High-permittivity dielectric particles with resonant magnetic properties are being explored as constitutive elements of new metamaterials and devices. Magnetic properties of low-loss dielectric nanoparticles in the visible or infrared are not expected due to intrinsic low refractive index of optical media in these regimes. Here we analyze the dipolar electric and magnetic response of lossless dielectric spheres made of moderate permittivity materials. For low material refractive index (<∼3) there are no sharp resonances due to strong overlapping between different multipole contributions. However, we find that Silicon particles with index of refraction∼3.5 and radius∼200 nm present strong electric and magnetic dipolar resonances in telecom and near-infrared frequencies, (i.e. at wavelengths≈1.2-2 mm) without spectral overlap with quadrupolar and higher order resonances. The light scattered by these Si particles can then be perfectly described by dipolar electric and magnetic fields.

Superresolution microscopy unravels the nanoscale properties of densely packed stimuli-responsive polymer microgels.

required more than 35 years. We believe that the retention of an ectothermal physiology constrained Deinosuchus to the deposition of slow-forming somatic tissues (such as lamellar bone) throughout development, necessitating a greater developmental time to reach dinosaurian proportions.

We study photonic band gap formation in two-dimensional high-refractive-index disordered materials where the dielectric structure is derived from packing disks in real and reciprocal space. Numerical calculations of the photonic density of states demonstrate the presence of a band gap for all polarizations in both cases. We find that the band gap width is controlled by the increase in positional correlation inducing short-range order and hyperuniformity concurrently. Our findings suggest that the optimization of shortrange order, in particular the tailoring of Bragg scattering at the isotropic Brillouin zone, are of key importance for designing disordered PBG materials. DOI: 10.1103/PhysRevLett.117.053902 Photonic band gap (PBG) materials exhibit frequency bands where the propagation of light is strictly prohibited. Such materials are usually designed by arranging highrefractive-index dielectric material on a crystal lattice [1,2]. The description of wave transport in a periodically repeating environment provides a clear physical mechanism for the emergence of PBGs, in analogy to common electronic semiconductors. It is also known that certain aperiodic dielectric structures, such as quasicrystals [3][4][5], can display a full PBG. Over the last decade disordered or amorphous photonic materials have gained growing attention [6][7][8][9][10][11][12][13][14][15][16][17][18]. This trend is motivated by the many disordered photonic materials found in nature that reveal fascinating structural color effects in plants, insects, and mammals [19]. At the same time, fabricating perfect crystalline structures with photonic properties at optical wavelengths has proven to be more difficult than initially anticipated [20]. It has been argued that disordered PBG materials should be less sensitive to fabrication errors or defects and thus promise a more robust design platform [15]. Moreover PBGs in disordered dielectrics are isotropic, which could make it easier to achieve a full PBG while at the same time offering better performance in wave guiding, design of noniridescent stable pigments, and display applications [21][22][23][24].Yet, until recently, direct evidence for the existence of full PBGs in disordered photonic materials had been scarce and the fabrication principles and physical-optical mechanism leading to PBG formation remained obscure. Although the importance of appropriate short-range order for the development of PBGs in disordered photonic materials was discovered early on [6-9], a strategy to maximize the PBG width was lacking.In 2009 Florescu and co-workers [25] proposed a new approach for the design of disordered PBG materials that has attracted widespread attention. They introduced the concept of hyperuniformity for photonic structures, which enforces a certain type of short-range order. In particular, so-called stealthy hyperuniform (SHU) disordered patterns were reported to be fully transparent to incident long-wavelength radiation [26,27] and lead to strong isotropic PBGs at shorter wavelengths [25]. Ot...

The frequency-dependent shear modulus of aqueous wormlike micellar solutions of cetylpyridinium chloride (CPyCl) and sodium salicylate (NaSal) has been measured over a broad frequency range from 10 -2 to 10 6 rad/s using diffusing wave spectroscopy (DWS) based tracer microrheology as well as mechanical techniques including rotational rheometry and oscillatory squeeze flow. Good agreement between mechanical and optical techniques is found in the frequency range from 10 -1 to 10 5 rad/s (Willenbacher, N.; Oelschlaeger, C.; Schopferer, M.; Fischer, P.; Cardinaux, F.; Scheffold, F. Phys. ReV. Lett. 2007, 99 (6), 068302). At intermediate frequencies between 10 and 10 4 rad/s, squeeze flow provides most accurate data and is used to determine the plateau modulus G 0 , which is related to the cross-link density or mesh size of the entanglement network, as well as the scission energy E sciss , which is deduced from the temperature dependence of the shear moduli in the plateau zone. In the frequency range above 10 4 rad/s, DWS including a new inertia correction is most reliable and is used to determine the persistence length l p . The system CPyCl/NaSal is known to exhibit two maxima in zero-shear viscosity and terminal relaxation time as the salt/surfactant ratio R is varied (Rehage, H.; Hoffman, H. J. Phys. Chem. 1988, 92 (16), 4712-4719). The first maximum is attributed to a transition from linear to branched micelles (Lequeux, F. Europhys. Lett. 1992, 19 (8), 675-681), and the second one is accompanied by a charge reversal due to strongly binding counterions. Here, we discuss the variation of G 0 , E sciss , and l p with salt/ surfactant ratio R at constant surfactant concentration of 100 mM CPyCl. G 0 increases at the linear-to-branched micelles transition, and this is attributed to the additional contribution of branching points to the cross-link density. E sciss exhibits two maxima analogous to the zero-shear viscosity, which can be understood in terms of the variation of micellar length and variation of the amount of branched micelles and contour length between branching points consistent with the results of a comprehensive cryo-transmission electron microscopy (TEM) study (Abezgauz, L.; Ramon, O.; Danino, D. Department of Biotechnology and Food Engineering, Technion, Haifa, Israel. European Colloid and Interface Society, Geneva, 2007). The persistence length decreases with increasing R. This decrease is stronger than expected from the decrease of Debye length according to the Odijk-Skolnick-Fixman (OSF) theory and is attributed to the penetration of salicylate ions into the micelles; the linear-to-branched transition obviously does not have an effect on l p .

The optical and structural properties of dense colloidal suspensions in the presence of long-range electrostatic repulsion are determined from both light and small-angle neutron scattering experiments. Short-range structural order induces an enhancement of the scattering strength while at the same time the total transmission shows strong wavelength dependence, reminiscent of a photonic crystal. Interestingly, the interplay between diffusive scattering and local order leads to negative values of the scattering anisotropy parameter. The tunable optical properties of these liquids furthermore suggest potential applications such as transparency switches or filters. DOI: 10.1103/PhysRevLett.93.073903 PACS numbers: 42.25.Bs, 42.70.Qs, 82.70.Dd When light is incident on a nonabsorbing material, its further propagation is strongly influenced by the microscopic structure of the material itself. For a bulk homogeneous medium, light is refracted according to Snell's law. Local variations in the dielectric properties lead to isolated scattering events that disperse the light beam. The scatterer density and cross section define the scattering mean free path l. As the number of scattering events increases, the transport of light becomes diffusive and the material appears turbid or ''white'' [1,2]. The relevant scattering length for diffusive light transport is the transport mean free path l . Both quantities are connected by the scattering anisotropy parameter g defined as the average of the cosine of the scattering angle g hcosi, l=l 1 ÿ g. Our current understanding of the diffusive transport is based on the knowledge of these key quantities. In the absence of positional correlations, l is usually equal to or larger than l [2 -5]. For instance, Mie particles (or human tissue [1]) scatter strongly in the forward direction (small scattering angles ) and hence g ' 1 while for Rayleigh scatterers g ' 0. Here we show that these common properties of diffusive transport can be manipulated by tuning the interaction between scatterers. By the appropriate control of the Coulomb repulsion between highly charged particles in suspension, we are now able to access the whole possible interval of g values (from forward scattering g ! 1 to the unusual case of strong backscattering g ! ÿ1).When mesoscopic variations of the dielectric constant can be neatly controlled over macroscopic distances, totally new, so-called photonic properties may appear [6]. At the core of the design of new photonic materials lies the intelligent way structures are assembled on length scales comparable to the wavelength of light. There are two main promising concepts to achieve lossless guidance and manipulation of light based on seemingly opposite principles: order or disorder. Photonic band gap materials are based on periodic structures predicted to inhibit light propagation completely [6]. In the case of disorder, light cannot propagate in the material due to recurrent interference called strong Anderson localization [7].Tailoring microstructures with an app...

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 viscoelasticity 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 suspensions of micron sized soft particles 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 can explain the rheology of these widely studied soft particle assemblies from the onset of elasticity deep into the overpacked regime. Interestingly, our results suggest that the friction between the microgels is reduced due to lubrification mediated by the polymer brush-like corona before the onset of interpenetration.

Using a simplified microstructural picture we show that interactions between thermosensitive microgel particles can be described by a polymer brushlike corona decorating the dense core. The softness of the potential is set by the relative thickness L 0 of the compliant corona with respect to the overall size of the swollen particle R. The elastic modulus in quenched solid phases derived from the potential is found to be in excellent agreement with diffusing wave spectroscopy data and mechanical rheometry. Our model thus provides design rules for the microgel architecture and opens a route to tailor rheological properties of pasty materials.PACS numbers: 83.80. Kn, 82.70.Dd, 82.70.Gg Thermoresponsive microgel particles are a hybrid between a colloidal and a polymeric system with properties that can be tuned externally [1][2][3]. Most of the previously studied stimuli responsive microgel systems are based on poly(N-isopropyl-acrylamide) (PNIPAM), a polymer which has a lower critical solution temperature (LCST) of approximately 33 C [1]. Above the LCST, the microgel particles expel water and are collapsed. The typical size of a collapsed microgel particle is in the range of 0:2-1 m. Upon lowering the temperature the particles swell to about twice their original size. Responsive microgels thus provide the possibility to fabricate smart colloidal materials for applications as viscosity modifiers, carrier systems, bioseparators, optical switches, or sensors [1,2,4]. Moreover, due to their tunability they are ideal model systems to study the phase behavior, glass transition, and jamming in dense colloidal dispersions [5][6][7].Particles come into contact and form a viscoelastic paste below the LCST if the polymer density inside the swollen particles approaches the total polymer density. A number of rheological studies have revealed the apparent divergence of the viscosity at the transition point and the emergence of an elastic shear modulus [2,3]. Since the particles consist of polymers and solvent in equilibrium, they are elastically compliant, and thus elastic moduli do not diverge at the jamming transition, unlike the behavior of rigid colloidal particles [7,8]. Several experimental studies show that in this regime the bulk modulus as a function of the effective volume fraction scales as a power law G p / 1þn=3 eff with values of n ranging from n ¼ 9 to n ¼ 22 [2,3]. These findings indicate that the particle interaction potential c ðrÞ / r Àn also follows a power law, at least over some significant range of length scales [2,3]. Unfortunately, a detailed model for the origin of these interactions has been missing. This is mainly due to the fact that modeling the interaction between swollen particles is complicated by the heterogeneous microstructure arising from the faster reaction rate of cross-linking compared to polymerization [9]. In this Letter we show that interactions between thermosensitive microgel particles can be modeled by a polymer brushlike corona decorating the dense core [10][11][12]. The softness ...