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2003, Frontiers in Optics
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My group has recently demonstrated a number of strategies for selfassembling spherical colloids into complex structures for various types of applications. In one example, spherical colloids have been organized into uniform, polygonal or polyhedral clusters (such as dimers, trimers, squares, pentagons, hexagons, and tetrahedrons) with well-controlled shapes and structures. These well-defined clusters provide a good model system to study the light scattering or hydrodynamics of nonspherical colloids. In another example, spherical colloids have been assembled into three-dimensionally ordered lattices that exhibit interesting photonic bandgap properties. We can control the orientations of these photonic crystals by templating against relief structures etched in the surfaces of silicon wafers. In this presentation, I am going to briefly discuss the procedure, capability, advantages, disadvantages, and future directions for each approach.
NPG Asia Materials, 2011
Figure 1. Colloidal crystals for photonics applications. (Left) Colloidal crystals can be formed into any shape to achieve desired refl ectance properties. (Center) Inverse structures can be obtained by infi ltrating the crystal with various materials and then removing the template particles (colloidal templating). (Right) Short-range-ordered structures of colloidal particles or colloidal glasses can be assembled to achieve angle-independent coherent optical scattering.
Advanced Functional Materials, 2009
A scalable method for site-selective, directed self-assembly of colloidal opals on topologically patterned substrates is presented. Here, such substrate contains optical waveguides which couple to the colloidal crystal. The siteselectivity is achieved by a capillary network, whereas the self-assembly process is based on controlled solvent evaporation. In the deposition process, a suspension of colloidal microspheres is dispensed on the substrate and driven into the desired crystallization sites by capillary flow. The method has been applied to realize colloidal crystals from monodisperse dielectric spheres with diameters ranging from 290 to 890 nm. The method can be implemented in an industrial wafer-scale process.
Applied Physics Letters, 2002
Optical Trapping and Optical Micromanipulation VIII, 2011
The force of light on objects provides tremendous flexibility for nanoscale manipulation. While conventional optical tweezers use the optical gradient force of a focused laser, 1-3 recent work has leveraged the strong field gradients near microphotonic devices for particle trapping. 4-15 However, such work has focused on trapping single or few particles. We have proposed to use optical forces near microphotonic devices for a fundamentally different purpose: to assemble periodic arrays of nanoparticles resembling synthetic, reconfigurable 2D crystals. Our approach, called light-assisted, templated self-assembly (LATS), exploits photonic-crystal slabs to create resonantlyenhanced optical forces orders of magnitude larger than radiation pressure. Here we provide the first experimental demonstration of LATS, assembling a square array of over 100 polystyrene particles near a silicon photonic-crystal slab. Our method, ideally suited for on-chip integration, should provide a platform for flowthrough, serial fabrication of 2D or 3D-nanostructured materials, all-optically tunable photonic devices, and lab-ona-chip applications.
Nano Letters, 2013
The force of light on objects provides tremendous flexibility for nanoscale manipulation. While conventional optical tweezers use the optical gradient force of a focused laser, 1-3 recent work has leveraged the strong field gradients near microphotonic devices for particle trapping. 4-15 However, such work has focused on trapping single or few particles. We have proposed to use optical forces near microphotonic devices for a fundamentally different purpose: to assemble periodic arrays of nanoparticles resembling synthetic, reconfigurable 2D crystals. Our approach, called light-assisted, templated self-assembly (LATS), exploits photonic-crystal slabs to create resonantlyenhanced optical forces orders of magnitude larger than radiation pressure. Here we provide the first experimental demonstration of LATS, assembling a square array of over 100 polystyrene particles near a silicon photonic-crystal slab. Our method, ideally suited for on-chip integration, should provide a platform for flowthrough, serial fabrication of 2D or 3D-nanostructured materials, all-optically tunable photonic devices, and lab-ona-chip applications.
Langmuir, 2006
We report on the fabrication of high-quality opaline photonic crystals from large silica spheres (diameter of 890 nm), self-assembled in hydrophilic trenches of silicon wafers by using a novel technique coined a combination of "lifting and stirring". The achievements reported here comprise a spatial selectivity of opal crystallization without special treatment of the wafer surface, a filling of the trenches up to the top, leading to a spatially uniform film thickness, particularly an absence of cracks within the size of the trenches, and finally a good 3D order of the opal lattice even in trenches with a complex confined geometry, verified using optical measurements. The opal lattice was found to match the pattern precisely in width as well as depth, providing an important step toward applications of opals in integrated optics.
Physics of The Solid State, 2011
A method for formation of photonic crystals has been proposed. The method is based on convective deposition of colloidal particles onto vertical substrates in the presence of a direct-current electric field directed perpendicular to the surface of the formed film and an alternating-current electric field applied parallel to the substrate plane. The structure and optical properties of the prepared colloidal crystals have been investigated using scanning electron microscopy, high resolution small-angle X-ray diffraction, and optical spectroscopy.
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2005
Colloidal crystals (CC) are closely related to photonic crystals (PC) as both of them can be considered as composite materials with a spatial periodic distribution of building blocks. In the case of CC those building blocks are submicrometric particles. Here we will report on recent advances made in our laboratory to fabricate new colloidal crystals with the aim to find new optical properties in relation to PC. We will focus on the following novel topics: (1) Non-close packed face centred cubic (FCC) structures by chemical etching techniques. Most results on solid colloidal crystals concern close packed systems. However, theory predicts that a non-close packed crystal structure presents stronger light scattering properties as compared to the close packed one. Here we shall show results on the fabrication method and optical characterization of non-close packed FCC colloidal crystals. (2) Diamond structures made from nanorobotics. This technique helps to build up complicate 3D crystal structures not achievable, so far, from colloidal self-assembly methods. We will report on the construction method of a crystal aggregate with diamond symmetry.
Three dimensional (3D) photonic crystals (PhCs) are important for many applications such as photon confinement, optical switching, bio-sensing etc. Their unique property arises from the periodic arrangement of micro/submicro-meter particles. There is a lack of simple method to fabricate the same. Here we present a simple method to fabricate uniform, large area 3D PhCs using polystyrene (PS) nanospheres by self-assembly method. PS nanospheres (150-200 nm) were synthesized using styrene monomer by wet chemical method. Size tunability of PS nanospheres has been achieved by changing the cross linking agent concentration. Finally self-assembly was utilized for PhC fabrication.
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