# Suspended Spot-Size Converters for Scalable Single-Photon Devices

## TL;DR

This paper presents a highly efficient, scalable optical spot-size converter for single-photon devices, enabling improved coupling between quantum dot sources and fibers in quantum photonics.

## Contribution

It introduces a novel inverted taper and epoxy overlay design for efficient end-face coupling of single photons from GaAs waveguides.

## Key findings

- Achieved a collection rate of 5.84 MHz for single photons.
- Estimated a chip-to-fiber coupling efficiency of approximately 48%.
- Demonstrated stability at cryogenic temperatures.

## Abstract

We report on the realization of a highly efficient optical spot-size converter for the end-face coupling of single photons from GaAs-based nanophotonic waveguides with embedded quantum dots. The converter is realized using an inverted taper and an epoxy polymer overlay providing a 1.3~$\mu$m output mode field diameter. We demonstrate the collection of single photons from a quantum dot into a lensed fiber with a rate of 5.84$\pm0.01$~MHz and estimate a chip-to-fiber coupling efficiency of $\sim48$~\%. The stability and compatibility with cryogenic temperatures make the epoxy waveguides a promising material to realize efficient and scalable interconnects between heterogeneous quantum photonic integrated circuits.