Mon 11 December, 2017

A Laser Cooled He* Beam

The laser cooling process
Figure 1: Momentum transfer in the laser cooling process.

Laser Cooling

Due to the neutrality of He* atoms they cannot be manipulated with electric or magnetic fields. Therefore to produce a high-intensity He* beam we use resonant laser light forces to collimate and focus the He* atoms. Laser cooling is based on the principle of momentum exchange from photons to atoms (figure 1). An atom exposed to resonant laser light will absorb photons along with their momentum in the direction of the laser. Emission of the photon will then occur spontaneously in a random direction. After many absorption/emission events the momentum transfer due to spontaneous emission adds to zero so that overall there is a net transfer of momentum to the atom in the direction of the laser. The force resulting from such momentum exchanges is called the spontaneous, or radiative force and can be used to manipulate He* atoms.

The He* Source

To produce the He* atoms a cold-cathode DC discharge source is used. A copper cold finger, attached to a hollow copper cathode, is cooled using liquid nitrogen. Helium gas, inlet into the cathode at a pressure of ~10 mbar, emerges through an aperture and expands supersonically into the laser cooling vacuum chambers. The helium atoms are excited into the metastable state by electron collisions in a DC discharge that emerges through the nozzle aperture. The resulting He* beam flux is characterised using a Faraday cup with typical flux values of 2x1014atomss-1sr-1 readily achieved. Time of flight measurements indicate a velocity distribution centred around 1000ms-1.

He* Source
Figure 2: The cold cathode DC discharge He* source.

Collimation

The He* beam emerging from the source is diverging in a 1/r2 fashion reducing the intensity with which to perform Metastable De-excitation Spectroscopy (MDS). To prevent this divergence and collimate the He* beam resonant laser light forces are applied using counter-propagating laser beams. In order to increase the laser-atom interaction time and thus obtain enough absorption/emission cycles for collimation, the lasers are continually reflected off four nearly parallel curved gold mirrors (figure 3). As the lasers reflect the angle with which they cross the atomic beam increases until they are almost perpendicular at the end of the collimation section. This is necessary to compensate for the changing Doppler shift of the He* atoms.

Collimation of the He* Beam
Figure 3: Collimation of the He* beam.

Before construction of the collimator, Monte Carlo simulations of atomic trajectories were performed. Figure 4a) shows how the trajectory of the He* atoms are bent towards the beam axis as they travel along the collimator. These simulations allow us to estimate that atoms with transverse velocities in the range +80ms-1 to -80ms-1 will be collimated within a distance of ~30cm. The transverse velocity of the He* atoms is reduced to <1ms-1 as shown in figure 4b). The simulations include the effects of spontaneous emission which may cause some atoms to be shifted out of resonance with the cooling lasers.

Simulation of the collimation process.
Figure 4: Simulations showing a) atomic trajectories along the collimator and b) the reduction in the transverse velocity width.

Characterisation of the He* beam is carried out using a Micro-Channel Plate (MCP) and phosphor screen assembly positioned perpendicular to the beam axis approximately 1.5m away from the source. Figure 5 shows images of the cross-section of the beam taken using a CCD camera. 5a) shows the diverging beam with no laser light forces applied. The diameter of the beam at this point is ~15cm representing a divergence angle of ~50mrad. A Faraday cup is used to measure the beam flux and the shadow of this can be seen in figure 5b). Typical uncollimated beam fluxes of 2x1014 atomss-1sr-1 are readily achieved. This flux is dramatically increased by using laser light forces to collimate the He* beam first in one dimension (5c) and then in two (5d). With lasers incident from four directions the He* beam is compressed into a square. Collimation increases the beam flux to ~1x1015atomss-1sr-1, a 500% increase. This represents a beam intensity of 6x1010 atomss-1cm-2.

To see a movie of the collimation process click here (QuickTime) or here (Windows Media Player). This shows the cross-section of the He* beam being compressed into a square as the laser frequency is scanned around the transition frequency.

He* beam cross-section
Figure 5: Cross-section of He* beam with a) no laser light force, b) Faraday cup in situ for beam flux measurement, c) collimation in one-dimension, and d) collimation in two-dimensions.

The linewidth of the 23S1-23P2 cooling transition is 1.6 MHz and so the collimation laser has to be stabilised using saturated absorption spectroscopy.

Focussing

Using a collimated He* beam to perform MDS significantly increases the ejected electron energy yield. However, an even more intense He* beam may be produced by focussing the collimated He* atoms to a point on the surface under study. This is again carried out using laser light forces with a Magneto-Optic Lens (MOL), the two-dimensional equivalent of the MOTs used to trap atoms and produce Bose-Einstein Condensates. The radiative force used in collimating the He* beam (optical molasses) is a velocity dependent force. To bend the collimated atoms to the beam axis a positional dependent force is required and this is produced by Zeeman shifting the cooling transition (23S1-23P2) using a quadrupole field created with permanent rare-earth magnets. Then, by exposing the He* beam from four sides with resonant laser light, the atoms experience a force directed towards the beam axis.

Focussing of the He* beam is currently underway. More details as things progress...

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