arXiv subjects

Masoud Taleb

Publications and source records attributed to Masoud Taleb.

13 recordsLinked to original sources

arXiv2026

Electron beam driven collective self hybridized exciton polaritons

Self-hybridized exciton polaritons in layered semiconductors emerge from the hybridization of excitonic resonances with confined photonic modes in films and provide a versatile platform for controlling light--matter interactions at the nanoscale. Yet, the nature of exciton--photon interactions and the collective and synchronous strong coupling of multiple excitons to the same photonic mode remain largely unexplored. Here, we use complementary momentum-resolved photoluminescence and cathodoluminescence spectroscopy to compare self-hybridized exciton polaritons in Ruddlesden--Popper perovskite flakes under optical and electron-beam excitation. We find that cathodoluminescence exhibits remarkably larger polaritonic level repulsion than photoluminescence, revealing an enhanced effective coupling strength under electron-beam excitation. We attribute this enhancement to the collective coherent excitation of multiple excitons by fast electrons, which couple to the same photonic mode and increase the interaction strength according to a $g_N=g_1\sqrt{N}$ scaling, with an effective number of up to $N\approx23$ coherently coupled excitonic oscillators. By reducing the electron kinetic energy, we further uncover a crossover to a partially incoherent excitation regime in which higher-order Fabry--P\'erot exciton-polariton resonances disappear, leaving only the lowest polariton branches while coherent transition radiation persists. This transition arises because low-energy electrons excite excitons over extended spatial volumes, washing out the phase coherence required to sustain collective coupling and form polaritonic resonances. Our work shows that electron beams can actively modify collective light--matter coupling in layered semiconductors and establishes cathodoluminescence as a route to access excitation regimes beyond all-optical excitation schemes.

quant-ph↗

arXiv2026

Boosting self hybridized exciton polaritons with metal clad WS2 waveguides

The formation of Fabry Perot and guided wave self hybridized exciton polaritons in two dimensional materials results in long range exciton energy transfer and strong exciton exciton interactions. Here, we demonstrate that the coupling strength between photonic modes and excitons is significantly boosted by embedding the active excitonic layer in a metal clad WS2 waveguide. The photonic modes in this waveguide exhibit modified dispersion properties for both Fabry Perot type and guided wave exciton polaritons compared to pure WS2 flakes, and show an increased couplingr strength. Our results provide a robust approach for controlling exciton photon interactions and their coupling strength in hybrid heterostructures.

quant-ph↗

arXiv2026

Plasmon exciton coupling enhances second order nonlinear response in borophene ZnO hybrid structures

Nonlinear optical processes in low dimensional materials are often weak or symmetry forbidden, limiting their use in nanoscale light sources and on chip frequency conversion. Here, we show that combining two weakly nonlinear systems, anisotropic borophene and excitonic zinc oxide, yields an enhanced and resonant nonlinear response. In borophene ZnO heterostructures, cathodoluminescence reveals a two orders of magnitude enhancement at 400 nm and 800 nm, due to an enhanced two photon absorption process. Under tunable near infrared excitation, a clear second harmonic signal emerges with quadratic power dependence and strong resonance near 800 nm. We attribute this to nonlinear plasmon exciton coupling, which reshapes the excitonic response and enables efficient hybrid pathways for frequency conversion. These results establish anisotropic plasmon exciton hybridization as a route to controlling nonlinear optical responses in low dimensional heterostructures.

physics.optics↗

arXiv2025

Exciton Energy Routing via Defect Networks in hBN/2D-Perovskite Hybrids

Excitons in two dimensional Ruddlesden Popper perovskites (RPPs) exhibit large and tunable binding energies, making them promising candidates for optoelectronic applications. In particular, long-range exciton energy transfer in these materi-als holds potential for light-harvesting technologies and nanoscale interconnects. Here, using cathodoluminescence spectros-copy, we demonstrate that exciton energy can be transferred over ultralong distances, up to 150 micrometers, in heterostructures composed of hexagonal boron nitride (hBN) and RPPs. This transfer is enabled by efficient exciton coupling to defect centers in hBN and subsequent defect defect interactions. This mechanism not only facilitates long-range energy transfer, but also leads to enhanced luminescence intensity, narrower emission linewidths, extended exciton lifetimes, and reduced electron-beam-induced degradation. Owing to the high density of emitters within the hBN layers, the investigated van der Waals heterostructure emerges as a robust and stable hybrid platform. Our findings open promising pathways for room-temperature excitonic devices with enhanced performance, including quantum transducers, light-harvesting systems, and optoelectronic interconnects.

cond-mat.mes-hall↗

arXiv2025

Phase-matched electron-photon interactions enabled by 3D-printed helical waveguides

The Smith-Purcell effect enables electromagnetic radiation across arbitrary spectral ranges by phase-matching the diffraction orders of an optical grating with the near-field of a moving electron. In this work, we introduce a novel approach using a helically shaped waveguide, where phase-matching is achieved through guided light within a helical optical fiber fabricated via two-photon polymerization using a 3D printer. Our results demonstrate that radiation from these structures precisely satisfies the phase-matching condition and is emitted directionally at specific angles, contrasting with the broad angular distribution characteristic of the traditional Smith-Purcell effect. Helical electron-driven photon sources establish a new paradigm, enabling 3D-printed structures to control electron-beam-induced radiation and, inversely, to facilitate light-induced efficient electron beam shaping and acceleration.

physics.optics↗

arXiv2024

Two-dimensional borophene: In-plane hyperbolic polaritons in the visible spectral range

Two-dimensional metals, such as graphene, have undergone extensive exploration, with graphene exhibiting a metallic response limited to the infrared spectral range. Overcoming the challenge of extending the electron mobility in two-dimensional metals to achieve plasmonic behaviors in the visible range necessitates innovative synthesis procedures. In this study, we showcase the successful realization of the phase of \c{hi}_3 borophene on diverse substrates using aluminum-based chemical vapor deposition. Leveraging first-principle density-functional theory alongside advanced deep-subwavelength cathodoluminescence spectroscopy, we reveal the extreme anisotropic response of this material in the visible range, transitioning from hyperbolic polaritonic to an elliptic wavefront. Our calculations substantiate the experimental findings, positioning borophene as an unprecedented candidate for the in-plane hyperbolic response in the visible range. These results open avenues for pioneering optoelectronic applications in the visible spectrum, particularly through the incorporation of borophene into hybrid metallic-semiconducting heterostructures.

cond-mat.mes-hall↗

arXiv2024

Ultrafast phonon-mediated dephasing of color centers in hexagonal boron nitride probed by electron beams

Defect centers in hexagonal boron nitride have been extensively studied as room temperature single photon sources. The electronic structure of these defects exhibits strong coupling to phonons, as evidenced by the observation of phonon sidebands in both photoluminescence and cathodoluminescence spectra. However, the dynamics of the electron phonon coupling as well as phonon mediated dephasing of the color centers in hexagonal boron nitride remain unexplored. Here, we apply a novel time resolved CL spectroscopy technique to explore the population decay to phonon states and the dephasing time T2 with sub femtosecond time resolution. We demonstrate an ultrafast dephasing time of only 200 fs and a radiative decay of about 585 fs at room temperature, in contrast with all optical time resolved photoluminescence techniques that report a decay of a few nanoseconds. This behavior is attributed to efficient electron-beam excitation of coherent phonon polaritons in hexagonal boron nitride, resulting in faster dephasing of electronic transitions. Our results demonstrate the capability of our sequential cathodoluminescence spectroscopy technique to probe the ultrafast dephasing time of single emitters in quantum materials with sub femtosecond time resolution, heralding access to quantum path interferences in single emitters coupled to their complex environment.

quant-ph↗

arXiv2024

Cathodoluminescence of quantized energy states at AlGaN/GaN interface

Recent progress in manufacturing high-electron-mobility transistors and optoelectronic devices highlights the necessity of understanding the charge dynamics and its impact on the optical properties inside heterostructures. Herein, we study the optical properties of GaN/AlGaN/GaN heterostructures using cathodoluminescence and photoluminescence spectroscopy. We explore the influence of generated secondary carriers after electron illumination and their penetration depth on the luminescence spectra. Our findings indicate that a higher laser power intensifies the photoluminescence response and establishes Fabry-Perot-like resonances. Furthermore, the intensity of the cathodoluminescence response shows a linear behavior versus the acceleration voltage and the current of electron beams for the yellow luminescence peak. A near-infrared cathodoluminescence peak (740 nm) is observed only when illuminating the sample with high currents that is attributed to the trapping of the secondary electrons within the Schottky barrier and the manipulation of the two-dimensional electron gas and the quantum-confined states within the barrier. Self-consistent Poisson-Schr\"odinger simulations verify this aspect. This research unveils the intricate charge dynamics associated with the interaction of electron beams with heterostructure systems, paving the way for innovative optoelectronic applications in semiconductor devices.

physics.ins-det↗

arXiv2022

Phase-locked photon-electron interaction without a laser

Ultrafast electron-photon spectroscopy in electron microscopes commonly requires ultrafast laser setups. Photoemission from an engineered electron source is used to generate pulsed electrons, interacting with a sample that is excited by the ultrafast laser pulse at a specified time delay. Thus, developing an ultrafast electron microscope demands the exploitation of extrinsic laser excitations and complex synchronization schemes. Here, we present an inverse approach based on cathodoluminescence spectroscopy to introduce internal radiation sources in an electron microscope. Our method is based on a sequential interaction of the electron beam with an electron-driven photon source (EDPHS) and the investigated sample. An electron-driven photon source in an electron microscope generates phase-locked photons that are mutually coherent with the near-field distribution of the swift electron. Due to their different velocities, one can readily change the delay between the photons and electrons arriving at the sample by changing the distance between the EDPHS and the sample. We demonstrate the mutual coherence between the radiations from the EDPHS and the sample by performing interferometry with a combined system of an EDPHS and a WSe2 flake. We assert the mutual frequency and momentum-dependent correlation of the EDPHS and sample radiation, and determine experimentally the degree of mutual coherence of up to 27%. This level of mutual coherence allows us to perform spectral interferometry with an electron microscope. Our method has the advantage of being simple, compact and operating with continuous electron beams. It will open the door to local electron-photon correlation spectroscopy of quantum materials, single photon systems, and coherent exciton-polaritonic samples with nanometric resolution.

cond-mat.mes-hall↗

arXiv2022

Spin-orbit interactions in noncentrosymmetric plasmonic crystals probed by site-selective cathodoluminescence spectroscopy

The study of spin-orbit coupling (SOC) of light is crucial to explore the light-matter interactions in sub-wavelength nanostructures with broken symmetries. In noncentrosymmetric photonic crystals, the SOC results in the splitting of the otherwise degenerate energy bands. Herein, we explore the SOC in a noncentrosymmetric plasmonic crystal, both theoretically and experimentally. Cathodoluminescence (CL) spectroscopy combined with the numerically calculated photonic band structure reveals an energy band splitting that is ascribed to the broken symmetries in the noncentrosymmetric plasmonic crystal. By shifting the impact position of the electron beam throughout a unit cell of the plasmonic crystal, we show that the emergence of the energy band splitting strongly depends on the excitation position of the surface plasmon (SP) waves on the crystal. Moreover, we exploit angle-resolved CL and dark-field polarimetry to demonstrate polarization-dependent scattering of SP waves interacting with the plasmonic crystal. The scattering direction of a given polarization is determined by the transverse spin angular momentum inherently carried by the SP wave, which is in turn locked to the direction of SP propagation. Our study gives insight into the design of novel plasmonic devices with polarization-dependent directionality of the Bloch plasmons. We expect spin-orbit plasmonics will find much more scientific interests and potential applications with the continuous development of nanofabrication methodologies and uncovering new aspects of spin-orbit interactions.

physics.optics↗

arXiv2022

Tailoring the band structure of plexcitonic crystals by strong coupling

Transition-metal dichalcogenides with their exciton-dominated optical behavior emerge as promising materials for realizing strong light-matter interactions in the visible range and at ambient conditions. When these materials are combined with metals, the energy confining ability of plasmon polaritons in metals below the diffraction limit, allows for further enhancing and tailoring the light-matter interaction, due to the formation of plexcitons in hybrid metal-TMDC structures at the interface. Herein, we demonstrate that the coupling between quasi-propagating plasmons in plasmonic crystals and excitons in WSe2, provides a multi-oscillator playground for tailoring the band structure of plasmonic crystal structures and results in emerging flat bands. The cathodoluminescence spectroscopy and angle-resolved measurements combined with the numerically calculated photonic band structure confirm a strong exciton-plasmon coupling, leading to significant changes in the band diagram of the hybrid lattice and the ability to tailor the band diagram via strong coupling. The hybrid plexcitonic crystal structures investigated here sustain optical waves with remarkably low group velocities. These results could be used for designing tunable slow-light structures based on the strong-coupling effect and pave the way toward plexcitonic topological photonic structures.

physics.optics↗

arXiv2021

Strong Interaction of Cherenkov Radiation with Excitons in WSe2 Crystals

The optical responses of semiconducting transition metal dichalcogenides are dominated by excitons. Being able to strongly interact with light and other materials excitations, excitons in semiconductors are prototypes for investigating many-particle and strong-field physics, including exciton-exciton, exciton-photon, and exciton-phonon interactions. Strong exciton-photon interactions, in particular, can lead to the emergence of exciton-polariton hybrid quasiparticles with peculiar characteristics, and a tendency toward macroscopic and spontaneous coherence. Normally, far-field and near-field optical spectroscopy techniques are used to investigate exciton-photon interactions. Here, we demonstrate that the radiation generated by moving electrons in transition metal dichalcogenides, namely Cherenkov radiation, can strongly interact with excitons. We investigate the coherence properties and spectral signatures of exciton-photon interactions in TMDC bulk crystals, using cathodoluminescence spectroscopy. Our findings lay the ground for cathodoluminescence spectroscopy and in particular electron-beam techniques as probes of exciton-polariton spontaneous coherence in semiconductors, beyond the well-known plasmonic investigations.

cond-mat.mes-hall↗

arXiv2021

Charting the Exciton-Polariton Landscape in WSe2 Thin Flakes by Cathodoluminescence Spectroscopy

Semiconducting transition-metal dichalcogenides (TMDCs) provide a fascinating discovery platform for strong light-matter interaction effects in the visible spectrum at ambient conditions. While most of the work has focused on hybridizing excitons with resonant photonic modes of external mirrors, cavities, or nanostructures, intriguingly, TMDC flakes of sub-wavelength thickness can themselves act as nanocavities. Here, we determine the optical response of such freestanding planar waveguides of WSe$_2$, by means of cathodoluminescence spectroscopy. We reveal strong exciton-photon interaction effects that foster long-range propagating exciton-polaritons and enable direct imaging of the energy transfer dynamics originating from cavity-like Fabry-Perot resonances. Furthermore, confinement effects due to discontinuities in the flakes are demonstrated as an efficient means to tailor the exciton-photon coupling strength, along the edges of natural flakes. Our combined experimental and theoretical results provide a deeper understanding of exciton-photon self-hybridization in semiconducting TMDCs and may pave the way to optoelectronic nanocircuits exploiting exciton-photon interaction.

physics.app-ph↗