LOADING OF A KRYPTON MAGNETO-OPTICAL TRAP WITH TWO HOLLOW LASER BEAMS IN A ZEEMAN SLOWER
S. Singh* V. B. Tiwari, S. R. Mishra, H. S. Rawat
Laser Physics Applications Section, Raja Ramanna Centre for Advanced Technology, Indore-4 52013, India
Received April 11, 2014
A significant enhancement in the number of cold atoms in an atomic-beam-loaded magneto-optical trap (MOT) for metastable krypton atoms is observed when hollow laser beams are used in a Zeeman slower instead of a Gaussian laser beam. In the Zeeman slower setup, a combination of two hollow laser beams, i.e., a variablediameter hollow beam generated using a pair of axicon lenses superimposed on a fixed-diameter hollow beam, has been used to reduce the longitudinal velocity of the atoms in the atomic beam below the capture speed of the MOT. The observed enhancement in the number of atoms in the MOT is attributed to reduced destruction of the atom cloud in the MOT and increased cooling of the off-axis atoms in the atomic beam, resulting from the use of hollow beams in the Zeeman slower.
DOI: 10.7868/S0044451014090065
1. INTRODUCTION
The laser cooling of noble gas atoms in the excited state is an attractive area of research to study cold atom collisions, ionization physics, nanolithography, and atom trap trace analysis (ATTA) [1,2]. Compared to alkali atoms, which are cooled in the ground state, noble gas atoms are laser cooled in a metastable excited state. The excitation to this metastable state can be achieved by radio-frequency (RF) excitation [3,4]. The metastable state atoms generated in the discharge section are transported to the magneto-optical trap (MOT) chamber in the form of an atomic beam. Capturing these atoms in the MOT requires reducing the longitudinal velocity of the atoms in the atomic beam below the capture velocity of the MOT. A high flux of the slowed metastable atoms in the atomic beam is important for efficient loading of the MOT, which is a workhorse for many experiments including Bose Einstein condensation (BEC), atom optics, and atomic physics [5,6]. The slowing of the atomic beam is conveniently achieved by using a Zeeman slower device [7], which is used as a decelerating unit for the atomic beam before its entry to the MOT chamber. The Zeeman slower decelerates the atoms through Doppler laser
E-mail: surendra'flircat.gov.in
cooling of atoms in the presence of a spatially varying magnetic field. The deceleration results from the scattering force on an atom due to a laser beam propagating opposite to the atomic beam direction. The variation of the magnetic field in the Zeeman slower is kept in such a way that the Zeeman shift in the atomic transition frequency compensates the Doppler shift in the laser frequency for a moving atom. Thus, laser beam interacts resonantly with the atoms throughout the length of the Zeeman slower, which leads to effectively slowing the atomic beam for the loading of the MOT.
In a Zeeman slower based atomic-beam-loaded MOT, a known difficulty is the destruction of the MOT atom cloud by the Zeeman slower laser beam, as the MOT cloud is formed on the atomic beam axis to which the Zeeman slower laser beam is aligned from the opposite direction. This laser beam, being close to the resonance with the atomic transition, thus knocks out the trapped atoms from the MOT at its higher power. This perturbation to the MOT cloud can be reduced by the off-axis alignment of the Zeeman slower laser beam such that the MOT cloud does not come in the way of the laser beam. But this also results in a decrease in the number of atoms in the MOT due to poor loading of the MOT caused by the less effective cooling in the Zeeman slower. In an important work [8], this problem was tackled by using a hollow Zeeman slower laser beam
aligned axially opposite to the atomic beam, such that the MOT cloud was formed in the dark central region of the hollow beam. The hollow beam was generated by using a dark spot in the central region of the transverse cross section of a Gaussian beam. This resulted in an enhancement in the number of atoms accumulated in the MOT. In another work [9], the decelerating Zeeman slower laser beam was tightly focused at a position slightly away (in the transverse direction) from the MOT cloud, such that its perturbation to the MOT cloud was as small as possible. But in this geometry, the number of atoms is expected to be very sensitive to the position of the focus of the Zeeman slower beam.
In this paper, we use two hollow laser beams of different dark diameters and ring widths in the Zee-man slower of an atomic-beam-loaded krypton MOT. The first hollow beam is generated using a dark spot in the path of a Gaussian beam. With this hollow beam used as a Zeeman slower beam, the observed enhancement in the number of atoms in the MOT is « 30 % of the number observed with a Gaussian Zeeman slower beam of same power. With the use of an additional second hollow beam of a larger dark diameter, we observe a further enhancement by « 30 % in the number of cold atoms in the MOT. This enhancement in the number of atoms in the MOT is attributed to more efficient cooling of the off-axis atoms in the atomic beam due to a specific intensity profile of the second hollow beam. Thus, combining two hollow beams in the Zee-man slower results in a larger number of atoms in the MOT cloud due to more efficient cooling of the atomic beam with lesser destruction of the MOT cloud compared with the use of a regular Gaussian beam in the Zeeman slower.
2. EXPERIMENTAL SETUP
The schematic of our experimental setup for the generation of two hollow beams for the Zeeman slower is shown in Fig. 1. The first hollow beam was a fixed-diameter hollow beam (HB1) generated by keeping a transparent glass slide with a dark spot in the path of a Gaussian laser beam from an extended cavity diode laser (ECDL) system having the maximum power of « 50 mW. The optimum size of the dark spot to generate HB1 was found by varying the size of the dark spot for a given « 30 mW power in the beam (before the dark spot) and measuring the number of atoms in the MOT with the HB1 beam used in the Zeeman slower. The optimum size of the dark spot was ~ 1 mm, and hence HB1 with this dark spot was used in further ex-
PBS GB
HWPl
HBl )
ZS-ECDL
BS
HBl + HB2
HB2 Output Q laser beam
HWP2
Fig. 1. Schematic of the experimental setup for the generation of hollow beams. ZS-ECDL — Zeeman slower-extended cavity diode laser; LI, L2, L3, and L4 — lenses of appropriate focal lengths; HWPl and HWP2 — half-wave plates; PBS — polarizing beam splitter cube; GB — Gaussian beam; DS — dark spot on the transparent glass plate; BS — beam splitter; Ml and M2 — mirrors; AX1 and AX2 — axicon lenses; HBl — hollow beam generated using the dark spot; HB2 — hollow beam generated using the axicon lenses
periments. The second hollow beam was a variablediameter hollow beam (HB2) generated by taking a part of a laser beam and passing it through a pair of axicon lenses (with the cone angle 176°) mounted on a calibrated translation stage (Fig. 1). These lenses were facing each other in the path of the slowing laser beam [10]. The dark diameter of the hollow beam (d) can be varied by varying the separation between the axicon lenses, similarly to an earlier work that used a pair of axicon mirrors to generate a variable-diameter collimated hollow beam [11].
The experiments reported here were performed on our magneto-optical trap setup for metastable Ivr atoms, which is also described elsewhere [12]. The schematic of this experimental setup for cooling and trapping of 84Ivr* atoms is shown in Fig. 2. The krypton gas first flows into the RF discharge glass tube through the inlet chamber (CI) with the pressure ~ 10-3 Torr. The glass tube has the inner diameter of 10 mm and the length of 150 mm. The Ivr* atoms produced in this tube by the RF-driven discharge (frequency « 30 MHz) pass through the analysis chamber (C2) with the pressure ~ 10-5 Torr. A Zee-man slower (length « 80 cm) along with an extraction coil was connected between the pumping chamber (C3) with the pressure ~ 10-6 Torr and the MOT chamber (C4) with the pressure ~ 10-8 Torr. It slows down the 84Ivr* atomic beam before cooling and trapping in the MOT chamber. The Zeeman slower laser beam that we used was composed of two hollow beams as shown in Fig. 1. This laser beam propagates opposite to the atomic beam and is <r+ polarized. Its frequency was de-
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Extraction coil
Control
CI
C2
Zooman slower coils
MOT beams
RF discharge
Kr gas
£ Pump
C4 if
*■ MOT coils
Zeeman slower beams
Kr* atomic beam
La 2 m
-<->-
Fig. 2. Schematic of the experimental setup for cooling and trapping metastable Kr atoms. CI — inlet cham ber (10-3 Torr), C2 — analysis chamber (1CT'J Torr), C3 — pumping chambers (10_o Torr), C4 — MOT chamber (1CT8 Torr)
tuned by « 80 MHz to the red of the 84Ivr* transition between 5,s[3/2]2 and 5p[5/2]3 states. The cooling laser beam for the MOT was split into three beams, each having a power of « 5 mW (the 1/e2 radius « 3 mm). These beams were used in a retro-reflection geometry to obtain the desired six beams for the MOT. The frequency of the MOT cooling laser was kept at « 6 MHz red-detuned to the 5,s[3/2]2 5p[5/2]3 transition of 84Ivr*. A pair of anti-Hclmholtz magnetic coils provided the magnetic field gradient of ~ 10 G/'cm for the MOT formation.
In the Zeeman slower, the magnetic field variation along the atomic beam direction was designed to provide an effective cooling of the longitudinal velocity of an atomic beam. In the magnetic field of the Zeeman slower, the Zeeman-shifted atomic transition frequency-is equal to the Doppler-shifted frequency of the counter-propagating laser beam. This resonance condition can be expressed as u>o + /¡bB-JH = u>l + kv, where u>o is the atomic transition frequency without any magnetic field, u>l is the slower laser frequency, fig
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