Waveguides can be laser-written into silicon carbide for photonic circuits

Optical waveguides can be fabricated in glass and other optical materials by focusing light from an ultrafast laser into the material so that a threshold peak optical power beneath the surface is reached, above which a change in refractive index occurs. In this way, the focused spot can be moved around within the optical material to create waveguides of various geometries. Now, silicon carbide (SiC), which is useful for integrated photonics, can be processed in this way also. Researchers from Shandong University (Jinan, China) and Nankai University (Tianjin, China) are using a laser producing a train of 400 fs linearly polarized pulses at a repetition rate of 25 Hz and a center wavelength of 1064 nm to write waveguides at a depth of 175 μm in 6H-SiC (a hexagonal form of SiC). The mode profile of the waveguides can be tailored by altering the writing parameters.

The SiC crystals are cut into wafers with dimensions of 10 × 10 × 0.33 mm, which are optically polished. The sample is placed on a precision stage that is scanned at 0.5 mm/s, with the polarization direction of the laser parallel to the translation direction. (The researchers have not yet tested waveguide writing at other polarization angles.) Because the laser light induces a negative refractive-index change, the waveguides have either a dual-line or rectangular depressed-cladding structure. To test the waveguides, light from a continuous-wave (CW) laser focused by a microscope objective with a 0.20 numerical aperture was end-fired into the waveguides; another objective collected output light and channeled it to a near-infrared CMOS camera to see the mode shapes. The maximum change in refractive index induced by the laser-writing is estimated to be about 1.5 × 10^-3. Reference: B. Zhang et al., Appl. Phys. Lett. (2020); https://doi.org/10.1063/1.5145025.