In this Phase-I project, a buffer-gas-free rubidium vapor cell is fabricated using microfabrication methods, for the target application area of vapor-cell optical atomic clocks. The cell has inner dimensions of 3mm x 3mm x 3mm, with anti-reflection (AR) coatings at 780nm and a bandwidth of 10nm on the glass windows and contains a thermal Rb vapor. The cell structure consists of a glass-silicon-glass structure. The fabrication involves the application of interior and exterior AR coatings to the glass layers, etching of the cell cavity into the 3-mm-thick double-side-polished silicon wafer, and anodic bonding and filling with Rb vapor in a vacuum bonding and filling system. A large fraction of the project is devoted to the actual fabrication effort, which will leverage Rydberg TechnologiesÂ’ experience in vapor cell fabrication. The fabrication effort includes the identification of materials with low helium permeability, as well as refining the detailed cell fabrication process. The metric of success for the fabrication component of this Phase-I project resides in meeting the specifications for the AR coatings of the cell, for rubidium vapor content, and for the absence of collisional line broadening. A reflectivity of less than about 0.1% over the 10nm bandwidth of the coating, centered at 780nm is targeted. Reflection measurements will be employed to characterize coating performance versus wavelength. Further, absorption spectroscopy on the Rb D2 line will be employed to verify the presence of Rb vapor in thermal equilibrium within the cell. The desired metric is an absorption coefficient of 1 to 2cm-1 at a temperature of 50C on the Rb-85 F=3 hyperfine component of the D2 line. The observation of saturated hyperfine lines of the 5P3/2 state with a width in the sub-10MHz range in saturated spectra will demonstrate the absence of line broadening due to unwanted trace gases, on an approximately 1MHz scale. The Phase-I project work is concluded with the design of a fully integrated vapor cell vacuum package with a volume of 10mm x 10mm x 10mm, with low helium permeation, electrical heating up to 120C with sub-10mK temperature stability and an exterior temperature of 30C in a 22C air environment, and with residual magnetic fields < 1nT. The package includes a box for the shielding of environmental magnetic fields, vacuum insulation to reach the required heat resistance between the package interior and the package surface, and apertures for optical beams and electrical wires for heaters and other electronic components. The design work will include detailed heat transport simulations, as well as simulations of the magnetic fields inside the package due to electric currents inside the package and external magnetic fields leaking through the apertures. Consideration will be given to aspects as to how the design can be structured to make it suitable for eventual mass fabrication of the cell packages.
