Abstract:
A method to reversibly assembly micro and nanostructures with a force provided by light inside an embedding medium which behaves as solid during steady state and as a fluid during assembly state. The embedding medium is a material capable to change from solid to fluid state during assembly and from fluid to solid state during fixation. The change in state at the embedding medium is controlled with the temperature and/or shear strain.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    This invention refers to the article published on the journal nature communications DOI: 10.1038/ncomms12002. 
         [0004]    This invention relates to programmable materials that change their physical properties dynamically at the micro and nanoscale. This document presents a new type of programmable material which can be configured through light beams. This occurs when dispersed solid micro or nanostructures migrate (or rotate) inside an embedding material with the action of light. Mechanisms to configure nanostructures in three-dimensional (3D) space are essential in nanotechnology, photonics, and materials science. Various nanopatterning techniques based on photoactive materials or photoablation have been used to produce static photonic crystals, lasers, metamaterials, holograms, storage devices and sensors. Common optical nanofabrication methods rely on light-sensitive materials such as silver halides and photoresists. Additionally, highly intense laser pulses have been applied to ablate materials with spatial selectivity. However, these nanofabrication techniques produce only static structures. This invention relates to a strategy, based on non-ablative laser beam, that arrange structures in three-dimensional (3D) space inside transparent or semitransparent materials with appropriate mechanical characteristics. At least two materials are necessary to form the programmable composite. One solid material corresponding to non-aggregated suspended micro or nanostructures and a second material working as an embedding medium which can behave as solid, semi-solid or fluid. Structures, such as nanoparticles embedded in a medium, can displace or rotate thanks to the momentum provided by light. Primarily, the photon momentum produces forces because of the scattering and absorption of the nanoparticles. However, thermal and acoustic forces also occur due to gradients of temperature in the medium. In thermophoresis (or thermodiffusion), the entropy of nanoparticles results in migration and/or rotation from high to low intensity regions. In acoustophoresis, a transient of temperature creates a mechanical pressure producing migration/or rotation to low intensity regions. 
         [0005]    An ideal embedding medium is a material which can change from a fluid state to a solid state and vice versa rapidly and locally with a change in temperate or shear strain. Hence, at fluid phase structures are arranged and at solid state fixated. Some materials have a well-defined transition of phase (melting point and freezing point), other solid materials have a more complex rheological characteristics and their viscosity is affected by temperature and shear strain. For example, some material with shear-thinning rheology can decrease their stiffness when a force is applied to the embedded structures. Therefore, a change in temperature and shear strain at the embedding medium, can provide displacement (or rotation) and fixation of nanoparticles according to the temperature and shear strain applied. In the present invention, the embedded elements are arranged with a force provided by light, while the temperature and shear strain in is provided by any source (including the light beam). 
         [0006]    The next section describes patents that are related to the invention. Every patent is described with three characteristics: number, title and a summary. 
         [0007]    U.S. Pat. No. 9,063,434 Tittle: Sub-diffraction-limited patterning and imaging via multi-step photoswitching. 
         [0000]    Summary: A photoresist can be configured for spectrally selective reversible transitions between at least two transition states based on a first wavelength band of illumination and a second wavelength band of illumination. 
         [0008]    U.S. Pat. No. 8,883,719. Title: Bacteriorhodopsin protein variants and methods of use for long term data storage. 
         [0000]    Summary: Bacteriorhodopsin variants for performance in holographic and three-dimensional (3D) memory storage devices are described. 
         [0009]    U.S. Pat. No. 8,605,175 Tittle: Solid-state image capturing device including a photochromic film having a variable light transmittance, and electronic device including the solid-state image capturing device. 
         [0000]    Summary: A solid-state image capturing device includes: a semiconductor substrate having a photosensitive surface with a matrix of pixels as respective photoelectric converters. 
         [0010]    U.S. Pat. No. 8,529,997. Title: Methods for preparing structured organic film micro-features by inkjet printing. 
         [0000]    Summary: The invention discloses a method for making a substrate comprising structured organic film. 
         [0011]    U.S. Pat. No. 8,462,603. Title: Device for recording and reading data on a multi-layer optical disc. 
         [0000]    Summary: A device for recording/reading information on a multilayer optical disc, comprising an optical disc positioning system. 
         [0012]    U.S. Pat. No. 9,242,248. Title: Methods and devices for optical sorting of microspheres based on their resonant optical properties. 
         [0000]    Summary: Microspheres brought within vicinity of the surface are subjected to forces that result from a coupling of the evanescent field to whispering gallery modes (WGM) in the microspheres. Microspheres are sorted by this resonant light pressure effects. 
         [0013]    U.S. Pat. No. 8,999,731. Title: Programmable illumination pattern for transporting microparticles. 
         [0000]    Summary: The invention combines provides a method and apparatus facilitating the spatial manipulation of colloidal particles and molecules at an interface between a light sensitive electrode and an electrolyte solution. 
         [0014]    U.S. Pat. No. 8,816,234. Title: Acousto-optic sorting. 
         [0000]    Summary: A method for sorting particles in a fluid. The method involves generating an optical landscape using an acousto-optic device. Preferably, the optical landscape is arranged to sort particles based on size or refractive index or shape. 
         [0015]    U.S. Pat. No. 8,767,216. Title: Holographically illuminated imaging devices. 
         [0000]    Summary: An imaging system which consist of: a holographic element that transforms an illumination beam into a focal array of light spots, a scanning mechanism that moves an object across the focal array of light spots, and a light detector that generates data from the received light. 
         [0016]    U.S. Pat. No. T8,964,518. Title: Data recording apparatus and method using 3D optical memory, and authentication apparatus and method using 3D optical memory. 
         [0000]    Summary: A data recording apparatus and method using three dimensional (3D) optical memory and an authentication. The data recording apparatus includes a recording excitation light splitting unit, a condition storage unit, and a data recording unit. 
         [0017]    U.S. Pat. No. 8,634,119. Title: System for holography. 
         [0000]    Summary: The present invention provides systems of recording holograms that reduce the writing time, increase the diffraction efficiency, improve the resolution, or restitute color. 
         [0018]    U.S. Pat. No. 8,203,780. Title: Systems and methods for improving the performance of a photorefractive device. 
         [0000]    Summary: The device comprises a layered structure in which one or more polymer layers are interposed between a photorefractive material and at least one transparent electrode layer. 
         [0019]    U.S. Pat. No. 5,800,950. Title: Recording medium 
         [0000]    Summary: This invention relates to a recording element for a photorefractive polymer, having a recording layer in which an inner electric field is adapted. 
         [0020]    U.S. Pat. No. 5,289,407. Title: Method for three dimensional optical data storage and retrieval. 
       BRIEF SUMMARY OF THE INVENTION 
       [0021]    A purpose of the invention is to provide a new mechanism to arrange solid structures (in the 3D space) inside an embedding medium which behave as fluid during writing state and as solid during fixation state. To achieve this, it is necessary a composite of dispersed micro or nanostructures suspended in an embedding transparent or semi-transparent medium. The energy to displace (or rotate) the embedded structures is provided by light. Although the displacement and rotation of particles by action of light is well established, this invention introduces the concept of arranging micro or nanostructures inside solid or semi-sold materials. In contrast to liquid crystals and photrefractive (or photochromic) materials, the manipulation of the structures does not occur at the molecular level nor it includes a change in the molecular structure. In the present invention, the chemical configuration of both, the embedded structures and the embedding medium, should be maintain to allow reversibility. The transformation of the composite occurs at physical level without affecting the covalent or ionic bonding. 
         [0022]    In the present invention, the energy to displace (or rotate) the micro or nanostructures is provided by light. When an incidental light beam interacts with the structures, different forces arise: optical, thermal and/or acoustic. The optical force (“optical tweezers”) can be formed at the gradients of the electromagnetic field, or in the direction of the propagation of the ray. For example, two interfering coherent light sources can produce strong intensity gradients in the three dimensional space. These fringe patterns can be controlled with the propagated wavefront of the interfering beams. Complex arrangements can be assembled with the gradients produced by interfering rays. 
         [0023]    A thermal force appears with light absorption. In this case, a gradient of temperatures can produce a force in the embedded element from the high temperature point to the low temperature point (thermophoresis). Another consequence of this effect is the temporal expansion of the materials, which induces an acoustic force (acustophoresis). 
         [0024]    Similarly, the rotation of the nanostructure can be observed by a torque produced by optical pressure, thermal force or acoustic force. Furthermore, a light beam with angular momentum (or with light momentum induced by the structure) can produce a torque on the nanoparticle. In practice, these phenomena occur simultaneously when a beam of light interacts with the composite. Although these phenomena are well established, the energy provided by light to configure the composite is just an element of this invention. 
         [0025]    The mechanism involved is a technique to structure composites at the micro and nanoscale in 3D patterns. In this context, a composite is a mixture of two or more materials which do not produce a chemical bonding. These materials can be separated and displaced physically. At least one of the components should be capable to reversibly transform from a solid state to fluid state. This component is referred as embedding medium and it provides the required plasticity. Similarly, it is necessary a second material, which is structured at the micro or nanoscale. This second material is commonly composed of nanoparticules that can be displeased or rotated inside the embedding medium. In broad terms, a preferred embodiment of the composite is comprised of at least one nanostructured material and one embedding medium with the suitable viscoelastic characteristics. 
         [0026]    The embedding medium should have the capability to be fluid during writing time, but rigid enough to maintain the structures fixated in stable positions after the writing period. The transition between these two states is controlled through two types of physical mechanisms: temperature and force. Most materials trend to reduce their viscosity at high temperatures. Some of them have a specific melting point where the embedded nansotructures are fluid. However, most solid materials have a more complex rheological behavior and their viscoelasticity is a function of temperature and shear strain. The temperature is provided to the composite in an extrinsic or intrinsic manner. Extrinsic refers to heat transferred from external sources through conduction, convection or radiation. Intrinsic refers to heat transferred from the light source to arrange the structures. In the intrinsic case the light source heats the medium, the particles or both. For instance, when a pulsed light source heats nanoparticles embedded in a solid medium, it remains localized for a period of time before it dissipates. In general, intrinsically heated media can be controlled more efficiently with pulsed light beams as the temperature heat is confined before diffusion takes place. An important advantage of intrinsically heated configurations, is that different elements can be displaced or rotated independently without affecting the rest of the assembly. 
         [0027]    An embedding medium with shear-thinning characteristics is ideal because it reduces its viscosity with both: temperature and shear strain. For instance, the shape-memory characteristics of some types of materials (including glass, gelatine or polymers) can be applied as embedding medium. The displacement of a nanostructure embedded in a shear-thinning solid is facilitated when the force provided by light is applied. In this scenario, the light should be preferably pulsed so the energy is released with large force in a short period of time. 
         [0028]    One advantage of this invention in contrast with other technologies such as photorefractive or photochromic materials, is that these type of composite can maintain the configuration without the aid of external energy. As a consequence, the configuration is maintained indefinitely until a new writing process starts. Another advantage of the invention is that large arrays of particles can be assembled simultaneously and/or locally according to the requirements of the process. 
         [0029]    In broad terms, a preferred embodiment of the mechanism of assembly is comprised of the following steps: an initial state where structures are randomly distributed, a transition state where structures migrate to lower-energy configurations through the momentum provided by light, and a final assembly state where structures are located in new stable positions. An optional intermediate state of heating the composite might be necessary to facilitate the process. 
         [0030]    Further aspects of the invention will become apparent from consideration of the drawings and the ensuing description of preferred embodiments of the invention. A person skilled in the art will realize that other embodiments of the invention are possible and that the details of the invention can be modified in a number of respects, all without departing from the inventive concept. Thus, the following drawings and description are to be regarded as illustrative in nature and not restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a graph of the rheology of a typical thermoplastic (e.g. the complex shear modulus (G′+i G″) of pHEMA changes from 1.4×109+i 2.0×107 Pa in the glassy regime to 2.9×104+i 2.0×104 Pa well above its glass transition temperature (Tg) of 300° C.). 
           [0032]      FIG. 2  is a representation of the dynamics of nanoparticle displacement in a standing wave. The force is proportional to the gradient of the intensity. Ag nanoparticles in the negative regime are pushed towards the minimum intensity regions. 
           [0033]      FIG. 3  is a measure of a relative maximum force acting on Ag nanoparticles of different diameters with a standing wave of 532 nm. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0034]    The invention refers in broad terms to assemblies of micro or nanostructures in solid or semi-solid media with the action of light. The reduction of the adhesion that fixated structures is induced by temperature or by the force applied to the embedding element. This includes composites containing a transparent or semi-transparent material and a second micro or nanostructured material. For example, nanoparticles fixated by frozen water. If nanoparticles absorb light, they can increase locally the temperature to produce a transition of phase. Furthermore, when heat is transferred with pulsed light before dissipation, it is possible to locate the transition of phase just at the boundary of nanoparticles while they are displaced or rotated. A similar effect can be induced in gelatines, but with the additional effect of reduction in viscosity due the shear strain applied at the boundaries of nanoparticles. A composite has demonstrated to work with silver nanoparticles embedded hydrogel. An expanded example is described in the following section, however the invention is not limited to the embodiment. 
       Working Example 
       [0035]    An assembly of nanoparticles embedded in a solid is reconfigurable with optical forces when the viscoelasticity of the medium permits the migration and the stabilization in a reversible manner. When the optical force passes a threshold, nanoparticles overcome surface adhesion, elastic forces, and the static friction induced by the medium. This phenomenon is analogous to “the knife in the butter”, where the medium changes its stiffness according to the temperature of the metal. The example is described in three sections: overview, and experimental demonstration. 
       Overview 
       [0036]    Ag nanoparticles are arranged dynamically in a three-dimensional (3D) space with a poly(2-hydroxyethyl methacrylate) (pHEMA) as the embedding medium. Ag nanoparticles were considered due its high optical scattering and absorption. PHEMA was chosen as embedding medium due its unique rheological characteristics. PHEMA matrix can reversibly transform from its glass state to its rubber state by increasing the temperature at the nanoparticle boundaries. The glass and the rubber behaviours correspond to the fixated and fluid states respectively. When the pHEMA matrix increases in temperature the viscoelasticity reduces allowing the migration of the nanoparticles. Notice that the phase transition temperature of Ag nanoparticles is lower than bulk Ag but still higher than the degradation temperature of pHEMA, both of which are slightly above 300° C. (S. A. Little, et al. Appl. Phys. Lett., vol. 100, no. 5, p. 51107, January 2012; M. Co         kun, et al. Polym. Degrad. Stab., vol. 61, no. 3, pp. 493-497, 1998). The temperature at the boundary of the nanoparticles dictates the mechanical properties of the surrounding medium. Since pHEMA has low heat conduction, the high temperature at the nanoparticle-pHEMA boundary allows the pHEMA matrix to behave like viscoelastic rubber. Furthermore, this effect is present as long as the heat of the metal diffuses in the pHEMA matrix. A pHEMA matrix that transforms from its glass state to its rubber state by increasing the temperature at the nanoparticle boundaries was rationally designed  FIG. 1 . 
         [0037]    When nanoparticle boundaries temperature is increased, nanoparticles move their position. Depending on the size, nanoparticles settle at the maximum intensity or minimum intensity regions of the interference fringe (P. Zemánek, et al. J. Opt. Soc. Am. A, vol. 19, no. 5, pp. 1025-1034, May 2002). We use optical standing waves to control heat and optical force to arrange nanoparticles in different 3D configurations. Dielectric and metal nanoparticles in viscoelastic media have a complex behavior in the presence of radiation gradients. An optical force (tractor force) results from the momentum transfer associated with the spatially asymmetric light scattering and absorption of a nanostructure (O. Brzobohaý, et al. Nat. Photonics, vol. 7, no. 2, pp. 123-127, February 2013). Electromagnetic forces in gradients can push particles toward regions of maximum intensity (positive force) or minimum intensity (negative forces) (M. {hacek over (S)}iler, et al. J. Quant. Spectrosc. Radiat. Transf., vol. 126, pp. 84-90, September 2013). In dielectrics, the force can be positive or negative when the nanoparticle has higher or lower refractive indexes than the medium, respectively (K. C. Neuman and S. M. Block, Rev. Sci. Instrum., vol. 75, no. 9, pp. 2787-2809, September 2004). The phase shift of the scattering dictates the direction of the force. In metal nanoparticles, the phase and intensity of the scattering depends on the Surface Plasmon Resonance (SPR) produced by the free electron cloud. Hence, the direction of the optical force is dictated by different factors including geometry, size and material of the nanoparticle, the surrounding medium, and the wavelength of the applied field (A. Dogariu, et al. Nat. Photonics, vol. 7, no. 1, pp. 24-27, January 2013; K.-S. Lee and M. A. EI-Sayed, J. Phys. Chem. B, vol. 109, no. 43, pp. 20331-20338, November 2005). 
         [0038]    Arbitrary standing waves were defined with the interference of two counter-propagating. The phase of the standing wave was controlled with the relative phase difference between the beams. Hence, the force exerted by the standing wave of two counter-propagating beams is proportional to the gradient of the intensity of the beam  FIG. 2  showed the displacement of nanoparticles from bright regions to dark regions in a standing wave produced by a negative force. 
         [0039]    In order to calculate the migration, it was applied the generalized Stokes&#39; law for a nanoparticle of radius r embedded in a complex viscoelastic medium with a shear modulus G=2.9×10 4 +i2.0×10 4 .  FIG. 3  shows the relative maximum force of Ag nanoparticles using a standing wave from a 532 nm light source. 
       Experimental Demonstration 
       [0040]    In order to demonstrate a 3D nanoassembly, a Nd:YAG (532 nm, 5 ns) pulsed laser was used to form a standing wave. The nanoparticle displacement was increased by repeating the number of pulses. Ag nanoparticles were arranged in a slanted 3D structure with a periodicity of ˜λ/2. We recorded multilayer structures by titling the sample at different angles with respect to the standing wave. The fabricated nanostructure served as a narrow-band wavelength-selective filter to diffract an intense color at 8° away from the sample normal. 
         [0041]    To demonstrate reversibility, we recorded a grating at 5° from the surface plane and erased it (recorded at 0°) iteratively several times. The holographic patterning technique can be used to configure different crystal plane orientations. Bragg planes were superposed at 5°, 10°, 15°, 20°, and 25° to form photonic crystals. To erase the pattern, pHEMA matrix was aligned parallel to the surface plane of the object (i.e. front-surface mirror) at 0°. This configuration aligned the multilayer structure with the specular reflection (zero order). Crystal structures in binary configurations were also recorded and erased to demonstrate volumetric data storage. The composite was also utilized to fabricate dynamic lenses and holographic reconstruction of coins and other objects. 
         [0042]    Although some embodiments are shown to include certain features, the applicant(s) specifically contemplate that any feature disclosed herein may be used together or in combination with any other feature on any embodiment of the invention. It is also contemplated that any feature may be specifically excluded from any embodiment of an invention.