# A three-dimensional polarization-insensitive grating coupler tailored for 3D nanoprinting

## TL;DR

This paper introduces a three-dimensional, polarization-insensitive grating coupler with over 80% efficiency in both polarizations, enabling more robust and integrated 3D photonic devices using nanoprinting techniques.

## Contribution

It presents a novel 3D design and optimization method for polarization-insensitive grating couplers suitable for 3D nanoprinting, outperforming traditional 2D designs.

## Key findings

- Achieves over 80% coupling efficiency in both polarizations.
- Design is feasible for fabrication using density-based topology optimization.
- Enables robust, integrated 3D photonic devices.

## Abstract

Efficiently coupling light from optical fibers into photonic integrated circuits is a key step toward practical photonic devices. While a notable coupling can be achieved by out of plane couplers such as grating couplers, their basic planar geometry typically tends to be sensitive to the polarization of light. This is partly due to the fact that the design spaces of such grating structures typically fabricated with techniques such as electron beam lithography are only two dimensional with a simple extrusion into the vertical dimension. This makes it challenging to optimize for both polarizations simultaneously, as performance typically degrades when trying to achieve high efficiency in both. As a result, conventional approaches either suffer from increased losses or require additional filtering components to account for different polarizations. In this work, we present a fully three dimensional, polarization insensitive grating coupler which has a highly efficient simulated coupling efficiency of over 80% in both polarizations. This performance matches that of state of the art couplers that are performant for one polarization only. This comes at the cost of a moderately larger size due to the lower refractive index materials typically available in 3D nanoprinting. Our design method uses density based topology optimization with a multi objective approach that combines electromagnetic simulations with a fictitious heat conduction model acting as a soft constraint to promote structural integrity. This ensures that the designed structures are feasible for fabrication. Our work opens new possibilities for robust 3D photonic devices, enabling advanced integration, fabrication, and applications across next generation photonics and electronics.