Bose 2.2 User Manual download pdf (Page 45)

3.1.2 Procedure

We create a beam of chromium atoms by sublimation in an eﬀusion cell

at a temperature

T ∼

1450

◦

. The atomic beam is ﬁrst collimated by a set of apertures, and then radially

cooled by a two-dimensional optical molasses [129, Ch.9.3]. For the radial cooling, we

use blue laser light at a power

P ∼

120

and at the wavelength

= 425

, close

to the

↔

transition (see Fig. 3.2(a)). We have chosen a single beam setup in a

back-reﬂected geometry to maximally use the available laser power.

After the radial cooling stage, the atoms enter a spin-ﬂip Zeeman slower (ZS) [124,

Ch.3.4]. There, the atoms are decelerated by scattering the light from a counterpropagating

resonant laser beam. The eﬃcient deceleration from a maximum velocity

max

∼

580

m/s

to a ﬁnal velocity

∼

m/s

is obtained by designing a magnetic gradient ﬁeld such

that the Zeeman-shift exactly compensates the Doppler-shift at any position inside the

ZS. This leads to persistent resonant absorption of the laser light [130, 131].

MOT & ZS

425,6 nm

spontaneous

decay

(a)

repumper

663,2 nm

(b)

(c)

opt. pumping

427,6 nm

ODT

Fig. 3.2, Cooling and trapping of

Cr :

(a) The magnetic quadrupole ﬁeld created

by the cloverleaf coils is used to operate the magneto-optical trap (MOT) on

the

↔

transition. The ﬁeld also provides a magnetic trap (MT) for

the atoms in the meta-stable

state. (b) To transfer back the atoms from

the

state to the

ground-state, a repump laser is used at a wavelength

= 663

, resonant with the transition to the

excited state. (c) After

loading the atoms from the MT to the optical dipole trap (ODT), the atoms

are optically pumped from the

= +3 to the

−

3 magnetic sublevel of

the

ground-state. Figure taken from [113].

Supplier: CreaTec Fischer & Co. GmbH, part number: HTC-40-10-2000-284-SHM + WK-63-40-267.

We additionally use crucibles made of ZrO

(stabilized by Y

) inside the tungsten crucible of the

eﬀusion cell, to avoid an chromium-tungsten alloy to form [123, Ch.4.2.1]. Supplier of crucibles: Cesima

Ceramics. Internal link to technical drawings of oven and crucibles.

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Bose 2.2 User Manual Page 45

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