WO2009060166A1

WO2009060166A1 - Control of lattice spacing within photonic crystals

Info

Publication number: WO2009060166A1
Application number: PCT/GB2008/003310
Authority: WO
Inventors: Christopher Bower; David Snoswell; Brian Vincent; Jeremy Baumberg
Original assignee: Eastman Kodak Co
Current assignee: Eastman Kodak Co
Priority date: 2007-11-10
Filing date: 2008-10-01
Publication date: 2009-05-14
Anticipated expiration: 2010-05-10
Also published as: GB0722131D0

Abstract

A method of controlling the particle spacing of a regular lattice of substantially monodisperse particles in a solution based photonic crystal, by use of a high frequency alternating electric field, the lattice being formed between parallel plate electrodes with a separation of at least twice the particle diameter, at least one of the electrodes being largely transparent.

Description

CONTROL OF LATTICE SPACING WITHIN PHOTONIC CRYSTALS

FIELD OF THE INVENTION

The invention relates to the field of photonic crystals, in particular to the rapid control of the lattice spacing between the particles in solution based crystals.

BACKGROUND OF THE INVENTION

It is known in the prior art that photonic crystals have a wide variety of applications in optoelectronics, lasers, metamaterials, 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. It is further known that 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. Aksay, Nature, vol 404, p56, 2000) and references cited therein. Further examples of colloidal crystals assembled by using an AC voltage applied to two planar electrodes can be found in "Electric Field-Reversible Three-Dimensional Colloidal Crystals" Tieying Gong, David T. Wu, and David W. M. Marr, Langmuir, vol 19 p5967, 2003 and "Two-Dimensional Crystallization of Microspheres by a Coplanar AC Electric Field", Simon O. Lumsdon, Eric W. Kaler, and Orlin D. Veley, Langmuir, vol 20, p2108, 2004. Some of the earliest work in this area was performed by Sanford Asher in the 1980's, using charged Polystyrene spheres in deionized water, see for example US 4632517, in which a filter device is constructed from the assembly of spheres into an ordered 3D crystal that can be mechanically tilted to allow different wavelengths to be filtered. The Asher group also pioneered much of the early use of polymeric photonic crystals, in which a photonic crystal is assembled within a polymeric matrix, to allow the crystal structure to be controlled by the interaction of the polymer with external stimulus such as glucose level, pH, or electrochemistry. See for instance "Polymerized colloidal crystal hydrogel films as intelligent chemical sensing materials" Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. More recently Arsenault et al used a similar approach to create a tunable display element that uses a solvent swellable polymer matrix with an embedded photonic crystal to create an electrically tunable display element, see Arsenault, A. C; Puzzo, D. P.; Manners, I.; Ozin, G. A. Nature Photonics 2007, 1, 468.

Typically the lattice spacing of the crystal is determined by the diameter of the close packed, monodisperse 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 in US 5281370 and also more recently US 2004/0131799. However both of these methods of changing the lattice spacing are realized with a photonic crystal embedded in a polymer matrix that is geometrically deformed. This is significantly different from the present invention, which uses an electric field to interactively control the spacing of a photonic crystal in liquid suspension. The range over which the lattice spacing can be tuned within previous systems is limited by the flexibility of the polymer matrix, which restricts the wavelength range over which a device might operate. Furthermore, the speed with which the lattice spacing can be changed is also dependent upon how rapidly the polymer matrix can be compressed or extended. Typically times in the order of 0.5 — Is are required which makes the photonic crystal in a polymer matrix arrangement unsuitable for a wide range of electro-optical devices, such as optical switches and displays for video-rate applications that require response times in the order of milliseconds or less. Although using a liquid suspension for the tunable crystal improves the switching time, there are other issues in these systems such as unwanted electro- hydrodynamic flows that are generated by the application of electric fields and cause bulk flow within the liquid which can disrupt the crystalline order. The flows are caused by electro-migration of dissolved ion species that leads to a build up of electrolyte near the electrodes, sometimes termed concentration polarization. See for example Levich, V. G. Physiochemical Hydrodynamics, Prentice Hall: Englewood Cliffs, NJ, 1962. In the case of an applied DC field the origin is electro-osmotic. One method of avoiding these problems is to confine the suspension to a very narrow gap and so constrain the crystal to an essentially 2D structure. WO 02/091028 describes such an embodiment in which the colloidal particles used to form the crystal are constrained by parallel plates with a separation of less than twice the particle diameter. Although this avoids some of the issues with unwanted flows the confinement to 2D crystals reduces the efficiency of the filter since there is only a single layer of particles available to diffract the incident light. A more efficient filter device should ideally have a number of such layers so as to form a 3D diffraction grating.

The benefits of using a photonic crystal as an optical filter within reflective displays have been suggested in WO 00/77566, and also in EP 1359459. However, use of the current invention in such a reflective display device offers further improvements in terms of manufacturability and performance, since instead of requiring three separate photonic crystal filters for red, green and blue pixels there is now the opportunity to use a single tuneable photonic crystal to provide all three colour responses, with fast switching rates that were not possible with polymer embedded photonic crystals.

It is known that photonic crystals can be used to create metamaterials, since a metamaterial in the broadest sense is a material that is structured with features much smaller than the wavelength of the electromagnetic radiation of interest and results in material properties that cannot be achieved by conventional materials. In particular the use of structured metallic/dielectric regions with a scale five to ten times below the free space excitation wavelength, that show negative refraction, have been suggested by C Luo, S Johnson, J Joannopoulos, J Pendry , "Negative refraction without negative index in metallic photonic crystals" Optics Express, 2003. A further method to create negative index materials has been demonstrated by D. R. Smith, W. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, "A composite medium with simultaneously negative permeability and permittivity," Phys. Rev. Lett. 84, 4184-4187 (2000), using a series of split-ring resonators. These structures can be mimicked by creating photonic crystals consisting of metallic particles or core-shell particles with alternating metal / dielectric layers in which the core-shell particle is a split ring resonator. Furthermore an actively tunable photonic crystal structure might be used to create a tunable metamaterial.

PROBLEM TO BE SOLVED BY THE INVENTION

A limitation with the methods described in the prior art is the speed with which the crystal lattice spacing can be tuned. In particular the methods that utilise a polymer matrix are limited to the speed with which the polymer matrix can be deformed or swelled. This therefore limits the applications for which these methods are suitable, since for example, in order to create a tunable display element for use in displaying video images, the tuning rate must be on the order of a few milliseconds to avoid blurring of the images.

SUMMARY OF THE INVENTION

The aim of the invention is to provide a method of rapidly controlling the lattice spacing of particles in a liquid 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 in a solution based photonic crystal, by use of a high frequency alternating electric field, the lattice being formed between parallel plate electrodes with a separation of at least twice the particle diameter, at least one of the electrodes being largely transparent. The present invention allows the rapid, dynamic, reversible control of particle spacing within crystals. 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 few milliseconds.

ADVANTAGEOUS EFFECT OF THE INVENTION

The present invention allows rapid, accurate, reversible, dynamic positioning of the particles in a suspension. The spacing can be controlled in a rapid, reversible and reproducible manner. Furthermore, the invention allows for a greater separation of the electrodes thereby increasing the number of layers in the photonic crystal structure which leads to improved optical properties of the device.

The features and advantages of the present invention will become apparent from the following description, in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying drawing in which: Figures Ia and Ib are schematic overhead and side views respectively of the layout of the electrodes used in an embodiment of the invention;

Figure 2 is a graph illustrating switching speed of the photonic crystal structure;

Figure 3 is a graph illustrating tuning of the diffraction peak; Figure 4 is a graph also illustrating tuning of the diffraction peak, for a more concentrated colloid solution; and

Figure 5 is a schematic side view of a display device with a panchromatic light source.

DETAILED DESCRIPTION OF THE INVENTION Figure 1 illustrates the layout of the parallel plate electrodes used to demonstrate the method of the invention.

Two electrodes 1, 2 are arranged parallel to each other with a liquid suspension 7 of particles situated in the gap therebetween. The gap is at least twice the particle diameter. The particles in the suspension are substantially monodisperse with a standard deviation of 5% or less. The particles can be polymeric, such as Polystyrene PS, PolyMethylMethAcrylate PMMA or other polymer or mix of polymers, inorganic such as Silica, Titania, Zinc Sulphide or other inorganic. The particles may have a uniform composition or they may have a layered core-shell structure in which layers of different material are present, such as alternating metal and dielectric layers, inorganic and polymer layers or possibly hollow particles with polymeric or inorganic shells. Monodisperse liquid drops, which can be created by passing through a narrow capillary or micro fluidic device, may also be used if coalescence of the drops is prevented by using charge adsorbed particles or steric stabilisation of the droplets. The droplets can be a liquid crystal solution. Alternatively the particles can be dispersed within a liquid crystal solution to further increase the tunability of the crystal.

The particle suspension 7 is in contact with both electrodes 1, 2 and is held in place by seals or a gasket 6 that prevents liquid from leaking out. The seals or gasket can be made from any inert material that does not react with the particle suspension and is not a source of ions or other contaminants that might change the overall salt concentration or conductivity of the suspension, for instance, photoresist, Polytetratfluoroethane (PTFE), Polydimethylsioloxane (PDMS), or other inert polymer materials are all suitable. At least one of the electrodes is largely transparent. This may be achieved by using a transparent conductive coating such as Indium Tin Oxide (ITO) or Aluminium doped Zinc Oxide, ALZnO, thin metal layer, conductive polymer, or fine mesh of metal lines with conductive polymer overcoat, on the inside of a glass substrate 3, 4. The electrodes are connected to a high frequency AC source 5, to create a field with an amplitude in the range of 0.01 V/μm to lOV/μm.

Figure 2 shows switching speed data taken from 1.5 wt% suspensions of 200nm polystyrene latex spheres studied by angle resolved spectrometry (detector 65 degrees from normal, 650nm wavelength illumination) and video microscopy. The applied ac voltage is amplitude modulated by a 500Hz square wave. The use of a high intensity white light laser enabled low noise data to be collected. Using field strengths between 0.9 and 1.8 million Vp-p per m (40OkHz sinewave), pillar-like crystal structures could be formed between two parallel plate ITO electrodes (area 4x4mm, gap 15 micron) as observed by video microscope. Lateral spacing of the mobile crystal pillars reached an equilibrium after approximately one minute giving rise to some enhanced scattering close to the reflected beam. Vertical ordering at maximum field strength resulted in Bragg-like diffraction, confirmed by a linear plot of (peak scattering wavelength)² vs sin²(θ). The diffraction caused by vertical ordering was measured to evolve within 0.5 milliseconds of the electric field being applied.

Figure 3 shows tuning of the diffraction peak from 600nm to 800nm by varying the electric field intensity by varying the applied field strength in a 1.5% w/w PS solution. Detector angle is +65 degrees from normal, with sample illuminated at 45 degrees from normal. Figure 4 shows tuning of the diffraction peak in a more concentrated particles suspension of 10% w/w PS. The peak in the attenuation of transmitted light (normal incidence) is tuned by the application of an AC electric field. Dashed lines indicates no field, a solid line indicates an electric field is applied

Figure 4 shows that high concentration (10 wt%) suspensions of 173nm particles in contact with ion exchange resin have been found to crystallise when sandwiched between parallel ITO electrodes with a gap of approximately 30 microns. The low ion concentration (equivalent to -0.00 ImM KCl) reduces electrostatic shielding and results in long range electrostatic repulsions between charged particles. Calculations reveal that although these particles are held in crystal formations, surface separations of neighbouring particles are approximately 160nm. Particles ordered this way produce high quality photonic crystals exhibiting attenuation of transmitted light of up to 92% at the stop band wavelength of 728nm. These crystals could be 'squeezed' by the application of field intensities of ~1 million V/m (40OkHz sinewave), thereby shifting the stopband to 714nm.

The salt concentration can be adjusted to control the equilibrium particle separation, since higher concentrations will further shield the electrostatic charge on the particles and therefore decrease the separation. However, at concentrations of more than around 0.0 ImM, the charged particles are now so close together that the tunable range is severely reduced. At even higher salt concentrations, the particles may start to aggregate.

When the gap between the two electrodes is greater than twice the particle diameter, electrohydrodynamic flows can lead to disruption of the ordered crystal structure, particularly when the applied AC field is around IkHz or less. Such unwanted flows were avoided by using a much higher driving frequency of greater than 1 kHz.

The experiment described above demonstrates the rapid assembly of colloidal crystals in an electric field. In addition, it demonstrates the dynamic, rapid, reversible control over the lattice spacing. 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. A further variation on this mode of display or capture would be to use additional colour fields to suit a particular application or to improve the colour gamut, for instance in addition to the red, green, blue fields, one might have a yellow field. In the case of field sequential emissive display devices, an array of pixels, each with broad spectral emission, typically white, is combined with a large area tunable colour filter which covers the whole of the pixel array. For each colour field, grey scale is provided by varying the intensity of emission of each pixel and colour is provided by tuning the filter to the appropriate spectral band. A complete image frame comprises sub- frames made up of separate colour records, presented sequentially at a rate which is below the integration time and above the flicker frequency threshold of the eye. Suitable emissive pixel technologies would include white organic light emitting diode (OLED) including both small molecule OLED and polymeric technologies, plasma panels, quantum dot emitters, white LEDs and field emission. A large-area tunable colour filter might also be used in conjunction with a broad spectral light source to form the basis of a colour tunable element used in solid state lighting, with variable colour temperature.

Figure 5 shows a display device having a panchromatic light source with a photonic crystal colour tunable filter operating in field sequential mode. In Figure 5 the panchromatic light source 51 is a white OLED device which has been pixelated so that the intensity of the pixel output can be controlled to create grey scale as required for image display. This is combined with a photonic crystal filter device 52, in which the entire filter is switched sequentially to allow red, green or blue light to be transmitted from the pixels in the OLED device. Extra colour fields such as a yellow, cyan or violet might be added to the red, green and blue fields to improve the colour gamut of the display.

By choosing the size of the colloidal particles appropriately 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.

The particles described in the examples have a fixed charge on their surface, equivalent to a zeta potential of at least 1OmV, preferably greater than 4OmV, which provides the repulsive force that keeps them separated. This force is balanced by the attractive dipole forces generated by the electric field. However, 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.

The invention has been described in detail with reference to preferred embodiments thereof. It will be understood by those skilled in the art that variations and modifications can be effected within the scope of the invention.

Claims

CLAIMS:

1. A method of controlling the particle spacing of a regular lattice of substantially monodisperse particles in a solution based photonic crystal, by use of a high frequency alternating electric field, the lattice being formed between parallel plate electrodes with a separation of at least twice the particle diameter, at least one of the electrodes being largely transparent.

2. A method as claimed in claim 1 wherein the separation of the parallel plate electrodes is more than 10 times the particle diameter.

3. A method as claimed in claim 1 or 2 wherein the frequency of the applied field is between IkHz and IMHz.

4. A method as claimed in claim 3 wherein the frequency of the applied field is between 100kHz and 50OkHz.

5. A method as claimed in any of claims 1 to 4 wherein the particle size is in the range of 50nm to 600nm.

6. A method as claimed in any of claims 1 to 5 wherein the particle size is in the range of 600nm - lOOOμm.

7. A method as claimed in any of claims 1 to 6 wherein the particle size is in the range of 1 mm - 10mm.

8. A method as claimed in any of claims 1 to 7 wherein the particles have a surface charge of at least ±lOmV zeta potential to render them mutually repulsive.

9. A method as claimed in claim 8 wherein the particles have a surface charge of ±40mV or greater zeta potential to render them mutually repulsive.

10 A method as claimed in any of claims 1 to 9 wherein the particles have a layer or layers comprising surfactant or oligomer or polymer or of smaller charged particles to create a steric repulsion between particles that renders the particles mutually repulsive.

11. A method as claimed in any of claims 1 to 10 wherein the electric field strength is in the range from 10000 to 10000000 Vm^"1

12. A method as claimed in claim 1 to 11 wherein the salt concentration of the particle suspension is in the range of 0.00 ImM to 0.0 ImM.

13. A method as claimed in any of claims 1 to 12 wherein a detectable change in lattice spacing of the crystal occurs in less than 1 millisecond.

14. A method as claimed in any of claims 1 to 13 wherein a detectable change in lattice spacing of the crystal occurs in less than 0.5 millisecond

15. A method as claimed in any of claims 1 to 14 wherein the particles used to assemble the lattice are hollow.

16. A method as claimed in any of claims 1 to 15 wherein the particles used to assemble the lattice have multiple layers of metal or dielectric materials or a combination of metal and dielectric materials.

17. A method as claimed in any of claims 1 to 16 wherein the particles used to assemble the photonic crystal comprise polymer, organic, inorganic, ceramic, metal, metal oxide or metal salts or metal coated particles.

18. A method as claimed in any of claims 1 to 17 wherein the particles used are monodisperse liquid drops of a limited coalescence emulsion stabilised by adsorption of charged particles at the interface.

19. A method as claimed in claim 18 wherein the liquid drops consist of or contain a liquid crystal material to allow further tuning of the optical response.

20. A method as claimed in any of claims 1 to 19 wherein the particles are suspended within a liquid crystal material to allow further tuning of the optical response.

21. A suspension based photonic crystal device having reversibly tuneable photonic properties the lattice dimension being controlled by the method according to claim 1.

22. A suspension based photonic crystal colour filter device having reversibly tuneable photonic properties, the lattice dimension being controlled by the method according to claim 1, that can be used to selectively reflect or transmit certain wavelengths of light, as part of a reflective or emissive display, as part of a filter array on a CMOS sensor or CCD or other image capture device or as partOf a lighting element or as part of an image capture or display device operated in field sequential mode.

23. An array of independently controlled tuneable photonic crystal colour filter devices as claimed in claim 22 for use in a reflective or emissive display device.

24. A device as claimed in claim 22 or 23, forming a field sequential emissive display device and having an OLED as a light source

25. A suspension based photonic crystal device having reversibly tuneable photonic properties the lattice dimension being controlled by the method according to claim 1 using materials to create a tuneable metamaterial or flat lens device.