Design and fabrication of ultrahigh-Q LiNbO3 microdisk optical resonator for sensitive transduction between microwave and optical photons

Problem Statement and Approach: Sensitive detection of electromagnetic (EM) fields at microwave frequencies underlies crucially many high-energy physics (HEP) experiments that rely on the measurement of weak couplings between ultralight dark matter and electromagnetism. These HEP experiments require the sensing of EM fields down to single photon level, which, unfortunately, remains a significant challenge. Moreover, highly efficient transduction between microwave photons and optical photons is crucial for a distributed quantum network that is built upon a combination of vastly different quantum systems that operate based upon the quantum states of microwave and optical photons. One elegant approach to resolve this challenge is to convert microwave photons into the optical domain with high efficiency. We proposed to design and fabricate ultrahigh-Q optical microresonators on chipscale thin-film lithium niobate (LN) platform that exhibit optimal electro-optic coupling for applications in ultra-sensitive transduction between microwave and optical photons. The proposed device is expected to offer optical Q > 107 in the telecom band. It enables compact optoelectronic integration that will offer strong transduction between the two spectral regimes. In compared with conventional approaches, the proposed device is expected to offer unprecedented performance with more than 100? reduction of the device footprint and 100? increase of microwave-optical conversion efficiency. In particular, the chipscale fully integrated approach enables wafer-scale mass production that will dramatically reduce the cost. Phase I Accomplishments: Phase I project focused on developing and prototyping high-Q LN microresonators that not only exhibited high optical Q but were also well suited for efficient microwaveto- optic transduction. We have accomplished all our Phase I goals: 1) identify the device geometry that not only supports high optical Q >107, but also produces optimal electro-optic coupling; 2) optimize the fabrication process to improve the optical Q. We have achieved all key objectives of Phase I: 1) Design of device structure for ultrahigh optical Q and small mode volume; 2) Design of device structure for best electro-optic coupling; 3) Fabrication of resonator device and optimization for high optical Q. Phase II Efforts: In Phase II, we will perform in-depth investigation to further improve the device optical quality by further optimization of the fabrication process, particularly on the wafer scale. We will also develop appropriate packaging processes for low-loss chip-fiber coupling and for laser-chip integration. Commercial Applications and Other Benefits: The proposed project, if successful, will offer to the market the first commercially available cost-effective ultrahigh-Q LN microresonator devices that can be produced massively on the whole wafer scale. These devices will offer an immediate solution to the HEP experiments for ultrasensitive detection of microwave photons. Moreover, the commercial availability of the proposed device with high performance is of immense importance for the development and implementation of distributed quantum network that builds upon disparate quantum systems operating in vastly different spectral regimes from microwave to visible. This SBIR effort aims to directly fulfill this emerging market demand. On the other hand, the proposed devices are of great potential for broad classical applications in radar, biomedicine, infrared/THz imaging, etc., where sensitive detection of microwaves (and electric field in general) is extremely critical. The resonator devices developed in this SBIR program are expected to have a profound commercial impact on a broad market.