Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Two-photon quantum well infrared photodetectors (QWIPs) are based on quantum wells comprising three energetically equidistant states, namely two bound subbands and one resonant state in the continuum. This configuration is extremely efficient for two-photon transitions, giving rise to strong resonant enhancement and to optical nonlinearities which are six orders of magnitude stronger than in bulk crystals. Using this approach, extremely sensitive quadratic detectors for mid-infrared and THz radiation can thus be realized. These devices are very useful for quadratic autocorrelation measurements of THz radiation from free-electron lasers, nonlinear optical converters, and modelocked quantum cascade lasers.
Exploiting the two-photon QWIP approach, we have investigated various detectors based on different epitaxial structures, covering operation wavelengths from the mid-wavelength infrared to the THz regime. In particular, we have demonstrated a two-photon QWIP which operates below the Reststrahlen band at 42 μm (7.1 THz) [1]. At shorter wavelengths, quadratic detection also works at higher temperatures, which is crucial for applications in practical systems. In particular, room temperature operation has been achieved at 5.5 μm (54.5 THz) [2].
In this talk, we will concentrate on the detection properties of two-photon QWIPs. We confirm their quadratic intensity dependence and demonstrate autocorrelation measurements and pulse monitoring of far-infrared picosecond pulses from the free-electron laser at the Forschungszentrum Dresden Rossendorf. Huge optical nonlinearities allow for quadratic detection at THz pulse energies as low as a few pJ. The time resolution of this detector is limited by the intersubband dynamics associated with the intermediate state. Besides applications in THz pulse diagnostics, the two-photon QWIP thus also provides interesting opportunities for studies of the associated intersubband population and polarization lifetimes.
[1] H. Schneider, H. C. Liu, S. Winnerl, C. Y. Song, M. Walther, M. Helm, Opt. Express 17, 12279 (2009)
[2] H. Schneider, H. C. Liu, S. Winnerl, O. Drachenko, M. Helm, J. Faist, Appl. Phys. Lett. 93, 101114 (2008)