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Autocorrelation measurements of the FELBE free-electron laser and photocurrent saturation study in two-photon QWIPs

Autokorrelationsmessung des FELBE freien Elektronenlasers und Photostrom-Sättigung in Zwei-Photonen-QWIPs
: Schneider, H.; Drachenko, O.; Winnerl, S.; Helm, M.; Walther, M.


Tsen, K.-T. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Ultrafast phenomena in semiconductors and nanostructure materials XI and semiconductor photodetectors IV : 22 - 24 January 2007, San Jose, California, USA
Bellingham, WA: SPIE, 2007 (SPIE Proceedings Series 6471)
ISBN: 978-0-8194-6584-9
Paper 64710T
Conference "Ultrafast Phenomena in Semiconductors and Nanostructure Materials" <11, 2007, San Jose/Calif.>
Conference Paper
Fraunhofer IAF ()
time resolved spectroscopy; zeitaufgelöste Spektroskopie; heterodyne detection; Heterodyn-Detektion; free-electron lasers; freier Elektronenlaser; Quantum Well Infrared Photodetector (QWIP); Quantumwell Infrarot-Photodetektor; GaAs/AlGaAs

The two-photon QWIP approach involves three equidistant subbands, two of which are bound in the quantum well, and the third state is located in the continuum. The intermediate subband induces a resonantly enhanced optical nonlinearity, which is about six orders of magnitude stronger than in usual semiconductors. 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. Both properties make this device very promising for pulse diagnostics of pulsed mid-infrared lasers. We have performed autocorrelation measurements of ps optical pulses from the free-electron laser (FEL) facility FELBE at the Forschungszentrum Dresden Rossendorf. Using a rapid-scan autocorrelation scheme at a scan frequency of 20 Hz, high-quality quadratic autocorrelation traces are obtained, yielding ratios close to the theoretically expected value of 8:1 between zero delay and large delay for interferometric autocorrelation, and 3:1 for intensity autocorrelation. Thus, two-photon QWIPs provide an excellent new technique for online pulse monitoring of the FEL. In addition, we have investigated the saturation mechanism of the photocurrent signal, which is due to internal space charges generated in the detector.