Pick-Flip-And-Place Hybrid Integration of a Single-Crystal Diamond Nanobeam with an Ensemble of Nitrogen-Vacancy Centers

Nitrogen-vacancy (NV) centers in single-crystal diamond have been intensively studied for various quantum applications such as quantum sensing, quantum communication, and quantum simulation. To accelerate advances in quantum technologies based on NV centers, implementing micro/nanostructures in diamonds is essential. Because of recent advancements in etching technologies for diamond photonics, various nanophotonic structures, including photonic crystals, ring resonators, and disk resonators, have been successfully fabricated to date.<b> </b><br/>Combining color centers in diamond with well-developed integrated photonics using hybrid integration techniques such as transfer printing provides a powerful route toward large-scale and multifunctional quantum devices. However, the existing etching technology for diamond photonics is inherently not compatible with reported hybrid integration approaches. For example, the bottom of a diamond nanostructure fabricated using angled etching is a triangular shape that makes it difficult to place on other substrates using conventional pick-and-place techniques. Therefore, it is significant to develop hybrid integration techniques with high yields for scalable diamond quantum photonics, irrespective of the sample structure.<br/>In this work, we propose “pick-flip-and-place” transfer printing for the hybrid integration of a diamond NV triangular nanobeam on chip in a deterministic manner. This approach provides a flat diamond-chip interface and allows for the integration of NV centers on chip with a near-unity success rate, irrespective of the width and shape of the diamond nanostructures. In experiment, we demonstrate that an integrated diamond nanobeam containing an ensemble of NV centers can function as a nanoscale quantum sensor. We believe that the proposed approach paves the way for the realization of scalable diamond quantum photonics assisted by cutting-edge integrated photonic technologies.<br/>We describe the basic procedure for the proposed nanofabrication and hybrid integration processes based on transfer printing. First, triangular nanobeam containing NV centers with high density (300 ppb) were prepared via vertical dry etching of diamond substrate, followed by a diamond undercut process based on angled etching. We then used transfer printing to select a suitable diamond nanostructure from the processed diamond substrate using a polydimethylsiloxane (PDMS) film with weak adhesion. This “weak” PDMS was flipped to lift off the nanostructure from the flipped film using a PDMS film with strong adhesion. This process allows for a flat interface between the diamond nanostructures and photonic chip, which is necessary for high-yield integration. Finally, the transferred diamond nanobeam was integrated by slowly peeling off the film onto an SiO₂ substrate, which is a common low-refractive-index material in integrated photonics for efficient light confinement in a waveguide. It is notable that we can perform hybrid integration irrespective of device structure and target materials by using the proposed “pick-flip-and-place” integration approach, enabling us to combine diamond quantum photonics and state-of-the-art photonic integration technology.<br/>To experimentally show that the fabricated device could function as a nanoscale quantum sensor, we performed optically detected magnetic resonance (ODMR) measurements on integrated NV centers. We confirmed an ODMR dip at a microwave frequency of 2.88 GHz with an ODMR contrast of ~0.006 and the linewidth of 7.8 MHz, which is comparable to those measured for bulk diamond. Therefore, we, for the first time, succeeded in the hybrid integration of a diamond nanobeam with an ensemble of NV centers on a chip for quantum sensing. The proposed approach based on transfer printing provides a deterministic route toward a scalable combination of diamond quantum photonics and cutting-edge integrated photonics technologies.