Li Wang

By Li Wang

In designing the terahertz quantum cascade lasers, electron injection manner indeed plays a significant role to achieve the population inversion. The resonant tunneling process is commonly employed for this injection process but waste more than 50% fraction of populations out of the active region owing to resonance alignment, and the injection efficiency is obviously degraded due to thermal incoherence. An alternative approach is to consider the phonon-assisted injection process that basically contributes to most of the populations to the upper lasing level. However, this manner is still not realized in experiments if a short-period design only containing two quantum wells is used. In this work, it is found in this design that the population inversion is indeed well improved; however, the optical gain is inherently low even at a low temperature. Those two opposite trends are ascribed to a strong parasitic absorption overlapping the gain. The magnitude of this overlap is closely related to the lasing frequency, where frequencies below 3 THz suffer from fewer effects.

Part of the book: Terahertz, Ultrafast Lasers and Their Medical and Industrial Applications

By Li Wang and Hideki Hirayama

Presently, terahertz quantum cascade lasers still suffer from operations below room temperature, which prohibits extensive applications in terahertz spectra. The past continuous contributions to improving the operating temperatures were by clarifying the main thermal degradation process and proposing different designs with the optical gain demonstrating higher temperature cut-offs. Recent designs have attempted to employ a narrow period length with a simplified and clean state system, and reach renewed operating temperatures above 200 K. This study reveals how historic designs approach such narrow-period designs, discus the limitations within those designs, and show further possible designs for higher operating temperatures.

Part of the book: Light-Emitting Diodes