Optically detected nuclear quadrupolar interaction of 14N in   nitrogen-vacancy centers in diamond
  We report sensitive detection of the nuclear quadrupolar interaction of the 14N nuclear spin of the nitrogen-vacancy (NV) center using the electron spin echo envelope modulation technique. We applied a weak transverse magnetic field to the spin system so that certain forbidden transitions became weakly allowed due to second-order effects involving the nonsecular terms of the hyperfine interaction. The weak transitions cause modulation of the electron spin-echo signal, and a theoretical analysis suggests that the modulation frequency is primarily determined by the nuclear quadrupolar frequency; numerical simulations confirm the analytical results and show excellent quantitative agreement with experiments. This is an experimentally simple method of detecting quadrupolar interactions, and it can be used to study spin systems with an energy structure similar to that of the nitrogen vacancy center. 
Chang S. Shin, Mark C. Butler, Hai-Jing Wang, Claudia E. Avalos, Scott   J. Seltzer, Ren-Bao Liu, Alexander Pines, and Vikra...
quant-ph