Søren Raza

Controlling light emission at the nanoscale has important applications in solid-state lighting, displays, and quantum light sources. Achieving this control requires both enhanced local electromagnetic fields to boost emission intensity and engineered radiation patterns to direct photons efficiently. Mie voids, consisting of an air cavity surrounded by a high-index semiconductor, are particularly suited for this purpose because they expose their strongest fields in an accessible region for nearby emitters while supporting resonances that shape directional emission through interference. Here, we demonstrate an all-van der Waals nanophotonic platform that couples excitons in atomically thin WS ₂ to Mie void resonators formed in WSe ₂ . Guided by electromagnetic simulations, we identify void geometries that maximize photoluminescence through synergistic enhancement of excitation and emission processes. We also introduce a two-step fabrication approach that leverages van der Waals assembly to independently control the void diameter and depth. Experimentally, we observe up to a 600-fold increase in photoluminescence intensity from monolayer WS ₂ placed on individual voids compared to unstructured WSe ₂ , along with pronounced out-of-plane beaming of light that yields a forward-to-off-axis enhancement of 2.6 dB. These results establish Mie voids in van der Waals semiconductors as a versatile platform for controlling light-matter interactions at the nanoscale.

We present a reflection-based method for measuring the in-plane refractive index of transparent van der Waals (vdW) materials. The approach enables the characterization of as small as 10 μm ² exfoliated flakes on a non-transmissive substrate without assuming any specific spectral shape of the refractive index. Exfoliated flakes are most commonly obtained through mechanical exfoliation, which generally produces vdW flakes with tens-of-micron lateral dimensions. As a result, conventional ellipsometry—which depends on large, uniform areas, and specific spectral models—becomes challenging to apply. Our method determines the refractive index directly from the spectral position of reflection minima, provided the flake thickness and the substrate complex refractive index are known. We demonstrate the technique on hafnium disulfide (HfS ₂ ), a vdW semiconductor with high refractive index and low absorption, retrieving its in-plane refractive index across the visible range. The results both validate previous ellipsometry measurements and establish this method as an accessible and spectral-model-free alternative for refractive index characterization of vdW materials.

The past decade has seen rapid growth in the number of experimentally realized two-dimensional (2D) materials with diverse chemical and physical properties. However, information on their crystal structure, synthesis routes, and measured or predicted properties remains scattered across thousands of publications. Here, we consolidate this fragmented knowledge by establishing X2DB─an open infrastructure that integrates experimental and computational data on 2D materials. Using extensive literature mining and direct community uploads, we identify 370 unique 2D materials that have been realized in monolayer or few-layer form and link them to their digital counterparts in computational databases, enabling consistent ab initio characterization of their properties across monolayer, bilayer, and bulk forms. We describe the structure and content of the database, highlight its support for community uploads, illustrate how it can be used to generate scientific insight, and introduce a hierarchical classification of the known set of 2D materials. Our work supports the integration and cross-fertilization of experimental and theoretical knowledge and contributes to data-driven and predictive synthesis of 2D materials.

Photonic devices depend critically on the dielectric materials from which they are made, with higher refractive indices and lower absorption losses enabling new functionalities and higher performance. However, these two material properties are intrinsically linked through the empirical Moss rule, which states that the refractive index of a dielectric decreases as its band gap energy increases. Materials that surpass this rule, termed super-Mossian dielectrics, combine large refractive indices with wide optical transparency and are therefore ideal candidates for advanced photonic applications. This Review surveys the expanding landscape of high-index dielectric and semiconductor materials, with a particular focus on those that surpass the Moss rule. We discuss how electronic band structures with a large joint density of states near the band edge give rise to super-Mossian behavior and how first-principles computational screening can accelerate their discovery. Finally, we establish how the refractive index sets the performance limits of nanoresonators, waveguides, and metasurfaces, highlighting super-Mossian dielectrics as a promising route toward the next performance leap in photonic technologies.

Controlling light emission at the nanoscale has important applications in solid-state lighting, displays, and quantum light sources. Achieving this control requires both enhanced local electromagnetic fields to boost emission intensity and engineered radiation patterns to direct photons efficiently. Mie voids, consisting of an air cavity surrounded by a high-index semiconductor, are particularly suited for this purpose because they expose their strongest fields in an accessible region for nearby emitters while supporting resonances that shape directional emission through interference. Here, we demonstrate an all-van der Waals nanophotonic platform that couples excitons in atomically thin WS$_2$ to Mie void resonators formed in WSe$_2$. Guided by electromagnetic simulations, we identify void geometries that maximize photoluminescence through synergistic enhancement of excitation and emission processes. We also develop a two-step fabrication strategy that enables independent control of void diameter and depth, providing a route to systematically tune the optical response. Experimentally, we observe up to a 600-fold increase in photoluminescence intensity from monolayer WS$_2$ placed on individual voids compared to on an unstructured WSe$_2$, along with pronounced out-of-plane beaming of light that yields a forward-to-off-axis enhancement of 2.6 dB. Our results establish Mie voids in van der Waals semiconductors as a new platform for controlling light-matter interactions and realizing compact, directional, and efficient nanoscale light sources.

High–refractive index dielectric materials can enhance many optical technologies by enabling efficient light manipulation in waveguides, metasurfaces, and nanoscale resonators. Van der Waals materials, which are anisotropic semiconductor materials, are particularly promising due to their excitonic response and strong in-plane polarizability. Here, we perform ab initio calculations to determine the refractive index of over a hundred anisotropic semiconductor materials, many of them van der Waals in nature. Our computational screening reveals both established and less-explored promising materials, including hafnium disulfide (HfS ₂ ), which exhibits an in-plane refractive index above 3 and large anisotropy in the visible range. We confirm these properties through ellipsometry and develop a nanofabrication process for HfS ₂ , demonstrating Mie-resonant nanodisks. This is achieved by mitigating the air sensitivity of HfS ₂ through storage in controlled environment or encapsulation. Our work provides a comparative overview of high-index van der Waals materials and establishes HfS ₂ as a promising material for visible-range photonics.

Scanning probe microscopy (SPM) is a powerful technique for mapping nanoscale surface properties through tip–sample interactions. Thermal scanning-probe lithography (tSPL) is an advanced SPM variant that uses a silicon tip on a heated cantilever to sculpt and measure the topography of polymer films with nanometer precision. The surfaces produced by tSPL—smooth topographic landscapes—allow mathematically defined contours to be fabricated on the nanoscale, enabling sophisticated functionalities for photonic, electronic, chemical and biological technologies. Evaluating the physical effects of a landscape requires fitting arbitrary mathematical functions to SPM datasets; however, this capability does not exist in standard analysis programs. Here, we provide an open-source software package (FunFit) to fit analytical functions to SPM data and develop a fabrication and characterization protocol based on this analysis. We demonstrate the benefit of this approach by patterning periodic and quasi-periodic landscapes in a polymer resist with tSPL, which we transfer to hexagonal boron nitride (hBN) flakes with high fidelity via reactive ion etching. The topographic landscapes in polymers and hBN are measured with tSPL and atomic force microscopy, respectively. Within the FunFit program, the datasets are corrected for artifacts, fit with analytical functions and compared, providing critical feedback on the fabrication procedure. This approach can improve analysis, reproducibility and process development for a broad range of SPM experiments. The protocol can be performed within a working day by a trained graduate student or researcher, where fabrication and characterization take a few hours, and software analysis takes a few minutes.

Thin films of high-refractive-index excitonic materials enable self-coupling by simultaneously supporting intrinsic excitonic transitions and optical resonances. These optical resonances take the form of Fabry-Perot resonances in thick films and absorption resonances in ultrathin films placed on metallic substrates. Here, we investigate whether these optical resonances lead to true exciton-photon hybridization. Using far-field reflectance and spectrally resolved photocurrent measurements, we study tungsten disulfide (WS$_2$) flakes on both metallic and dielectric substrates across a range of thicknesses. While reflectance spectra for ultrathin flakes exhibit resonance splitting between excitons and absorption resonances, our photocurrent measurements reveal only excitonic peaks, indicating that no polaritons are formed. In contrast, thicker flakes exhibit Fabry-Perot resonances that strongly couple to excitons, resulting in clear splitting in both reflectance and photocurrent spectra, and providing evidence of polariton formation. We further show that the polariton resonances can be tuned through the reflection phase at the WS$_2$-substrate interface by changing the substrate material. In addition to coupling with the strong A-exciton, we observe polariton formation involving the weaker B-exciton at shorter wavelengths, as well as higher-order hybridization where both excitons interact simultaneously with a single Fabry-Perot resonance. These findings clarify the distinction between apparent and true strong coupling in excitonic materials and demonstrate how reflection phase and flake thickness can be used to engineer light-matter interactions.