The radiation emitted by atoms and molecules is usually spectrally broadened by the movement of the emitters, an effect called Doppler diffusion. Overcoming this effect is a difficult task, especially for molecules. One way to reduce molecular motion is to create deep potential traps of small dimensions. So far this has been achieved, albeit with limited success, by e.g. several opposing beams were arranged in a complicated structure.
The researchers of the co-operation between Max Born Institute and Xlim Institute show that the subwavelength localization and the reduction of the line width in a very simple arrangement by self-assembly of Raman-active gas (molecular hydrogen) are possible in a crystalline, photonic hollow fiber , Raman scattering converts the pump light into so-called Stokes sidebands. Reflections at the fiber ends cause these sidebands to reciprocate in the fiber forming a stationary interference pattern: a standing wave with alternating high and low light fields [Fig. 1]. In the high-field regions, the Raman transition is saturated and inactive. The molecules have a high potential energy because they are partially in the excited state. In the low-field region, the molecules are Raman-active. They have a low potential energy because they are close to the ground state. These low field regions form a lattice of approximately 40,000 narrow, heavy traps containing localized Raman-active molecules. The size of these traps is about 100 nm (1 nm = 10-9 m), which is much smaller than the 1130 nm light wavelength. Therefore, the emitted Stokes sidebands have a very narrow spectral width of only 15 kHz - 10,000 times narrower than the double broadened sidebands under the same conditions!