  We present an interband cascade laser (ICL) emitting at 5.2 {\mu}m consisting
of an 8-stage active region and a hybrid cladding composed of outer
plasmon-enhanced InAs_0.915 Sb_0.085 and inner InAs/AlSb superlattice
claddings. The hybrid cladding architecture shows an increase in
mode-confinement in the active region by 11.2 % according to the simulation.
This is a consequence of a significantly lower refractive index of
plasmon-enhanced claddings. The threshold current density is 242 A/cm^2 in
pulsed operation at room temperature. This is the lowest value reported to date
for ICLs emitting at wavelengths longer than 5 {\mu}m. We also report close to
record value threshold power density of 840 W/cm^2 for ICLs at such
wavelengths.
