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Efficient sub-Doppler Transverse Laser Cooling of an Indium Atomic Beam

Jae-Ihn Kim, K. Maier, J. I. Kim, D. Haubrich

Published Jan 1, 2009• 0 citations
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Abstract

Laser cooled atomic gases and atomic beams are widely studied samples in experimental research in atomic and optical physics. For the application of ultra cold gases as model systems for e.g. quantum many particle systems, the atomic species is not very important. Thus this field is dominated by alkaline, earthalkaline elements which are easily accessible with conventional laser sources and have convenient closed cooling transition. On the otherhand, laser cooled atoms may also be interesting for technological applications, for instance for the creation of novel materials by atomic nanofabrication (ANF). There it will be important to use technologically relevant materials. As an example, using group III atoms of the periodical table in ANF may open a route to generate fully 3D structured composite materials. The minimal requirement in such an ANF experiment is the collimation of an atomic beam which is accessible by one dimensional laser cooling. In this dissertation, I describe transverse laser cooling of an Indium atomic beam. For efficient laser cooling on a cycling transition, I have built a tunable, continuous-wave coherent ultraviolet source at 326 nm based on frequency tripling. For this purpose, two independent high power Yb-doped fiber amplifiers for the generation of the fundamental radiation at 977 nm have been constructed. I have observed sub-Doppler transverse laser cooling of an Indium atomic beam on a cycling transition of In by introducing a polarization gradient in the linear-perpendicular-linear configuration. The transverse velocity spread of a laser-cooled In atomic beam at full width at half maximum was achieved to be 13.5 ± 3.8 cm/s yielding a full divergence of only 0.48 ± 0.13 mrad. In addition, nonlinear spectroscopy of a 3-level, Lambda-type level system driven by a pump and a probe beam has been investigated in order to understand the absorption line shapes used as a frequency reference in a previous two-color spectroscopy experiment. For the analysis of this atomic system, I have applied a density matrix theory providing an excellent basis for understanding the observed line shapes.

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Annual Meeting Optical Society of America•Jan 1, 1989•0 citations
Laser cooled atomic gases and atomic beams are widely studied samples in experimental research in atomic and optical physics. For the application of ultra cold gases as model systems for e.g. quantum many particle systems, the atomic species is not very important. Thus this field is dominated by alkaline, earthalkaline elements which are easily accessible with conventional laser sources and have convenient closed cooling transition. On the otherhand, laser cooled atoms may also be interesting for technological applications, for instance for the creation of novel materials by atomic nanofabrication (ANF). There it will be important to use technologically relevant materials. As an example, using group III atoms of the periodical table in ANF may open a route to generate fully 3D structured composite materials. The minimal requirement in such an ANF experiment is the collimation of an atomic beam which is accessible by one dimensional laser cooling. In this dissertation, I describe transverse laser cooling of an Indium atomic beam. For efficient laser cooling on a cycling transition, I have built a tunable, continuous-wave coherent ultraviolet source at 326 nm based on frequency tripling. For this purpose, two independent high power Yb-doped fiber amplifiers for the generation of the fundamental radiation at 977 nm have been constructed. I have observed sub-Doppler transverse laser cooling of an Indium atomic beam on a cycling transition of In by introducing a polarization gradient in the linear-perpendicular-linear configuration. The transverse velocity spread of a laser-cooled In atomic beam at full width at half maximum was achieved to be 13.5 ± 3.8 cm/s yielding a full divergence of only 0.48 ± 0.13 mrad. In addition, nonlinear spectroscopy of a 3-level, Lambda-type level system driven by a pump and a probe beam has been investigated in order to understand the absorption line shapes used as a frequency reference in a previous two-color spectroscopy experiment. For the analysis of this atomic system, I have applied a density matrix theory providing an excellent basis for understanding the observed line shapes.
We have realized efficient transverse cooling of an Indium atomic beam by combining optical pumping with a closed cycle UV laser cooling transition at 325.6 nm. The transverse velocity of the atomic beam is reduced to 13.5+/-3.8 cm/s, well below the Doppler cooling limit. The fraction of laser-cooled In atoms is enhanced to 12+/-3 % by optical pumping in the present experiment. It can be scaled up to approach 100% efficiency in cooling, providing high brightness atomic beams for further applications. Our results establish In on the map of elements suitable for applications involving laser cooling.
We have laser collimated a gallium atomic beam in one dimension using the linear-perpendicular-linear polarization gradient technique. Operating on the cycling $4{p}^{\phantom{\rule{0.2em}{0ex}}2}{P}_{3∕2}(F=3)\ensuremath{\rightarrow}4{d}^{\phantom{\rule{0.3em}{0ex}}2}{D}_{5∕2}(F=4)$ transition at 294.45nm294.45\phantom{\rule{0.3em}{0ex}}\text{nm}294.45nm, the full angular divergence of the atomic beam was reduced to 0.3mrad0.3\phantom{\rule{0.3em}{0ex}}\text{mrad}0.3mrad, corresponding to a transverse velocity of $\ifmmode\pm\else\textpm\fi{}11\phantom{\rule{0.3em}{0ex}}\text{cm}∕\mathrm{s}$, about one-half the Doppler cooling limit. The dependence of the cooling efficiency on the laser detuning and power was investigated. Optical pumping of the atoms out of the 4p2P3∕2(F=3)4{p}^{\phantom{\rule{0.2em}{0ex}}2}{P}_{3∕2}(F=3)4p2P3∕2​(F=3) state by the cooling laser was observed. Repumping schemes were investigated and found to successfully repopulate the cooled F=3F=3F=3 hyperfine state.
Indium is a technologically interesting element in the semicoductor and opto-electronics industry. Instead of other usually applied methods, we try to fabricate nano-structures of indium with atom lithography. The simulations show that the feature size of the structures is strongly dependent of the divergence of the indium atomic beam. Thus laser cooling is applied for transverse collimation. For laser cooling of indium, two wavelengths, i.e., 410nm and 451nm, of laser light are used. Light at 410nm is accessed by GaN diode lasers, while light at 451nm is generated by frequency-doubling a Ti-Sa laser emmiting at 902nm. The laser frequnecies are stabilized on the atomic resonance lines with the spectroscopic methods. We observed the optical manipulation of the indium atomic beam with laser light at both wavelengths and even one wavelength at 410nm. The possible mechanisms are also discussed in the thesis.
The linear-perpendicular-linear polarization gradient technique was used for investigating laser collimation of a gallium atomic beam in one dimension. The full angular divergence of the atomic beam was reduced to 0.3 mrad by operating on a particular electron transition at 294.45 nm. The transverse velocity of the atoms was reduced to 11 cm/s, which was about half of the Doppler cooling limit. The one-dimensional kinetic energy of atoms was reduced to 6 neV. The transition state exhibited optical pumping of the atoms by the cooling laser.
We have discovered a new cooling process that uses an applied magnetic field to mix differentially light shifted atomic ground state sublevels. In related experiments cooling is achieved by mixing caused by polarization gradients. Because the energy loss mechanism stems from the light shift and not the momentum transfer of a scattering event, the accessible temperature is much lower than the limit of Doppler cooling. We used this method for optical collimation of an 85Rb atomic beam to a transverse rms speed of 3 cm/s, well below the v = ( 7 ℏ γ / 20 M ) ≅ 10 cm / s of the Doppler limit for two-level atoms in this 1-D optical molasses. This low velocity, limited only by experimental geometry, is achieved only when a magnetic field of ~20 µT is applied perpendicular to the axis of the circularly polarized diode laser beam. We use a thermal beam of natural Rb produced by an oven at T ~ 150°C with an ~0.33-mm diam aperture and a defining aperture of ~0.33-mm diam about 24 cm away. The atoms are optically collimated by laser beams transverse to their motion that excite the transition at λ = 780 nm. The atomic beam profile is measured with a vertically oriented scanning hot tungsten wire, 25 µm in diameter, 1.3 m away.