Two-Color Laser Emission from Optically Cascaded Intersubband Transitions from 1.3 THz to 2 THz and Above 3 THz

Challenges for THz Quantum Cascade Lasers (QCLs) emitting below 2 THz arise due to the low energy spacing between radiative states. This reduces both the oscillator strength of the optical transition and the achievable gain. By designing and growing a homogeneous GaAs/Al<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</inf>Ga<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</inf>As active region with two optically cascaded transitions using molecular beam epitaxy, a QCL capable of emitting in two distinct regions of the THz spectrum-below 2 THz and above 3 THz-has been successfully developed. The lasing process involves two radiative transitions [1] [2], where the lower energy level of the first transition is at the same time the upper energy level of the second transition (Fig. 1.a). In this configuration, population inversion for the first transition is achieved through electrical injection of electrons into the quantum well. The lasing process for the second transition starts only after the upper radiative transition induces population inversion in the following energy level. Ridge double-metal devices with varying lengths and widths were fabricated using the described active region and measured with a vacuum Fourier-transform (FT) spectrometer at 20 K. Just above the threshold current, the laser emits from the first optical transition at approximately 3 THz. As the injection current increases, the photon population inside the cavity grows and the stimulated emission forces more electrons to cascade into the lower energy level, establishing population inversion for the second transition as well. Consequently, lasing emission from both transitions has been observed. The beginning of the lasing emission on the second optical cascaded intersubband transition can be appreciated observing the variation in power slope efficiency with the current. The laser's light-current (LI) curve (Fig. 1.b) exhibits three distinct regions: the first dominated by lasing on the initial optical transition (Fig. 1.c), the second featuring contributions from both transitions (Fig. 1.d), and the third where lasing on the second transition becomes dominant (Fig. 1.e). Lasing on both transitions has been measured up to the temperature of 90 K.