The relation between free carrier absorption and intersubband transitions in
semiconductor heterostructures is resolved by comparing a sequence of
structures. Our numerical and analytical results show how free carrier
absorption evolves from the intersubband transitions in the limit of an
infinite number of wells with vanishing barrier width. It is explicitly shown
that the integral of the absorption over frequency matches the value obtained
by the f-sum rule. This shows that a proper treatment of intersubband
transitions is fully sufficient to simulate the entire electronic absorption in
heterostructure THz devices.Comment: 6 pages, accepted by Physical Review
