Speaker
Description
Ultraviolet (UV) photonic integrated circuits (PICs) have many interesting applications, such as biochemical sensing, atomic clocks, and UV Raman spectroscopy. For realizing PICs in the UV spectral range, materials with a large bandgap are required to facilitate low optical losses. One exciting option is AlGaN as it allows the monolithic integration of AlGaN based UV light emitting diodes (UV-LEDs), UV lasers, and photo detectors. However, the optical properties of AlGaN materials, especially the optical losses have not yet been studied in detail in the UV-C spectral range. In this work, the optical losses in 220$\,$μm wide n-Al$_{0.76}$Ga$_{0.24}$N waveguides are determined from LED–detector pairs with waveguide lengths from 400$\,$μm – 1000$\,$μm. The PIC consisting of waveguides, LEDs and photodetectors were fabricated from an AlGaN heterostructure grown by metal-organic vapor phase epitaxy (MOVPE) on sapphire substrates. Our measurements show that the detector signal actually exhibits two different exponential dependencies on the waveguide length with a much smaller slope for waveguide lengths longer than 1000 μm. Monte-Carlo ray tracing simulations showed that scattering at the substrate’s backside can dominate the signal for long waveguide lengths, which we could confirm by applying an absorbing layer on the backside of the sample and measuring the LED-detector pairs again. By ray tracing, we also identified different contributions of the optical losses, namely, the propagation losses and scattering losses. Here, a major challenge is the implementation of the waveguide surface roughness that was measured by atomic force microscopy. In conclusion, we find a total waveguide loss of 30$\,$cm$^{-1}$ to which scattering losses contribute with 6$\,$cm$^{-1}$ and propagation losses with 24$\,$cm$^{-1}$.
