Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having growth from a solution comprising a solvent which is...
Reexamination Certificate
2003-08-20
2004-09-28
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having growth from a solution comprising a solvent which is...
C117S069000, C117S070000
Reexamination Certificate
active
06797057
ABSTRACT:
This application is the US national phase of international application PCT/GB00/03405 filed 5 Sep. 2000, which designated the US.
The present invention relate to colloidal photonic crystals, to a method of growing robust large area colloidal crystals and devices produced thereby.
For the purposes of this patent, a photonic crystal shall be defined as an object whose optical properties are spatially periodic.
It has been known for some time that colloidal suspensions can be made to crystallise under certain conditions to produce colloidal crystals which exhibit interesting optical properties.
Such photonic colloidal crystals are capable of modifying the propagation of light due to the fact that the crystal structure is periodic on the scale of the wavelength of light. Accordingly, colloidal photonic crystals find applications in a variety of optical devices including optical filters and limiters. The reflective properties of colloidal photonic crystals can also be controlled offering further opportunities for exploitation in optical devices.
Bulk samples of such crystals are however usually polycrystalline and comprise many hundreds of crystals of the order of 100 microns in size which are randomly oriented. The crystals may also possess a variety of crystalline structures including face-centred-cubic (fcc), hexagonally-close-packed (hcp), and random-close-packed (rcp). These and other imperfections within the crystal impair the optical characteristics of the crystal and make the crystal unsuitable for materials applications.
An improved method for growing colloidal photonic crystals has been proposed by P. N. Pusey and B. Ackerson, in patent GB 2 260 714. The improved method reduces the imperfections in the crystal by melting and aligning the crystal into a preferred structure. Specifically, this method relates to a suspension of monosized polymer colloidal spheres and consists of aligning the colloid into a face-centred-cubic crystal structure by applying a rectilinear shearing force usually by inducing flow in the liquid.
The photonic crystals formed by this method are essentially perfect face-centred-cubic structures. The method enables single crystals to be grown over areas larger than 1 cm
2
.
At the end of the growth process, after sufficient shearing force has been applied to establish a substantially single crystal structure, the structure may be sealed to retain the carrier liquid. Alternatively, some form of gelling agent may be added to the carrier liquid to improve the stability of the structure or the carrier liquid may be allowed to evaporate to a leave a self-supporting structure of colloidal particles.
Shearing produces two types of single face-centred-cubic structure, one type being produced on the forward shear, the second type being produced on the reverse shear. When the shearing is stopped the colloid relaxes into a twinned face-centred-cubic structure. In the case of the twinned arrangement both forms of face-centred-cubic structure coexist within the crystal, one on top of the other.
Whilst the twinned face-centred-cubic structure exhibits useful optical characteristics, the single face-centred-cubic arrangement provides improved optical properties over the former. For example, the single face-centred-cubic structure demonstrates improved photonic band-gap properties and can be optimised to be reflective for a large range of angles of incident radiation and polarisation angles (for incident radiation within a limited wavelength range); see for example Yablonovitch et al, “Three-dimensional photonic band structure”, [(Yablonovitch, E., Gmitter, T. J., Leung, K. M., Meade, R. D., Rappe, A. M., Brommer, K. D., Joannopoulos, J. D., “Three-dimensional photonic band structure”, Opt. & Qu. Elect., 24, S273, 1992] and references therein. The single face-centred-cubic structure therefore offers a greater potential for high quality optical devices but cannot be made by the aforementioned linear shearing method because of the tendency of the crystal to relax into the twinned face-centred cubic structure.
Further limitations of current colloidal photonic crystals relate to the physical properties of the crystals. Current colloidal photonic crystals are relatively fragile and lack permanence, largely due to the fact that the crystalline layers are merely held in place between rigid parallel plates. With reference to configurations in which the carrier fluid is retained within the colloidal crystal, the sealing can become compromised allowing unwanted evaporation of carrier liquid leading to degradation of the crystalline structure. Further, the physical dimensions of the crystals remain relatively small precluding widespread adoption of colloidal photonic crystals in optical applications.
It is an object of the present invention to provide an improved method for producing robust large area colloidal photonic crystals.
According to the present invention, a method of growing an essentially perfect colloidal photonic crystal exhibiting a single face-centred-cubic structure comprises the steps of:
preparing a suspension of monosized colloidal spheres having a volume concentration that produces spontaneous local crystallisation in a suitable dispersion medium,
inserting the colloidal suspension into a gap between two substantially parallel surfaces,
subjecting the surfaces to relative oscillating motion parallel to their surfaces and,
subjecting the surfaces to a series of small linear displacements relative to each other, the displacements being parallel to their surfaces and in two dimensions, comprising the sequence of applying a linear displacement to one of the surfaces with respect to the other surface, rotating the direction in which the linear displacement is applied to the surface by substantially 120 degrees in a single constant direction and applying a further linear displacement to the surface, the sequence being repeated until the colloidal photonic crystal has been purified into a single face-centred-cubic structure.
Preferably the dispersion medium is one that can be changed from a liquid phase to a solid phase in order to fix the colloidal crystalline structure.
The direction of rotation may be either clockwise or anticdockwise.
In an another arrangement, the method of growing an essentially perfect colloidal photonic crystal exhibiting a single face-centred-cubic structure comprises the steps of:
preparing a suspension of monosized colloidal spheres having a volume concentration that produces spontaneous local crystallisation, in a dispersion medium that can be changed from a liquid phase to a solid phase in order to fix the colloidal crystalline structure
inserting the colloidal suspension into a gap between two substantially parallel surfaces, and
subjecting the surfaces to relative oscillating motion parallel to their surfaces.
In particular, the magnitude of the small linear displacements applied to the surfaces is substantially equal to the product of the diameter of the colloidal spheres and the number of crystalline layers in the crystal.
In one preferred arrangement the surfaces are displaced with respect to each other in an equilateral triangle.
The minimum volume fraction of monosized colloidal spheres is advantageously 0.49 and preferably the radius of the monosized colloidal spheres is in the range 0.01 &mgr;m to 100 &mgr;m.
Preferably the radius of the monosized colloidal spheres is in the range 0.05 &mgr;m to 10 &mgr;m.
The colloidal spheres may comprise at least one of a polymer, a nonlinear material, a magnetic material, a metal, a semiconductor, glass doped with an active dye, polymer doped with an active dye, and silica. In particular the colloidal spheres may be polymethylmethacrylate.
The material used for the dispersion medium is preferably at least one of an adhesive, a polymer, a resin, a non-linear optical material, an active optical material, octanol.
In a preferred embodiment the active optical material used for the dispersion medium is a liquid crystal material.
In one arrangement the dispersion medium is subsequently
Amos Richard M
Kitson Stephen C
Rarity John G
Shepherd Terence J
Tapster Paul R
Kunemund Robert
Nixon & Vanderhye P.C.
Qinetiq Limited
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