On-chip natural assembly of silicon photonic bandgap crystals
Original Entry: Ian Bruce Burgess Fall 2009
1. Y.A. Vlasov, X.-Z. Bo, J.C. Sturm, D.J. Norris, Nature 414, 289-293 (2001).
2. A. Blanco et al., Nature 405, 437-440 (2000).
This paper describes the fabrication of a silicon 3D photonic crystal with sufficiently low defect densities to maintain the complete bandgap over a large volume. The structure is fabricated by infiltrating a thin-layer silica colloidal crystal with Si by low pressure chemical vapor deposition, and then removing the template of the colloidal crystal using buffered oxide etching. This additional step is required to achieve the refractive index contrast required for a complete 3D bandgap (>2.8:1). What allows improved crystal quality in the colloidal opals over previous work , is the use of the meniscus-driven vertical deposition technique as opposed to gravitationally-driven sedimentation. The figure below shows the colloidal crystal (left) and the inverted Si photonic crystal (right).
The photonic crystals were characterized by optical reflection spectroscopy in both the  and  directions. A region of 100% reflection was observed in both directions, confirming the prediction from modeling of an omnidirectional bandgap. Defects in such a crystal allow for wavelength-scale confinement of light in the defect region, and can behave as high-quality-factor optical resonators with ultrasmall modal volumes. The authors demonstrate such defect engineering by adding spheres of a smaller size to the solution before deposition. This translates into randomly placed point defects in the opal and thus the inverse opal. The authors further demonstrate that these photonic crystals can be patterned on a large scale on chip using conventional photolithography and reactive-ion etching of Si.
This paper demonstrates a powerful use for the phenomenon of colloidal crystallization covered in this week's lectures. More importantly, it demonstrates how the thickness and long range quality of colloidal crystals can be controlled by controlling the effective inter-sphere forces. In sedimentation crystallization, the suspension concentrates over time near the bottom due to gravity. At a certain point the concentration crosses the critical volume fraction and ordering in the colloidal suspension appears. Upon removal of the solvent (drying), the opal photonic crystal template is left behind. However, the domain size and the number of layers (thickness) cannot be easily controlled using this method. As was covered in the previous section on phase transitions, a system that simultaneously undergoes a phase transition at all locations tends to from domains of a characteristic size, which is undesirable for photonic crystal fabrication since domain boundaries act as defects which add undesirable optical modes to the bandstructure. In the vertical deposition technique, the crystal assembles due to strong capillary forces near the meniscus on the evaporation front. There is an added effective attractive force between the spheres due to the surface tension and the transition from colloidal gas (solution) to crystal occurs at only one location at a time. The authors speculate that there is a meniscus-induced shear that aligns the crystal planes preferentially into one orientation. In addition, vertical deposition allows precise thickness control of the crystal by tuning the colloidal concentration. The concentration determines the flux at which the spheres diffuse into the meniscus region (where packing occurs), and thus the number of layers that form during an incremental movement of the meniscus.