arXiv subjects

Edmund J. R. Kelleher

Publications and source records attributed to Edmund J. R. Kelleher.

6 recordsLinked to original sources

arXiv2026

Terahertz s-SNOM reveals nonlocal nanoscale conductivity of graphene

As photonic and electronic technologies approach nanometre length scales and terahertz operating speeds, electrical conductivity can no longer be treated as a purely local material parameter. In this regime, charge transport becomes intrinsically nonlocal, with conductivity depending on both frequency and momentum, $σ(ω,q)$, fundamentally limiting field confinement, dispersion, and loss in nanoscale devices. Here, we directly measure the nonlocal nanoscale conductivity of graphene using terahertz scattering-type near-field optical microscopy. By combining broadband THz near-field spectroscopy with quantitative electrodynamic modelling, we extract the complex conductivity of single- and few-layer graphene with $\sim$50 nm spatial resolution. We find that nonlocal response dominates the terahertz conductivity of monolayer graphene even at length scales comparable to practical device dimensions. These results establish nonlocal conductivity as a measurable and design-relevant material property in the terahertz regime, providing a quantitative foundation for predicting performance limits in ultracompact photonic and electronic systems.

physics.optics↗

arXiv2025

Ultrafast control of coherent acoustic lattice dynamics in the transition metal dichalcogenide alloy WSSe

Coherent acoustic phonons (CAPs)$-$propagating strain waves that can dynamically modify the structure and symmetry of a crystal$-$offer unique opportunities for controlling material properties. We investigate CAP generation in the Janus-like layered alloy tungsten sulfide selenide (WS$_x$Se$_{1-x}$, hereafter WSSe). Employing high-fluence photoexcitation at 400 nm combined with ultrafast transient reflection spectroscopy, we capture the carrier-lattice dynamics governed by a cascade of processes including rapid exciton formation, phonon recycling, and thermoelastic deformation. These phenomena precede the emergence of a robust CAP mode at 27 GHz. Notably, the CAP amplitude in WSSe substantially exceeds that observed in the symmetric parent crystals WS$_2$ and WSe$_2$, which we attribute to an enhanced coupling mediated by a built-in out-of-plane electric field arising from the inversion asymmetry of the WSSe alloy. Furthermore, the implementation of a tailored two-pulse excitation sequence enables optical control of the CAP, underscoring the potential of WSSe and related Janus-like layered alloys as versatile building blocks in optomechanical and nanoacoustic device applications.

cond-mat.mtrl-sci↗

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

Towards Intense Ultra-Broadband High Repetition Rate Terahertz Sources Based on Organic Crystals [Invited]

Increasing the average power of broadband, few-cycle terahertz (THz) sources is currently a topic of intense investigation, fueled by recent immense progress in high average power femtosecond laser driving sources at 1030 nm. However, many crucial applications would benefit not only from an increase in average power, but also from ultra-broad bandwidth, while maintaining high dynamic range at these frequencies. This calls for the challenging combination of high repetition rates and high average power simultaneously. Here, we discuss the recent progress in the promising approach enabled by organic crystals for THz-generation. Specifically, this review article discusses advances with the most commonly used organic crystals BNA, DAST, DSTMS, OH1 and HMQ-TMS. We place special emphasis on nonlinear and thermal properties and discuss future directions for this field.

physics.optics↗

arXiv2016

Theory of edge-state optical absorption in two-dimensional transition metal dichalcogenide flakes

We develop an analytical model to describe sub-bandgap optical absorption in two-dimensional semiconducting transition metal dichalcogenide (s-TMD) nanoflakes. The material system represents an array of few-layer molybdenum disulfide crystals, randomly orientated in a polymer matrix. We propose that optical absorption involves direct transitions between electronic edge-states and bulk-bands, depends strongly on the carrier population, and is saturable with sufficient fluence. For excitation energies above half the bandgap, the excess energy is absorbed by the edge-state electrons, elevating their effective temperature. Our analytical expressions for the linear and nonlinear absorption could prove useful tools in the design of practical photonic devices based on s-TMDs.

cond-mat.mes-hall↗

arXiv2015

Surfactant-aided exfoliation of molydenum disulphide for ultrafast pulse generation through edge-state saturable absorption

We use liquid phase exfoliation to produce dispersions of molybdenum disulphide (MoS2) nanoflakes in aqueous surfactant solutions. The chemical structures of the bile salt surfactants play a crucial role in the exfoliation and stabilization of MoS2. The resultant MoS2 dispersions are heavily enriched in single and few (<6) layer flakes with large edge to surface area ratio. We use the dispersions to fabricate free-standing polymer composite wide-band saturable absorbers to develop mode-locked and Q- switched fibre lasers, tunable from 1535-1565 and 1030-1070 nm, respectively. We attribute this sub-bandgap optical absorption and its nonlinear saturation behaviour to edge-mediated states introduced within the material band-gap of the exfoliated MoS2 nanoflakes.

cond-mat.mtrl-sci↗