  Steady-state access to intraband transitions in colloidal quantum dots
(CQDs), via heavy doping, allows exploiting the electromagnetic spectrum at
energies below the band gap. CQD intraband optoelectronics opens up a new path
to cheap mid- and long-wavelength infrared photodetectors and light-emitting
devices, which today employ mostly epitaxial materials. As a recent field of
experimental research, thorough studies of the basic properties of intraband
transitions in CQDs are still lacking. In this work, we investigate the size
and temperature dependence of the intraband transition in heavily n-doped PbS
quantum dot (QD) films. We measure the absorption coefficient of the intraband
transition to be in the order of $10^4$ cm$^{-1}$, which is comparable to the
value of the interband absorption coefficient. Additionally, we determine the
size-dependence of the oscillator strength of the intraband transition. We
demonstrate a negative dependence of the intraband energy with temperature, in
contrast to the positive dependence of the interband transition.
