  Photon-assisted tunneling frequently provides detailed information on the
underlying charge-transfer process. In particular, the Tien-Gordon approach and
its extensions predict that the sideband spacing in bias voltage is a direct
fingerprint of the number of electrons transferred in a single tunneling event.
Here, we analyze photon-assisted tunneling into subgap states in
superconductors in the limit of small temperatures and bias voltages where
tunneling is dominated by resonant Andreev processes and does not conform to
the predictions of simple Tien-Gordon theory. Our analysis is based on a
systematic Keldysh calculation of the subgap conductance and provides a
detailed analytical understanding of photon-assisted tunneling into subgap
states, in excellent agreement with a recent experiment. We focus on tunneling
from superconducting electrodes and into Yu-Shiba-Rusinov states associated
with magnetic impurities or adatoms, but we also explicitly extend our results
to include normal-metal electrodes or other types of subgap states in
superconductors. In particular, we argue that photon-assisted Andreev
reflections provide a high-accuracy method to measure small, but nonzero
energies of subgap states which can be important for distinguishing
conventional subgap states from Majorana bound states.

Mesoscale and Nanoscale Physics;Superconductivity