Radio-frequency Atomic Magnetometer Research Articles

Radio-Frequency Magnetometry Based on Parametric Resonances.

We propose and demonstrate a radio-frequency (rf) atomic magnetometer based on parametric resonances. Previously, most rf atomic magnetometers are based on magnetic resonances and their sensitivities are often limited by spin-exchange relaxation. Here, we introduce a novel scheme for an rf magnetometer where the rf magnetic field is measured by exciting the parametric resonances instead of magnetic resonances using parametric modulation fields. In this way, the spin-exchange relaxation is almost eliminated. Benefiting from the low spin relaxation rate, the parametric resonance scheme exhibits a narrower linewidth and stronger signal, which results in a higher sensitivity. With a 6×6×3 mm^{3} Rb atomic vapor cell, we developed an rf atomic magnetometer with a noise floor of 2 fT/Hz^{1/2}, which is about one order of magnitude higher than the sensitivity achieved in the magnetic-resonance-based scheme. The presented rf detection scheme holds promise in advancing rf atomic magnetometers and brings new insight into their various applications.

Neural network-aided optimisation of a radio-frequency atomic magnetometer.

Efficient unsupervised optimisation of atomic magnetometers is a requirement in many applications, where direct intervention of an operator is not feasible. The efficient extraction of the optimal operating conditions from a small sample of experimental data requires a robust automated regression of the available data. Here we address this issue and propose the use of general regression neural networks as a tool for the optimisation of atomic magnetometers which does not require human supervision and is efficient, as it is ideally suited to operating with a small sample of data as input. As a case study, we specifically demonstrate the optimisation of an unshielded radio-frequency atomic magnetometer by using a general regression neural network which establishes a mapping between three input variables, the cell temperature, the pump beam power and the probe beam power, and one output variable, the AC sensitivity. The optimisation results into an AC sensitivity of 44 fT/Hz at 26 kHz.

Two-photon electromagnetic induction imaging with an atomic magnetometer

Electromagnetic induction imaging (EMI) is a contactless, nondestructive evaluation technique based on sensing the response of a target to oscillating magnetic fields as they penetrate into materials. Leveraging the enhanced performance of radio frequency atomic magnetometers (RF-AMs) at low frequencies can enable highly sensitive through-barrier EMI measurements, which, for example, can reveal concealed weaponry or inspect subsurface material defects. However, deriving this advantage requires precise control of a well-defined, low bias magnetic field with respect to the background magnetic field texture, which presents a cumbersome challenge to stabilize in real-world unshielded scenarios. Here, we implement a two-photon RF-AM scheme in a portable setup to bypass the requirement of a low bias field and achieve stable, repeatable resonances in the sub-kHz regime. The improved accessibility to lower primary field frequencies offer greater skin-depth in target materials and facilitates an enhancement of a factor of 8 in skin penetration with this portable system, detecting features behind an Al shield of 3.2 mm. The scheme also reduces the need of large compensation coils to stabilize the bias field, facilitating the implementation of compact devices.

Rapid Electromagnetic Induction Imaging With an Optically Raster-Scanned Atomic Magnetometer

We present an apparatus to overcome the limitations of mechanical raster-scanning in electromagnetic induction imaging (EMI) techniques by instead performing a 2D optical raster-scan within the vapour cell of a radio-frequency atomic magnetometer (RF-AM). A large cuboidal <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">87</sup> Rb vapour cell is employed to act as the medium of an RF-AM with the pump and probe beams translated in the cell via acousto-optics. The technique is shown to give robust and repeatable magnetic measurements over the cell volume and successfully resolves conductive targets with EMI. Optical raster-scanning removes the limitation of slow mechanical actuation and a fast imaging procedure is enacted resolving conductive targets at a rate of 40 ms/pixel.

High-sensitivity operation of an unshielded single cell radio-frequency atomic magnetometer: erratum.

An erratum is presented to include the right vertical scale and label in Fig.2 and Fig.7, which were omitted in our published manuscript ["High-sensitivity operation of an unshielded single cell radio-frequency atomic magnetometer" Opt. Express30 (23), 42015 (2022) 10.1364/OE.476016].

Different Configurations of Radio-Frequency Atomic Magnetometers-A Comparative Study.

We comprehensively explore different optical configurations of a radio-frequency atomic magnetometer in the context of sensor miniaturisation. Similarities and differences in operation principles of the magnetometer arrangements are discussed. Through analysis of the radio-frequency and noise spectra, we demonstrate that all configurations provide the same level of atomic polarisation and signal-to-noise ratio, but the optimum performance is achieved for significantly different laser powers and frequencies. We conclude with possible strategies for system miniaturisation.

High-sensitivity operation of an unshielded single cell radio-frequency atomic magnetometer

Real-world applications of atomic magnetometers require the ability to operate them with high-sensitivity in the presence of magnetic noise. In the present work, high-sensitivity operation of unshielded atomic magnetometers in a magnetically noisy environment is demonstrated. The distinguishing feature of the demonstrated approach is the implementation of active in-situ bias field stabilization using multiple fluxgate magnetometers. This is combined with the use of a counter-propagating pump and triple-pass probe configuration, to maximize the atomic polarization and the probe rotation respectively, so to reach high-sensitivity. The improvement in sensitivity of the unshielded system with respect to previous realizations is fully characterized, with the contributions of the different modifications of the apparatus individually quantified. The presented set-up is suitable for the detection of long-range magnetic fields, where shielding or differential measurements using multi-sensor set-ups do not constitute viable options.

Optimisation of a radio-frequency atomic magnetometer: a Uniform Design approach.

High-sensitivity operation of a radio-frequency atomic magnetometer (RF-AM) requires careful setting of the system parameters, including the lasers intensity and detuning, and the vapour cell temperature. The identification of the optimal operating parameters, which ensures high sensitivity, is typically performed empirically and is often a lengthy process, which is especially labour intensive if frequent retuning of the magnetometer is required to perform different tasks. This paper demonstrates an efficient approach to RF-AM performance optimisation which relies on an open-loop optimisation technique based on Uniform Design (UD). This paper specifically describes the optimisation of an unshielded RF-AM based on a 4-factor-12-level UD of the experimental parameters space. The proposed procedure is shown to lead to the efficient optimisation of the atomic magnetometer at different frequencies, and is applicable to both AC and DC sensitivity optimisation. The procedure does not require any detailed knowledge of the model underlying the operation of the RF-AM and is effective in reducing the number of experimental runs required for the optimisation. It is ideally suited to self-calibration of devices without human supervision.

Object detection with an alkali-metal spin maser

We present magnetic induction tomography measurements performed with a radio-frequency atomic magnetometer operating in the spin maser mode. We discuss different regimes of spin maser operation that are defined by the gain within the system’s feedback, i.e., below threshold, unsaturated, and saturated gain. Two configurations are explored: first, where an rf field continuously drives the spin maser operation and the object’s presence changes the gain in the system’s feedback loop, and the second, where the spin maser operation is triggered and sustained directly by the object response. The advantages and disadvantages of each configuration are discussed in the context of object detection.

Identification of object composition with magnetic inductive tomography.

The inductive response of an object to an oscillating magnetic field reveals information about its electrical conductivity and magnetic permeability. Here, we introduce a technique that uses measurements of the angular, frequency, and spatial dependence of the inductive signal to determine the object composition. Identification is performed by referencing an object's inductive response to that of materials with mutually exclusive properties such as copper (high electrical conductivity and negligible magnetic permeability) and ferrite (negligible electrical conductivity and high magnetic permeability). The technique uses a sensor with anisotropic sensitivity to discriminate between the different characters of the eddy current and magnetization driven object responses. Experimental validation of the method is performed using magnetic induction tomography measurement with a radio-frequency atomic magnetometer. Possible applications of the technique in security screening devices are discussed.

Optimization of a Radio-frequency Atomic Magnetometer Toward Very Low Frequency Signal Reception

We describe a single-channel rubidium (Rb) radio-frequency atomic magnetometer (RFAM) as a receiver that takes magnetic signal resonating with Zeeman splitting of the ground state of Rb. We optimize the performance of the RFAM by recording the response signal and signal-to-noise ratio (SNR) in various parameters and obtain a noise level of 159 fT/Hz around 30 kHz. When a resonant radio-frequency magnetic field with a peak amplitude of 8.0 nT is applied, the bandwidth and signal-to-noise ratio are about 650 Hz and 88 dB, respectively. It is a good agreement that RFAM using alkali atoms is suitable for receiving signals in the very low frequency (VLF) carrier band, ranging from 3 kHz to 30 kHz. This study shows the new capabilities of the RFAM in communications applications based on magnetic signals with the VLF carrier band. Such communication can be expected to expand the communication space by overcoming obstacles through the high magnetic sensitive RFAM.

Inductive Imaging of the Concealed Defects with Radio-Frequency Atomic Magnetometers

We explore the capabilities of the radio-frequency atomic magnetometers in the non-destructive detection of concealed defects. We present results from the systematic magnetic inductive measurement of various defect types in an electrically conductive object at different rf field frequencies (0.4–12 kHz) that indicate the presence of an optimum operational frequency of the sensor. The optimum in the frequency dependence of the amplitude/phase contrast for defects under a 0.5–1.5 mm conductive barrier was observed within the 1–2 kHz frequency range. The experiments are performed in the self-compensated configuration that automatically removes the background signal created by the rf field producing object response.

Role of the probe beam in a radio-frequency atomic magnetometer.

We explore the role of the linearly polarized probe beam in the radio-frequency atomic magnetometer. Two regimes of coupling of the linearly polarized light to the atomic ensemble, near and off-resonant, are demonstrated and discussed. Our studies cover three types of interaction between the atomic spin system and the linearly polarized beam, i.e., absorptive production of spin polarization, dispersive spin state readout, and nonlinear spin couplings. The work is performed with the outlook of optimization and simplification of magnetometer operation. Operation of a magnetometer with a single beam, generating and probing the atomic orientation, is presented. This single-beam configuration, which combines optical pumping, nonlinear spin couplings, and spin-exchange collisions, creates the option for a portable device with simple instrumentation.

Object surveillance with radio-frequency atomic magnetometers.

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.

Generation of atomic spin orientation with a linearly polarized beam in room-temperature alkali-metal vapor

Traditionally, atomic spin orientation is achieved by the transfer of angular momentum from polarised light to an atomic system. We demonstrate the mechanism of orientation generation in room-temperature caesium vapours that combines three elements: optical pumping, non-linear spin dynamics and spin-exchange collisions. Through the variation of the spin-exchange relaxation rate, the transition between an aligned and an oriented atomic sample is presented. The observation is performed by monitoring the atomic radio-frequency spectra. The measurement configuration discussed, paves the way to simple and robust radio-frequency atomic magnetometers that are based on a single low power laser diode that approach the performance of multi-laser pump-probe systems.

Alkali-metal spin maser for non-destructive tests

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.

Enhanced material defect imaging with a radio-frequency atomic magnetometer

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.

A cold atom radio-frequency magnetometer

We propose and demonstrate a radio-frequency atomic magnetometer with sub-Doppler laser cooled rubidium-87. With a simple and compact design, our system demonstrates a sensitivity of 330 pT/Hz in an unshielded environment, thus matching or surpassing previously reported cold atom designs. By merging the multiple uses and the robustness of radio-frequency atomic magnetometers with the detailed control of laser cooling, our cold atom radio-frequency magnetometer has potential for moving applications of atomic magnetometry to high spatial resolutions. A direct impact in metrology for applied sciences, materials characterization, and nanotechnology can be anticipated.

Field-Polarization Sensitivity in rf Atomic Magnetometers

We demonstrate the sensitivity of a sensor based on an optically-pumped radio-frequency (RF) atomic magnetometer to the polarization state of the detected RF magnetic field, and measure $>$36 dB difference in amplitude sensitivity for opposite circular field polarizations. This sensitivity could be used to create novel sensors that would allow signal detection while suppressing the ambient noise, regardless of the distance between the sources of signal and noise, in contrast to traditional gradiometry configurations. Additionally, such sensor will be orientation-sensitive, as the phase of the detected signal is shown to depend on the angle between the sensor's detection axis and the direction to the transmitter.

Imaging of material defects with a radio-frequency atomic magnetometer.

Non-destructive inductive testing of defects in metal plates using the magnetic resonance signal of a radio-frequency atomic magnetometer is demonstrated. The shape and amplitude of the spatial profile of the signal features, which represent structural defects, are explored. By comparing numerical and experimental results on a series of benchmark aluminium plates, we show correspondence between the properties of the secondary field and those of the magnetometer signal. In particular, we show that two components of the secondary field are mapped onto the amplitude and phase of the atomic magnetometer signal. Hence, a magnetic field measurement with the atomic magnetometer, although scalar in its nature, provides semi-vectorial information on the secondary field. Moreover, we demonstrate a robust process for determining defect dimensions, which is not limited by the size of the sensor. We prove that the amplitude and phase contrast of the observed profiles enables us to reliably measure defect depth.