  We propose using tunneling spectroscopy with a superconducting electrode to
probe the collective modes of unconventional superconductors. The modes are
predicted to appear as peaks in dI/dV at voltages given by eV = {\omega}i/2
where {\omega}i denotes the mode frequencies. This may prove to be a powerful
tool to investigate the pairing symmetry of unconventional superconductors. The
peaks associated with the collective modes appear at fourth order in the single
particle tunneling matrix element. At the same fourth order, multiple Andreev
reflection (MAR) leads to peaks at voltage equal to the energy gaps, which, in
BCS superconductors, coincides with the expected position of the amplitude
(Higgs) mode. The peaks stemming from the collective modes of unconventional
superconductors do not suffer from this coincidence. For scanning tunneling
microscopes (STM), we estimate that the magnitude of the collective mode
contribution is smaller than the MAR contribution by the ratio of the energy
gap to the Fermi energy. Moreover, there is no access to the mode dispersion.
Conversely, for planar tunnel junctions the collective mode peak is expected to
dominate over the MAR peak, and the mode dispersion can be measured. We discuss
systems where the search for such collective modes is promising.
