arXiv subjects

Frank Scheffold

Publications and source records attributed to Frank Scheffold.

At least 19 recordsLinked to original sources

arXiv2026

Strong suppression of the photonic density of states in three-dimensional disordered silicon networks

Photonic bandgaps can open not only in crystalline dielectric materials but also in amorphous structures. Self-uniform amorphous gyroid networks have been proposed as promising disordered photonic architectures. Motivated by numerical studies, we combine direct laser-writing lithography with advanced materials processing to fabricate these structures from high-refractive-index silicon. Spectroscopic measurements at mid-infrared wavelengths reveal pronounced transmission minima. To investigate the formation of photonic band gaps in amorphous structures, we develop polarization-resolved transmission spectroscopy that separates ballistic and multiply scattered contributions, enabling direct identification of the underlying transport regimes. We observe a strong suppression of diffuse transmission, providing direct experimental evidence for a breakdown of conventional diffuse transport associated with a strongly reduced photonic density of states. Together with large-scale numerical simulations, our results establish the presence of a deep photonic pseudogap in an amorphous three-dimensional dielectric material and open new opportunities for observing disorder-induced localization phenomena, including Anderson localization of light.

physics.optics↗

arXiv2025

Tuning pair interactions in colloidal systems using random light fields

We propose a method to tune interactions between absorptionless colloidal particle pairs. This is achieved via optimization of the spectral energy density of a homogeneous random optical field. Several standard and more exotic interaction potentials, as well as their negative counterparts, are shown to be successfully tuned. We show that the effective dimensionality of the space of potential functions that can be created by this means can reach up to several tens.

cond-mat.soft↗

arXiv2025

Expanding the reach of diffusing wave spectroscopy and tracer bead microrheology

Diffusing Wave Spectroscopy (DWS) is an extension of standard dynamic light scattering (DLS), applied to soft materials that are turbid or opaque. The propagation of light is modeled using light diffusion, characterized by a light diffusion coefficient that depends on the transport mean free path l* of the medium. DWS is highly sensitive to small particle displacements or other local fluctuations in the scattering properties and can probe sub-nanometer displacements. Analyzing the motion of beads in a viscoelastic matrix, known as one-bead microrheology, is one of the most common applications of DWS. Despite significant advancements since its invention in the late 1980s, including two-cell and multispeckle DWS, challenges such as merging single- and multispeckle data and limited accuracy for short correlation times persist. Here, we address these issues by improving the two-cell echo DWS scheme. We propose a calibration-free method to blend and merge echo and two-cell DWS data and demonstrate the use of an exponential basis fit to enhance data quality, in particular at very short times. Building on this, we introduce stable corrections for bead and fluid inertia, significantly improving the quality of microrheology data at high frequencies.

cond-mat.soft↗

arXiv2024

Assessing the pair interactions of pNIPAM microgel particles using optical tweezers

We experimentally study the pairwise interactions between poly(N-isopropylacrylamide) (pNIPAM) microgel particles using line optical tweezers. To measure their interaction potentials under controlled conditions, we trap two microgel particles in the tweezer and analyze their thermal positional fluctuations near contact. The pair interaction potential is modeled using a Hertzian core polymer brush corona framework, capturing the complexity of particle interactions by accounting for both the deformable, elastic core and the steric effects of the polymer brush-like shell. Our experimental results demonstrate that the brush corona interactions soften as the system approaches the lower critical solution temperature (LCST) of the microgels, consistent with the decay of the polymer's second virial coefficient.

cond-mat.soft↗

arXiv2024

Mesoscopic light transport in nonlinear disordered media

Nonlinear disordered media uniquely combine multiple scattering and second-harmonic generation. Here, we investigate the statistical properties of the nonlinear light generated within such media. We report super-Rayleigh statistics of the second-harmonic speckle intensity, and demonstrate that it is caused by the mesoscopic correlations arising in extreme scattering conditions. The measured conductance is the lowest ever observed in an isotropically scattering 3D medium, with applications in broadband second-harmonic generation, wavefront shaping in nonlinear disordered media, and photonic computing.

physics.optics↗

arXiv2024

A Free Energy Model for the Plateau Shear Modulus in Thermosensitive Microgel Suspensions

Polymer microgels exhibit intriguing macroscopic flow properties arising from their unique microscopic structure. Microgel colloids usually comprise a crosslinked polymer network with a radially decaying density profile, resulting in a dense core surrounded by a fuzzy corona. Notably, microgels synthesized from poly(N-isopropyl acrylamide) (PNIPAM) are thermoresponsive, capable of adjusting their size and density profile based on temperature. Above the lower critical solution temperature ($T_\text{LCST} \sim 33$ $^\circ$C), the microgel's polymer network collapses, expulsing water through a reversible process. Conversely, below $33$ $^\circ$C, the microgel's network swells, becoming highly compressible and allowing overpacking to effective volume fractions exceeding one. Under conditions of dense packing, microgels undergo deformation in distinct stages: corona compression and faceting, interpenetration, and finally, isotropic compression. Each stage exhibits a characteristic signature in the dense microgel suspensions' yield stress and elastic modulus. Here, we introduce a model for the linear elastic shear modulus by minimizing a quasi-equilibrium free energy, encompassing all relevant energetic contributions. We validate our model by comparing its predictions to experimental results from oscillatory shear rheology tests on microgel suspensions at different densities and temperatures. Our findings demonstrate that combining macroscopic rheological measurements with the model allows for temperature-dependent characterization of polymer interaction parameters.

cond-mat.soft↗

arXiv2023

Efficient structural color from pigment-loaded nanostructures

Color can originate from wavelength-dependence in the absorption of pigments or the scattering of nanostructures. While synthetic colors are dominated by the former, vivid structural colors found in nature have inspired much research on the latter. However, many of the most vibrant colors in nature involve the interactions of structure and pigment. Here, we demonstrate that pigment can be exploited to efficiently create bright structural color at wavelengths outside its absorption band. We created pigment-enhanced Bragg reflectors by sequentially spin-coating layers of poly-vinyl alcohol (PVA) and polystyrene (PS) loaded with $\beta$-carotene (BC). With only 10 double layers, we acheived a peak reflectance over $0.8$ at 550 nm and normal incidence. A pigment-free multilayer made of the same materials would require 25 double layers to achieve the same reflectance. Further, pigment loading suppressed the Bragg reflector's characteristic iridescence. Using numerical simulations, we further show that similar pigment loadings could significantly expand the gamut of non-iridescent colors addressable by photonic glasses.

cond-mat.soft↗

arXiv2023

Bandgap fluctuations and robustness in two-dimensional hyperuniform dielectric materials

We numerically study the statistical fluctuations of photonic band gaps in ensembles of stealthy hyperuniform disordered patterns. We find that at low stealthiness, where correlations are weak, band gaps of different system realizations appear over a wide frequency range, are narrow, and generally do not overlap. Interestingly, above a critical value of stealthiness $\chi \gtrsim 0.35$, the bandgaps become large and overlap significantly from realization to realization, while a second gap appears. These observations extend our understanding of photonic bandgaps in disordered systems and provide information on the robustness of gaps in practical applications.

physics.class-ph↗

arXiv2023

Probing temperature-responsivity of microgels and its interplay with a solid surface by superresolution microscopy and numerical simulations

Superresolution microscopy has become a powerful tool to investigate the internal structure of complex colloidal and polymeric systems, such as microgels, at the nanometer scale. The ability to monitor microgels response to temperature changes in situ opens new and exciting opportunities to design and precisely control their behaviour for various applications. When performing advanced microscopy experiments, interactions between the particle and the environment can be important. Often microgels are deposited on a substrate since they have to remain still for several minutes during the experiment. This study uses dSTORM microscopy and advanced coarse-grained molecular dynamics simulations to investigate, for the first time, how individual microgels anchored on hydrophilic and hydrophobic surfaces undergo their volume phase transition in temperature. We find that, in the presence of a hydrophilic substrate, the structure of the microgel is unperturbed and the resulting density profiles quantitatively agree with simulations performed in bulk conditions. Instead, when a hydrophobic surface is used, the microgel spreads at the interface and an interesting competition between the two hydrophobic strengths -- monomer-monomer vs monomer-surface -- comes into play at high temperatures. The remarkable agreement between experiments and simulations makes the present study a fundamental step to establish this high-resolution monitoring technique as a platform for investigating more complex systems, being these either macromolecules with peculiar internal structure or nanocomplexes where molecules of interest can be encapsulated in the microgel network and controllably released with temperature.

cond-mat.soft↗

arXiv2022

Light transport through amorphous photonic materials with localization and bandgap regimes

We propose a framework that unifies the description of light transport in three-dimensional amorphous dielectric materials that exhibit both localization and a photonic bandgap. To this end, we argue that coherent reflection near and in the bandgap attenuates the generation of diffuse or localized photons. Using the self-consistent theory of localization and considering the density of states of photons, we can quantitatively describe all transport regimes: Transparency, light diffusion, localization, and bandgap. Comparing the model with numerical data on optical transport in hyperuniform dielectric networks confirms our findings.

physics.optics↗

arXiv2021

Light in correlated disordered media

The optics of correlated disordered media is a fascinating research topic emerging at the interface between the physics of waves in complex media and nanophotonics. Inspired by photonic structures in nature and enabled by advances in nanofabrication processes, recent investigations have unveiled how the design of structural correlations down to the subwavelength scale could be exploited to control the scattering, transport and localization of light in matter. From optical transparency to superdiffusive light transport to photonic gaps, the optics of correlated disordered media challenges our physical intuition and offers new perspectives for applications. This article reviews the theoretical foundations, state-of-the-art experimental techniques and major achievements in the study of light interaction with correlated disorder, covering a wide range of systems -- from short-range correlated photonic liquids, to L\'evy glasses containing fractal heterogeneities, to hyperuniform disordered photonic materials. The mechanisms underlying light scattering and transport phenomena are elucidated on the basis of rigorous theoretical arguments. We overview the exciting ongoing research on mesoscopic phenomena, such as transport phase transitions and speckle statistics, and the current development of disorder engineering for applications such as light-energy management and visual appearance design. Special efforts are finally made to identify the main theoretical and experimental challenges to address in the near future.

physics.optics↗

arXiv2021

Enhancing the Refractive Index of Polymers with a Plant-Based Pigment

Polymeric materials are prized for their formability, low density, and functional versatility. However, the refractive indices of common polymers fall in a relatively narrow range between 1.4 and 1.6. Here, we demonstrate that loading commercially-available polymers with large concentrations of a plant-based pigment can effectively enhance their refractive index.For polystyrene loaded with 67w/w\% $\beta$-carotene, we achieve a peak value of 2.2 near the absorption edge at $531~\mathrm{nm}$, while maintaining values above 1.75 across longer wavelengths of the visible spectrum. Despite high pigment loadings, this blend maintains the thermoforming ability of polystyrene, and $\beta$-carotene remains molecularly dispersed. Similar results are demonstrated for the plant-derived polymer ethyl cellulose. Since the refractive index enhancement is intimately connected to the introduction of strong absorption, it is best suited to applications where light travels short distances through the material, such as reflectors and nanophotonic systems.We experimentally demonstrate enhanced reflectance from films, as large as seven-fold for ethyl cellulose at selected wavelengths. Theoretical calculations that highlight that this simple strategy can significantly increase light scattering by nanoparticles and enhance the performance of Bragg reflectors.

cond-mat.soft↗

arXiv2020

Angular resolved light scattering from micron-sized colloidal assemblies

Disordered dielectrics with structural correlations on length scales comparable to visible light wavelengths exhibit complex optical properties. Such materials exist in nature, leading to beautiful structural non-iridescent color, and they are also increasingly used as building blocks for optical materials and coatings. In this article, we study the single-scattering properties of micron-sized, disordered colloidal assemblies. The aggregates act as structurally colored supraparticles or as building blocks for macroscopic photonic glasses. We present experimental data for the differential scattering and transport cross-section. We show how we can adapt existing macroscopic models to describe the scattering from small colloidal assemblies outside the weak-scattering limit and entering the Lorentz-Mie regime.

physics.optics↗

arXiv2020

Scattering from controlled defects in woodpile photonic crystals

Photonic crystals with a sufficiently high refractive index contrast display partial or full band gaps. However, imperfections in the metamaterial cause light scattering and extinction of the interfering propagating waves. Positive as well as negative defect volumes may contribute to this kind of optical perturbation. In this study, we fabricate and characterize three-dimensional woodpile photonic crystals, with a pseudo-bandgap for near-infrared optical wavelengths. By direct laser writing, we intentionally introduce random defects in the periodic structure. We show that we can model random defect scattering by considering the difference between the disordered and the regular structure. Our findings pave the way towards better control and understanding of the role of defects in photonic materials that will be crucial for their usability in potential applications.

physics.optics↗

arXiv2020

Experimental Tuning of Transport Regimes in Hyperuniform Disordered Photonic Materials

We present wave transport experiments in hyperuniform disordered arrays of cylinders with high dielectric permittivity. Using microwaves, we show that the same material can display transparency, photon diffusion, Anderson localization, or a full band gap, depending on the frequency $\nu$ of the electromagnetic wave. Interestingly, we find a second weaker band gap, which appears to be related to the second peak of the structure factor. Our results emphasize the importance of spatial correlations on different length scales for the formation of photonic band gaps.

cond-mat.dis-nn↗

arXiv2018

Transition from Light Diffusion to Localization in Three-Dimensional Amorphous Dielectric Networks near the Band Edge

Localization of light is the photon analog of electron localization in disordered lattices for whose discovery Anderson received the Nobel prize in 1977. The question about its existence in open three-dimensional materials has eluded an experimental and full theoretical verification for decades. Here we study numerically electromagnetic vector wave transmittance through realistic digital representations of hyperuniform dielectric networks, a new class of highly correlated but disordered photonic band gap materials. We identify the evanescent decay of the transmitted power in the gap and diffusive transport far from the gap. Near the gap, we find that transport sets off diffusive but, with increasing slab thickness, crosses over gradually to a faster decay, signaling localization. We show that we can describe the transition to localization at the mobility edge using the self-consistent theory of localization based on the concept of a position-dependent diffusion coefficient.

physics.optics↗

arXiv2018

Relationship between Rheology and Structure of Interpenetrating, Deforming and Compressing Microgels

Thermosensitive microgels are widely studied hybrid systems combining properties of polymers and colloidal particles in a unique way. Due to their complex morphology their interactions and packing, and consequentially the viscoelastcity of suspensions made from microgels, are still not fully understood, in particular under dense packing conditions. Here we study the frequency-dependent linear viscoelastic properties of dense microgel suspensions in conjunction with an analysis of the local particle structure and morphology based on superresolution microscopy. By identifying the dominating mechanisms that control the elastic and dissipative response, we propose a unified framework that can explain the rheology of these widely studied soft particle assemblies from the onset of elasticity deep into the overpacked regime. Our results clarify the transition and coupling between the regime dominated by fuzzy shell interactions and the one controlled by the densely cross-linked core.

cond-mat.soft↗

arXiv2018

Observation of strongly heterogeneous dynamics at the depinning transition in a colloidal glass

We study experimentally the origin of heterogeneous dynamics in strongly driven glass-forming systems. Thereto, we apply a well-defined force with a laser line trap on individual colloidal polystyrene probe particles seeded in an emulsion glass composed of droplets of the same size. Fluid and glass states can be probed. We monitor the trajectories of the probe and measure displacements and their distributions. Our experiments reveal intermittent dynamics around a depinning transition at a threshold force. For smaller forces, linear response connects mean displacement and quiescent mean squared displacement. Mode coupling theory calculations rationalize the observations.

cond-mat.soft↗