a) Setup
We use a standard Zeeman slower and trapping configuration to load our blue MOT :
b) Blue light
To obtain the 461 trapping light we start with an external cavity grating
stabilized diode laser. We are currently using an SLI laser which
freely runs at 912 nm and requires significant heating to reach 922 nm.
This provides
about 10 mW of 922 nm light which we use to inject a SDL 925 nm tapered
amplifier. The output of the amplifier feeds a doubling cavity using
KNbO3 in a type I critical phase-matching configuration at about 26 degrees
celsius.
This very compact scheme allows us to have at our disposal 150mW of 461nm light,
all made from diode lasers!
A portion of the 461 nm output of the cavity will be used
to lock the laser to a strontium atomic beam using the absorbtion signal.
Currently we are able to scan a doppler broadened
Sr signal using the optogalvanic effect in a Sr coated hollow cathode lamp.
This is to broad to serve as a locking signal but assures us that we can
tune over the 461 nm resonance.
c) Atomic source
We chose to load the MOT from an atomic beam rather than a vapor due
to the technical difficulties of maintaining the entire vacuum chamber
at several hundred degrees in order to supply a sufficient vapor pressure
of strontium in the cell. We will use an oven operating at
about 500 K in conjuction with a Zeeman slower to provide approximately
109 atoms/sec to the MOT. We plan to deflect the slowed
atoms to an offset MOT in order to prevent unslowed portion of the beam
from hitting the trap.
d) Repumping
While the blue trapping transition almost closed (i.e. almost all atoms
exited in the cooling process fall quickly back to the ground state) ,
there is a small loss of atoms by optical pumping to the meta-stable 5p3P2
state (see term
diagram in the Strontium facts section) which are no longer
trapped. It is not neccessary to close this loss channel to obtain
a reasonable MOT, but closing it does can provide an order of magnitude
increase number of atoms trapped.
This channel is closed by repumping. On drives a transition
from 5p3P2 to a higher state which has a good
chance to decay down to the ground state in time to remain trapped.
This has been accomplished previously by using two red lasers to drive
both of the meta-stable triplet transitions (5p3P0,2
)
to the 6s3S1 state which can decay to the J
= 1 state and then the ground state. Another possibility, depicted
in the table schematic, is to drive the blue transition to the 5d3D2
state. This has not been tried yet, and all of the decay rates have
yet to be determined.
e) Results
After all this, what we get is a nice blue MOT
with 107 atoms.
We have also looked at the cloud expansion
after a variable delay following the cutting the trapping and cooling fields
and found a temperature of a few mK for our cold trapped atoms.
The MOT used for our coherent backscattering experiments with Sr had
an optical thickness of up to 3
