Two-photon quantum well infrared photodetectors below 6 THz

• Terahertz QWIPs optimized for two-photon absorption were fabricated and characterized.
• Dark current showed discontinuities over five orders of magnitude.
• Intensity dependent THz photocurrent measurements using a free-electron laser showed nonlinear behavior for all samples.
• Interferometric autocorrelation was measured at THz frequencies between 3 and 6 THz.

Two-photon quantum well infrared photodetectors (QWIPs) are nonlinear detectors for the mid-infrared and terahertz regimes optimized for resonant two-photon absorption. Here we present first results on two-photon QWIP samples based on the GaAs/AlGaAs material system with intersubband energies between 25 and 12 meV (6–3 THz) confirmed by photocurrent spectra. The dark current showed large discontinuities, presumably caused by impact ionization. We performed interferometric autocorrelation experiments at the free-electron laser FELBE and observed nonlinear interferograms for all samples.