Study Of NMOR Atomic Magnetometer Based Ultra-low Field NMR Spectrometer Working At Room-temperature

Nuclear magnetic resonance(NMR)is a powerful spectral analysis method,through which the information of material composition and structure can be provided accurately and non-destructively.So far,the NMR measurements are mainly performed via high-field NMR spectrometers with superconducting magnets,which provides a high magnetic field,because the signal-to-noise ratio of the NMR spectrometers is proportional to γ5/2B3/2.However,the superconductor-based NMR spectrometers are usually bulky and expensive.Moreover,the technical difficulty of a shimming system increases sharply with the magnetic field strength,restricting the improvement of spectrometers’ measurement accuracy.The magnetic field of ultralow-field NMR is usually below 10 μT.In such a low field,an extremely homogeneous magnetic field can be achieved,leading to a narrow spectral linewidth of tens of mHz.Therefore,the ultralow-field NMR can be used for determining J-coupling constants accurately and as a complementary method to high-field NMR.The crucial point to realize the ultralowfield NMR is the high-sensitive sensor for detecting low-frequency magnetic signals.The atomic magnetometer is one of the most sensitive magnetic field sensors.It measures magnetic field via the interaction between atoms and laser in the presence of a magnetic field.It has the advantages of small size,portability,and cryogen-free,which make it an ideal choice for ultralow-field NMR.The atomic magnetometer based on nonlinear magneto-optical rotation(NMOR)can achieve a sensitivity better than hundreds of fT/Hz1/2 when working at room temperature.This feature is suitable for the detection of biological samples and biomagnetic fields.This thesis focuses on the study of NMOR atomic magnetometer based ultralowfield NMR spectrometer working at room temperature.The single-beam NMOR atomic magnetometer,ultralow-field NMR spectrometer,and its applications are described in detail,respectively.The main progress and significant results are summarized as follows:Firstly,the single-beam NMOR atomic magnetometer was studied,including the optics,coils,heater,and so on.The parameters of laser power,wavelength,working temperature,and residual magnetic field were optimized.The amplitude-modulated(AM)NMOR experiment was carried out and a method that could match the modulation frequency with Larmor frequency was studied.Finally,the sensitivity of the NMOR magnetometer was optimized to 94.2 fT/Hz1/2(@15Hz).Secondly,the NMOR based on the alignment-to-orientation conversion(AOC)effect in 133Cs D1 lines was studied.The experimental results showed that the AOCrelated NMOR could achieve a 1.7 fold enhancement of signal amplitude compared to coherence-related NMOR at the 133Cs 6 2S1/2 F=3→6 2P1/2 F’=4 transition,benefiting from narrow linewidth and ultraweak power broadening.The bandwidth of the magnetometer at high laser power is about one order of magnitude higher than that at the low laser power case,which is greatly helpful for the detection of high-frequency magnetic signals in zero-field J-spectroscopy.Thirdly,a method based on AM-NMOR was proposed to achieve the ultralowfield FID measurement.The influence of magnetic field inhomogeneity was suppressed by placing the vaper cell and NMR sample in the same homogenous magnetic field,which leads to an improvement of the maximum working magnetic field from 202 nT to 428 nT.Such a method facilitates the detection of the nuclear magnetic moments,especially for the low-gyromagnetic-ratio nuclei.Fourthly,the ultralow-field NMR spectrometer based on NMOR magnetometer working at room temperature was developed.After the optimization of sub-systems,by using the spectrometer,the ultralow-field(27～428 nT)free induction decay(FID)measurement,the zero-and the near-zero-field(3.1～50.2 nT)J-spectra measurement at room temperature range(25～28℃)were demonstrated.The signal-to-noise ratio of the ultralow-field NMR spectrometer is more than 10,and the linewidth as narrow as 0.2 Hz can be achieved.Finally,using the high-resolution ultralow-field NMR spectrometer,we proposed an application of nondemolition evaluation of local hydrogen-bonding networks in solution systems.We found that the J-spectra of methanol solutions with different components and concentrations were slightly shifted.By the theoretical analysis via molecular dynamics and density functional theory,we found that this variation of Jcoupling constants of methyl was caused by the hydrogen bonds around the methanol molecules,and can be utilized as a sensitive probe of the local hydrogen bond network.