Extending the Supercontinuum Spectrum in Fully Etched Thick Aluminum Nitride Waveguides

The engineering of aluminum nitride (AlN) waveguides allows for broad supercontinuum generation (SCG) within its wide transparency window (200 nm - <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$5.5 \mu \mathrm{m}$</tex>) enabling applications in precision spectroscopy and metrology fields. An efficient coverage from 500 to 3500 nm was shown by pumping crystalline AlN waveguides with a femtosecond laser centered at 1560 nm [1]. UV and visible coverage was reached by pumping around 800 nm and by chirping the waveguide [2] or by exploiting the modal dispersion of higher-order modes [3]. An effective approach to further expand the SC range to the MIR lies in engineering the waveguide dispersion adjusting both its height and width, to extend the anomalous dispersion around the pump and to increase the spectral separation of long and short-dispersion waves (LWDW, SWDW). While the waveguide width is determined by the lithographic process used to transfer the chip design to the material substrate and is easily tunable with high precision, the height is fixed by the epitaxial growth process of the epilayer, limiting the choice of this dimension to the available commercial standards. Thanks to in-house crystalline AlN epitaxial layer growth facilities, we obtain high quality AlN layers up to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.3\mu \mathrm{m}$</tex> thick on <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$c$</tex> -plane sapphire. This gives us an additional degree of freedom in the dispersion engineering of waveguides for SCG. In addition, we developed the fabrication process to obtain fully etched waveguides to improve mode confinement. This required adjusting the thickness of the lithographic mask without compromising the quality of the sidewalls. We achieve propagation losses of 0.24 dB/cm at 1550 nm as shown in the ring <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$Q$</tex> -factor measurement in Fig. 1(a).