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Chapter 4 Optical-Fiber-Based Measurement of an Ultrasmall Volume , High-Q Silicon Photonic Crystal Microcavity

This chapter details the measurements of photonic crystal microcavities within silicon membranes. The cavity design is the graded square lattice geometry described in chapter 2 and examined experimentally within InP-based multi-quantum-well membranes in the previous chapter. From a device performance perspective, the primary result of importance in this chapter is the demonstration of a quality factor Q ∼ 40, 000 in a wavelength-scale PC cavity. Of equal importance, however, is the development of an optical-fiber-based probing technique that is utilized to study the spectral and spatial properties of resonant modes within these PC cavities. This technique, which essentially involves evanescent coupling between an optical fiber taper waveguide and the PC cavity, allows for quantitative measurements of two of the most important properties of a cavity mode, its Q and spatial localization (related to V eff). From an experimentalist's perspective, other benefits of this fiber-based probing method are that it is relatively easy to implement and can be used to rapidly characterize all of the devices on a chip. The majority of this chapter (sections 4.2-4.7) is largely based on three articles; (i) ref. [52], which describes our initial use of the fiber taper to demonstrate a high Q (∼ 40, 000) and spatial localization consistent with an ultrasmall V eff (∼ 0.9(λ/n) 3) in a PC cavity, (ii) ref. [53], which uses the fiber-based probe to study the robustness of the cavity Q in our graded lattice design with respect to perturbations in the lattice, and (iii) ref. [54], which is a review article that we have written on the use of the fiber taper as a probe for PC microcavities (and wavelength-scale semiconductor cavities