  We are the first to show with clear experimental results that photons that
have energy lower than the transition energy between quantum dots states and
valance band can still contribute greatly to the photocurrent via both
one-photon absorption process (1PA) and two-photon absorption process (2PA).
The tailing states function as both the energy states for low energy photon
absorption and the photocarriers extraction pathway. One of the biggest
advantages of our method is that it can clearly differentiate the photocurrent
due to 1PA process and 2PA process. Both 1PA and 2PA photocurrent generation
efficiency in an InAs/GaAs QD device with photon excitation at 1550 nm have
been quantitatively evaluated.
