Classifications

• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y20/00—Nanooptics, e.g. quantum optics or photonic crystals
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
  • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
  • G02B6/122—Basic optical elements, e.g. light-guiding paths
  • G02B6/1225—Basic optical elements, e.g. light-guiding paths comprising photonic band-gap structures or photonic lattices
• • G—PHYSICS
  • G02—OPTICS
  • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
  • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
  • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
  • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
  • G02B2006/12166—Manufacturing methods
• • G—PHYSICS
  • G02—OPTICS
  • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
  • G02F2202/00—Materials and properties
  • G02F2202/32—Photonic crystals

Definitions

• the invention relates to the field of crystals, in particular to the control of the lattice spacing between the particles in the crystals.
• photonic crystals have a wide variety of applications in optoelectronics, lasers, flat lenses, sensors, wavelength filters and display devices.
• a common route to fabrication of photonic crystals is to use self- assembly of colloids into colloidal crystals. This self-assembly process can be achieved by a range of different methods such as sedimentation, centrifugation, filtration, shear alignment or evaporative deposition.
• electric fields can be used to assemble close packed arrays of colloids. For example see (Electrophoretic assembly of colloidal crystals with optically tunable micropatterns R. C. Hayward, D. A. Saville & I. A.
• the lattice spacing of the crystal is determined by the diameter of the close packed, monodispersed spheres, and remains fixed once the crystal structure has formed. It is useful to be able to control the lattice spacing of a photonic crystal since this parameter determines the position of the optical stop band, and therefore the wavelength of light that will be reflected since propagation within the crystal is forbidden. The ability to interactively tune the lattice spacing within a photonic crystal is therefore a desirable property since it allows for the creation of a variety of electro-optical devices. A method of creating a tuneable photonic crystal has been described inUS5281370 and also more recently US20040131799.
• both of these methods of changing the lattice spacing are realized with a photonic crystal embedded in a polymer matrix which is geometrically deformed. This is significantly different from the present invention which uses an electrostatic field to interactively control the spacing of a photonic crystal in liquid suspension.
• a limitation of embedding the photonic crystal within a polymer matrix is that the crystals tend to be polycrystalline in nature. This leads to an increase in the width, reduction in the intensity and uncertainty in the position of the reflected peak.
• the range over which the lattice spacing can be tuned within these systems is limited by the flexibility of the polymer matrix, which restricts the wavelength range over which a device might operate.
• the speed with which the lattice spacing can be changed is also dependent upon how rapidly the polymer matrix can be compressed or extended.
• the aim of the invention is to provide a method of controlling the lattice spacing of particles in a suspension that does not suffer from the problems and limitations of the methods known in the prior art.
• the present invention uses an electric field to interactively control the spacing of a photonic crystal in liquid suspension. According to the present invention there is provided a method of controlling the particle spacing of a regular lattice of substantially monodisperse particles or a mixture of particles by use of an electric field.
• the present invention allows the dynamic, reversible control of particle spacing within crystals along two independent axes. As the particles are charged electrostatic forces prevent the surfaces from touching. However the particles are held in a hexagonal close packed (HCP) pattern by temporary dipoles induced by the electric field. Since the separation of the particles within the crystal is controlled by the electric field changing the field intensity can change the lattice spacing. The changes to the lattice spacing are reversible and rapid, occurring within a fraction of a second.
• HCP hexagonal close packed
• the present invention allows accurate, reversible, dynamic positioning of the particles in a suspension.
• the spacing can be controlled in a rapid, reversible and reproducible manner.
• the present invention also allows the aspect ratio to be controlled, i.e. the spacing can be different along different axes.
• Figure 1 is a schematic view of the layout of the electrodes used in an embodiment of the present invention.
• Figure 2 is a graph illustrating particle to particle separation versus field strength using a non rotating electric field
• Figure 3 is a graph illustrating particle to particle separation versus field strength using a rotating electric field
• Figure 4 is a further graph illustrating lattice spacing versus applied field strength.
• Figure 1 illustrates the layout of the electrodes used to demonstrate the method of the invention.
• Electrodes 1 and 2 are connected to a signal amplifier 5. Electrodes 3 and 4 are connected to a signal amplifier 6. The four electrodes are co-planar. In the experiments conducted the distance between electrodes 1, 4 and 2, 3 are 159 ⁇ m. The distance between electrodes 1, 3 and 2, 4 are 142 ⁇ m. However, the gap can be adjusted as required. Smaller distances mean lower voltages to achieve the desired effect, i.e. a field strength of order 30000Vm "1 .
• the electrodes consist of a 40 mn thick layer of platinum, sputter coated onto a glass microscope slide.
• Adjacent crystals periodically drifted and connected together, increasing the size of the crystal and simultaneously decreasing its rotational speed. If one of the signal amplifiers was disconnected, the spinning stopped immediately and portions of the crystals delaminated into chains. Crystals that drifted away from the central region between the electrodes were also observed to gradually delaminate into chains. The speed of rotation was observed to be proportional to the field strength. Switching the relative phase shift to, 270° could reverse the direction of the rotation. Alternating the relative phase shift between 90° and 270° every cycle, or halving the frequency of one voltage source prevents rotation of the spinning crystals.
• the crystals were asymmetric (elongated) because the attractive forces between chains were significantly less than between particles in each chain. This was caused by the sub-optimal alignment and restricted positioning of the dipoles hi adjacent chains.
• a coplanar quadrapole electrode has been used to generate a low frequency (1600 Hz) rotating electric field.
• frequencies in the range of 100Hz up to 2OkHz can be used. It will be understood by those skilled in the art that it is not essential to the invention that the electric field is rotating, but it is essential that there is a time dependent change in the field vector.
• the combined effect on the HCP crystal structure was to stretch it along one axis.
• the presence of fluid flow during the experiments was noted to skew the HCP structure, causing it to approach a cubic close packed (CCP) configuration.
• CCP cubic close packed
• the ability to distort the lattice in this manner can be used to enhance the size of the photonic band gap.
• the lattice spacing of the crystal was determined by two different methods; first, from optical microscopy images of the PS spheres in-situ, and second by observation and measurement of the spacing of the first order diffraction spots obtained by focusing a 635nm light from a diode laser through the 2D crystal. The results are shown in Figure 4.
• Figure 4 illustrates that the lattice spacing determined by laser diffraction (open squares) is consistently higher by around 20nm that that determined from optical microscopy (solid squares). However spacing determined by both methods shows the same response to field strength, i.e. as field strength is increased the lattice spacing of the crystal decreases.
• the monodisperse spheres are assembled into chains, aligned along the electric field direction.
• This arrangement to actively control the alignment of the chains it is possible to tune the wavelength of the reflected light.
• the ensemble of chains acts as a diffraction grating with a grating period dependent on the angle subtended by the incident light and the long axis of the chains.
• a further benefit of this arrangement is that the selected wavelength of light scattered normal to the spheres shows little variation with viewing angle.
• the experiments described above demonstrate the rapid assembly of colloidal crystals in an electric field. In addition, they demonstrate the control over the rotation of the crystals and the dynamic, rapid, reversible control over the lattice spacing along independent axes.
• the ability to interactively tune the lattice spacing of a photonic crystal is of particular use in optoelectronics for tuneable filter elements, or flat lenses with tuneable optical properties, and also in the display industry where it can be used as part of a tuneable colour element in a display or as tuneable optical filter for a CCD, CMOS or other image capture device, for example film camera or thermal imager.
• An alternative approach might use a field sequential mode of capture or display wherein the red, green and blue fields are either captured or displayed sequentially.
• the device can be used to control different regions of the electromagnetic spectrum. For instance, particles in the size range of 100-600nm might be used for a device to operate in the visible part of the spectrum, whilst particles in the micrometer size range would be used to make a device operate in the infrared region of the spectrum. Use of even larger particles would allow operation in the terahertz and microwave region of the spectrum.
• monodisperse spheres of polystyrene or silica fuiictionalised spheres might also be used, or spheres that have a core particle with a shell of different material or materials such as ceramics, metal oxides or salts, polymers or a layer of metal to manipulate surface plasmons or enhance the photonic band gap.
• hollow particles or bubbles to provide a greater dielectric contrast between the suspending liquid and the particles could be used. Hollow particles also provide the assembled lattice with two distinct length scales for the inside and outside of the shell, which can be utilised to improve the band gap.
• a further refinement would be to use hollow particles with a plurality of alternating layers of material with different dielectric constant to create multiple, controllable length scales.
• Another method to achieve a larger band gap is to use two distinct sizes of monodisperse spheres and adjust the ratio of the amounts of each size to alter the resultant packing structure of the lattice.
• a variation on this approach is to use asymmetric particles such as oval, rod or plate shaped particles with an aspect ratio greater than unity to change the packing symmetry. These differently shaped particles may be used separately or in combination.
• the droplets can be given surface charge by using stabilising particles that develop a surface charge.
• the droplets could consist of or contain a liquid crystal material that changes its dielectric properties upon application of an electric field, .offering further- opportunities to selectively tune the optical response of the photonic crystal.
• the particles described in the examples have a fixed charge on their surface, which provides the repulsive force that keeps them separated. This force is balanced by the attractive dipole forces generated by the electric field.
• the minimum requirement is a mutual repulsion of the particles that can be provided by other means such as steric repulsion due to an adsorbed layer or layers, comprising surfactant or oligomer or polymer, or of charged particles or other dispersant on the particle surface for instance, thus relaxing the requirement for a permanent surface charge.

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• Physics & Mathematics (AREA)
• Optics & Photonics (AREA)
• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• General Physics & Mathematics (AREA)
• Nonlinear Science (AREA)
• Nanotechnology (AREA)
• Life Sciences & Earth Sciences (AREA)
• Biophysics (AREA)
• Crystallography & Structural Chemistry (AREA)
• Microelectronics & Electronic Packaging (AREA)
• Liquid Crystal (AREA)
• Electrochromic Elements, Electrophoresis, Or Variable Reflection Or Absorption Elements (AREA)
• Crystals, And After-Treatments Of Crystals (AREA)
• Thermistors And Varistors (AREA)

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• 2004
  • 2004-12-23 GB GBGB0428261.2A patent/GB0428261D0/en not_active Ceased
• 2005
  • 2005-12-22 JP JP2007547656A patent/JP2008525836A/ja not_active Withdrawn
  • 2005-12-22 US US11/813,487 patent/US20080230752A1/en not_active Abandoned
  • 2005-12-22 EP EP05843717A patent/EP1828823A2/en not_active Withdrawn
  • 2005-12-22 CN CNA2005800439450A patent/CN101084459A/zh active Pending
  • 2005-12-22 WO PCT/GB2005/005029 patent/WO2006067482A2/en not_active Ceased

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