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Japanese Journal of Applied Physics

Transfer-printed nanophotonic waveguide on an erbium doped crystal towards hybrid photonic quantum devices

Taiyu Okajima, Ryuichi Ohta, Takumi Sato, Yuma Tachizaki, Xuejun Xu, Hajime Okamoto, Yasutomo Ota

Published Feb 12, 2025• 4 citations
Publisher

Abstract

Abstract We report the design, fabrication and optical characterization of silicon nanophotonic waveguides on an erbium doped crystal. The silicon waveguide allows for efficient optical addressing of erbium ions embedded near the crystal surface. We employed the transfer printing method to assemble the heterogeneous photonic structure in a simple pick-and-place manner. We observed coupling of the emission from erbium ions into the waveguide in the telecommunication wavelength band. The developed methodology opens the way to functional hybrid quantum devices mediating between photons, electrons and phonons.

DOI
10.35848/1347-4065/adb54e

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Abstract We report the design, fabrication and optical characterization of silicon nanophotonic waveguides on an erbium doped crystal. The silicon waveguide allows for efficient optical addressing of erbium ions embedded near the crystal surface. We employed the transfer printing method to assemble the heterogeneous photonic structure in a simple pick-and-place manner. We observed coupling of the emission from erbium ions into the waveguide in the telecommunication wavelength band. The developed methodology opens the way to functional hybrid quantum devices mediating between photons, electrons and phonons.
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We report the broadband luminescence enhancement of erbium ions embedded in a non-resonant hybrid waveguide photon-emitter interface on silicon. The measured emission rate enhancement of > 300 is enabled by a large radiative Purcell factor across the spectrum in the telecommunications C-band. This enables the shortening of the lifetime of erbium from 3 ms to 8 μs, effectively providing brighter on-chip photon sources. The platform efficiently collects the photon emission of erbium in a single-mode waveguide and out-couples it to a low-loss photonic waveguide using a reverse nano-focusing method. This hybrid approach opens a large range of possible emitter-waveguide platform combinations for heterogeneous and hybrid quantum emitter integration, towards scalable on-chip quantum networks.
With an assortment of narrow line-width transitions spanning the visible and IR spectrum and long spin coherence times, rare-earth doped crystals are the leading material system for solid-state quantum memories. Integrating these materials in an on-chip optical platform would create opportunities for highly integrated light-matter interfaces for quantum communication and quantum computing. Nano-photonic resonators with high quality factors and small mode volumes are required for efficient on-chip coupling to the small dipole moment of rare-earth ion transitions. However, direct fabrication of optical cavities in these crystals with current nanofabrication techniques is difficult and unparallelized, as either exotic etch chemistries or physical milling processes are required. We fabricated hybrid devices by mechanically transferring a nanoscale membrane of gallium arsenide (GaAs) onto a neodymium-doped yttrium silicon oxide (Y2SiO5) crystal and then using electron beam lithography and standard III-V dry etching to pattern nanobeam photonic crystal cavities and ring resonator cavities, a technique that is easily adapted to other frequency ranges for arbitrary dopants in any rare earth host system. Single crystalline GaAs was chosen for its low loss and high refractive index at the transition wavelength. We demonstrated the potential to evanescently couple between the cavity field and the 883 nm 4I9/2- 4F3/2 transition of nearby neodymium impurities in the host crystal by examining transmission spectra through a waveguide coupled to the resonator with a custom-built confocal microscope. The prospects and requirements for using this system for scalable quantum networks are discussed.
A scalable platform for on-chip optical quantum networks will rely on standard top-down nanofabrication techniques and solid-state emitters with long coherence times. We present a new hybrid platform that integrates amorphous silicon photonic waveguides and microresonators fabricated on top of a yttrium orthosilicate substrate doped with erbium ions. The quality factor of one such resonator was measured to exceed 100,000 and the ensemble cooperativity was measured to be 0.54. The resonator-coupled ions exhibited spontaneous emission rate enhancement and increased coupling to the input field, as required for further development of on-chip quantum light-matter interfaces.
Quantum networks are a rapidly developing field of quantum information processing that have the potential to enable long-range entanglement between future quantum computers as well as the implementation of secure communication through quantum key distribution. Two key components of such networks are quantum memories to store entangled photon pairs for use in the quantum repeater protocol and quantum light-matter interfaces to efficiently interconvert between stationary qubits and flying qubits in the form of photons. Rare-earth ion-doped crystals are a promising solid-state platform that show promise for both of these applications due to their long optical and spin coherence times. Due to their relatively weak optical transitions, rare-earth ions have been coupled with nanophotonic resonators to enhance their transition strengths, with past work in the Faraon group utilizing focused ion beam milled photonic crystal resonators with 45-degree angled couplers to couple light in and out. Such resonators have the disadvantage of requiring manual alignment to fabricate, and the couplers are also relatively inefficient which limits the performance of such devices. It is therefore desirable to move towards silicon photonics, where mature techniques such as electron-beam lithography can allow for scalable fabrication of nanophotonic cavities together with high coupling efficiencies. In this thesis, we demonstrate significant progress towards the usage of acid-etched tapered optical fibers as an efficient interface for coupling light into tapered silicon nanophotonic waveguides. We show comprehensive simulations of the taper geometries required to achieve adiabatic coupling with theoretical efficiencies of more than 99%, and design a silicon photonic crystal mirror to be used in the measurement of the fiber-waveguide coupling efficiency. We then optimize the hydrofluoric acid fiber etching process and demonstrate the ability to make tapered fibers which are 200 microns long with a taper half-angle of 2 degrees and a tip diameter of 50 nm. Using these tapered fibers to couple light into tapered silicon waveguides fabricated using electron-beam lithography shows a moderately high coupling efficiency of 11.4% with the potential for improvement. This method of tapered fiber coupling shows promise to be integrated into silicon nanophotonic resonators on rare-earth ion doped crystals and allow for highly efficient quantum memories and quantum light-matter interfaces in the solid-state.
The hybrid of silicon and erbium-doped crystals provides a promising photonic platform for the advancement of quantum technologies, combining scalability, telecom compatibility, and complex quantum functionalities. However, the inherent high refractive index of silicon leads to optical field confinement within the silicon, limiting the number of optically active erbium ions and the achievable light absorption, which are essential for various quantum applications. To fully utilize the potential of erbium-doped crystals, we implemented bound states in the continuum within the hybrid silicon structures, which achieve substantial optical energy confinement within the lower-index crystal. Our structure, which is applicable to both transverse magnetic and transverse electric polarization modes, effectively increases the number of optically active erbium ions and demonstrates simultaneous minimal radiative losses and enhanced erbium absorption. Moreover, the core design principle is extensible to other low-refractive-index materials.