Although the potential of quantum-well infrared photodetectors for thermal imaging has been known for some time, significantly less research has been focused on the nonlinear behavior of these devices and on the potential applications of that behavior. Now scientists at Fraunhofer Institut für Angewandte Festkörperphysik in Freiburg, Germany, and at the National Research Council in Ottawa have developed quantum-well IR photodetectors with the ability to characterize mid-IR pulses in the picojoule range.Figure 1. The quantum-well infrared photodetectors have three equidistant subbands in the conduction band, with both the ground and intermediate bands bound in the quantum well. The third subband is a resonance in the continuum slightly above the barrier conduction band edge.According to Thomas Maier of the Fraunhofer institute, high sensitivity and subpicosecond response time are critical features of the photodetectors, which include three equidistant energy levels (Figure 1). Resonant nonlinearity is such that there is a quadratic power dependence of the photocurrent down to excitation power densities as low as 0.1 W/cm2."Epitaxial structures with equidistant bound energy levels have been studied for some time now, especially in the context of second harmonic generation," he said. "In our detectors, the final state is located in the continuum so that the structures can be used as photodetectors. The important thing is to realize exactly equidistant levels in order to obtain the resonantly enhanced nonlinearity."Figure 2. The highest two-photon photoemission occurs with energy levels equidistant. The inset illustrates a related intensity autocorrelation trace and a schematic of the two-photon quantum-well IR photodetector.The scientists created the artificial three-level system using the narrowband characteristics of intersubband transitions. Maier reported that the photodetector nonlinearity is more than six orders of magnitude higher than that of the bulk material used to manufacture nonlinear crystals. Compared with off-resonant detection concepts, device absorption is more than three orders of magnitude higher.The team analyzed samples grown by molecular beam epitaxy on [100]-oriented semi-insulating GaAs substrates. One sample, with peak detection at 10.2 µm, has 7.6-nm-wide GaAs quantum wells N-doped with Si that are sandwiched between 47-nm-wide AlGaAs barriers. The other, with a peak transition wavelength of 8 µm, has 6.8-nm-wide InGaAs wells separated by 47-nm-wide AlGaAs barriers. Maier said that each sample has an active region containing 20 periods and is embedded between N-type contact layers.Autocorrelation experiments were performed on laser pulses obtained by difference frequency generation in a nonlinear crystal of two output beams from an optical parametric oscillator. This was pumped by a Ti:sapphire mode-locked laser to produce pulses with energy of a few picojoules and a duration of 165 fs. The scientists then split this output into two identical pulses using a Michelson interferometer with a corner-cube mirror at the end of each arm. The setup enabled them to illuminate the two-photon quantum-well IR photodetector collinearly using variable delay times. They also delayed pulsing with a change of interferometer arm length.They mounted the corner cube mirror of the second arm onto a mechanical shaker. By setting the amplitude of the oscillations to a few microns, it was possible to average out the phase between two pulses to produce an intensity autocorrelation for the process.According to Maier, this is essentially the main application for these devices, but interferometric autocorrelation also has the potential to provide information on pulse shape and chirp. Ongoing research efforts include work to improve detector response time.Although the photodetectors were designed for the mid-IR, the detection concept might be applied to longer wavelengths -- "even in the highly interesting terahertz regime," Maier said. "One could also realize detector arrays that provide spatial information on the pulses."