  Combining high-contrast imaging with medium-resolution spectroscopy has been
shown to significantly boost the direct detection of exoplanets. HARMONI, one
of the first-light instruments to be mounted on ESO's ELT, will be equipped
with a single-conjugated adaptive optics system to reach the diffraction limit
of the ELT in H and K bands, a high-contrast module dedicated to exoplanet
imaging, and a medium-resolution (up to R = 17 000) optical and near-infrared
integral field spectrograph. Combined together, these systems will provide
unprecedented contrast limits at separations between 50 and 400 mas. In this
paper, we estimate the capabilities of the HARMONI high-contrast module for the
direct detection of young giant exoplanets. We use an end-to-end model of the
instrument to simulate observations based on realistic observing scenarios and
conditions. We analyze these data with the so-called "molecule mapping"
technique combined to a matched-filter approach, in order to disentangle the
companions from the host star and tellurics, and increase the S/N of the
planetary signal. We detect planets above 5-sigma at contrasts up to 16 mag and
separations down to 75 mas in several spectral configurations of the
instrument. We show that molecule mapping allows the detection of companions up
to 2.5 mag fainter compared to state-of-the-art high-contrast imaging
techniques based on angular differential imaging. We also demonstrate that the
performance is not strongly affected by the spectral type of the host star, and
that we reach close sensitivities for the best three quartiles of observing
conditions at Armazones, which means that HARMONI could be used in
near-critical observations during 60 to 70% of telescope time at the ELT.
Finally, we simulate planets from population synthesis models to further
explore the parameter space that HARMONI and its high-contrast module will soon
open.
