Anisotropic Optical Properties Research Articles

Anisotropic optical properties in a square quantum well wire under different polarizations of intense laser fields

In this paper, the polarization angle of intense laser field and incident light effects on the intersubband optical absorption and refractive index change in the square quantum well wire are investigated theoretically by Kramers–Henneberger approximation. It is found that the polarization angle of the intense laser field can influence the energy gap and exchange electronic configuration by breaking symmetry. Hence, the incident light polarization angle should be considered. The variation of optical absorption and refractive index change are given with different polarization-intense laser fields and probe light, which provides the possibility of designing various polarization-sensitive devices.

Transfer and Amplification of Iodine-Based Diacetylene Amphiphiles to Anisotropic Optical Properties by Uniaxial Orientation in Thin Films.

For flexible displays, there is a desperate need for a broadband coatable polarizer that can absorb light in a specific direction. Conventional polarizers fabricated by the polymer stretching process are too thick (50-200 μm) to be used as polarizers that can be applied to antireflective films in flexible displays. For the development of the broadband coatable thin film polarizer, diacetylene (DA) amphiphiles containing I- or I3- are newly designed and synthesized, and the content of DA amphiphiles in the 4,6-decadiyne solvent is optimized to form a lyotropic liquid crystal (LLC) phase. Topochemical polymerization of uniaxially oriented iodine-based DA not only stabilizes the film but also broadens the polarization light region from 350 to 700 nm. The transfer and amplification of iodine and DA functions in uniaxially oriented thin films enable the fabrication of broadband coatable thin film polarizers.

Effect of terahertz laser field on anisotropic optical absorption and refractive index changes of coaxial square quantum well wires

The influence of terahertz laser field on nonlinear anisotropic optical properties, such as optical absorption and refractive index changes is studied. By solving the Schrödinger equation of the system in the framework of effective mass with the Kramers–Henneberger (KH) transformation, the eigenfunctions and energy eigenvalues of the system are obtained. The optical absorption and refractive index changes are deduced by the compact density matrix form and the scheme of iterative method. The results show that the polarization angle and intensity of terahertz laser field have a strongly influence on the peak of optical absorption and refractive index change in different directions respectively.

Birefringence and Dichroism in Quasi‐1D Transition Metal Trichalcogenides: Direct Experimental Investigation

Birefringence and dichroism are very important properties in optical anisotropy. Understanding the intrinsic birefringence and dichroism of a material can provide great help to utilize its optical anisotropy. But the direct experimental investigation of birefringence in nanoscale materials is rarely reported. As typical anisotropic transition metals trichalcogenides (TMTCs) materials with quasi-1D structure, TiS3 and ZrS3 have attracted extensive attention due to their special crystal structure and optical anisotropy characteristics. Here, the optical anisotropy properties such as birefringence and dichroism of two kinds of quasi-1D TMTCs, TiS3 and ZrS3 , are theoretically and experimentally studied. In experimental results, the anisotropic refraction and anisotropic reflection of TiS3 and ZrS3 are studied by polarization-resolved optical microscopy and azimuth-dependent reflectance difference microscopy, respectively. In addition, the birefringence and dichroism of ZrS3 nanoribbon in experiment are directly measured by spectrometric ellipsometry measurements, and a reasonable result is obtained. This work provides the basic optical anisotropy information of TiS3 and ZrS3 . It lays a foundation for the further study of the optical anisotropy of these two materials and provides a feasible method for the study of birefringence and dichroism of other nanomaterials in the future.

In-plane anisotropic optical and mechanical properties of two-dimensional MoO3

Molybdenum trioxide (MoO3) in-plane anisotropy has increasingly attracted the attention of the scientific community in the last few years. Many of the observed in-plane anisotropic properties stem from the anisotropic refractive index and elastic constants of the material but a comprehensive analysis of these fundamental properties is still lacking. Here we employ Raman and micro-reflectance measurements, using polarized light, to determine the angular dependence of the refractive index of thin MoO3 flakes and we study the directional dependence of the MoO3 Young’s modulus using the buckling metrology method. We found that MoO3 displays one of the largest in-plane anisotropic mechanical properties reported for 2D materials so far.

NaNbO3/NaTaO3 Superlattices: Cation-Ordering Improved Band-Edge Alignment for Water Splitting and CO2 Photocatalysis.

Perovskite oxide heterostructures have been extensively investigated for their excellent photocatalytic properties. Here, through hybrid density functional theory calculations, we systematically investigate the formation of NaNbO3-NaTaO3 (NBO-NTO) heterostructures. The sequential cations replacement in the superlattices reveals the Nb-Ta ratio range that allows the effective formation of heterostructures, which occurs through a spontaneous polarization mechanism induced by the electrostatic potential discontinuity in the interface. The resulting cation ordering is responsible for the sawtooth-like potential distribution that spatially separates valence and conduction charges and reduces the heterostructure bandgap. The symmetric NBO5/NTO5 junction has the smallest bandgap (2.50 eV) whose transitions are associated with Nb 5dxy orbitals on the interfacial plane. Such a relaxation mechanism provides the heterostructure with anisotropic optical properties and interface absorption peaks closer to the visible light spectrum. The phenomena strongly suggest the use of these heterostructures in photocatalytic reactions, supported by their coherent band-edge alignment with both water splitting and CO2 reforming potentials.

A tunable ultrasensitive plasmonic biosensor based on α-MoO3/graphene hybrid architecture

Two-dimensional (2D) materials attract wide attention due to their unique exciting physical properties, which offer new opportunities to design novel devices with enhanced or multiple functionalities. In particular, α-phase molybdenum trioxide (α-MoO3) is an emerging 2D material and exhibits strong anisotropic optical properties and low optical losses in the visible region, making it a promising candidate in tunable optical devices. Here, we proposed a tunable plasmonic biosensor based on α-MoO3/graphene hybrid architectures. By optimizing Au film thickness, the number of α-MoO3 layers and rotation angle, our proposed biosensor can achieve a high phase detection sensitivity of 1.5172 × 105 deg RIU−1 with a biosensor configuration of SF11 prism/47 nm Au/6-layer α-MoO3/monolayer graphene at the rotation angle ϕ = 60°. In addition, the proposed biosensor represents tunable phase detection sensitivity since α-MoO3 can act as a polarizer. Our approach offers a new direction in the development of tunable ultrasensitive plasmonic biosensors for label-free detection and ultralow-concentration analytes.

Insights into electronic and optical properties of AGdS2 (A = Li, Na, K, Rb and Cs) ternary gadolinium sulfides

The ternary rare-earth sulfides ALnS2 have been widely applied in phosphors or scintillator materials. This study focused on systematically discussing the structural, electronic and optical properties of AGdS2 (A = Li, Na, K, Rb and Cs) ternary gadolinium sulfides using First-principles calculations for a better understanding of the physical properties of AGdS2. The ΔH values of all these sulfides reveal that they are phase stable, and LiGdS2 has the best phase stability. Electronic properties, including band structure, DOS, electron density difference and Mulliken population, provide that these sulfides are indirect band-gap semiconductors and have strong Gd–S covalent bonds and weak A-S ionic-covalent mixed bonds. The calculated absorption coefficients and reflectivity indicate that these AGdS2 sulfides are appropriate for the longer wavelength lasers. The anisotropy in optical properties for AGdS2 sulfides was studied through the polycrystalline and directional static dielectric constants ε1(0) and static refractive indexes n(0), and the order of optical anisotropy can be obtained as LiGdS2 > NaGdS2 > KGdS2 > RbGdS2 > CsGdS2.

Flux Method Growth of Large Size Group IV–V 2D GeP Single Crystals and Photoresponse Application

Two-dimensional (2D) materials driven by their unique electronic and optoelectronic properties have opened up possibilities for their various applications. The large and high-quality single crystals are essential to fabricate high-performance 2D devices for practical applications. Herein, IV-V 2D GeP single crystals with high-quality and large size of 20 × 15 × 5 mm3 were successfully grown by the Bi flux growth method. The crystalline quality of GeP was confirmed by high-resolution X-ray diffraction (HRXRD), Laue diffraction, electron probe microanalysis (EPMA) and Raman spectroscopy. Additionally, intrinsic anisotropic optical properties were investigated by angle-resolved polarized Raman spectroscopy (ARPRS) and transmission spectra in detail. Furthermore, we fabricated high-performance photodetectors based on GeP, presenting a relatively large photocurrent over 3 mA. More generally, our results will significantly contribute the GeP crystal to the wide optoelectronic applications.

Growth of two-dimensional material black phosphorus guided by gold substrates and its growth mechanism

<p indent=0mm>Two-dimensional (2D) materials are the frontier research direction of material sciences and condensed matter physics in recent years. Among different kinds of 2D materials, black phosphorus (BP) has received extensive attention due to its unique band gap and excellent optical and electrical anisotropy properties. Compared with graphene and transition metal dichalchogenides (TMDCs), BP stands out for its unique properties with widely tunable direct bandgaps ranging from <sc>0.3–1.7 eV,</sc> covering the wavelength from ~600 to <sc>4000 nm,</sc> which fill in the gap between graphene <sc>(0 eV)</sc> and TMDCs (~1.0–2.5 eV) and is considered the ideal material for optoelectronic applications, such as light detection and light modulation. For example, photodetectors are basic devices for optoelectronics, as they transform the information stored in light into electric signals. BP based photodetectors not only stand out for their wide detection range reaching far infrared (IR) region, but also for their high responsivity and high gain, demonstrating their excellent performance for mid-IR detection. In order to achieve above-mentioned applications, few or even monolayer BP flakes or continuous BP films are required. However, how to control the preparation of thin layer of BP is one of the biggest challenges in this field. In this paper, we carried out a systematic study on the growth of BP on different substrates and found that Au can guide and promote the growth of BP, thereby controlling its growth. Based on the above findings, the growth of BP nanosheets on the Au-Si substrate is achieved by using a Si substrate deposited with Au thin film on top, and by designing a strategy with gradient temperature and in confined space. We observed that the BP nanosheets grown by this system exhibited different shapes such as ribbons and flakes, and their thicknesses can be reduced to be about <sc>51 nm</sc> with a lateral size of around <sc>10 µm.</sc> In addition, by systematically tuning the growth parameters in the system, we found that the growth temperatures and the amount of red phosphorous loading are the key parameters for the growth of BP. With certain temperatures and precursor loadings, we achieve the growth of BP nanosheets, while by-products would be obtained on the substrate with other parameters. In our previous work, we found that the temperature gradients play a crucial role in the yield, purity, and crystal quality of bulk BP growth. Therefore, further optimization of temperature distribution is of great importance for the growth of BP nanosheets. In addition, we observed in this paper that Au would guide the nucleation and growth of BP. Therefore, it is important to regulate Au-P compounds during growth of BP, as on the one hand, it can control the by-products during the reaction and on the other hand, it can also improve the controllability of BP nucleation. Based on this, we believe that optimization of the temperature distribution and the growth regulation of Au-P compounds are important research directions, as to precisely control and reduce the thickness of BP nanosheets to grow few layer BP. The growth system proposed in this paper improves the chemical vapor transport (CVT) growth of traditional bulk BP from three aspects: Substrate design, confined space design, and temperature gradient control, which have paved the way for the future controllability growth of BP nanosheets and have deepened the understanding of BP growth mechanism.

Anisotropic 2D SiAs for High-Performance UV-Visible Photodetectors.

Recently, extensive efforts have been directed at finding novel 2D-layered structures with anisotropic crystal structures. Herein, the in-plane anisotropic optical and (photo)electrical properties of 2D SiAs nanosheets synthesized using a solid-state reaction and subsequent mechanical exfoliation are reported. The angle-resolved polarized Raman spectrum shows high in-plane anisotropy of the phonon vibration modes, which are consistent with the theoretical prediction. Field-effect transistor devices fabricated using the SiAs nanosheets demonstrate significant anisotropy in the hole mobility with an anisotropic ratio as high as 5.5. Photodetectors fabricated with single SiAs nanosheet exhibit high sensitivity in the UV-visible region, and the anisotropic ratio of the photocurrent reaches 5.3 at 514.5 nm and 2.3 at 325 nm. This work lays the foundation for future research in anisotropic 2D materials.

Crystal structures, electrical properties, and electron energy-loss spectroscopy of the sodium and potassium tetragonal tungsten bronzes

The tungsten bronzes (MxWO3) are non-stoichiometric metal oxides which have attracted interest for their potential applications in plasmonics. Although the cubic structures of the sodium tungsten bronzes (NaxWO3) have been extensively studied, reports into the tetragonal-II structures (TII-) of any tungsten bronze are comparatively rare. In this work, TII-NaxWO3 and TII-KxWO3 were prepared by a furnace-assisted method, and characterised by X-ray and neutron powder diffraction, selected-area electron diffraction and electron energy-loss spectroscopy (EELS). A structural determination of TII-NaxWO3 was performed in the space group I4∕m, in a 2×2×2 supercell of the P4∕mbm TII-KxWO3 structure. Density functional theory is then used to calculate the valence electronic structure and optical properties to support the EELS measurements. Similar to the cubic structures, the conduction band of TII- is composed of hybridised O 2p and W 5d states, which is filled by electrons donated from the inserted Na or K. The O sites which are in-plane with the W sites are found to have highly localised bands, resulting in low-energy interband transitions in the a∕b direction of the unit cell. In contrast, purely free-electron behaviour is observed in the c-direction below 2 eV. High-quality plasmon resonances are thus only observed in the c-direction, with energy and quality similar to those of the cubic structures. These anisotropic optical properties make TII-NaxWO3 and TII-KxWO3 interesting materials for further study as potential plasmonic materials.

Orientation-independent reaction activity monitoring with single particle and data analytics

Single-particle analysis is the most powerful method to obtain accurate local information for understanding and monitoring chemical reactions. However, investigations about obtaining comprehensive information at the single-particle level to overcome individual errors and sampling randomness have not been reported to date. Plasmonic nanorods, which have excellent anisotropic optical and chemical properties, make us in situ acquisition of conformation and dynamics of the biological information. On the basis of their anisotropic optical properties of the plasmonic nanorods such as Au nanorods (AuNRs) and data analytics, herein we developed a high-throughput resonance scattering imaging method of AuNRs under dark-field microscopy (DFM) to monitor orientation-independent reaction activity of AuNRs. Data analytics are introduced to determine a large number of AuNRs orientation obtained from a series of polarized DFM images, allowing us to real-time monitor reaction activity of AuNRs at all orientations, and also makes it possible to study the global and local reaction processes of AuNRs at single-particle level. Our method is expected to provide a new strategy for analytical study and single-particle sensing in chemistry.

Electrical control of anisotropic and tightly bound excitons in bilayer phosphorene

Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with the linear-dichroic light-matter interaction and the widely tunable direct band gap in the infrared frequency range. Despite recent theoretical predictions of strongly bound excitons with unique properties, it remains experimentally challenging to probe excitonic quasiparticles due to the severe oxidation that occurs during device fabrication. In this study, we report the observation of strongly bound excitons and trions with highly anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN) in a field-effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic optical spectra from anisotropic excitons and trions in the hBN-encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) \ensuremath{\sim}30 meV below the optical band gap of the charge-neutral system. Our work shows exciting possibilities for monolayer and few-layer phosphorene as a platform to explore many-body physics and photonics and optoelectronics based on strongly bound excitons with twofold anisotropy.

Type‐II AsP/As van der Waals Heterostructures: Tunable Anisotropic Electronic Structures and Optical Properties

Abstractvan der Waals (vdW) heterostructures, formed by stacking layered materials, have great potential for next‐generation nanoelectronics and optoelectronics with unique flexibility and high performance. Using density functional theory, it is demonstrated that all AA‐ and AB‐AsP/As heterostructures are semiconductors with intrinsic type‐II band alignments, facilitating the separation of photogenerated electron–hole pairs. The anisotropic electronic structures of the puckered AsP/As vdW heterostructures can be effectively modulated by applying tensile strains. The bandgaps exhibit nonmonotonic variations and monotonic changes with the strains exerted along x and y direction, respectively, both experiencing an indirect‐to‐direct bandgap transition. The external electric field gives rise to significant modulation on the bandgaps of the AsP/As vdW heterostructures. Specifically, the buckled AsP/As heterostructures can be changed from indirect to direct semiconductors with a small positive electric field from 0.3 to 0.6 V Å−1, which cannot be achieved under tensile strains. In addition, AsP/As vdW heterostructures exhibit excellent anisotropic light‐harvesting performances in visible light and ultraviolet regions and can be efficiently tuned by external strains, suggesting the potential applications in photovoltaics. The tunable anisotropic electronic and optical properties of the AsP/As vdW heterostructures may facilitate their practical applications for nanoelectronic devices and photovoltaics.

Strain-engineering on GeSe: Raman spectroscopy study.

Among the IV-VI compounds, GeSe has wide applications in nanoelectronics due to its unique photoelectric properties and adjustable band gap. Even though modulation of its physical characteristics, including the band gap, by an external field will be useful for designing novel devices, experimental work is still rare. Here, we report a detailed anisotropic Raman response of GeSe flakes under uniaxial tension strain. Based on theoretical analysis, the anisotropy of the phonon response is attributed to a change in anisotropic bond length and bond angle under in-plane uniaxial strain. An enhancement in anisotropy and band gap is found due to strain along the ZZ or AC directions. This study shows that strain-engineering is an effective method for controlling the GeSe lattice, and paves the way for modulating the anisotropic electric and optical properties of GeSe.

Strong anisotropic optical properties of 8-Pmmn borophene: a many-body perturbation study.

Using first-principles many-body perturbation theory, we investigate the optical properties of 8-Pmmn borophene at two levels of approximations; the GW method considering only the electron-electron interaction and the GW in combination with the Bethe-Salpeter equation including electron-hole coupling. The band structure exhibits anisotropic Dirac cones with semimetallic character. The optical absorption spectra are obtained for different light polarizations and we predict strong optical absorbance anisotropy. The absorption peaks undergo a global redshift when the electron-hole interaction is taken into account due to the formation of bound excitons which have an anisotropic excitonic wave function.

Enhancement and active mediation of near-field radiative heat transfer through multiple nonreciprocal graphene surface plasmons

Metasurfaces, the two-dimensional counterparts of three-dimensional metamaterials, have recently attracted much attention due to their interesting properties, such as negative refraction, hyperbolic dispersion, and the ability to manipulate the evanescent spectrum. In this work, we propose a theoretical model for near-field radiative heat transfer between two multilayered systems consisting of anisotropic metasurfaces. The choice of metasurface is graphene, with an adjustable drift current, since this provides an ideal platform to support a high density of modes around the plasmon frequency. In this configuration, multiple nonreciprocal surface plasmon polaritons are excited, providing a strong way for near-field energy transport. The resulting heat transfer, assisted by the graphene's multilayered structure and with a high drift-current velocity, is more than 36 times stronger than that of a graphene monolayer structure without a drift current for the same vacuum gap. By adjusting the vacuum gap and the thickness of a dielectric spacer, this enhanced effect can be modulated over a large range, and can even turn into a suppression. Our findings provide a powerful way to enhance and regulate energy transport, and in turn, open up a way to enrich the moir\'e physics inherent to the anisotropic optical properties of a metasurface.

In-Plane Optical and Electrical Anisotropy of 2D Black Arsenic.

Low-symmetry two-dimensional (2D) semiconductors have attracted great attention because of their rich in-plane anisotropic optical, electrical, and thermoelectric properties and potential applications in multifunctional nanoelectronic and optoelectronic devices. However, anisotropic 2D semiconductors with high performance are still very limited. Here, we report the systematic study of in-plane anisotropic properties in few-layered b-As that is a narrow-gap semiconductor, based on the experimental and theoretical investigations. According to experimental results, we have come up with a simple method for identifying the orientation of b-As crystals. Meanwhile, we show that the maximum mobility of electrons and holes was measured in the in-plane armchair (AC) direction. The measured maximum electron mobility ratio is about 2.68, and the hole mobility ratio is about 1.79.

Tetrahex carbides: Two-dimensional group-IV materials for nanoelectronics and photocatalytic water splitting

We report a series of novel two-dimensional (2D) group-IV carbides with perfectly ordered arrangement of tetragons and hexagons. The 2D tetrahex carbides possess robust energetic, dynamical, thermal, and mechanical stabilities. They have strong anisotropic mechanical, electronic, and optical properties. The negative Poisson’s ratio (PR) in the 2D tetrahex carbides emerges by small tensile strain (2–5%). The feature of sign-tunable PR enables them to be non-auxetic, auxetic, and partially-auxetic nanomaterials. They exhibit ultrahigh ideal strength that can even outperform graphene. They are natural semiconductors with direct and indirect band gaps which can be tuned by strain engineering. Beyond the binary carbides, the ternary alloying of group-IV elements further provides the ability to engineer their structural, mechanical, optical, and electronic properties. Excitingly, the alloying offers a continuous band gap energy, band edge modulation, and indirect-to-direct band gap transition. The 2D tetrahex-carbides exhibit high and directionally anisotropic carrier mobility, favorable for the separation of photo-generated carriers, and has very good optical absorption performance in the visible-light region. The position of band edges in the 2D tetrahex carbides fits perfectly the water oxidation and reduction potentials. Therefore, the 2D tetrahex carbides have potential applications not only in photocatalytic water-splitting but also in designing 2D opto-electro-mechanical devices.