Lithium Faraday Filter: Some Like It Hot

TL;DR

This work addresses the challenge of achieving high-fidelity, ultra-narrowband optical filtering near atomic resonances using hot lithium vapor. The authors construct a Li-based Faraday filter operating around 671 nm and extend the ElecSus toolkit to treat both the closely spaced D1/D2 transitions (and isotopic overlaps) with power-broadening and saturation corrections. The experimental setup combines a high-temperature heat pipe, a longitudinal magnetic field, and polarization analysis to realize a tunable bandpass filter, achieving approximately $0.82$ peak transmission at an optimal point ($T=264\,^{\circ}\mathrm{C}$, $B=269\ \mathrm{G}$) with ENBW $=5.32\ \mathrm{GHz}$ and FOM $=0.154\ \mathrm{GHz}^{-1}$. The results demonstrate the viability of lithium-based Faraday filtering for high-SNR optical detection and state readout in quantum-optics experiments, and the publicly available Li-extension of ElecSus enables broader adoption of this approach.

Abstract

Magnetically induced rotation of linearly polarized light near an atomic resonance, combined with Doppler-broadened absorption windows, enables narrowband transmission of optical frequencies. An ultra-narrowband lithium vapor Faraday filter at about 671 nm is investigated experimentally and theoretically. The resulting Faraday filter transmittance is demonstrated using a lithium heat pipe oven under longitudinal magnetic fields ranging from 0 to 300 G. Optimization of the lithium Faraday filter performance reveals an optimal operating point at 264 °C and an external magnetic field of 269 G, yielding a peak transmission of approx. 82%. The lithium D$_1$- and D$_2$-transitions are only 10 GHz apart and temperature broadening leads to an overlap of the isotopes D-lines. Thus, the applied theoretical model needs to consider both transitions simultaneously. For this purpose, we extended an existing Python library (ElecSus), which now allows for the calculation of the atomic susceptibilities of lithium.

Figures (3)

• Figure 1: The setup of the lithium Faraday filter. (a) An evacuated metal pipe with windows is placed between two polarizing beamsplitters (PBS). A cut along the pipe shows the heating tape around the lithium filled cartridge with a nickel mesh. On the outside the copper solenoid is placed to generate an longitudinal magnetic field $\vec{B}_z$ that causes the vapor atoms to rotate the linear polarized light (arrows). Two photodiodes measure the intensity of the rotated light $I_y$ and the unrotated light $I_x$. (b) The experimental setup with the catching container below the steaming hot pipe with a wet bandage.
• Figure 2: Transmission spectrum and Faraday filter spectrum of lithium. The light gray background serves only as a visual guide. (vapor temperature = 264 °C, Doppler temperature = 417 °C, length = 150 mm, magnetic field = 269 G) (a) The transmission spectrum is reconstructed from both photodiodes with all $D$-lines of Lithium 6 and 7. Light on the $^6$Li-transitions is not fully absorbed because of the natural abundance of 7.58 % and the high probe intensity. (b) and (e) The calculated residuals for the transmission spectrum and for the Faraday filter spectrum are lower than ±4 % at the steep edges. (c) The calculated dispersion for the corresponding parameter set. (d) The Faraday filter spectrum with a maximum transmittance of approx. 82 %.
• Figure 3: Density plot of the Faraday filter transmittance for a temperature of 302 °C and optical path length of 150 mm for various magnetic fields. The Doppler temperature corresponds to 505 °C. The red dotted line indicates a spectrum with a peak transmission around 92 %. (a) The simulated data. (b) The experimental data.