Chip Scale Atomic Magnetometer Based On Serf

Chip Scale Atomic Magnetometer Based on SERF Song Xing*, Dong Haifeng, Fang Jiancheng Key Lab of Fundamental Science for National Defense, School of Instrument Science and Opto-electronics Engineering Beijing University of Aeronautics and Astronautics, China

  Abstract — We are developing a new detection scheme for magnetometers based on atomic vapor confined in millimeter-scale micromachined enclosures and atomic Zeeman transitions. By adding three micro-reflective mirrors and modifying the light path arrangement, the new detection scheme effectively hurdles the main obstacle which prevents SERF from being applied in chip scale sensors, i.e. the difficulty to implement the transverse laser fields using MEMS fabrication methods. Instead of guiding the pumping laser beam parallel to the wafer plates that is not compatible with MEMS fabrication, such scheme rotates both pumping and probing laser beams, and maintains the orthogonal relationship between them. Finally, several fabrication process suited for this scheme are discussed. Keywords SERF

— Atomic Magnetometer, MEMS, Micro mirrors,

I.

INTRODUCTION

Sensitive magnetometers find a wide range of applications, from discovery of space science [1] to measurements of biological magnetic fields [2]. So far, the highest sensitivity of atomic magnetometers comes from the sensors using the spin-exchange-relaxation free (SERF) method developed by the Romalis group. The sensitivity of the technique with alkali metals has been experimentally demonstrated to be 0.54fT ⋅ Hz -1/2 with the potential for improvements to the 10-3 fT level in a large cell. Their fundamental sensitivity is due to shot-noise. δB =

1 γ nT2Vt

(1)

where n is the density of atoms, γ is their gyromagnetic ratio, T2 is the transverse spin relaxation time, V is the cell volume, and t is the measurement time. It is anticipated that a shot noise limited sensitivity for a micro-fabricated SERF magnetometer with a 1mm3 cell will be less than 0.3fT ⋅ Hz -1/2 [3]. There has been much progress recently in miniaturizing atomic vapor cell based devices such as Chip Scale Atomic Clocks (CSAC) [4] and magnetometers [5]. The techniques of Micro-Electro-Mechanical Systems (MEMS) allows the devices to be orders of magnitude smaller and consume considerably less power than conventional atom-based devices while performing well and offering huge cost savings per unit device [6]. These advantages will largely broaden the applications of atomic devices, such as remote sensing, underground surveys [7], and bio-magnetic applications [8].  

——————————————————————— This project was funded by the Major Programs of China National Space Administration (Project code: D2120060013) and the Key Programs of National Natural Science Foundation of China (Project code: 60736025). *Contacting Author: Song Xing Email: [email protected]

Unfortunately, there are several shortcomings go with the above advantages because of the limitation of micro fabrication technics, such as difficulty in applying transverse laser field which is the key point for SERF magnetometer and the volume limitation of vapor cell [5]. So far, the sensitivity of chip-scale magnetometers based on Coherent Population Trapping (CPT) and Nonlinear Magneto Optical Rotation (NMOR) can just achieve 5pT ⋅ Hz -1/2 with a 1mm3 cell [9]. A chip scale magnetometer based on SERF with a 6mm3 cell and single low-power laser described in refs [10] can achieve 70fT ⋅ Hz -1/2 , still having a long distance from the goal sensitivity of 0.3fT ⋅ Hz -1/2 (the sensitivity of chip scale atomic magnetometer based on SERF using an orthogonal pump-probe configuration). The mainly limitation is the excess amplitude noise of the laser beam. Here, we describe a new solution for arrangement scheme of MEMS atomic magnetometer based on SERF and also compare the optional fabrication processes for the micro mirrors which are all compatible with those for atomic vapor cell. II.

MECHANISM OF MAGNETIC FIELD DETECTION

When polarized alkali atoms were exposed in magnetic field, the Zeeman sublevels of the ground state were shifted in energy by proportional to B [11]. As a result, such energy level shift changed the character of interaction between polarized atoms and circularly polarized light as shown in fig. 1. 2g μ B =Γ

Fig. 1 the Zeeman sublevels of the ground state are shifted in energy by factor of gμB. This leads to a difference in resonance frequencies for left- and right-circularly polarized light (σ±).

 

Linear polarized light can be decomposed to left-circularly (σ+) and right-circularly (σ-) polarized light with the same amplitude. The difference of refractive index ( Δn = n+ − n− ) due to magnetic field B changes the phase transition between o and e light, leading to polarization angle shift as shown in fig. 2. The changed angle of linear polarization is given by

ϕ =(n+-n-) πl λ

ϕ

laser field using MEMS fabrication methods.

Signal +/PD1 Atomic Vapor Cell

πl ϕ =(n l –optical +-n-)length of the sample λ

PD2 Polarization Analyzer Bo

Vapor cell

λ/4

Polarizator ND Magnetic Field

VCSEL1 Linear Polarization

 

Fig. 2 the difference of refractive index between left and right circularly polarized light leads to the angle shift of polarization. The angle of rotation is proportional to the external magnetic field (l –optical length of the sample).

ϕ∝

2 g F μ0 B / γ rel l 2l0 1 + ( 2 g F μ0 B / γ rel )2

(2)

Where g F — Land’e factor μ0 — Bohr magneton l0 — absorption length

γ rel — relaxation rate of the atomic polarization Supposed that the angle between pumping and probing laser beams is θ . In the axis of probing, the exciting action component from pumping beam is proportional to cosθ and the energy of single photon in pumping beam. According to above analysis, the difference of refractive index ( Δn ) as well as the rotation angle of polarization plane ( ϕ ) is susceptive to energy change. As a result, for high sensitive magnetometers, it is important to maintain the orthogonal relationship between pumping and probing laser beams.

VCSEL2

Fig.3 Conventional arrangement of laser beam in micro model for magnetometer based on SERF—Pumping laser beam, Probing laser beam and vector of magnetic field are orthogonal to each other.

In order to overcome the above difficulty of conventional chip scale magnetometer based on SERF, we develop an improved scheme for this kind of sensors, which is to maintain orthogonal relationship between the pumping and probing lasers using a novel arrangement of laser beams which is compatible to MEMS fabrication. The components used in this experiment are similar to those used in the miniature atomic magnetometers described in refs [5] and [6], while two mirror plates with three micro reflective mirrors were implanted into the optical structure. The inclinations of these three mirrors were respectively designed as +54.74o , −54.74o and 80.26o as shown in Fig. 4.

III. ARRANGEMENT OF LASER BEAM The conventional micro model for magnetometer based on SERF is arranged as shown in Fig.3. The magnetometer mainly consists of alkali metal vapor cell, one micro mirror and two Vertical Cavity Surface Emitting Lasers (VCSELs). The laser from VCSEL2 reflects at the micro mirror and pumps the alkali metal vapor. At the same time, the linear polarized laser beam from VCSEL1 passes through the polarized atomic vapor, as probing laser. Such lay out completely reflects the idea of atomic magnetometer setup using MEMS fabrication and well maintains the orthogonal requirement in this kind of atomic magnetometers, while the largest difficulty of this scheme is applying the transverse

λ/4

  Fig. 4 The improved schematic of the experimental setup — The structure is similar to that of conventional ones, while the adding micro mirrors help to rotate both laser beams without breaking the orthogonal relationship.

 

Micro mirrors M1 and M3 are parallel to each other, so the probing laser beam passed through the atomic vapor cell does not change the direction, still being vertical to polarization analyzer plate. Because of micro mirror M2, the pumping laser beam can be orthogonal with the probing beam in the atomic vapor cell. The upper and lower glass flakes of vapor cell both have parallel surface and are extremely thin, so the optical path distortion can be neglected and the laser beams can be considered as passing through them without changing direction. At the same time, the structure character of monocrystalline silicon makes the angle of ±54.74o easy to be achieved. Considering the symmetry of M1 and M3, these two micro mirrors plates can be fabricated using the same procedure, just bonding with opposite direction. The schematic blocks of improved atomic magnetometer are shown in Fig.5.

speed, while it currently does not suit for mass production. Dilute wet etching is a normal process and has the ability of etching various inclination angles, while etching speed and roughness are not satisfying. The effective images of mesa structure etched by ICP and highly reflective micro mirror etched by DEM are respectively shown in Fig. 6 and Fig. 7. In order to improve the optic performance of the micro mirrors, Chemical Mechanical Planarization (CMP) can be used to post-disposing the surface [15], and the roughness can reach as low as 2~8nm.

Fig. 6 SEM cross-sectional image of a mesa structure etched using ICP [13].

  Fig. 7 SEM image of highly reflective mirror etched by DEM [14].

  Fig.5 Schematic blocks of the magnetic sensor. The components are (1) Silicon Basement; (2) VCSEL; (3) polyimide spacer; (4) optics package including (from bottom to top) a neutral density filter, polarizer, a quartz quarter wave plate, and a mirror plate; (5) vapor cell with ITO above and below it;(6) mirror plate and photodiode assembly.

IV.

DISCUSSION ON THE INCLINATION ETCHING

The inclination and surface optical smoothness of mirror with angular ±54.74o can be easily achieved using wet etch method with KOH solution. There are several methods to get the expected mirror with angular 80.26o , such as inductively coupled plasma(ICP) [12][13], Deep-etching, Electro-forming, Micro-replication (DEM) [14] and dilute wet etching. ICP etching has the ability to control the vertical and lateral etch components individually. By adjusting a combination of parameters, sidewalls can be formed, with inclination angles ranging from 25o to 40o . While the shortcoming is that the inclination angle range does not include angle 80.26o . DEM technology is a novel and integrated process. DEM has the ability to etch fine inclination surface with large size and high

V.

SUMMARY

An improved laser beams arrangement in atomic magnetometers based on SERF is presented, whose fabrication is compatible with MEMS process and rarely adds the manufacture difficulty. Such improvement makes the MEMS atomic magnetometers based on SERF feasible. The advantages of the use of compound micro mirrors as laser modifiers in such a system are higher sensitivity, when compared with setups based on CPT, NMOR and SERF with single laser, and less difficulty in micro fabrication when compared with common micro model based on SERF. It is promising that such scheme will be an effective way to fabricate high performance chip scale magnetometers. Future research will focus on the fabrication technology of micro mirrors. REFERENCES [1] H. Acuna, “Space-based magnetometers,” Rev. Sci. Instrum, vol. 73, 3717–3736, 2002. [2] L. Fagaly, “Superconducting quantum interference device instruments and applications,” Rev. Sci. Instrum, vol. 77, 101101, 2006. [3] J. Allred, T. Kornack, and M. Romalis, “high-sensitivity atomic magnetometer unaffected by spin-exchange relaxation,” Physical Review

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