  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.
