We demonstrate imaging of ferromagnetic carbon steel samples and we detect the thinning of their profile with a sensitivity of 0.1 mm using a Cs radio-frequency atomic magnetometer. Images are obtained at room temperature, in magnetically unscreened environments. By using a dedicated arrangement of the setup and active compensation of background fields, the magnetic disturbance created by the samples' magnetization is compensated. Proof-of-concept demonstrations of non-destructive structural evaluation in the presence of concealing conductive barriers are also provided. Relevant impact for steelwork inspection and health and usage monitoring without disruption of operation is envisaged, with direct benefit for industry, from welding in construction, to pipelines inspection and corrosion under insulation in the energy sector.

Imaging of structural defects in a material can be realized with a radio-frequency atomic magnetometer by monitoring the material's response to a radio-frequency excitation field. We demonstrate two measurement configurations that enable the increase of the amplitude and phase contrast in images that represent a structural defect in electrically conductive and magnetically permeable samples. Both concepts involve the elimination of the excitation field component, orthogonal to the sample surface, from the atomic magnetometer signal. The first method relies on the implementation of a set of coils that directly compensates the excitation field component in the magnetometer signal. The second takes advantage of the fact that the radio-frequency magnetometer is not sensitive to the magnetic field oscillating along one of its axes. Results from simple modelling confirm the experimental observation and are discussed in detail. Published under license by AIP Publishing. https://doi.Published under license by AIP Publishing.FIG. 3. The modelled change in the signal phase (dotted black line) and the amplitude (solid red line) of the magnetic resonance signal over a recess recorded by a magnetometer for various amplitudes of the primary field components. The vertical axis of the image array represents changes in the vertical component, while the horizontal axis represents changes in the horizontal component of the primary field. Amplitude is expressed in units of b y,max .Journal of Applied Physics ARTICLE scitation.org/journal/jap

Imaging of structural defects in a material can be realized with a radio-frequency atomic magnetometer by monitoring the material's response to a radio-frequency excitation field. We demonstrate two measurement configurations that enable the increase of the amplitude and phase contrast in images that represent a structural defect in highly electrically conductive and high magnetic permeability samples. Both concepts involve the elimination of the excitation field component, orthogonal to the sample surface, from the atomic magnetometer signal. The first method relies on the implementation of a set of coils that directly compensates the excitation field component in the magnetometer signal. The second takes advantage of the fact that the radio-frequency magnetometer is not sensitive to the magnetic field oscillating along one of its axes. Results from simple modelling confirm the experimental observation and are discussed in detail.

Knowing, and controlling, the spatial polarization distribution of a beam is of importance in applications such as optical tweezing, imaging, material processing and communications. Here we show how the polarization distribution is affected by both linear and nonlinear (self-focussing) propagation. We derive an analytical expression for the polarization rotation of fully-structured light (FSL) beams during linear propagation and show that the observed rotation is due entirely to the difference in Gouy phase between the two eigenmodes comprising the FSL beams, in excellent agreement with numerical simulations. We also explore the effect of cross-phase modulation due to self-focusing (Kerr) nonlinearity and show that polarization rotation can be controlled by changing the eigenmodes of the superposition, and physical parameters such as the beam size, the amount of Kerr nonlinearity and the input power. Finally, we show that by biasing cylindrical vector (CV) beams to have elliptical polarization, we can vary the polarization state from radial through spiral to azimuthal using nonlinear propagation.

Radio-frequency atomic magnetometers offer attractive alternatives to standard detection methods in nondestructive testing, which are based on inductive measurements. We demonstrate a magnetometer in the so-called spin maser configuration, which addresses two important challenges of the technique: shifts in the radio frequency resonance position caused by magnetically permeable samples and the sensor bandwidth. Key properties of the self-oscillating sensor are presented in both a magnetically shielded and an open environment. Demonstration of defect detection via magnetic induction tomography in a ferromagnetic carbon steel sample is presented. The configuration discussed paves the way for a simple, rapid, and robust nondestructive material defect detection system based on an atomic magnetometer.

Implementation of an alkali–metal spin maser in magnetic induction tomography is explored. While the spin maser vastly improves the detection speed and solves the problem of imperfect bias magnetic field stabilization in non-destructive testing, it provides only partial information about the spatial extent of the defect. We demonstrate two ways in which the whole image of the defect can be reconstructed and experimentally demonstrate that the amplitude of the spin maser signal can be used as an indicator of defect depth. Additionally, the spatial extent of the imaging of the defect is increased by the application of a spin maser operating at two frequencies. A significant benefit of operating in the spin maser mode is that the system follows any fluctuations in the Larmor frequency due to changes in the bias magnetic field strength. This removes the need for active stabilization of the bias magnetic field, greatly reducing the complexity of the system.

The capabilities of a radio-frequency atomic magnetometer for object detection based on magnetic induction tomography are explored. The determination of object orientation is demonstrated by utilizing the measurement geometry. The self-compensation configuration of the atomic magnetometer is implemented to address the issue of saturation of the sensor response by the radio-frequency primary field that generates the object signature. Three methods of “covert” detection are investigated as a testbed for exploring the functionalities of this sensor, where (1) the operational frequency of the sensor is continuously changed, (2) the primary field has non-monochromatic frequency distribution, and (3) the sensor operates in the so-called spin maser mode. The results of the measurements are also discussed in terms of possible magnetic field communication.

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