Silicon is one of the best known materials in the world. It is available in high purity and the processing technologies for silicon are by now very well established. While the properties of silicon as a semiconductor are well explored, it is not much used in photonic technology. However, Si is an excellent optical waveguide material, as it is completely transparent at 1.5 µm. In this work we have exploited silicon as a substrate and optical waveguide material in a variety of novel photonic structures. Our experiments focussed on a) the design of two-dimensional photonic bandgap structures, b) measurements on the spontaneous emission rate of atoms to probe the local optical density of states.
2-D photonic crystal waveguides are fabricated in Si by reactive ion etching of Si pillars that are 5 mm tall, 205 nm in diameter, and arranged in a square lattice with a 570 nm pitch. Bandstructure calculations predict a gap for TM modes at a wavelength of 1.5 mm with a gap width equal to 37 % of the center energy. In order to meet the severe nano-tolerance requirements in such a device, the SF6/O2 electron cyclotron resonance plasma process conditions at reduced temperature are optimized. By removing an array of Si pillars, the waveguide is defined. In order to achieve confinement in the third dimension, a 2 µm thick region of the Si structure was amorphized before processing, using 4 MeV Xe irradiation. Input and output waveguides are integrated with the structure.
In order the measure the local density of states in such photonic structures, optical probe ions must be incorporated. We have studied Er ion implantation into Si and found the optimum implantation and annealing conditions to obtain optically active Er, luminescing at 1.535 µm in Si. The luminescence shows strong quenching above 200 K, but lifetimes as long as 1 ms are achieved at 77 K. We have also studied the coating of the Si pillar structures with a novel wet chemical process, in which we have used a reaction between tetraethoxysilane, ammonia, and water to nucleate and grow well-controlled layers of SiO2 on surfaces with extreme aspect ratio. Using confocal microscopy we show the uniform coating of such layers doped with an eosin dye.
Finally, we show how we can strongly affect the radiative lifetime of Er probe ions implanted into silica colloidal particles with a diameter of 300 nm, by comparing three geometries, 1) placed on a Si substrate, 2) arranged in a 3-D crystal, or 3) surrounded by an index-matching fluid.