The natural world is a colorful environment. Stunning displays of coloration have evolved throughout nature to optimize camouflage, warning, and communication. The resulting flamboyant visual effects and remarkable dynamic properties, often caused by an intricate structural design at the nano- and microscale, continue to inspire scientists to unravel the underlying physics and to recreate the observed effects. Here, the methodologies to create bioinspired photonic pigments using colloidal self-assembly approaches are considered. The physics governing the interaction of light with structural features and natural examples of structural coloration are briefly introduced. It is then outlined how the self-assembly of colloidal particles, acting as wavelength-scale building blocks, can be particularly useful to replicate coloration from nature. Different coloration effects that result from the defined structure of the self-assembled colloids are introduced and it is highlighted how these optical properties can be translated into photonic pigments by modifications of the assembly processes. The importance of absorbing elements, as well as the role of surface chemistry and wettability to control structural coloration is discussed. Finally, approaches to integrate dynamic control of coloration into such self-assembled photonic pigments are outlined.

Colloidal assemblies have applications as photonic crystals and templates for functional porous materials. While there has been significant progress in controlling colloidal assemblies into defined structures, their 3D order remains difficult to characterize. Simple, low‐cost techniques are sought that characterize colloidal structures and assist optimization of process parameters. Here, structural color is presented to image the structure and dynamics of colloidal clusters prepared by a confined self‐assembly process in emulsion droplets. It is shown that characteristic anisotropic structural color motifs such as circles, stripes, triangles, or bowties arise from the defined interior grain geometry of such colloidal clusters. The optical detection of these motifs reliably distinguishes icosahedral, decahedral, and face‐centered cubic colloidal clusters and thus enables a simple yet precise characterization of their internal structure. In addition, the rotational motion and dynamics of such micrometer‐scale clusters suspended in a liquid can be followed in real time via their anisotropic coloration. Finally, monitoring the evolution of structural color provides real‐time information about the crystallization pathway within the confining emulsion droplet. Together, this work demonstrates that structural color is a simple and versatile tool to characterize the structure and dynamic properties of colloidal clusters.

Colloidal lithography is a cost-efficient method to produce large-scale nanostructured arrays on surfaces. Typically, colloidal particles are assembled into hexagonal close-packed monolayers at liquid interfaces and deposited onto a solid substrate. Many applications, however, require non close-packed monolayers, which are more difficult to fabricate. Preassembly at the oil/water interface provides non close-packed colloidal assemblies but these are difficult to transfer to a solid substrate without compromising the ordering due to capillary forces acting upon drying. Alternatively, plasma etching can reduce a close-packed monolayer into a non close-packed arrangement, however, with limited interparticle distance and compromised particle shape. Here, we present a simple alternative approach toward non close-packed colloidal monolayers with tailored interparticle distance, high order, and retained spherical particle shape. We preassemble poly(N-isopropylacrylamide)-silica (SiO2@PNiPAm) core–shell particles at the air/water interface, transfer the interfacial spacer to a solid substrate, and use the polymer shell as a sacrificial layer that can be thermally removed to leave a non close-packed silica monolayer. The shell thickness, cross-linking density, and the phase behavior upon compression of these complex particles at the air/water interface provide parameters to precisely control the lattice spacing in these surface nanostructures. We achieve hexagonal non close-packed arrays of silica spheres with interparticle distances between 400 and 1280 nm, up to 8 times their diameter. The retained spherical shape is advantageous for surface nanostructuring, which we demonstrate by the fabrication of gold nanocrescent arrays via colloidal lithography and silicon nanopillar arrays via metal-assisted chemical etching.

resonances. [5,6] A collective excitation (SLR) possesses a reduced linewidth as compared to the excitation of the LSPRs in nanoparticle ensembles. [1][2][3][4] Thus, SLRs can provide high quality-factor metasurfaces with substantial impact in nanophotonic and sensing applications. [1][2][3][4] Oblique incidence on planar metasurfaces constructed of an array of split-ring resonators showed that SLRs can also selectively respond to the handedness of circularly polarized light, causing sharp lattice-mode assisted extrinsic circular dichroism. [7,8] The spectral position of these SLRs can be tuned by the angle of incidence, leading to the emergence of extrinsic chiral surface lattice resonances. [9][10][11][12] Strong optical activity can also result from plasmonic 3D nanostructures without mirror symmetry. However, SLRs from arrays of 3D building-blocks with intrinsic chirality remain unexplored, but promise decreased losses and enhanced optical activity. [1,13] The fabrication of chiral 3D nanostructures is challenging for both, bottom-up and top-down methods. [14][15][16] Recently, the excitation of SLRs was seen in self-assembled, large area colloidal systems. [17,18] Such systems offer the possibility for fast manufacturing and covering large-areas on different substrate materials. [19] Colloid-based materials allow for embedding of nanoparticle arrays into flexible free-standing polymer films, [20] enabling the design of mechanically tunable [21] and stimuliresponsive hydrogel membranes. [22] Here, we use colloidal lithography [23][24][25] to fabricate arrays of 3D crescents with a selective response to the handedness of incident circularly polarized light, and we experimentally demonstrate handedness-dependent (chiral) SLRs at normal incidence. Colloidal lithography [23][24][25] is an experimentally simple and fast process to fabricate 3D chiral plasmonic nanostructures. [26][27][28] However, a necessary condition to observe SLRs in such nanostructure arrays is that their lattice constant must be in the range of the wavelength of the LSPRs of the individual nanostructures. [1][2][3][4] For objects with resonances in the near-infrared, such as, for example, split ring resonators, this requires large interparticle distances approaching the micrometer range. For conventional colloidal lithography processes, which depend on the controlled shrinkage of a polymer particle matrix, such distances are not easily achievable. [12,[28][29][30] Therefore, we use core-shell particles with rigid cores (silica) and soft, deformable polymer shells. [31,32] The particles selfassemble into hexagonally ordered monolayers at the air/water Collective excitation of periodic arrays of metallic nanoparticles by coupling localized surface plasmon resonances to grazing diffraction orders leads to surface lattice resonances with narrow line width. These resonances may find numerous applications in optical sensing and information processing. Here, a new degree of freedom of surface lattice resonances is experimentally investigated by dem...

Plasmonic effects in noble metal nanostructures are probably one of the most widely recognized forms of nanotechnology. The tremendous success and interdisciplinary use of plasmonic Chiral plasmonics is a fascinating research field that is attractive to scientists from diverse backgrounds. Physicists study light-matter interactions, chemists seek ways to analyze enantiomeric molecules, biologists study living objects, and material engineers focus on scalable production processes. Successful access to this emergent field for an interdisciplinary community depends on overcoming three main issues. First, understanding the physical background of chiral plasmonics requires proper introduction in easy language. Second, pitfalls in the characterization of chiroplasmonic features can prevent accurate interpretation. Third, simple and robust methods capable of covering macroscopic substrate areas must be available. This tutorial-style review addresses these issues with the goal to provide a comprehensive introduction into chiral plasmonic nanostructures. It starts with a brief introduction of the relevant physics involved in chiral light−matter interactions. A brief guide about how to adequately characterize samples follows. Subsequently, an overview of fabrication techniques that produce chiral substrates over large areas is given, and the strengths and weaknesses of the different approaches are discussed. The focus is on simple and robust processes that do not require clean room facilities and can be implemented by a much larger scientific audience.