Aleksandr S. Slavich

The miniaturization of polarization-controlling optical components is one of the central pursuits in nanophotonics. While traditional anisotropic materials require large propagation lengths to achieve the desired phase shifts, metasurfaces mitigate this size constraint but often introduce narrow operational bandwidths and high fabrication complexities. To bridge this gap, we introduce MoOCl2 as a promising material for ultracompact and broadband phase retardation. Building on its giant optical anisotropy, we experimentally demonstrate MoOCl2 quarter-wave plates with thicknesses down to 65 nm. These flakes exhibit achromatic quarter-wave retardation across broad visible and near-infrared (510−900 nm) spectral windows, surpassing the fundamental thickness and bandwidth limitations of both conventional optical materials and artificial nanostructures. Moreover, MoOCl2 wave plates demonstrate a measured retardance tolerance reaching λ/4500 at their experimentally determined quarter-wave wavelengths. As a result, this study establishes MoOCl2 as a building block for ultracompact polarization optics.

The development of nanophotonics is hindered by a fundamental trade‐off between a material's refractive index ( n ) and its electronic bandgap ( E _g ), which severely restricts the choice of materials for short‐wavelength applications. This challenge is particularly acute in the visible and ultraviolet (UV) spectra, where high‐performance devices require materials that are simultaneously highly refractive and transparent. Here, we report on the van der Waals (vdW) crystal cadmium phosphorus trisulfide (CdPS ₃ ) as a solution to this long‐standing problem. Through comprehensive optical and structural characterization, we show that CdPS ₃ possesses an anomalously high in‐plane refractive index across the visible spectrum approaching three in the near‐UV, combined with a wide indirect bandgap. This combination of properties is validated by first‐principles calculations and direct near‐field imaging of highly confined waveguide modes. These findings establish CdPS ₃ as a leading material for UV–visible photonics, opening a new pathway for the development of high‐density integrated circuits and metasurfaces.

The growing demand for high-performance, nanoscale building blocks for printing technologies necessitates a scalable method to produce clean, stable nano-inks without performance-degrading additives. Layered van der Waals (vdW) materials are promising candidates due to their unique properties and can be used to create such nano-inks. Current approaches often rely on surfactant residues or high-boiling point solvents to achieve colloidal stability, which can be incompatible with chemically sensitive substrates. A robust methodology based on pulsed laser ablation in liquid (PLAL) provides a purely physical, single-step alternative to circumvent these limitations. This approach directly generates a portfolio of additive-free, spherical vdW nanoparticles that exhibit exceptional, long-term colloidal stability, an intrinsic property that stems from a native surface charging mechanism, eliminating the need for stabilizing agents. Our work establishes a universal platform for vdW nano-inks, offering a powerful new tool for next-generation optoelectronics, sensing technologies, and flexible devices.

The realization of extreme optical anisotropy is foundational for nanoscale light manipulation. The van der Waals (vdW) crystal MoOCl₂ has emerged as a promising candidate for this quest, hosting hyperbolic plasmon polaritons in the visible and near-infrared wavelengths. However, the fundamental anisotropic dielectric tensor governing this behavior has remained elusive. Here, we resolve this problem by providing the first experimental determination of the full dielectric tensor of hyperbolic vdW MoOCl₂. Via spectroscopic ellipsometry, Mueller matrix, and reflectance measurements, we quantify the material's optical duality: a metallic optical response (ε₁ < 0) along the crystallographic a-axis and a dielectric response (ε₁ > 0) along the orthogonal directions. This dichotomy drives an epsilon-near-zero (ENZ) condition at ≈512 nm and results in a giant in-plane birefringence of Δn ≈ 2.2 for MoOCl₂. As a result, our work provides the critical missing experimental parameters for MoOCl₂, establishing it as a benchmark hyperbolic and ENZ material.

Thanks to their record high refractive index and giant optical anisotropy, van der Waals (vdW) materials have accelerated the development of nanophotonics. However, traditional high refractive index materials, such as titanium dioxide (TiO₂), still dominate in the most important visible range. This is due to the current lack of transparent vdW materials across the entire visible spectrum. In this context, we propose that germanium disulfide (GeS₂) could offer a significant breakthrough. With its high refractive index, negligible losses, and biaxial optical anisotropy across the whole visible range, GeS₂ has the potential to complement TiO₂ and close the application gap of vdW materials in the visible spectrum. The addition of GeS₂ could have a profound impact on the design of van der Waals nanophotonic circuits for any operation wavelength from ultraviolet to infrared, emphasizing the significance of the potential impact of GeS₂ on the field of nanophotonics.

Van der Waals topological insulators, characterized by their high-index dielectric response, offer a promising materials platform for nanophotonics. Among these materials, Bi2Te3 has one of the highest refractive indices and extinction coefficients. However, the precise determination of Bi2Te3 optical properties remains challenging owing to its complicated physical model, which includes an oxide layer, topological conducting states, and optical anisotropy. Here, we resolve this problem and develop an accurate optical model for Bi2Te3 in a broad (450–1500 nm) spectral range. Our study shows that an oxide layer plays a major role in optical model for these wavelengths, while the influence of topological conducting states and optical anisotropy is minimal. Our model allows us to obtain accurate Bi2Te3 optical constants and demonstrate their use in biosensors, thermal theranostics, and topological phase singularities. Moreover, we observe a polarization transition of topological phase singularity for Bi2Se3, which opens a new direction for the development of topological phase effects. Therefore, our results open new avenues for photonic applications of Bi2Te3 optical properties.

Recent advancements in nanophotonic devices rely on materials with high refractive index and large optical anisotropy. Among them, van der Waals (vdW) crystals have garnered considerable interest due to their record-value optical properties originating from their complex layered structure. Moreover, the number of vdW crystals increases rapidly every year, but their traditional optical characterization is time-consuming and cannot keep up with vdW crystals’ appearance. Here, we introduce an express method for the determination of vdW optical properties to resolve this issue by taking into account their anisotropy. Our approach relies on polarized micro-transmittance, which allows us to demonstrate the superior optical response of vdW crystals on the example of hBN and GeS₂. Hence, our developed technique offers a universal tool for the characterization of vdW materials’ optical properties for current and next-generation vdW-based photonic devices.

The 2D materials are the fundamental building blocks for modern optoelectronics and photonics. Optically anisotropic monolayers give even more flexibility in device design and performance. However, the random orientation of optical axes in the large‐scale samples prevents anisotropic monolayers from widespread use. The alternative structure is a monolayer of aligned single‐walled carbon nanotubes (SWCNTs) with an anisotropic dielectric tensor. Herein, aligned SWCNTs monolayer anisotropic optical constants in a broad spectral range (250–1700 nm) are measured for the first time. It is discovered that it has a large birefringence of Δn ≈ 0.2 and a high dichroism of Δk ≈ 0.4. Moreover, it is demonstrated that aligned SWCNTs monolayer optical response can be described by an effective medium approximation using the graphene dielectric function. In addition, it gives a universal approach for a determination of carbon concentration in nanotubes structures. It also applies for other types of carbon nanotubes, such as multi‐walled and randomly oriented carbon nanotubes arrays. Hence, in the results, aligned SWCNTs monolayer optical constants are added to the optical anisotropy database, which facilitates the longstanding challenge of using 1D structures in two dimensions, and a rapid characterization method for carbon nanotubes is provided.

Dynamicallyprogrammable metasurfaces capable of manipulating terahertz (THz) wavefronts in various manners depending on external controls are highly desired for next-generation wireless communication systems and new tools for THz diagnostics. Such metasurfaces may utilize the insulator-to-metal transition in VO₂, which can be induced both electrically and optically. Optical control is especially convenient for individual addressing to each meta-atom, but it is hampered by the high optical switching threshold of VO₂. We experimentally realize VO₂-based THz metasurfaces with hybrid electro-optical control when the metasurface is brought close to the transition point by an almost-threshold current, and then is easily switched by unfocused continuous-wave light. We were able to control the metasurface THz transmission by 0.4W/cm² near-IR light, while purely optical switching required tightly focused light with an intensity of >3×10⁵W/cm². After correcting for the fact that a tightly focused spot dissipates heat easier, we estimate that the optical switching threshold reduction due to the electric current alone is ∼2 orders of magnitude. Finally, coating the metasurface with Au nanoparticles further reduced the threshold by 30% due to plasmonic effects.