arXiv subjects

Farsane Tabataba-Vakili

Publications and source records attributed to Farsane Tabataba-Vakili.

11 recordsLinked to original sources

arXiv2026

Optimizing strong light-matter coupling of plasmonic lattices and monolayer semiconductors

Exciton-polaritons provide a versatile platform for the study of a wide range of phenomena, including polariton lasers, topological polaritons, and bosonic condensation. Transition metal dichalcogenide monolayers host excitons with large oscillator strength and binding energies constituting a robust matter constituent that forms polaritons from cryogenic to room temperature when embedded in optical microcavities. Plasmonic nanoparticles arranged in lattice geometries offer strong field-confinement and high quality factors. However, the high sensitivity of monolayer excitons to strain and dielectric disorder necessitates encapsulation in atomically flat hBN to ensure a high optical quality, rendering plasmonics more challenging. Here, we employ our recently developed fabrication method for embedding gold nanodisk arrays into van der Waals heterostructures and compare two samples with opposite layer order. We observe that strain and etching-induced surface contamination can reduce the exciton quality and thus the light-matter interaction strength significantly. Our fabrication approach reduces interfacial irregularities and enables homogeneous large-area polariton lattices for a wide range of applications, such as polarization-control or topological polaritonics.

cond-mat.mes-hall↗

arXiv2025

Cavity-mediated exciton hopping in a dielectrically engineered polariton system

Exciton-polaritons - coherently hybridized states of excitons and photons - are instrumental for solid-state nonlinear optics and quantum simulations. To enable engineered polariton energy landscapes and interactions, local control over the particle-like states can be achieved by tuning the properties of the exciton constituent. Monolayer transition metal dichalcogenides stand out in this respect, as they readily allow for a deterministic, flexible and scalable control of excitons, and thus of hybrid exciton-polaritons, via environmental dielectric engineering. Here, we demonstrate the realization of mesoscopic exciton-polariton domains in a structured dielectric exciton environment, and establish an effective long-range exciton hopping in the dispersive regime of cavity-coupling. Our results represent a crucial step toward interacting polaritonic networks and quantum simulations in exciton-polariton lattices based on dielectrically tailored two-dimensional semiconductors.

cond-mat.mes-hall↗

arXiv2024

Doping-control of excitons and magnetism in few-layer CrSBr

Magnetism in two-dimensional materials reveals phenomena distinct from bulk magnetic crystals, with sensitivity to charge doping and electric fields in monolayer and bilayer van der Waals magnet CrI3. Within the class of layered magnets, semiconducting CrSBr stands out by featuring stability under ambient conditions, correlating excitons with magnetic order and thus providing strong magnon-exciton coupling, and exhibiting peculiar magneto-optics of exciton-polaritons. Here, we demonstrate that both exciton and magnetic transitions in bilayer and trilayer CrSBr are sensitive to voltage-controlled field-effect charging, exhibiting bound exciton-charge complexes and doping-induced metamagnetic transitions. Moreover, we demonstrate how these unique properties enable optical probes of local magnetic order, visualizing magnetic domains of competing phases across metamagnetic transitions induced by magnetic field or electrostatic doping. Our work identifies few-layer CrSBr as a rich platform for exploring collaborative effects of charge, optical excitations, and magnetism.

cond-mat.mes-hall↗

arXiv2024

Metasurface of strongly coupled excitons and nanoplasmonic arrays

Metasurfaces allow to manipulate light at the nanoscale. Integrating metasurfaces with transition metal dichalcogenide monolayers provides additional functionality to ultrathin optics, including tunable optical properties with enhanced light-matter interactions. In this work, we demonstrate the realization of a polaritonic metasurface utilizing the sizable light-matter coupling of excitons in monolayer WSe2 and the collective lattice resonances of nanoplasmonic gold arrays. To this end, we developed a novel fabrication method to integrate gold nanodisk arrays in hexagonal boron nitride and thus simultaneously ensure spectrally narrow exciton transitions and their immediate proximity to the near-field of array surface lattice resonances. In the regime of strong light-matter coupling, the resulting van der Waals metasurface exhibits all key characteristics of lattice polaritons, with a directional and linearly-polarized far-field emission profile dictated by the underlying nanoplasmonic lattice. Our work can be straightforwardly adapted to other lattice geometries, establishing structured van der Waals metasurfaces as means to engineer polaritonic lattices.

physics.optics↗

arXiv2023

Lattice reconstruction in MoSe$_2$-WSe$_2$ heterobilayers synthesized by chemical vapor deposition

Vertical van der Waals heterostructures of semiconducting transition metal dichalcogenides realize moiré systems with rich correlated electron phases and moiré exciton phenomena. For material combinations with small lattice mismatch and twist angles as in MoSe$_2$-WSe$_2$, however, lattice reconstruction eliminates the canonical moiré pattern and instead gives rise to arrays of periodically reconstructed nanoscale domains and mesoscopically extended areas of one atomic registry. Here, we elucidate the role of atomic reconstruction in MoSe$_2$-WSe$_2$ heterostructures synthesized by chemical vapor deposition. With complementary imaging down to the atomic scale, simulations, and optical spectroscopy methods we identify the coexistence of moiré-type cores and extended moiré-free regions in heterostacks with parallel and antiparallel alignment. Our work highlights the potential of chemical vapor deposition for applications requiring laterally extended heterosystems of one atomic registry or exciton-confining heterostack arrays.

cond-mat.mes-hall↗

arXiv2020

Analysis of low-threshold optically pumped III-nitride microdisk lasers

Low-threshold lasing under pulsed optical pumping is demonstrated at room temperature for III-nitride microdisks with InGaN/GaN quantum wells on Si in the blue spectral range. Thresholds in the range of 18 kW/cm2 have been achieved along with narrow linewidths of 0.07 nm and a large peak to background dynamic of 300. We compare this threshold range with the one that can be calculated using a rate equation model. We show that thresholds in the few kW/cm2 range constitute the best that can be achieved with III-nitride quantum wells at room temperature. The sensitivity of lasing on the fabrication process is also discussed.

physics.app-ph↗

arXiv2020

Monolithic integration of ultraviolet microdisk lasers into photonic circuits in a III-nitride-on-silicon platform

Ultraviolet microdisk lasers are integrated monolithically into photonic circuits using a III-nitride on silicon platform with gallium nitride (GaN) as the main waveguiding layer. The photonic circuits consist of a microdisk and a pulley waveguide terminated by out-coupling gratings. We measure quality factors up to 3500 under continuous-wave excitation. Lasing is observed from 374 nm to 399 nm under pulsed excitation, achieving low threshold energies of $0.14 ~\text{mJ/cm}^2$ per pulse (threshold peak powers of $35 ~\text{kW/cm}^2$). A large peak to background dynamic of around 200 is observed at the out-coupling grating for small gaps of 50 nm between the disk and waveguide. These devices operate at the limit of what can be achieved with GaN in terms of operation wavelength.

physics.app-ph↗

arXiv2019

Demonstration of critical coupling in an active III-nitride microdisk photonic circuit on silicon

On-chip microlaser sources in the blue constitute an important building block for complex integrated photonic circuits on silicon. We have developed photonic circuits operating in the blue spectral range based on microdisks and bus waveguides in III-nitride on silicon. We report on the interplay between microdisk-waveguide coupling and its optical properties. We observe critical coupling and phase matching, i.e. the most efficient energy transfer scheme, for very short gap sizes and thin waveguides (g = 45 nm and w = 170 nm) in the spontaneous emission regime. Whispering gallery mode lasing is demonstrated for a wide range of parameters with a strong dependence of the threshold on the loaded quality factor. We show the dependence and high sensitivity of the output signal on the coupling. Lastly, we observe the impact of processing on the tuning of mode resonances due to the very short coupling distances. Such small footprint on-chip integrated microlasers providing maximum energy transfer into a photonic circuit have important potential applications for visible-light communication and lab-on-chip bio-sensors.

physics.app-ph↗

arXiv2019

Blue Microlasers Integrated on a Photonic Platform on Silicon

The main interest of group-III-nitride nanophotonic circuits is the integration of active structures and laser sources. A photonic platform of group-III-nitride microdisk lasers integrated on silicon and emitting in the blue spectral range is demonstrated. The active microdisks are side-coupled to suspended bus waveguides, and the coupled emission is guided and outcoupled to free space using grating couplers. A small gap size of less than 100 nm between the disk and the waveguide is required in the blue spectral range for optimal evanescent coupling. To avoid reabsorption of the microdisk emission in the waveguide, the quantum wells are etched away from the waveguide. Under continuous-wave excitation, loaded quality factors greater than 2000 are observed for the whispering gallery modes for devices with small gaps and large waveguide bending angles. Under pulsed excitation conditions, lasing is evidenced for 3 $μ$m diameter microdisks integrated in a full photonic circuit. We thus present a first demonstration of a III-nitride microlaser coupled to a nanophotonic circuit.

physics.app-ph↗

arXiv2019

Q factor limitation at short wavelength (around 300 nm) in III-nitride-on-silicon photonic crystal cavities

III-nitride-on-silicon L3 photonic crystal cavities with resonances down to 315 nm and quality factors (Q) up to 1085 at 337 nm have been demonstrated. The reduction of the quality factor with decreasing wavelength is investigated. Besides the quantum well absorption below 340 nm, a noteworthy contribution is attributed to the residual absorption present in thin AlN layers grown on silicon, as measured by spectroscopic ellipsometry. This residual absorption ultimately limits the Q factor to around 2000 at 300 nm when no active layer is present.

physics.app-ph↗

arXiv2019

III-nitride on silicon electrically injected microrings for nanophotonic circuits

Nanophotonic circuits using group III-nitrides on silicon are still lacking one key component: efficient electrical injection. In this paper we demonstrate an electrical injection scheme using a metal microbridge contact in thin III-nitride on silicon mushroom-type microrings that is compatible with integrated nanophotonic circuits with the goal of achieving electrically injected lasing. Using a central buried n-contact to bypass the insulating buffer layers, we are able to underetch the microring, which is essential for maintaining vertical confinement in a thin disk. We demonstrate direct current room-temperature electroluminescence with 440 mW/cm$^2$ output power density at 20 mA from such microrings with diameters of 30 to 50 $μ$m. The first steps towards achieving an integrated photonic circuit are demonstrated.

physics.app-ph↗