Design strategy for terahertz quantum dot cascade lasers.

Benjamin A Burnett,Benjamin S Williams

doi:10.1364/oe.24.025471

Abstract

The development of quantum dot cascade lasers has been proposed as a path to obtain terahertz semiconductor lasers that operate at room temperature. The expected benefit is due to the suppression of nonradiative electron-phonon scattering and reduced dephasing that accompanies discretization of the electronic energy spectrum. We present numerical modeling which predicts that simple scaling of conventional quantum well based designs to the quantum dot regime will likely fail due to electrical instability associated with high-field domain formation. A design strategy adapted for terahertz quantum dot cascade lasers is presented which avoids these problems. Counterintuitively, this involves the resonant depopulation of the laser's upper state with the LO-phonon energy. The strategy is tested theoretically using a density matrix model of transport and gain, which predicts sufficient gain for lasing at stable operating points. Finally, the effect of quantum dot size inhomogeneity on the optical lineshape is explored, suggesting that the design concept is robust to a moderate amount of statistical variation.

Highlights

Terahertz quantum cascade lasers (QCLs) are increasingly viable coherent sources in the 15 THz range, having advanced to pulsed peak power above 1 W [1] and CW power above 100 mW [2, 3]
It has been proposed that replacing the quantum wells in a QCL with quantum dots could offer a solution due to the discretization of the electronic subbands into “sublevels.” In principle this would lead to a so-called “phonon bottleneck,” where LO-phonon scattering is suppressed between pairs of sublevels provided they are not resonant with ELO [8,9,10]
The candidate geometries for such a quantum dot cascade laser (QDCL) include self-assembled quantum dots [13, 14], nanopillars etched from the top-down into QCL active material [15,16,17], and nanopillars epitaxially grown from the bottom-up [18, 19]

Summary

Introduction

Terahertz quantum cascade lasers (QCLs) are increasingly viable coherent sources in the 15 THz range, having advanced to pulsed peak power above 1 W [1] and CW power above 100 mW [2, 3]. It has been proposed that replacing the quantum wells in a QCL with quantum dots could offer a solution due to the discretization of the electronic subbands into “sublevels.” In principle this would lead to a so-called “phonon bottleneck,” where LO-phonon scattering is suppressed between pairs of sublevels provided they are not resonant with ELO [8,9,10] The existence of this effect has been experimentally confirmed by directly measuring THz intersublevel relaxation times as long as 1 ns in self-assembled quantum dots at 10 K (extrapolated to tens of picoseconds at room temperature) [11], as well as the raising of a THz QCL’s operating temperature from 160 K to 225 K by applying a strong magnetic field to discretize the subbands into Landau levels [12]. The candidate geometries for such a quantum dot cascade laser (QDCL) include self-assembled quantum dots [13, 14], nanopillars etched from the top-down into QCL active material [15,16,17], and nanopillars epitaxially grown from the bottom-up [18, 19]

Methods

Results

Conclusion