Liquid Crystal–Based Photonic Devices: LC Photonet

LC Photonet is a BRITE‐Euram Thematic Network that has been running since November 1997. It has 19 partners, 11 from the academic world and 8 from industrial groups. Geographically, it covers the following EU countries: Sweden, Denmark, United Kingdom, Germany, France, Italy, and Spain. The main goal of LC Photonet is to address the specific problems that must be solved in order to enlarge the range of applications of liquid crystals (LCs) for the realization of optical and electro‐optical devices. Applications of such devices can be found in the following fields: integrated communication technology, optical imaging and processing, light shutters and modulators, optical memories, and large area optical devices. The expertise of our partners spans from: (a) chemistry—new materials development and testing; (b) physics—study of new optical and electro‐optical effects, device design, and measurements; (c) engineering—device realization and testing, device optimization. We have divided our activities into four main tasks.     

Electroactive Soft Photonic Devices for the Synesthetic Perception of Color and Sound

Color, as perceived through the eye, transcends mere information in the visible range of electromagnetism and serves as an agent for communication and entertainment. Mechanochromic systems have thus far only aimed at satisfying the sense of vision and have overlooked the possibility of generating acoustic vibrations in concert with their visual color responses that would enable the simultaneous satisfaction of the auditory system. Transcending the boundaries of the two senses (i.e., sound and color), herein a strategy for their concurrent and synesthetic fulfillment is elucidated by electrically actuating an organogel photonic device, controlled by a single input signal. This new class of devices that integrate a color module with a speaker is fabricated from a mechanochromic layer that comprises close‐packed photonic lattice with an organogel matrix pervading the void fraction. Exploiting a dielectric elastomer actuator, the system's mechanical response permits the simultaneous, yet independent, exploration of visible‐light reflection alongside audible sound‐wave generation. Large areal strains at low frequencies of actuation tune the photonic stop‐band, whereas the layer remains incompressible and exhibits negligible strain when actuated at higher frequencies (e.g., tens of Hz), thereby making it amenable to modulate sound and color simultaneously yet independently.     

Photonic Bandgaps in Disordered Inverse‐Opal Photonic Crystals

Three‐dimensional photonic crystals with full bandgaps at optical wavelengths can be fabricated with inverse‐opal techniques. We have shown that the bandgap is extremely sensitive to the presence of geometric disorder in the crystals (see Figure). The bandgap closes completely with a disorder strength as small as under two percent of the lattice constant. This fragility persists even at very high refractive index contrasts and is attributed to the creation of a bandgap at high frequency bands (8–9 bands) in inverse‐opal crystals. This should impose severe demand on the quality of lattice uniformity.     

Photonic Microcapsules Containing Single‐Crystal Colloidal Arrays with Optical Anisotropy

Colloidal particles with a repulsive interparticle potential spontaneously form crystalline lattices, which are used as a motif for photonic materials. It is difficult to predict the crystal arrangement in spherical volume as lattices are incompatible with a spherical surface. Here, the optimum arrangement of charged colloids is experimentally investigated by encapsulating them in double‐emulsion drops. Under conditions of strong interparticle repulsion, the colloidal crystal rapidly grows from the surface toward the center of the microcapsule, forming an onion‐like arrangement. By contrast, for weak repulsion, crystallites slowly grow and fuse through rearrangement to form a single‐crystal phase. Single‐crystal structure is energetically favorable even for strong repulsion. Nevertheless, a high energy barrier to colloidal rearrangement kinetically arrests the onion‐like structure formed by heterogeneous nucleation. Unlike the isotropic onion‐shaped product, the anisotropic single‐crystal‐containing microcapsules selectively display—at certain orientations but not others—one of the distinct colors from the various crystal planes.     

Silicon‐Based Photonic Crystals

Photonic crystals can be thought of as optical analogues of semiconductors. Here recent advances in photonic crystals based on silicon are reviewed. After summarizing the theory of photonic bandgap materials, the preparation and linear optical properties of 1D, 2D, and 3D silicon‐based photonic crystals are discussed. Laterally structured porous silicon with a defect line is shown in the Figure.     

Perfecting Imperfection—Designer Defects in Colloidal Photonic Crystals

Current progress in the exciting and burgeoning field of functional defects in colloidal photonic crystals (CPCs) is reported. After a brief introduction into the importance and nature of defects in CPCs the state‐of‐the‐art in fabricating point, line, and planar defects is described. Measurement and characterization techniques as well as the corresponding theory are discussed. Besides normal, passive defects, the recent development of reversibly tunable defects adds important functionality. In particular, the addition of chemical functionality is demonstrated to open a path to a wide range of color readout devices for ultrasensitive optical detection of biomolecules and pharmaceuticals.     

Self‐Assembled Photonic Structures

Photonic crystals have proven their potential and are nowadays a familiar concept. They have been approached from many scientific and technological flanks. Among the many techniques devised to implement this technology self‐assembly has always been one of great popularity surely due to its ease of access and the richness of results offered. Self‐assembly is also probably the approach entailing more materials aspects owing to the fact that they lend themselves to be fabricated by a great many, very different methods on a vast variety of materials and to multiple purposes. To these well‐known material systems a new sibling has been born (photonic glass) expanding the paradigm of optical materials inspired by solid state physics crystal concept. It is expected that they may become an important player in the near future not only because they complement the properties of photonic crystals but because they entice the researchers’ curiosity. In this review a panorama is presented of the state of the art in this field with the view to serve a broad community concerned with materials aspects of photonic structures and more so those interested in self‐assembly.