Design of inverse opals with controlled structures and wettability for photonic bandgap engineering

Abstract

This thesis focuses on colloidal photonic crystal for the anti-counterfeiting purposes. Structural colors have attracted considerable attention in a variety of research areas because of their unique characteristics that differ from those of chemical pigments or dyes. Although pigments and dyes owe their color to the absorption of light, structural color has its origin in geometric structures capable of manipulating the diffraction or reflection of light through periodically arranged photonic nanostructures. Photonic crystals can be considered as periodic arrangements of regularly shaped with different dielectric constants. They have distinct wavelengths of reflection that are governed by the distance between neighboring lattices, refractive index difference between particle and matrix. If the refractive index of the photonic crystal is modulated, for example, by a certain stimulus, the wavelength of maximum reflectance also will change. Moreover, similar to semiconductors, the controlled insertion of extrinsic defects into photonic crystals could manipulate their optical properties and enrich their functionalities by creating optical states within the complete or pseudophotonic band gap.

In Chapter 2, we report a simple method to encrypt polymeric inverse opals with a combinatorial code of micropatterned graphic and unique spectrum. To accomplish this, we prepared micropattern of photoresist on the top surface of hydrophobic inverse opal by photolithography, which served as a shadow mask for directional reactive ion etching with oxygen gas. The selective infiltration provides unique spectral code with two peaks composed of the original and the shifted, where position of the shifted peak is determined by refractive index of the aqueous solution. Therefore, the decoded inverse opals deliver unique combinatorial code which is revealed only when aqueous solution agreed in advance is used for decoding. In addition, the photonic structures are chemically stable, maintaining the invariant combinatorial codes for many cycles of uses and a long storage period. Moreover, the inverse opal film can be released from substrate to be freestanding, which can be further transferred into any surfaces for anti-counterfeiting purpose. This photonic encryption material will provide new opportunity in a wide range of security applications.

In Chapter 3, we propose and demonstrate a novel method for incorporating defect layers into SU-8 inverse opal. We can prepare inverse opal films with defect layer combining capillary wetting with coating method to create a homogeneous defect layer of uniform width between two inverse opal films of controlled thickness. Microscopy and optical characterization results for these new structures indicate that they have high structural and optical quality and in particular that the planar defect behaves as a trapped layer of light. The appearance of allowed states within the photonic stop band is observed and is in good agreement with theoretical predictions. We also show that a wide range of planar defect thickness and color of inverse opals. The shapes of spectral code are all different depending on thickness of defect layer, however we can’t distinguish among them in the naked eyes. Furthermore, conventional photolithography enables patterning of various shapes. Complex spectral codes in the spectrum provide high security level, thereby being potentially useful for anti-counterfeiting materials.