Tunable Diode Lasers and Frequency Combs for Atom Laser Cooling & Trapping

Laser cooling and magneto-optical trapping of atoms

  • Doppler cooling
  • Sisyphus / polarization gradient cooling & optical molasses
  • Raman cooling, sideband cooling and exotic laser cooling schemes
  • Magneto-optical trapping (MOT)
  • Atomic fountains
  • Atom lithography
  • 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 microkelvin 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 atoms are really a cool thing! They are at the very heart of applied quantum technology and used to study many aspects of fundamental quantum technology, such as the atom light interaction in general, Bose-Einstein condensation or degenerate Fermi gases.

Laser cooling

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 loose kinetic energy. They either experience 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 lower energy. The latter schemes are arranged such that in the lowest motional state the atoms do no longer scatter photons. The most commonly used laser cooling schemes in atom optics are Doppler cooling and polarization gradient or Sisyphus cooling. Raman laser cooling and (resolved) sideband cooling are 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 (velocity selective coherent population trapping) and depolarization/demagnetization cooling.

Laser cooled neutral atomic elements sorted by atomic number

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

Magneto-optical trapping

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 intersect at the center of the magneto-optical trap (MOT), the location where the magnetic field is zero. The therefore 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 millikelvin range and densities of typically 108 to 1011 atoms/cm3.

Atom lithography and optical dipole traps

Atom lithography and optical dipole traps are based on the so-called dipole force. The laser induces an oscillating electric dipole in 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 then 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 laser cooling-based quantum technologies

Quantum technology applications require special or even customized lasers with always constantly evolving demands according to the newest developments and changing scientific research topics: The laser systems have to provide enough power at the desired  wavelength. The linewidth has to be below the linewidth of the atomic transition (typ. MHz but in recent experiments also in the kHz or even Hz 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. Remote, digital control of the laser is another feature becoming increasingly essential.
TOPTICA is key supplier of such laser systems to most research groups and to quantum technology industry all around the world. Since more than two decades, 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. You will find experts at TOPTICA with profound quantum technology background that understand your application and your requirements. Please contact us to discuss details of your plans.