A journey through nonlinear magneto-optics
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Abstract
This thesis contributes to the research on nonlinear magneto-optical effects, specifically focusing on atomic magnetometry based on nonlinear magneto-optical rotation and mirrorless lasing (based on amplified spontaneous emission) – while encountering also other nonlinear effects, such as the Kerr effect.
Measuring magnetic fields has been of great importance since the bronze age and has been crucial to the development of the human civilization: the first magnetometer was the compass, a magnetometer that measures only the direction of the earth’s magnetic field and is so important that it has a place amongst the four great inventions of ancient China. Measuring the magnitude of a magnetic field is a more modern story that starts with Carl Friedrich Gauss measuring the Earth’s magnetic field in 1833.
Among the techniques existing nowadays, for precisely measuring magnetic fields, optically pumped magnetometry (OPM) stands out for its sensitivity, size, robustness and low cost. After the pioneers set the foundations more than half a century ago, diode laser technology allowed optical magnetometers to become a workhorse for magnetometry. OPM magnetometers are potentially as sensitive as SQUIDs (Superconducting Quantum Interference Device) and do not require cryogenics. Applications span over a wide range of fields: geophysics, bio-magnetic measurements and fundamental physics. OPM research in recent years has shifted from working in the laboratory to applications in the field and a useful step towards commercialization is the self-oscillating configuration. The basic operation principle is based on using the detected signal to sustain continuous oscillation at the resonant frequency. Such systems have a broad dynamic range, can follow field fluctuations and are simple.
Although OPMs and especially SERF (spin exchange relaxation-free) type magnetometers are highly sensitive, they need to operate in low fields and hence require magnetic shielding from the Earth’s field and other noise sources. Being able to measure in the geophysical field range or earth field with high sensitivity could open the path to low-cost bio-magnetic measurements, space-magnetometry, non-destructive testing and imaging and magnetometry on rapidly moving platforms.
This thesis focuses on Earth-field optical magnetometry and addresses challenges arising from the Earth’s magnetic field by using techniques like spin locking or creating a device free of classic Earth-field magnetometry issues, such as heading error.
The second part of this thesis is dedicated to mirrorless lasing. Since their invention in the 60s, lasers (light amplification by stimulated emission of radiation) have played a huge role in many areas of scientific research, industry, and everyday life and continue to grow. There are three principal components usually attributed to a laser: a gain medium, a pumping process and a feedback loop, although there is a debate over whether a feedback loop is always required. Lasing is often distinguished from processes such as Amplified Spontaneous Emission (ASE), Superradiance (SR) and Superflouorescence (SF), but this is not the case for the work presented here. Typically, lasers follow a conventional structure that includes an optical resonator setup. This setup uses mirrors to amplify light over multiple round trips in the gain medium. In mirrorless laser setups, the gain
medium serves as the resonator, and the feedback loop would typically happen through multiple scattering processes in systems with varying degrees of disorder. Optical feedback through scattering can also create random lasers. The system we are studying does not involve scattering mechanisms and we use the term lasing interchangeably with Amplified Spontaneous Emission (ASE). We define mirrorless lasing as as directed monochromatic emission from an ensemble of atoms or molecules
pumped with a laser light. Experiments in alkali metal vapor have shown gain through the
phenomenon of amplified spontaneous emission (ASE). This thesis focuses on the phenomenon of amplification of spontaneous emission and degenerate mirrorless lasing in alkali atoms with magnetically degenerate hyperfine states.