18.10.2017 Heat is the new light! Quantum cascade structures are changing the way we perceive heat in semiconductor lasers

Dr. Yuri V. Flores, Research Laboratory of Electronics at Massachusetts Institute of Technology, Cambridge, Massachusetts, USA

Date: Wednesday, 25.10.2017, 15:00 h
Location: WZMW, Hans-Meerwein-Str. 6 (Lahnberge), seminar room 02D36

Abstract

Terahertz light-emitting quantum cascade lasers (THz-QCLs) represent by far the best combination of power and compactness in lasers emitting within the THz spectral range (0.3-10 THz, 30-1000 μm). These devices consist on alternated layers of typically 2-3 types of nanometer-thick III-V semiconductor compound materials with different energy bandgaps. Light generation in QCLs is achieved by radiative transitions between quantum-well states within the conduction band of the heterostructure. Under a certain bias, the ideal operation of a QCL assumes that an electron injected externally into the device will generate multiple photons - one in each "energy cascade" - while transporting through the heterostructure. By tailoring the layer thicknesses and through appropriate doping, one can engineer device parameters as the photon energy, oscillator strengths, and scattering rates. Moreover, further diversification of THz-QCL-based technology strongly depends on the improvement of key laser performance indicators as the maximum operating temperature T_max. Although laser performance characteristics of THz-QCLs continuously improved over the years, reported T_max values converged to 200 K (–73 ºC), not least because of an incomplete understanding of the physics of temperature-driven electron transport in these devices.

In this talk, I will review recent progress in understanding the physics of temperature-driven electron-transport in THz-QCLs. We conduct a detailed analysis of the light-output power versus temperature characteristics of THz-QCLs and point out to new aspects of the electron-phonon dynamics in these devices. Contrary to common sense expectation, we find that heat is not always bad for the laser performance so that an increase of the device temperature can even promote population inversion. These findings are changing the way that heat is perceived in semiconductor lasers and, naturally, this development will lead to novel semiconductor device concepts. An example here are the envisioned thermophotonic intersubband lasers.

References:
1.   A. Albo and Y. V. Flores, "Temperature-driven enhancement of the stimulated emission rate in terahertz quantum cascade lasers," IEEE J. Quantum Electron. 53 (1), 2300105 (2017).
2.   Y. V. Flores and A. Albo, "Impact of interface roughness scattering on the performance of GaAs/Al_xGa_1-xAs terahertz quantum cascade lasers," IEEE J. Quantum Electron. 53 (3), 2300208 (2017).
3.   A. Albo and Y. V. Flores, "Carrier leakage dynamics in terahertz quantum cascade lasers," IEEE J. Quantum Electron. 53 (5), 8500508 (2017).