Optical detection of low frequency NQR signals (Vol. 48, No. 3)

Optical detection of NQR signal

Nuclear quadrupole resonance (NQR) is a branch of radiofrequency (RF) spectroscopy. It became a promising tool in detecting illicit substances like explosives, narcotics and counterfeit medicines. Many of these substances contain 14N nuclei and are detectable by the NQR spectrometer. Practically all 14N NQR frequencies are in the range below 5 MHz and correspondingly the spectrometer sensitivity is low. One of possible improvements is a combination of the very sensitive potassium (K) pumped optical magnetometer (KPOM) and the pulsed NQR spectrometer. The linearly polarized probe laser beam detects the magnetic part of the low frequency 14N RF signal. This results in a rotation of the probe beam polarization plane after the beam leaves the K-cell. This rotation is measured and is proportional to the NQR signal. Combination of the classic RF excitation of the sample 14N nuclei and a subsequent optical detection of the sample response leads to a S/N improvement of up to a factor of 10 as it was demonstrated in the case study of some difficult-to-detect illicit substances. An efficient magnetic shielding may be necessary.

S. Begus, J. Pirnat, V. Jazbinsek and Z Trontelj, Optical detection of low frequency NQR signals: a step forward from conventional NQR, J. Phys. D: Appl. Phys. 50, 095601 (2017)