I describe mid/far-infrared photodetectors based on frequency upconversion in a near-resonant cascade of interband and intersubband transitions in high optical non-linearity asymmetric quantum well structures. Such structures can yield high detectivity and responsivity in the mid/far-infrared range. Resonant uponversion detectors can be designed for both collinear and perpendicular pump and signal beams. They can be integrated with semiconductor pump lasers to yield compact devices. Single photon counting is also achieved by these detectors. I present specific device designs based on GaAs/AlGaAs and InGaAs/AlInAs heterostructures and calculations of their expected figures of merit. This includes a study of the intersubband nonlinear absorption of asymmetric double quantum wells designed for mid/far-IR range. The dependance of second order nonlinear susceptibility on various parameters of the structure is studied. In particular, different values for barrier and well widths are considered. The nonlinear absorption can be obtained by using perturbative calculation of the linear susceptibility up to second order with density matrix approach. The intersubband linear and nonlinear asymmetric double quantum well can be tuned using two design parameters. One is the width of the barrier between the wells that controls the coupling and the second is the width of the narrow well that controls the asymmetry of the structure. As the barrier width narrows the energy gap at the anticrossing increases. The asymmetry of the two well potentials is essential for sum frequency and difference frequency generation since in a symmetric well hZ31i = 0 due to the same parity of the ground and second excited states so that Â(2) = 0. In our detection scheme using frequency up-conversion we demonstrate that these devices can achieve high detectivity, very low noise and high value for Â(2) hence good efficiency. This can be an important advantage for low signal detection and single photon counting.