Novel alumina optical waveguides have very low loss in the blue and near-UV

Researchers at the Massachusetts Institute of Technology (MIT; Cambridge, MA) and MIT Lincoln Laboratory (Lexington, MA) have developed optical waveguides that have low loss at blue and near-UV wavelengths.¹ Made of aluminum oxide (Al₂O₃, or alumina) fabricated via atomic-layer deposition, the fully etched waveguides have losses of less than 3 dB/cm and 2 dB/cm at 371 nm and 405 nm wavelengths, respectively. The researchers say that this is more than an order-of-magnitude improvement over previous waveguides created for this wavelength region.

Films of alumina were grown on bare silicon and on silicon dioxide (SiO₂) on silicon with growth-process parameters optimized for defect-free growth, producing a film with refractive index of 1.65 at 633 nm and 1.72 at 260 nm; the researchers say that, although the refractive index could be boosted by high-temperature annealing, this would change to alumina to a polycrystalline form with high optical loss (greater than 20 dB/cm).

Plasma-enhanced chemical vapor deposition (PECVD) SiO₂ was patterned on top of the alumina and the alumina plasma-etched, then a layer of PECVD SiO₂ added to clad the waveguides. The researchers fabricated waveguides with widths of 400 to 600 nm, which are single-mode for all test wavelengths (371, 405, 419, and 458 nm) except for the 600 nm width at 371 nm, which is multimode.

A ring resonator with a 500 nm waveguide width a 90 um radius was fabricated. A tunable 405 nm laser source was used to find resonance and a Q (quality) factor of 470,200 was measured at 405 nm.

The researchers say that, because their measurements showed that the waveguide loss was likely dominated by scattering, it should be straightforward to further reduce loss via improved fabrication.

REFERENCE:

1. Gavin N. West et al., arXiv:180800429v1 [physics.optics] 1 Aug. 2018.