In atomic vapor cells, atoms collide with the inner surface, causing their spin to randomize on the walls. This wall-depolarizing effect can be diffusive and it becomes more pronounced in smaller vapor cells under high temperatures. In this work we investigate the polarization of optically pumped alkali-metal atoms in a millimeter-sized cell heated to $150{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\text{C}$. We consider two extreme boundary conditions, fully depolarizing and nondepolarizing boundaries, and we provide an analytical estimation of the polarization difference between them. In the nondepolarizing case, the pump beam's absorption is proportional to the average atomic polarization. However, for fully depolarizing walls, the absorption peak may correspond to a polarization minimum. To mitigate the wall effect, we propose reducing the pump beam's diameter while maintaining the pump power to prevent illumination of the cell wall and increase the pump intensity in the central area. This is crucial for compact vapor-cell devices where the laser frequency cannot be detuned since it is locked to the absorption peaks. Additionally, we analyze the wall-depolarizing effect on the performance of an alkali-metal atomic magnetometer operating in the spin-exchange relaxation-free regime. We show that the signal strength is highly limited by wall collisions and we provide an upper bound for it.
We experimentally demonstrate hyperfine optical pumping of rubidium atoms probed by an evanescent electromagnetic field at a dielectric-vapor interface. This light-atom interaction at the nanoscale is investigated using a right angle prism integrated with a vapor cell and excited by evanescent wave under total internal reflection. An efficient hyperfine optical pumping, leading to almost complete suppression of absorption on the probed evanescent signal, is observed when a pump laser beam is sent at normal incidence to the interface. In contrast, when the pump and probe beams are co-propagating in the integrated prism-vapor cell, no clear evidence of optical pumping is observed. The experimental results are supported by a detailed model based on the optical Bloch equation of a four atomic-level structure, which also includes a treatment of transit relaxation and wall collision with relaxation rates that were obtained directly from the thermal velocities of the atoms and the penetration depth of the evanescent wave. The obtained highly efficient optical pumping at the nanoscale is regarded as an important step in the quest for applications such as optical switching, magnetometry, and quantum memory.
We report on the spectroscopy in a resonant vapor film, in a regime in which the inhomogeneous transit time induces a velocity selection. For cell thickness (10--1000 \ensuremath{\mu}m) smaller than the transverse dimensions and for low enough atomic density, the atomic trajectories are essentially free flights from wall to wall. Hence, a single laser beam irradiation is sufficient to observe sub-Doppler resonances because of the transient atomic response, which enhances the response of the slowest atoms flying nearly parallel to the wall. The peculiar features of this velocity selection are presented in the linear regime as well as in the case of saturation induced by the optical pumping. Also, we report, in the case of the Cs
D2 line (852 nm), on the passage from a two-level linear-response regime to an optical pumping regime when increasing the incident intensity. We also describe the influence of several other parameters, including cell thickness and laser beam size and shape, on the experimental line shapes. An extension to Doppler-free polarization spectroscopy in a thin cell with a single beam, and to saturated absorption, is also reported.