arXiv subjects

Ganesh Ghimire

Publications and source records attributed to Ganesh Ghimire.

4 recordsLinked to original sources

arXiv2025

Enhanced and directional light emission from two-dimensional excitons using Mie voids

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.

physics.optics↗

arXiv2025

Dielectric permittivity extraction of MoS$_2$ nanoribbons using THz nanoscopy

The nanoscale optical properties of high-quality MoS$_2$ nanoribbons are investigated using THz nanoscopy based on a scattering-type scanning probe. The nanoribbons comprise a multi-layer core, surrounded by monolayer edges. A featureless complex permittivity spectrum covering the range 0.6-1.6 THz is extracted from experimental time-domain measurements through a minimization procedure, adopting an extended finite-dipole model of the probe-sample interaction. Real-space mapping of the nanoribbon reveals variations in the local permittivity down to the instrument-limited resolution, on the order of 30 nm. Clustering analysis statistically identifies regions of lower apparent permittivity that we attribute to a high curvature at the edges of the nanoribbon causing an increase in local material strain or cross-talk in the measured signal with topography-induced measurement artifacts. The core of the nanoribbon contains two regions that follow tightly distributed, but slightly shifted Gaussian statistics in complex permittivity space, with the real part mean of both distributions lying around 5.4 and compatible with literature values of the static permittivity of thin-film MoS$_2$ reported previously. Our results show that the nanoribbons exhibit a modest degree of dielectric variation at the nanoscale that could be explained by heterogeneous doping or variations in the local defect density. We believe that our approach could be useful for the direct real-space measurement of dielectric disorder in other low-dimensional semiconducting material systems.

physics.optics↗

arXiv2023

A High-Performance Quasi-1D MoS$_2$ Nanoribbon Photodetector

Molybdenum disulfide (MoS$_2$) nanoribbons have attracted increased interest due to their properties which can be tailored by tuning their dimensions. Herein, we demonstrate the growth of highly crystalline quasi-one-dimensional (1D)MoS$_2$ nanoribbons and aligned 3D triangular crystals with predominantly 3R or 2H stacking orientation. The synthesis method relies on the reaction between an ultra-thin MoO3-x film grown by Pulsed Laser Deposition (PLD) and NaF in a sulfur-rich environment. The quasi-1D MoS$_2$ nanoribbons can reach several micrometres in length, and feature single-layer (1L) edges aligned with the nanoribbon core, thereby forming a 1L-multilayer (ML) homojunction due to abrupt discontinuity in thickness. The 1L edges of the nanostructures show a pronounced second harmonic generation (SHG) due to the symmetry breaking, in contrast to the centrosymmetric ML structure, which is unsusceptible to the second-order nonlinear process. A pronounced splitting of the Raman spectra was observed in quasi-1D MoS$_2$ nanoribbons arising from distinct contributions from the 2D edges and a multilayer core. Nanoscale imaging reveals the blue-shifted exciton emission of the monolayer nanoribbon compared to the triangular MoS$_2$ counterpart due to built-in local strain and disorder. We further report on a versatile and ultrasensitive photodetector made of a single quasi-1D MoS$_2$ nanoribbon with a maximum responsivity of 872 A/W at the wavelength of 532 nm. The optoelectronic performance of the MoS$_2$ nanoribbon is superior to the previously reported single-nanoribbon photodetectors. Our findings can inspire the design of TMD semiconductors with tunable geometries for efficient optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Bandgap renormalization in monolayer MoS_2 on CsPbBr_3 quantum dot via charge transfer at room temperature

Many-body effect and strong Coulomb interaction in monolayer transition metal dichalcogenides lead to shrink the intrinsic bandgap, originating from the renormalization of electrical/optical bandgap, exciton binding energy, and spin-orbit splitting. This renormalization phenomenon has been commonly observed at low temperature and requires high photon excitation density. Here, we present the augmented bandgap renormalization in monolayer MoS_2 anchored on CsPbBr_3 perovskite quantum dots at room temperature via charge transfer. The amount of electrons significantly transferred from perovskite gives rise to the large plasma screening in MoS_2. The bandgap in heterostructure is red-shifted by 84 meV with minimal pump fluence, the highest bandgap renormalization in monolayer MoS_2 at room temperature, which saturates with further increase of pump fluence. We further find that the magnitude of bandgap renormalization inversely relates to Thomas-Fermi screening length. This provides plenty of room to explore the bandgap renormalization within existing vast libraries of large bandgap van der Waals heterostructure towards practical devices such as solar cells, photodetectors and light-emitting-diodes.

cond-mat.mes-hall↗