Saturated Absorption Spectroscopy

Saturated absorption spectroscopy is a technique used to achieve high-resolution spectroscopy in gas-phase atoms. It addresses the issue of Doppler broadening, which is the spreading of spectral lines due to the varying velocities of atoms in a gas. In this method, two laser beams—a pump beam and a counter-propagating probe beam—are directed through a rubidium vapor cell.

The pump beam saturates the transition of rubidium atoms moving with zero velocity component along the beam axis. When the probe beam encounters these saturated atoms, it experiences reduced absorption, creating a sharp, narrow peak in the absorption spectrum superimposed on the broader Doppler-broadened background. This peak corresponds to the exact transition frequency of stationary rubidium atoms.

This technique is particularly useful in laser cooling experiments, where precise control of laser frequency is crucial. For laser cooling rubidium atoms, an External Cavity Diode Laser (ECDL) is often used. By using saturated absorption spectroscopy, the frequency of the ECDL can be locked to the narrow peak in the absorption spectrum. This ensures that the laser frequency remains stable and resonant with the rubidium atoms' transition, which is essential for efficient laser cooling and trapping. This stability enables the manipulation of atomic motion, cooling the atoms to extremely low temperatures and allowing for further experiments in atomic physics and quantum mechanics.