Microfabricated vapor cells with tunable buffer gas mixtures using laser-actuated break-seal reservoirs

Microfabricated cells are the core component of a wide variety of chip-scale atomic devices,such as clocks or magnetometers [1]. In those instruments, the cell is generally filled with a pressureof buffer gas. The presence of buffer gas reduces the atom-wall collision rate, enhances theatomic coherence lifetime and therefore improves the stability or sensitivity of the device.In microwave clocks, buffer gas mixtures are often used. Each buffer gas shifts the hyperfineclock frequency with opposite signs such that an inversion temperature, around which the clockfrequency dependence is cancelled at the first order, can be obtained by tuning properly the buffergas pressure ratio. Stabilizing the cell at the inversion temperature is then beneficial to improvethe long-term stability of the clock frequency.Wafer-level vapor cells are in general filled with buffer gas by backfilling the bonding chamberwith buffer gas prior to the final cell sealing. Several approaches have been explored for the cellfilling including the thermal decomposition of BaN6-RbCl precursors [2], the employment ofUV-decomposed alkali azides [3] or the use of alkali pill dispensers [4]. In general, the targetpressure for all cells of a given wafer is the same while some non-negligible dispersion on thegas pressure measured in each cell of the wafer can be observed. In [5], the tunability of nitrogenpressure in micromachined cells was demonstrated through the activation of getter compounds inan alkali pill dispenser. Nevertheless, this approach might not be efficient with other types of gaswhile both decomposition and adsorption processes of N2 might continue, even at small rate, limitingthe long-term stability of atomic clocks.In this work, we report on the development and characterization of Cs vapor microfabricatedcells filled with tunable He-Ne buffer gas mixture using laser-actuated break seal reservoirs. Theidea is here to release He gas, pre-loaded into reservoirs annexed to the main science cavity, prefilledwith neon, using femtosecond laser ablation of a silicon wall membrane. With this approach,we demonstrated the progressive increase of He gas in the main cavity by consecutiveopenings of He reservoirs around the cell, confirmed by the progressive increase of the clockfrequency inversion temperature, adjusted here up to almost 100°C. A cell, tuned at 95°C, wasused on a CPT clock setup, achieving a fractional frequency stability of 9x10-11 at 1 day. Theseresults demonstrate the possibility to develop wafer-level microfabricated cells with tunablebuffer gas mixtures [6]. We plan to develop cells with alternative buffer gas mixtures in the futureusing this approach. A summary of results will be presented at the conference.