Optical detection of low frequency NQR signals: a step forward from conventional NQR

In searching for the more sensitive ¹⁴N nuclear quadrupole resonance (NQR) detecting system for illicit substances, a promising combination of a classic RF pulse NQR spectrometer and a K optically pumped magnetometer was tested. The initial results are encouraging. The principles of such a combination are described, and the detection limits in the low frequency RF region, where the ¹⁴N pulse NQR frequencies are usually positioned, are presented. Several illicit substances which are difficult to detect with a classic pulse NQR spectrometer were detected with both types of spectrometers. We noticed that with the proposed combination of classic RF excitation of ¹⁴N nuclei, using a pulse NQR spectrometer and subsequent optical detection of the sample's response, a gain in S/N of up to a factor of 10 was possible. (paper)