Self-assembled Photonic Crystals Research Articles

Layer Transfer Approach to Opaline Hetero Photonic Crystals

By taking advantage of the hydrophobicity of dry polystyrene colloidal crystal (opal) films and the large surface tension of water, a convectively self-assembled polystyrene opal film on a hydrophilic glass substrate can be peeled off from the substrate and floated on the water surface. A layer transfer technique was developed to sequentially stack floating opal films of different sphere sizes, resulting in opaline hetero photonic crystals. The feasibility of this technique to planar defect engineering in a self-assembled colloidal photonic crystal was also demonstrated. Both structural observation and optical characterization confirmed the crystalline integrity of the resultant opaline heterostructures.

Acclaimed defects

Defect engineering is crucial for realizing all-optical integrated circuits from self-assembled photonic crystals. A two-photon polymerization strategy paves the way towards incorporation of arbitrary defects in silicon inverse opal photonic crystals.

Fabrication of a simultaneous red–green–blue reflector using single-pitched cholesteric liquid crystals

A cholesteric liquid crystal (CLC) is a self-assembled photonic crystal formed by rodlike molecules, including chiral molecules, that arrange themselves in a helical fashion. The CLC has a single photonic bandgap and an associated one-colour reflection band for circularly polarized light with the same handedness as the CLC helix (selective reflection). These optical characteristics, particularly the circular polarization of the reflected light, are attractive for applications in reflective colour displays without using a backlight, for use as polarizers or colour filters and for mirrorless lasing. Recently, we showed by numerical simulation that simultaneous multicolour reflection is possible by introducing fibonaccian phase defects. Here, we design and fabricate a CLC system consisting of thin isotropic films and of polymeric CLC films, and demonstrate experimentally simultaneous red, green and blue reflections (multiple photonic bandgaps) using the single-pitched polymeric CLC films. The experimental reflection spectra are well simulated by calculations. The presented system can extend applications of CLCs to a wide-band region and could give rise to new photonic devices, in which white or multicolour light is manipulated.

Observation of higher-order diffraction features in self-assembled photonic crystals

The optical response of high quality three dimensionally (3D) ordered photonic crystals is analyzed in the high energy region. By tuning the reflectance with the angle of incidence of light, the peaks in the reflection spectrum that correspond to the first, second, and third order photonic stop bands and the van Hove singular point in the photon density of states are clearly distinguished. The high energy features have been observed for photonic crystals made from colloids of different diameters, having different index contrast and fabricated by two different self-assembly routes. The observation of van Hove singularity at near-normal incidence of light and its presence even in low index-contrast photonic crystals provide conclusive evidence that these high energy features are due to the perfect periodic ordering present in the photonic crystals with less defects and disorder.

Self-Assembled Photonic Crystals and Templated Nanomaterials

Self-Assembled Photonic Crystals and Templated Nanomaterials

Tailoring Self-Assembled Metallic Photonic Crystals for Modified Thermal Emission

We predict that modified thermal emission can result from three-dimensional, self-assembled photonic crystals. In previous tungsten structures, known as inverse opals, strong absorption prevented any influence of the periodicity. We consider the origin of this effect and show how to tailor both absorption and surface coupling in experimentally realizable metallic inverse opals. Calculations for tungsten, molybdenum, and tantalum crystals show that their optical properties can be similar to or even better than the tungsten woodpile, where modified thermal emission has already been seen.

Effects of Paraffin Addition on Optical Properties and Self-Assembly of SiO<sub>2</sub> Photonic Crystal

The effects of paraffin addition on the optical properties and crystallinity of selfassembled SiO2 photonic crystal (PC) were examined using a silica nanopowder. The silica nanopowder was fabricated using the well-known Stöber process. During the evaporation process for self-assembly of PC, the nanopowder content was 0.05 or 0.2 wt%, the paraffin addition was fixed to 0.001 wt%, the process temperature was 80oC and the heat-treatment temperature after evaporation was 900oC. The addition of paraffin reduced the defects in the PC layer, thereby resulting in good crystallinity and transmittance. Our results suggested that paraffin can enhance the bonding force between silica nanopowders.

Defect Engineering in Self-Assembled 3D Photonic Crystals

This work describes the combination of photolithography and self-assembly methods for fabrication of 3D photonic crystals (PCs) with well-defined micron-scale line defects embedded in the PCs. Line defects with different dimensions, shapes, and compositions have been introduced into the 3D PCs by choosing different photoresists, masks, and template-directed assembly techniques. Infiltration of carbon using high-temperature chemical vapor deposition (CVD) technique showed that the fabrication procedure offers an ideal approach to functional 3D photonic devices from self-assembled photonic crystals.

Defect Engineering in Self-Assembled 3D Photonic Crystals

This work describes the combination of photolithography and self-assembly methods for fabrication of 3D photonic crystals (PCs) with well-defined micron-scale line defects embedded in the PCs. Line defects with different dimensions, shapes, and compositions have been introduced into the 3D PCs by choosing different photoresists, masks, and template-directed assembly techniques. Infiltration of carbon using high-temperature chemical vapor deposition (CVD) technique showed that the fabrication procedure offers an ideal approach to functional 3D photonic devices from self-assembled photonic crystals.

Rapid synthesis of monodisperse polymer spheres for self-assembled photonic crystals

In order to make both the laboratory and the industry preparation of monodisperse spheres easy, a boiling temperature soap-free emulsion polymerization was developed. A number of polymer and copolymer spheres with a size of several hundreds of nanometer were synthesized. Comparing with the conventional synthesis, this method has following advantages: (1) the experimental set-up and operation are simplified, (2) the reaction time is shortened, (3) ease of control of sphere size and size distribution with good repeatability, (4) a number of polymer spheres, including PAN, PS, PMMA and PVA, can be synthesized, and (5) spheres with over 20% cross-linkage can be synthesized. Application of the spheres in the construction of self-assembled photonic crystal films was demonstrated.

Towards Si-based photonic circuits: Integrating photonic crystals in silicon-on-insulator platforms

In the context of Si-based photonics, we report on a strategy to integrate two optical components, a 3D photonic crystal light emitter and a waveguide, in a silicon-on-insulator patterned substrate. Self-assembled colloidal photonic crystals are produced with high crystalline quality and spatial selectivity. Plane wave expansion and finite-difference time-domain have been used to find suitable configurations for positioning emitters and waveguides. The first steps toward the realisation of these configurations are presented.

Slow to superluminal light waves in thin 3D photonic crystals

Phase measurements on self-assembled three-dimensional photonic crystals show that the group velocity of light can flip from small positive (slow) to negative (superluminal) values in samples of a few mum size. This phenomenon takes place in a narrow spectral range around the second-order stop band and follows from coupling to weakly dispersive photonic bands associated with multiple Bragg diffraction. The observations are well accounted for by theoretical calculations of the phase delay and of photonic states in the finite-sized systems.

Fluorescence amplification using colloidal photonic crystal platform in sensing dye-labeled deoxyribonucleic acids

We applied self-assembled silica colloidal photonic crystals (CPCs) for a biosensing platform. Thin-film CPCs were tested for detecting dye-labeled deoxyribonucleic acid hybridization reactions. Due to the high porosity and large internal surface area, we observed a fluorescence signal enhanced by two orders of magnitude from CPC films composed of tens of silica sphere layers in the [1 1 1]-direction. This observation agrees with the theoretical enhancement factor of 2 π / 3 per silica sphere layer. The proof-of-the-concept experiment on fluorescence amplification was then followed by other complementary experiments that clarified the concerns regarding the practicality of the CPC sensor platform, such as film thickness dependence, and the effect of non-hybridized DNAs. We also learned that the silica sphere size should be chosen carefully, since the L-point photonic bandgap of the CPC may suppress the dye fluorescence.

Defect Healing of Self-Assembled SiO2 Layer by Heat-Treatment and Multi-Coating

This study examined the effect of heat-treatment and multi-coating on the microstructure and optical properties of photonic crystals (PCs) that had been self-assembled using mono-dispersed spherical SiO2 nanoparticles. When the heat-treatment temperature was increased, the reflectance peak due to the photonic bandgap moved to a shorter wavelength direction, and the peak intensity of the Fabry-Perot fringes increased. The highest reflectance peak intensity was obtained in the sample heat-treated at 250-300 degrees C. Heat-treatment reduced the average particle size and number of defects, and increased the packing density of the PC. When the heat-treatment temperatures were increased up to 900 degrees C, a large crack with an average size of approximately 1.3 microm was formed through the self-assembled layer. A multi-coating could effectively fill the open space of this crack and thereby reduce the total number of defects. The crystallinity and optical properties of the self-assembled PC were improved by the heat-treatment and multi-coating.

Integration of Self-Assembled Three-Dimensional Photonic Crystals onto Structured Silicon Wafers

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.

Building Nanocrystalline Planar Defects within Self‐Assembled Photonic Crystals by Spin‐Coating

Building Nanocrystalline Planar Defects within Self‐Assembled Photonic Crystals by Spin‐Coating

Patterning of silica microsphere monolayers with focused femtosecond laser pulses

We demonstrate the patterning of monolayer silica microsphere lattices with tightly focused femtosecond laser pulses. We selectively removed microspheres from a lattice and characterized the effect on the lattice and the substrate. The proposed physical mechanism for the patterning process is laser-induced breakdown followed by ablation of material. We show that a microsphere focuses radiation in its interior and in the near field. This effect plays an important role in the patterning process by enhancing resolution and accuracy and by reducing the pulse energy threshold for damage. Microsphere patterning could create controlled defects within self-assembled opal photonic crystals.

Self-assembled zno colloidal photonic crystals: Light channels in cracks

Large-area films of colloidal photonic crystals (PCs) were prepared by self-assembly technique. The films contain ordered PC domains separated by cracks. The size of the domains and cracks can be controlled by the drying process. The angular dependence of the stop-band was studied by changing the angle of incidence θ between the beam and the (1 1 1) direction, from 0° to 60°. The observed stop-bands are almost independent of the density of cracks. It is seen that as the angle increases the stop-band shifts to shorter wavelengths. We demonstrate that the photoluminescence emission spectra depend on the PC domain size and on the concentration of cracks. For a film with small density of cracks, we observed light channeling through cracks probably due to light inhibition.

3-D Photonic Band Structure Engineering in Self-Assembled Macroporous Photonic Crystals

AbstractBy applying directional pressure along the (111) crystal axis of opaline photonic crystals under controlled temperatures, inverse opals with symmetry broken structures are fabricated. This selective deformation results in strongly modified photonic band structures and hence optical properties of the photonic crystals. Experimental data are accompanied by theoretical band structure calculations that confirm the experimental results and are used to predict new structures with optimized band gap properties.

Precise Fabrication of Point Defects in Self-Assembled Three-Dimensional Macroporous Photonic Crystals

We demonstrate a new and simple method of precisely fabricating defects in three-dimensional (3D) CdS macroporous photonic crystals (PCs) with a variable pressure scanning electron microscope. Well-defined point defects, not only vacancies but also an impurity (a reduced-size sphere), were directly fabricated by electron-beam irradiation under a gas atmosphere. This provides a convenient and straightforward method of introducing various designed defects into 3D PCs for photonic-band-gap-based applications.