arXiv subjects

Toby Severs Millard

Publications and source records attributed to Toby Severs Millard.

7 recordsLinked to original sources

arXiv2026

Revealing antenna-molecule coupling using virtual gain from synthetic complex frequency analysis

Synthetic complex frequency waves (CFWs) enhance spectral and spatial resolution by compensating loss through virtual gain. This approach is particularly useful in nano-photonics where loss is intrinsically linked to strong field confinement and system size. Here, we apply CFWs to metasurfaces composed of Fano-resonant plasmonic antennas with lithographically tunable coupling between the weak and strong coupling regimes and where loss may be reduced to the Drude damping limit. We quantitatively assess the impact of the virtual gain to find that, while reducing loss, internal coupling rates remain invariant. While the exceptional point associated with strong light-matter coupling is invariant under virtual gain, the cooperativity of Fano interference is enhanced. When the metasurfaces are functionalised with a molecular monolayer, CFWs are thus able to reveal a molecule's vibrational modes, even when obscured by loss, and allow direct extraction of the plasmon-molecule coupling strength. We thus use CFW analysis as a quantitative tool for revealing and assessing light matter-coupling.

physics.optics↗

arXiv2025

Optical and electrical probing of plasmonic metal-molecule interactions

Plasmonic nanostructures enable efficient light-to-energy conversion by concentrating optical energy into nanoscale volumes. A key mechanism in this process is chemical interface damping (CID), where surface plasmons are damped by adsorbed molecules, enabling the transfer of charge to adsorbed molecules. In this study, we investigate the relationship between CID and adsorbate-induced changes in DC electrical resistivity for four molecular adsorbates-adenine, 4-aminothiophenol (ATP), biphenyl thiol (BPT), and 1-dodecanethiol (DDT)-on gold surfaces. Our results reveal two distinct CID regimes. BPT causes CID via direct electronic transitions to the lowest unoccupied molecular orbital (LUMO), which is centered at approx. 2 eV above the Fermi level and can be resonantly excited by the plasmon. This mechanism is dependent on plasmon energy. In contrast, ATP, adenine and DDT lead to plasmon damping through inelastic electron scattering at the metal-molecule interface. This regime shows a weaker dependency on plasmon energy since it does not involve resonant electron excitation between hybridized metal-molecule states. This same mechanism contributes to adsorbate-induced changes in DC resistivity, suggesting that resistivity measurements can serve as a probe of plasmonic energy transfer, as highlighted by the good correlation between the two effects. These findings provide new insights into the microscopic origins of plasmon damping and offer a unified framework for understanding metal-adsorbate energy transfer.

physics.optics↗

arXiv2025

Phase-Matching-Free Sensing with Undetected Light Using a Nonlinear Thin-Film Metasurface

In this article, we report classical sensing with undetected light at a wavelength of 1500 nm, detected on a silicon camera at 580 nm, using four-wave mixing from a plasmonic metasurface. The bidirectional nonlinear scattering due to inherent reflections from such thin nonlinear materials modifies their operation within a nonlinear interferometer. The theoretical model for visibility accounting for such bidirectionality as well as pulsed illumination accurately predicts visibility in the system as a function of transmission in the near-infrared seed (idler) arm. Spectrally resolving the visible signal emission evaluates the total dispersion within the interferometer, highlighting the prospect of ultrafast sensing with undetected photons.

physics.optics↗

arXiv2024

Surface lattice resonance lasers with epitaxial InP gain medium

Surface lattice resonance (SLR) lasers, where gain is supplied by a thin film active material and the feedback comes from multiple scattering by plasmonic nanoparticles, have shown both low threshold lasing and tunability of the angular and spectral emission. However, typically used materials such as organic dyes and QD films suffer from photo-degradation which hampers practical applications. Here, we demonstrate photo-stable single-mode lasing of SLR modes sustained in an epitaxial solid-state InP slab waveguide. The nanoparticle array is weakly coupled to the optical modes, which decreases the scattering losses and hence the experimental lasing threshold is as low as 90 $μ$J/cm$^{2}$. The nanoparticle periodicity defines the lasing wavelength and enables tuneable emission wavelengths over a 70 nm spectral range. Combining plasmonic nanoparticles with an epitaxial solid-state gain medium paves the way for large-area on-chip integrated SLR lasers for applications including optical communication, optical computing, sensing, and LiDAR.

physics.optics↗

arXiv2022

Van der Waals Materials for Applications in Nanophotonics

Numerous optical phenomena and applications have been enabled by nanophotonic structures. Their current fabrication from high refractive index dielectrics, such as silicon or gallium phosphide, pose restricting fabrication challenges, while metals, relying on plasmons and thus exhibiting high ohmic losses, limit the achievable applications. Here, we present an emerging class of layered so-called van der Waals (vdW) crystals as a viable nanophotonics platform. We extract the dielectric response of 11 mechanically exfoliated thin-film (20-200 nm) van der Waals crystals, revealing high refractive indices up to n = 5, pronounced birefringence up to $Δ$n = 3, sharp absorption resonances, and a range of transparency windows from ultraviolet to near-infrared. We then fabricate nanoantennas on SiO$_2$ and gold utilizing the compatibility of vdW thin films with a variety of substrates. We observe pronounced Mie resonances due to the high refractive index contrast on SiO$_2$ leading to a strong exciton-photon coupling regime as well as largely unexplored high-quality-factor, hybrid Mie-plasmon modes on gold. We demonstrate further vdW-material-specific degrees of freedom in fabrication by realizing nanoantennas from stacked twisted crystalline thin-films, enabling control of nonlinear optical properties, and post-fabrication nanostructure transfer, important for nano-optics with sensitive materials.

physics.optics↗

arXiv2021

Transition metal dichalcogenide dimer nano-antennas with ultra-small gaps

Transition metal dichalcogenides have emerged as promising materials for nano-photonic resonators due to their large refractive index, low absorption within a large portion of the visible spectrum and compatibility with a wide range of substrates. Here we use these properties to fabricate WS$_2$ double-pillar nano-antennas in a variety of geometries enabled by the anisotropy in the crystal structure. Using dark field spectroscopy, we reveal multiple Mie resonances, to which we couple WSe$_2$ monolayer photoluminescence and achieve Purcell enhancement and an increased fluorescence by factors up to 240. We introduce post-fabrication atomic force microscope repositioning and rotation of dimer nano-antennas, achieving gaps as small as 10$\pm$5 nm, opening the possibility to a host of potential applications including strong Purcell enhancement of single photon emitters and optical trapping, which we study in simulations. Our findings highlight the advantages of using transition metal dichalcogenides for nano-photonics by exploring new applications enabled by their unique properties.

physics.app-ph↗

arXiv2020

Strong Exciton-Photon Coupling in Large Area MoSe$_2$ and WSe$_2$ Heterostructures Fabricated from Two-Dimensional Materials Grown by Chemical Vapor Deposition

Two-dimensional semiconducting transition metal dichalcogenides embedded in optical microcavities in the strong exciton-photon coupling regime may lead to promising applications in spin and valley addressable polaritonic logic gates and circuits. One significant obstacle for their realization is the inherent lack of scalability associated with the mechanical exfoliation commonly used for fabrication of two-dimensional materials and their heterostructures. Chemical vapor deposition offers an alternative scalable fabrication method for both monolayer semiconductors and other two-dimensional materials, such as hexagonal boron nitride. Observation of the strong light-matter coupling in chemical vapor grown transition metal dichalcogenides has been demonstrated so far in a handful of experiments with monolayer molybdenum disulfide and tungsten disulfide. Here we instead demonstrate the strong exciton-photon coupling in microcavities comprising large area transition metal dichalcogenide / hexagonal boron nitride heterostructures made from chemical vapor deposition grown molybdenum diselenide and tungsten diselenide encapsulated on one or both sides in continuous few-layer boron nitride films also grown by chemical vapor deposition. These transition metal dichalcogenide / hexagonal boron nitride heterostructures show high optical quality comparable with mechanically exfoliated samples, allowing operation in the strong coupling regime in a wide range of temperatures down to 4 Kelvin in tunable and monolithic microcavities, and demonstrating the possibility to successfully develop large area transition metal dichalcogenide based polariton devices.

physics.optics↗