Recently, the joint research team from Anhui Institute of Optics and Fine Mechanics, HFIPS, Chinese Academy of Sciences, Institute of Applied Ecology, Chinese Academy of Sciences, University of Science and Technology of China, and Université du Littoral Côte d’Opale  published a NO2 Sensor Based on Faraday Rotation Spectroscopy Using Ring Array Permanent Magnets.

Farraday Rotational Spectroscopy (FRS) achieves highly selective and sensitive detection of paramagnetic molecules by detecting the changes in polarization state of linearly polarized light induced by the gas medium immersed in an external longitudinal magnetic field. This spectroscopic detection method exhibits inherent immunity to diamagnetic molecules such as water vapor and CO2, which results in a high degree of sample specificity. Additionally, the implementation of a pair of closely spaced orthogonal polarizers effectively suppresses laser noise, thus providing FRS with a very high detection sensitivity.

Usually, a solenoid coil is used to provide a longitudinal magnetic field to produce themagneto-optical effect. However, such a method has the disadvantages of excessive power consumption and susceptibility to electromagnetic interference. The research team proposed a novel FRS approach based on a combination of a neodymium iron boron permanent magnet ring array and a Herriott multipass absorption cell is proposed. A longitudinal magnetic field was generated by using 14 identical neodymium iron boron permanent magnet rings combined in a non-equidistant form according to their magnetic field’s spatial distribution characteristics. The average magnetic field strength within a length of 380 mm was 346 gauss. HealthyPhoton Co.,Ltd provided an integrated TE-cooled mercury cadmium telluride (MCT) infrared detector with front-end amplification(HPPD-B-08-10-150 K) for this project. A quantum cascade laser was used to target the optimum 441 ← 440 Q-branch nitrogen dioxide transition at 1613.25 cm–1 (6.2 μm) with an optical power of 40 mW. Coupling to a Herriott multipass absorption cell, a minimum detection limit of 0.4 ppb was achieved with an integration time of 70 s. The low-power FRS nitrogen dioxide sensor proposed in this work is expected to be developed into a robust field-deployable environment monitoring system.

Static magnetic field Faraday rotation spectral sensing device

Integrated preamplifier and cryocooler type mercury cadmium telluride (MCT) infrared detector

Circular array permanent magnets and their longitudinal magnetic field distribution characteristics

(a) Simulated distribution of the central longitudinal magnetic field for an equidistant NdFeB permanent magnet ring array;

(b) simulated (black line) and measured (red line) distributions of the central longitudinal magnetic field for a non-equidistant NdFeB permanent magnet ring array;

(c) schematic configuration of the Herriott cell and the non-equidistant NdFeB permanent magnet ring array.

The Relationship between FRS signal and its SNR and the Deflection Angle of the Polarizer

(a) FRS signal amplitude and

(b) SNR as a function of the analyzer angle α.

Reference:

Yuan Cao, Kun Liu, Ruifeng Wang, Xiaoming Gao, Ronghua Kang, Yunting Fang, Weidong Chen，NO₂ Sensor Based on Faraday Rotation Spectroscopy Using Ring Array Permanent Magnets, Anal. Chem. 2023, 95, 2, 1680–1685

https://doi.org/10.1021/acs.analchem.2c04821

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