Two-photon electromagnetic induction imaging with an atomic magnetometer

Abstract

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.