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Nonlinear Magneto-Optical Rotation in Rubidium Vapour

Nathanial Wilson

Published Jan 1, 2020• 0 citations
OpenAlex

Abstract

The field of magnetometry has played a pivotal role in developing our understanding of the world around us, by providing a wealth of unique information via non-invasive magnetic measurements. In many applications the characteristics of magnetic fields — their strength and temporal dependence, for example — constitute a unique fingerprint of their source, and provide a window into both their origin and any underlying physical processes that produced them. The invention of the laser, and recent advancements in anti-relaxation coatings for vapour cells, has revolutionised the field of magnetometry and given rise to a myriad of optical magnetometers which demonstrate exquisite sensitivity. This has seen a movement away from traditional cryogenic magnetometry techniques — which utilise superconducting quantum interference devices — and has ushered in a slew of optical techniques which offer a whole host of benefits such as room-temperature operability, intrinsic accuracy, and portability. Applications which were previously inaccessible due to demanding requirements such as high sensitivity at ambient temperatures, are now permissible using these newly developed optical techniques. This thesis details the development of an ultrastable optical magnetometer based on nonlinear magneto-optical rotation in rubidium vapour, that has been optimised for high sensitivity over long timescales. Additionally, a novel measurement technique — which relies on tracking the instantaneous phase of atomic spin precession — has been developed and demonstrated, resulting in significant improvements to both the amplitude and frequency response of the device when compared to conventional measurement techniques described in the literature. Significant attention is paid to technical and fundamental noise sources, which must be minimised in order to achieve high performance — especially over long timescales. The latter chapters discuss the development of the magnetometer into a sensitive and reliable system, capable of measuring extremely small magnetic-field fluctuations, and transient fields with arbitrarily complex temporal dependence. The capability of the device to measure minute field fluctuations, as well as rapid transient disturbances, may find use in real-world applications such as geophysical exploration, medical diagnostics and imaging, and magnetic anomaly detection. Furthermore, any fundamental physics applications which demand accurate and precise measurements of magnetic fields may find application of this device — and the novel measurement techniques developed within this thesis—extremely beneficial.

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The field of magnetometry has played a pivotal role in developing our understanding of the world around us, by providing a wealth of unique information via non-invasive magnetic measurements. In many applications the characteristics of magnetic fields — their strength and temporal dependence, for example — constitute a unique fingerprint of their source, and provide a window into both their origin and any underlying physical processes that produced them. The invention of the laser, and recent advancements in anti-relaxation coatings for vapour cells, has revolutionised the field of magnetometry and given rise to a myriad of optical magnetometers which demonstrate exquisite sensitivity. This has seen a movement away from traditional cryogenic magnetometry techniques — which utilise superconducting quantum interference devices — and has ushered in a slew of optical techniques which offer a whole host of benefits such as room-temperature operability, intrinsic accuracy, and portability. Applications which were previously inaccessible due to demanding requirements such as high sensitivity at ambient temperatures, are now permissible using these newly developed optical techniques. This thesis details the development of an ultrastable optical magnetometer based on nonlinear magneto-optical rotation in rubidium vapour, that has been optimised for high sensitivity over long timescales. Additionally, a novel measurement technique — which relies on tracking the instantaneous phase of atomic spin precession — has been developed and demonstrated, resulting in significant improvements to both the amplitude and frequency response of the device when compared to conventional measurement techniques described in the literature. Significant attention is paid to technical and fundamental noise sources, which must be minimised in order to achieve high performance — especially over long timescales. The latter chapters discuss the development of the magnetometer into a sensitive and reliable system, capable of measuring extremely small magnetic-field fluctuations, and transient fields with arbitrarily complex temporal dependence. The capability of the device to measure minute field fluctuations, as well as rapid transient disturbances, may find use in real-world applications such as geophysical exploration, medical diagnostics and imaging, and magnetic anomaly detection. Furthermore, any fundamental physics applications which demand accurate and precise measurements of magnetic fields may find application of this device — and the novel measurement techniques developed within this thesis—extremely beneficial.
Optical pumping in an alkali metal vapour produces long-standing coherences of ground state Zeeman sub-levels. These coherences produce large nonlinear magneto-optic rotation (NMOR). Techniques using optical coherences for sensitive magnetometry have been in vogue for a long time. However, the sensitivity achieved has been three orders of magnitude lower than that of a SQUID magnetometer. The NMOR technique has yielded signals having a very narrow width of a few micro-Gauss. In principle, it has been demonstrated that with NMOR magnetometers one can obtain a sensitivity matching, or better than the sensitivity of a Squid magnetometer. There are still many problems to be resolved before these magnetometers can be put to use in geomagnetic and biological applications, where small magnetic fields need to be measured.
The development of an experimental apparatus to produce Bose-Einstein condensates (BECs) of ⁸⁷Rb atoms and their application to magnetometry are discussed. Optical detection of atomic Larmor precession is a widely explored method for high-sensitivity measurements of magnetic fields. In this context, short laser/atom interaction time, atomic thermal diffusion and decoherence effects are among the main limitations. In this thesis, we overcome such problems by using spin-polarised ⁸⁷Rb ultra-cold atoms as the sensing element. After the atoms are polarised, a resonant pulse of radio-frequency excites Larmor precession, which is sensitive to external magnetic fields. By measuring the perturbations of the radio-frequency induced spin precession, information on the magnetic fields of interest. This is achieved by monitoring the polarisation plane’s rotation of a linearly polarised resonant laser probe. In the first part of this thesis, the building and optimisation of a laser-cooling set up to obtain a BEC in a hybrid trap is reported. In order to achieve the Phase Space Density (PSD) required for BEC, several different stages of trapping and cooling are necessary. Each phase has been implemented and optimised. The first step consists in the magneto-optical trap (MOT). Here a velocity dependent damping force and a spatially dependent confining force give the largest changes in PSD. Then atoms are loaded into a hybrid trap obtained by overlapping a quadrupole magnetic potential and a far detuned optical crossed dipole trap. The final stage for the condensation consists of forced evaporative cooling, both via magnetic and optical evaporation. In the second part of the thesis, a general overview of the principles of optical atomic magnetometry is provided and the advantages of using ultra-cold atoms with respect to conventional thermal vapours are discussed. The implementation, operation and a preliminary characterisation of the ultra-cold atom magnetometer are described along with the preliminary results collected. Finally, a plan for future improvements of its sensitivity is presented.
A self-oscillating magnetometer based on nonlinear magneto-optical rotation using amplitude-modulated pump light and unmodulated probe light (AM-NMOR) in 87Rb has been constructed and tested towards a goal of airborne detection of magnetic anomalies. In AM-NMOR, stroboscopic optical pumping via amplitude modulation of the pump beam creates alignment of the ground electronic state of the rubidium atoms. The Larmor precession causes an ac rotation of the polarization of a separate probe beam; the polarization rotation frequency provides a measure of the magnetic field. An anti-relaxation coating on the walls of the atomic vapor cell results in a long lifetime of 56 ms for the alignment, which enables precise measurement of the precession frequency. Light is delivered to the magnetometer by polarization-maintaining optical fibers. Tests of the sensitivity include directly measuring the beat frequency between the magnetometer and a commercial instrument and measurements of Earth's field under magnetically quiet conditions, indicating a sensitivity of at least 5 pT/νHz. Rotating the sensor indicates a heading error of less than 1 nT, limited in part by residual magnetism of the sensor.
There has been permanent interest in investigations of new magnetic sensors and their various applications (Edelstein, 2007). Last years there is a rapid progress in development of magneto-optical sensors because of their sensitivity and potential for miniaturization. Magnetometers, based on magneto-optical sensors have high sensitivity comparable to, or even surpassing this of the SQUIDs (Superconducting Quantum Interference Devices) (Kominis et al., 2003; Dang et al., 2010; Savukov, 2010; Knappe, 2010). Microfabrication of components using the techniques of Micro-Electro-Mechanical Systems (MEMS) developed for atomic clocks (Knappe, 2004) gives the opportunity for building small, low consuming, low cost and non-cryogenic (as SQUIDs) sensors (Griffith et al., 2010). Coherent optical effects can be applied for magnetic field detection and offer perspectives for development of high-precision optical magnetometers (Cox et al., 2011; Kitching et al., 2011). These magnetometers are appropriate for geomagnetic, space, nuclear and biological magnetic field measurements (cardio and brain magnetic field imaging), environmental monitoring, magnetic microscopy, investigations of fundamental physics, etc. Coherent magneto-optical resonances have many applications not only in magnetometry, but in high-resolution spectroscopy, lasing without inversion, laser cooling, ultraslow group velocity propagation of light, etc. (Gao, 2009).
This work presents new methods of optical magnetometry developed in the Center for Magneto-Optical Research at the Jagiellonian University. Using rubidium atoms contained in cm-long glass cells and amplitude-modulated laser we have recorded magneto-optical signals of extremely small widths. These signals allow measurements of magnetic fields with a sensitivity comparable to that achieved with magnetometers based on Superconducting Quantum Interference Devices (SQUIDs). Full Text: PDF References: M. Faraday,Experimental Researches in Electricity. Nineteenth Series, Trans. R. Soc. London 136, 1 (1846). [CrossRef] K. Barczak and S. Pustelny,Magnetooptic properties of special optical type D fibers, J. Phys. IV 137, 15 (2006). [CrossRef] W. Gawlik, J. Kowalski, R. Neumann, and F. Trager, Observation of the electric hexadecapole moment of free Na atoms in a forward scattering experiment, Opt. Commun. 12, 400 (1974). [CrossRef] W. Gawlik, S. Pustelny, in New Trends in Quantum Coherence and Optics, R. Drampyan Ed., 47 (New York, Nova Publishers, 2009). D. W. Gawlik, D. F. Kimball, S. M. Rochester, A. Weis, and V. V. Yashchuk,Resonant nonlinear magneto-optical effects in atoms, Rev. Mod. Phys. 74, 1153 (2002). [CrossRef] D. V.V. Yashchuk, and M. Zolotorev,Nonlinear Magneto-optic Effects with Ultranarrow Widths, Phys. Rev. Lett. 81(26), 5788 (1998). [CrossRef] D. D. F. Kimball, S. M. Rochester, V. V. Yashchuk, and M. Zolotorev,Sensitive magnetometry based on nonlinear magneto-optical rotation, Phys. Rev. A 62, 043403 (2000). [CrossRef] G. Wąsik, W. Gawlik, J. Zachorowski, and W. Zawadzki, Laser frequency stabilization by Doppler-free magnetic dichroism, Appl. Phys. B 75, 613 (2002). [CrossRef] W. Gawlik, L. Krzemien, S. Pustelny, D. Sangla, J. Zachorowski, M. Graf, A. O. Sushkov, and D. Budker, magneto-optical rotation with amplitude modulated light, Appl. Phys. Lett. 88 131108 (2006). [CrossRef] D. D. F. Kimball, V. V. Yashchuk, M. Zolotorev,Nonlinear magneto-optical rotation with frequency-modulated light, Phys. Rev. A 65, 055403 (2002). [CrossRef] J. M. Higbie, E. Corsini, and D. Robust, high-speed, all-optical atomic magnetometer, Rev. Sci. Instr. 77, 113106 (2006). [CrossRef] S. Pustelny, A. Wojciechowski, M. Kotyrba, K. Sycz, J. Zachorowski, W. Gawlik, A. Cingoz, N. Leefer, J. M. Higbie, E. Corsini, M. P. Ledbetter, S. M. Rochester, A. O. Sushkov, and D. All-optical atomic magnetometers based on nonlinear magneto-optical rotation with amplitude modulated light, Proc. of SPIE 6604, 660404 (2007). [CrossRef] S. Pustelny, A. Wojciechowski, M. Gring, M. Kotyrba, J. Zachorowski, W. Gawlik, Magnetometry based on nonlinear magneto-optical rotation with amplitude-modulated light, J. Appl. Phys. 103, 063108 (2008). [CrossRef] S. Pustelny, S. M. Rochester, D. F. Jackson Kimball, V. V. Yashchuk, D. and W. Gawlik, Nonlinear magneto-optical rotation with modulated light in tilted magnetic fields, Phys. Rev. A 74, 063420 (2006). [CrossRef]
Quantum optical sensing is a maturing field that offers the capability to achieve extraor- dinary sensitivity. Atomic magnetometers, for example, now have the capability to detect brain signals and to precisely measure Earth’s gravity field. In optical atomic magnetome- ters, the electronic spins of the atoms in vapor form are aligned in the same direction by “pumping” the ensemble using laser light at a select frequency. Any magnetic field alters the spin states, which changes the optical properties of the vapor, such as the transmission of the laser light, providing a way to measure magnetic fields. After reviewing the theory of atomic structure and atom-light interactions relevant to the research presented here, I describe in the first part of this thesis a vector magnetometry method using microwave-assisted optical pumping, in which a microwave field resonant to the ground state hyperfine splitting of 87Rb pumps an ensemble of warm rubidium vapor, in addition to a strong optical pump beam, and a weaker optical probe beam. I present theoretical details as well as preliminary data demonstrating magnetic field measurements of a DC magnetic field. The measurements were taken when the DC field was aligned at various angles relative to the microwave magnetic field alignment. I compare theoretical and experimental differences in the measurements. With new methods for quantum devices comes the need for portability. In the second part of this thesis, we demonstrate the fabrication of millimeter-sized rubidium vapor cells using silicon micro-machining techniques. This method is relatively inexpensive compared to glass-blowing methods and allows rapid, large-scale production. I describe our process flow and demonstrate the presence of rubidium atoms inside our vapor cells. To conclude, suggestions for improvement are discussed.