  Stellar population studies in the infrared (IR) wavelength range have two
main advantages with respect to the optical regime: they probe different
populations, because most of the light in the IR comes from redder and
generally older stars, and they allow us to see through dust because IR light
is less affected by extinction. Unfortunately, IR modeling work was halted by
the lack of adequate stellar libraries, but this has changed in the recent
years. Our project investigates the sensitivity of various spectral features in
the 1--5\,$\mu$m wavelength range to the physical properties of stars
($T_{eff}$, [Fe/H], log g and aims to objectively define spectral indices that
can characterize the age and metallicity of unresolved stellar populations. We
implemented a method that uses derivatives of the indices as functions of
$T_{eff}$, [Fe/H] or log g across the entire available wavelength range to
reveal the most sensitive indices to these parameters and the ranges in which
these indices work. Here, we complement the previous work in the I and K bands,
reporting a new system of 14, 12, 22, and 12 indices for Y, J, H, and L
atmospheric windows, respectively, and describe their behavior. Our analysis
indicates that features sensitive to the effective temperature are present and
measurable in all the investigated atmospheric windows at the spectral
resolution and in the metallicity range of the IRTF library for a
signal-to-noise ratio greater than 20-30. The surface gravity is more
challenging and only indices in the H and J windows are best suited for this.
The metallicity range of the stars with available spectra is too narrow to
search for suitable diagnostics. For the spectra of unresolved galaxies, the
defined indices are valuable tools in tracing the properties of the stars in
the IR-dominant stellar populations.
