Hybrid integration of blue laser diodes on SiN photonic integrated circuits by flip-chip bonding

In this work, we demonstrate the hybrid integration of GaN LDs operating at 488 nm on SiN PICs via flip-chip bonding. Specialized fork-shaped edge couplers were employed to mode match the laser spatial profile of LD with the high aspect ratio of horizontal to vertical dimensions. The edge couplers were patterned using electron-beam lithography and reactive ion etching, achieving a low coupling loss of 2.2 dB at the optimal coupling position. The pedestals with precise height were fabricated in front of the edge couplers using a two-step lithography process, enabling the vertical (z-axis) alignment through mechanical contact with LDs. The pedestal height was co-designed with LDs, and defined by controlling the etching time, resulting in a height variation below 50 nm. For flip-chip bonding, the LDs were picked up and accurately placed on the pedestals after fine horizontal (x-y plane) alignment, reaching a total coupling loss of ~6 dB. Driven by a pulse current of 400 mA with a 1% duty cycle, 0.2 mW of optical power in waveguide is observed at 488 nm. This work provides a straightforward route realizing blue integrated lasers in the visible photonics platform, which has strong potential to support a wide range of applications for the life science and quantum computing.