Travis C. Briles, Time and Frequency Division, National Institute of Standards and Technology; David R. Carlson, Octave Photonics, Louisville; Jennifer Black, Time and Frequency Division, National Institute of Standards and Technology; Jizhao Zang, Grisha Spektor, Yan Jin, Scott Papp, Time and Frequency Division, National Institute of Standards and Technology & University of Colorado

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The development of laboratory optical atomic clocks has led to unprecedented accuracy that now reaches the 1E-18 level and opens the door for a new class of fundamental physics experiments and diverse applications in precision timing, geodesy and navigation. Fully realizing these applications requires the development of low-SWaP, portable optical atomic clocks to move this technology out specialized metrology labs. Architectures for such a portable clock include compact systems with total volume ~ 0.1 m3 that are constructed from robust atomic and photonic subsystems potentially even fully integrated, chip-scale systems with volumes ~ 1 cm3. Both systems will benefit from a flexible platform for nonlinear integrated photonics that supports the 1) optical routing between semiconductor laser sources and photodetectors, 2) generation of optical frequency combs, and 3) small footprint, beam formation for optical interrogation of atomic vapors. In this abstract, we report on the development of such a photonics platform that is based on thin-film tantala (Ta2O5, TaO hereafter) waveguides and metasurfaces. While TaO is commonly used as a high-index contrast material in ultra-high-reflectivity mirror coatings due to its low optical absorption, it has seen limited use in nonlinear integrated photonics. From a fabrication and integration perspective, TaO has several key advantages over alternative platforms such as silicon nitride, including reduced film stress and versatile deposition/annealing for wide process compatibility. The lower film stress allows for thick films (>800 nm) to be deposited without developing significant cracks in the waveguide layer, which is difficult to achieve in silicon nitride without significant additional processing steps. Additionally, the low impurities associated with the TaO deposition process means that the lowest optical losses can be obtained at significantly lower annealing temperatures (<500 C) than silicon nitride (1100 C), a material that is more widely used for low-loss waveguides. The reduced annealing temperature is important for downstream integration with more sensitive materials which may undergo catastrophic transitions at elevated temperatures. Recent work from our group has shown that its combination of low loss (0.1 dB/cm at 1550 nm) and high-nonlinearity allow broadband Kerr comb generation in microresonators at threshold powers as low as 5 mW and supercontinuum generation in straight waveguides that spans 500 - 2500 nm with < 100 pJ input pulse energies [1]. Both of these accomplishments rely on engineering of the group velocity dispersion using lithographic control of the resonator and waveguide geometries [2]. In particular, we report the generation of near-octave spanning, spontaneous soliton combs [3] and the efficient generation of optical supercontinuum spectra with a dispersive wave tuned to the Rb atomic transition wavelength of 780 nm (input power only 60 pJ) [1]. Recent progress on supercontinuum generation reaching the 689 nm clock transition will be reported by Andrew Ferdinand in a separate submission. We have also demonstrated solitons in photonic crystal microresonators with a waveguide PhC reflector for pump recirculation that display near-unit efficiency nonlinear conversion of the pump laser power to the frequency comb lines. Such combs are ideally suited for low SWaP telecommunications applications [4] and will be reported by Jizhao Zang in a separate submission to this conference. A significant challenge in creating portable optical clocks is compressing the bulky footprint of the traditional optical systems used to interrogate atomic vapor. Metasurface optics composed of sub-wavelength nanostructures are capable of a higher degree of spatially control of the optical phase than is generally possible with traditional optics. This means that a single metasurface optic can combine the functionality of multiple bulk optics, resulting in more compact optical systems with reduced sensitivity to environmental perturbations. Our target system is a Sr blue MOT operating at a wavelength of 461 nm. This challenging, visible wavelength is well suited to two types of metasurfaces created from 570 nm TaO films on SiO2 substrates. The first type is an array of nanopillars that combines the functionality of a traditional lens (f=1 mm) and quarter-wave plate in a single metasurface optic. Such devices are well suited to a mode converter for the output of a semiconductor laser that also converts the polarization from linear to circular. The second type is a novel sub-wavelength grating structure that achieves beam splitting, polarization rotation, and beam expansion. In particular, this device exploits successive total internal reflections within the SiO2 substrate to reduce the effective propagation distance by a factor of 20x-50x. Both types of metasurfaces rely on the lithographic definition of TaO features ranging from 80-180 nm with a minimum gap between features of 70 nm. Details on the nanofabrication process for such small features, including the development of an Al2O3 hard etch mask will be discussed.