  The role of nematic fluctuations in the pairing mechanism of iron-based
superconductors is frequently debated. Here we present a novel method to reveal
such fluctuations by identifying energy and momentum of the corresponding
nematic boson through the detection of a boson-assisted resonant amplification
of Friedel oscillations. Using Fourier-transform scanning tunneling
spectroscopy, we observe for the unconventional superconductor LiFeAs strong
signatures of bosonic states at momentum $q\sim 0$ and energy
$\Omega\approx8$~meV. We show that these bosonic states survive in the normal
conducting state, and, moreover, that they are in perfect agreement with
well-known strong above-gap anomalies in the tunneling spectra. Attributing
these small-$q$ boson modes to nematic fluctuations we provide the first
spectroscopic approach to the nematic boson in an unconventional
superconductor.
