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Key engineering materials

Combining Micro-Nanotechnology with Atomic Spin Devices

Hai Feng Dong, Binquan Zhou

Published Jul 1, 2013• 0 citations
Publisher

Abstract

In recent year, the sensitivities of atomic spin devices are improved greatly with the realization of spin exchange relaxation free (SERF) regime. Usually the SERF regime is realized using orthogonal beams scheme, i.e. one pump beam to polarize the atoms and the other orthogonal probe beam to measure the polarization. Due to the requirement of four optical windows for the atomic vapor cell, the orthogonal beams scheme has difficulties for micro fabrication. In this paper, we research a new scheme for SERF realization using only one beam, which facilitates the micro fabrication greatly. Furthermore, the fabrication processes of the MEMS atomic vapor cell with two out-of-plane optical windows are designed and performed. In the end, the possibility of increasing the relaxation time by nanotechnology is discussed.

DOI
10.4028/www.scientific.net/kem.562-565.1088

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In recent year, the sensitivities of atomic spin devices are improved greatly with the realization of spin exchange relaxation free (SERF) regime. Usually the SERF regime is realized using orthogonal beams scheme, i.e. one pump beam to polarize the atoms and the other orthogonal probe beam to measure the polarization. Due to the requirement of four optical windows for the atomic vapor cell, the orthogonal beams scheme has difficulties for micro fabrication. In this paper, we research a new scheme for SERF realization using only one beam, which facilitates the micro fabrication greatly. Furthermore, the fabrication processes of the MEMS atomic vapor cell with two out-of-plane optical windows are designed and performed. In the end, the possibility of increasing the relaxation time by nanotechnology is discussed.
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.
Abstract The chip-scale hybrid optical pumping spin-exchange relaxation-free (SERF) atomic magnetometer with a single-beam arrangement has prominent applications in biomagnetic measurements because of its outstanding features, including ultrahigh sensitivity, an enhanced signal-to-noise ratio, homogeneous spin polarization and a much simpler optical configuration than other devices. In this work, a miniaturized single-beam hybrid optical pumping SERF atomic magnetometer based on a microfabricated atomic vapor cell is demonstrated. Although the optically thin Cs atoms are spin-polarized, the dense Rb atoms determine the experimental results. The enhanced signal strength and narrowed resonance linewidth are experimentally proven, which shows the superiority of the proposed magnetometer scheme. By using a differential detection scheme, we effectively suppress optical noise with an approximate five-fold improvement. Moreover, the cell temperature markedly affects the performance of the magnetometer. We systematically investigate the effects of temperature on the magnetometer parameters. The theoretical basis for these effects is explained in detail. The developed miniaturized magnetometer has an optimal magnetic sensitivity of 20 fT/Hz 1/2 . The presented work provides a foundation for the chip-scale integration of ultrahighly sensitive quantum magnetometers that can be used for forward-looking magnetocardiography (MCG) and magnetoencephalography (MEG) applications.
Compared with superconducting quantum interference devices, chip-scale atomic magnetometers using the wafer-level micro alkali vapor cells can be used more conveniently in the applications of geophysical mapping, health care and remote monitoring. Drastic wall collisions and decreased polarization lifetime are the main barriers for the performance of the chip-scale atomic magnetometers. In this study, OTS organic films coated on the inner surface of the micro vapor cells were used to suppress wall collisions and improve polarization lifetime of the alkali atoms in the micro cells. First of all, the fabrication process of wafer-level micro OTS coated vapor cells is introduced, the process is based on anodic bonding technology. Then the prepared wafer-level micro OTS coated vapor cells have been characterized by the water-contact angle test and measured in the micro-machined spin-exchange relaxation free(SERF) atomic magnetometers. Results show that the OTS coating is effective after anodic bonding from the water contact angle test. Results also indicate that the spin exchange relaxation free atomic magnetometer using the micro OTS-coated vapor cell can operate successfully.
The ultra-sensitivity magnetometer plays an important role in fundamental physics and applications, including magnetoencephalography (MEG) and magnetocardiography (MCG). The spin-exchange relaxation-free (SERF) atomic magnetometer (AM) has great sensitivity but is limited by the relatively complicated dual-beam structure for application, leading to the development of the compact AM based on one single laser beam. Compared with the single-beam optically pumped magnetometer (OPM) detecting the absorption of the circularly polarized beam, the SERF AM based on an elliptically polarized single beam detecting the optical rotation angle of polarization has a better response, lower laser power, and no need for signal modulation or demodulation. Here, we would like to report an elliptically-polarized single-beam SERF AM enhanced by reflecting the probe beam, increasing the light-atom interaction. Moreover, unlike the previous reflecting AM using the beam splitter, we used a D-shape mirror to extract the reflected beam, avoiding power losses of both incident and reflected beam and destruction of polarization. The sensitivity of our device in the reflecting scheme reaches 5 fT/Hz1/2 at 9–60 Hz, satisfying the need to measure the human MEG/MCG or search for the fifth force. Our research paves the way for developing the compact SERF AM for multiple future applications.
The designs of microfabricated alkali vapor cells play an important role when the atomic magnetometers are developing toward on-chip integration. This paper presents microfabricated spherical alkali vapor cells with improved sensitivity for chip-scale atomic magnetometers. A microfabricated spherical rubidium vapor cell and a traditional microfabricated planar rubidium vapor cell are designed for comparison, which are of the same volume and interconnected by a microchannel to acquire equal inner buffer gas pressure. In chip-scale spin-exchange relaxation-free (SERF) atomic magnetometers, the rubidium polarization lifetimes of three pairs of spherical and planar vapor cells at different buffer gas pressures of 0.35, 0.73, and 2.00 amg are measured, respectively. The sensitivity of SERF atomic magnetometers using the vapor cells at a 0.35-amg buffer gas pressure is also measured. The results show that the transverse polarization lifetime of the spherical vapor cell is 2.46, 1.44, and 1.05 times that of the planar one at 0.35-, 0.73-, and 2.00-amg buffer gas pressures, respectively. Combining the improved polarization lifetime and increased optical path length, chip-scale SERF atomic magnetometer using the microspherical vapor cell has the sensitivity of 60 fT/Hz1/2from 5 to 15 Hz, while that based on the planar one is demonstrated with the sensitivity of 400 fT/Hz1/2from 5 to 15 Hz.
High sensitive SERF(spin-exchange-relaxation-free) atomic magnetometers play an important role in many fields including geophysical mapping, space science, nuclear magnetic resonance, health care and perimeter and remote monitoring. This paper presents a novel approach for the system integration of a SERF atomic magnetometer. First of all, Rb vapor cell integrated with micro heater and Helmholtz coils was selected as physic part of the system and the vapor cell was characterized by pressure broadening effect to analyze nitrogen buffer gas pressure in the vapor cell. Then optical module including laser, micro-lens to collimate light beam emitting from the laser, quarter-wave plate to circularly polarize laser beam was taken as optical pumping and probing source simultaneously, and the interaction of circularly polarized laser beam with rubidium atoms was detected by silicon PIN photo-detectors. At last, Laser module, physics part of the system and silicon PIN photo-detector were integrated on a glass wafer substrate benefiting from the characteristics of non-magnetic material and good flatness, based on the integrated SERF atomic magnetometer, weak magnetic field could be measured. Results demonstrate that the SERF atomic magnetometer is integrated successfully and could be used for the detection of weak magnetic field.