Two-photon excitation provides a means of activating chemical or physical processes with high spatial resolution in three dimensions and has made possible the development of three-dimensional fluorescence imaging, optical data storage, and lithographic microfabrication. These applications take advantage of the fact that the two-photon absorption probability depends quadratically on intensity, so under tight-focusing conditions, the absorption is confined at the focus to a volume of order λ3 (where λ is the laser wavelength). Any subsequent process, such as fluorescence or a photoinduced chemical reaction, is also localized in this small volume. Although three-dimensional data storage and microfabrication have been illustrated using two-photon-initiated polymerization of resins incorporating conventional ultraviolet-absorbing initiators, such photopolymer systems exhibit low photosensitivity as the initiators have small two-photon absorption cross-sections (δ). Consequently, this approach requires high laser power, and its widespread use remains impractical. Here we report on a class of π;-conjugated compounds that exhibit large δ (as high as 1, 250 × 10−50 cm4 s per photon) and enhanced two-photon sensitivity relative to ultraviolet initiators. Two-photon excitable resins based on these new initiators have been developed and used to demonstrate a scheme for three-dimensional data storage which permits fluorescent and refractive read-out, and the fabrication of three-dimensional micro-optical and micromechanical structures, including photonic-bandgap-type structures.

Two-photon polymerization initiators for three-dimensional optical data storage and microfabrication

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.

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Self-assembled photonic structures.

3D photonic crystals (PhCs) and photonic bandgap (PBG) materials have attracted considerable scientific and technological interest. In order to provide functionality to PhCs, the introduction of controlled defects is necessary; the importance of defects in PhCs is comparable to that of dopants in semiconductors. Over the past few years, significant advances have been achieved through a diverse set of fabrication techniques. While for some routes to 3D PhCs, such as conventional lithography, the incorporation of defects is relatively straightforward; other methods, for example, self-assembly of colloidal crystals (CCs) or holography, require new external methods for defect incorporation. In this review, we will cover the state of the art in the design and fabrication of defects within 3D PhCs. The figure displays a fluorescence laser scanning confocal microscopy image of a y-splitter defect formed through two-photon polymerization within a CC.

Introducing Defects in 3D Photonic Crystals: State of the Art

A variety of methods have been successfully used to produce crystals of colloidal particles made of polymeric (or silica) microspheres. Achieving highly accurate growth and control of the packing symmetry, packing efficiency and packing quality of these crystals is of paramount importance for many applications, for example in photonics. If colloidal crystals are formed in self-assembly processes, it is usually the most densely packed (111) set of planes that terminates the crystal–air interface. However, often exposure of (given) different packing facets is required at the crystal surface. In addition, there is (e.g. in photonics) need for crystals exhibiting lower than the tightest packing, and possessing also lower degrees of packing symmetry. These requirements demand development of various engineering approaches for controlled particle assembly in regular structures. The synthesis of polymer colloidal particles with different sizes, shapes and surface charge density is first briefly outlined. The various interactions and forces that control growth for a broad range of colloidal crystals are subsequently discussed. In the main section of this review we give an account of various template-assisted, graphoepitaxial assembly approaches to produce colloidal crystals with tailored packing structures and controlled crystal orientation with respect to the topologically patterned substrates used to direct the assembly process. In the outlook we also describe various selected emerging approaches, which have the potential to produce crystals with low degree of packing symmetries, for example using direct one-to-one colloidal particle assembly.

Colloidal crystal assembly on topologically patterned templates

Colloidal crystals (CCs) are ordered arrays of monodisperse colloidal particles. Because of the presence of a periodic superlattice of colloids and voids, CCs bear many unique structures and fascinating properties which are distinctly different from those of standard crystals with atoms, ions or molecules as repeating subunits. The research on CCs has surprisingly blossomed during recent decades, and many practical materials based on CCs with potential applications in photonic devices, material science and biomedical engineering have been generated. In this review, we give a systematic, balanced and comprehensive summary of the main aspects of CCs related to their preparation and application, and propose perspectives for the future developments of CCs.

Current status and future developments in preparation and application of colloidal crystals

An electron-beam lithography technique is described capable of structuring three-dimensional self-assembled photonic crystals. It is shown that the control of the writing depth can be achieved by varying the electron acceleration voltage. Microscopic structures with a depth from 0.4 up to 2 μm are fabricated with a typical lateral resolution of 0.4 μm. The relevance of this technique for the fabrication of deterministic defects sites in opal photonic crystals is discussed and its extension towards buried structures is suggested.

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Structuring of self-assembled three-dimensional photonic crystals by direct electron-beam lithography

The growth of an opal-like polymer photonic crystal (PhC) on deeply etched silicon wafers is reported It is shown that 10 μm deep trenches, as narrow as 10 μm can be uniformly filled by self-assembly of microspheres, in a close-packed face-centered-cubic lattice These observations are confirmed by optical reflectance measurements in the visible range, in agreement with theoretical calculations of the photonic band gap A slight fluctuation of the lattice parameter is noticed in the case of the narrowest channels The possibility to detach the PhC from the substrate is also demonstrated The potential of this approach for building complex PhC-based complex architectures is discussed

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Self-assembly of three-dimensional photonic crystals on structured silicon wafers

We present a robust method for computing the reflection of arbitrarily shaped and sized beams from finite thickness photonic crystals. The method is based on dividing the incident beam into plane waves, each of which can be solved individually using Bloch periodic boundary conditions. This procedure allows us to take a full advantage of the crystal symmetry and also leads to a linear scaling of the computation time with respect to the number of plane waves needed to expand the incident beam. The algorithm for computing the reflection of an individual plane wave is also reviewed. Finally, we find an excellent agreement between the computational results and measurement data obtained from opals that are synthesized using polystyrene and poly(methyl methacrylate) microspheres.

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Reflection of focused beams from opal photonic crystals.

Optical properties of several heterostructures representing two-layer opaline photonic crystals have been examined. Two separate stop-bands have been observed both in transmission and emission spectra. The effect of the interface disorder on the optical spectra was not observed, probably, due to the insufficient degree of order of the opaline layers.

M. Egen

Papers

Structuring of self-assembled three-dimensional photonic crystals by direct electron-beam lithography

Self-assembly of three-dimensional photonic crystals on structured silicon wafers

Reflection of focused beams from opal photonic crystals.

Photonic crystals based on two-layer opaline heterostructures

Self-assembly of three-dimensional photonc crystals on structured silicon substrates