subDoppler

SubDoppler refers to phenomena or techniques that achieve spectral resolution or cooling below the Doppler limit set by thermal motion of atoms or molecules. In spectroscopy, Doppler broadening from thermal motion produces wide lines; sub-Doppler methods exploit velocity-selective processes or coherence to resolve narrower features. In laser cooling, sub-Doppler cooling reaches temperatures below the Doppler limit for a two-level system by exploiting the multi-level structure of real atoms and polarization gradients.

Sub-Doppler cooling mechanisms include Sisyphus cooling and polarization gradient cooling in optical molasses. They rely on spatially varying light polarization that creates position-dependent optical pumping among magnetic sublevels, removing kinetic energy more efficiently than Doppler cooling alone. This approach is especially effective in alkali atoms (for example rubidium and cesium). Temperatures typically reach tens to hundreds of microkelvin, and advanced techniques can reach microkelvin to sub-microkelvin regimes in optical lattices or with Raman sideband cooling.

Sub-Doppler spectroscopy uses methods such as saturated-absorption (Lamb dip), Doppler-free two-photon spectroscopy, and polarization spectroscopy to reveal narrow features within Doppler-broadened profiles. These often employ counter-propagating beams to cancel first-order Doppler shifts or rely on coherent effects such as coherent population trapping or electromagnetically induced transparency to produce narrow resonances.

Applications include precision metrology, atomic clocks, quantum information, and fundamental studies of atomic structure. Limitations include dependence on level structure and polarization control, sensitivity to magnetic fields, and challenges with heating in dense samples.