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.

Investigations of Optical Functions and Optical Transitions of 2D Semiconductors by Spectroscopic Ellipsometry and DFT.

Optical functions and transitions are essential for a material to reveal the light-matter interactions and promote its applications. Here, we propose a quantitative strategy to systematically identify the critical point (CP) optical transitions of 2D semiconductors by combining the spectroscopic ellipsometry (SE) and DFT calculations. Optical functions and CPs are determined by SE, and connected to DFT band structure and projected density of states via equal-energy and equal-momentum lines. The combination of SE and DFT provides a powerful tool to investigate the CP optical transitions, including the transition energies and positions in Brillouin zone (BZ), and the involved energy bands and carries. As an example, the single-crystal monolayer WS2 is investigated by the proposed method. Results indicate that six excitonic-type CPs can be quantitatively distinguished in optical function of the monolayer WS2 over the spectral range of 245-1000 nm. These CPs are identified as direct optical transitions from three highest valence bands to three lowest conduction bands at high symmetry points in BZ contributed by electrons in S-3p and W-5d orbitals. Results and discussion on the monolayer WS2 demonstrate the effectiveness and advantages of the proposed method, which is general and can be easily extended to other materials.

Quantitatively Exploring Giant Optical Anisotropy of Quasi-One-Dimensional Ta2NiS5.

Optical anisotropy offers a heightened degree of flexibility in shaping optical properties and designing cutting-edge devices. Quasi-one-dimensional Ta2NiS5, with giant optical anisotropy, has been used in the development of new lasers and sensors. In this research endeavor, we successfully acquired the complete dielectric tensor of Ta2NiS5, utilizing the advanced technique of Mueller matrix spectroscopic ellipsometry, enabling a rigorous quantitative assessment of its optical anisotropy. The results indicate that Ta2NiS5 demonstrates giant birefringence and dichroism, with Δnmax = 1.54 and Δkmax = 1.80. This pursuit also delves into the fundamental underpinnings of this optical anisotropy, drawing upon a fusion of first-principles calculations and critical points analysis. The anisotropy of Ta2NiS5 arises from differences in optical transitions in different directions and is shown to be due to van Hove singularities without exciton effects. Its giant optical anisotropy is expected to be useful in the design of novel optical devices, and the revelation of the physical mechanism facilitates the modulation of its optical properties.

Giant in-plane optical and electronic anisotropy of tellurene: a quantitative exploration.

Tellurene's giant in-plane optical anisotropy brings richer physics and an extra degree of freedom to regulate its optical properties for designing novel and unique polarization-sensitive devices. Here, we quantitatively evaluate the in-plane optical anisotropy of tellurene and further reveal its physical origins by combining imaging Mueller matrix spectroscopic ellipsometry (MMSE) and first-principles calculations. The anisotropic complex refractive indices and dielectric functions, as well as the derived giant birefringence (|Δn|max = 0.48) and dichroism (Δk > 0.4), are accurately determined by imaging MMSE to quantitatively evaluate the in-plane optical anisotropy of tellurene. With density functional theory (DFT), tellurene's optical anisotropy is connected to its low-symmetry lattice structure with electrical anisotropy (including the anisotropic effective mass, partial charge density, and carrier mobility), leading to anisotropic electric polarization and ultimately optical anisotropy. This work provides a general and quantitative way to explore the optical anisotropy and also helps to comprehend the connection between the lattice structure and the optical anisotropy of tellurene and even other emerging low-symmetry materials, which will further promote their polarization-sensitive optical applications.

Broadband and Incident-Angle-Modulation Near-Infrared Polarizers Based on Optically Anisotropic SnSe.

Optical anisotropy offers an extra degree of freedom to dynamically and reversibly regulate polarizing optical components, such as polarizers, without extra energy consumption and with high modulating efficiency. In this paper, we theoretically and numerically design broadband and incident-angle-modulation near-infrared polarizers, based on the SnSe, whose optical anisotropy is quantitatively evaluated by the complete dielectric tensor, complex refractive index tensor, and derived birefringence (~|Δn|max = 0.4) and dichroism (~|Δk|max = 0.4). The bandwidth of a broadband polarizer is 324 nm, from 1262 nm to 1586 nm, with an average extinction ratio above 23 dB. For the incident-angle-modulation near-infrared polarizer, the high incident angles dynamically and reversibly modulate its working wavelength with a maximum extinction ratio of 71 dB. Numerical simulations and theoretical calculations reveal that the considerable absorption for p light and continuously and relatively low absorption of s light lead to the broadband polarizer, while the incident-angle-modulation one mainly arises from the blue shift of corresponding wavelength of p light's minimum reflectance. The proposed novel design of polarizers based on SnSe are likely to be mass-produced and integrated into an on-chip system, which opens up a new thought to design polarizing optical components by utilizing other low-symmetry materials.

Wide-spectrum polarization-sensitive and fast-response photodetector based on 2D group IV-VI semiconductor tin selenide.

Tin selenide (SnSe) has attracted considerable interest recently on account of its low-symmetry lattice structure, great compatibility with key semiconductor technology, and remarkable electrical and optical performance. SnSe-based polarization-sensitive photodetectors show promising application prospects because of their fast response and excellent photoelectric performance. Here, an in-plane anisotropic SnSe nanosheet was synthesized and reported in detail by applying angle-resolved polarized Raman spectroscopy (ARPRS), polarization-resolved optical microscopy(PROM), angle-resolved optical absorption spectroscopy (AROAS), and other crystal structure characterization methods. Moreover, SnSe crystals exhibit superior polarization detection performance with a high anisotropic photocurrent ratio (2.31 at 1064 nm) due to the structure formed by the Van der Waals superposition of covalently bonded atomic layers. Furthermore, SnSe-based photodetectors have high responsivity (9.27 A/W), high detectivity (4.08 × 1010 Jones), and fast response (in the order of nanoseconds). These results suggest a new method for fabricating 2D fast-response polarization-sensitive photodetectors in the future.

Rapid mass production of two-dimensional metal oxides and hydroxides via the molten salts method.

Because of their exotic electronic properties and abundant active sites, two-dimensional (2D) materials have potential in various fields. Pursuing a general synthesis methodology of 2D materials and advancing it from the laboratory to industry is of great importance. This type of method should be low cost, rapid and highly efficient. Here, we report the high-yield synthesis of 2D metal oxides and hydroxides via a molten salts method. We obtained a high-yield of 2D ion-intercalated metal oxides and hydroxides, such as cation-intercalated manganese oxides (Na0.55Mn2O4·1.5H2O and K0.27MnO2·0.54H2O), cation-intercalated tungsten oxides (Li2WO4 and Na2W4O13), and anion-intercalated metal hydroxides (Zn5(OH)8(NO3)2·2H2O and Cu2(OH)3NO3), with a large lateral size and nanometre thickness in a short time. Using 2D Na2W4O13 as an electrode, a high performance electrochemical supercapacitor is achieved. We anticipate that our method will enable new path to the high-yield synthesis of 2D materials for applications in energy-related fields and beyond.