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Applied Physics Letters

Evanescent wave magnetometer

Kaifeng Zhao, Z. Wu

Published Dec 25, 2006• 25 citations
Publisher

Abstract

The authors describe an atomic magnetometer, the evanescent wave magnetometer, which uses an evanescent wave to measure the Larmor frequency of Rb atoms near the cell surface. The submicron penetration depth of the probe beam allows the evanescent wave magnetometer to achieve a spatial resolution of several tens of microns or better, and greatly reduces the inhomogeneous broadeningdue to magnetic field inhomogeneities. Its noise density in the present experiment is ∼100pT∕Hzfor frequencies between 1 and 10mHz, and decreases to less than 10pT∕Hz as the frequency increases to 25Hz.

DOI
10.1063/1.2424657

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arXiv:1208.1236•Applied Physics Letters•Aug 20, 2012•45 citations
The authors describe an atomic magnetometer, the evanescent wave magnetometer, which uses an evanescent wave to measure the Larmor frequency of Rb atoms near the cell surface. The submicron penetration depth of the probe beam allows the evanescent wave magnetometer to achieve a spatial resolution of several tens of microns or better, and greatly reduces the inhomogeneous broadeningdue to magnetic field inhomogeneities. Its noise density in the present experiment is ∼100pT∕Hzfor frequencies between 1 and 10mHz, and decreases to less than 10pT∕Hz as the frequency increases to 25Hz.
The authors demonstrate a novel atomic magnetometer that uses differential detection of the spatially diverging components of a light field to monitor the Larmor precession frequency of atoms in a thermal vapor. The design is implemented in compact form with a micromachined alkali vapor cell and a naturally divergent light field emitted by a vertical-cavity surface-emitting laser. Operating the magnetometer in differential mode cancels common-mode noise and improves the sensitivity by a factor of 26 over single-channel operation. They also suggest ways in which the current sensitivity of 28pT∕√Hz may be improved further without sacrificing size or simplicity.
It is shown experimentally that use of fluorescence and transmission spectra obtained from nanocells with the thickness of column of rubidium atomic vapor L = λ/2 and L = λ, respectively (λ = 794 nm is the wavelength of laser radiation close to resonance with D 1-line transition of Rb atoms), by means of a narrowband diode laser allows spectral separation and study of variations of probabilities of atomic transitions between ground and excited states of hfs of D 1 lines of 85Rb and 87Rb atoms in the range of magnetic fields from 10 to 5000 G. Small thickness of atomic vapor column (∼390 nm and ∼794 nm) allows applying permanent magnets simplifying essentially creation of strong magnetic fields. Advantages of this technique are discussed as compared with the technique of saturated absorption. The obtained results show that a nanocell with submicrom thickness of vapor column may serve as a basis for designing a magnetometer with submicron local spatial resolution which is important in case of measuring strongly inhomogeneous magnetic fields. Experimental data are in good agreement with the theoretical results.
This paper presents a novel method to measure the magnetic gradient by an Mz atomic magnetometer using a micro-fabricated shaped rubidium vapor cell. The shaped vapor cell is fabricated by a chemical foaming process. Four micro radio frequency (RF) coils are mounted on the shaped rubidium vapor cell. Magnetic fields at the four points corresponding to the RF coils along the laser beam propagation are obtained by measuring Larmor frequency of the rubidium atoms inside the vapor cell. The magnetic gradients are also acquired with the measured magnetic fields. The measurement is performed in a magnetic shield environment. A dc electrical coil is taken to produce an existed magnetic field for the measurement and the theoretical magnetic field distribution is calculated. It is demonstrated that the measured magnetic fields and magnetic gradients correspond well with the calculated ones. Results show that the absolute error of the magnetic field is less than 0.1 μT, the relative error of the magnetic field is below 0.8%, and the relative error of the magnetic gradient is below 5.5%. Results also indicate that the atomic magnetometer has a sensitivity of 5 pT/Hz1/2from 1 to 2 Hz. The Mz atomic magnetometer is potentially useful for applications requiring gradient field information.
Summary form only given. Evanescent wave atomic mirrors have been shown to be very sensitive to the surface roughness of the glass substrate on which the evanescent wave is formed. Non-specular reflection of slow atoms at normal incidence has been observed for surface roughness as low as 0.1 nm. We interpret this behaviour in terms of diffraction of the atomic De Broglie wave on the modulated potential created by the interference of the evanescent wave with light scattered by the rough surface. However these experiments could not separate the specular peak of reflected atoms from the diffuse part. We have developed a new method to measure the velocity distribution of the atoms after the bounce with a much better resolution, using Raman transitions. Two phase locked counterpropagating laser beams induce a Raman transition between the ground state hyperfine levels of the /sup 85/Rb atoms falling from a magneto-optical trap. The resonance condition is only fulfilled for atoms with a transverse velocity controlled by the interaction time between the atoms and the beams. These velocity selected atoms bounce on the mirror and their velocity distribution after reflection can be measured using the same Raman beams, by scanning their difference in frequency.
Atomic magnetometry was performed at Earth’s magnetic field over a free-space distance of ten meters. Two laser beams aimed at a distant alkali-vapor cell excited and detected the 87Rb magnetic resonance, allowing the magnetic field within the cell to be interrogated remotely. Operated as a driven oscillator, the magnetometer measured the geomagnetic field with ≲3.5 pT precision in a ∼2 s data acquisition; this precision was likely limited by ambient field fluctuations. The sensor was also operated in self-oscillating mode with a 5.3 pT/Hz noise floor. Further optimization will yield a high-bandwidth, fully remote magnetometer with sub-pT sensitivity.