Summary
This project consists of building a deep optical dipole trap or lattice,
with depths of a few tens of milliKelvin, in order to trap calcium atoms
already cooled in a magneto-optical trap (MOT). The goal is to trap
atoms which have undergone only regular Doppler cooling to a few
miliKelvin, as is the usual case of some alkaline-earth atoms such as
calcium and magnesium. We intend to perform resolved sideband
cooling on the trapped atoms, using for example the calcium
intercombination transition at 657 nm. In addition, we intend to
investigate the use of evaporative cooling by lowering the trap
potential. Both techniques are aimed to reduce significantly the atom
temperature, while holding the atoms close in space in a possible
route towards Bose-Einstein condensation.
In an initial implementation, we have built a high quality Fabry-Perot
cavity (diagram shown in fig. 1) formed by two concave mirrors (ROC =
30 cm), with reflectivities of 99.6 % at 532 nm, the wavelength of the
pump laser. The mode-matching efficiency into the TEM00 mode of
the cavity was optimized and measured to be better than 97 %. This
cavity amplified the light of a 5.5 W single-frequency laser at 532 nm
(Coherent Verdi) with a power enhancement factor of 150. Therefore,
circulating powers in excess of 500 Watts are obtained in this way
and, combined with a waist size of about 15 μm, this apparatus
provides traps depths near 20 mK. Alternatively, for a waist size of 100
m with the same intra-cavity power we reach a potential depth of 0.8
mK, near of the calcium Doppler limit. A deep optical trap allows the
use of waist sizes of several hundred microns, yielding better overlap
with the size of a magneto-optical trap (MOT) and thus increasing the
atom transfer efficiency from the MOT.
One of the mirrors has a piezo stack that allows tuning the cavity and
locking it to resonance, using the Pound-Drever-Hall technique. This
is based on the phase-sensitive detection of the light reflected from
the cavity. The laser frequency is modulated into an electro-optical
modulator (EOM). The beat signal between this modulation frequency
and this same modulation frequency, phase-shifted by the cavity and
detected into a photodetector gives an error signal (Figure 2) that is
applied to a PI servo-control and piezo driver in order to lock the cavity
resonance to the laser frequency.
This linear cavity will be placed outside the MOT chamber. An
alternative cavity built in ring geometry allows traveling waves, suitable
for a deep optical dipole trap.
This project consists of building a deep optical dipole trap or lattice,
with depths of a few tens of milliKelvin, in order to trap calcium atoms
already cooled in a magneto-optical trap (MOT). The goal is to trap
atoms which have undergone only regular Doppler cooling to a few
miliKelvin, as is the usual case of some alkaline-earth atoms such as
calcium and magnesium. We intend to perform resolved sideband
cooling on the trapped atoms, using for example the calcium
intercombination transition at 657 nm. In addition, we intend to
investigate the use of evaporative cooling by lowering the trap
potential. Both techniques are aimed to reduce significantly the atom
temperature, while holding the atoms close in space in a possible
route towards Bose-Einstein condensation.
In an initial implementation, we have built a high quality Fabry-Perot
cavity (diagram shown in fig. 1) formed by two concave mirrors (ROC =
30 cm), with reflectivities of 99.6 % at 532 nm, the wavelength of the
pump laser. The mode-matching efficiency into the TEM00 mode of
the cavity was optimized and measured to be better than 97 %. This
cavity amplified the light of a 5.5 W single-frequency laser at 532 nm
(Coherent Verdi) with a power enhancement factor of 150. Therefore,
circulating powers in excess of 500 Watts are obtained in this way
and, combined with a waist size of about 15 μm, this apparatus
provides traps depths near 20 mK. Alternatively, for a waist size of 100
m with the same intra-cavity power we reach a potential depth of 0.8
mK, near of the calcium Doppler limit. A deep optical trap allows the
use of waist sizes of several hundred microns, yielding better overlap
with the size of a magneto-optical trap (MOT) and thus increasing the
atom transfer efficiency from the MOT.
One of the mirrors has a piezo stack that allows tuning the cavity and
locking it to resonance, using the Pound-Drever-Hall technique. This
is based on the phase-sensitive detection of the light reflected from
the cavity. The laser frequency is modulated into an electro-optical
modulator (EOM). The beat signal between this modulation frequency
and this same modulation frequency, phase-shifted by the cavity and
detected into a photodetector gives an error signal (Figure 2) that is
applied to a PI servo-control and piezo driver in order to lock the cavity
resonance to the laser frequency.
This linear cavity will be placed outside the MOT chamber. An
alternative cavity built in ring geometry allows traveling waves, suitable
for a deep optical dipole trap.
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Deep Optical Dipole Trap
