Trapping and cooling of potassium isotopes in a double-magneto-optical-trap apparatus
M. Prevedelli, F. S. Cataliotti, E. A. Cornell,* J. R. Ensher,* C. Fort, L. Ricci,
²
G. M. Tino,
‡
and M. Inguscio
INFM, European Laboratory for Non Linear Spectroscopy (LENS) and Dipartimento di Fisica, Universita ` di Firenze,
Largo E. Fermi, 2, I-50125 Firenze, Italy
Received 6 July 1998
We report on the efficient operation of a double-magneto-optical-trap MOT apparatus for potassium
isotopes. Mechanisms for the cooling and the transfer between the two MOTs are studied. A magnetic quad-
rupole trap has been loaded from the second MOT; density and temperature appear to be promising for starting
runaway evaporative cooling. S1050-29479903201-1
PACS numbers: 32.80.Pj, 42.50.Vk
Experiments on cooling and trapping of potassium iso-
topes are strongly motivated by the occurrence in nature of
three isotopes (
39
K,
40
K,
41
K with a relative abundance of
93.26%, 0.012%, and 6.73%, respectively. Two of them,
39
K and
41
K, are bosons while
40
K is a fermion. Potassium
therefore offers the opportunity to investigate the properties
of different bosonic isotopes and eventually could allow the
study of Bose condensates, a degenerate Fermi gas, or both.
Unlike other alkali-metal atoms, potassium has not been
widely investigated. Magneto-optical traps MOTs for the
most abundant isotopes do not have a long history 1–5 and
only recently was trapping reported for the rare fermion from
a natural abundance sample 6,7 and for unstable isotopes
from an on-line isotope separator 8. Theoretical predictions
for the collisional behavior at ultralow temperature reported
so far are contradictory 9,10. As a consequence, experi-
mental investigations on potassium are challenging but also
stimulating because of the large amount of original informa-
tion which they are likely to provide.
With the final goal of cooling atoms at high densities
starting from rare or expensive enriched isotopes, we have
developed a double-MOT apparatus 11 which allows effi-
cient loading and long trapping lifetime. An alternative to
this approach is a MOT loaded by a slow beam generated in
an atomic funnel 7. In view of a possible future sympa-
thetic cooling of
40
K by evaporatively cooled bosons, it is
necessary to start working with the bosonic isotopes of po-
tassium. However, the physics of laser cooling and the trans-
fer process between the two MOTs for
39
K and
41
K are
different than for alkali-metal atoms such as Rb. Similar to
7
Li, the hyperfine level spacing in the upper state is compa-
rable to the homogeneous broadening and it is not possible to
isolate a single cooling transition. For potassium further
complications arise from the noninverted hyperfine structure
in the excited level. Hence, two laser frequencies
1
and
2
red detuned with respect to the F
g
=1 →F
e
=2 and
F
g
=2 →F
e
=3 transitions are necessary 1. The optimal
loading rate and number of atoms in MOTs of
39
K or
41
K are
achieved when both laser fields are detuned with respect to
the whole hyperfine structure of the 4
2
P
3/2
state using rela-
tively high intensity. With this configuration, atoms are
trapped at low density and high temperature ( 1 mK) be-
cause sub-Doppler forces are suppressed. Recently we have
found that by reducing intensities and detunings we can re-
cover some of the usual characteristics of cooling other
alkali-metal atoms 5.
We demonstrate that the double-MOT scheme is success-
ful for potassium and we report on a systematic study of the
transfer of atoms between the two MOTs and the loading of
the second MOT, with either
39
K or
41
K. The atoms were
also transferred to a quadrupole magnetic trap with a lifetime
in the trap of about 30 s.
Laser light requirements for
39
K and
41
K double MOTs
are somewhat complicated since intense radiation at two dif-
ferent frequencies is necessary for the two MOTs. Two fre-
quencies are also necessary for the transfer. As is schemati-
cally shown in Fig. 1, all the laser light used in our
experiment is provided by a cw Ti:sapphire laser Coherent
model 899-21 with an output power of up to 1 W. The laser
frequency is offset locked using an acousto-optic modulator
AOM1 in double pass configuration to a saturated absorp-
tion signal obtained in a potassium vapor cell. Using six
different AOMs we obtain independent beams for the two
*Permanent address: JILA, National Institute of Standards and
Technology, University of Colorado, and Department of Physics,
University of Colorado, Boulder, CO 80309-0440.
²
Permanent address: INFM, Dipartimento di Fisica, Universita ` di
Trento, I-38050 Povo, Italy.
‡
Permanent address: INFM, Dipartimento di Scienze Fisiche,
Universita ` di Napoli ‘‘Federico II,’’ Complesso Universitario di
Monte S. Angelo, via Cintia, I-80126 Napoli, Italy.
FIG. 1. Experimental apparatus of the double-MOT system for
potassium isotopes. Ti:sa, titanium sapphire laser; QWP, quarter
wave plate; HWP, half wave plate; AOM, acousto-optic modulator;
PD, photodiode. Lenses are not shown for clarity.
PHYSICAL REVIEW A JANUARY 1999 VOLUME 59, NUMBER 1
PRA 59 1050-2947/99/591/8863/$15.00 886 ©1999 The American Physical Society