Nitrogen-Pressure Dependence of Coherent Population Trapping Signal Characteristics in Microfabricated Rubidium Vapor Cells

Abstract

Controlling buffer-gas conditions in vapor cells is essential for frequency stabilization and atomic-clock performance. This study investigated the dependence of coherent population trapping (CPT) signal characteristics on nitrogen (N ${}_{2}$ ) buffer-gas pressure in micro-fabricated vapor cells containing natural-abundance rubidium (Rb) generated from Rb azide (RbN ${}_{3}$ ) for chip-scale atomic clocks. A wafer-level fabrication method enabling individual control of N2 pressure was developed by inkjet patterning of RbN3 and subsequent krypton fluoride (KrF) excimer laser decomposition. An increasing N2 pressure was found to reduce the CPT linewidth and cause a significant decrease in the signal amplitude, which resulted in monotonic degradation in the CPT figure of merit with higher pressures. A CPT-based laser frequency stabilization method that locked the laser wavelength to the CPT resonance peak yielded significantly larger signal amplitudes than a conventional absorption-based method in the low-pressure regime below 8 kPa. These results show that CPT performance in micro-fabricated Rb vapor cells is more restricted by the signal amplitude rather than the linewidth. They also offer practical buffer-gas design guidelines for enhancing short-term frequency stability in chip-scale atomic clocks.