arXiv subjects

Sander J. W. Vonk

Publications and source records attributed to Sander J. W. Vonk.

9 recordsLinked to original sources

arXiv2026

When Integrated Photonics Should Be Wavy

Increasing demand for high-performance optical devices drives the search for improved fabrication and design paradigms. While photonic circuits have traditionally used structures with two discrete 'binary' height levels, grayscale 'wavy' interfaces, or optical Fourier surfaces, have recently become possible. They provide precise control over the Fourier components that govern the optical response. This capability raises the question: When does a wavy device improve performance and why? Here, we show that wavy integrated processors exhibit superior accuracy and efficiency to binary analogs. Inverse-designed wavy interferometers have the design freedom to minimize outscattering to free space and backreflections. They reach a five-fold lower transmission error than binary counterparts and enable bandwidths up to 300 nm. We formalize these findings for other distinct integrated devices, such as photonic crystals, nanocavities, and beam emitters. A Fourier-optics analysis identifies a trade-off: wavy profiles manipulate light more accurately, whereas binary profiles excel in interaction strength. As such, tailored wavy profiles emit high-quality beams. However, binary profiles remain preferable in photonic crystals and nanocavities, where maximal index contrast is required and binarization-induced higher harmonics are benign. Thus, for quantum information, optical computing, and sensing, optical Fourier surfaces offer a route to miniaturized integrated circuits with improved performance.

physics.optics↗

arXiv2026

Plasmonic Fourier Surfaces Revisited: Relating Bandgaps with Bound States in the Continuum

Periodically corrugated metal interfaces supporting surface plasmon polaritons (SPPs) belong to the earliest nanoplasmonic platforms. Even simplest reliefs described by a few harmonics - plasmonic Fourier surfaces - display markedly different far-field signatures depending on the corrugation depth and symmetry: shallow reliefs exhibit a plasmonic bandgap (PBG) between two hybridized SPP standing waves, while deeper reliefs support asymmetry-induced single sharp resonances, termed over the last decade as quasi-bound states in the continuum (qBICs). Although these spectral features have long been observed experimentally and treated empirically, the underlying eigenmode evolution connecting the shallow and deep corrugation regimes has remained largely unexplored. Here we revisit this long-standing problem by analyzing it in terms of modern eigenstate formalism. Starting from the Rayleigh hypothesis, we develop a concise first-principle analytical description that explicitly captures how one eigenmode transforms from a dark bound state into an observable qBIC, while the other turns from bright into an overcoupled, unobservable state - thus unifying the SPP manifestations featuring PBG and qBIC within the same eigenmode framework. Finally, we demonstrate the practical relevance of the theory by showing how precise eigenstate engineering can enhance the SPP refractive index sensitivity.

physics.optics↗

arXiv2026

Broadband, compact, and training-free optical processors for parallel image classification

As artificial intelligence becomes increasingly prevalent, the demand for faster and more energy-efficient computing approaches grows. While optical computing offers intrinsic advantages in bandwidth and power consumption, existing implementations remain bulky, wavelength-specific, and dependent on complex training procedures, limiting scalability and parallel operation. In this work, we demonstrate a compact, training-free optical processor based on wavy diffractive features, known as Fourier surfaces, for parallel image classification. Our device achieves classification accuracies of up to 84% for digit datasets and 66% for fashion datasets within a 40$\times$40 $μ$m$^2$ footprint. The diffractive layer inherently separates incident wavelengths into distinct output directions, enabling broadband operation and allowing multiple colors to function as independent computation channels. As a result, this passive system supports up to 20 simultaneous computations within a single optical pass. These results highlight the potential of nanoscale diffractive systems to achieve high compute densities, paving the way for scalable, low-power optical processors for machine learning and image-recognition applications.

physics.optics↗

arXiv2026

Fourier pixels for reciprocal light control

Digital cameras and displays utilise picture elements (pixels) that perform a single function: detecting or emitting light intensity. To exploit the full information content of electromagnetic waves, more advanced elements are required. This has driven the development of multifunctional components, which for example, simultaneously detect and emit intensity or extract intensity and spectral information. However, no pixel exists that both senses and generates optical wavefronts with full control over amplitude, phase, and polarisation, limiting reciprocal control and feedback of sophisticated light fields. Here we present a route to such pixels by demonstrating a versatile platform of miniaturised diffractive elements based on Fourier optics. We exploit plasmonic surface waves, which propagate coherently and efficiently across metallic surfaces. When these plasmons are launched towards wavy microstructures designed with simple Fourier analysis, arbitrary and background-free optical wavefronts are generated. Conversely, incoming light can be sensed and its amplitude, phase, and polarisation fully characterised. By combining or superposing several such components, we create multifunctional 'Fourier pixels' that provide compact and accurate control over the optical field. Our approach, which could also use photonic waveguide modes, establishes a scalable, universal architecture for vectorially programmable pixels with applications in adaptive optics, holographic displays, optical communication, and quantum-information processing.

physics.optics↗

arXiv2026

Band-Edge Carrier Trapping Limits Light Emission in WSe$_2$

Monolayers of transition metal dichalcogenides (TMDs) exhibit bright photoluminescence, a desirable property for light-emitting diodes and single-photon emitters. Because the emission intensity is heavily influenced by factors such as defect density and oxidation, it is critical to understand how they affect photoluminescence efficiency. However, due to the time-consuming process of identifying individual monolayers, studies of high-quality exfoliated TMDs have been limited to only a few samples. Here, we present an investigation of excited-state lifetimes and spectra for over 200 WSe$_2$ exfoliated monolayers at room temperature. We find a linear correlation between photoluminescence lifetime and intensity across hundreds of monolayers and within individual monolayers. Results from intentional photooxidation experiments indicate that this correlation is due to photoinduced band-edge carrier traps, which introduce a nonradiative decay pathway that competes with exciton emission. Our work highlights the importance of controlling such traps, as they are the primary limitation of bright photoluminescence.

cond-mat.mes-hall↗

arXiv2025

Vanishing quantum confinement enables bright and thermally excited multi-carrier emission from semiconductor nanocrystals

Recently, nanocrystals in the regime of vanishing quantum confinement-termed bulk nanocrystals (BNCs)-have demonstrated remarkable optical gain characteristics. While their high-power lasing performance was demonstrated convincingly, the photophysics at low and intermediate powers-where charge-carrier populations are discrete-remain unexplored. Using single-photon avalanche diode (SPAD) array technology, we resolve the dynamics and energetics of six multi-carrier excited states in individual CdSe/CdS core/shell BNCs, containing up to four electrons and two holes. Each state exhibits bimodal emission, indicative of thermal equilibrium between closely spaced electron and hole levels, confirmed via temperature-dependent single-particle measurements. Quantification of radiative and nonradiative decay channels reveals strongly suppressed Auger recombination through both the negative- and positive-trion pathways. We present a model that combines statistical scaling of rate constants with Fermi-Dirac thermal occupations of electron and hole levels, bridging the transitional regime between quantum-confined and bulk nanocrystals, and providing a comprehensive framework for understanding this emerging class of materials.

cond-mat.mtrl-sci↗

arXiv2025

Controlling viscosity to engineer focal conic domains in photonic cellulose nanocrystal films

Cellulose nanocrystals (CNCs) form cholesteric architectures that can have color specific reflectivity and enable sustainable photonic films. However, achieving uniform color, suppressing iridescence, and accessing ordered defect structures such as focal conic domains remain challenging. Here, we control the photonic properties of CNC films by steering the self assembly process. Across 24 dish-cast films with varying salt concentrations and sonication doses, we combine viscosity measurements, timelapse polarized optical microscopy, and angle-resolved reflectance spectroscopy to correlate evaporation dynamics with photonic structure. We show that viscosity, jointly controlled by NaCl-mediated electrostatic screening and sonication-induced bundle fragmentation, dictates the extent of tactoid coalescence. Low-viscosity suspensions generate large, homogeneous cholesteric domains and narrow spectral responses, while high viscosity leads to arrested, heterogenous domains and increased diffuse light reflection. Critically, within a narrow parameter window of intermediate ionic strength and moderate sonication, we reproducibly engineer photonically active focal conic domains. These results identify viscosity-driven flow as a key, previously underappreciated factor in CNC self-assembly and establish design rules for producing structurally colored films with tunable photonic response, reduced iridescence, and controllable defect architectures.

cond-mat.soft↗

arXiv2024

Hot-carrier trapping preserves high quantum yields but limits optical gain in InP-based quantum dots

Indium phosphide is the leading material for commercial applications of colloidal quantum dots. To date, however, the community has failed to achieve successful operation under strong excitation conditions, contrasting sharply with other materials. Here, we report how the unusual photophysics of state-of-the-art InP-based quantum dots make them unattractive as a gain material. A combination of ensemble-based time-resolved spectroscopy over timescales from femtoseconds to microseconds and single-quantum-dot spectroscopy reveals ultrafast trapping of hot charge carriers. This process leads to charge-carrier losses, thereby reducing the achievable population inversion which limits amplification of light in a gain material. Interestingly, fluorescence is only delayed, not quenched, by hot charge-carrier trapping, explaining why InP-based quantum dots are successful as bright luminescent colour convertors for low-intensity applications. Comparison with other popular quantum-dot materials, such as CdSe, Pb-halide perovskites, and CuInS2, indicate that the hot-carrier dynamics observed are unique to InP.

cond-mat.mtrl-sci↗

arXiv2019

Optical Fourier surfaces

Gratings and holograms are patterned surfaces that tailor optical signals by diffraction. Despite their long history, variants with remarkable functionalities continue to be discovered. Further advances could exploit Fourier optics, which specifies the surface pattern that generates a desired diffracted output through its Fourier transform. To shape the optical wavefront, the ideal surface profile should contain a precise sum of sinusoidal waves, each with a well-defined amplitude, spatial frequency, and phase. However, because fabrication techniques typically yield profiles with at most a few depth levels, complex 'wavy' surfaces cannot be obtained, limiting the straightforward mathematical design and implementation of sophisticated diffractive optics. Here we present a simple yet powerful approach to eliminate this design-fabrication mismatch by demonstrating optical surfaces that contain an arbitrary number of specified sinusoids. We combine thermal scanning-probe lithography and templating to create periodic and aperiodic surface patterns with continuous depth control and sub-wavelength spatial resolution. Multicomponent linear gratings allow precise manipulation of electromagnetic signals through Fourier-spectrum engineering. Consequently, we immediately resolve an important problem in photonics by creating a single-layer grating that simultaneously couples red, green, and blue light at the same angle of incidence. More broadly, we analytically design and accurately replicate intricate two-dimensional moiré patterns, quasicrystals, and holograms, demonstrating a variety of previously impossible diffractive surfaces. Therefore, this approach provides instant benefit for optical devices (biosensors, lasers, metasurfaces, and modulators) and emerging topics in photonics (topological structures, transformation optics, and valleytronics).

physics.optics↗