Due to shortcomings in emerging alkali-based atomic physics based systems, a need to investigate alkali resistant materials has arisen. There is interest in alkali based systems such as atomic clocks and diode pumped alkali laser (DPAL) systems. In the case of atomic clocks and DPALs, alkali metal vapor, such as Rb, is the active part of the systems. The alkali vapor is confined in some manner of housing, but the transmission of electromagnetic radiation is required in the cells. This requires the incorporation of windows into the cell. The current window material, however, have been shown to degrade over time, thus reducing the effectiveness of these systems. It is believed that the alkali atoms diffuse into the bulk of the housing material. This diffusion results in changes of optical and, in some cases, structural properties of the material. These changes lead to the degradation of window materials in these alkali-based systems.
In an effort to improve the longevity of alkali-based systems, a material study was conducted to identify window material that could resist diffusion-based changes in optical properties. Candidate materials were selected based on their structure, optical properties, and/or density. All candidate materials underwent baseline characterization. Baseline characterization techniques included atomic force microscopy, spectrophotometry, reflectometry, ellipsometry, and X-ray diffraction spectroscopy. Once baseline data was collected, the candidate materials were exposed to Rb at high temperatures for an extended period of time to simulate atomic physics devices. Exposure was achieved by heating the Rb source to ~ 550 °C while the candidate materials were kept at ~ 450 °C. This created a 100 °C temperature gradient to thoroughly expose the materials to gaseous Rb. After exposure, the materials underwent the same analysis techniques to ascertain the changes in structural and optical properties. Additionally, time of flight secondary ion mass spectroscopy depth profiling was conducted to quantitatively determine the depth of Rb into the bulk of the material.
The results of this research effort found that highly crystalline materials were capable of resisting alkali diffusion better than amorphous materials, often only tens of nm. Their optical properties were also relatively unchanged. Amorphous materials were not able to resist the diffusion of Rb; diffusion depths were shown to be on the order of microns. Based on this research effort, aluminum oxynitride, MgAl2O4, MgO, and ZrO2 are being recommended as materials that will improve the longevity of emerging atomic physics systems. A vapor cell made from ZrO2 was fabricated and is being evaluated for use in atomic clock systems. For DPAL systems, window materials will need to be further tested to determine whether it can resist the high fluence laser radiation after being exposed to Rb.