Abstract:A small depression is created in a straight optical fiber taper to form a local probe suitable for studying closely spaced, planar microphotonic devices. The tension of the "dimpled" taper controls the probe-sample interaction length and the level of noise present during coupling measurements. Practical demonstrations with high-Q silicon microcavities include testing a dense array of undercut microdisks (maximum Q = 3.3×10 6 ) and a planar microring (Q = 4.8×10 6 ).

Encapsulation layers are explored for passivating the surfaces of silicon to reduce optical absorption in the 1500-nm wavelength band. Surface-sensitive test structures consisting of microdisk resonators are fabricated for this purpose. Based on previous work in silicon photovoltaics, coatings of SiNx and SiO2 are applied under varying deposition and annealing conditions. A short dry thermal oxidation followed by a long high-temperature N2 anneal is found to be most effective at long-term encapsulation and reduction of interface absorption. Minimization of the optical loss is attributed to simultaneous reduction in sub-bandgap silicon surface states and hydrogen in the capping material.The integration of high-speed photonics and electronics on a silicon platform is currently being actively pursued due to the soaring demand for bandwidth and the difficulties in maintaining Moore's Law scaling of microelectronics [1,2]. Due to the thermal and power limitations already faced by the electronics industry [3,4], future systems incorporating on-chip optical processing and communication will likely put a heavy premium on low optical power levels and high optical efficiency. With the many benefits of Si microphotonic circuits, such as the ability to create active and nonlinear optical elements, there is also the adverse effect of increased optical loss in comparison to conventional glass-based photonic circuits. In addition to the intrinsic nonlinear absorption present in micron-scale Si waveguides [5], the high-index contrast of such structures results in significant optical scattering loss for surface-roughness at the nanometerscale. Over the past several years there has been considerable effort put forth to improve the quality of Si microphotonic device fabrication, with optical scattering loss having been reduced to a level of 1 dB/cm in single-mode waveguides [6]. This is still several orders of magnitude above the bulk absorption limit of moderately doped Si in the 1.3-1.5 µm wavelength band [7], and it is interesting to consider whether further reductions in optical loss can be made. One possible impedement is the absorption present at the Si surfaces of current highindex contrast and, consequently, high surface-to-volume ratio Si microphotonics.In previous work we measured the effects of surface chemistry on the optical losses in smoothly etched, high-Q Si microdisk resonators [8,9] and found that for losses below the dB/cm level, surface absorption begins to play a role. In that study, hydrogen-passivated surfaces showed marked improvement with optical losses measured as low as 0.15 dB/cm; however, the passivation was temporary and easily spoiled by the atmosphere. In this Letter we investigate, for photonics applications, several techniques originally developed for electronic surface passivation of Si solar cells. The reliability standards of Si photovoltaics fabrication [10,11,12,13] routinely call for various passivation layers to be deposited over the Si surfaces in order to preserve the lifetime of the minor...