Toward Achieving Critical Coupling in Phononic Integrated Circuits

Critical coupling is crucial for effective energy transfer between a waveguide and a microring. Current lithography methods face challenges in achieving the necessary resolution. The proposed solutions include employing atomic layer deposition (ALD) of Al<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> to decrease the effective coupling length (L<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cp</inf>), thereby maintaining critical coupling within the constraints of existing capabilities. These strategies are essential for ensuring the successful integration of these components in phononic devices. The deposition of a 25 nm Al<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> layer has been shown to reduce the necessity for critical coupling. Simulation and experiment results indicate that ALD of 25 nm alumina decreases the critical L<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cp</inf>. The normalized transmission measurement of the waveguide-microring device exhibits enhanced transmission and additional distinct resonance dips after ALD. The extinction depth in the ring dips has increased, and the quality factors have remained stable, indicating an improvement yet still an undercoupled system.