Publications

Atomically thin transition metal dichalcogenides (TMDCs) present a promising platform for numerous photonic applications due to excitonic spectral features, possibility to tune their constants by external gating, doping, or light, and mechanical stability. Utilization of such materials for sensing or optical modulation purposes would require a clever optical design, as by itself the 2D materials can offer only a small optical phase delay - consequence of the atomic thickness. To address this issue, we combine films of 2D semiconductors which exhibit excitonic lines with the Fabry-Perot resonators of the standard commercial SiO2/Si substrate, in order to realize topological phase singularities in reflection. Around these singularities, reflection spectra demonstrate rapid phase changes while the structure behaves as a perfect absorber. Furthermore, we demonstrate that such topological phase singularities are ubiquitous for the entire class of atomically thin TMDCs and other high-refractive-index materials, making it a powerful tool for phase engineering in flat optics. As a practical demonstration, we employ PdSe2 topological phase singularities for a refractive index sensor and demonstrate its superior phase sensitivity compared to typical surface plasmon resonance sensors.

A 1.5 micrometer-long ballistic graphene Josephson junction has been successfully fabricated, demonstrating a large critical current density and multiple Andreev reflections.

Traditional magnetic phase diagram represents a transition between the ferromagnetic and paramagnetic states of a material under the influence of varied temperature, magnetic field, and pressure. So far, the ferromagnetic phase has been considered predominantly as a single type of magnetization texture extending macroscopically in the bulk of a crystal, existing as a ground state determined by the interactions between localized magnetic moments arranged in a lattice. Here, it is demonstrated that an unconventional magnetic order composed of vertically correlated planar magnetic sub-domains occurs intrinsically in mechanically exfoliated layers of van der Waals ferromagnet CrBr3. Based on the visualization of the magnetic textures through magnetic force microscopy in conjunction with the ab initio calculations of the crystal structure in the magnetic phase and micromagnetic simulations, the origin of the magnetic sub-domains is attributed to stacking faults isolating a van der Waals ferromagnetic well from the bulk film due to modifications in the interlayer exchange coupling. This enables to create a phase diagram describing the magnetic states unique to van der Waals ferromagnets in terms of the degree of correlation between the magnetic sub-domains, dependent on the exchange coupling constants and tuneable by magnetic field and temperature.

Abstract Chemical oscillators—such as the Belousov‐Zhabotinsky reaction—have long served as model systems for studying non‐equilibrium chemical dynamics and as analogues of biological oscillations. However, many biological processes rely on out‐of‐equilibrium, often oscillatory, ionic fluxes that do not involve chemical reactions. Examples include action potentials in neurons, muscle contraction, cardiac rhythmicity, intracellular calcium signaling, and calcium wave oscillations. Despite these parallels, the development of biomimetic systems compatible with neuromorphic interfaces remains a significant challenge. Here, a strategy is demonstrated to organize oscillating ionic currents by developing ionic transistors composed of graphene oxide and polyelectrolyte, and assembling them into all‐ionic integrated circuits. By driving these systems out of equilibrium using external voltages, periodic motion of various ions across defined interfaces is achieved. This behavior, governed by local electric fields arising from unbalanced ionic concentrations, closely mimics biological excitability, such as that observed in neuronal and cardiac systems. These ionic transistors serve as a foundational building block for neuromorphic interfaces, offering a universal platform to emulate complex biological ionic processes with high fidelity.

Microfabrication of graphene devices used in many experimental studies currently relies on the fact that graphene crystallites can be visualized using optical microscopy if prepared on top of Si wafers with a certain thickness of SiO2. The authors study graphene’s visibility and show that it depends strongly on both thickness of SiO2 and light wavelength. They have found that by using monochromatic illumination, graphene can be isolated for any SiO2 thickness, albeit 300nm (the current standard) and, especially, ≈100nm are most suitable for its visual detection. By using a Fresnel-law-based model, they quantitatively describe the experimental data.

Chirality is probably the most mysterious among all symmetry transformations. Very readily broken in biological systems, it is practically absent in naturally occurring inorganic materials and is very challenging to create artificially. Chiral optical wavefronts are often used for the identification, control and discrimination of left- and right-handed biological and other molecules. Thus, it is crucially important to create materials capable of chiral interaction with light, which would allow one to assign arbitrary chiral properties to a light field. In this paper, we utilized van der Waals technology to assemble helical homostructures with chiral properties (e. g. circular dichroism). Because of the large range of van der Waals materials available such helical homostructures can be assigned with very flexible optical properties. We demonstrate our approach by creating helical homostructures based on multilayer As$_2$S$_3$, which offers the most pronounced chiral properties even in thin structures due to its strong biaxial optically anisotropy. Our work showcases that the chirality of an electromagnetic system may emerge at an intermediate level between the molecular and the mesoscopic one due to the tailored arrangement of non-chiral layers of van der Waals crystals and without additional patterning.

No abstract is provided for this article.

We report on experimental investigations of an electrically driven WSe2 based light-emitting van der Waals heterostructure. We observe a threshold voltage for electroluminescence significantly lower than the corresponding single particle band gap of monolayer WSe2. This observation can be interpreted by considering the Coulomb interaction and a tunneling process involving excitons, well beyond the picture of independent charge carriers. An applied magnetic field reveals pronounced magneto-oscillations in the electroluminescence of the free exciton emission intensity with a 1/B periodicity. This effect is ascribed to a modulation of the tunneling probability resulting from the Landau quantization in the graphene electrodes. A sharp feature in the differential conductance indicates that the Fermi level is pinned and allows for an estimation of the acceptor binding energy.

Van der Waals (vdW) materials are at the core of modern optoelectronics and nanophotonics. However, relatively limited research attention is devoted to their giant optical anisotropy. Here, we demonstrate that the use of giant anisotropy leads to the next-generation integrated circuits and optical elements, determining the exact values of anisotropic dielectric permittivity tensor for the variety of vdW materials in the broad spectral range (250–1700 nm) using cross-validation of far- and near-field techniques, accompanied by first-principle calculations. Our results show high refractive index, transparency over the whole studied spectral range and giant optical anisotropy for all investigated vdW materials. Furthermore, we suggest applications of vdW materials for chiral optics and integrated photonics.

We have measured the quantum Hall activation gaps in bilayer graphene at filling factors ν = ± 4 and ν = ± 8 in high magnetic fields up to 30 T. We find that energy levels can be described by a 4-band relativistic hyperbolic dispersion. The Landau level width is found to contain a field independent background due to an intrinsic level broadening and a component which increases linearly with magnetic field.

With the advance of on-chip nanophotonics, there is a high demand for high-refractive-index and low-loss materials. Currently, this technology is dominated by silicon, but van der Waals (vdW) materials with a high refractive index can offer a very advanced alternative. Still, up to now, it was not clear if the optical anisotropy perpendicular to the layers might be a hindering factor for the development of vdW nanophotonics. Here, we studied WS2-based waveguides in terms of their optical properties and, particularly, in terms of possible crosstalk distance. Surprisingly, we discovered that the low refractive index in the direction perpendicular to the atomic layers improves the characteristics of such devices, mainly due to expanding the range of parameters at which single-mode propagation can be achieved. Thus, using anisotropic materials offers new opportunities and novel control knobs when designing nanophotonic devices.