Quadratic autocorrelation of free-electron laser radiation and photocurrent saturation in two-photon quantum well infrared photodetectors

Using the free-electron laser facility FELBE, the authors have studied the influence of the intensity on the quadratic autocorrelation measured with two-photon quantum well infrared photodetectors (QWIPs). At high illumination powers, the shape of the autocorrelation trace is affected by photocurrent saturation of the two-photon QWIP. They describe the saturation mechanism by different analytical models taking account of the photocurrent nonlinearity in analogy to linear QWIPs and give conditions where true quadratic behavior can be observed. While these studies were carried out at 77 K, properties of two-photon QWIPs at room temperature will also be addressed.