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Thermal Alkali Vapour Spectroscopy: Buffer Gas Enhanced K Systems and Magneto-Optical Rb Applications

ALQARNI, SHARAA,AHMED,S

Published Jan 1, 2025• 0 citations
OpenAlex

Abstract

We present experimental and theoretical work on atomic bandpass filters and spectroscopic characterisations of alkali vapour systems for magneto-optical applications. We demonstrate a new method for generating arbitrary angle magnetic fields in atomic filters using Rb vapour. This involves a fixed of permanent magnets in a Voigt geometry combined with a solenoid. We compare this method with the previously used method of rotating the permanent magnet pair. This setup offers more precise and flexible angle control. While both methods generate similar transmission profiles, the new setup allows larger angles and supports longer vapour cells, enhancing magneto-optical filters. Next, we investigate K D1 transition in the presence of neon buffer gas, in particular the pressure induced broadening and frequency shift, and the Zeeman splitting in the hyperfine Paschen-Back (HPB) regime. We use dual-control temperature systems to independently adjust Doppler and collisional effects, and we achieve excellent agreement with literature values for potassium-neon collisions. For the first time, buffer gas effects integrate into our theoretical model ElecSus, producing accurate prediction of modified Voigt profiles. We conduct a comprehensive experimental and theoretical study into the Stokes polarimetry of potassium atomic vapour with neon buffer gas, focusing on the temperature and magnetic field effects. This work represents the first application of ElecSus to model the buffer gas polarimetry of K D1 transitions. This study provides new insights into the effects of buffer gases on Stokes parameters, and indicates advances in a theoretical framework for understanding atom-light interactions in buffer gas environments.

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We present experimental and theoretical work on atomic bandpass filters and spectroscopic characterisations of alkali vapour systems for magneto-optical applications. We demonstrate a new method for generating arbitrary angle magnetic fields in atomic filters using Rb vapour. This involves a fixed of permanent magnets in a Voigt geometry combined with a solenoid. We compare this method with the previously used method of rotating the permanent magnet pair. This setup offers more precise and flexible angle control. While both methods generate similar transmission profiles, the new setup allows larger angles and supports longer vapour cells, enhancing magneto-optical filters. Next, we investigate K D1 transition in the presence of neon buffer gas, in particular the pressure induced broadening and frequency shift, and the Zeeman splitting in the hyperfine Paschen-Back (HPB) regime. We use dual-control temperature systems to independently adjust Doppler and collisional effects, and we achieve excellent agreement with literature values for potassium-neon collisions. For the first time, buffer gas effects integrate into our theoretical model ElecSus, producing accurate prediction of modified Voigt profiles. We conduct a comprehensive experimental and theoretical study into the Stokes polarimetry of potassium atomic vapour with neon buffer gas, focusing on the temperature and magnetic field effects. This work represents the first application of ElecSus to model the buffer gas polarimetry of K D1 transitions. This study provides new insights into the effects of buffer gases on Stokes parameters, and indicates advances in a theoretical framework for understanding atom-light interactions in buffer gas environments.
Abstract In this study, we investigate the effect of buffer gas and magnetic field on the spectral line shapes of the potassium D1 transition using sealed vapour cells filled with varying amounts of neon as a buffer gas. Employing a dual-temperature control system, we independently manipulate the cell body and stem temperatures to explore Doppler and collisional effects on the spectrum. Our results show how the Voigt spectral profile changes from Gaussian- to Lorentzian-dominated forms due to pressure broadening and shifts caused by collisions between potassium atoms and neon. Our measurements are in excellent agreement with the literature values for potassium-neon collisions. For the first time we were able to incorporate the buffer-gas shift and broadening into the modified Voigt profile via the ElecSus code, and found excellent agreement between the predicted and measured line profiles. We also analyse the potassium D1 spectral lines in the hyperfine Paschen–Back regime using strong magnetic fields, demonstrating how Zeeman splitting modifies the pressure-broadened line shape. This work provides valuable insights into collision-induced broadening and shifts, enhancing our understanding of potassium spectroscopy and its application in the development of advanced magneto-optical filters for solar physics and other applications.
Abstract This study presents a comprehensive experimental and theoretical characterisation of Stokes polarimetry in potassium (K) vapour on the D1 line. Measurements were performed in the weak-probe regime, investigating the influence of neon buffer gas in the presence of an applied magnetic field in the Faraday geometry. While previous Stokes polarimetry studies in alkali-metal vapours have been conducted, the specific effects of buffer gas-induced broadening and shifts on the observed Stokes parameters remained largely underexplored. Here, experimental measurements of absolute absorption and dispersion were compared with a theoretical model for the electric susceptibility of the vapour, calculated using the established software package ElecSus . This work marks the first application of ElecSus to model buffer gas polarimetry of the potassium D1 line, with validation performed against experimental spectra for magnetic fields up to 1.2 kG. Our findings provide new insight into how the presence of buffer gas influences the observed Stokes parameters, thereby enhancing the predictive capabilities of theoretical frameworks for atom-light interactions in buffer-gas environments.
We present investigations of magneto-optical phenomena using a 87^{87}87Rb thermal vapour in large magnetic fields. At this point, in the hyperfine Paschen-Back regime, optical transitions can be separated by more than their Doppler-broadened linewidth, providing a high level of control over the atomic system and a simpler theoretical model for studying atom-light-magnetic field interactions. In this context, we study the spectra of 87^{87}87Rb at magnetic fields of 0.4 0.4~0.4 T and 1.5 1.5~1.5 T in the Voigt geometry. In both cases we find excellent agreement between experimental and theoretical absorption spectra. With the 1.5 1.5~1.5 T field we also study polarisation changes induced in the light by the atoms subject to a large magnetic field. Using this approach, we consider the practical applications of our system as a vector magnetometer. We present a scheme that eliminates most sources of systematic errors in the measurement of the field strength by using pairs of transitions that cancel excited-state terms in the energy shifts. Finally, we extend the concepts and ideas investigated to an application in precision thermometry. We discuss how good understanding of the spectra and the magnetic field strength potentially allow detection of small differences in different spectral features, which could then be traced to the effect of the Boltzmann factor, krmBTk_{\\rm{B}}TkrmB​T, on the atomic populations.
Optical pumping is fundamental to high-precision measurement using thermal alkali-metal atoms in vapor cells. In applications such as atomic magnetometry, buffer gases (e.g., N2\mathrm{N}_2N2​ or He\mathrm{He}He) at specific pressures are introduced to quench fluorescence and mitigate wall relaxation. In the high-pressure limit (e.g., the N2\mathrm{N}_2N2​ pressure pN2>1p_{\mathrm{N}_2}> 1pN2​​>1~atm), where collisional broadening exceeds hyperfine splittings of the atoms, optical pumping theory provides a clear description of the angular momentum exchange between photons and atomic spins. However, in many magnetic sensing scenarios, the high-pressure approximation becomes inadequate as its pressure conditions are not strictly satisfied. Consequently, an explicit description of optical pumping under realistic pressures is critical for selecting operating points and enhancing system performance. To address this, we develop a unified theoretical framework of optical pumping in the quasi-high-pressure regime, where collisional broadening is comparable to the ground-state hyperfine splitting. We demonstrate that light absorption, spin polarization, and magnetic-resonance linewidth in this regime differ significantly from those predicted by the high-pressure limit and offer favorable operating conditions. Our study extends conventional modeling and offers critical guidance for atomic magnetometry operating under realistic buffer gas pressures.
This thesis presents both a computational and experimental investigation into light propagation in hot alkali-metal vapours, with a particular focus on utilizing the Faraday effect for practical applications. A model to calculate various spectra for a weak-probe laser beam in an atomic medium with an applied axial magnetic field is presented. A computer program (ElecSus) was developed which implements this model efficiently. Using ElecSus we design optical devices such as Faraday filters and laser frequency stabilizing references. The design of Faraday filters utilizing compact vapour cells is shown, along with excellent agreement with experiment. The importance of including the effect of self broadening in the model is shown for these short path length vapour cells. Also, a Faraday filter is presented that can potentially be used for quantum optics experiments on the caesium D1_11​ line (894~nm). The filter displays the highest ratio of transmission to equivalent noise bandwidth to date for a linear Faraday filter, demonstrating the power of computerized optimization for this application. Furthermore, a Faraday filter is presented for use as an intra-cavity element in an external-cavity diode laser. A proof-of-principle experiment is demonstrated which shows that using a short external cavity with the Faraday filter eliminates mode-hops. Experimentally and theoretically the Faraday effect is investigated in large magnetic fields where alkali-metal atoms enter the hyperfine Paschen-Back regime. This hyperfine Paschen-Back Faraday effect is shown to allow a direct measure of the refractive indices for left and right circular polarized light. Furthermore, fitting the weak-probe spectra using ElecSus is found to give measures of the magnetic field with a fractional precision of the order of 10−410^{-4}10−4. In addition we study slow-light pulse propagation in a high density rubidium vapour, showing that our theoretical model for the electric susceptibility is valid for short pulses as well as continuous-wave light. This shows that the model is accurate for predicting weak-probe pulse propagation.
We present a detailed spectroscopic investigation of a thermal 87 Rb atomic vapour in magnetic fields up to 0.4 T in the Voigt geometry. We fit experimental spectra with our theoretical model ElecSus and find excellent quantitative agreement, with RMS errors of ∼0.3%. We extract the magnetic field strength and the angle between the polarisation of the light and the magnetic field from the atomic signal and find excellent agreement to within ∼1% with a commercial Hall probe. Finally, we present an investigation of the relative sensitivity of this technique to variations in the field strength and angle with a view to enabling atom-based high-field vector magnetometry.