PhD Defence Soheila Mardani | AlN and Al2O3 for Broadband Integrated Photonics

AlN and Al2O3 for Broadband Integrated Photonics

Soheila Mardani is a PhD student in the department Optical Sciences. (Co)Promotors are prof.dr. S.M. Garcia Blanco and dr. L. Chang from the faculty Science & Technology (TNW), University of Twente.

Broadband photonic integrated circuits (PICs) are essential for applications spanning ultraviolet (UV), visible, and infrared (IR) wavelengths, including telecommunications, biosensing, quantum optics, and nonlinear photonics. The development of such systems demands material platforms with wide transparency windows, low propagation losses, and compatibility with active and passive photonic components. Traditional materials, such as silicon and silicon nitride, offer partial solutions but fall short of achieving full-spectrum performance. In contrast, aluminum oxide (Al₂O₃) and aluminum nitride (AlN) stand out as unique candidates. Al₂O₃ provides deep UV to mid-IR transparency with ultra-low loss, while AlN combines broadband transparency with an intrinsic Pockels effect, making it one of the few CMOS-compatible materials suitable for integrated electro-optic modulation. 

This thesis presents the development and characterization of low-loss Al₂O₃ and AlN photonic platforms. Al₂O₃ was first explored as a passive platform and demonstrated exceptional performance after RF sputtering deposition and chemical mechanical polishing (CMP), achieving slab propagation losses as low as 0.6 dB/cm at 377 nm and waveguide propagation losses as low as 2.3 dB/cm at 369 nm. These results demonstrate Al₂O₃’s viability for UV-visible photonic circuits and its potential integration with active layers (Chapter 2). 

Building upon this, AlN thin films were deposited using RF reactive sputtering and annealed at 600 °C in a nitrogen environment to improve film quality. This process reduced the oxygen content, resulting in slab propagation losses as low as 1.6 dB/cm at 633 nm (Chapter 3), and waveguide propagation losses of 4.4 dB/cm at 1550 nm. Microring resonators showed Q-factors up to 12,000, confirming the platform’s low-loss potential (Chapter 4). 

Finally, AlN Mach-Zehnder modulators (MZMs) were fabricated with top electrodes to explore electro-optic modulation. Although modulation via the Pockels effect was not observed due to limited optical-electric field overlap, the design and fabrication workflow were validated (Chapter 5). 

The results presented in this thesis, along with the structural and optical insights gained, establish the experimental and conceptual foundation for developing thermally robust, low-loss AlN waveguides. These advancements underpin the innovative material and process techniques protected by patent NL2033304B1. The integration of Al₂O₃ and AlN is proposed as a promising path toward broadband, low-loss, and actively tunable photonic platforms.