AbstractPhotonic crystals currently hold exciting potential in waveguiding applications with the ability to greatly reduce waveguide bend radii, allowing for the realization of photonic networks with increased complexity and device density. Square spiral photonic crystals are comprised of periodic arrays of slanted cylindrical columns with periodic abrupt 90° changes in the column growth direction. The abrupt changes produce elbows which correspond to points in the three-dimensional diamond lattice, which are commonly referred to as diamond:n photonic crystal structures based on the nth nearest neighbor lattice point between column arms. The diamond:n structures are characterized by four parameters: column separation, spiral pitch, column diameter, and column growth angle. These structures have been shown theoretically to have a three-dimensional relative bandgap of approximately 15% for a direct silicon diamond:1 structure and approximately 24% for an inverse silicon diamond:5 structure with circular column cross sections. Silicon square spiral photonic crystals can be grown by a single-step thin film deposition process known as glancing angle deposition (GLAD), which utilizes rotation of lithographically patterned substrates and advanced control algorithms. While using GLAD to grow square spiral photonic crystals in a single-step process is desirable, it does have inherent limitations. Physical vapor deposited films deposited at oblique incidence angles form columns with angle ≤ 60° relative to the substrate normal. This deviates from the optimal diamond:1 and diamond:5 structures which require film growth angles of approximately 64° and 74°, respectively. Here we present SEM images of periodic silicon square spiral films grown with a vapor source incidence angle of 85° that indicate an average column tilt angle of 57˚ for films grown with the basic GLAD process and an average column tilt angle between 61˚ and 65˚ using an ion-assisted GLAD process. Photonic simulations indicating that square spiral films with complete three-dimensional photonic bandgaps are only achievable via ion-assisted GLAD are also presented.
Fabrication of Coatings with Structural Color on a Wood Surface
