Photonics in Low Index Media

Many applications in photonics rely on the ability to confine light in small volumes. This is commonly achieved by utilizing two or more materials with a large refractive index contrast, such as silicon and silicon dioxide, diamond and air, etc. However, techniques available to fabricate sub-wavelength structures in these materials, including electron beam lithography followed by etching, or focused ion beam milling, are often costly and time consuming. In addition, there are few options to tune the optical response of fabricated devices. A panoply of new photonic applications can be unlocked by taking advantage of the versatility of so-called ”soft” materials. Though they typically have a lower index contrast, they can be manipulated by a variety of accurate and rapid techniques, in addition to the standard cleanroom approaches. Polymers and colloids are thus attractive materials for photonics because of the large toolbox available for their fabrication on length scales comparable to the wavelength of light. In this thesis, photonic applications based on three different platforms are presented, each of which comes with a unique fabrication approach: (1) colloidal self-assembly of three dimensional periodic structures, (2) roll-to-roll nano-imprint lithography (R2RNIL) of polymers towards functional photonic devices such as colorimetric sensors and on-chip spectrometers and (3) biopolymer (silk) microspheres. Spherical polystyrene colloids are self-assembled into a 3D face-centered cubic (fcc) lattice and are embedded in SiO2 or TiO2. Once the colloids are burnt out, a porous structure which preserves the fcc arrangement remains. It possesses optical properties of a photonic crystal, which are modified by infiltrating liquids that create partially filled patterns throughout the structure. The evolution of the photonic properties are investigated as deviations from perfect periodicity increase. Furthermore, the opportunity to realize optically pumped lasers (e.g. band edge or random lasers) in this material platform is discussed. Polymers have numerous desirable properties for the future of photonic devices. However, they suffer from a low refractive index contrast. The ability to create functional polymer photonic devices for chip-scale operation is demonstrated, and simulations and experiments are conducted for various photonic components, including waveguides, gratings, and ring resonators. Two dimensional photonic crystals with 100 nm feature sizes are produced by the R2RNIL and they display tunable structural color. Photonic elements for on-chip photonic integration are also fabricated with R2RNIL. S-shaped waveguides are coupled to ring resonators and ring resonator quality (Q) factors close to 60,000 are measured. A grating coupler setup is built and tested by measuring silicon-on-insulator devices featuring rings coupled to the waveguide, and Qs of 20,000- 30,000 are obtained. Silk is a special polymer in the sense that it is biocompatible and thus interfacing photonic components with internal organs can be envisaged. Here, investigations of silk’s photonic properties in the context of microspheres are performed. A tapered fiber is used to couple in and out of the whispering gallery optical resonances of silk microspheres. Qs of 500-1000 are determined from transmission measurements; simulations and scanning electron micrographs confirm that the reduction in Q is due to deviations from a completely spherical shape.Chemistry and Chemical Biolog

Nanocrystalline Precursors for the Co-Assembly of Crack-Free Metal Oxide Inverse Opals

Inorganic microstructured materials are ubiquitous in nature. However, their formation in artificial self‐assembly systems is challenging as it involves a complex interplay of competing forces during and after assembly. For example, colloidal assembly requires fine‐tuning of factors such as the size and surface charge of the particles and electrolyte strength of the solvent to enable successful self‐assembly and minimize crack formation. Co‐assembly of templating colloidal particles together with a sol–gel matrix precursor material helps to release stresses that accumulate during drying and solidification, as previously shown for the formation of high‐quality inverse opal (IO) films out of amorphous silica. Expanding this methodology to crystalline materials would result in microscale architectures with enhanced photonic, electronic, and catalytic properties. This work describes tailoring the crystallinity of metal oxide precursors that enable the formation of highly ordered, large‐area (mm2) crack‐free titania, zirconia, and alumina IO films. The same bioinspired approach can be applied to other crystalline materials as well as structures beyond IOs.Chemistry and Chemical Biolog