Hafnia-based platform for high-index visible and UV integrated photonics

Integrated photonics at visible and ultraviolet wavelengths stand to enable applications spanning quantum systems based on individual atoms, ions, and defects in solids, bio-chemical spectroscopy, and neural stimulation and probing. Operation at visible and ultraviolet wavelengths requires different material platforms and device architectures as compared to the comparatively mature near infrared, where communications applications have spurred significant development. The lack of wide-bandgap materials platforms transmissive to these high energy photons has been a major limitation on development of integrated photonics at these short wavelengths. This program develops a new CMOS-compatible platform for ultraviolet and visible photonics based on thin-film deposited hafnia, whose relatively high refractive index is a critical advantage over the current alternative for ultraviolet integrated photonics, alumina. This program explores strategies to inhibit crystallization of hafnia, which has been a major obstacle to realization of low optical losses, by controlled incorporation of other elements into films. The resulting composite films maintain the bulk of hafnia's advantage in refractive index, while reducing losses by orders of magnitude. The work further explores and develops materials processing and fabrication techniques to pattern this material into low-loss photonic devices, and develops novel photonic device concepts leveraging the new platform motivated by application in atomic quantum systems. This work will enable significantly higher performance photonics at blue/UV wavelengths, and due to the CMOS compatibility of the proposed platform, enables rapid integration into state-of-the-art manufacturing platforms. Undergraduate education and participation in the research is a key component of the program, as are outreach efforts exposing K-12 students to cutting-edge work happening in optical and atomic science and technology. The program explores routes to optimizing material loss and index in HfO2/Al2O3 composites formed by atomic layer deposition, develops nanofabrication processes for these composites to enable lithographically defined nanophotonics, and pursues detailed characterization of the developed platform. The high refractive index of the composite material as compared to the current best alternative, pure Al2O3, is a significant enabling advantage for multiple photonic components including grating couplers, photonic crystals, microresonators, and active electro- and acousto-optic devices. Furthermore, this work introduces novel device architectures leveraging the platform capability to address key challenges in application to atomic quantum systems. In particular, a novel concept is introduced to enable robust, broadband generation of pure circularly polarized radiation (>99.9% purity in relative intensity) from passive integrated photonics. This concept takes advantage of the high refractive index contrast enabled by the developed platform, and will be pursued in theory and demonstrated in experiment. The work also explores novel hybrid schemes for blue/ultraviolet parallel electro-optic control enabled by HfO2’s high index, via integration with bulk beta barium borate. This work opens future directions in efficient integrated short-wavelength nonlinear optics, as well as architectures for integrated acousto-optics in the blue/ultraviolet. The program addresses key challenges in scaling quantum systems, with advances spanning photonic materials/processing, as well as in passive and active optical functionalities. Due to the material's compatibility with CMOS fabrication, this work stands to rapidly impact integrated photonics for atomic quantum systems and at blue/ultraviolet wavelengths more broadly. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.