Li-Anne Liew

A microfabricated atomic clock

Fabrication techniques usually applied to microelectromechanical systems (MEMS) are used to reduce the size and operating power of the core physics assembly of an atomic clock. With a volume of 9.5mm3, a fractional frequency instability of 2.5×10−10 at 1s of integration, and dissipating less than 75mW of power, the device has the potential to bring atomically precise timing to hand-held, battery-operated devices. In addition, the design and fabrication process allows for wafer-level assembly of the structures, enabling low-cost mass-production of thousands of identical units with the same process sequence, and easy integration with other electronics.

Chip-scale atomic magnetometer

Using the techniques of microelectromechanical systems, we have constructed a small low-power magnetic sensor based on alkali atoms. We use a coherent population trapping resonance to probe the interaction of the atoms’ magnetic moment with a magnetic field, and we detect changes in the magnetic flux density with a sensitivity of 50pTHz−1∕2 at 10Hz. The magnetic sensor has a size of 12mm3 and dissipates 195mW of power. Further improvements in size, power dissipation, and magnetic field sensitivity are immediately foreseeable, and such a device could provide a hand-held battery-operated magnetometer with an atom shot-noise limited sensitivity of 0.05pTHz−1∕2.

Microfabricated alkali atom vapor cells

We describe the fabrication of chip-sized alkali atom vapor cells using silicon micromachining and anodic bonding technology. Such cells may find use in highly miniaturized atomic frequency references or magnetometers. The cells consist of cavities etched in silicon, with internal volumes as small as 1 mm3. Two techniques for introducing cesium and a buffer gas into the cells are described: one based on chemical reaction between cesium chloride and barium azide, and the other based on direct injection of elemental cesium within a controlled anaerobic environment. Cesium optical absorption and coherent population trapping resonances were measured in the cells.

A chip-scale atomic clock based on 87Rb with improved frequency stability.

We demonstrate a microfabricated atomic clock physics package based on coherent population trapping (CPT) on the D1 line of 87Rb atoms. The package occupies a volume of 12 mm3 and requires 195 mW of power to operate at an ambient temperature of 200 degrees C. Compared to a previous microfabricated clock exciting the D2 transition in Cs [1], this 87Rb clock shows significantly improved short- and long-term stability. The instability at short times is 4 x?10-11 / tau?/2 and the improvement over the Cs device is due mainly to an increase in resonance amplitude. At longer times (tau?> 50 s), the improvement results from the reduction of a slow drift to ?5 x 10-9 / day. The drift is most likely caused by a chemical reaction of nitrogen and barium inside the cell. When probing the atoms on the D1 line, spin-exchange collisions between Rb atoms and optical pumping appear to have increased importance compared to the D2 line.

Chip-scale atomic magnetometer with improved sensitivity by use of the Mx technique

The fabrication and performance of a miniature optically pumped atomic magnetometer constructed with microfabricated components are discussed. This device measures the spin precession frequency of Rb87 atoms to determine the magnetic field by use of the Mx technique. It has a demonstrated sensitivity to magnetic fields of 5pT∕Hz1∕2 for a bandwidth from 1to100Hz, nearly an order of magnitude improvement over our previous chip-scale magnetometer. The 3dB bandwidth has also been increased to 1kHz by reconfiguring the miniature vapor cell heater.

Wafer-level filling of microfabricated atomic vapor cells based on thin-film deposition and photolysis of cesium azide

The thin-film deposition and photodecomposition of cesium azide are demonstrated and used to fill arrays of miniaturized atomic resonance cells with cesium and nitrogen buffer gas for chip-scale atomic-based instruments. Arrays of silicon cells are batch fabricated on wafers into which cesium azide is deposited by vacuum thermal evaporation. After vacuum sealing, the cells are irradiated with ultraviolet radiation, causing the azide to photodissociate into pure cesium and nitrogen in situ. This technology integrates the vapor-cell fabrication and filling procedures into one continuous and wafer-level parallel process, and results in cells that are optically transparent and chemically pure.

Atomic vapor cells for miniature frequency references

We report on the fabrication of millimeter-sized vapor cells and their performance in atomic clocks based on coherent population trapping (CPT). We discuss two fabrication techniques. The first one is based on hollow-core pyrex fibers, fused with a CO/sub 2/ laser or micro-torch, and the second one involves anodic bonding of micro-machined silicon wafers to pyrex. Key aspects of the discussion are the performance of the cell in frequency references, the potential for further miniaturization of the cells and the ability to manufacture them on a large scale with reproducible performance.

Microfabricated atomic frequency references

Using microfabrication processes, we have been able to construct physics packages for vapour cell atomic frequency references 100× smaller than previously existing versions, with a corresponding reduction in power consumption. In addition, the devices offer the potential for wafer-level fabrication and assembly, which would substantially reduce manufacturing costs. It is anticipated that a complete frequency reference could be constructed based on these physics packages with a total volume below 1 cm3, a power dissipation near 30 mW and a fractional frequency instability below 10−11 over time periods from hours to days. Such a device would enable the use of atomically precise timing in applications that require battery operation and portability, such as hand-held global positioning system receivers and wireless communication systems.

Chip Scale Atomic Magnetometers

An optically pumped magnetometer was drastically miniaturized, by taking advantage of MEMS techniques, producing the chip-scale atomic magnetometers (CSAM) physics package. The key component of the package is an alkali vapor cell. To probe the magnetic field experienced by the atoms, the injection current to the VCSEL was modulated at 3.4 GHz near half the hyperfine frequency of 87Rb. Lock-in detection is used to determine the center of the resonance, and the magnetic field is determined by counting the modulation frequency, which is related to the Larmor precession frequency under these locked conditions. The ITO heaters create a large magnetic field gradient, which broadens the resonance and reduces the sensitivity.

Microfabricated atomic clocks

We summarize the development of microfabricated atomic frequency references at NIST. The physics packages of these devices have volumes near 10 mm/sup 3/ and power dissipation below 150 mW, and can potentially achieve a fractional frequency instability in the range of 10/sup -11/ over long periods. A fully integrated frequency reference could find application in portable, battery operated devices such as global positioning system (GPS) receivers and cellular telephones.