LONG WAVELENGTH (λ≈12-16 um) AND CASCADED TRANSITION QUANTUM CASCADE LASERS

Abstract

Quantum Cascade (QC) lasers have been undergoing rapid development since its invention in 1994, leading to high power continuous-wave (CW) operation in the 3.8-10.6 µm range. However, there is still a strong requirement for high performance long-wavelength (λ ≈ 12- 16 µm) QC lasers, which are crucial devices for improving the detection sensitivity for important gases including BTEX (benzene, toluene, ethylbenzene, and xylenes) or uranium hexafluoride.

By gain optimization, a high-performance QC laser emitting at ~ 14 µm is achieved, optimized by employing a diagonal optical transition and a two-phonon-continuum depletion scheme. It shows a high power of 336 mW, a low threshold current density of 2.0 kA/cm2, as well as temperature-insensitive performance (characteristic temperature ~ 310 K) over a wide temperature range around room temperature (240- 390K). In order to optimize the ridge profile and fabrication related waveguide loss, the ridge-width dependence of threshold of ~ 14 µm QC lasers by both wet etching and dry etching is studied. The main challenge for narrowing wet-etched ridges is the high loss caused by mode coupling to surface plasmon modes at the insulator/metal interface of sloped sidewalls. Conversely, dry-etched ridges avoid surface plasmon mode coupling due to the absence of transverse magnetic (TM) polarization for the vertical insulator and metal layers.

In addition to laser gain and loss, the microscopic electrical properties are also investigated via simulations on the self-consistent process of interaction between local electrons and photons. In QC lasers, local electron photon interaction leads to nonuniform current density distribution, which depends on the local photon density, in the lateral direction. The nonuniform current density distribution is simulated, and the corresponding spatial hole burning is investigated. Furthermore, multiple-transverse-mode operation of the QC laser is also studied.

Conventional QC lasers are based on intersubband transitions in repeated stages of precisely engineered coupled quantum wells, with one electron emitting at most one photon in each stage. While this is a major achievement enabling the QC laser to have become a most useful, powerful, efficient mid-infrared laser source, it is by far not the last innovation driving this emergent field. We develop a same-wavelength (λ ~ 14.2 µm) cascaded-transition QC structure, with two subsequent cascaded optical transitions in each stage. In addition to reusing electrons for optical transitions in each stage to improve the efficiency and power, cascaded-transition QC structures have novel physical effects, i.e., unique light-intensity dependent characteristics in population inversion, gain, pumping and depopulation rates. This broadens our perspective on "engineering" QC lasers, not only limited to wavelength, gain, and electron transport, but also leading to novel and interesting interactions between light and matter.