arXiv subjects

Renaud A. L. Vallée

Publications and source records attributed to Renaud A. L. Vallée.

5 recordsLinked to original sources

arXiv2026

Three-dimensional photon transport in spinodal photocatalytic aerogels: how bicontinuous morphology controls kinetic rate constants

Anatase TiO2 dispersed in a silica aerogel matrix forms porous monolithic photocatalysts promising for indoor air purification and VOC photo-degradation. Their bicontinuous spinodal architecture provides high surface area, structural integrity, and intense UV multiple scattering. Extracting kinetic descriptors from apparent rate constants requires an optical model for the photon fluence, and current practice replaces the 3D pore network by a homogeneous 1D slab with effective optical coefficients -- an approximation never quantified for spinodal geometries. We combine realistic 3D spinodal pore masks from Cahn-Hilliard spectral integration with 3D GPU-accelerated Monte Carlo (MC) photon-migration simulations, introducing the solid-phase-weighted estimator $Φ_{3D}$ and comparing it with the conventional volume average $Φ_{1D}$ and the diffusion-approximation (DA) prediction $Φ_{DA}$. The ratio $Φ_{3D}/Φ_{1D}$ equals 1.515 at porosity $ϕ=0.70$ and 1.774 at $ϕ=0.90$: the solid phase is preferentially illuminated, traced to optical phase contrast. Since pores neither absorb nor scatter, photons cross quasi-ballistically, and the void phase sustains only $r=0.514$ of the solid-phase fluence, diluting the volume average with dark voxels. This ratio is porosity-independent, so the concentration ratio obeys the exact relation $1/[ϕr+(1-ϕ)]$, reproducing both simulated values. The kinetic descriptor $k_{surf}\cdot K_{ads}$ is underestimated by 20% via the DA route and overestimated by 51.5% via the volume-averaged route. Homogeneous-slab MC simulations at matched optical thickness show that 84% of the fluence-weighted RMS discrepancy between MC and DA is intrinsic to the bicontinuous architecture and beyond any effective-medium DA. These results give a quantitative correction for kinetic extraction and design rules linking porosity, pore size and photon utilisation.

physics.optics↗

arXiv2026

Band-Selective LDOS Engineering of Yb/Er Upconversion: an Electromagnetic-Kinetic Diagnostic Framework

A persistent challenge in plasmonic upconversion is decoupling pump-field enhancement from emission-side local-density-of-optical-states (LDOS) engineering to achieve selective band manipulation. Here, we show that a corrugated SU8/Au/Al2O3 grating coated with a NaYF4:Yb/Er upconversion nanoparticle (UCNP) monolayer realizes a truly band-selective platform. A broad plasmonic resonance near 670 nm modulates the red Er 3 + 3+ decay rate by $\pm$15% as a function of the Al2O3 spacer thickness, while leaving the green transition experimentally invariant (< 1% change). Simultaneously, the 980 nm pump field is monotonically suppressed below free-space levels, ensuring that steady-state and time-resolved observables cleanly probe the emission-side LDOS without pump interference. We analyze this system using a coupled electromagnetic-kinetic framework that integrates finite-difference time-domain (FDTD) calculations of Purcell factors and pump fields with a six-level Yb/Er rate-equation model. The framework quantitatively reproduces the 670 nm plasmonic resonance, the red-band decay-rate modulation, and the monotonic decrease of the green/red intensity ratio. Crucially, the model serves as a powerful diagnostic tool: it overpredicts a green-band rate reduction, but systematic parametric testing rules out geometric (apex smoothing) and material (grain-boundary damping, interband loss) imperfections as the cause. Instead, it isolates the residual discrepancy to measurement-versus-model factors (finite-aperture angular averaging) and missing non-radiative kinetic channels, establishing a clear roadmap for the rational design and validation of future plasmonic-UCNP architectures.

physics.optics↗

arXiv2026

Characterization of Photopolymerized Microscopic Chiral Structures Using Photonic Orbital Angular Momentum

The controlled fabrication and chiroptical characterization of microscale chiral structures remain central challenges in photonics, sensing, and metamaterial engineering. Here we demonstrate an accessible, low-cost platform that combines digital micromirror device-enabled maskless photolithography with capillarity-induced self-assembly to produce polymer chiral microstructures of deterministic handedness, and a liquid-crystal spatial light modulator to generate vortex beams for their characterization via helical dichroism (HD). Using a standard 532 nm laser, we observe HD signals of approximately 30% for microstructures with a characteristic diameter of about 15 micrometers. Rigorous finite-difference time-domain simulations performed on three-dimensional geometries reconstructed from high-resolution Scanning Electron Microscopy data reproduce the experimental HD spectra and confirm the role of structural handedness in driving the differential orbital angular momentum (OAM) response. Near-mirror-symmetric HD spectra for opposite-handed enantiomers, combined with a vanishing response for achiral controls, establish OAM as a robust and spatially selective chiral probe at the microscale. Crucially, both fabrication and characterization rely on equipment standard in an optics laboratory, without recourse to femtosecond sources, plasmonic substrates, or costly photoresists. These results open practical pathways toward OAM-driven chiral sensing, enantioselective detection, and photonic logic devices.

physics.optics↗

arXiv2026

From Light Diffusion to Photocatalytic Rates: Compact Scaling Laws for Strongly Scattering Porous Slabs

Light transport in strongly scattering porous photocatalytic materials governs the spatial distribution of absorbed photons and therefore the generation of charge carriers driving photocatalytic reactions. Yet translating measured optical properties of such media into intrinsic reaction rate constants remains challenging, as it requires simultaneously accounting for multiple scattering, boundary losses, photochemical efficiency, and surface kinetics. Here we develop a compact analytical framework that unifies these processes for nanoparticle-loaded photocatalytic slabs. Using a finite-slab diffusion model with extrapolated boundaries, we derive closed-form expressions for the fluence field and couple them to a photochemical quantum efficiency and first-order surface kinetics. The resulting predictors yield intrinsic volumetric and areal rate constants whose dependence on the transport mean free path, optical thickness, and surface-to-volume ratio emerges transparently. Validation against Monte-Carlo photon-migration simulations shows that the diffusion approximation reproduces the fluence and generation-rate profiles with a modest multiplicative mismatch, typically within a factor of about 1.20-1.39, depending on the anisotropy and scattering phase function. This level of agreement is consistent with the known limits of the diffusion approximation and is sufficient to enable reliable, design-oriented predictions. The analytical descriptors introduced here, such as l*, S/V and the extrapolation length zb are general, physically interpretable, and directly integrable into data-driven optimisation and geometry-engineered reactor design. The framework thus provides a versatile and physically grounded tool for photocatalytic systems across diverse applications, including VOC photo-degradation, indoor air purification, and solar-fuel production.

physics.optics↗

arXiv2019

Energy transfer and interference by collective electromagnetic coupling

The physics of collective optical response of molecular assemblies, pioneered by Dicke in 1954, has long been at the center of theoretical and experimental scrutiny. The influence of the environment on such phenomena is also of great interest due to various important applications in e.g. energy conversion devices. In this manuscript we demonstrate both experimentally and theoretically the spatial modulations of the collective decay rates of molecules placed in proximity to a metal interface. We show in a very simple framework how the cooperative optical response can be analyzed in terms of intermolecular correlations causing interference between the response of different molecules and the polarization induced on a nearby metallic boundary and predict similar collective interference phenomena in excitation energy transfer between molecular aggregates.

cond-mat.mes-hall↗