Femtosecond carrier dynamics and nonlinear effects in quantum cascade lasers

Sheng Liu, Elaine Lalanne, Peter Q. Liu, Xiaojun Wang, Claire F. Gmachl, Anthony M. Johnson

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


Quantum cascade lasers (QCLs) are complicated unipolar semiconductor devices based on intersubband transitions and resonant tunneling. In this study, femtosecond mid-infrared (Mid-IR) pulses are employed to investigate the nature of carrier transport through the active and injector regions of a room temperature, pulse biased ultrastrong coupling design QCL. Despite the low average power (<1 mW) of femtosecond Mid-IR pulses, the efficient coupling of these pulses into the QCL waveguide made the study of nonlinear effects in QCLs possible. Biased just below threshold, we observed ultrafast gain recovery within the first 200 f s mainly contributed by electrons resonant tunneling through a much thinner injector barrier than that of conventional designed QCLs, which overcomes the interface-roughness-induced detuning of resonant tunneling. Oscillation or overshooting within the first picosecond is caused by electron relaxation from continuum region excited by strong pump beam, as well as coherent electron tunneling transport from injector to active region. The former feature is supported by the observation of second harmonic generation (SHG) with emission of 2.2 m pulses and measured positive photoconductivity. The transport of electrons through the injector region contributes to a slower gain recovery. A much longer recovery (hundreds of picoseconds) can be explained as electrons are depleted from upper stages down to lower stages in real space.

Original languageEnglish (US)
Article number6145139
Pages (from-to)92-104
Number of pages13
JournalIEEE Journal on Selected Topics in Quantum Electronics
Issue number1
StatePublished - 2012

All Science Journal Classification (ASJC) codes

  • Atomic and Molecular Physics, and Optics
  • Electrical and Electronic Engineering


  • Carrier dynamics
  • hot carriers
  • intersubband transitions
  • quantum cascade laser
  • resonant tunneling
  • second-harmonic generation
  • two-photon absorption


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