Autocorrelation measurements of free-electron laser radiation using a two-photon QWIP

The two-photon QWIP approach involves three equidistant subbands, namely two bound states localized in the quantum well and an extended state in the continuum. Here the intermediate state generates a huge optical nonlinearity, six orders of magnitude larger than in usual bulk semiconductors, which makes this device very promising for quadratic autocorrelation measurements of pulsed mid-infrared lasers. In addition, temporal resolution is only limited by the sub-ps intrinsic time constants of the quantum wells, namely the intersubband relaxation time and the dephasing time of the intersubband polarization [1]. We will report here on autocorrelation measurements of ps optical pulses from a free-electron laser (FEL). Due to the intense nonlinear signal, two-photon QWIP operation is possible at room temperature. The device is thus ideally suited for standard diagnostics of the FEL pulse shape.
[1] H. Schneider, T. Maier, H. C. Liu, M. Walther, P. Koidl, Ultra-sensitive femtosecond two-photon detector with resonantly enhanced nonlinear absorption, Optics Letters 30, 287 (2005).