  The X-ray binary, black hole candidate, and microquasar H1743-322 exhibited a
series of X-ray outbursts between 2003 and 2008. We took optical/infrared (OIR)
observations with the ESO/NTT telescope during 3 of these outbursts (2003,
2004, and 2008), to study its spectral energy distribution (SED).
  We detect rapid flares of duration ~5 mn in the high time-resolution IR
lightcurve. We identify H and He emission lines in the IR spectra, coming from
the accretion disk. The IR SED exhibits the spectral index typically associated
with the X-ray high, soft state in our observations taken during the 2003 and
2004 outbursts, while the index changes to one that is typical of the X-ray
low, hard state during the 2008 outburst. During this last outburst, we
detected a change of slope in the NIR spectrum between the J and Ks bands,
where the JH part is characteristic of an optically thick disk emission, while
the HKs part is typical of optically thin synchrotron emission. Furthermore,
the comparison of our IR data with radio and X-ray data shows that H1743-322
exhibits a faint jet both in radio and NIR domains. Finally, we suggest that
the companion star is a late-type main sequence star located in the Galactic
bulge.
  These OIR photometric and spectroscopic observations of the microquasar
H1743-322, the first of this source to be published in a broad multiwavelength
context, allow us to unambiguously identify two spectra of different origins in
the OIR domain, evolving from optically thick thermal emission to optically
thin synchrotron emission toward longer wavelengths. Comparing these OIR
observations with other black hole candidates suggests that H1743-322 behaves
like a radio-quiet and NIR-dim black hole in the low, hard state. This study
will be useful when quantitatively comparing the overall contribution of the
compact jet and accretion flow in the energy budget of microquasars.
