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IEEE Sensors Journal

Indium Tin Oxide Non-Magnetic Heating Film for Miniaturized SERF Gradient Magnetometer

Zhi Liu, Chen Zhang, Heng Yuan, Kaifeng Yin, Yang Yang, Yeguang Yan, Yaxiang Wang, Binquan Zhou, Xiaolin Ning, Bangcheng Han

Published Apr 28, 2021• 21 citations
Publisher

Abstract

We propose non-magnetic heating film based on indium tin oxide (ITO) and apply it in a miniaturized spin exchange relaxation free (SERF) gradient magnetometer. To achieve ultrahigh sensitivity, alkali metal atoms inside the vapor cell of the magnetometer require an elevated temperature field that is stable and uniform. ITO is a transparent and electrically conductive material which is suitable for such application and can be used as resistance wire to form the heating film. A film structure composed of an insulation layer sandwiched between two ITO layers with the same optimized pattern is used to configure the heating current to double back on itself to reduce the magnetic noise caused by it. Experiments show the proposed film can be heated to over 200 °C, which is sufficient for vaporizing alkali metal atoms such as potassium, rubidium, and cesium. The magnetic current coefficients of the heating films are below 0.2 nT/mA and with the high frequency heating current, low-frequency magnetic noise can be further reduced. We compare ITO-based heating films and conventional flexible printed circuit (FPC) heating films side by side in the same gradient magnetometer and with ITO films a more uniform temperature field and better intrinsic magnetic field sensitivity of 22 fT/cm/Hz $^{1/2}$ is achieved. The proposed ITO-based non-magnetic heating film can be used not only in atomic magnetometers but also in other atomic devices such as chip scale gyroscopes and atomic clocks.

DOI
10.1109/jsen.2021.3076133

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Microsystems & Nanoengineering•Aug 30, 2024•62 citations
We propose non-magnetic heating film based on indium tin oxide (ITO) and apply it in a miniaturized spin exchange relaxation free (SERF) gradient magnetometer. To achieve ultrahigh sensitivity, alkali metal atoms inside the vapor cell of the magnetometer require an elevated temperature field that is stable and uniform. ITO is a transparent and electrically conductive material which is suitable for such application and can be used as resistance wire to form the heating film. A film structure composed of an insulation layer sandwiched between two ITO layers with the same optimized pattern is used to configure the heating current to double back on itself to reduce the magnetic noise caused by it. Experiments show the proposed film can be heated to over 200 °C, which is sufficient for vaporizing alkali metal atoms such as potassium, rubidium, and cesium. The magnetic current coefficients of the heating films are below 0.2 nT/mA and with the high frequency heating current, low-frequency magnetic noise can be further reduced. We compare ITO-based heating films and conventional flexible printed circuit (FPC) heating films side by side in the same gradient magnetometer and with ITO films a more uniform temperature field and better intrinsic magnetic field sensitivity of 22 fT/cm/Hz 1/2^{1/2}1/2 is achieved. The proposed ITO-based non-magnetic heating film can be used not only in atomic magnetometers but also in other atomic devices such as chip scale gyroscopes and atomic clocks.
Wafer-level integration of micro heater on an alkali vapor cell for chip-scale SERF magnetometers is presented. In order to obtain a suitable resistivity, polysilicon and aluminum are mixed as the target material. Magnetron sputtering is adopted to fabricate the heater on the surface of the alkali vapor cell. Various voltages are applied to the heater, of which the temperatures are measured by thermal sensitive sensors. Spin exchange relaxation free (SERF) atomic magnetometer with the heater integrated on the alkali vapor cell is characterized. Results show that the micro heater is successfully integrated on the alkali vapor cell. With the integrated heater, the rubidium vapor cell is heated up to 120°C by a 445kHz AC current. The sensitivity of the SERF magnetometer is 6.3pT/Hz1/2. Results also indicate that it provides an effective and low-cost solution for the miniaturization of atomic magnetometers.
Alkali metal atomic cells are crucial components of atomic instruments, such as atomic magnetometers, atomic gyroscopes, and atomic clocks. A highly uniform and stable heating structure can ensure the stability of the alkali metal atom density. The vapor cell of an atomic magnetometer that uses laser heating has no magnetic field interference and ease of miniaturization, making it superior to hot air heating and AC electric heating. However, the current laser heating structure suffers from low heating efficiency and uneven temperature distribution inside the vapor cell. In this paper, we designed a non-magnetic heating structure based on the laser heating principle. We studied the temperature distribution of the heating structure using the finite element method (FEM) and analyzed the conversion and transfer of laser energy. We found that the heat conduction between the vapor cell and the heating chips (colored filters) is poor, resulting in uneven temperature distribution and low heating efficiency in the vapor cell. Therefore, the addition of graphite film to the four surfaces of the vapor cell was an important improvement. This addition helped to balance the temperature distribution and improve the conduction efficiency of the heating structure. It was measured that the power of the heating laser remained unchanged. After the addition of the graphite film, the temperature difference coefficient (CVT) used to evaluate the internal temperature uniformity of the vapor cell was reduced from 0.1308 to 0.0426. This research paper is crucial for improving the heating efficiency of the non-magnetic heating structure and the temperature uniformity of the vapor cell.
Heating the alkali metal vapor cell to obtain high atomic density is a prerequisite for realizing the high-precision measurement for spin-exchange relaxation-free (SERF) co-magnetometer. However, the thermo-magnetic and thermo-optical coupling noise caused by the temperature diffusion from the alkali vapor cell is one of the main factors restricting the co-magnetometer performance at present. Here the influence of heating on the SERF co-magnetometer was comprehensively studied, and the thermal error model of the system was established. A novel heat insulation structure for the co-magnetometer was designed, and the effectiveness was verified through monitoring of characteristic temperature points. Compared to the conventional oven support, the average maximum temperature difference within 12 h of the optical path and magnetic shielding systems (OPMSS) decreased by 52%, the bias instability of the co-magnetometer decreased by 24%, and the noise of the co-magnetometer at 1 Hz decreased by 33%. The experimental results demonstrate that this method reduces the temperature fluctuation of the OPMSS, effectively suppresses the thermal coupling noise, and thus improves the performance of the SERF co-magnetometer. Moreover, this method is also applicable to other quantum sensors based on alkali metal vapor cell.
Atomic magnetometers based on spin-exchange relaxation free (SERF) regime typically operate with the high optical density and low magnetic fields. We present SERF magnetometer with a compact heating system for compact and efficient temperature control. A resistive heating system with high frequency alternating current was used to achieve high optical density. The current frequency was carefully chosen to avoid resonance with the atomic magnetometer. This resulted in a compact heating system that also allows independent operation of multiple heating systems. Optical rotation signal was obtained at 200 oC and width was about 3 nT with pump laser power of 55 mW. Bandwidth of the magnetometer with the compact heater was 40 Hz. Consequently, the compact resistive heating system proved to be a good alternative to the air-flow heating system. Additionally, residual field effect and signal-to-noise yielded a sensitivity of 50 fT/Hz1/2 at 10 Hz.
The spin-exchange relaxation-free (SERF) atomic magnetometer is one of the ultra-sensitive magnetometers, which must be operated in a low magnetic field and heated more than 100°C. One of the main engineering difficulties in fabricating a SERF atomic magnetometer is to develop a heater that introduces no additional magnetic noise into the measurement system. In this paper, a novel compact current-heating scheme and apparatus for the SERF atomic magnetometer are proposed. The thermal and magnetic simulations of this apparatus are carried out and the results of the simulations demonstrate the feasibility of this heating apparatus: the vapor cell can be heated to a temperature of 103°C and magnetic field introduced in the magnetometer can be limited in 10nT.
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.