Quantum device for shift current generation (IMAGE)
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Fig. 1. Ultrafast shift current generation in asymmetric semiconductor quantum wells and optical geometry. (a) Electrons are confined within a 13-nm wide quantum well made from AlxGa1-xAs. By varying the aluminum content one obtains an asymmetric (‘triangular’) potential, sandwiched between potential barriers. Quantum confinement leads to the formation of discrete energy levels 1 and 2 of electrons along the c axis. The contours are the spatially shifted probability distributions of electrons in the ground state (blue) and in the first excited state (red). The blue and red arrows mark the centers of gravity of the respective distribution. Electrons are promoted from state 1 to state 2 by an ultrashort mid-infrared pulse, resulting in a time-dependent spatial shift of electron position, which represents a time-dependent electric current. This so-called shift current emits a mono-cycle THz pulse. (b) Optical geometry of the THz source. 20 quantum wells are stacked at the bottom of a prism structure. They are excited by a femtosecond mid-infrared pulse (red transient, left). The THz pulses emitted in forward direction (green transient, right) are analyzed in the experiment. The height of the prism structure is approximately 350 µm.
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