|
The goal of this project is to study the interaction between atoms and magnetic resonant microstructures. A spin-polarized atom has a magnetic moment that, for example, precesses when placed in a DC magnetic field. This precession can be excited by an oscillating magnetic field with a frequency equal to the that of the atomic precession and oriented perpendicular to the DC magnetic field. Alternatively, the magnetic field generated by and ensemble of precessing atoms (or perhaps even a single precessing atom) can be detected by a sensitive magnetometer.
Micromachining techniques based on photolithographic patterning and etching of Si-based materials have developed rapidly over the last few years. One class of devices that have received considerable attention is that of mechanically resonant microstructures, such as cantilevers, torsional resonators and disk resonators. The atomic force microscope is perhaps the most famous example of an application of a mechanically resonant microstructure. In an atomic force microscope, van der Waals interactions between a surface and a nanometer-scale mechanically resonant device alter the resonant frequency of the device, the motion of which is detected optically. A related field, magnetic force microscopy, involves magnetic interactions between a resonant microstructure with a magnetic tip and a magnetic surface. More information on work at NIST in magnetic force microscopy can be found at the Nanoprobe Imaging group. Since a moving mechanical resonator with a magnetic tip can create an oscillating magnetic field some distance from the tip, spin precession in atoms near the tip can be excited by the motion of the cantilever. This excitation is most effective if the resonant frequency of the cantilever is the same as the spin precession frequency of the atoms since a small driving force can excite a large amplitude of motion for the cantilever and therefore generate a large oscillating magnetic field. In order to create a resonant interaction, that is, one that involves multiple oscillations of the cantilever and atomic spin, the atoms must be moving slowly enough that the period they spend in the oscillating field longer than several oscillation periods. Free room-temperature atoms, moving with velocities exceeding 100 m/s, spend only a few microseconds near the millimeter-scale microstructure, not long enough to interact in a resonant manner. The experiment is therefore carried out with laser-cooled atoms, which have thermal velocities of a few centimeters per second and therefore spend a few tens of milliseconds near the cantilever tip. The basic experimental apparatus is shown above at right. Atoms are laser-cooled to a few microkelvins and trapped in a standard magneto-optic trap. They are then released from the trap and fall towards the cantilever placed below the MOT. On their way to the cantilever the atoms are optically pumped by a laser beam into a single spin state. They then fall past the cantilever, and the spin precession is excited. After passing the cantilever, the state of the atoms is detected by a combination of microwave and optical excitation. Cantilevers fabricated for this experiment are shown in the figure below.
|
||||||||||||||||||||
