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Atom Laser Cooling & Trapping

Atom optics

  • Doppler cooling
  • Sisyphus / polarization gradient cooling & optical molasses
  • Raman cooling and exotic laser cooling schemes
  • Magneto-optical trapping (MOT)
  • Atomic fountains
  • Atom lithography and optical dipole traps

Over the last century, many experiments and theoretical studies on matter waves and the interaction between light and atoms have paved the way to modern atom optics and its application in research & industry. Laser cooling of neutral atoms was first demonstrated around 1985. In 1997 the Nobel Prize was awarded to Steven Chu, Claude Cohen-Tannoudji and William D. Phillips “for development of methods to cool and trap atoms with laser light”. Subsequently, many different laser cooling and trapping mechanisms have been demonstrated and even more atomic species have been laser cooled. Final temperatures are in the Micro-Kelvin range, so many orders of magnitude colder than any place in outer space and only a few millionth of a degree above the absolute zero. Laser cooled atomic clouds are really a cool thing! They are used to study the atom light interaction in general and serve as cold atom sources for other experiments like Bose-Einstein condensation & degenerate Fermi gases, atom interferometry, collision studies and metrology like precision measurements of time & frequency, accelerations and rotations, isotope ratios, fundamental constants.

Laser cooling is achieved by applying special light fields or light pulses to an ensemble of atoms, e.g. stemming from the background vapor or thermal beams. Energy and momentum is exchanged between photons and the atoms in such a way that atoms experience either a friction force leading to direct cooling of the atoms or atoms are optically pumped into a specific quantum mechanical state (e.g. vibrational state in a harmonic trap) with low momentum in which they are trapped because they do no longer scatter photons. The most commonly used laser cooling schemes in atom optics are Doppler cooling and polarization gradient or Sisyphus cooling (see application notes). Raman laser cooling is sometimes used together with trapped atoms and more frequently applied for ions in traps. More exotic but physically also interesting laser cooling schemes are VSCPT and depolarization/demagnetization cooling, probably the oldest proposed light cooling scheme (demonstrated in 2006).

Laser cooled neutral atomic elements sorted by atomic number:(feel free to inform us about missing species and win a TOPTICA cup)He*, Li, Ne*, Na, Mg, Al, Ar*, K, Ca, Cr, Fe, Ga, Kr, Rb, Sr, Ag, Cd, In, Xe, Cs, Ba, Dy, Er, Tm, Yb, Hg, Fr, Ra (to be continued)

Magneto-optical trapping combines the friction force of laser cooling with a “restoring” force depending on the atomic position. In order to realize the position dependent force, a magnetic quadrupole field combined with three orthogonal pairs of counter-propagating laser beams intersecting at the center of the magneto-optical trap (MOT), the location where the magnetic field is zero. The position dependent Zeeman effect interplays with the circular polarization and the frequency of the laser beams such that atoms are always pushed towards the MOT center. Most of the laser cooled elements have also been trapped in MOTs. Typical atom numbers range from a few thousand to a few billion of atoms at temperatures in the micro to Milli-Kelvin range and densities of typically 10^8 to 1011 atoms/cm3.

Atomic fountains can be generated out of MOTs by switching of the magnetic field and switching on a frequency difference between upward and downward traveling lasers after the desired number of atoms has accumulated and thermalised in the MOT. This way, atoms are cooled in a moving frame and move up with a well defined velocity. Gravity decelerates atoms on their way up until they fall down again after reaching their turning point. The main advantage of such an atomic fountain is a dramatically increased observation or interaction of released atoms up to typ. 1 s, a big advantage for precision measurements. Atoms can eventually be recaptured and re-launched allowing for atom juggeling!

Atom lithography and optical dipole traps are based on the so-called dipole force. The laser induces an oscillating electric dipole within the atom which interacts with the electric field of the laser beam itself to form a conservative optical potential. The optical potential varies with the laser intensity and its gradient gives rise to the dipole force. If the laser frequency is below the atomic resonance frequency the atomic dipole and the electric field oscillate in phase and the optical potential is negative. Atoms experience a dipole force towards the intensity maximum of the light field. A tightly focused laser beam can trap atoms at the center of its focus and form a so-called optical tweezer or optical dipole trap. More complex optical traps are created by counter-propagating laser beams in one, two or even three dimensions forming 1-d, 2-d or even 3-d optical lattices. These optical lattices – perfect crystals made out of light with intensity maxima at every half of the optical wavelength (typ. < 1 µm) – are used to study and simulate solid state physics with trapped atoms. Optical potentials can also be used to generate lenses for rays of atoms reversing the traditional roles of matter and light where an arrangement of matter (e.g. a glass lens) is used to focus a ray of light. In atom lithography, a beam of atoms is sent through a special light configuration and focused by this light lens onto a substrate where it forms atomic structures on the nm scale.

TOPTICA’s added value in atom optics
Atom optics is a challenging field for laser applications. Special or even customized lasers are needed with always changing demands according to the newest scientific research topics. The laser systems have to provide enough power at the desired (and quickly changing) wavelength. The linewidth has to be below the linewidth of the atomic transition (typ. MHz but in recent experiments also in the kHz range) or the difference between atomic transition and desired laser frequency. Mode-hop free fine tuning, that is very precise adjustment of the laser frequency, allows one to set or even stabilize the laser frequency at a well defined position close to the atomic transition frequency. Even complex laser systems have to be easy to operate since nowadays more and more lasers have to work simultaneously and reliably in order to have experimental success.
TOPTICA is key supplier of such laser systems not only to Nobel Laureates but also to most research groups all around the world. Since more than a decade, we are well known for the quality of our products always at the front line of research and for our flexibility. We are constantly developing new laser systems and are open for special solutions according to customers’ demands. Last but not least, although being a professionally organized company with quality management, service department and profound industrial/OEM competence, we are scientist at heart. Living partnerships with many research groups around the world is not only must for us but a pleasure since we are still excited about science. You will find experts at TOPTICA with profound atom optics background that understand your application and your requirements. Please contact us to discuss details of your experiment.