In2Se3‐based synaptic device features multimode operations, enabled by the coupled ferroelectric polarization under various voltage pulses applied at both drain and gate terminals. Moreover, the energy consumption can be reduced to ~ 1 pJ by using high‐κdielectric (Al2O3). The successful control of ferroelectric polarizations in α‐In2Se3and its application in artificial synapses are expected to inspire the implementation of 2D ferroelectric materials for future neuromorphic systems.

Role of intraband dynamics in the generation of circularly polarized high harmonics from solids

Recent studies have demonstrated that the polarization states of high harmonics from solids can differ from those of the driving pulses. To gain insights on the microscopic origin of this behavior, we perform one-particle intraband-only calculations and reproduce some of the most striking observations. For instance, our calculations yield circularly polarized harmonics from elliptically polarized pulses that sensitively depend on the driving conditions. Furthermore, we perform experiments on ZnS and find partly similar characteristics as reported from silicon. Comparison to our intraband-only calculations shows reasonable qualitative agreement for a below-band-gap harmonic. We show that intraband dynamics predict depolarization effects for higher field strengths. For harmonics above the band gap, interband dynamics become important and the high-harmonic response to elliptical excitation looks systematically different. Our work proposes a method to distinguish between different high-harmonic generation mechanisms and it could pave the way to compact solid-state high-harmonic sources with controllable polarization states.

Optically tunable terahertz chiral metasurface based on multi-layered graphene

Active manipulation of the polarization states at terahertz frequencies is crucially helpful for polarization-sensitive spectroscopy, having significant applications such as non-contact Hall measurements, vibrational circular dichroism measurements and anisotropy imaging. The weakness of polarization manipulation provided by natural materials can be overcomed by chiral metamaterials. Chiral metamaterials have a huge potential to achieve the necessary polarization effects, hence they provide the basis for applications such as ultracompact polarization components. Terahertz chiral metamaterials that allow dynamic polarization modulation of terahertz waves are of great practical interest and still challenging. Here, we show that terahertz metasurface based on the four conjugated “petal” resonators integrated with multi-layered graphene (MLG) can enable dynamically tunable chiroptical response using optical pumping. In particular, a change of ellipticity angle of 20° is observed around 0.76 THz under optical pumping by a 980 nm continuous wave (CW) laser. Furthermore, using temporal coupled-mode theory, our study also reveals that the chiroptical response of the proposed multi-layered graphene-based metasurface is strongly dependent on the influence of optical pumping on the loss parameters of resonance modes, leading to actively controllable polarization states of the transmitted terahertz waves. The present work paves the way for the realization of fundamental terahertz components capable for active polarization manipulation.

High-purity polarized multi-beams from polarization-twisting meta-surface Cassegrain systems.

Bi-functional meta-surfaces capable of simultaneously controlling polarization states and wave-fronts of electromagnetic fields are introduced into the design of Cassegrain system for the synthesis of multi-beams. More specifically, electromagnetic fields reflected by the secondary meta-surface with tailored diverged wave-fronts would be collimated by the primary meta-surface into multi-beams with transformed polarization states that can directly go through the secondary meta-surface without any blockage. Especially, we show that such a polarization-twisting meta-surface Cassegrain system can possess much more compact configuration by properly devising the phase distribution over the secondary meta-surface, and can also achieve high-purity polarized multiple radiations when we enlarge the secondary meta-surface as a radome. The present approach of integrating two bi-functional meta-surfaces into the design of Cassegrain system for the generation of multi-beams should pave the way for building up more advanced meta-surface based architectures with specific characteristics.

Polarization-Tailored Raman Frequency Conversion in Chiral Gas-Filled Hollow-Core Photonic Crystal Fibers.

Broadband-tunable sources of circularly polarized light are crucial in fields such as laser science, biomedicine, and spectroscopy. Conventional sources rely on nonlinear wavelength conversion and polarization control using standard optical components and are limited by the availability of suitably transparent crystals and glasses. Although a gas-filled hollow-core photonic crystal fiber provides pressure-tunable dispersion, long well-controlled optical path lengths, and high Raman conversion efficiency, it is unable to preserve a circular polarization state, typically exhibiting weak linear birefringence. Here we report a revolutionary approach based on a helically twisted hollow-core photonic crystal fiber, which displays circular birefringence, thus robustly maintaining a circular polarization state against external perturbations. This makes it possible to generate pure circularly polarized Stokes and anti-Stokes signals by rotational Raman scattering in hydrogen. The polarization state of the frequency-shifted Raman bands can be continuously varied by tuning the gas pressure in the vicinity of the gain-suppression point. The results pave the way to a new generation of compact and efficient fiber-based sources of broadband light with a fully controllable polarization state.

Multichannel Spatially Nonhomogeneous Focused Vector Vortex Beams for Quantum Experiments

AbstractThe utilization of vector vortex beams (VVBs) in quantum experiments has recently become a topic of tremendous interest. However, achieving a focused, high‐resolution, transmission‐type, multichannel, and nonhomogeneously polarized VVB array remains a great challenge. Here, by introducing a degree of freedom on a high‐order Poincaré sphere in the form of latitude‐polarization control and locally tailoring the phase and transverse polarization distributions at the subwavelength scale, a single transmission‐type dielectric metalens design for producing a multichannel, spatially nonhomogeneous, focused VVB array is demonstrated. Three special kinds of lattices (triangle, square, and rhombus) are demonstrated, yielding independently controllable orbital angular momentum states and polarization states of a tightly focused field in each channel. Furthermore, by using only the proposed metalens and a polarizer, a mode‐demultiplexing system is realized with a mode crosstalk as low as −34.8 dB for all four channels. The results offer a robust technological foundation for exploiting spatial degrees of freedom of structured light in the miniaturization and integration of quantum photonics devices.

Highly efficient generation of arbitrary vector beams with tunable polarization, phase, and amplitude

We propose an efficient and robust method to generate tunable vector beams by employing a single phase-type spatial light modulator (SLM). With this method, a linearly polarized Gaussian beam can be converted into a vector beam with arbitrarily controllable polarization state, phase, and amplitude. The energy loss during the conversion is greatly reduced and depends mainly on the reflectivity of the SLM. We experimentally demonstrate that conversion efficiency of about 47% is achieved by using an SLM with reflectivity of 62%. Several typical vector beams, including cylindrical vector beams, vector beams on higher order Poincare spheres, and arbitrary vector beams attached with phases and with tunable amplitude, are generated and verified experimentally. This method is also expected to create high-power vector beams and play important roles in optical fabrication and light trapping.

Active Terahertz Chiral Metamaterials Based on Phase Transition of Vanadium Dioxide (VO2)

Compared with natural materials, chiral metamaterials have been demonstrated with orders of magnitude stronger chiroptical response, which provides the basis for applications such as ultracompact polarization components and plasmonic-enhanced biosensing. Terahertz chiral metamaterials that allow dynamic polarization control of terahertz waves are of great practical interest, but remain extremely rare. Here, we show that hybrid metamaterials integrated with vanadium dioxide (VO2) exhibiting phase transition can enable dynamically tunable chiroptical responses at terahertz frequencies. In particular, a circular dichroism of ~40° and a maximum polarization rotation of ~200°/λ are observed around 0.7 THz. Furthermore, our study also reveals that the chiroptical response from the proposed metamaterials is strongly dependent on the phase transition of VO2, leading to actively controllable polarization states of the transmitted terahertz waves. This work paves the way for the development of terahertz metadevices capable of enabling active polarization manipulation.

Shaping of optical vector beams in three dimensions.

We present a method of shaping three-dimensional (3D) vector beams with prescribed intensity distribution and controllable polarization state variation along arbitrary curves in three dimensions. By employing a non-iterative 3D beam-shaping method developed for the scalar field, we use two curved laser beams with mutually orthogonal polarization serving as base vector components with a high-intensity gradient and controllable phase variation, so that they are collinearly superposed to produce a 3D vector beam. We experimentally demonstrate the generation of 3D vector beams that have a polarization gradient (spatially continuous variant polarization state) along 3D curves, which may find applications in polarization-mediated processes, such as to drive the motion of micro-particles.

Design of metasurface polarizers based on two-dimensional cold atomic arrays.

Engineering light-matter interaction using cold atomic arrays is one of the central topics in modern optics. Here we have demonstrated the capability of two-dimensional asymmetric cold atomic arrays as microscopic metasurfaces for controlling polarization states of light. The designed linear polarizer can lead to an extinction ratio over 20dB as well as a high transmittance over 0.8 for the permitted polarization at zero detuning. For detuned driving light, changing lattice constants can also achieve high performance linear polarizers. We have also accomplished a circular polarizer by manipulating the phases of transmitted light. A theoretical analysis based on Bloch theorem shows the underlying mechanism for this performance is actually attributed to cooperative effects in periodic lattices. Finally, we discuss in detail the effects of system size, lattice imperfection and nonzero driving light linewidth in practical implementation. The present study paves a way to design extremely miniaturized metasurfaces using cold atoms and other two-level systems, showing great potential in quantum information and quantum metrology sciences as well as the fundamental physics of light-matter interaction.

Bright high-order harmonic generation with controllable polarization from a relativistic plasma mirror.

Ultrafast extreme ultraviolet (XUV) sources with a controllable polarization state are powerful tools for investigating the structural and electronic as well as the magnetic properties of materials. However, such light sources are still limited to only a few free-electron laser facilities and, very recently, to high-order harmonic generation from noble gases. Here we propose and numerically demonstrate a laser–plasma scheme to generate bright XUV pulses with fully controlled polarization. In this scheme, an elliptically polarized laser pulse is obliquely incident on a plasma surface, and the reflected radiation contains pulse trains and isolated circularly or highly elliptically polarized attosecond XUV pulses. The harmonic polarization state is fully controlled by the laser–plasma parameters. The mechanism can be explained within the relativistically oscillating mirror model. This scheme opens a practical and promising route to generate bright attosecond XUV pulses with desirable ellipticities in a straightforward and efficient way for a number of applications.