Measurement of Alkali Metal Vapor Number Density Based on Surface Plasmon Resonance

Alkali metal vapor cells are essential components of atomic magnetometers for magnetic field measurements. In this letter, a novel measurement method based on the surface plasmon resonance (SPR) effect to determine the number density and temperature of alkali metal vapor in cells is proposed. Polarized alkali metal atoms with different number densities exhibit slight refractive index differences, which can be measured by SPR to calculate the vapor density. Temperature information is obtained by coating a polydimethylsiloxane (PDMS) film at a fixed location above the metallic layer in the vapor cell. A Kretschmann-type prism-coupled SPR structure with angular modulation is analyzed using the finite element method for its suitability in measuring the number density and temperature of rubidium (Rb) vapor cells. The results show that within the resonance angle range of 43.08 ° to 46 °, the rubidium vapor density ranges from 2.48×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> to 1.7×1016 cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup>, with a density sensitivity of 1.63×10-16 °/cm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> and a temperature sensitivity of 6.5×10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> °/K, covering most operating conditions of atomic magnetometers. The proposed sensing structure offers a novel, real-time, non-contact, and spatially distributed method for measuring alkali metal gas density and temperature, significantly advancing the development of atomic magnetometers..