Laser-written depressed-cladding waveguides deep inside bulk silicon

Laser-written waveguides created inside transparent materials are important components for integrated optics. Here, we demonstrate that subsurface modifications induced by nanosecond pulses can be used to fabricate tubular-shaped “in-chip” or buried waveguides inside silicon. We first demonstrate single-line modifications, which are characterized to yield a refractive index depression of ≈2 × 10 compared to that of the unmodified crystal. Combining these in a circular geometry, we realized 2.9-mm-long, 30-μm core-diameter waveguides inside the wafer. The waveguides operate in a single-mode regime at a wavelength of 1300 nm. We use near- and far-field imaging to confirm waveguiding and for optical index characterization. The waveguide loss is estimated from scattering experiments as 2.2 dB/cm.