Photonic bandgap crystals can reflect light for any direction of propagation in specific wavelength ranges1,2,3. This property, which can be used to confine, manipulate and guide photons, should allow the creation of all-optical integrated circuits. To achieve this goal, conventional semiconductor nanofabrication techniques have been adapted to make photonic crystals4,5,6,7,8,9. A potentially simpler and cheaper approach for creating three-dimensional periodic structures is the natural assembly of colloidal microspheres10,11,12,13,14,15. However, this approach yields irregular, polycrystalline photonic crystals that are difficult to incorporate into a device. More importantly, it leads to many structural defects that can destroy the photonic bandgap16,17. Here we show that by assembling a thin layer of colloidal spheres on a silicon substrate, we can obtain planar, single-crystalline silicon photonic crystals that have defect densities sufficiently low that the bandgap survives. As expected from theory, we observe unity reflectance in two crystalline directions of our photonic crystals around a wavelength of 1.3 micrometres. We also show that additional fabrication steps, intentional doping and patterning, can be performed, so demonstrating the potential for specific device applications.

On-chip natural assembly of silicon photonic bandgap crystals

We describe the recent experimental progress in the control of spontaneous emission by manipulating optical modes with photonic crystals. It has been clearly demonstrated that the spontaneous emission from light emitters embedded in photonic crystals can be suppressed by the so-called photonic bandgap, whereas the emission efficiency in the direction where optical modes exist can be enhanced. Also, when an artificial defect is introduced into the photonic crystal, a photonic nanocavity is produced that can interact with light emitters. Cavity quality factors, or Q factors, of up to 2 million have been realized while maintaining very small mode volumes, and both spontaneous-emission modification (the Purcell effect) and strong-coupling phenomena have been demonstrated. The use of photonic crystals and nanocavities to manipulate spontaneous emission will contribute to the evolution of a variety of applications, including illumination, display, optical communication, solar energy and even quantum-information systems.

Spontaneous-emission control by photonic crystals and nanocavities

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.

/pdf/self-assembled-photonic-structures-4qp8mc5drj.pdf

Self-assembled photonic structures.

Inverse opals or three-dimensionally ordered macroporous (3DOM) materials, prepared by colloidal crystal templating (CCT) methods, possess distinctive structural features that are important for the design of photonic crystals, sensors, power sources, catalysts, and various other applications. In the past decade, vast progress has been made in shaping these materials at various length scales. It is now possible to prepare 3DOM materials in a variety of simple and complex compositions, in which pore sizes, pore shapes, and skeletal geometries can be adjusted statically and sometimes even dynamically to alter the physical properties of the material. Hierarchical pore structures can be achieved by combining CCT with additional templating techniques. Furthermore, shaped porous nanoparticles may be synthesized using CCT methods. This review will highlight recent advances in controlling both the internal structure of inverse opals and their external morphology, enhancing compositional complexity, endowing these ...

Morphological Control in Colloidal Crystal Templating of Inverse Opals, Hierarchical Structures, and Shaped Particles†

Thin opaline films (see Figure), which are self-assembled from colloidal microspheres, have recently been used to explore the properties of photonic crystals. However, the detailed growth mechanism of these opals is not known. Here the underlying issues involved in the growth are discussed, and it is suggested that solvent flow, which to date has not been seriously considered, could play a critical role in controlling the formation process.

Opaline Photonic Crystals: How Does Self‐Assembly Work?

We show how optical microscopy can be used to study the optical properties of a single crystalline domain in a self-assembled photonic crystal. By measuring spatially resolved reflection and emission spectra from a synthetic opal, inhomogeneities due to averaging over inherent disorder can be avoided. From “defect-free” reflection and emission spectra, the intrinsic photonic band structure can be extracted and inhibition of spontaneous emission can be verified.

Single-domain spectroscopy of self-assembled photonic crystals

Summary from only given. Three-dimensional silicon photonic band gap crystals are obtained by combining planar self-assembly of colloidal spheres with standard silicon deposition techniques. This simple and inexpensive approach, which is compatible with conventional microelectronics fabrication, yields high refractive- index inverted opals of controllable thickness that are integrated directly onto a silicon wafer. These photonic bandgap structures, which operate at telecommunications wavelengths, are planar single-crystals over large areas with very low defect densities.

On-chip assembly of silicon photonic band gap crystals

Summary form only given. Recently, several groups have reported inverted opals that may satisfy the requirements for a complete photonic band gap (PBG). These photonic crystals, which represent a face-centered cubic (FCC) lattice of spherical voids in a high refractive index material, were made from materials with sufficiently high refractive index to open the PBG. However, a fundamental question remains about how the intrinsic disorder present in opal photonic crystals will affect the PBG. Deviations in the lattice constant, point defects in the lattice, and the tendency to form stacking faults during the self-assembly of inverted opals, results in significant inhomogeneous broadening of the stop-bands and eventually can eliminate the PBG. We report quantitative measurements of the higher frequency photonic band structure in opal photonic crystals in the region of the eighth and ninth bands. These bands should evolve toward the complete PBG with increase of the refractive index contrast.

Evolution of the higher order band structure in FCC photonic crystals

Summary form only given Here we show how to use self-organization to make three-dimensional semiconductor photonic crystals operating at optical wavelengths Our approach combines colloidal chemistry with two-steps of hierarchical self-assembly First monodisperse submicron silica spheres slowly settle onto a flat substrate and self-organize as a (mostly) face-centered cubic (fcc) lattice that is periodic on an optical length scale Second, this template serves as a three-dimensional scaffolding for the self-assembly of nanometer-scale CdSe colloidal quantum dots Subsequently, the silica template is removed by selective etching and a three-dimensionally patterned material consisting solely of densely-packed semiconductor quantum dots is obtained Therefore, the exact three-dimensional structure of the template is imprinted in the final material

Semiconductor quantum-dot photonic crystals

Response to “Comment on ‘Enhancement of the optical gain of semiconductors embedded in three-dimensional photonic crystals’ ” [Appl. Phys. Lett. 73, 550 (1997)]

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