arXiv subjects

Abhay V. Agrawal

Publications and source records attributed to Abhay V. Agrawal.

5 recordsLinked to original sources

arXiv2026

Precise one-dimensional nanochannels in transition metal dichalcogenides as building blocks for advanced nanophotonics

Atomically sharp edges are essential for future high-index nanophotonic structures, yet conventional lithography and dry etching methods inevitably introduce edge roughness that limits optical confinement and reproducibility. Recently, anisotropic wet etching of multilayer van der Waals crystals, such as transition metal dichalcogenides (TMDs), has enabled crystallographically defined, atomically sharp zigzag edges, eliminating the edge-roughness problem. However, the process is intrinsically limited to confined geometries such as isolated triangular or hexagonal features dictated by crystal stacking symmetry. Here, we demonstrate a lithography-guided anisotropic etching framework that drives TMDs etching beyond isolated confined geometries by enforcing controlled interaction of neighboring etched nanoholes regions. In multilayer 2H-WS2, merging of anisotropic etch fronts enables sustained long-range propagation of zigzag facets, introducing a previously inaccessible 180-degree edge alignment and a crystallographically defined design space combining 120-degree and 180-degree junctions. Using this approach, we fabricate extended nanophotonic structures with ultrasharp sidewalls, including sub-100-nm-gap one-dimensional gratings, waveguides, defect-engineered photonic cavities, angle programmed photonic lattices, and diffractive zone plates. Back-focal-plane reflection spectroscopy of atomically sharp 1D periodic 2H-WS2 gratings demonstrates their photonic functionality, revealing symmetry-protected bound states in the continuum (SP-BICs) and strong exciton-photon coupling in multilayer WS2. Finally, we fabricate ultrathin, ultranarrow, and ultralong nanoribbons with record-high aspect ratios. Together, these results demonstrate edge merging as a generic route to fabricate edge-defined, atomically sharp nanophotonic and nanoelectronic architectures in layered van der Waals platforms.

physics.optics↗

arXiv2024

Humidity-enhanced NO$_2$ gas sensing using atomically sharp edges in multilayer MoS$_2$

Ambient humidity poses a significant challenge in the development of practical room temperature NO$_2$ gas sensors. Here, we employ atomically precise zigzag edges in multilayer MoS$_2$, fabricated using electron beam lithography and anisotropic wet etching, to achieve highly sensitive and selective gas sensing performance that is humidity-tolerant at elevated temperatures and humidity-enhanced at room temperature under ultraviolet illumination. Notably, exposure to 2.5 parts per billion (ppb) NO$_2$ at 70% relative humidity under ultraviolet illumination and at room-temperature resulted in a 33-fold increase in response and a 6-fold faster recovery compared to 0% relative humidity, leading to response values exceeding 1100%. The optimized samples demonstrated a theoretical detection limit ranging from 4 to 400 parts per trillion (ppt) NO$_2$. The enhanced NO$_2$ sensing capabilities of MoS$_2$ edges have been further confirmed through first-principles calculations. Our study expands the applications of nanostructured MoS$_2$ and highlights its potential for detecting NO$_2$ at sub-ppb levels in complex scenarios, such as high humidity conditions.

physics.app-ph↗

arXiv2024

Ultrathin 3R-MoS$_2$ metasurfaces with atomically precise edges for efficient nonlinear nanophotonics

Dielectric metasurfaces that combine high-index materials with optical nonlinearities are widely recognized for their potential in various quantum and classical nanophotonic applications. However, the fabrication of high-quality metasurfaces poses significant material-dependent challenges, as their designs are often susceptible to disorder, defects, and scattering losses, which are particularly prone to occur at the edges of nanostructured features. Additionally, the choice of the material platforms featuring second-order optical nonlinearities, $χ^{(2)}$, is limited to broken-inversion symmetry crystals such as GaAs, GaP, LiNbO$_3$, and various bulk van der Waals materials, including GaSe and NbOCl$_2$. Here, we use a combination of top-down lithography and anisotropic wet etching of a specially stacked van der Waals crystal -- 3R-MoS$_2$, which exhibits both a high refractive index and exceptional $χ^{(2)}$ nonlinearity, to produce metasurfaces consisting of perfect equilateral triangle nanoholes with atomically precise zigzag edges. Due to the geometry of the triangle, the etching process is accompanied by a transition from an in-plane $C_4$ symmetric structure to a broken-in-plane symmetry configuration, thereby allowing for the realization of the quasi-bound-state-in-the-continuum (q-BIC) concept. The resulting ultrathin metasurface ($\sim$ 20-25 nm) demonstrates a remarkable enhancement in second-harmonic generation (SHG) -- over three orders of magnitude at specific wavelengths and linear polarization directions compared to a host flake.

physics.optics↗

arXiv2024

Defect-assisted reversible phase transition in mono- and few-layer ReS$_2$

Transition metal dichalcogenide (TMD) materials have attracted substantial interest due to their remarkable excitonic, optical, electrical, and mechanical properties, which are highly dependent on their crystal structure. Controlling the crystal structure of these materials is essential for fine-tuning their performance, $\textit{e.g.}$, linear and nonlinear optical, as well as charge transport properties. While various phase-switching TMD materials, like molybdenum telluride (MoTe$_2$), are available, their transitions are often irreversible. Here, we investigate the mechanism of a light-induced reversible phase transition in mono- and bilayer flakes of rhenium disulfide (ReS$_2$). Our observations, based on scanning transmission electron microscopy, nonlinear spectroscopy, and density functional theory calculations, reveal a transition from the ground T$''$ (double distorted T) to the metastable H$'$ (distorted H) phase under femtosecond laser irradiation or influence of highly-energetic electrons. We show that the formation of sulfur vacancies facilitates this phenomenon. Our findings pave the way towards actively manipulating the crystal structure of ReS$_2$ and possibly its heterostructures.

physics.optics↗

arXiv2024

Quantum trapping and rotational self-alignment in triangular Casimir microcavities

Casimir torque -- a rotational motion caused by the minimization of the zero-point energy -- is a problem that attracts significant theoretical and experimental interest. Recently, it has been realized using liquid crystal phases and natural anisotropic substrates. However, for natural materials, the torque reaches substantial values only at van der Waals distances of ~10 nm. Here, we employ Casimir self-assembly using templated gold nanostructures of triangular symmetry for the purpose of rotational self-alignment at truly Casimir distances (100 -- 200 nm separation). The joint action of repulsive electrostatic and attractive Casimir potentials leads to the formation of a stable quantum trap, giving rise to a tunable Fabry-Perot microcavity. This cavity self aligns both laterally and rotationally to maximize the overlap area between the templated and floating triangular flakes. The rotational self-alignment is remarkably sensitive to the equilibrium distance between the two triangles as well as their area, which opens possibilities for active control through manipulating the electrostatic screening. Our self-assembled and self-aligned Casimir microcavities could find future use as a versatile and tunable platform for nanophotonic, polaritonic, and optomechanical applications.

cond-mat.mes-hall↗