Ph.D. Thesis - Physics

5.3.2 Lasers and imaging
Two lasers are required for Doppler cooling and detection of 88 Sr + ; these have wavelengths
of 422 nm and 1092 nm. The 422 nm laser addresses the 5S 1/2 →5P 1/2 transition and
provides the momentum transfer for Doppler cooling. Also, 422 nm photons are detected
when imaging ions. The 1092 nm laser is a “repumper” that addresses the 4D 3/2 →5P 1/2
transition and prevents optical pumping to the metastable 4D 3/2 state. The level structure
is depicted in Fig. 5-5.
Both lasers are extended cavity diode lasers. The laser diode is mounted on a temperature-
stabilized baseplate, and current (50 - 100 mA) is passed through it to produce laser radi-
ation. An “extended cavity” is formed by using a grating to reflect the radiation back into
the diode. This permits additional tuning of the laser frequency.
After leaving the grating, the beam passes through an optical isolator, which is a device
that prevents light from the far side of the isolator from being reflected back into the diode.
This is essential for preventing any effects due to an unintended cavity being formed by some
surface other than the grating; it also prevents overloading of the diode due to excessive
feedback. Finally, mode-matching lenses are used to couple the beam into a single-mode
fiber patch cord for delivery to the ion trap. Coupling efficiencies of ≈ 50 % are typical.
The beams are outcoupled from the fibers onto fast achromatic lens pairs from Thorlabs
for collimation. A telescope and a final focusing lens are used to produce the desired beam
waists at the trap site. 1/e 2 waists of about 50 µm are normally used.
Photoionization (PI) of neutral Sr atoms is done by a two-photon process. The first
photon, at 461 nm, pumps the atoms from the 5s5s ground state into the 5s5p excited
state, where the lowercase letters refer to the orbital angular momenta of each of the two
valence electrons in the neutral atom. The second photon addresses a broad (≈ 3 nm)
transition that pumps the atom from its excited state into an autoionizing state which lies
above the dissociation energy. Thus, one electron is removed. This process was described
in Ref. [BLW + 07].
The 460 nm radiation is produced by doubling the 920 nm output of a titanium sapphire
(Ti-Saph) laser (Coherent model no. MBR-110), which is pumped by a 5 W Spectra Physics
Millennia Pro solid-state laser. A Spectra-Physics WaveTrain doubler is used. The 405 nm
laser is quite a bit simpler; we use the output of an ECDL with a readily-available 405 nm
diode. Due to the width of the transition, no further frequency stabilization is needed.
Level diagrams for the ionic and PI transitions are presented in Fig. 5-5.
For the atomic ion trap experiments in Ch. 5-7, we used a CCD camera (part no. ST-
3200ME) from Santa Barbara Instrument Group. It features a 2184 × 1472 array of 6.8 µm
× 6.8 µm pixels, and can be cooled to -10 ◦ C. The quantum efficiency at 422 nm is about
60 %.
Light is imaged onto the CCD using a simple system of two 2 in. diameter achromatic
doublets that are mounted vertically above the vacuum chamber. The lens closest to the
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