Field Ionization of Cold Atoms near the Wall of a Single Carbon Nanotube

We observe the capture and field ionization of individual atoms near the side wall of a single suspended nanotube. Extremely large cross sections for ionization from an atomic beam are observed at modest voltages due to the nanotube’s small radius and extended length. The effects of the field strength on both the atomic capture and the ionization process are clearly distinguished in the data, as are prompt and delayed ionizations related to the locations at which they occur. Efficient and sensitive neutral atom detectors can be based on the nanotube capture and wall ionization processes.

Figure 1

Suspended nanotube for atom capture. (a) SEM image of the top surface of the sample, showing the nanotube suspended between rigid membrane arms of silicon oxide and silicon nitride. This sample yielded the data presented here. (b) Schematic cross section of the sample chip with trajectory of an atom that is captured and ionized.

Figure 2

Cold-atom nanotube apparatus. Rubidium atoms are loaded and laser cooled in a stationary MOT. The $550\mathrm{\text{−}}\mu \mathrm{m}$ diameter atom cloud (fluorescence image shown at bottom) is launched and the pulse travels 22 mm to the nanotube. Ions are detected 25 mm above the sample by the CEM (kept at $-2.3\mathrm{kV}$). A heater coil controls the sample temperature, and a shield around the ion detector minimizes background counts.

Figure 3

Optical and nanotube detection of launched cold atoms. The ion signal is measured with the CEM after atoms are ionized by the nanotube kept at high voltage ($>200\mathrm{V}$). The data show the temporal profile of an atom cloud as it arrives at the sample, and are compared to the optical density (OD) profile measured with the laser beam. The histogram has a bin size of $50\mu \sec$ and a total of 259 single ions. For direct comparison, the OD signal is offset by 0.33 msec, the time of flight for atoms between the laser probe and the sample.

Figure 4

Number of ions vs nanotube voltage, and interaction dynamics at the nanoscale between cold atoms and a nanotube. (a) Experimental measurements (points) and theoretical model (solid curve) of the number of ions detected from launched atom pulses with velocity $5.3\mathrm{m}/\mathrm{s}$ and peak $\mathrm{OD}=10\%$. The model has a nanotube radius of 3.3 nm. The threshold behavior (125–150 V) shows that a minimum electric field is needed for efficient electron tunneling, as described in the text. The arrow marks the voltage above which ion arrivals are prompt indicating that electron tunneling takes place outside the image-potential barrier. The dashed line reflects the linear voltage dependence of the critical impact parameter for atom capture and corresponds to what we would measure if atoms were ionized with 100% probability at all voltages. It agrees with the data in the high-voltage regime. Error bars (s.d.) are determined from counting statistics. (b), (c) Measured ion arrival times relative to the atom launch at $t=0\mathrm{msec}$, with $100\mu \sec$ bin size. Background counts are negligible as the time frame from 0–3 msec shows.