Polar Crystals Research Articles

High efficiency continuous wave and Q-switched laser based on an orthorhombic Yb:GdScO3 crystal in the (112) direction with a broad emission bandwidth of 93 nm

Herein, we demonstrate a broad emission bandwidth of 93 nm with the orthorhombic Yb:GdScO3 crystal along the (112) direction, which is about 3.7–7.8 times wider than that of cubic sesquioxide crystals. By investigating the distorted RE2Ox ligands and their association with lattice vibrations, the inhomogeneous broadening was found to be dependent on local crystal polarization and electron-phonon interaction. Moreover, the (112) direction sample has a small emission cross-section of 0.42 × 10−20 cm2 and better thermal symmetry, which allow it to achieve a continuous wave (CW) laser with a slope efficiency of 62%, a passively PdSe2 Q-switched laser with an average output power of 1.69 W, a slope efficiency of 43%, and a pulse width of 272.9 ns. We believe this to be the first investigation of the effective spectral broadening and Q-switch operations based on the orthorhombic Yb:GdScO3 crystal in the (112) direction.

High-performance dual-core photonic crystal fiber polarization beam splitter based on gold-titania composite film coating

Abstract A novel gold-titania composite film-coated dual-core photonic crystal fiber polarization beam splitter (GT-DC-PCF PBS) is proposed for the first time. The titania film and gold film are sequentially deposited on the inner wall of the central air hole to improve the polarization splitting performance and enhance the adsorption strength between the gold film and the optical fiber. Based on the mode coupling theory and surface plasmon resonance effect, the transmission characteristics of x-polarization (X-pol) and y-polarization (Y-pol) odd and even modes in the near-infrared band are analyzed by finite element method. Meanwhile, the variation of coupling length and coupling length ratio for X-pol and Y-pol light under different PCF structural parameters at the wavelength of 1550 nm are also studied to optimize the performance of PBS. The results show that at the 1550 nm communication window, the extinction ratio of the PBS is up to -175.82 dB for the polarization splitting length of 2.76 mm, with an operating bandwidth of 179 nm and a maximum insertion loss of 0.23 dB. This paper aims to propose a novel design method to improve the performance of DC-PCF PBS, which has profound guiding significance for the development of high-performance PBS.

Freely suspended nematic and smectic films and free-standing smectic filaments in the ferroelectric nematic realm.

We show that stable, freely suspended liquid crystal films can be made from the ferroelectric nematic (NF) phase and from the recently discovered polar, lamellar SmZA and SmAF phases. The NF films display two-dimensional, smectic-like parabolic focal conic textures comprising director/polarization bend that are a manifestation of the electrostatic suppression of director splay in the film plane. In the SmZA and SmAF phases, the smectic layers orient preferentially normal to the film surfaces, a condition never found in typical thermotropic or lyotropic lamellar LC phases, with the SmZA films exhibiting focal-conic fan textures mimicking the appearance of typical smectics in glass cells when the layers are oriented normal to the plates, and the SmAF films showing a texture of plaquettes of uniform in-plane orientation where both bend and splay are suppressed, separated by grain boundaries. The SmAF phase can also be drawn into thin filaments, in which X-ray scattering reveals that the smectic layer planes are normal to the filament axis. Remarkably, the filaments are mechanically stable even if they break, forming free-standing, fluid filaments supported only at one end. The unique architectures of these films and filaments are stabilized by the electrostatic self-interaction of the liquid crystal polarization field, which enables the formation of confined, fluid structures that are fundamentally different from those of their counterparts made using previously known liquid crystal phases.

Tunable waveguide mode of hyperbolic phonon polaritons in α-MoO3 flake/gold slit composite structure

Hyperbolic phonon polaritons (HPhPs) in van der Waals layered polar crystals exhibit extreme light confinement capability, providing an unprecedented research opportunity to manipulate nanoscale midinfrared photons. Precise tuning of HPhPs propagation characteristics and near-field energy routing is crucial for applying polaritonic devices. Here, we demonstrate a widely tunable waveguide mode of HPhPs in an α-MoO3 flake/gold slit composite structure. By varying the width of the gold slit from 3 μm to 220 nm, the compression ratio of HPhPs wavelength relative to the incident light wavelength can be adjusted from 35% to 8.6%, respectively. This is attributed to the introduction of the gold slit, which can effectively excite and guide HPhPs within an α-MoO3 flake, forming a confined waveguide mode. Notably, the excitation efficiency of HPhPs in the nanoscale ultranarrow waveguide can be enhanced by integrating an extended port at the waveguide port. In addition, the routing of polaritons in a Y-shaped waveguide is realized by modulating the frequency of the incident light. This work presents a promising platform for manipulating deep subwavelength polaritons in planar photonic devices for infrared applications.

Polar Order in a Fluid Like Ferroelectric with a Tilted Lamellar Structure – Observation of a Polar Smectic C (SmCP) Phase

AbstractThe spontaneous polarisation of the SmCP phase is over two orders of magnitude larger than that found in conventional ferroelectric SmC phase of chiral materials used in some LCD devices. Fully atomistic molecular dynamics simulations faithfully and spontaneously reproduce the proposed structure and associated bulk properties; comparison of experimental and simulated X‐ray scattering patterns shows excellent agreement. The materials disclosed here have significantly smaller dipole moments than typical polar liquid crystals such as RM734 which suggests the role of molecular electrical polarity in generating polar order is perhaps overstated, a view supported by consideration of other molecular systems.

Zipper-Like Dynamic Switching of Coordination Bonds Gives a Polar Bimetallic Halide Toward Self-Driven X-Ray Detection.

Polar molecular crystals hold a promise for controlling bulk physical properties originated in their unique switchable polarity via structural transformation. However, the mechanisms for switching polarization are mainly limited to displacive and disorder-order phase transitions, which rarely involve the reconstruction of chemical bonds. Here, we have switched and tuned electric polarization in a bimetallic halide, (Neopentylammonium)4AgBiBr8 (1), as verified by light-excited pyroelectric effect. Most notably, its Ag-Br coordination bonds show a zipper-like dynamic switching behavior from the 'locked' to 'unlocked' state, namely, reconstruction of chemical bonds. Coupling with the dynamic ordering of organic cations, this bond-switching transition makes a contribution to switchable polarization of 1. As expected, its polarity creates pyroelectric effect for self-driven X-ray detection with high sensitivity (3.8×103 μC Gy-1 cm-2) and low limit of detection (4.8 nGy s-1). This work on the bond-switching mechanism provides an avenue to design polar molecular candidate for smart optoelectronic devices.

Combining graph deep learning and London dispersion interatomic potentials: A case study on pnictogen chalcohalides.

Machine-learning interatomic potential models based on graph neural network architectures have the potential to make atomistic materials modeling widely accessible due to their computational efficiency, scalability, and broad applicability. The training datasets for many such models are derived from density-functional theory calculations, typically using a semilocal exchange-correlation functional. As a result, long-range interactions such as London dispersion are often missing in these models. We investigate whether this missing component can be addressed by combining a graph deep learning potential with semiempirical dispersion models. We assess this combination by deriving the equations of state for layered pnictogen chalcohalides BiTeBr and BiTeI and performing crystal structure optimizations for a broader set of V-VI-VII compounds with various stoichiometries, many of which possess van der Waals gaps. We characterize the optimized crystal structures by calculating their x-ray diffraction patterns and radial distribution function histograms, which are also used to compute Earth mover's distances to quantify the dissimilarity between the optimized and corresponding experimental structures. We find that dispersion-corrected graph deep learning potentials generally (though not universally) provide a more realistic description of these compounds due to the inclusion of van der Waals attractions. In particular, their use results in systematic improvements in predicting not only the van der Waals gap but also the layer thickness in layered V-VI-VII compounds. Our results demonstrate that the combined potentials studied here, derived from a straightforward approach that neither requires fine-tuning the training nor refitting the potential parameters, can significantly improve the description of layered polar crystals.

Polar Order in a Fluid Like Ferroelectric with a Tilted Lamellar Structure - Observation of a Polar Smectic C (SmCP) Phase.

The spontaneous polarisation of the SmCP phase is over two orders of magnitude larger than that found in conventional ferroelectric SmC phase of chiral materials used in some LCD devices. Fully atomistic molecular dynamics simulations faithfully and spontaneously reproduce the proposed structure and associated bulk properties; comparison of experimental and simulated X-ray scattering patterns shows excellent agreement. The materials disclosed here have significantly smaller dipole moments than typical polar liquid crystals such as RM734 which suggests the role of molecular electrical polarity in generating polar order is perhaps overstated, a view supported by consideration of other molecular systems.

Fabrication of hydrothermal PPKTP and its spontaneous parametric down-conversion characteristics

High-voltage electric field polarization, selective corrosion, current monitoring, and other methods are used to develop hydrothermal PPKTP. It is found that the reversal domain nucleation of hydrothermal-grown KTP crystals has four kinds of microscopic morphology, namely, strip shape, spindle shape, bullet shape and irregular shape, in the process of high voltage electric field polarization. Compared with the leakage current in the process of polarization of flux-grown KTP crystals, the hydrothermal-grown KTP crystals only exist when the applied electric field is much larger than the coercive field. The leakage current can be suppressed by multiple loading of short pulses, and the PPKTP with a poled period of 10 μm and 46 μm were fabricated, respectively. Subsequently, based on the fabricated PPKTP, the SPDC characteristics are studied. To the PPKTP with a poled period of 10 μm, the quantum entanglement source whose brightness exceeds 2.1 kHz @ 810 nm is prepared by the SPDC technique. Otherwise, the PPKTP with a poled period of 46 μm is used for SPDC, and the brightness of entangled photon pairs in channel 1 and channel 2 are 3.21 kHz @ 1560 nm and 5.31 kHz @ 1560 nm, respectively.

Tunable spatial shifts of reflected beam from a polar crystal film sandwiched by dual-twisted black phosphorus gratings

Tunable spatial shifts of reflected beam from a polar crystal film sandwiched by dual-twisted black phosphorus gratings

Breakdown of Volterra's Elasticity Theory of Dislocations in Polar Skyrmion Lattices.

Emerging polar skyrmion crystals (SkX) have raised much interest for technological applications owing to their nontrivial topologies of electric dipoles, quasiparticle-like behaviors, and unique electrical responses. Understanding SkX defects, especially dislocations, is crucial for their unique lattice dynamics and responses; however, it still remains elusive. Here, we have not only demonstrated that a SkX dislocation exhibits an anomalously deformed core structure with over 50% elongation of skyrmions but also discovered that Volterra's elasticity theory of dislocation is broken down in SkX. Our phase-field simulation reveals that these distinct features of SkX dislocation arise from a rigid to soft quasiparticle transition of skyrmions depending on the electric field and temperature. In SkX, there exist inherent mechanics that mitigate the mismatch by both migration and deformation of skyrmions. This work provides novel insights into a new class of lattice mechanics and related functionality arising from the unique properties of quasi-particle SkX.

Oriented Crystal Polarization Tuning Bulk Charge and Single-Site Chemical State for Exceptional Hydrogen Photo-Production.

Rapid bulk charge recombination and mediocre surface catalytic sites harshly restrict the photocatalytic activities. Herein, the aforementioned concerns are well addressed by coupling macroscopic spontaneous polarization and atomic-site engineering of CdS single-crystal nanorods for superb H2 photo-production. The oriented growth of CdS nanorods along the polar axis, vectorially superimposing substantial polar units with orderly arrangement, renders a strong polarization electric field (20.1 times enhancement), which boosts bulk charge separation with an efficiency up to 72.4% (80.4-fold). Remarkably, polarization electric field alters the chemical state of Pt single sites by orderly reducing the binding energy of Pt atom with stepwise polarization enhancement of CdS substrate, which increases the onsite electron density of Pt from 10.232 to 10.261e- and *H key intermediates, providing preponderant Volmer-Tafel/Volmer-Heyrovsky reaction pathways with significantly decreased energy barriers for H2 production. Thus, highly polarized CdS nanorods with atomically dispersed Pt sites perform an outstanding H2 space-time yield of 118.5mmol g-1 h-1 and apparent quantum efficiency of 57.7% at λ = 420nm, and a record-high H2 turnover frequency of 57798.4 h-1, being one of the best catalysts for photocatalytic H2 evolution. This work highlights the function of polarization in manipulating charge separation and catalytic reaction.

Synthesis, characterization and evaluation of optically active polar semi-organic glycine-cobalt chloride crystals

Synthesis, characterization and evaluation of optically active polar semi-organic glycine-cobalt chloride crystals

Robust optical singularity detection enabled by spin-synchronized shearing interference

Singular beams carrying phase and polarization singularities have been extensively studied for applications in optical communication, quantum information processing, optical imaging, and other fields. However, developing a robust method to detect optical singularity that is insensitive to wavelength, polarization, and illumination position remains a significant challenge. In this study, we propose and demonstrate a robust optical singularity detection scheme based on spin-synchronized shearing interference (SSSI) using spin-dependent gradient phase modulation of a quarter-wave liquid–crystal polarization grating. Since the spin-dependent gradient phase modulation of the polarization grating is wavelength-independent, and the two circular bases of the incident field can be measured simultaneously, the proposed method is robust to wavelength and polarization. Additionally, we have verified that SSSI performs well in detecting multi-singularity cylindrical vector beam. This work provides a practical and straightforward scheme for robust optical singularity detection.

Design and fabrication of an achromatic liquid crystal polarization grating with large deflection angle for visible wavelength

Design and fabrication of an achromatic liquid crystal polarization grating with large deflection angle for visible wavelength

Nonsymmetrical thermal conductivity along the director field in ferroelectric nematic liquid crystals.

The synthesis of ferroelectric nematic liquid crystals (FNLCs) concludes the long wait for their existence and potential usage in multiple liquid crystal based applications. In FNLCs, electric polarization in the nematic phase significantly decreases the switching time of in-on display pixels. In this article, we report the occurrence of translation symmetry breaking for heat propagation along the director field n[over ̂] in the ferroelectric nematic phase. Due to the C_{∞V} symmetry of such a phase and close similarity to the bent-core polar liquid crystal phase, a rank-3 tensor describes its scalar order parameter and algebraic deductions. The finite element simulations show the occurrence of the nonsymmetrical thermal conductivity along n[over ̂]. The preferential heat transport in FNLCs can allow them to work as an all-thermal monophase non-nanostructured single-material thermal rectifier. We expect that this study will contribute towards the FNLCs application as functional layers and inks.

Compact binocular holographic near-eye 3D display system based on a liquid crystal polarization grating

Holographic technology is considered as one of the most ideal 3D display technologies and has significant potential in near-eye display. However, binocular holographic near-eye 3D display still faces challenges such as serious visual fatigue and complex system. Here, we propose a compact binocular holographic near-eye 3D display system based on a liquid crystal polarization grating (LCPG). Based on the LCPG, the reconstructed light can be deflected respectively to left and right viewing areas. The proposed system achieves binocular display with only one phase-only spatial light modulator used to load computer-generated hologram, which makes the binocular display system more compact. Simultaneously, the proposed system reduces mismatches in the images perceived by two eyes, thus achieving a better binocular display effect. Experimental results demonstrate the feasibility of the proposed binocular holographic near-eye 3D display system with dual viewing areas. Additionally, based on the time-division multiplexing method, dynamic binocular holographic near-eye display can be achieved. The proposed system has potential applications in holographic virtual reality and augmented reality display.

Optically Isotropic Polymer Stabilized Liquid Crystals with Fast and Large Field‐Induced Refractive Index Variation

AbstractOptically isotropic polymer stabilized liquid crystals with low haze (≈1% at 550 nm), large (≈0.09) field induced refractive index variation, and fast (<0.1 ms) relaxation time are reported. The impressive improvement compared to previous studies using conventional nematic liquid crystals are due to both the addition of novel highly polar ferroelectric nematic liquid crystals that decrease the required field to achieve the theoretically predicted refractive index variation and of the 6 wt.% of chiral dopant with high helical twisting power that suppressed the haze and made the system reversible with fast relaxation. It is also shown that the field induced refractive index variation can be increased over to 0.1 by using high intensity UV light for polymer stabilization. Although such polymer structure is not completely isotropic, it still provides polarization independent optical properties for normal incidence. Since fast and large switching of focal length is essential in AR/VR glasses, these systems offer significant potential for applications in highly tunable microlens arrays.

Triggering Overall Crystal Polarization by Octahedron Elongation Distortion for Highly Boosted and Selective Aerobic Photo‐Oxidation

AbstractHigh carrier recombination causes a huge waste of photogenic charge, which severely restrains the photocatalytic efficiency. Creating polarization electric field by polar photocatalytic materials proves effective to promote charge separation, while developing efficient polarization strategy is challenging and the related micromechanism remains obscure. Here, overall polarization is achieved in Bi2MoO6 single crystal photocatalyst by lateral growth of the nanoplate along {010} facets. Gradual elongation of equatorial Mo─O bonds along the MoO6 octahedral layer occurs, while the length of apical Mo─O bonds basically remains unchanged, thus allowing the large asymmetric distortion of MoO6 octahedron for producing the microscopic polarization. Meanwhile, the lateral growth of Bi2MoO6 nanoplates enables the accumulation of distorted polar units, further increasing the polarization intensities. The inside‐out crystal polarization significantly promotes the separation and migration of photogenerated charge carriers. The finely polarized Bi2MoO6 nanoplates exhibit an exceptional photocatalytic selective oxidation rate of benzyl alcohol (BA) to benzaldehyde (BAD) under visible light irradiation (Conversion (Con.) ≈100% within 30 min, Selectivity (Sel.) ≥95%), over 70.7 times that of original Bi2MoO6 nanosheets, on the condition of comparable O2 adsorption capability. This study advances atomic‐level mechanistic insight into polarized materials toward precise design of high‐performance photocatalysts.

Exo-6b2-Methyl-Substituted Pentabenzocorannulene: Synthesis, Structural Analysis, and Properties.

exo-6b2-Methyl-substituted pentabenzocorannulene (exo-PBC-Me) was synthesized by the palladium-catalyzed cyclization of 1,2,3-triaryl-1H-cyclopenta[l]phenanthrene. Its bowl-shaped geometry with an sp3 carbon atom in the backbone and a methyl group located at the convex (exo) face was verified by X-ray crystallography. According to DFT calculations, the observed conformer is energetically more favorable than the endo one by 39.9 kcal/mol. Compared to the nitrogen-doped analogs with intact π-conjugated backbones (see the main text), exo-PBC-Me displayed a deeper bowl depth (avg. 1.93 Å), redshifted and broader absorption (250-620 nm) and emission (from 585 to more than 850 nm) bands and a smaller optical HOMO-LUMO gap (2.01 eV). exo-PBC-Me formed polar crystals where all bowl-in-bowl stacking with close π ⋅ ⋅ ⋅ π contacts is arranged unidirectionally, providing the potential for applications as organic semiconductors and pyroelectric materials. This unusual structural feature, molecular packing, and properties are most likely associated with the assistance of the methyl group and the sp3 carbon atom in the backbone.