Nano-trap lattice and linewidth reduction in a Raman-active gas

Reducing the emission linewidth of a molecule is one of the main objectives of precision spectroscopy. One approach is based on the cooling of molecules close to absolute zero. An alternative possibility is the localization of the molecules on the subwavelength scale. A novel approach in this direction has recently been proposed by a joint team of the Max Born Institute (A. Husakou) and the Xlim Institute in Limoges. This approach uses a standing wave in a gas-filled hollow fiber for localization. It produces a lattice of deep nanometer-scale lattices for Raman-active molecules, resulting in a line width reduction of a factor of 10,000.

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!

Fig. 1: The pump light changes on the macroscopic scale into forward-looking Stokes radiation (FS), which is partially reflected by the fiber end and becomes reverse-directed Stokes radiation (BS). The latter is also reinforced by the pump light. In the area where both FS and BS are strong, they form a standing wave interference pattern, which is shown on the microscopic scale. In the low-field regions (characterized by red-colored molecules), the molecules are in the ground state and are strongly localized, as shown by the potential in the lower part. It is these "trapped" molecules that are Raman active, which leads to a reduction in linewidth.

The self-organization of the gas also manifests itself on the macroscopic scale. First of all, the calculations show that the Raman process mainly takes place precisely in the fiber section where the standing wave is formed, as shown in the upper part of Fig. 1. Furthermore, the macroscopic gradient of the potential for the flow of gas leads to the fiber ends, which can be observed with the naked eye in the experiment. This strong localization and the narrowing of the linewidth can lead to different applications e.g. in spectroscopy. However, it can also be used as a method of periodically modulating the gas density, which is suitable for the development of quasi-phase matched arrangements for other nonlinear processes such as e.g. for the effective generation of high harmonics.

Original publication

Raman gas self-organizing into deep nano-trap lattice

M. Alharbi, A. Husakou, M. Chafer, B. Debord, F. Gerome, F. Benabid

Nature Communications 7 (2016) 12779/1-9