Laser cooling with nanostructured grating chips

Unlike ions, neutral atoms require laser light from all spatial directions to be trapped and cooled by means of lasers. The wavelength of the laser light must be adapted to the atom to be trapped and cooled, since each atom has different resonances – its own signature, so to speak. However, radiating laser light from several spatial directions requires complex optomechanical setups that must be adjusted very accurately. Nanostructured grating chips are a promising approach to miniaturizing a magneto-optical trap (MOT). From a single incident laser beam, the grating chip uses diffraction to generate the other laser beams that are necessary to build a trap. The adequate diffraction angle and the intensity balance of all beams are ensured by optimizing the grating structure.

However, there is one crucial point where grating MOTs have reached their limits: Up till now, one grating chip could only address one laser wavelength, which is sufficient for many atomic species. Specific interesting applications such as optical clocks or special gravimeters, however, use atoms that are laser-cooled by means of several, very different wavelengths. These different wavelengths cause the direction and intensity of the laser beams to differ, depending on their color, after being diffracted at the grating, which makes the operation inefficient. The challenge consisting in enabling the use of this technology also for cooling with several laser wavelengths has now been solved within the scope of a cooperation project between TU Braunschweig, DLR and PTB: A new grating chip allows strontium atoms to be cooled down to a temperature of a few microkelvins by means of blue and red laser light. With this contribution to research, the team has allowed highly accurate quantum sensors to be further miniaturized.