Sara Elrafei - Academia.edu (original) (raw)

Papers by Sara Elrafei

Research paper thumbnail of Monolayer Semiconductor Superlattices with High Optical Absorption

ACS photonics, Jun 17, 2024

Research paper thumbnail of Approaching the absorption limit with monolayer semiconductor superlattices

arXiv (Cornell University), Feb 15, 2024

Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly abs... more Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly absorbing materials are thus needed for efficient and miniaturized devices. There exists, however, a fundamental limit of 50% absorptance for any ultrathin film in a symmetric environment. Although deviating from these conditions allows higher absorption, finding the thinnest possible material with the highest intrinsic absorption is still desirable. Here, we demonstrate strong absorption approaching the fundamental limit by artificially stacking WS 2 monolayers into superlattices. We compare three simple approaches based on different spacer materials to surpass the record peak absorptance of WS 2 monolayers, which stands at 16% on ideal substrates. Through direct monolayer stacking without an intentional spacer, we reach a transmittance contrast of 30% for an artificial bilayer, although with limited control over interlayer distance. Using a molecular spacer via spin coating, we demonstrate controllable spacer thickness in a bilayer, reaching 28% transmittance contrast while increasing photoluminescence thanks to doping. Finally, we exploit atomic layer deposition of alumina spacers to boost the transmittance contrast to 36% for a 4-monolayer superlattice. Our results demonstrate that monolayer superlattices are a powerful platform directly applicable to improve exciton-polariton phenomena such as strong light-matter coupling and nanophotonic devices such as modulators and photodetectors.

Research paper thumbnail of Guiding light with surface exciton-polaritons in atomically thin superlattices

Nanophotonics IX

Two-dimensional materials give access to the ultimate physical limits of photonics with appealing... more Two-dimensional materials give access to the ultimate physical limits of photonics with appealing properties for ultracompact optical components such as waveguides and modulators. Specifically, in monolayer semiconductors, a strong excitonic resonance leads to a sharp oscillation in permittivity from positive to even negative values. This extreme optical response enables surface exciton-polaritons to guide visible light bound to an atomically thin layer. However, such ultrathin waveguides support a transverse electric (TE) mode with low confinement and a transverse magnetic (TM) mode with short propagation. Here, we propose that realistic semiconductor-insulator-semiconductor superlattices comprising monolayer WS 2 and hexagonal boron nitride (hBN) can improve the properties of both TE and TM modes. Compared to a single monolayer, a heterostructure with a 1-nm hBN spacer separating two monolayers enhances the confinement of the TE mode from 1.2 to around 0.5 μm, while the out-of-plane extension of the TM mode increases from 25 to 50 nm. We propose two simple additivity rules for mode confinement valid in the ultrathin film approximation for heterostructures with increasing spacer thickness. Stack

Research paper thumbnail of Developing optical up-conversion process via embedded silver nanostructures

Journal of Luminescence, 2021

Research paper thumbnail of Impact of plasmonic nanoparticles on up-conversion luminescence and efficiency of erbium-doped ceria nanoparticles under 780 nm excitation

Journal of Luminescence, 2018

Up-conversion process is being widely studied due to its enormous applications such as bioimaging... more Up-conversion process is being widely studied due to its enormous applications such as bioimaging, energy harvesting, and optical sensors. However, the low conversion efficiency can limit these applications. In this paper, a detailed study about the effect of using plasmonic metallic nanostructures is presented, aiming to support the up-conversion process under 780 nm excitation. The targeted material is erbium-doped ceria material due to its promising optical properties along with the relatively low-phonon nature of ceria host. At an average distance of 0.67 nm between Er ions, the efficiency can be improved from 1.58% to 13.52% for green emission and from 0.007% to 0.234% for the red one, which corresponds to 30-fold and 8.5-fold, respectively. Also, some other parameters such as irradiance and multi-phonon relaxations and their impact on the efficiency, according to the added gold nanoparticles are studied. Such promising modeling results could help in further studies regarding t...

Research paper thumbnail of Temperature impact on upconversion efficiency and luminescence of Erbium-doped ceria-plasmonic nanostructure

Ceria nanoparticles have been proved to be one of the most promising optical conversion host stru... more Ceria nanoparticles have been proved to be one of the most promising optical conversion host structures, due to its low-phonon nature and non-stoichiometric structure. In up-conversion, erbium has been extensively used as the main optical center for converting low-photon energy into higher ones. This paper studies the effect of introducing plasmonic nanostructure, such as gold, for enhancing the optical upconversion quantum yield efficiency of erbium-doped-ceria nanoparticles. The numerical results show that the efficiency experienced a significant enhancement as a result of existence of metal nanostructures. In addition, the temperature influence upon the nanocomposite is studied in detail. The numerical calculations show that the temperature change has a remarkable influence on the luminescence parameters and quantum yield efficiency of the up-conversion structure.

Research paper thumbnail of Efficiency improvement of up-conversion process of plasmonic-enhanced Er-doped-NaYF4 nanoparticles under IR excitation

Optics Express, Sep 14, 2018

The up-conversion process is extensively studied because of its wide variety of applications such... more The up-conversion process is extensively studied because of its wide variety of applications such as bioimaging, energy harvesting, and optical sensors. However, the optical conversion efficiency is still relatively low and needs to be improved. Therefore, this paper introduces a detailed study of improving the up-conversion emission efficiency through adding plasmonic metallic nanostructures to the up-conversion optical centers. Our idea is to couple the optical plasmonic resonance with the visible emission of the optical centers under IR excitation. The optical centers are erbium ions hosted by fluoride low-phonon environment. Our calculations consider most possible transitions that can occur between the optical centers; tri-valent erbium ions, through Judd-Ofelt analysis. In addition, the effect of changing some parametric values is discussed, such as irradiance, and multi-phonon relaxations, to show their optimum values which correspond to best quantum yield efficiency. By increasing the diameter of added gold nanoparticles (Au NPs), the probability of occupation has been increased, and consequently, both the luminescence and up-conversion efficiency have been increased.

Research paper thumbnail of Monolayer Semiconductor Superlattices with High Optical Absorption

ACS photonics, Jun 17, 2024

Research paper thumbnail of Approaching the absorption limit with monolayer semiconductor superlattices

arXiv (Cornell University), Feb 15, 2024

Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly abs... more Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly absorbing materials are thus needed for efficient and miniaturized devices. There exists, however, a fundamental limit of 50% absorptance for any ultrathin film in a symmetric environment. Although deviating from these conditions allows higher absorption, finding the thinnest possible material with the highest intrinsic absorption is still desirable. Here, we demonstrate strong absorption approaching the fundamental limit by artificially stacking WS 2 monolayers into superlattices. We compare three simple approaches based on different spacer materials to surpass the record peak absorptance of WS 2 monolayers, which stands at 16% on ideal substrates. Through direct monolayer stacking without an intentional spacer, we reach a transmittance contrast of 30% for an artificial bilayer, although with limited control over interlayer distance. Using a molecular spacer via spin coating, we demonstrate controllable spacer thickness in a bilayer, reaching 28% transmittance contrast while increasing photoluminescence thanks to doping. Finally, we exploit atomic layer deposition of alumina spacers to boost the transmittance contrast to 36% for a 4-monolayer superlattice. Our results demonstrate that monolayer superlattices are a powerful platform directly applicable to improve exciton-polariton phenomena such as strong light-matter coupling and nanophotonic devices such as modulators and photodetectors.

Research paper thumbnail of Guiding light with surface exciton-polaritons in atomically thin superlattices

Nanophotonics IX

Two-dimensional materials give access to the ultimate physical limits of photonics with appealing... more Two-dimensional materials give access to the ultimate physical limits of photonics with appealing properties for ultracompact optical components such as waveguides and modulators. Specifically, in monolayer semiconductors, a strong excitonic resonance leads to a sharp oscillation in permittivity from positive to even negative values. This extreme optical response enables surface exciton-polaritons to guide visible light bound to an atomically thin layer. However, such ultrathin waveguides support a transverse electric (TE) mode with low confinement and a transverse magnetic (TM) mode with short propagation. Here, we propose that realistic semiconductor-insulator-semiconductor superlattices comprising monolayer WS 2 and hexagonal boron nitride (hBN) can improve the properties of both TE and TM modes. Compared to a single monolayer, a heterostructure with a 1-nm hBN spacer separating two monolayers enhances the confinement of the TE mode from 1.2 to around 0.5 μm, while the out-of-plane extension of the TM mode increases from 25 to 50 nm. We propose two simple additivity rules for mode confinement valid in the ultrathin film approximation for heterostructures with increasing spacer thickness. Stack

Research paper thumbnail of Developing optical up-conversion process via embedded silver nanostructures

Journal of Luminescence, 2021

Research paper thumbnail of Impact of plasmonic nanoparticles on up-conversion luminescence and efficiency of erbium-doped ceria nanoparticles under 780 nm excitation

Journal of Luminescence, 2018

Up-conversion process is being widely studied due to its enormous applications such as bioimaging... more Up-conversion process is being widely studied due to its enormous applications such as bioimaging, energy harvesting, and optical sensors. However, the low conversion efficiency can limit these applications. In this paper, a detailed study about the effect of using plasmonic metallic nanostructures is presented, aiming to support the up-conversion process under 780 nm excitation. The targeted material is erbium-doped ceria material due to its promising optical properties along with the relatively low-phonon nature of ceria host. At an average distance of 0.67 nm between Er ions, the efficiency can be improved from 1.58% to 13.52% for green emission and from 0.007% to 0.234% for the red one, which corresponds to 30-fold and 8.5-fold, respectively. Also, some other parameters such as irradiance and multi-phonon relaxations and their impact on the efficiency, according to the added gold nanoparticles are studied. Such promising modeling results could help in further studies regarding t...

Research paper thumbnail of Temperature impact on upconversion efficiency and luminescence of Erbium-doped ceria-plasmonic nanostructure

Ceria nanoparticles have been proved to be one of the most promising optical conversion host stru... more Ceria nanoparticles have been proved to be one of the most promising optical conversion host structures, due to its low-phonon nature and non-stoichiometric structure. In up-conversion, erbium has been extensively used as the main optical center for converting low-photon energy into higher ones. This paper studies the effect of introducing plasmonic nanostructure, such as gold, for enhancing the optical upconversion quantum yield efficiency of erbium-doped-ceria nanoparticles. The numerical results show that the efficiency experienced a significant enhancement as a result of existence of metal nanostructures. In addition, the temperature influence upon the nanocomposite is studied in detail. The numerical calculations show that the temperature change has a remarkable influence on the luminescence parameters and quantum yield efficiency of the up-conversion structure.

Research paper thumbnail of Efficiency improvement of up-conversion process of plasmonic-enhanced Er-doped-NaYF4 nanoparticles under IR excitation

Optics Express, Sep 14, 2018

The up-conversion process is extensively studied because of its wide variety of applications such... more The up-conversion process is extensively studied because of its wide variety of applications such as bioimaging, energy harvesting, and optical sensors. However, the optical conversion efficiency is still relatively low and needs to be improved. Therefore, this paper introduces a detailed study of improving the up-conversion emission efficiency through adding plasmonic metallic nanostructures to the up-conversion optical centers. Our idea is to couple the optical plasmonic resonance with the visible emission of the optical centers under IR excitation. The optical centers are erbium ions hosted by fluoride low-phonon environment. Our calculations consider most possible transitions that can occur between the optical centers; tri-valent erbium ions, through Judd-Ofelt analysis. In addition, the effect of changing some parametric values is discussed, such as irradiance, and multi-phonon relaxations, to show their optimum values which correspond to best quantum yield efficiency. By increasing the diameter of added gold nanoparticles (Au NPs), the probability of occupation has been increased, and consequently, both the luminescence and up-conversion efficiency have been increased.