Optical Constants and Structural Properties of Epitaxial MoS2 Monolayers

Hexagonal boron nitride nanophotonics: a record-breaking material for the ultraviolet and visible spectral ranges

A global trend towards miniaturization and multiwavelength performance of nanophotonic devices drives research on novel phenomena, such as bound states in the continuum and Mietronics, as well as surveys for high-refractive index and strongly anisotropic materials and metasurfaces. Hexagonal boron nitride (hBN) is one of the promising materials for future nanophotonics owing to its inherent anisotropy and prospects of high-quality monocrystal growth with an atomically flat surface. Here, we present highly accurate optical constants of hBN in the broad wavelength range of 250-1700 nm combining imaging ellipsometry measurements, scanning near-field optical microscopy and first-principles quantum mechanical computations. hBN's high refractive index, up to 2.75 in the ultraviolet (UV) and visible range, broadband birefringence of ∼0.7, and negligible optical losses make it an outstanding material for UV and visible range photonics. Based on our measurement results, we propose and design novel optical elements: handedness-preserving mirrors and subwavelength waveguides with dimensions of 40 nm operating in the visible and UV ranges, respectively. Remarkably, our results offer a unique opportunity to bridge the size gap between photonics and electronics.

Broadband Optical Properties of Bi2Se3

Materials with high optical constants are of paramount importance for efficient light manipulation in nanophotonics applications. Recent advances in materials science have revealed that van der Waals (vdW) materials have large optical responses owing to strong in-plane covalent bonding and weak out-of-plane vdW interactions. However, the optical constants of vdW materials depend on numerous factors, e.g., synthesis and transfer method. Here, we demonstrate that in a broad spectral range (290-3300 nm) the refractive index n and the extinction coefficient k of Bi₂Se₃ are almost independent of synthesis technology, with only a ~10% difference in n and k between synthesis approaches, unlike other vdW materials, such as MoS₂, which has a ~60% difference between synthesis approaches. As a practical demonstration, we showed, using the examples of biosensors and therapeutic nanoparticles, that this slight difference in optical constants results in reproducible efficiency in Bi₂Se₃-based photonic devices.

The effects of spin-orbit coupling on optical properties of monolayer [Formula: see text] due to mechanical strains

We have studied the optical conductivity of a quasi two-dimensional [Formula: see text] in the presence of external magnetic field and spin-orbit coupling. Specially, we address the frequency dependence of optical conductivity due to spin-orbit interaction. Using linear response theory the behavior of optical conductivity has been obtained within Green's function method. We have also considered the effects of uniaxial and biaxial in-plane strain on the optical absorption of [Formula: see text] layer. In the absence of external magnetic field with negative uniaxial strain parameter, optical conductivity includes Drude weight at zero frequency limit while Drude weight vanishes for [Formula: see text] layer under positive uniaxial strain. Our results show that the increase of uniaxial positive strain parameter causes to move the position peak to the higher frequencies. In contrast to uniaxial strain case, the Drude weight in optical conductivity appears at positive biaxial strain value 0.15. Also we have studied the effects of magnetic field, electron doping, hole doping in the presence of spin-orbit coupling on frequency dependence of optical conductivity of [Formula: see text] in details. The magnetic field dependence of optical absorption shows a monotonic decreasing behavior for each value of temperature in the absence of strain parameter.

Non-Additive Optical Response in Transition Metal Dichalcogenides Heterostructures

Van der Waals (vdW) heterostructures pave the way to achieve the desired material properties for a variety of applications. In this way, new scientific and industrial challenges and fundamental questions arise. One of them is whether vdW materials preserve their original optical response when assembled in a heterostructure. Here, we resolve this issue for four exemplary monolayer heterostructures: MoS₂/Gr, MoS₂/hBN, WS₂/Gr, and WS₂/hBN. Through joint Raman, ellipsometry, and reflectance spectroscopies, we discovered that heterostructures alter MoS₂ and WS₂ optical constants. Furthermore, despite the similarity of MoS₂ and WS₂ monolayers, their behavior in heterostructures is markedly different. While MoS₂ has large changes, particularly above 3 eV, WS₂ experiences modest changes in optical constants. We also detected a transformation from dark into bright exciton for MoS₂/Gr heterostructure. In summary, our findings provide clear evidence that the optical response of heterostructures is not the sum of optical properties of its constituents.

Large-Scale MoS2 Pixel Array for Imaging Sensor

Two-dimensional molybdenum disulfide (MoS₂) has been extensively investigated in the field of optoelectronic devices. However, most reported MoS₂ phototransistors are fabricated using the mechanical exfoliation method to obtain micro-scale MoS₂ flakes, which is laboratory- feasible but not practical for the future industrial fabrication of large-scale pixel arrays. Recently, wafer-scale MoS₂ growth has been rapidly developed, but few results of uniform large-scale photoelectric devices were reported. Here, we designed a 12 × 12 pixels pixel array image sensor fabricated on a 2 cm × 2 cm monolayer MoS₂ film grown by chemical vapor deposition (CVD). The photogating effect induced by the formation of trap states ensures a high photoresponsivity of 364 AW^-1, which is considerably superior to traditional CMOS sensors (≈0.1 AW^-1). Experimental results also show highly uniform photoelectric properties in this array. Finally, the concatenated image obtained by laser lighting stencil and photolithography mask demonstrates the promising potential of 2D MoS₂ for future optoelectrical applications.

Optical Anisotropy and Excitons in MoS2 Interfaces for Sensitive Surface Plasmon Resonance Biosensors

The use of ultra-thin spacer layers above metal has become a popular approach to the enhancement of optical sensitivity and immobilization efficiency of label-free SPR sensors. At the same time, the giant optical anisotropy inherent to transition metal dichalcogenides may significantly affect characteristics of the studied sensors. Here, we present a systematic study of the optical sensitivity of an SPR biosensor platform with auxiliary layers of MoS₂. By performing the analysis in a broad spectral range, we reveal the effect of exciton-driven dielectric response of MoS₂ and its anisotropy on the sensitivity characteristics. The excitons are responsible for the decrease in the optimal thickness of MoS₂. Furthermore, despite the anisotropy being at record height, it affects the sensitivity only slightly, although the effect becomes stronger in the near-infrared spectral range, where it may lead to considerable change in the optimal design of the biosensor.

Broadband Optical Constants and Nonlinear Properties of SnS2 and SnSe2

SnS₂ and SnSe₂ have recently been shown to have a wide range of applications in photonic and optoelectronic devices. However, because of incomplete knowledge about their optical characteristics, the use of SnS₂ and SnSe₂ in optical engineering remains challenging. Here, we addressed this problem by establishing SnS₂ and SnSe₂ linear and nonlinear optical properties in the broad (300-3300 nm) spectral range. Coupled with the first-principle calculations, our experimental study unveiled the full dielectric tensor of SnS₂ and SnSe₂. Furthermore, we established that SnS₂ is a promising material for visible high refractive index nanophotonics. Meanwhile, SnSe₂ demonstrates a stronger nonlinear response compared with SnS₂. Our results create a solid ground for current and next-generation SnS₂- and SnSe₂-based devices.

Broadband Optical Properties of Atomically Thin PtS2 and PtSe2

Noble transition metal dichalcogenides (TMDCs) such as PtS2 and PtSe2 show significant potential in a wide range of optoelectronic and photonic applications. Noble TMDCs, unlike standard TMDCs such as MoS2 and WS2, operate in the ultrawide spectral range from ultraviolet to mid-infrared wavelengths; however, their properties remain largely unexplored. Here, we measured the broadband (245-3300 nm) optical constants of ultrathin PtS2 and PtSe2 films to eliminate this gap and provide a foundation for optoelectronic device simulation. We discovered their broadband absorption and high refractive index both theoretically and experimentally. Based on first-principle calculations, we also predicted their giant out-of-plane optical anisotropy for monocrystals. As a practical illustration of the obtained optical properties, we demonstrated surface plasmon resonance biosensors with PtS2 or PtSe2 functional layers, which dramatically improves sensor sensitivity by 60 and 30%, respectively.