An semiconductor structure comprises a first quantum well having a first state (E3) with an energy that is greater than an energy of a lower state (E1) by a first energy difference, a quantum well structure (100) adjacent to the first quantum well and having at least a second state (E4) having an energy level which is resonant with the first state (E3) of the first quantum well, a second quantum well having at least a third state (E2) to collect electrons from the second state (E4) of the quantum well structure through a non-radiative mechanism and a fourth state (E5). An energy of the fourth state of the second quantum well is greater than an energy of the third state (E2) by a second energy difference. The structure can absorb two photons, one in the first quantum well by excitation of a carrier from the lower energy state (E1) to the first state (E3) and another in the second quantum well by excitation of a carrier from the third state (E2) to the fourth state (E5). The quantum well structure provides efficient extraction of excited carriers from the first quantum well.