Imaging of relaxation times and microwave field strength in a microfabricated vapor cell

Abstract

We present a characterization technique for atomic vapor cells, combining time-domain measurements with absorption imaging to obtain spatially resolved information on decay times, atomic diffusion, and coherent dynamics. The technique is used to characterize a 5-mm-diameter, 2-mm-thick microfabricated Rb vapor cell, with N${}_{2}$ buffer gas, placed inside a microwave cavity. Time-domain Franzen and Ramsey measurements are used to produce high-resolution images of the population (${T}_{1}$) and coherence (${T}_{2}$) lifetimes in the cell, while Rabi measurements yield images of the ${\ensuremath{\sigma}}_{\ensuremath{-}}$, $\ensuremath{\pi}$, and ${\ensuremath{\sigma}}_{+}$ components of the applied microwave magnetic field. For a cell temperature of $90{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}$C, the ${T}_{1}$ times across the cell center are found to be a roughly uniform $265\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}$s, while the ${T}_{2}$ times peak at around $350\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}$s. We observe a ``skin'' of reduced ${T}_{1}$ and ${T}_{2}$ times around the edge of the cell due to the depolarization of Rb after collisions with the silicon cell walls. Our observations suggest that these collisions are far from being 100$%$ depolarizing, consistent with earlier observations made with Na and glass walls. Images of the microwave magnetic field reveal regions of optimal field homogeneity, and thus coherence. Our technique is useful for vapor cell characterization in atomic clocks, atomic sensors, and quantum information experiments.