Determining the Two-Photon Absorption Cross-Section for the 5~2S_(1/2)→5~2D_(5/2) Transition in Naturally Occurring Rubidium

摘要

Recently we demonstrated lasing at 420 nm and 455 nm inside of rubidium and cesium vapor cells respectively, by non-resonant, two photon absorption at near infrared pump wavelengths. Two photon absorption from the ground ~2S_(1/2) state to excited ~2D_(3/2,5/2) or higher lying ~2S_(1/2) state via virtual states and subsequent infrared stimulated emission pumps the upper laser levels, the second excited ~2P_(1/2,3/2) states. To better understand the mechanism and phenomenology of this type of laser, this paper presents a method for determining the two-photon absorption (TPA) cross-section for the 5~2S_(1/2) → 5~2D_(5/2) transition in naturally occurring rubidium. For an incident pump intensity of 392w/cm~2 and a rubidium concentration of 3.34 × 10~(15)atoms/cm~3, a peak absorbance of 0.10 is observed. The concentration of the Rb in the test cell is determined by one photon absorption spectra of the D_2, indicating a vapor curve shifted by 14°C from the exterior wall temperature. The incident laser beam is focused to a waist of 75 μm. The beam quality is consistent with an M~2 = 1.05 - 1.09. A model for the intensity as a function of propagation distance is developed to interpret the observed absorption spectrum and determine a cross-section for the two photon absorption.