May 09, 2011

Self-assembled colloidal structures for photonics

Colloidal crystals for photonics applications. (Left) Colloidal crystals can be formed into any shape to achieve desired reflectance properties. (Center) Inverse structures can be obtained by infiltrating the crystal with various materials and then removing the template particles (colloidal templating). (Right) Short-range-ordered structures of colloidal particles or colloidal glasses can be assembled to achieve angle-independent coherent optical scattering.

Colloidal self-assembly has been investigated as a promising and practical approach for the fabrication of photonic nanostructures, including colloidal crystals, composite and inverse opals, and photonic glasses. Depending on the interactions between the colloidal particles, colloidal structures can be affected dramatically and modulated by applying an additional external field. Furthermore, in contrast to other approaches, self-assembled nanostructures with large areas or designed shapes can be prepared at low cost. As a result, the use of such colloidal systems has been investigated in many practical photonic applications. In this review article, we describe the colloidal self-assembly of periodic and non-periodic photonic nanostructures in brief and then summarize recent achievements in the field of colloidal photonic nanostructures and their applications, which include displays, optical devices, photochemistry and biological sensors.

Schematic summary of display devices based on colloidal-structured photonics. (a) Passive-mode display with pixelated photonic crystals. (b) Active-mode display based on dynamic modulation of lattice constant. (c) Gyricon display based on rotation of Janus photonic microspheres. (d) Display device with isotropic colloidal structure of short-range order for angle-independent structural color.
Over the past few decades, several different approaches have been employed to produce colloidal nanostructures in investigations of photonic applications. This research, conducted by many different research groups, has expanded the range of applications to include nonphotonic materials, which might result for instance in the convergence of photonics and biomedical applications and in energy-harvesting devices. In particular, colloidal crystals and composite and inverse opals have been investigated and modified for specific applications. In spite of their limited optical response, or in other words their incomplete pseudo-photonic bandgap, they can be successfully implemented in relatively simple applications. By adding functional molecules into such colloidal nanostructures, photonic crystal-based sensor devices have been developed and their technological level has in some cases matured close to the point of commercialization or the opening of new markets. New types of photonic nanostructures, such as photonic glasses, have been introduced for angle-independent light reflection, random lasing and other applications.

In some original applications of photonic crystals, several research groups have fabricated optical circuits out of colloidal particles and demonstrated their great potential. However, it is still difficult to fabricate defect-free colloidal crystals and then to create prescribed defects because of the inherent defects that arise during self-assembly. As a result, research activity in this area has gradually decreased. However, in a new approach, the use of selective chemical glue on particle surfaces such as DNA or peptides has been proposed. Further developments might enable the fabrication of better photonic devices based on colloidal particles. Moreover, recent progress in the synthesis of nonspherical particles is accelerating research into the colloidal crystallization of new crystal phases. One promising strategy with strong potential for producing new photonic properties is the use of directional interparticle interaction with chemical patterns on a colloid surface to create and design new types of colloidal lattices.

Patterning of photonic crystals. (a) Patterning process for two different colloidal crystals by combining the MIMIC technique and vertical deposition. (b) The pixelation method for red, green and blue (RGB) colors. Inverse opal with pores filled with SU-8 is exposed twice under UV light via a line-pattern photomask at 0° and 90°. (c) Patterning of one-dimensional photonic crystals of colloidal chain-like structures. Superparamagnetic particles are aligned into chains under an external magnetic field. Selective localized UV exposure then fixes the structure. (d) Optical microscopy image of blue and yellow dot patterns forming the butterfly image inset.

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