Method of self-assembly and optical applications of crystalline colloidal patterns on substrates

#The present invention relates to methods of synthesis and optical applications of composite materials comprised of crystalline colloidal patterns on substrates, and more particularly the present invention is related to a method of synthesizing crystalline photonic crystals on the surfaces of, or embedded within, substrates and optical utility thereof.

#Photonics is the science of molding the flow of light. A photonics revolution is currently taking place and it is anticipated that the use of photons to carry information will be as important as electrons are in microelectronics. To realize this fundamental change photonic materials need to be developed that enable for example the generation, guiding, detection, amplification, attenuation, modulation, switching, filtering, coupling, splitting, shifting, frequency doubling and tripling and storing processing operations of light [Made to Measure: New Materials for the 21st Century, P. Ball, Princeton U Press, 1997].

#A new material (and devices derived therefrom) that is expected to prove useful in this regard is the photonic crystal (PC). The structure of a photonic crystal is based on for example a lattice filled with regular arrays of micron size air holes. When the refractive index contrast (RIC) between the voids and the material comprising the lattice is sufficiently large the photonic crystal has the property of diffracting light according to Bragg's law. This leads to the development of a photonic band gap (PBG) in the photon density of states (PDS) and the inability of light to propagate in the photonic crystal at wavelengths corresponding to the PBG.

#The methods of synthesis of colloidal crystals on patterned substrates according to the present invention will now be described, by way of example only, reference being made to the accompanying drawings, in which:

#FIG. 1 shows diagrammatic representation illustrating an exemplary, non-limiting synthetic strategy illustrating vectorial control of thickness, area, lattice structure, orientation and registry of silica colloidal crystals in silicon wafers for lab-on-chip and photonic chip technologies, the procedure for making the Si (100) substrate patterned with V-shaped grooves comprises several steps including

#step (i) an ethanolic hexadecanethiol solution (2 mM) is inked on the PDMS master;

#The present invention discloses six, straightforward, rapid, reproducible and inexpensive methods that combine microfluidic and spin-coating, dip coating and two new methodologies with colloidal crystal self-assembly and lithography to produce patterned crystalline colloidal crystals embedded on the surface of or within substrates, such as but not limited to, semiconductor wafers. These techniques enable the planarization of micron scale colloidal crystal-based 3-D PCs that are demonstrated to function as ideal single crystal 1-, 2- and 3-D diffractive optical components. This invention has led to the creation of 1-, 2- and 3-D micro-photonic crystal devices that are packaged in a planar substrate and which may provide the enabling technology for highly compact optical circuits in which 3-D micro-photonic crystals are integrated with optical waveguides and optical fibers. Colloidal crystal patterns in or on substrates can be used for templating micron scale, structural defects in photonic crystals (PC) and integrating PC devices within photonic chips that may be coupled to optical waveguides including but not limited to optical fibers. Since structural defects in photonic crystals can be designed to localize and guide light (E. Yablonovitch et al. Phys. Rev. Lett. 67, 3380 1991; M. Bayindir et al., Phys. Rev. B 61, R11855, 2000), this invention provides an enabling technology for the positioning, assembly and interconnection of micro-optical devices in photonic chips. These devices may include low threshold lasers, optical switches, wavelength dispersion compensators and wavelength division multiplexers that can be linked by circuits of light and coupled to glass fiber optic or other waveguide systems for applications in optical chips, computers and optical telecommunication systems.

#The processes disclosed herein for growing colloidal crystal patterns on substrates is surprisingly straightforward, and very advantageously is easily integrated into existing chip fabrication facilities and readily amenable to mass production. The methods combine self-assembly of colloidal particles and lithographic patterning to grow and position colloidal crystals of known thickness and orientation in a wafer, which may be used as a template for the production of photonic crystals in photonic devices produced by the combination of substrate and embedded photonic crystal. Because the methods disclosed herein of creating the colloidal crystal patterned wafers are extremely facile, rapid, reproducible and cheap, they are easily adapted for engineering the lattice dimensions, refractive index contrast, structural defects, location and integration of PC on photonic chips. The photonic chips may be seamlessly coupled to optical fiber and waveguide systems to complete an all-optical planarized highly compact telecommunication system.

#Soft lithography encompasses a suite of techniques that have proven to be remarkably useful for the fabrication of micron to sub-micron patterns on a range of planar and curved substrates, see Y. Xia and G. M. Whitesides, Angew. Chem. Intl. Ed. 1998, 37, 551. The procedures are facile and can be readily handled in an ordinary laboratory environment. In the context of the work described in this study, a line patterned polydimethylsiloxane (PDMS) master is obtained by casting a pre-polymer gel (Sylgard 184, Corning) from the corresponding lithographically defined photo-resist pattern. When access to lithographic instrumentation is not available, PDMS masters may be easily made by non-lithographic methods, such as the use of transmission electron microscopy (TEM) grids or optical gratings as templates for making micro-molds (see below).