Kazuhiro Ban

On-chip fabrication of alkali-metal vapor cells utilizing an alkali-metal source tablet

We describe a novel on-chip microfabrication technique for the alkali-metal vapor cell of an optically pumped atomic magnetometer (OPAM), utilizing an alkali-metal source tablet (AMST). The newly proposed AMST is a millimeter-sized piece of porous alumina whose considerable surface area holds deposited alkali-metal chloride (KCl) and barium azide (BaN6), source materials that effectively produce alkali-metal vapor at less than 400 °C. Our experiments indicated that the most effective pore size of the AMST is between 60 and 170 µm. The thickness of an insulating glass spacer holding the AMST was designed to confine generated alkali metal to the interior of the vapor cell during its production, and an integrated silicon heater was designed to seal the device using a glass frit, melted at an optimum temperature range of 460–490 °C that was determined by finite element method thermal simulation. The proposed design and AMST were used to successfully fabricate a K cell that was then operated as an OPAM with a measured sensitivity of 50 pT. These results demonstrate that the proposed concept for on-chip microfabrication of alkali-metal vapor cells may lead to effective replacement of conventional glassworking approaches.

Development of high-sensitivity portable optically pumped atomic magnetometer with orthogonal pump and probe laser beams

Optically pumped atomic magnetometers (OPAMs) have been demonstrated to show sensitivities better than the order of a few femtoteslas. In this study, we developed a portable potassium OPAM module using an electrically heated oven and orthogonal pump and probe beams coupled from polarization-maintaining optical fibers. The OPAM module shared a volume as small as 700 cm3, and its footprint was as small as 60 × 60 mm2; it operated with a single-channel sensitivity of 14 fTrms/Hz1/2 at 100 Hz when the 2-cm cubic potassium cell was heated to 180 °C. In our module, the optical beam path was folded along the magnetic field to be measured. The development of a magnetic sensor device comprising an array of OPAM modules will be an important step toward realizing a biomagnetic imaging system based on OPAMs. The orthogonal two-beam OPAMs will be beneficial for realizing a flexible placement of the array.

A plateau in the sensitivity of a compact optically pumped atomic magnetometer

In a compact optically pumped atomic magnetometer (OPAM), there is a plateau in the sensitivity where the dependence of the sensitivity on pumping power is small compared with that predicted by the uniform polarization model. The mechanism that generates this plateau was explained by numerical analysis. The distribution of spin polarization in the alkali metal cell of an OPAM was modeled using the Bloch equation incorporating a diffusion term and an equation for the attenuation of the pump beam. The model was well-fitted to the experimental results for a module with a cubic cell with 20 mm sides and pump and probe beams with 8 mm diameter. On the plateau, strong magnetic response was generated at the regions that were not illuminated directly by the intense pump beam, while at the same time spin polarization as large as 0.5 was maintained due to diffusion of the spin-polarized atoms. Thus, the sensitivity of the magnetometer monitored with a probe beam decreases only slightly with increasing pump beam int...

Characterization of alkali-metal vapor cells fabricated with an alkali-metal source tablet

Optically pumped magnetometers (OPMs) that use alkali metal vapor cells can measure weak magnetic fields generated by the human body. A multichannel detector head with alkali-metal vapor cells mounted in arrays is employed to assure spatial resolution for real-time biomagnetic imaging of various body surfaces. However, further development of alkali metal vapor cell fabrication processes is required to obtain cells with uniform magnetic field sensitivities together with sufficient sensitivity of each individual cell. Herein, the authors propose the fabrication of alkali metal vapor cells for OPM arrays using alkali metal source tablets (AMSTs) as alkali metal dispensers. An AMST is a three-dimensional microstructure that contains precise quantities of the chemical precursors that are used to produce pure alkali metals by low temperature thermal decomposition and to fill reproducible quantities of these metals into cells. In this work, the K production efficiency was characterized with respect to the partic...

Alkali metal source tablet for vapor cells of atomic magnetometers

An array system of optically pumped atomic magnetometers (OPAMs) that uses potassium vapor cells requires the quantity of potassium enclosed in the cells to be uniform. We describe a tablet-shaped potassium source, which generates a fixed amount of potassium, for fabricating homogeneous vapor cells. The tablet consists of a tiny microstructured plate covered with a layer of raw materials. Two sets of raw materials were chosen: one was a mixture of KCl and BaN6, and the other was KN3. The efficiency of potassium generation depended on the shape and size of the microstructure for each raw material, and a maximum potassium yield of 65% was observed for KN3 tablets. The source tablets containing KCl and BaN6 were used to build vapor cells by a microfabrication technique, and the source tablets containing KN3 were used to introduce potassium into glass cells, which were confirmed to work in highly sensitive OPAMs.