Study For Fabricating Polymer Photonic Crystals And The Optical Properties

Photons have many advantages over electrons as carriers of information. They are faster and can convey huge amounts of data with low power losses. A new class of materials called photonic crystals has the potential to steer light in the same way as electrons are manipulated in semiconductor. Photonic crystals, also called photonic band-gap (PBG) materials or PhCs , are a new class of artificial optical materials composed of periodic dielectric structures with different permittivity and feature size on the order of optical wavelengths. PhCs have unusual optical properties and promise to provide revolutionary solutions to the miniaturization of photonic devices. Photonic crystals have the property of preventing light from propagating in certain directions with specified energies, creating a photonic band gap which is analogous to the electronic band gap in semiconductors. The photons are either allowed or non-allowed states of the lattice depending on their energy. Applications for PCs fall into two generic categories based on either the reflective or transmissive attributes of a photonic crystal. So,for different applications of photonic crystals, the choice of material composition, lattice periodicity and symmetry as well as the deliberate creation of defect structures embedded in PhCs allows the control over the properties of this novel class of optical materials.Since it was invented by Yablonovitch and John , photonic crystal has been one of the most active research areas. Photonic crystal not only provides new opportunity for integration and minimization of photonic instruments, but also shows some new physical phenomena. The study of this dissertation is to focus on the properties and these unusual optical characteristics for two-dimensional and three-dimensional photonic crystals, furthermore, to research and analyze the polymer photonic crystals, their superprism effect and structure in depth. We studied the photonic crystals fabricating methods in systematization, using soft lithography and laser holographic lithography, two-dimensional and three-dimensional polymer photonic crystals were fabricated successfully, and their optical properties were investigated. The main research works and innovations were expatiated as followings:â… Design and simulation for two-dimensional polymer triangle lattice photonic crystal superprism structures in low refractive indexHere we design and calculate the polymer photonic crystal superprism structure with a triangle array of air holes on the polymer slab. The refractive index of background polymer is 1.475 at 1550nm. To optimize the design of superprism for polymer photonic crystals, the complete band structure and the dispersion surface were calculated and analyzed by the plane wave expansion. (Chapter 2)â…¡Simulating and demonstrating three-dimensional FCT woodpile structure superprism effectIn following the same method for two-dimensional photonic crystal superprism, we directly compute the dispersion surface based on the full three-dimensional photonic band structure, and then determine the propagation angles from the gradient of this surface. The calculations were performed for light incident to the y-z plane. Due to the conservation of the parallel component of the k vector at the transition from one medium to another, the light inside the photonic crystals will be confined to the y-z plane. Then we can simplify the three-dimensional dispersion surface to two-dimensional-like dispersion surface.The band structure is calculated using BANDSOLVE software package which utilizes the plane wave expansion method. The calculation shows the photonic band structure for FCT lattice woodpile with w/d=0.266 and h/d=0.305 in a polymer/SiO₂ medium and withâ–³n=0.17. There is no complete band gap in the polymer woodpile due to low refractive index.We demonstrated low refractive index two-dimensional polymer triangle lattice and three-dimensional FCT woodpile structure superprism effect and the propagation property for these structures. The propagation angle is very sensitive to the incident wavelength and incident angle. The following two-dimensional and three-dimensional photonic crystal structures fabrication are based on these design and simulation. (Chapter 2)â…¢Fabrication of two-dimensional polymer photonic crystals by soft lithography and demonstration for superprism effectImprint lithography as an alternative fabrication technique provides low-cost, high throughput and high resolution advantages over the next generation lithography. Many applications in micro- and nano-structures were demonstrated. An important application for imprint lithography is soft lithography, using this technology, two-dimensional photonic crystals and superprism structures that can manipulate laser beam steering were fabricated successfully. Polymer photonic crystals usually do not exhibit complete photonic band gaps because of low dielectric constants of polymers. However, certain optical devices such as superprism do not require the photonic crystals to have a bandgap. The theoretical calculation shows the strong superprism effects in the polymer photonic crystals due to anisotropic dispersion surface. Soft lithography offers a convenient, effective and low-cost technology for fabricating micro-and nano-structures.We introduced some key fabricating technologies. There are three steps in the soft lithography procedure: baseplate fabrication, PDMS template formation, and pattern transfer. The method and principle for fabricating PDMS template were set forth, and used this PDMS template to obtain two-dimensional polymer photonic crystals with micro-capillaries. The two-dimensional triangular photonic crystal superprism structures were fabricated by soft lithography provided not only an aspect ratio of 1.25 with 300nm air holes, but also the depth of the baseplate and that of molded patterns have only 3% difference. Meanwhile we also obtained the photonic crystal structures with 450nm air holes in diameter and 900nm in lattice constant. And we demonstrated the superprism effect in two-dimensional 900nm lattice constant photonic crystal structure at near-infrared wavelength by optical measuration. The beam propagation angle changed from positive to negative when the input incident angle was varied from 15°to 11°, and the experimental results are in good agreement with the simulation.Furthermore the physical mechanism behind the superprism phenomenon and effect was analyzed with the anisotropic dispersion surface .The soft lithography with the baseplate, PDMS templates and molded patterns offers a simple, low cost and dependable technique for fabricating fine feature structure as small as 150nm in line-width without distortions and defects over a large patterned area. (Chapter 3)â…£Fabrication of holographic three-dimensional polymer photonic crystals in near-infrared band and study for the optical propertyThe self-designed computer simulation of the multi-laser-beam interference is introduced to the study of the relationship between the polarization of light and the clarity of the interference pattern, which provides the optimal solution of the polarization on holographic lithography technology and improves the fabrication of sub-micrometer periodic structure much more efficiently. A three equilateral sidewalls prism holographic fabrication has been proved for three-dimensionalfcc(face-centered-cubic)-type polymer photonic crystal using negative photoresist. Special fabricating treatment has been introduced to make sure the stability of the fabricated area nanostructures. The SEM results testified the good dependability of the fabricated structures. The experimental results are consistent with the results of computer simulation. Based on the method of a prism interference, a submicron three-dimensional fcc-type polymer photonic crystal was fabricated by applying He-Cd 325nm continuous laser interaction with negative photoresist SU8. The structure was also simulated using computer simulation. By analysis of SEM images and FTIR transmission and reflection spectra, the fabricated photonic crystal structures are in good agreement with the simulation results. In order to simulate precisely the three-dimensional polymer photonic crystal holographic lithography process, the adsorption inside the photoresist needs to be taken into consideration. As the size gets smaller, the effect of adsorption on crystal dependability gets bigger. We also obtained the simulating results of a fcc-type photonic crystal with adsorption and without adsorption. (Chapter 4) â…¤Primary study for fabricating three-dimensional polymer woodpile photonic crystals by nano-imprintThe woodpile photonic crystal is one of the most popular three-dimensional lattices and the polymer photonic crystals need to be fabricated using layer-by-layer stacking method. The structure consists of layers one-dimensional rods, stacking according to certain crystal symmetry to form a lattice structure. We first write the alignment marks on the silicon substrate by E-beam. And each layer patterns is written at a correct position referencing to the alignment marks fabricated in the first step. Then using layer-by-layer stacking method, by repeating the process ,we can obtain the woodpile structure and finally fabricate three-dimensional low refractive index polymer/SiO₂ photonic crystal that has low loss at 1550nm wavelength and thermal stability. (Chapter 5)...