arXiv subjects

Sampath Gamage

Publications and source records attributed to Sampath Gamage.

9 recordsLinked to original sources

arXiv2026

Nanoscopy of Excitons in Atomically Thin In-Plane Heterostructures with Nanointerfaces

Atomically sharp 2D in-plane heterostructures with nanoscale interfaces provide a powerful platform for tailoring optical and electrical properties at the nanoscale, enabling novel device engineering and the exploration of new physical phenomena. However, direct experimental correlation between local dielectric response and excitonic properties across such interfaces has remained elusive. Here, we probed the nanoscale complex dielectric function and the corresponding localized photoluminescence (PL) modulations in heterostructure domains of lateral monolayer MoxW1-xS2 - WxMo1-xS2, synthesized using a liquid-phase precursor-assisted approach. Near-field nano imaging across the visible-near-infrared range enables real space mapping of sharp amplitude and phase changes at the heterointerface, resolving the local complex dielectric function with nano-meter scale spatial resolution. Excitation energy-dependent nano spectroscopy reveals a reversal of dielectric contrast between Mo-rich and W-rich domains at their respective excitonic resonances, consistent with Lorentz-oscillator fits. Complementary hyperspectral nano-PL mapping resolves the evolution of excitonic emission across the lateral heterointerface, with neutral-exciton intensities varying continuously from W-rich to Mo-rich regions. Effective-medium theory modeling of the imaginary part of the effective dielectric function of the heterostructure as a function of photon energy and Mo filling fraction reproduces the observed excitonic trends, linking the PL evolution to a composition-dependent dielectric response. Together, these results provide direct nanoscale correlation between dielectric and excitonic boundaries in laterally stitched monolayer heterostructures and establish a multimodal near-field spectroscopy framework for probing excitonic phenomena at the nanoscale.

cond-mat.mtrl-sci↗

arXiv2023

Infrared Nanoimaging of Hydrogenated Perovskite Nickelate Synaptic Devices

Solid-state devices made from correlated oxides such as perovskite nickelates are promising for neuromorphic computing by mimicking biological synaptic function. However, comprehending dopant action at the nanoscale poses a formidable challenge to understanding the elementary mechanisms involved. Here, we perform operando infrared nanoimaging of hydrogen-doped correlated perovskite, neodymium nickel oxide (H-NdNiO3) devices and reveal how an applied field perturbs dopant distribution at the nanoscale. This perturbation leads to stripe phases of varying conductivity perpendicular to the applied field, which define the macroscale electrical characteristics of the devices. Hyperspectral nano-FTIR imaging in conjunction with density functional theory calculations unveil a real-space map of multiple vibrational states of H-NNO associated with OH stretching modes and their dependence on the dopant concentration. Moreover, the localization of excess charges induces an out-of-plane lattice expansion in NNO which was confirmed by in-situ - x-ray diffraction and creates a strain that acts as a barrier against further diffusion. Our results and the techniques presented here hold great potential to the rapidly growing field of memristors and neuromorphic devices wherein nanoscale ion motion is fundamentally responsible for function.

physics.app-ph↗

arXiv2023

Photodegradation and Thermal Effects in Violet Phosphorus

Violet phosphorus (VP) has garnered attention for its appealing physical properties and potential applications in optoelectronics. We present a comprehensive investigation of the photo-degradation and thermal effects of exfoliated VP on SiO2 substrate. The degradation rate of VP was found to be strongly influenced by the excitation wavelength and light exposure duration. Light exposure to the above bandgap ({\lambda} > 532 nm) leads to faster degradation, attributed to interactions with reactive oxygen species. Power-dependent photoluminescence (PL) measurements at low temperature (T=4 K) showed neutral exciton (X0) and trion (T) intensities linearly increased with excitation power, while the energy difference between their peak energies decreased, indicating changes in the exciton energy gap due to degradation. At room temperature X0 and T peaks were observed with higher X0 spectral weight, indicating reduced thermal stability of T. As the temperature decreased to 4 K, both X0 and T emissions intensified with blue-shifted peak positions. The T/X0 spectral weight ratio increased from 0.28 at 300 K to 0.69 at 4 K, suggesting enhanced T formation due to reduced phonon scattering. Temperature-dependent Raman spectroscopy revealed the presence of VP up to 673K. By tracking the peak position of 9 Raman modes with temperature the linear first-order temperature coefficient were obtained and found to be linear for all modes up to 673 K. Our results provide a deeper understanding of VP's degradation behavior and implications for optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2023

Substrate Induced van der Waals Force Effect on the Stability of Violet Phosphorous

The van der Waals (vdWs) forces between monolayers has been a unique distinguishing feature of exfoliable materials since the first isolation of graphene. However, the vdWs interaction of exfoliable materials with their substrates and how this interface force influences their interaction with the environment is yet to be well understood.Here, we experimentally and theoretically unravel the role of vdWs forces between the recently rediscovered wide band gap p-type vdW semiconductor violet phosphorus (VP), with various substrates (including, SiO$_2$, mica, Si, Au) and quantify how VP stability in air and its interaction with its surroundings is influenced by the interface force.Using a combination of infrared nanoimaging and theoretical modeling we find the vdWs force at the interface to be a main factor that influences how VP interacts with its surroundings.In addition, the hydrophobicity of the substrate and the substrate surface roughness modify the vdWs force there by influencing VP stability. Our results could guide in the selection of substrates when vdW materials are prepared and more generally highlight the key role of interface force effects that could significantly alter physical properties of vdWs materials.

cond-mat.mtrl-sci↗

arXiv2021

Redox-tunable structural colour images based on UV-patterned conducting polymers

Precise manipulation of light-matter interaction has enabled a wide variety of approaches to create bright and vivid structural colours. Techniques utilizing photonic crystals, Fabry-Pérot cavities, plasmonics, or high-refractive index dielectric metasurfaces have been studied for applications ranging from optical coatings to reflective displays. However, complicated fabrication procedures for sub-wavelength nanostructures, limited active areas, and inherent absence of tunability with these approaches significantly impede their further developments towards flexible, large-scale, and switchable devices compatible with facile and cost-effective production. Herein, we present a way to generate structural colours based on conducting polymer thin films prepared on metallic surfaces via vapour phase polymerization and ultraviolet (UV) light patterning. Varying the UV dose leads to synergistic variation of film absorption and thickness, which generates controllable colours from violet to red. Together with greyscale photomasks this enables fabrication of high-resolution colour images using single exposure steps. We further demonstrate spatiotemporal tuning of the structurally coloured surfaces and images via electrochemical modulation of the polymer redox state. The simple structure, facile fabrication, wide colour gamut, and dynamic colour tuning make this concept competitive for future multi-functional and smart displays.

physics.optics↗

arXiv2018

Near-field Surface Waves in Few-Layer MoS2

Recently emerged layered transition metal dichalcogenides have attracted great interest due to their intriguing fundamental physical properties and potential applications in optoelectronics. Using scattering-type scanning near-field optical microscope (s-SNOM) and theoretical modeling, we study propagating surface waves in the visible spectral range that are excited at sharp edges of layered transition metal dichalcogenides (TMDC) such as molybdenum disulfide and tungsten diselenide. These surface waves form fringes in s-SNOM measurements. By measuring how the fringes change when the sample is rotated with respect to the incident beam, we obtain evidence that exfoliated MoS2 on a silicon substrate supports two types of Zenneck surface waves that are predicted to exist in materials with large real and imaginary parts of the permittivity. We have compared MoS2 interference fringes with those formed on layered insulator such as hexagonal boron nitride where only leaky modes are possible due to its small permittivity. Interpretation of experimental data is supported by theoretical models. Our results could pave the way to the investigation of surface waves on TMDCs and other van der Waals materials and their novel photonics applications.

cond-mat.mes-hall↗

arXiv2017

Near-field edge fringes at sharp material boundaries

We have studied the formation of near-field fringes when sharp edges of materials are imaged using scattering-type scanning near-field optical microscope (s-SNOM). Materials we have investigated include dielectrics, metals, near-perfect conductor, and those that possess anisotropic permittivity and hyperbolic dispersion. For our theoretical analysis, we use a technique that combines full-wave numerical simulations of tip-sample near-field interaction and signal demodulation at higher orders akin to what is done in typical s-SNOM experiments. Unlike previous tip-sample interaction near-field models, our advanced technique allows simulation of the realistic tip and sample structure. Our analysis clarifies edge imaging of recently emerged layered materials such as hexagonal boron nitride and transition metal dichalcogenides (in particular, molybdenum disulfide), as well as traditional plasmonic materials such as gold. Hexagonal boron nitride is studied at several wavelengths, including the wavelength where it possesses excitation of phonon-polaritons and hyperbolic dispersion. Based on our results of s-SNOM imaging in different demodulation orders, we specify resonant and non-resonant types of edges and describe the edge fringes for each case. We clarify near-field edge-fringe formation at material sharp boundaries, both outside bright fringes and the low-contrast region at the edge, and elaborate on the necessity of separating them from propagating waves on the surface of polaritonic materials.

physics.optics↗

arXiv2015

Nanoscopy Reveals Metallic Black Phosphorus

Layered and two-dimensional (2D) materials such as graphene, boron nitride, transition metal dichalcogenides(TMDCs), and black phosphorus (BP) have intriguing fundamental physical properties and bear promise of numerous important applications in electronics and optics. Of them, BP is a novel 2D material that has been theoretically predicted to acquire plasmonic behavior for frequencies below ~0.4 eV when highly doped. The electronic properties of BP are unique due to an anisotropic structure, which could strongly influence collective electronic excitations. Advantages of BP as a material for nanoelectronics and nanooptics are due to the fact that, in contrast to metals, the free carrier density in it can be dynamically controlled by electrostatic gating, which has been demonstrated by its use in field-effect transistors. Despite all the interest that BP attracts, near-field and plasmonic properties of BP have not yet been investigated experimentally. Here we report the first observation of nanoscopic near-field properties of BP. We have discovered near field patterns of outside bright fringes and high surface polarizability of nanofilm BP consistent with its surface-metallic, plasmonic behavior at mid-infrared (mid-IR) frequencies. This behavior is highly frequency-dispersive, disappearing above frequency, ω =1070 cm-1, which allowed us to estimate the plasma frequency and carrier density. We have also observed similar behavior in other 2D semiconductors such as TMDCs but not in 2D insulators such as boron nitride. This new phenomenon is attributed to surface charging of the semiconductor nanofilms. This discovery opens up a new field of research and potential applications in nanoplasmonics and optoelectronics.

cond-mat.mes-hall↗

arXiv2015

Control of plasmonic nanoantennas by reversible metal-insulator transition

Nanophotonic (nanoplasmonic) structures confine, guide, and concentrate light on the nanoscale. Advancement of nanophotonics critically depends on active nanoscale control of these phenomena. Localized control of the insulator and metallic phases of vanadium dioxide (VO2) would open up a universe of applications in nanophotonics via modulation of the local dielectric environment of nanophotonic structures allowing them to function as active devices. Here we show dynamic reversible control of VO2 insulator-to-metal transition (IMT) locally on the scale of 15 nm or less and control of nanoantennas, observed in the near-field for the first time. Using polarization-selective near-field imaging techniques, we monitor simultaneously the IMT in VO2 and the change of plasmons on gold infrared nanoantennas. Structured nanodomains of the metallic VO2 locally and reversibly transform infrared plasmonic dipolar antennas to monopole antennas. Fundamentally, the IMT in VO2 can be triggered on femtosecond timescale to allow ultrafast nanoscale control of optical phenomena. These unique capabilities open up exciting novel applications in active nanophotonics.

cond-mat.mes-hall↗