Bottom-reflector Enhanced Grating Couplers Micro-transfer Printed on Silicon Nitride Photonic Integrated Circuits

We demonstrate a new post-processing approach to efficiently couple light in silicon nitride (Si
$_{3}$
N
$_{4}$
) photonic integrated circuits (PIC) via grating couplers add-ons. Thin-film coupons of apodised Si
$_{3}$
N
$_{4}$
grating couplers enhanced with metallic bottom reflectors and adiabatic couplers are micro-transfer printed onto the input and output of Si
$_{3}$
N
$_{4}$
waveguides, providing efficient coupling without impacting the rest of the PIC. Two-dimensional FDTD simulations predict coupling efficiencies as large as 62.4
$\%$
(-2.0 dB) at a wavelength of 785 nm with a coupling optical fiber tilted at 10
$^\circ$
. As a proof-of-concept, we report an experimental coupling efficiency of -2.1 dB which is 3 dB better than the value obtained with standard designs of grating couplers.