arXiv subjects

A. Louise Bradley

Publications and source records attributed to A. Louise Bradley.

15 recordsLinked to original sources

arXiv2025

Integration of quantum dots at the tips of single plasmonic bipyramid nanoantennas for strong coupling at room temperature

Achieving strong coupling between excitons of colloidal semiconductor quantum dots (QDs) and localized surface plasmon polaritons (LSPs) is critical for advanced room-temperature quantum emitter and sensing applications. A key challenge is to have precise control of the emitters position with respect to an individual plasmonic nanostructure. Here, we present room temperature strong coupling between QDs and a single gold nano-bipyramid (BPs). The selection of the bipyramid plasmonic nanocavity offers access to a single hotspot with a very small mode volume. The localization of QDs at a single hotspot is achieved via plasmon-triggered two-photon polymerization. This technique exploits the enhanced electric field at the BP tip to selectively polymerize a photosensitive QD-containing formulation. Room-temperature scattering spectra of a 3-QD-BP system reveal Rabi splitting of 349.3 meV and a coupling strength of 175.68 meV. The with distinct anti-crossing behavior is confirmed by simulations. This approach simplifies QD integration for strong coupling systems compared to previous methods. These results indicate a scalable platform for solid-state quantum technologies with colloidal QDs, enabling explora-tion of exciton-plasmon interactions and further advance-ment of applications in quantum optics and quantum sensing under ambient conditions.

quant-ph↗

arXiv2024

Friedrich-Wintgen bound states in the continuum in dimerized dielectric metasurfaces

Bound states in the continuum (BIC) are trapped eigenmodes with infinite $Q$ factors that are confined in the system. In this work, we propose a simple design for engineering a Friedrich-Wintgen BIC through the interference between a symmetry protected BIC and a surface lattice mode in a dimerized dielectric metasurface. The meta-atoms are comprised a symmetric double bar dimer. Using incident angle tuning an avoided crossing between the symmetry protected BIC and surface lattice mode is observed. At a specific detuning, the lower energy resonance vanishes, resulting in the formation of a Friedrich-Wintgen BIC. Investigations using coupled mode theory elucidate the role of the damping rate and coupling strength in the formation of the Friedrich-Wintgen BIC in the dimerized bar metasurface. By tuning the spacing between the two bars, Friedrich-Wintgen BIC can be engineered with controlled energy and damping rate. It is shown that the damping rate of the coupled modes can be considerably suppressed in this system. We envision that these results will not only enhance the understanding of strongly coupled interactions in metasurfaces but also indicate new paths for actively and passively controlling metasurface resonators for photonic applications.

physics.optics↗

arXiv2023

Supercavity Modes in Stacked Identical Mie-resonant Metasurfaces

Modes with a high-$Q$ factor are crucial for photonic metadevices with advanced functionalities. In sharp contrast to recent techniques which generate a supercavity mode by bound states in the continuum via symmetry breaking, we reveal a general and new route, by stacking two parallel and identical Mie-resonant metasurfaces with an air separation. The supercavity mode can be designed by the established theoretical model by overlapping Mie resonant modes with tailor-made Fabry-Pérot modes. The simplified system, with free-space field concentration which can exist in plane as well as out of plane of the metasurfaces, provides for ease of integration with added matter, creating exciting different opportunities for the study of fundamental light-matter coupling. This work deepens our understanding of light manipulation using metasurfaces. It paves a different and general route for generating supercavity modes which can be easily engineered and controlled for different applications of all dielectric metaoptics.

physics.optics↗

arXiv2022

Electrically driven reprogrammable vanadium dioxide metasurface using binary control for broadband beam-steering

Resonant optical phased arrays are a promising way to reach fully reconfigurable metasurfaces in the optical and NIR regimes with low energy consumption, low footprint and high reliability. Continuously tunable resonant structures suffer from inherent drawbacks such as low phase range, amplitude-phase correlation or extreme sensitivity that makes precise control at the individual element level very challenging. In order to bypass these issues, we use 1-bit (binary) control for beam steering for an innovative nano-resonator antenna and explore the theoretical capabilities of such phased arrays. A thermally realistic structure based on vanadium dioxide sandwiched in a metal-insulator-metal structure is proposed and optimized using inverse design to enhance its performance at 1550 nm. Continuous beam steering over 90° range is successfully achieved using binary control, with excellent agreement with predictions based on the theoretical first principles description of phased arrays. Furthermore a broadband response from 1500 nm to 1700 nm is achieved. The robustness of the design manufacturing imperfections is also demonstrated. This simplified approach can be implemented to optimize tunable nanophotonic phased array metasurfaces based on other materials or phased shifting mechanisms for various functionalities.

physics.optics↗

arXiv2022

Polaritonic Critical Coupling in a Hybrid Quasi-Bound States in the Continuum Cavity-WS$_2$ Monolayer System

We theoretically propose and numerically demonstrate that perfect feeding of a polaritonic system with full electromagnetic energy under one-port beam incidence, referred to as polaritonic critical coupling, can be achieved in a hybrid dielectric metasurface-WS$_2$ monolayer structure. Polaritonic critical coupling, where the critical coupling and strong coupling are simultaneously attained, is determined by the relative damping rates of the cavity resonance, $\rm γ_Q$, provided by a symmetry-protected quasi-bound states in the continuum, and excitonic resonance of WS$_2$ monolayer, $\rm γ_X$. We reveal that the population of the polariton states can be tuned by the asymmetric parameter of the quasi-bound states in the continuum. Furthermore, polaritonic critical coupling is achieved in the designed system while $\rm γ_Q=γ_X$ and only strong coupling is achieved while $\rm γ_Q\neqγ_X$. This work enriches the study of polaritonic physics with controlled absorbance and may guide the design and application of efficient polariton-based light-emitting or lasing devices.

physics.optics↗

arXiv2022

Absorbance Enhancement of Monolayer MoS$_2$ in a Perfect Absorbing System

We reveal numerically and experimentally that dielectric resonance can enhance the absorbance and emission of monolayer MoS$_2$. By quantifying the absorbance of the Si disk resonators and the monolayer MoS$_2$ separately, a model taking into account of absorbance as well as quantum efficiency modifications by the dielectric disk resonators successfully explains the observed emission enhancement under the normal light incidence. It is demonstrated that the experimentally observed emission enhancement at different pump wavelength results from the absorbance enhancement, which compensates the emission quenching by the disk resonators. In order to further maximize the absorbance value of monolayer MoS$_2$, a perfect absorbing structure is proposed. By placing a Au mirror beneath the Si nanodisks, the incident electromagnetic power is fully absorbed by the hybrid monolayer MoS$_2$-disk system. It is demonstrated that the electromagnetic power is re-distributed within the hybrid structure and 53\% of the total power is absorbed by the monolayer MoS$_2$ at the perfect absorbing wavelength.

physics.optics↗

arXiv2021

Ultra-Spatiotemporal Light Confinement in Dielectric Nanocavity Metasurfaces

Light concentration with strong temporal and spatial confinement is crucial for tailoring light-matter interaction. Electromagnetic cavity modes in photonic and plasmonic resonators provide platforms for optical field localization. Here, we propose a concept of quasi-bound states in the continuum gap cavity and reveal that ultra spatiotemporal confinements in free-space can be realized in a dielectric nanocavity metasurface. By introducing an asymmetric air slot in a nanodisk resonator, an ultra-high quality factor $\rm Q \sim 10^6$, accompanying an ultra-small effective mode volume, $\rm V_m \sim 10^{-2}$ $(λ/n)^3$ are achieved resulting in a Purcell factor of $\rm 10^6 (λ/n)^{-3}$ in the visible wavelength range. The toroidal dipole drives the electric and magnetic field concentration in the air gap with a generated vortex polarizing electric field. As an alternative to plasmonic and photonic crystal cavities, our study provides a more intriguing platform for engineering light-matter interaction to advance a plethora of fundamental studies and device applications, such as Purcell factor enhancement, room temperature strong coupling and nonlinear nanophotoncis.

physics.optics↗

arXiv2020

Constructive and Destructive Interference of Kerker-type Scattering in an Ultra-thin Silicon Huygens Metasurface

High refractive index dielectric nanoparticles have provided a new platform for exotic light manipulation through the interference of multipole modes. The Kerker effect is one example of a Huygens source design. Rather than exploiting interference between the electric dipole and magnetic dipole, as in many conventional Huygens source designs, we explore Kerker-type suppressed backward scattering mediated by the dominant electric dipole, toroidal dipole and magnetic quadrupole. These modes are provided by a designed and fabricated CMOS compatible ultra-thin Silicon nanodisk metasurface with a suppressed magnetic dipole contribution, and verified through multipole decomposition. The non-trivial substrate effect is considered using a semi-analytical transfer matrix model. The model successfully predicts the observed reflection dip. By applying a general criterion for constructive and destructive interference, it is shown that while constructive interference occurs between the electric and toroidal dipole contributions, the experimentally observed suppressed backward Kerker-type scattering arises from the destructive interference between backward scattered contributions due to the total electric dipole and the magnetic quadrupole. Our study paves the way towards new types of Huygens sources or metasurface design, such as for peculiar transverse Kerker scattering.

physics.optics↗

arXiv2017

Oxide-mediated self-limiting recovery of field effect mobility in plasma-treated MoS$_2$

Precise tunability of electronic properties of 2D nanomaterials is a key goal of current research in this field of materials science. Chemical modification of layered transition metal dichalcogenides leads to the creation of heterostructures of low-dimensional variants of these materials. In particular, the effect of oxygen-containing plasma treatment on molybdenum disulfide (MoS$_2$) has long been thought to be detrimental to the electrical performance of the material. Here we show that the mobility and conductivity of MoS$_2$ can be precisely controlled and improved by systematic exposure to oxygen:argon plasma, and characterise the material utilising advanced spectroscopy and microscopy. Through complementary theoretical modelling which confirms conductivity enhancement, we uncover the role of a two-dimensional phase of molybdenum trioxide (2D-MoO$_3$) in improving the electronic behaviour of the material. Deduction of the beneficial role of MoO$_3$ will serve to open the field to new approaches with regard to the tunability of 2D semiconductors by their low-dimensional oxides in nano-modified heterostructures.

cond-mat.mtrl-sci↗

arXiv2016

Influence of Plasmonic Array Geometry on Energy Transfer from a Quantum Well to a Quantum Dot Layer

A range of seven different Ag plasmonic arrays formed using nanostructures of varying shape, size and gap were fabricated using helium-ion lithography (HIL) on an InGaN/GaN quantum well (QW) substrate. The influence of the array geometry on plasmon-enhanced Förster resonance energy transfer (FRET) from a single InGaN QW to a ~ 80 nm layer of CdSe/ZnS quantum dots (QDs) embedded in a poly(methyl methacrylate) (PMMA) matrix is investigated. It is shown that the energy transfer efficiency is strongly dependent on the array properties and an efficiency of ~ 51% is observed for a nanoring array. There were no signatures of FRET in the absence of the arrays. The QD acceptor layer emission is highly sensitive to the array geometry. A model was developed to confirm that the increase in the QD emission on the QW substrate compared with a GaN substrate can be attributed solely to plasmon-enhanced FRET. The individual contributions of direct enhancement of the QD layer emission by the array and the plasmon-enhanced FRET are separated out, with the QD emission described by the product of an array emission factor and an energy transfer factor. It is shown that while the nanoring geometry results in an energy transfer factor of ~ 1.7 the competing quenching by the array, with an array emission factor of ~ 0.7, results in only an overall gain of ~ 14% in the QD emission. The QD emission was enhanced by ~ 71% for a nanobox array, resulting from the combination of a more modest energy transfer factor of 1.2 coupled with an array emission factor of ~ 1.4.

physics.optics↗

arXiv2016

Temperature Dependent Luminescent Decay Properties of CdTe Quantum Dot Monolayers: Impact of Concentration on Carrier Trapping

We have investigated the temperature dependence of the photoluminescence spectra and average photoluminescence decay rate of CdTe quantum dot monolayers of different sizes as a function of concentration in the range 77 K to 296 K. It is shown that a simple three level analytic model involving bright and dark exciton states can only describe the lower temperature data but is unable to satisfactorily fit the data over the full temperature range. An extended model which includes external trap states is necessary to fit the data above approximately 150 K. Parameters for the model are obtained using both temporal and spectral data. The model indicates that the efficiency of interaction with trap states increases as the QD monolayer concentration increases, which is likely due to an increase in the density of available traps.

cond-mat.mes-hall↗

arXiv2016

Ag colloids and arrays for plasmonic non-radiative energy transfer from quantum dots to a quantum well

Ag nanoparticles in the form of colloids and ordered arrays are used to demonstrate plasmon-mediated non-radiative energy transfer from quantum dots to quantum wells with varying top barrier thicknesses. Plasmon-mediated energy transfer efficiencies of up to ~25% are observed with the Ag colloids. The distance dependence of the plasmon-mediated energy transfer is found to follow the same d^{-4} dependence as the direct quantum dot to quantum well energy transfer. There is also evidence for an increase in the characteristic distance of the interaction, thus indicating that it follows a Förster-like model with the Ag nanoparticle-quantum dot acting as an enhanced donor dipole. Ordered Ag nanoparticle arrays display plasmon-mediated energy transfer efficiencies up to ~21%. To explore the tunability of the array system, two arrays with different geometries are presented. It is demonstrated that changing the geometry of the array allows a transition from overall quenching of the acceptor quantum well emission to enhancement, as well as control of the competition between the quantum dot donor quenching and energy transfer rates.

physics.optics↗

arXiv2016

Near-field relaxation of a quantum emitter to 2D semiconductors: surface dissipation and exciton polaritons

The total spontaneous emission rate of a quantum emitter in the presence of an infinite MoS\textsubscript{2} monolayer is enhanced by several orders of magnitude, compared to its free-space value, due to the excitation of surface exciton polariton modes and lossy modes. The spectral and distance dependence of the spontaneous emission rate are analyzed and the lossy-surface-wave, surface exciton polariton mode and radiative contributions are identified. The transverse magnetic and transverse electric exciton polariton modes can be excited for different emission frequencies of the quantum emitter, and their contributions to the total spontaneous emission rate are different. To calculate these different decay rates, we use the non-Hermitian description of light-matter interactions, employing a Green's tensor formalism. The distance dependence follows different trends depending on the emission energy of quantum emitter. For the case of the lossy surface waves, the distance dependence follows a $z^{-n}$, $n=2,3,4$, trend. When transverse magnetic exciton polariton modes are excited, they dominate and characterize the distance dependence of the spontaneous emission rate of a quantum emitter in the presence of the MoS\textsubscript{2} layers. The interaction between a quantum emitter and a MoS\textsubscript{2} superlattice is investigated and we observe a splitting of the modes supported by the superlattice. Moreover, a blue shift of the peak values of the spontaneous emission rate of a quantum emitter is observed as the number of layers is increased. The field distribution profiles, created by a quantum emitter, are used to explain this behavior.

physics.optics↗

arXiv2015

A Theoretical Investigation of Decay and Energy Transfer Rates and Efficiencies Near Gold Nanospheres

We consider the effect of Au nanospheres of subwavelength sizes on the decay and energy transfer rates of quantum systems placed in the proximity of these nanospheres. We find that, for the sphere sizes considered in this contribution, the radiative decay rate is barely affected by the presence of the nanosphere, whereas the non-radiative decay rate is greatly enhanced due to energy transfer from the quantum system to the nanosphere, leading to a strong quenching of the emission of the quantum system. The emission wavelength of the quantum emitter and its intrinsic quantum yield play an important role and the impact of both has to be considered together when investigating their effect on the non-radiative decay rate. The energy transfer process from the emitter to the nanosphere has a complicated distance dependence, with a 1/r^6 regime, characteristic of the Förster energy transfer mechanism, but also exhibiting other distance dependence regimes. In the case of a donor-acceptor pair in the presence of a Au nanosphere, the donor couples strongly to the nanosphere, acting as an enhanced dipole; the donor-acceptor energy transfer rate then follows a Förster trend, with an increased Förster radius. The angular dependence of the energy transfer efficiency between donor and acceptor has a strong dipole-dipole trend for small spheres and deviates from it for larger spheres, especially when the donor and acceptor are on opposite sides of the sphere. The spectral overlap of the donor emission, acceptor absorption and gold nanosphere extinction shows an interesting trend in that the largest Förster radius is obtained when the donor emission and acceptor absorption maxima are somewhat red-shifted from the localized surface plasmon peak in the extinction spectrum of the Au nanosphere, being located between it and the near-field scattering maximum.

physics.optics↗

arXiv2014

Dynamical Tuning of Energy Transfer Efficiency on a Graphene Monolayer

We present in this contribution a theoretical investigation of the spontaneous emission and energy transfer rates between quantum systems placed above a monolayer of conducting graphene. The conditions for strong and weak coupling between a quantum system and the surface plasmon-polariton of graphene are determined and, subsequently, we focus exclusively on the weak coupling regime. We then calculate the dispersion relation of the surface plasmon mode on graphene and, by varying the chemical potential, show a good control of its resonance frequency. Using a Green's tensor formalism, we calculate the spontaneous emission and energy transfer rates of quantum systems placed near the graphene monolayer. The spontaneous emission rate of a single quantum system is enhanced by several orders of magnitude close to the graphene monolayer and we show that this enhancement is due almost exclusively to excitation of the surface plasmon mode. When considering the energy transfer rate between two quantum systems, we find a similar enhancement of several orders of magnitude close to the graphene monolayer. The direct interaction between the donor and acceptor dominates when they are close to each other, but is modified from its free-space behavior due to the presence of the graphene monolayer. As the donor-acceptor separation is increased, their direct interaction is overshadowed by the interaction via the surface plasmon mode. Due to the large propagation length of surface plasmon mode on graphene -- hundreds of nanometers -- this enhancement of the energy transfer rate holds over large donor-acceptor separations along the graphene monolayer.

physics.optics↗