Optical Probe for Nondestructive Wafer-Scale Characterization of Photonic Elements

This letter describes fabrication and operation of a microfabricated photonic waveguide loop probe for nondestructive evanescent coupling and localized testing of nanophotonic devices and circuits. A microfabricated silicon (Si) waveguide ( $\approx 300$ nm $\times \approx 260$ nm cross section) loop forming a $\approx 10\mu \text{m}$ diameter half-circle is suspended in air at the tip of a cantilever extending from a single-mode fiber-connectorized silicon on insulator chip ( $\approx 1.5$ mm $\times \approx 3.5$ mm). The probe is designed for use with commercial scanning probe microscopes (SPM) for accurate positioning, controlled optical coupling, and chip-scale and wafer-scale testing. We demonstrate this by performing spectroscopy on a Si microdisk optical cavity with an optical quality factor of $\approx 10^{6}$ . The SPM enables precise measurement and stable control of sample-probe distance to vary the coupling rate from under- to over-coupled regimes.