In appraising the originality and significance of the work described in the ISI New Hot Paper published in Advanced Functional Materials 2002 paper, "Opal circuits of light – planarized microphotonic crystal chips" by Miguez, Yang, and Ozin , which was proceeded by two earlier trend-setting background papers (see below) from Yang and Ozin, "Race for the photonic chip, opal-patterned chips" in Advanced Functional Materials 2001, and "Opal-chips: vectorial growth of colloidal crystal patterns inside silicon wafers" in Chemical Communications 2000, it is important to keep in mind that for colloidal crystals built from microspheres (interestingly known for more than half a century) to realize their full potential in microphotonic crystal devices and, ultimately, optically integrated microphotonic chips and circuits, they have to be fashioned in the form of optically functional planarized architectures—such as switch, mirror, filter, waveguide, laser, or superprism. This is an achievable goal provided simple and reproducible methods can be devised to make colloidal crystal lattices into microphotonic crystal components in chips that have a sufficiently high level of microstructural perfection and optical quality for envisioned optical telecommunication applications. This is essentially what we have achieved in this paper.

I believe the work described in the paper describes both of these things. Let me amplify upon this statement.

The paper "Opal circuits of light – planarized microphotonic crystal chips" and its two earlier companion papers mentioned above, teach a portfolio of directed colloidal self-assembly methods that enable construction of micrometer-scale patterned colloidal photonic crystal 3-D diffractive optic components with complex form and of the type that offer optical functionality and optical quality, which make them potentially suitable for envisioned applications in microphotonics.

The approaches described in these papers to achieve this objective utilize a convolution of strategies in geometrically confined microsphere crystallization and soft-lithographic patterning of surface relief patterns in substrates, and its versatility and effectiveness are demonstrated by reference but not limited to the construction of (a) colloidal photonic crystal microwells, (b) uniform thickness colloidal photonic crystal microchannels with a rectangular-shaped cross section, (c) uniform thickness colloidal photonic crystal microchannels with a V-shaped cross section (d) modulated thickness colloidal photonic crystal microchannels with a rectangular-shaped cross section, (e) modulated lattice dimension colloidal photonic crystal microchannels with a rectangular-shaped cross section and (f) a colloidal photonic crystal Lincoln log architecture.

While one of the main contributions of the work described in these papers, as well as in more detail in later papers from our group, focuses attention purely on the synthesis and assembly of such kinds of patterned colloidal photonic crystals, another equally important contribution concerns novel strategies for making detailed, spatially resolved optical microscope spectroscopy measurements of these patterned colloidal photonic crystals with simple and complex form, which is specifically intended to demonstrate their high structural and optical quality, the capacity to crystal engineer their colloidal photonic crystal properties, and their ability to function as tunable colloidal photonic crystal optical Bragg filters and mirrors.

The work describes a culmination of novel strategies in self-assembly and microfabrication by which micrometer-dimension, structurally well ordered, controlled size, shape, and orientation microsphere-based colloidal photonic crystals are grown exclusively within the spatial confines of geometrically pre-defined surface relief patterns on a substrate. Different methods of confinement-facilitated colloidal crystallization are employed to achieve this goal; these methods are founded on the ability to spatially direct, organize and crystallize microspheres within geometrically well-defined surface relief patterns in a substrate by colloidal self-assembly methods, exemplified in this and later papers but not limited to (a) directed evaporation induced self-assembly, (b) assisted directed evaporation induced self-assembly, (c) microfluidic self-assembly, (d) dip coating self-assembly and (e) spin-coating self-assembly. These methods provide strategies for patterning microsphere-based colloidal photonic crystals with well-defined lattice geometry and a high degree of structural order, and through the use of a novel microscope optical spectroscopy measurement technique demonstrated optical quality and designed optical functionality.

Specifically, the approach described, namely confined colloidal crystal self-assembly exclusively within geometrically well-defined surface relief patterns in a substrate, enables control over colloidal crystal structural order, shape, size, orientation and location as well as their colloidal photonic crystal properties. The work delineates detailed procedures for spatially directing microspheres to surface relief patterns specifically to enable the fabrication of geometrically pre-determined micrometer scale colloidal photonic crystal patterns with demonstrated structural order, optical quality, and optical functionality that make them potentially interesting platform materials for possible construction of microphotonic crystal devices, chips, and circuits that can be coupled to optical waveguides and optical fibers for envisioned optical computing and optical telecommunication applications (see reviews and papers written by photonic crystal pioneers Sajeev John, Eli Yablonovitch).

In a nutshell, what it describes are simple, quick, reproducible, and inexpensive methods for making planarized microphotonic crystal chips with potential applications in optical chip and lab-on-chip technologies. Methods show how to combine soft lithography to define geometrically well-defined surface relief patterns in a chip and the use of these patterns to spatially confine and thereby control the nucleation and growth of microspheres to achieve the first examples of vectorial control of structural order, thickness, area, orientation, and registry of patterned single crystal colloidal photonic crystals integrated into wafers with demonstrated high optical quality and optical functionality.

Planarized microphotonic crystals of this genre could prove useful for diverse kinds of 3-D diffractive optical components coupled to waveguides in optical chips. In this context, the application that has created the most excitement is the use of these structures, as templates for making photonic crystals comprised of high refractive index semiconductors like silicon. Photonic crystals of this type have a 3-D photonic band gap in the wavelength range 1.5 microns required for fiberoptic telecommunications. Such 3-D silicon photonic crystals are touted as the optical analogue of the transistor, namely a semiconductor for photons rather than electrons.

Colloidal photonic crystal circuits of light based on planarized microphotonic crystals made by the methods described in this paper, namely directed self-assembly – a fusion of microsphere crystallization and templated assembly - now seems to be a realistic and attainable goal. The ability for example, to make Lincoln-Log colloidal photonic crystal architectures attests to the potential that colloidal photonic crystal optics have in microphotonics.

This is actually an amusing story. One of the pioneers of the theory of photonic crystals, Professor Sajeev John is a colleague of mine in Physics at the University of Toronto. He was traveling the globe telling everybody about his invention and trying to persuade various folk to have a go at making the inverse silicon colloidal photonic crystal, which according to his theory should have a 3-D photonic band gap around 1.5 microns. A friend and colleague told him during these travels that one of the best people to bring his theory to reality is actually working in the building next door at his own University, namely Geoffrey Ozin and his materials chemistry research group. He came over and introduced himself; I never had heard of him, as at that time I was neither in the field of photonic crystals nor knowledgeable of the fundamental scientific importance and technological relevance of the area. We spoke and rest is history!

The fruits of this collaboration were published in a paper in Nature:

Blanco, A., et al., "Self-assembly of a silicon photonic bandgap material with a complete three-dimensional gap at 1.5 microns," Nature 405: 437-440, 2000.

First reported synthesis, of a silicon photonic crystal, with a complete photonic band gap at 1.5 mm. This realizes a long-standing goal in photonics, opening a door for complete control of radiative emission from atoms and molecules, light localization and integration of micron scale photonic devices into all-optical microchips.

This work in Nature is now one of the most-cited papers in the field of photonic crystals. It was very obvious to me that the next step absolutely had to be to grow these silicon photonic crystals in chips. Planarization and integration was the name of the game! Just like what pioneer and Nobel Laureate Jack Kilby did for transistors and electrons in the planarized and integrated microelectronic silicon chip, it was crystal clear that we now had to figure out how to make the same kind of thing but for photonic crystals and photons in the planarized and integrated silicon microphotonic chip.

The work described in our ISI New Hot Advanced Functional Materials paper under discussion together with two earlier papers listed below, is a first step in this direction:

• Miguez, H., Yang, S.M., Ozin, G.A., "Opal circuits of light – planarized microphotonic crystal chips," Adv. Funct. Mat. 12: 425-431, 2002.
• Yang, S.M., Ozin, G. A., "Race for the photonic chip, opal-patterned chips," Adv. Funct. Mater. 11: 1-10, 2001.
• Yang, S.M., Ozin, G. A., "Opal-chips: vectorial growth of colloidal crystal patterns inside silicon wafers," Chem. Commun. 2507: 2000.

Professor Geoffrey A. Ozin
Government of Canada Research Chair in Materials Chemistry
Materials Chemistry Research Group
Chemistry Department
University of Toronto
Toronto, Ontario, Canada