Optical probe for nondestructive wafer-scale characterization of photonic elements

This article 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 (≈ 300 nm × ≈ 260 nm cross-section) loop forming a ≈ 10 μm diameter half-circle is suspended in air at the tip of a cantilever extending from a single-mode fiber-connectorized silicon on insulator (SOI) chip (≈ 1.5 mm × ≈ 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 ≈ 10⁶. The SPM enables precise measurement and stable control of sample-probe distance to vary the coupling rate from under- to over-coupled regimes.